Challenges in biogas valorization, effect of impurities on dry reforming process during H₂ generation: a review

Abstract

Growing concerns about our planet's sustainability have prompted the need to find alternative and renewable sources of energy. One example is the transformation of bio-waste (e.g., sewage sludge, forest leftovers, agriculture, and food processing) to generate biogas by methanisation process. Biogas is a very promising renewable energy source and is mostly composed of CH₄ and CO₂, and by far is mostly used as fuel in cogeneration systems, vehicle mobility, and household heating, among additional uses. The combustion of biogas releases greenhouse gases, so it is important to convert biogas into cleaner energy carriers such as hydrogen. Dry reforming is one of the possible ways to use the biogas as in this process two of the primary greenhouse gases found in biogas react to form syngas. The efficacy of the dry reforming process is impacted by the inability to completely remove some impurity gases from raw biogas during pretreatment. The main components of biogas are CH₄ (40–70%) and CO₂ (10–50%), with other impurities including water vapor (H₂O), nitrogen (N₂), oxygen (O₂), ammonia (NH₃), hydrogen sulphide (H₂S), carbon monoxide (CO), volatile organic compounds (VOCs; hydrocarbons, organic alcohols, aromatic hydrocarbons, halogenated compounds, and sulphur compounds like carbon disulphide and mercaptans) and volatile methyl siloxanes (VMSs) being the most common components among them. Moreover, the natural process of methanisation of organic waste produces biogas which vary in nature and impurity depending on several factors, such as the initial waste input, the microbial community involved in anaerobic digestion and the process conditions. The present study examines the effects of several contaminants on the dry reforming process, such as NH₃, H₂S, siloxanes, O₂, etc. Reported studies have focused on the development of sulfur-resistant catalysts, as H₂S has the potential to poison catalysts. In contrast, little research has been carried out into the impact of other contaminants present in biogas. This article provides a comprehensive summary of research into biogas impurities, covering their main effects, analytical and sampling techniques, and concentrations for dry reforming reactions for hydrogen production.

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1. Introduction

Global warming and energy depletion are the most pressing issues of the modern period. Rapid industrialisation and population growth have led to a surge in energy demand, which is expected to continue. Traditional fossil fuels like coal, petroleum, and natural gas continue to be heavily used, despite their substantial greenhouse gases (GHGs) emissions during burning. They release trapped carbon from the biosphere, contributing significantly to global warming and climate change.^1,2 A survey on GHGs emission as per the countries is duplicated in Fig. 1a and b stating China (13.94 billion tons), USA (6 billion tons), Germany (739.39 million tons) and France (375.93 million tons), respectively. Over the past decade, the energy sector has been the dominant source of GHG emissions. With time, the energy sector is generating global greenhouse gas emissions. The three greatest contributors account for more than 80% of GHG emissions from the energy field.⁴ However, fossil fuels cannot provide long-term energy demands because of their non-renewable nature. As a result, adopting renewable energy is essential for lowering GHG emissions. Since 2024, over 100 countries have policies and targets for renewable energy. Subsequently, extensive scientific research has been focused on the development of clean and sustainable energy sources as alternatives to fossil fuels. These approaches hold great potential for mitigating environmental impacts, shortening GHGs emissions, and ensuring a sustainable energy supply for future generations.⁵ Conventional waste treatment techniques, such as incineration and landfill, possess several disadvantages.⁶ Incineration, for instance, leads to the release of numerous toxic substances (such as dust, acid molecules and GHGs) into the atmosphere. Similarly, inadequate containment measures in landfill sites can result in the release of chemical and microbiological contaminants into the surrounding environment.⁷ Considering these challenges, anaerobic digestion presents a compelling solution as it offers a dual benefit of being a renewable energy generation process and an alternative method for waste treatment, all while mitigating the pollution risks associated with conventional treatment approaches.⁸ Anaerobic digestion involves the conversion of organic matter into its basic constituents, and this process results in the production of biogas.⁹ It is produced from multiple feedstocks, often divided into three generations. Biomass can be classified based on its origin. Biomass is primarily composed of proteins, carbohydrates, lipids, hemicelluloses, and cellulose holds significant potential as a substrate for biogas production. However, optimization is essential, as factors such as feedstock type, digestion process, and retention time directly influence the expected methane (CH₄) yield and the overall composition of the biogas.¹⁰ Seeds, grains, and sugars make up most of the first-generation feedstocks. The second-generation feedstock refers to lignocellulosic biomass, composed of lignocellulosic plants and agricultural wastes.¹¹ Algae are now gaining interest as a potential third-generation feedstock because of its rapid growth rate and high carbohydrate content.¹²

Fig. 1 Main greenhouse gas emissions in tons over a 100-year period worldwide (a) and by countries (b) (Jones et al. (2024) – with major processing by Our World in Data. “Annual greenhouse gas emissions” [https://ourworldindata.org/grapher/ghg-emissions-by-world-region]). Reproduced from ref. 3 with permission from Springer Nature, copyright 2023.

In general, raw biogas consist mainly of methane (CH₄) ∼40–70% and carbon dioxide (CO₂) 10–50%. Other minor components such as secondary impurities like nitrogen (N₂) about 0–10%, less than 0.6% of the following substances are present: carbon monoxide (CO), hydrogen sulphide (H₂S), 0–10 000 ppm, oxygen (O₂), ammonia (NH₃), siloxanes (0–41 mg_Si m⁻³), hydrocarbons (0–200 mg m⁻³), water (H₂O) are mostly considered as impurities, as their presence affect most of the possible use of the biogas. Interest in biogas production has grown significantly in many countries since the 1970s, mainly due to growing concerns about energy resources and environmental problems. Simultaneously, there has been a growing number of researchers investigating various pathways for using biogas. These pathways extend beyond traditional applications like combustion and/or energy generation and encompass emerging technologies such as converting biogas into hydrogen and liquid fuels.

Hydrogen (H₂) is an appealing substitute for fuel because of its high energy value and favorable environmental impact. H₂ is extensively used as a portable chemical fuel in a variety of applications, including domestic, automotive, industrial, and rocket propulsion. Non-renewable energy sources have considerable issues, including depletion and environmental effect. However, hydrogen is emerging as a promising answer for the future. Hydrogen energy is a promising alternative to dwindling fossil fuels due to its availability, high energy density, and lack of carbon emissions.¹³ Hydrogen can be obtained from biogas conversion using the dry reforming (DR) process. Common techniques for producing hydrogen include industrial reforming processes such as steam reforming of methane (SRM), partial oxidation of methane (POM), and dry reforming of methane (DRM).

SRM: CH₄ + H₂O ⇌ 3H₂ + CO ΔH⁰ = +206 kJ mol⁻¹

DRM: CH₄ + CO₂ ⇌ 2H₂ + 2CO ΔH⁰ = +247.3 kJ mol⁻¹

SRM is the most widely used technology for producing H₂, accounting for about 75% of the world's output.¹⁴ Recent developments in SRM concentrate on bio-alcohols, expanding its carbon-neutral uses to include butanol, ethylene glycol, glycerol, methanol, ethanol, propanol, and propanediol. There are differences between SRM, POM, and DRM in terms of the oxidant that is used, reaction kinetics, requirements for energy, and syngas ratio (H₂/CO). However, out all these approaches, DRM is the most promising as it makes use of abundant GHGs (CO₂ and CH₄) and offers an affordable way to lower net emissions into the environment.¹⁵ Extensive research has focused on dry reforming of methane (DRM) as a promising method to establish a renewable energy source.¹⁶ The DRM reaction is resulting in the production of syngas, which consists of an equimolar mixture of hydrogen (H₂) and carbon monoxide (CO). Syngas is an important reductive agent in the refinery industry as it is resulting from oil cracking process and serves then a wide range of chemical applications.¹⁷ DRM reaction is characterized as an endothermic process (ΔH = +247 kJ mol⁻¹) and consequently needs the use of appropriate catalysts to achieve desirable conversions, even at high reaction temperatures. However, the deposition of carbon on the catalysts during the DRM reaction poses a significant challenge for the implementation of DRM process in the industry. Carbon is produced by secondary reactions and leads to catalyst deactivation, by covering completely the catalyst active sites. Furthermore, catalyst formulation (its composition and the preparation method) has been developed and tested to develop a highly active and stable catalyst over DRM reaction.^18,19 The presence of the impurities may not only impact the performances over DRM, but they are also provoking equipment corrosion and the use of adapted setup equipment for the purification prior to the use of biogas. Consequently, the control of impurity levels through upgrading and purification technologies becomes crucial, tailored to the requirements of the specific technologies being employed. The presence of impurities remains a significant challenge in the exploitation of biogas.

Furthermore, a noticeable trend in recent years has been the progression of biogas utilization for various environmental applications such as fuel, heat, etc. with the aim of unlocking the full potential value of biogas and facilitating its industrialization and scalable utilization. Using the “Scopus” database, bibliometric analysis has been carried out to examine historical and contemporary advancements regarding biogas use and the impact of contaminants on the dry reforming process. The bibliometric description is a useful tool for assessing the historical development and quantitative tendencies of published scientific papers on certain subjects. As a results, we got 445 and 501 articles by using keywords “biogas dry reforming” and “biogas impurities” separately, from 2013 to 2024 (Fig. 2).

Fig. 2 Number of publications on Scopus data base by years using keywords “biogas dry reforming” and “biogas impurities” on 20-September-2024.

Most publications focus on the valorization of biogas through dry reforming, relatively few address the impact of impurities on the DRM process. Recognizing this gap, the present study aims to highlight both the advancements in biogas dry reforming and the challenges posed by the complex composition of biogas. To provide a comprehensive understanding, the study begins by exploring the origins, typical composition, and various valorization methods of biogas, offering readers a clear overview of its current utilization landscape. Despite its potential for hydrogen (H₂) production, this approach faces significant hurdles, such as catalyst deactivation and the inherently low H₂/CO ratio in the resulting syngas. The study examines current industrial DRM techniques, alongside potential integrated processes designed to overcome these limitations. Furthermore, it presents recent progress in catalyst development aimed at enhancing H₂ production via DRM. A thorough analysis is also conducted on the impact of specific impurities commonly found in biogas namely siloxanes, hydrogen sulfide (H₂S), volatile organic compounds (VOCs), oxygen (O₂), and water (H₂O) each of which poses unique challenges to the DRM process. Finally, the study concludes by outlining future research directions to advance the effective dry reforming of biogas.

2. Biogas: sources, composition, purification and valorization

2.1. Sources

The type of feedstock utilized in a certain biogas system is usually affected by the region's existing agricultural practices and industries. Several studies have calculated biogas potential from manure. In general, manure was classified by animal kind, with cattle, cows, sheep, goats, pigs, and poultry being the most included animals. Camel, donkey, buffalo, and horse were among the less frequent animals. Crop residues, or the non-food and non-fodder components of agricultural crops, were also often used to estimate potential.²⁰ The nature and shape of the leftovers were primarily determined by the crop involved, which were usually cereals like wheat and barley that produced straw or stalk as residues. Rice was also a widely researched crop, producing husk and straw as byproducts.²¹ Other research advised cultivating clover, ley grasses, intermediate crops, and catch crops to produce biogas. Crops were traditionally seen as part of single-oriented agricultural systems, although some research employed crop rotation systems to estimate their potential.²² Crop rotation is a key farming technique for food and feed systems because well-planned rotation systems can greatly enhance sustainability. Therefore, it is essential to consider crop rotation when evaluating the potential uses of agricultural substrates.^23,24 Municipal solid waste (MSW) was included in almost a third of the research. Biogas solutions for MSW can help with energy recovery while also improving trash management and nutrient recycling. Because the organic portion is utilized to produce biogas, MSW mostly consists of food waste from establishments including homes, restaurants, grocery stores, and workplaces. Recent research evaluated the possibility for producing biogas from sewage sludge, which is typically obtained from municipal wastewater treatment plants, in addition to MSW. The residues were mostly inedible components that were thrown out when processing animal goods (dairy and meat) or crops (seed oil and sugar). Biogas production can benefit from the presence of organic materials in the food industry effluent. A wide range of industrial sectors were addressed in the many articles, with slaughterhouses, sugar, milk processing, and alcohol production (such as breweries and wines) being the most often mentioned.²⁵ These primary materials are presented in Table 1.

Table 1 Possible feedstock in anaerobic processes

Sources
Agricultural origin	Industrial origin	Municipal origin	Aquatic biomass	Ref.
Animal waste	Wastewater	Sewage sludge	Microalgae	26–28
Crop waste	Sludge	Municipal solid waste	Macroalgae	27 and 28
Dedicated energy crops waste	Organic by-products			26 and 28

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2.2. Composition

The sources of organic materials are diverse and easily mobilizable for anaerobic digestion process. Indeed, anaerobic digestion (AD) or the methanisation process, is a microbiological process of degrading primary organic materials that have various origins: agricultural, industrial, and municipal waste, that take place in the absence of oxygen.²⁹ Most of the anaerobic digestion processes are working at temperatures in the range of 30 to 38 °C and with the use of mesophilic bacteria, such microorganism being the most active in this range of these temperature range. The methanisation process results in the production of gas as well as digestate. The digestate is used most of the time as a fertilizer for the soil, after maturation through composting.³⁰ The anaerobic digestion occurs in four main stages: hydrolysis, acidogenesis, acetogenesis, and methanogenesis as outlined in Fig. 3.³¹

Fig. 3 Graphic representation of the trophic chain of methanisation and its different stages.

Hydrolysis in AD is the first step in the process. During this stage, complex biopolymers and colloidal waste are broken down into soluble monomers or oligomers. This involves the breakdown of various organic polymers, including carbohydrates, proteins, and lipids. Hydrolytic enzymes, produced by fermentative bacteria, break down complex compounds into smaller, water-soluble substances such as sugars, amino acids, and long-chain fatty acids. By the end of hydrolysis, these organic compounds are available for further processing. In the next stage, acidogenesis, facultative and obligate anaerobic bacteria absorb and degrade these compounds, producing short-chain fatty acids and alcohols.³² Acetogenesis is the stage in anaerobic digestion (AD) where organic acids formed during the second phase are converted into acetic acid, acid derivatives and hydrogen. During this process, the products of hydrolysis are further broken down by various facultative fermentative microorganisms. This breakdown primarily produces weak acids, such as acetic acid, along with other volatile fatty acids like propionic and butyric acid. Additionally, lactic acid, alcohols, and hydrogen are also generated.³³ This stage is characterized by the generation of high hydrogen concentrations by acidogenic microorganisms, making it one of the fastest steps in a well-balanced anaerobic process. Acetogenesis is marked by the accumulation of intermediate products, such as lactate, ethanol, propionate, butyrate, and other higher volatile fatty acids, often referred to as electron sinks. The production of acetate by acetogenic microorganisms is a bacterial response to elevated hydrogen levels within the system.³⁴ Methanogenesis is a crucial phase in the anaerobic digestion (AD) process, significantly influencing its overall efficiency, as approximately 70% of the methane produced in AD originates from this stage.³⁵ During methanogenesis, two types of methanogens play key roles: carbon dioxide-reducing and hydrogen-oxidizing methanogens convert hydrogen and carbon dioxide into methane, while acetoclastic methanogens utilize acetate to produce methane.³⁶ Methanogens, a group of Archaea, primarily use substrates like acetate, hydrogen, and CO₂, and to a lesser extent methanol, methylamines, and formate, to generate methane and carbon dioxide.³⁷ The end products of this process serve as the main substrates for methanogenic bacteria, producing biogas which typically contain CH₄, CO₂, as well as small quantities of impurities such as nitrogen, hydrogen and hydrogen sulfide. The level of methanogenesis is an indicator of biological activity within an anaerobic system, reflecting the stability and performance of the digestion process. Higher methane production indicates a more stable and efficiently functioning system. However, biogas resulting from AD is rich in CH₄ (45–75%) and CO₂ (20–50%), which are two main greenhouse gases.³⁸ Generally, the volumetric composition of CH₄/CO₂ in biogas is close to 60/40. A wide variety of undesirable organic compounds, totaling no more than 1 vol%, are also observed. The intermediate organic molecules are issued from fermentation and are formed from: alcohols, acids, aromatics, aliphatics, ketones, esters, and hydrogen, or even thiols (mercaptans), heterocyclic compounds, terpenes, some of which are fluorinated or chlorinated.^39–41 The detailed composition of biogas is presented in Table 2.

Table 2 Biogas composition

Component	Unit	Agricultural waste	Landfills	Industrial waste	Ref.
Methane CH4	Vol%	49–69	45–55	44–67	42
Carbon dioxide CO2	Vol%	29–44	25–40	30–44	43
Hydrogen H2	Vol%	0–5	0–1	0–2	44
Nitrogen N2	Vol%	0.6–13	0–17	0.1–6	43
Oxygen O2	Vol%	<1	<1	<5	45
Carbon monoxide CO	Vol%	<2	Traces	<1	42
Ammonia NH3	Vol%	Traces	Traces	Traces	43
Hydrogen sulfide H2S	ppm	7–6570	0–5143	2–3174	46
Siloxanes	mg m^−3	<0.02	0.1–11	0–3.4	46
Aromatic & cyclic	mg m^−3	0–293	22–1614	52–705	45
Alkanes	mg m^−3	0–0.8	44–5390	9–65	45
Halogens	ppm	n.r.	1–318	n.r.	46
Water H2O	—	Saturate	Saturate	Saturate	43
Benzene	mg N^−1 m^−3	0.1–1.1	0.6–35.6	0.1–0.3	44
Toluene	mg N^−1 m^−3	0.2–7	1.7–287	2.8–11.8	45
Total chlorine	mg N^−1 m^−3	0–5	17.4–200	n.r.	42

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2.3. Purification

After biogas is produced, it requires pretreatment before it can be used. The objective of pretreatment is to remove undesirable compounds and traces of pollutants.⁴⁷ For example, for all applications, it is generally necessary to reduce the concentration of hydrogen sulfide (H₂S). H₂S and CO₂ in solution are corrosive.⁴⁸ Recently, biogas upgrading and purification technologies have been the subject of systematic research. Major biogas upgrading and purification methods include physical and chemical absorption, adsorption, membrane separation, cryogenic techniques and biological processes.⁴⁹ Absorption relies on the principle that gases exhibit varying solubilities in liquid solvents. In physical absorption, two main methods are high-pressure water scrubbing (HPWS) and organic physical scrubbing (OPS).⁵⁰ HPWS, a well-established industrial technique, uses water as a cost-effective solvent to remove CO₂ and H₂S from biogas under pressures of 6–10 bar.⁵¹ OPS operates similarly but uses organic solvents like methanol or propylene carbonate, which dissolve CO₂ and H₂S more effectively, reducing the need for solvent volume and energy consumption. Both methods efficiently separate unwanted gases from biogas. Chemical absorption involves reactions between gases and solvents, typically carried out in a counter-current system at lower pressures (1–2 bar). Alkanolamine or inorganic alkaline solutions, such as sodium and potassium hydroxide, are commonly used to remove CO₂ and H₂S.⁵² These absorbents react with the target gases, enabling efficient separation. Adsorption relies on variations in van der Waals forces between gases and adsorbent surfaces. This method is widely used in industry and can achieve CH₄ concentrations as high as 97%.⁵³ In the process, biogas is passed through an adsorption column where impurities like CO₂, H₂S, N₂ and water are captured by porous adsorbents under pressures of 4–10 bar. Once the adsorbent is saturated with CO₂, the biogas is directed to another column, and the pressure in the saturated column is reduced to release CO₂.⁵⁴ Common adsorbents include carbon molecular sieves, zeolites, and activated carbon. On the other hand, membrane separation is based on the selective permeability of membranes. The driving force for separation comes from the pressure differential between the two sides of the membrane. This method includes gas–gas separation at high pressures (typically 20–40 bar or sometimes 8–10 bar) and gas–liquid separation at near-atmospheric pressure.⁴⁵ During gas–gas separation, impurities like CO₂ and H₂S pass through the membrane while CH₄ is retained, resulting in higher methane concentrations. In gas–liquid separation, CO₂ and H₂S permeate the membrane and are absorbed by solutions, like chemical absorption. Common membranes are made from organic polymers like polyimide or inorganic materials like zeolites and MOFs. In addition, cryogenic separation is a more recent technique that relies on the differences between the boiling points of biogas components. By cooling the biogas to −70 °C at a pressure of around 40 bar, impurities such as CO₂, H₂S, N₂ and water are separated by condensation and distillation. This process produces high-purity methane and CO₂. However, this method requires the intensive use of heat exchangers and compressors, which considerably increases costs and energy consumption. Aiming at H₂S removal, traditional bio-desulfurization is widely used in the biogas industry. In this process, biogas is passed through a filter with 2–5% air, designing favorable conditions for sulfur-oxidizing bacteria to convert H₂S into elemental sulfur. This technique is often coupled with water scrubbing, where the filter offers sufficient surface area for gas–liquid interaction and bacterial growth. Shell–Paques technology, a two-stage process, enhances H₂S removal by combining adsorption and bio-oxidation.⁵² This method efficiently removes H₂S while leveraging biological processes to minimize chemical usage and operational complexity.

2.4. Dry reforming of biogas

Biogas is characterized by its density, lower calorific value of 23.1 MJ m⁻³, and Wobbe index of about 27 MJ N⁻¹ m⁻³.³⁰ Energy recovery is chemically linked to methane % in biogas, the only component that contributes to the higher calorific value (1 m³) of raw biogas in standard conditions (25 °C and 1 atm), which contains 60% methane produces 21.5 MJ of heat when burned.^29,44 The similar chemical composition of biogas and natural gas allows them to be transported on already existing networks and to be used for similar applications. However, the quality of biogas is tailored to the specific needs of the applied techniques, which is why biogas undergoes different treatments that are necessary for each final use. Direct use of raw biogas for lighting and cooking is possible. Indirect biogas utilisation includes physical, chemical, and biological methods that treat biogas to convert it into other forms which can be exploited. The process of transforming biogas into hydrogen contributes significantly to the reduction of greenhouse gas emissions. The production of hydrogen can be achieved through the dry reforming (DR) process, which converts biogas into syngas. DR is an energy-intensive process as could be seen from the high positive enthalpy of the reaction shown in eqn (1). The resulting gas can manufacture various chemicals, including methanol, gasoline, and Fischer–Tropsch oil. The reaction is endothermic, requiring high temperatures between 700 and 900 °C to promote the conversion of CH₄ and CO₂ into syngas. Along with the primary reaction, other side reactions may occur simultaneously during the DR process. One of the typical side reactions in DR is the reverse water gas shift (RWGS) (eqn (2)). This reaction can decrease the proportion of hydrogen to carbon monoxide as end product, which is not favorable for syngas production. However, the RWGS reaction can be advantageous in modifying the H₂/CO ratio to produce larger hydrocarbons.⁵⁵ Moreover, the RWGS reaction may lead to greater carbon dioxide conversion than methane.⁵⁶

CH₄ + CO₂ ⇌ 2H₂ + 2CO ΔH⁰ = +247.3 kJ mol⁻¹ (1)

CO₂ + H₂ ⇌ CO + H₂O ΔH⁰ = +41 kJ mol⁻¹ (2)

During the dry reforming process, other significant reactions lead to the formation of coke.⁵⁷ For example, decomposition of methane (eqn (3)), produces solid carbon that covers the catalyst surface and releases hydrogen gas. Eqn (4) illustrates the Boudouard reaction where carbon monoxide is disproportionate to produce solid carbon and carbon dioxide. Additionally, the hydrogenation of carbon dioxide and carbon monoxide occur during the process, where these compounds react with hydrogen gas to produce solid carbon and water vapors as shown in eqn (5) and (6).

CH₄ ⇌ C + 2H₂ ΔH⁰ = +74.9 kJ mol⁻¹ (3)

2CO ⇌ 2C + CO₂ ΔH⁰ = −172.4 kJ mol⁻¹ (4)

CO₂ + 2H₂ ⇌ C + 2H₂O ΔH⁰ = −90 kJ mol⁻¹ (5)

CO + H₂ ⇌ C + H₂O ΔH⁰ = −131.3 kJ mol⁻¹ (6)

Although biogas contains more CH₄ than CO₂, it is still possible to produce carbon (coke). Coke formation can occur between 553 °C and 700 °C caused by methane decomposition starting at > 550 °C and from the Boudouard reaction that occurs below 700 °C.⁵⁸ The probability of coke formation depends on the ratio of oxygen, carbon, and hydrogen from the gas composition. If the O/C and H/C ratios decrease, the chances of coke formation increase. To avoid carbon formation, the CO₂/CH₄ ratio in the biogas feed should be adjusted to be equal to or less than one.⁵⁹ Another limitation of dry reforming process is the slow kinetics of reaction, in this condition higher rection temperature are needed and consequently higher energy input. To improve biogas conversion efficiency, various types of dry reforming catalysts have been developed. In DRM, catalysts play various roles, such as decreasing reaction temperature, promoting higher conversion, directing the selectivity to avoid carbon as product and stability for longer reaction stream. The efficiency of the DR catalyst is influenced by various factors, including the choice of metal, the type of support material, the surface area of the support, the size of the metal particles, and the SMSI (strong metal support interaction) effect. Noble metals such as Rh, Ru, Pt, Pd, and Ir are commonly used due to their high catalytic activity and good resistance toward carbon production.⁶⁰ The catalytic activity of these materials follows the trend: Rh > Ru > Ir > Pd > Pt. However, their high cost and limited resources of noble metals limit their industrial application. Non-noble metals, such as nickel, have gained attention as low-cost alternatives due to their excellent activity and more abundant and cheap sources. Over time, nickel-based catalysts often suffer from fast sintering, that provokes carbon accumulation, leading to a fast decreasing of activity. However, the quantification of coke deposition on the catalysts follows a different order: Ni > Pd ≫ Ir > Pt > Ru, Rh. To increase the catalytic performances, Ni must be highly dispersed in combination with promoters. Bimetallic catalysts using transition metals (Co, Fe, Cu) have also been studied. Co, Fe, and Cu may not exhibit significant catalytic activity when utilized in monometallic catalysts, but they have the potential to act in synergy with a in bimetallic catalysts formulations. Moreover, bimetallic catalysts incorporating nickel with noble or transition metals are being developed to enhance catalyst activity.⁶¹ In addition, the synthesis of catalysts has been enhanced through the introduction of new designs such as core–shell structure and Janus particles, aimed at improving their stability toward sintering and carbon formation.^62–64

Supported catalysts, that consist of a metallic active phase and a support material that provides a large surface area for the metal nanoparticles to be dispersed on and to enhance their stability. During dry reforming, both CH₄ and CO₂ participate in a bifunctional mechanism where the metal phase adsorbs and provides enough energy for the dissociation of CH₄ and of CO₂. The supports used in dry reforming are generally composed of SiO₂, Al₂O₃, MgO, ZrO₂, TiO_2, or CeO₂. To change and balance the chemistry at the surface of the supported catalyst, apart from single element supports, researchers have also investigated catalysts based on supports composed by mixed oxides, such as MgO–Al₂O₃, ZrO₂–SiO₂, and CeO₂–ZrO₂, etc. In addition, mesoporous materials offer a new method for synthesizing catalysts. They have a large surface area of > 800 m² g⁻¹, high pore volume, and uniform pore size (from 3 to 15 nm), making them efficient for dispersing active metals nanoparticles. Mesoporous silicas support like MCM-41, SBA-15 and KIT-6 are types of mesoporous materials that are commonly used in making catalysts.⁶⁵

3. Impurities

The presence of impurities in biogas can significantly impact the efficiency and stability of the upgrading process. The strategy for managing these impurities depends on their chemical nature and concentration. In cases where impurities pose a high risk of catalyst poisoning or equipment corrosion, pre-treatment and purification of the biogas are required prior to utilization. By far some impurities found in biogas, such as sulfur compounds, siloxanes, VOCs, O₂, H₂O may bring significant drawbacks in the dry reforming process. Understanding the effects of these impurities is essential to optimize biogas reforming by synthesizing more efficient catalysts resistant to the impurities present in biogas, and to improve the viability of this sustainable energy source. This section presents a general discussion of the various impurities present in biogas and their effect on catalysts during the dry reforming process, based on various studies carried out in the presence of impurities.

3.1. Sulfur compounds

Impurities in biogas negatively impact the activity and stability of catalysts, regardless of their properties or reaction conditions. “Sulfur” is a term that encompasses a diverse range of compounds present in biogas. The prevalent forms include hydrogen sulfide, sulfur dioxide, thiophene, and its derivatives. H₂S, which is typically present in biogas, is a significant impurity that rapidly poisons catalysts, by reacting with active phase site and forming sulfides even at low concentrations and corrodes equipment. H₂S is formed during the anaerobic digestion process as microorganisms decompose sulfur-containing compounds in organic matter. The concentration of H₂S in biogas varies widely depending on the raw materials and digestion conditions. There are several methods that may aim H₂S to remove biogas, including biological, chemical, and physical treatments. Biological methods involve the use of microorganisms that oxidize H₂S to elemental sulfur or sulfate. For instance, in a study reported by Chaghouri et al.,⁶⁶ states that they completely removed H₂S from a synthetic biogas containing 1700 ppm of H₂S, which is similar to the concentration found in landfill biogas. They achieved this by using a H₂S-resistant fungal species called Trichoderma harzianum, which were isolated from industrial digestate. This fungus converted H₂S into various sulfur compounds, including SO₄²⁻, S₈, SO₂, and DMS, using a fungi-based biofilter made from biodegradable materials. The study highlighted the important role of fungal and bacterial species in the removal of H₂S from biogas. A widely used chemical method for hydrogen sulfide removal from biogas involves passing the gas through filters composed of iron oxide or activated carbon, which act as adsorbents. These materials capture and retain H₂S through adsorption processes. Iron oxide reacts with H₂S to form iron sulfide (Fe₂S₃), while activated carbon physically or chemically adsorbs the gas onto its high-surface-area structure. This process effectively reduces H₂S concentration in biogas, preventing corrosion, equipment damage, and toxic emissions, thereby improving the quality and usability of the biogas.^45,67 Although desulfurization is often applied, traces of sulfur compounds are always remaining after purification process.

3.1.1. Catalyst deactivation: mechanisms of sulfur poisoning. The presence of H₂S in biogas during dry reforming presents a major challenge due to its strong poisoning effect on catalysts. H₂S can react with active metal sites, forming metal sulfides (e.g., NiS) that reduce catalytic activity and stability. It also participates in side reactions, such as the formation of COS, CS₂, and elemental sulfur, which further block active sites and modify reaction pathways. These reactions not only deactivate the catalyst but also promote coke formation, accelerating the loss of performance. For biogas containing H₂S under dry reforming reaction, the possible reactions involved between H₂S and the other species are:^68,69

H₂S decomposition: 2H₂S ⇌ 2H₂ + S₂ ΔH_298K = +45.24 kJ mol⁻¹ (7)

CO desulfurization: CO + H₂S ⇌ COS + H₂ ΔH_298K = −56 kJ mol⁻¹ (8)

CS₂ partial hydrolysis: CS₂ + H₂O ⇌ COS + H₂S ΔH_298K = +33.19 kJ mol⁻¹ (10)

COS hydrolysis: H₂S + CO₂ ⇌ COS + H₂O ΔH_298K = +33.45 kJ mol⁻¹ (11)

CS₂ formation: CH₄ + 2S₂ ⇌ CS₂ + 2H₂S ΔH_298K = −98.59 kJ mol⁻¹ (12)

There are three primary types of mechanisms that can cause this deactivation: sulfidation, weathering, and carbon formation. These mechanisms can differ according to the reaction conditions.^70,71Fig. 4 illustrates these three mechanisms that are commonly observed. First of all, sulfur can interact with catalyst active sites through adsorption of sulfur compounds and the formation of metal sulphides. When hydrogen sulphide gas reacts with the active metal sites in the catalyst, metal sulfides are formed which block catalytic activity and generally have no significant catalytic properties for reforming hydrocarbons into syngas (Fig. 4(1)). Nickel-based catalysts are highly susceptible to deactivation by sulfide compounds, whatever the sulfur species involved. When H₂S is present in the feedstock, the deactivation mechanism is primarily influenced by H₂S concentration and operating temperature. At low H₂S concentrations, sulfur adsorbs onto the catalyst surface, preventing reagents from reaching the active metal sites. This type of deactivation is generally reversible, and catalytic activity can be restored by raising the temperature or reducing the sulfur content of the feed, which desorbs the sulfur from the surface.

Fig. 4 An illustration of the three main mechanisms of catalyst deactivation by sulfur (reproduced from ref. 70 with permission from Elsevier, license number 5873100720225, copyright 2019).

However, at higher H₂S concentrations, the increased chemical potential of sulfur promotes chemisorption, leading to sulfidation reactions that form metal sulfides. These reactions are generally irreversible and lead to a permanent loss of catalytic activity, as the transition metal active sites are permanently altered. The second mechanism is closely linked to the first, as sulfur compounds can interact with active metal sites in various ways, especially in catalysts with heterogeneous structures. Sulfur poisoning can significantly impact on catalyst functionality, altering its affinity, selectivity, and the reaction pathways of reagents adsorbed on the metal surface (Fig. 4(2)). These changes can lead to shifts in catalytic performance, affecting the overall efficiency and selectivity of the reactions.

The third mechanism involves the role of sulphur compounds in the growth and deposition of carbonaceous coke on the catalysts. Several studies propose that when catalysts are poisoned by sulfur, coke development accelerates.^70,72 This is because alkyl radicals preferentially interact with sulfur atoms to create R–S species, which quickly dehydrogenate and form stable coke deposits on the active sites of the catalyst surface (Fig. 4(3)). H₂S binds strongly to the active metal site, such as Ni, forming strong Ni–S bonds that are stronger than Ni–Ni bonds in the rest of the particle according to this reaction: H₂S + Ni ⇌ Ni–S + H₂ (This reaction is highly exothermic, thus increasing the temperature of the catalyst will shift the chemisorption equilibrium to the left-hand side of equation).⁷³ This forms a sulfide layer that blocks the surface for reactions, leading to deactivation. Nickel is more susceptible to sulfide formation than other metals of group eight due to its favorable sulfidation chemical equilibrium.⁷⁴ At 500 °C and H₂S partial pressure, a moderate sulfur coverage is obtained on nickel, which means that the sulfur in the feed will be fully retained until saturation.⁷⁵ To overcome sulfur poisoning, various strategies have been developed to improve catalyst resistance. These approaches aim to enhance the durability and activity of catalysts.

3.1.2. Sulphur resistance strategies. Sulfur resistance in reforming reactions is a key factor for ensuring stable operation under industrial conditions. To address this challenge, various strategies have been developed to enhance sulfur tolerance. These approaches are broadly classified into two categories: extrinsic improvements, such as process integration and feedstock treatment, and intrinsic improvements, which involve optimizing the composition and structure of the catalysts. This section explores some of the intrinsic strategies and provides an overview of current trends in the design of sulfur-resistant reforming catalysts. One of the most fundamental approaches to improving sulfur resistance in reforming systems is to modify the intrinsic properties of the catalytic material. Changing the supports and introducing or substituting various chemical elements (such as metal dopants, including noble metals, rare earths, alkali metals, and transition metals) known for their anti-sulfur properties into existing bimetallic catalyst designs is the simplest method to enhance catalysts.⁷⁶ These elements either prevent sulfur deposition on the catalyst, exhibit catalytic properties similar to existing materials but with greater sulfur resistance, or generate synergistic effects when combined with existing catalysts. Fig. 5 summarizes these strategies for improving sulfur tolerance.

Fig. 5 Design strategies for sulfur-tolerant Ni-based catalysts (reproduced from ref. 76 with permission from Elsevier, license number 5875840641798, copyright 2022).

The use of alumina as a catalyst support improves Ni dispersion, resulting in Ni particles of uniform size and better resistance to sulfur poisoning. However, other supports may offer better performance in sulfur-rich environments. Natural minerals such as dolomite and mayenite have shown promising sulfur tolerance.⁷⁷ Calcined dolomite, composed of calcium carbonate and magnesium, was found to be more effective against H₂S poisoning in the pyrolysis, gasification and two-stage reforming than Ni/Al₂O₃.⁷⁸ Srinakruang et al.⁷⁹ reported that Ni/dolomite exhibited better reforming activity, decreased coke formation and higher sulfur resistance compared to Ni supported on Al₂O₃ and SiO₂, due to the alkaline earth metals in dolomite adsorbing sulfide ions. In addition, Ni/mayenite showed higher reforming activity under 500 ppm H₂S compared with Ni/Al₂O₃, attributed to the adsorption of sulfide ions in mayenite, which enhanced oxygen mobility on the catalyst surface.⁸⁰

Introducing other metals into Ni catalysts to form bimetallic systems is a key approach for enhancing resistance to sulfur poisoning. Noble metals like Rh have been shown to significantly improve both catalytic activity and sulfur tolerance. For instance, Lakhapatri et al. demonstrated that adding Rh and Pd to Ni catalysts enhanced their performance, with Rh preventing rapid deactivation in sulfur-rich conditions.⁸¹ Rh also inhibits the growth of Ni crystallites during synthesis and steam reforming, maintaining catalyst stability. DFT calculations suggest that H₂S adsorbs more strongly to Rh–Ni (111) alloys than to Ni (111).⁸² However, it desorbs more easily from Rh, which reduces the formation of inactive Ni–S complexes.⁸³ Rh reduces the adsorption of dissociated sulfur species (HS, S, H), making it easier to remove them from the catalyst surface. Although Rh and other noble metals are effective in improving sulfur resistance, their high cost limits their economic feasibility in large-scale applications. Due to their high electro positivity, alkaline elements can donate electrons to sulfur ligands, thereby inhibiting the interaction between sulfur species and the active metal, helping to mitigate sulfur poisoning. Alkali and alkaline earth elements have been demonstrated to be effective in enhancing the ability of a catalyst to resist sulfur at low levels. Potassium, for example, has been found to serve different functions in reforming processes: it can enhance active sites and decrease activity while also preventing formation of coke. However, studies have shown that it can also reduce the surface coverage by sulfur of active nickel sites by weakening the bond Ni–S.^84,85 This approach is being explored as a potential solution to reduce the negative impact of sulfur poisoning on the materials used. Ceria tends to promote the formation of sulfur oxides at elevated temperatures because of its superior ability to move oxygen, allowing the oxygen anions to react with sulfur on the active metal sites. Transition metals have also been found to enhance the sulfur resistance of Ni catalysts. For exemple, the addition of tungsten (W) promotes sulfur dissociation by reacting with sulfur-passivated Ni to form a W–S complex.⁸⁶ This complex is easily reduced by H₂, facilitating the desorption of H₂S from the tungsten surface. The addition of boron to a nickel-based catalyst improves its performance in reforming reactions, even in the presence of sulfur. Boron enhances sulfur resistance by coating the nickel particles, which reduces direct interaction between nickel and sulfur. This prevents sulfur from deactivating the catalyst, leading to improved sulfur tolerance and overall better catalytic efficiency.⁸⁷

3.1.3. Catalyst regeneration and H₂S impact on DR performance. The performance of the catalyst is strongly affected by the presence of H₂S in the biogas, particularly in dry reforming (DR) reactions. Researchers have focused on understanding how H₂S affects catalysts and have worked to develop sulfur-resistant catalysts able to maintain their activity in sulfur-rich conditions. These studies aim to better understand how H₂S interacts with catalysts and to find ways to reduce its negative effects on performance, for example by using regeneration techniques to restore the activity of the catalyst and improve its lifespan. Chattanathan et al.,⁸⁸ examined the impact of H₂S on the conversion of CO₂ and CH₄, using H₂S concentrations ranging from 0.5 to 1.5 mol%. They observed that the introduction of 0.5 mol% H₂S resulted in a significant decrease in the conversion rates of CO₂ and CH₄ using a commercial catalyst (Reformax^® 250). Before the introduction of H₂S, the conversion rates of CO₂ and CH₄ were 87% and 67%. However, after the introduction of 0.5 mol% H₂S, the conversion rates decreased to 22% and 19%, respectively. The authors attributed this decrease in conversion rates to the formation of a sulfur layer on the catalyst surface during the dry reforming of biogas. Another study conducted on the dry reforming of synthetic biogas by Pawar et al.,⁸⁹ by using a 15 wt.%Ni/γ-Al₂O₃ catalyst at different H₂S concentrations (0, 5 ppm, and 10 ppm), two CH₄/CO₂ ratios (1.5 and 2), and two temperatures (700 °C and 800 °C). The results showed that the catalyst deactivation rate increased with increasing H₂S concentration in the biogas. However, when H₂S was eliminated, a slow regeneration of catalyst activity occurred at 800 °C and this was attributed to the unfavorable desorption kinetics of H₂S at lower temperatures. The amount of sulfur adsorbed on the catalyst surface indicated a decrease in the H₂/CO ratio of less than one. This decrease was attributed to higher CO₂ conversion than CH₄ and sulfur poisoning, which favored secondary reactions such as RWGS and coke degasification.

The method of regeneration is influenced by the catalyst since it can modify the structure and oxidation state of the catalyst.⁹⁰ In a particular experiment, subjecting a Ni/Al₂O₃ catalyst to air treatment at a temperature of 700 °C did not result in the regeneration of the catalyst. This was due to the formation Ni aluminate (NiAl₂O₄) compound.⁹¹ Consequently, even if the regeneration is a reversible process, the catalyst has altered catalytic properties at high temperature caused the formation of an undesirable compound that make it ineffective. Blanchard et al.,⁹² studied the impact of H₂S concentrations on dry reforming of methane using the Ni/Al₂O₃-YSZ (yttria-stabilized zirconia) catalyst. Their results showed that the catalyst was able to withstand lower concentration of H₂S (1.55 ppm) and higher temperature (900 °C), but experienced deactivation after 50 hours of exposure to concentrations ranging from 5 to 235 ppm of H₂S at temperatures ranging from 780 °C to 825 °C. While the catalyst Ni/Al₂O₃-YSZ was partially regenerated, its deactivation rate remained higher than that of the fresh catalyst even after calcination at 900 °C and cessation of the H₂S supply. To better understand the deactivation – regeneration performance and the sulfur poisoning mechanism during dry reforming of biogas, the group of Chen⁹³ conducted a study to examine the Ni/SiO₂ catalyst under various operating conditions by evaluating the effectiveness in reversing the deactivation caused by sulfur poisoning. They compared three methods of catalyst regeneration: stopping H₂S feeding, temperature-programmed calcination (TPC), and O₂ activation. The stability and catalytic activity tests revealed that the nickel become deactivated as the H₂S concentration increases from 0 to 50 and then to 100 ppm (Fig. 6a and b). The poisoning occurs due to the formation of Ni₇S₆, which blocks the active sites by forming Ni–S species. The diameter of metallic Ni nanoparticles deposited during the reaction decreased in the following order: original deactivation > stopping H₂S feeding > TPC > O₂ activation > freshly reduced catalyst. This suggests that sulfur poisoning can also promote catalyst sintering, and that O₂ activation is the most effective method for regenerating the catalysts. Similarly, the regeneration of sulfur poisoned 20 wt%Ni–5 wt%CeO₂/Al₂O₃ catalyst is invested by Chein et al.,⁷³ They identified two key factors that affect the poisoning: temperature and time. The researchers tried three methods to regenerate the catalyst: high-temperature reaction (700–900 °C), high-temperature bi-reforming (800 °C), and high-temperature oxidation (800 °C). It was found that increasing the temperature higher than 700 °C, reduced the amount of sulfur covering the catalyst surface, but regeneration was not possible at temperatures lower than 700 °C as the sulfur strongly adsorbed on the surface. A longer reaction time (10 h) was required to regenerate the catalyst using the high-temperature reaction method. This study also showed that high-temperature bi-reforming, which involves adding O₂ and H₂O to the reagent, prevented regeneration, while high-temperature oxidation effectively regenerated the catalyst. Saha et al.,⁹⁴ have developed a novel type of alumina-supported catalysts (15 wt% Ni + 0.5 wt% Co/M_aO_x–N_aO_x–Al₂O₃ where M = Mg, Ca, La, Y, Gd; N = Al, Zr) for dry reforming of biogas that can resist sulfur. To improve their deactivation capability, they added basic oxide dopants to the support structure to modify its basicity. Co addition enhanced the sulfur tolerance of the Ni species, while the addition of MgO and ZrO promoters boosted catalyst surface area, metal surface area, and pore volume. The test results showed that the bi-metallic (reverse stepwise Co → Ni) catalysts exhibited the highest activity after exposure to H₂S (100 ppm) at 900 °C under a 50 : 40 : 10 vol% of CH₄ : CO₂ : N₂ mixture, followed by bi-metallic (simultaneous Ni + Co), mono-metallic (Ni), and bi-metallic (stepwise Ni → Co) catalysts (Fig. 6c and d). Ocsachoque et al.,⁹⁵ have investigated the impact of metal phases (Rh, Ni) and CeO₂ support on sulfur deactivation of Rh/CeO₂ and Ni/CeO₂ catalysts during dry reforming of methane. Their research found that Rh/CeO₂ had a higher tolerance to sulfur and desorption of S–O species compared to Ni/CeO₂. The reason for this was the ability of Rh/CeO₂ to donate oxygen effectively, which facilitated the reduction of CeO₂, as indicated by TPR results. In contrast, the presence of O²⁻ species on the Rh/CeO₂ sample helped to eliminate sulfur in the form of SO₂ and prevent Rh–S interaction. Overall, the CeO₂ support served as a sacrificial trap, decreasing sulfur poisoning and the deactivation of Rh metal phase. Similarly, Rh helped to remove S–O species formed on the support, delaying sulfur poisoning of the Rh/CeO₂ catalyst. Similarly, in a study conducted by Chein et al.,⁹⁶ it was found that sulfur poisoning is regenerated at high temperatures (>700 °C). The impact of H₂S has been studied both experimentally and theoretically. The theoretical study used a new thermodynamic equilibrium model to determine the rate at which H₂S can be removed in the dry reforming of biogas containing H₂S. The study evaluates the effect of two sulfur sorbents, calcium oxide (CaO) and calcium carbonate (CaCO₃), and found out that the removal rate depends on the initial H₂S concentration and temperature. Furthermore, the work also revealed that CaO is more preferable than CaCO₃ due to its lower CO₂ conversion and thus lower CO yield, decreased reaction temperature, and finally its ability to adsorb CO₂.

Fig. 6 Influence of H₂S concentration on dry biogas reforming at 700 °C: (a) CH₄ and CO₂ conversion (reproduced from ref. 93 with permission from American Chemical Society, copyright 2017) and (b) H₂/CO ratio, and a long-term stability study on (c) 5Co →15Ni/MgO–Al₂O₃ and (d) 15Ni/CaO–Al₂O₃ catalyst for biogas reforming (T = 900C; CH₄/CO₂/N₂/H₂S = 50%/40%/10%/100 ppm) (reproduced from ref. 94 with permission from Elsevier, license number 5815851414890, copyright 2014).

On the other hand, Mancino et al.⁹⁷ revealed that removing sulfur from the feed is enough to regenerate the catalytic surface of Rh/γ-Al₂O₃ and restore its initial catalytic performances. Regardless of the type of sulfur-carrying compounds present, such as H₂S and SO₂, the catalyst becomes deactivated at low concentration (1 ppm of H₂S), and the surface reaches sulfur saturation at 10 ppm or higher concentration. Sulfur binds directly to Rh in the catalyst, but removing S from the feed causes the Rh to be released also, allowing the CH₄ molecules to be reactivated. The impact of Rh concentration on Ni-based reforming catalysts was investigated by Theofanidis et al.,⁸² by using a gas mixture corresponding to the effluent of a biomass gasifier with and without 7 ppm H₂S impurities. Four Ni–Rh catalysts were tested, with Ni : Rh molar ratios ranging from 18 to 82. The introduction of sulfur significantly reduced the catalyst activity, with all tested samples losing 90% of their activity for CH₄. The regeneration rate of the catalyst was influenced by the Ni : Rh molar ratio. This was attributed to the formation of a Ni–Rh surface alloy during the reduction process above 530 °C, which increased the activation energy for H₂S dissociation during dry reforming. As a result, the presence of Rh suppressed H₂S dissociation compared to pure Ni catalysts. The Ni–Rh catalyst supported on Mg–Al spinel was studied by Yin et al.,⁹⁸ to investigate the impact of low concentrations of H₂S (0, 2.5, 5 and 7.5 ppm) and NH₃ (0, 30 and 50 ppm) at 700 °C on the reforming of model biogas. The presence of H₂S was found to block the reforming sites, but its removal partially restored activity, with the extent of restoration depending on the concentration of H₂S. In contrast, NH₃ addition caused a gradual decline in catalyst activity, which was restored upon NH₃ removal. The researchers developed a detailed kinetic model that accounted for the effects of H₂S and NH₃ and validated it using experimental data. The catalyst's deactivation was attributed to a simple molecular adsorption step, and the rate parameter analysis revealed that H₂S and NH₃ dissociated on the Ni sites. However, the model underestimated activity restoration in cases of H₂S poisoning. Similarly, Gao and colleagues⁹⁹ conducted a study to assess the impact of the presence of NH₃ and H₂S onto the performance of Ni/MgO catalysts synthesized through wet impregnation in the dry reforming of biogas. They also examined the synergetic effect of both H₂S and NH₃ on the catalyst's performance. The study results demonstrated that as the concentration of H₂S increased from 1 to 50 ppm and the temperature rose, the rate of catalyst deactivation also increased significantly (Fig. 7). However, the deactivated catalysts were able to be restored to their original activity through air calcination at elevated temperatures of 500 °C. Furthermore, the study also revealed that when H₂S and NH₃ were present together: (1) the decline in biogas conversion rate was found faster than when only H₂S was present, (2) the surface oxygen of the catalyst was inhibited by the synergistic effect of H₂S and NH₃ than by H₂S alone, and (3) air calcination could also recover the activity of the catalyst that had been deactivated under the combined effect of H₂S and NH₃. A study was conducted by Liu and colleagues¹⁰⁰ on the deactivation of a Ni–Mo/SiO₂ catalyst used in the H₂S-induced dry reforming of methane process. Their objective was to examine the catalyst's sulfur tolerance performance and the role of MoO_x surface decoration in improving the catalyst resistance. Their findings showed that H₂S adsorbed on the active Ni sites, leading to the formation of different Ni–S phases that caused catalyst deactivation. However, the Mo-promoted Ni/SiO₂ catalysts exhibited better resistance to H₂S. The study revealed that an appropriate amount of Mo could enhance the catalyst's H₂S adsorption capacity, slowing down the sulfidation rate of the active Ni sites and protecting them from deactivation. Moreover, the Ni–Mo intermetallic formed resulted in “inactive” oxygen performance, leading to continued sulfur adsorption behavior. Therefore, the added Mo acted as an intermetallic promoter, improving the catalytic performance of the catalyst. Gaillard et al.,¹⁰¹ have prepared a novel catalyst based on molybdenum and evaluated its performance in the dry reforming of methane under conditions containing 50 ppm of H₂S in the feed. The results demonstrated that the molybdenum-based catalyst exhibited excellent resistance to sulfur accumulation. Additionally, the optimized catalysts consisting of molybdenum promoted by nickel exhibited a synergistic effect, leading to improved reducibility and stability in the presence of sulfur of 50 ppm. The effect of the solid-state synthesis at high temperatures (1200–1600 °C) to synthesize microcrystalline spinel-supported Ni, Ni–Co, and Ni–Cu catalysts with highly faceted oxides was evaluated by Misture et al.¹⁰² The presence of {1 1 1} facets facilitated the removal of sulfur and increased sulfur tolerance by improving oxygen transfer. The sulfur resistance of the catalysts was also influenced by the Ni/Co ratio, as quantum mechanical calculations showed that the adsorption energy of H₂S on metal surfaces is slightly higher for Co than for Ni, and that Co should therefore be more prone to sulfur poisoning. The researchers found that the activity and stability of the catalysts were significantly improved through regeneration. In particular, the Co_0.375Ni_0.375Mg_0.25Al₂O₄ catalyst achieved 94% CH₄ conversion in the presence of 20 ppm H₂S remaining stable for 12 hours after regeneration. A newly developed catalyst has been reported by Sato et al.,¹⁰³ Ni–WO₃/MgO–CaO, which incorporates WO₃ as a sulfur-resistant promoter. The catalyst has shown good sulfur activity and stability, even at high concentrations of 300 ppm. The addition of WO₃ has facilitated the decomposition of sulfur compounds, leading to accelerated reactions (NiS_x + W → Ni + WS_x and WS_x + xH₂ → W + xH₂S) and promoting the retention of active Ni within the catalyst. Jiang and colleagues¹⁰⁴ also conducted a study to assess the sulfur tolerance of Ni- and Co based Ce/Zr and Ce/La oxide dry reforming catalysts. The results indicated that only the catalysts containing both Co and Ni supported by Ce–Zr oxide demonstrated extended tolerance to 20–30 ppm of sulfur. This tolerance was attributed to the synergy between Co and Ni in the catalysts. A summary of the compositions of recently investigated sulfur-deactivated catalysts developed for syngas production, as reviewed in this section, is given in Table 3.

Fig. 7 Effect of different H₂S concentrations on CH₄ and CO₂ conversion, (a) 700 °C CH₄, (b) 700 °C CO₂, (c) 750 °C CH₄, (d) 750 °C CO₂, (e) 800 °C CH₄, (f) 800 °C CO₂, GHSV = 15 000 mL g_cat.⁻¹ h⁻¹ (reproduced from ref. 99 with permission from Springer Nature, license number 5815930902224, copyright 2022).

Table 3 Catalyst studied under sulfur for syngas production from methane reforming

Metal	Support	Loading wt%	Reforming Temp., °C	H2S concentration	Reforming condition	Conversion (%)			Sulfur deposition wt%	Ref.
						CH4	CO2	H2/CO
Ni	Al2O3	50	750	—	CO2/CH4 = 1.5, N2 = 2, GHSV = 300 cm^3 gcat^−1 min^−1, t = 5 h	67	86	0.8	0	88
				0.5%		23	18	n.r.	1.58
				1%		11	15	n.r.	1.4
				1.5%		16	16	0.4	1.25
NiAl2O4	Al2O3-YSZ	5	800	231 μL L^−1	CO2/CH4/H2O = 1/1/0.1, GHSV = 6800 mlSTP gcat^−1 h^−1, mcata = 0.3087 g, t = 6 h	10	n.r.	n.r.	n.r.	92
			825	26.3 μL L^−1		9	n.r.	n.r.	n.r.
			900	1.55 μL L^−1		88	n.r.	n.r.	n.r.
Ni	γ-Al2O3	15	700	—	CO2/CH4 = 1.5, GHSV = 16.2 m^3 kg^−1 h^−1, t = 6 h	48	59	1.1	n.r.	89
				5 ppm		27	36	1.02	n.r.
				10 ppm		16	18	0.84	n.r.
				—	CO2/CH4 = 2, GHSV = 16.2 m^3 kg^−1 h^−1, t = 6 h	30	56	1.09	0
				5 ppm		17	26	0.93	0.05
				10 ppm		15	22	0.91	0.04
			800	—	CO2/CH4 = 1.5, GHSV = 16.2 m^3 kg^−1 h^−1, t = 6 h	65	70	1.16	n.r.
				5 ppm		26	37	1	n.r.
				10 ppm		19	18	0.63	n.r.
				—	CO2/CH4 = 2, GHSV = 16.2 m^3 kg^−1 h^−1, t = 6 h	50	76	1.2	0
				5 ppm		15	23	0.88	0.12
				10 ppm		13	21	0.71	0.19
Ni	SiO2	10	700	—	CO2/CH4/N2 = 0.4/0.4/0.2, mcata = 100 mg (0.42–0.84 mm), GHSV = 24 000 mL g^−1 h^−1, t = 4–5 h	52	63	n.r.	n.r.	93
				50 ppm		5	10	0.42	n.r.
				100 ppm		≃0	≃0	≃0	n.r.
			750	—		82	84	n.r.	n.r.
				50 ppm		12	9	0.41	n.r.
				100 ppm		6	8	0.22	n.r.
			800	—		88	88	n.r.	n.r.
				50 ppm		15	21	0.23	n.r.
				100 ppm		9	16	0.11	n.r.
Ni	MgO	10	700	—	CO2/CH4 = 50%/50%, mcata = 200 mg, GHSV = 15 000 mL g^−1 h^−1, t = 12 h	62	67	n.r.	0	99
				1 ppm		18	30	n.r.	0.88
				5 ppm		3	8	n.r.	1.05
				10 ppm		5	10	n.r.	1.09
				20 ppm		4	9	n.r.	1.17
				50 ppm		3	7	n.r.	1.32
			750	—		65	75	n.r.	0
				1 ppm		29	41	n.r.	0.83
				5 ppm		2	5	n.r.	1.13
				10 ppm		5	3	n.r.	1.14
				20 ppm		4	5	n.r.	1.10
				50 ppm		≃0	6	n.r.	1.21
			800	—		82	93	n.r.	0
				1 ppm		8	14	n.r.	0.90
				5 ppm		3	5	n.r.	1.04
				10 ppm		5	9	n.r.	1.08
				20 ppm		4	8	n.r.	1.12
				50 ppm		5	11	n.r.	1.15

---

The presence of sulphur compounds in biogas, especially hydrogen sulphide (H₂S), constitutes a significant obstacle to the catalytic stability and efficiency of dry reforming reactions. Sulphur compounds deactivate catalysts by generating metal sulphides that block active sites, change reaction pathways, and enhance carbon deposition. This reduces the conversion rates of CH₄ and CO₂ and overall selectivity towards syngas. Nickel-based catalysts, which are commonly used for biogas reforming because of their high activity and cost-effectiveness, are very susceptible to sulphur poisoning, resulting in permanent deactivation under sulfur-rich conditions. Extensive research has been conducted to generate sulfur-resistant catalysts using both intrinsic and extrinsic alterations. Intrinsic techniques include the use of noble metals (e.g., Rh, Pd) and transition metals (e.g., Mo, W), which improve sulphur tolerance by altering the electronic structure of active sites and weakening sulphur sorption. Using alkaline earth metals (e.g., Mg, Ca), natural minerals (e.g., dolomite, mayenite) rare earth oxides (e.g., CeO₂) as supports improves sulphur resistance by increasing oxygen mobility and aiding sulphur desorption. More research is required to optimise catalyst compositions and increase economic feasibility. Extrinsic techniques, such as feedstock pretreatment and process adjustments, have been shown to improve catalytic lifetime when exposed to sulphur. Catalyst regeneration is an important part of ensuring long-term catalytic performance. High-temperature oxidation, temperature-programmed calcination, and O₂ activation can effectively restore catalytic activity following sulphur poisoning. Fig. 8 depicts a comprehensive overview of the various regeneration techniques. However, the regeneration efficiency is extremely dependent on the catalyst formulation and the level of sulphur coating. The catalyst's strength and surface characteristics are critical in deciding whether sulphur poisoning is reversible. Combining regeneration techniques with sulfur-tolerant catalyst design remains critical for long-term catalytic performance.

Fig. 8 Summary of regeneration strategies for sulfur-poisoned catalysts ((a) sulfur removal, (b) steam addition, (c) temperature evaluation and (d) oxidation).

Future advances in biogas dry reforming will necessitate a better mechanistic understanding of sulphur poisoning at the atomic level, as well as the development of prediction models for catalyst deactivation and regeneration. The development of bimetallic and trimetallic catalysts with optimised metal–support interactions and increased sulphur tolerance, together with improved process integration, will be critical for attaining sustainable and economically feasible biogas valorisation.

3.2. Siloxanes

Volatile methyl siloxanes (VMS) compounds, also referred as siloxanes, are one of the most challenging compounds present in biogas. Siloxanes have been used as raw materials for consumer products such as detergents, cosmetics, coating materials, and textiles.¹⁰⁵ Siloxanes are a type of organic silicon compounds that consist of alternating silicon–oxygen bonds and functional groups of hydrocarbons, typically methyl. They have two structures: chain (labeled as L) and cyclic (labeled as D) and the number refers to the number of Si atoms in the compounds. The methyl groups bound to the Si atom give siloxanes strong hydrophobic properties such as low solubility and thermal stability.¹⁰⁶ However, most siloxanes are highly volatile and disperse into the atmosphere, where they undergo decomposition (mostly hydroxyl-radical-initiated) into silanols (Si–OH) and various carbonyl compounds, ultimately leading to CO₂ oxidation.¹⁰⁷ Siloxane compounds present in biogas are derived from metabolism of polydimethylsiloxanes (PDMS) in anaerobic digestion processes. The quantity of siloxane found in biogas can differ depending on the particular feedstock employed. In recent years, the presence of VMS in different types of biogas has increased, as a consequence of the intensive use of PDMS.¹⁰⁸ Typically, the concentration of siloxanes in biogas obtained from landfill sites (landfill biogas) or generated through the anaerobic digestion of sludge in sewage treatment plants (sewage-sludge biogas) is 2–3 times higher than that detected in biogas produced from agriculture.¹⁰⁹Table 4 shows the different siloxane derivatives identified in the biogas and their physical and chemical properties. Siloxanes found in landfill biogas are mainly composed of L2, L3, D4, and D5, in the order of D4 > L2 > D5 > L3 with D4 constituting about 60% of the total siloxanes.^110,111 On the other hand, in biogas produced from sewage sludge, D4 and D5 are considerably more abundant than other types of siloxanes.¹¹² Generally, the L structure is more stable, while the D structure is more volatile and its concentration in the biogas is very high, resulting in greater Si deposition. The main reason for removing siloxanes from biogas is to prevent technical problems such as corrosion, erosion and clogging of the unit and pipes. Additionally, it protects the engines from degradation and the catalyst from deactivation by poisoning, because this compound decomposes into lower molecular weight siloxanes and partially oxidizes to silicones and silicon oxide (SiO₂) at high temperatures.¹¹³ The removal of siloxanes compounds from biogas, may be realized by various absorption methods: physical absorption by using mineral oils, chemical absorption by using strong acids and alkalis, biological and membrane methods, or by adsorption on zeolites or polymeric adsorbents.¹¹⁰

Table 4 Typical siloxanes compounds identified in biogas

Compound	Abbreviation	Physical properties	Density (g cm^−3) at 20 °C	Critical temperature (°C)	Critical pressure (atm)	Structural pattern	Molecular formula	Ref.
Hexamethylcyclotrisiloxane	D3	Solid, white, hydrocarbon odor	0.87	281.05	17.7		C6H18O3Si3	110
Octamethylcyclotetrasiloxane	D4	Liquid, colorless, oily, odorless	0.95	313.35	13.2		C8H24O4Si4	53 and 111
Decamethylcyclopentasiloxane	D5	Liquid, oily	0.95	346.05	11.5		C10H30O5Si5	110
Dodecamethylcyclohexasiloxane	D6	Liquid, colorless, faint, odorless	0.96	382.25	12.9		C12H36O6Si6	53
Hexamethyldisiloxane	L2	Liquid, colorless, odorless	0.77	245.15	19.4		C6H18OSi2	110
Octamethyltrisiloxane	L3	Liquid, colorless, odorless	0.82	297.95	17.3		C8H23O2Si3	111 and 110
Hexamethyltetrasiloxane	L4	Solid	0.85	333.35	14.9		C10H30O3Si4	53 and 111

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The chemical reactions involved in dry methane reforming with siloxanes are complex and vary depending on the specific type of siloxane compound and reaction conditions, including temperature and the existence of a catalyst. Two main reactions that siloxanes can undergo are pyrolysis and oxidation. Pyrolysis occurs at high temperatures and breaks down the organic groups in siloxane molecules into smaller hydrocarbons, while the silicon and oxygen atoms form SiO₂. In contrast, oxidation involves the reaction of siloxanes with oxygen, resulting in the formation of carbon dioxide, water, and SiO₂. However, the impact of siloxanes on dry reforming catalysts has not been extensively studied in research. Elsayed et al.,¹¹⁴ investigated the impact of silica deposition on catalyst performance during the dry reforming of methane, revealing that silica accumulation can lead to catalyst deactivation and diminished CH₄ and CO₂ conversion. Their study evaluated the influence of siloxanes compounds commonly present in landfill gas on two catalyst formulations: 1.3 wt% Ni–wt% Mg and 0.16 wt% Pt–1.3 wt% Ni–1 wt% Mg, both supported on ceria–zirconia oxide. The catalysts were used in the presence of siloxane with concentrations representative of landfill gas over timeframes of one week (1 week), one month (1 month), and six months (6 months). The findings demonstrated that even minimal silica deposition significantly impaired catalyst activity, reducing CH₄ and CO₂ conversion to syngas (as shown in Fig. 9), with the adverse effects intensifying with increased silica accumulation. Despite these observations, the exact mechanisms underlying silica-induced deactivation and the interaction between siloxanes and the catalyst surfaces remain unclear, necessitating further research. In summary, this study underscores the critical importance of removing silicon-containing species to maintain effective biogas reforming. Even trace amounts of silica were found to negatively affect catalyst longevity and efficiency, as reflected by decreasing CH₄ and CO₂ conversion rates with rising silica content. Understanding the mechanisms of silica poisoning is crucial for designing more durable and efficient catalysts. Detailed investigation into how siloxanes interact with catalyst surfaces will aid in developing strategies to mitigate deactivation, ensuring sustained performance in biogas reforming and enhancing the long-term viability of catalytic processes exposed to silicon-containing compounds.

Fig. 9 Hydrogen production on (a) Pt and (b) NiMg catalysts and carbon monoxide formation on (c) Pt and (d) NiMg catalysts as a function of temperature (reproduced from ref. 114 with permission from Elsevier, license number 5866990429425, copyright 2017).

3.3. Oxygen (O₂)

Oxygen (O₂) is introduced into biogas primarily through atmospheric ingress during collection or through intentional aeration applied during desulfurization to facilitate the oxidation and removal of hydrogen sulfide.¹¹⁵ Elevated levels of O₂ in biogas are uncommon, as most desulfurization techniques concurrently reduce trace oxygen concentrations. Furthermore, O₂ can be efficiently removed using adsorption-based purification methods, including activated carbon, molecular sieves, and membrane separation technologies. In typical biogas streams, the oxygen concentration is generally maintained below 5%, minimizing its impact on downstream catalytic processes and ensuring process stability. The presence of oxygen during dry reforming can lead to multiple reactions (eqn (13)–(17)), including partial oxidative reforming (POM), methane combustion (MC), carbon oxidation, carbon monoxide oxidation and hydrogen oxidation, depending on the amount of the oxygen present.¹¹⁶ Moreover, these side reactions are exothermic, which could help compensate for energy consumption.^117,118

MC: CH₄ + 2O₂ → CO₂ + 2H₂O ΔH_298K = −890.3 kJ mol⁻¹ (14)

Partial oxidative reforming is a process for transforming methane (CH₄) into syngas through the addition of oxygen (O₂). This process is exothermic and occurs most efficiently at temperatures below 600 °C.¹¹⁹ When combined with dry reforming, a balance can be achieved between heat release and absorption, resulting in a tri-reforming process. In a recent study, Chai et al.,¹²⁰ reported the investigation on Ni-foam-structured (NiO–MO_x–Al₂O₃ (M = Ce or Mg)) nanocomposite catalyst which can improve the partial oxidative reaction. Both NiO–CeO₂–Al₂O₃/Ni-foam and NiO–MgO–Al₂O₃/Ni-foam catalysts, despite having identical NiO contents, demonstrated comparable activity and selectivity in the oxidation reaction. The NiO–CeO₂–Al₂O₃/Ni-foam achieved a high CH₄ conversion (86.4%), with H₂/CO selectivity of 91.2%/89.0%, when tested with a CH₄/O₂ feed ratio of 1.8/1.0 at 700 °C. Notably, NiO–CeO₂–Al₂O₃/Ni-foam exhibited superior stability compared to the NiO–MgO–Al₂O₃/Ni-foam due to significantly enhanced carbon resistance. This improvement was attributed to the formation of CeAlO₃, which played a crucial role in enhancing carbon resistance. The Ce³⁺/Ce⁴⁺ chemical cycling associated with CeAlO₃ created an oxidative environment around Ni nanoparticles, effectively suppressing carbon deposition. In another study, Hassan et al.,¹²¹ have studied the partial oxidation of methane at 385 °C and 26 MPa. Their results suggest that having higher ratios of oxygen to methane (O₂/CH₄) could result in more effective production of syngas. Also, Chein et al.,¹²² investigated the partial oxidative reforming of biogas using various ratios of oxygen–methane respectively O₂/CH₄ = 0/2 but at an elevated pressure of 20 bar. Their results showed that CH₄ conversion increases with the increase in O₂/CH₄ ratio (Fig. 10a). For the case O₂/CH₄ = 2, CH₄ conversion is 100% for all reaction temperatures. This means that all CH₄ is completely oxidized and the products are CO₂ and H₂O according to eqn (14). However, CO₂ conversion decreases with increasing O₂/CH₄ (Fig. 10b), because CO₂ is produced from POM (eqn (13)) or MC (eqn (14)). On the other hand, Lucrédio et al.,¹²³ examined the dry reforming process using Ni/γ-Al₂O₃ and Ni/MgO catalysts with a feed gas containing CH₄, CO₂, and O₂ at a ratio of 1.5 : 1 : 0.25. They found that the presence of O₂ facilitated the oxidation of carbon into CO (eqn (15)).

Fig. 10 Effect of O₂ content on CH₄ (a) and CO₂ (b) conversions at 20 bar pressure (reproduced from ref. 122 with permission from Elsevier, license number 5867061096576, copyright 2015).

In addition, Kohn et al.,¹²⁴ explored the dry reforming process using a Rh/γ-Al₂O₃ monolith catalyst under the presence of O₂. They investigated the impact for CH₄/CO₂/O₂ ratio of 1 : 1 : 0.46 and they found that the resulting net enthalpy was zero. This means that there was no need for external heat, and the risk of carbon formation was eliminated. In another recent study, Chen et al.,¹²⁵ investigated the effects of O₂ content (ranging from 0–15%) on a Ni/SiO₂ catalyst during the dry reforming of biogas. The study demonstrated that increasing O₂ concentration from 0% to 15% led to higher CH₄ conversion while decreasing CO₂ conversion, a trend consistent with theoretical predictions. The presence of 5% O₂ in biogas enhanced CH₄ conversion, improved biogas reforming stability, and slightly reduced CO₂ conversion. Under these conditions, CH₄ and CO₂ conversion efficiencies reached 62%, with an H₂/CO ratio of 0.87 at 700 °C, reducing theoretical energy consumption by 18.7%. However, O₂ concentrations above 10% severely decreased CO₂ conversion and increased H₂O production. The use of an inexpensive Ni-based catalyst in biogas reforming demonstrated that O₂ had an anti-sintering effect by altering the reduction environment and nickel particle mobility. Additionally, carbon deposition was eliminated due to enhanced carbon activity in the presence of O₂. These findings strongly supported that maintaining O₂ concentrations below 5% provided an energy-efficient approach to syngas production from biogas by ensuring stable syngas yield, lowering energy consumption, and allowing the use of cost-effective metal catalysts without carbon deposition as shown in Table 5.

Table 5 Influence of O₂ concentration on catalytic properties during dry reforming of biogas at 700 °C (ref. 125)

O2 concentration (%)	Conversion (%)			H2O productivity (%)
	CH4	CO2	H2/CO
0	57	71	0.85	4.78
5	62	60	0.87	7.32
10	70	55	0.96	10.31
15	72	43	0.99	13.69

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Jahangiri et al.,¹²⁶ reported the coupling of O₂ with DRM using perovskites catalysts (LaNiO₃ and LaNi_1−xFe_xO₃ (x = 0.2, 0.4, 0.6, 0.8 and 1)) which were fabricated via sol–gel technique. The study was conducted under a stream of CH₄/CO₂/O₂ ratio of 1 : 1 : 0.5. The results showed that the addition of oxygen to the CH₄ + CO₂ system led to a decrease in reaction temperature and energy consumption, indicating that the heat liberated by methane combustion reaction (eqn (14)) (exothermic) promotes the DRM process (endothermic). Similarly, Chaudhary et al.,¹²⁷ conducted a study to investigate the impact of O₂ on dry reforming of methane using two catalysts, 10 wt% Ni/Al₂O₃ and 10 wt% Ni/MgAl₂O₄ (Fig. 11). Their results showed that the addition of O₂ increased CH₄ conversion and the H₂/CO ratio, without effect on CO₂ conversion. Furthermore, increasing temperature and O₂ concentration led to a reduction in carbon deposition. In another work, Foo et al.,¹²⁸ found that O₂ co-feeding during DRM resulted in a stable catalytic performance with higher conversion rates, minimal carbon deposition, and H₂ : CO of 1.26 for downstream processing. Previous research on dry reforming of methane (DRM) has predominantly utilized an O₂/CH₄ ratio of 0.5, corresponding to the stoichiometric requirement for partial oxidative reforming of methane. However, the influence of lower O₂ concentrations ranging from 0.01% to 5%, which are typically present in biogas on the catalytic performance, energy efficiency, and overall process dynamics of biogas reforming remains insufficiently explored. Moreover, limited studies have addressed the impact of this O₂ concentration range on the long-term stability and consistency of syngas production. For optimal dry reforming of biogas and efficient hydrogen generation, maintaining O₂ content below 5% is advisable, as elevated oxygen levels (≥10%) significantly reduce CO₂ conversion efficiency and promote increased H₂O formation, thereby impairing syngas quality and overall process efficiency.

Fig. 11 Effect of O₂ on CH₄ and CO₂ conversions and equilibrium conversions on Ni/Al₂O₃ and Ni/MgAl₂O₄ catalysts, reaction condition: (CH₄ : CO₂ : N₂ = 1 : 1 : 1) (a and b), reaction condition: (CH₄ : CO₂ : O₂ : N₂ = 1 : 1 : 0.08 : 0.92) (c and d), (captions: experimental conversions for Ni/Al₂O₃ catalyst at (a) 600 °C, (b) 750 °C and for Ni/MgAl₂O₄ catalyst at (c) 600 °C, (d) 750 °C; equilibrium conversions at (e) 600 °C, (f) 750 °C) (reproduced from ref. 127 with permission from Elsevier, license number 5815910869539, copyright 2020).

3.4. Volatile organic compounds (VOCs)

Volatile organic compounds (VOCs), known for their high vapor pressure and low solubility in water, are harmful to the environment. These compounds can be found in a variety of sources, including biogas. Various toxic VOCs are issued from household waste, such as cleaning compounds, pesticides, pharmaceuticals, plastics, synthetic textiles, and coatings.¹²⁹ Biogas can contain a range of different VOCs, including methanol, ethanol, pinene, styrene, limonene, terpenes, BTEX (benzene, toluene, ethylbenzene and xylene), hexane and other compounds. The composition and quantity of VOCs in biogas is dependent on the initial feedstock used. Landfill biogas, for example, typically contains higher amounts of minor compounds (benzene, toluene, Aromatic & cyclic…) compared to other types of biogases due to the lack of waste sorting beforehand.¹³⁰ Terpenes are primarily generated during the decomposition of green waste or through the volatilization of aerosols.¹³¹ On the other hand, BTEX are usually found in biogas because of organic solvents, food additives, petroleum-based products, and other sources.¹³² The removal of BTEX is crucial due to the toxicity of these substances and their potential to generate tropospheric ozone and other oxidized VOCs.¹³³ Where the latter two impurities can cause side reactions on the catalyst, which in turn impairs process efficiency and deactivates the catalyst. This is often accomplished by incorporating a purification step that involves elimination of VOCs on activated carbon. But this process involves additional costs for maintenance and regeneration.¹³⁴ During dry reforming, VOCs present in the biogas could interact with the catalyst used, particularly methanol, toluene and ethanol. This interaction facilitates the concurrent conversion of these individual compounds into syngas through reforming reactions.

3.4.1. Methanol. Methanol is widely used as an industrial solvent, but it poses health and environmental risks.¹³⁵ However, since methanol can be produced from syngas, there is no need to perform its dry reforming reaction to obtain syngas. If methanol is produced from biomass-derived sources, it can be considered a valuable source for generating a H₂-rich stream to feed fuel cell systems through the dry reforming reaction.¹³⁶ Although few studies have been reported on dry reforming of methanol (DRMe) due to its inefficiency in producing net energy, steam reforming of methanol is more attractive in terms of H₂ yield.¹³⁷ The most common reactions during DRMe involve eqn (18)–(21).¹³⁸ Recently, Zhang et al.,¹³⁹ investigated DRMe using plasma technology in a rotating sliding arc reactor to produce clean syngas with high CO₂ conversion. They showed that this technology has potential for producing clean syngas and achieving high-efficiency CO₂ conversion.

DRMe: CH₃OH + CO₂ → CH₂O + CO + H₂O ΔH_298K = 133.5 kJ mol⁻¹ (18)

DRMe: CH₃OH + 2CO₂ → 3CO + 2H₂O ΔH_298K = 172.7 kJ mol⁻¹ (19)

Methanol dehydrogenation: CH₃OH → CH₂O + H₂ ΔH_298K = 92.4 kJ mol⁻¹ (20)

Methanol decomposition: CH₃OH → CO + 2H₂ ΔH_298K = 90.5 kJ mol⁻¹ (21)

The combined steam and dry reforming of methanol process was conducted by Mosayebi et al.,¹⁴⁰ at temperatures between 400 °C and 900 °C, CO₂/H₂O ratios of 0.5–2.5, and (CO₂ + H₂O)/CH₃OH ratios of 0.5–2.5 at atmospheric pressure over a Pt/ZrO₂ catalyst in a fixed-bed reactor. The kinetic model, based on the Langmuir–Hinshelwood isotherm, showed a low error (7.97%) in predicting methanol conversion. Nearly complete methanol conversion occurred above 800 °C, except for a (CO₂ + H₂O)/CH₃OH ratio of 0.5. Temperature positively impacted CO and H₂ yields, with CO showing a stronger dependence. The reverse water–gas shift reaction caused deviations from thermodynamic equilibrium predictions at high temperatures. In another study, Mosayebi et al.¹⁴¹ developed a kinetic model for the dry reforming of methanol using a Cr–Mo–Mn/SiO₂ catalyst under varying conditions (500–900 °C and CO₂/CH₃OH ratio of 1–2.5). The model was based on the Langmuir–Hinshelwood approach, featured three reversible reactions on different active sites. Statistical analysis confirmed the model's significance and high precision, particularly in methanol conversion. Temperature and CO₂/methanol ratio affected CO yield and conversions of methanol and CO₂, with higher temperatures and CO₂ concentrations improving conversions while decreasing H₂ yield. The model showed increasing differences in CO₂ conversion and H₂ yield with rising temperature and CO₂/CH₃OH ratio.

However, several dry reforming studies have also focused on VOCs such as toluene, demonstrating that this compound can react with catalysts to generate syngas. Toluene can deposit on the surface of equipment, reactors, and catalysts, causing poisoning and deactivation, thereby maintenance costs would be high.

3.4.2. Toluene. Toluene is an aromatic hydrocarbon present in biogas. The production of syngas by dry reforming of toluene (DRTo) can promote the use of hydrogen as a substitute for fuels or fossil fuels. The dry reforming of toluene is a complex process that necessitates precise control over reaction conditions and careful selection of catalysts to optimize product yields and minimize the formation of undesired by-products. This process is governed by the thermodynamics of equilibrium reactions. As a result, a thorough understanding of the thermodynamics involved in the dry reforming of toluene is crucial for achieving efficient and selective conversion.¹⁴² Therefore, it is essential to study the thermodynamics of dry reforming of toluene. Toluene reacts with carbon dioxide according to the following reactions:

DRTo: C₇H₈ + 7CO₂ → 14CO + 4H₂ ΔH_1073K = 1105 kJ mol⁻¹ (22)

DRTo: C₇H₈ + 11CO₂ → 18CO + 4H₂O ΔH_1073K = 1236 kJ mol⁻¹ (23)

Dry reforming reaction of toluene are endothermic and are not spontaneous below 500 °C (eqn (22)) and 520 °C (eqn (23)). On the other hand, when rising the temperature below 700 °C reactions may occur:

Hydrocracking: C₇H₈ + 10H₂ → 7CH₄ ΔH_1073K = −731 kJ mol⁻¹ (24)

Hydrodealkylation: C₇H₈ + H₂ → C₆H₆ + CH₄ ΔH_1073K = −104 kJ mol⁻¹ (25)

Thermal cracking: nC₇H₈ → mC_xH_y + zH₂ (26)

Carbon formation: C₇H₈ → 7C + 4H₂ ΔH_1073K = −73 kJ mol⁻¹ (27)

DRTo are carried out at temperatures between 300 and 800 °C, to be able to study the conversion of toluene, the selectivity of the products and the influence of the side reactions on the formation of carbon in the presence of catalysts. The use of an appropriate catalyst will control the reaction mechanisms, play a role in balancing the reactions and direct the reactions toward desired products. Kong et al.,¹⁴³ conducted a study on various supported catalysts: Ni/MgO, Ni/γ-Al₂O₃, Ni/α-Al₂O₃, Ni/SiO₂ and Ni/ZrO₂, in order to examine their performance in dry reforming of toluene. They revealed that the activity of these catalysts is mainly influenced by the type of support used. Among the catalysts tested, Ni/MgO demonstrated the most promising catalytic performance, achieving an 85% conversion rate of toluene at a temperature of 600 °C. This success was attributed to the strong interaction between NiO and MgO, which forms a solid solution known as Mg–Ni–O. Additionally, the high dispersion of nickel particles in a basic environment contributed to the favorable results. They also explored the impact of reduction in temperature and determined that an adequate amount of surface nickel is crucial for achieving improved activity. Consequently, higher temperature reduction led to enhanced toluene conversion. However, sintering of nickel particles occurs making Ni/MgO not stable.¹⁴⁴ In subsequent work, they found that the calcination temperature of the Ni/MgO catalyst (550, 600, 700 and 800 °C for a duration of 6 hours in an air environment) significantly influences its catalytic activity. Specifically, the catalyst Ni/MgO calcined at 600 °C exhibited the highest level of activity. This catalyst also displayed the highest concentration of surface-reduced nickel, which likely accounts for its superior performance.¹⁴⁵ Bao et al.,¹⁴⁶ evaluated the catalytic performance of Co/MgO catalyst in dry reforming of toluene with different cobalt contents. The conversion of toluene over reduced Co/MgO catalysts at 700 °C increased with increasing cobalt content showing that the metallic Co formed from the reduction of cobalt oxides and MgCo₂O₄ was responsible for the catalytic activity in dry reforming of toluene. The deactivation of the Co/MgO catalyst was mainly due to the partial oxidation Co metal by CO₂ and which could not be reduced again by H₂ and/or CO at the reaction temperature (570 °C). The dry reforming of toluene on Ni/Palygorskite catalysts was studied by Chen et al.¹⁴⁷ Palygorskite (Mg, Al)₂Si₄O₁₀(OH).4(H₂O), a natural mineral material known for its large specific surface area and mesoporous structure, was employed as a support due to its favorable nanoscale properties. They have shown that the adsorption of CO₂ is favored with the increase in the nickel content and that the maximum yield of H₂ is obtained for the 5% Ni/palygorskite catalyst. However, the increase in the temperature and/or the CO₂ charge slightly decreases the H₂ yield while inhibiting the deposition of carbon on the catalyst. In addition, Oh et al.,¹⁴⁸ are studying the properties of DRTo using 3%Ca 20%Ni 0.6%Ru/Al₂O₃, 1%Ca 20%Ni 0.6%Ru/Al₂O₃, and 1%Mn 20%Ni 0.6%Ru/Al₂O₃ catalysts. They found that the conversion of toluene and the concentrations of H₂ and CO in the product gas increased with temperature. The catalytic activity was enhanced with higher Ca content, and the 3%Ca 20%Ni 0.6%Ru/Al₂O₃ catalyst exhibited the highest activity among the tested catalysts. A summary of the compositions of the studied and developed catalysts to produce syngas by dry reforming of toluene, is present in Table 6.

Table 6 Catalysts and their performance in recent studies on toluene reforming

Metal	Support	Loading wt%	Reforming Temp., °C	Toluene concentration	Reforming condition	X T conversion (%)	Ref.
Ni	MgO	5	600	0.7%	0.5 mL catalyst, t = 7 h, 11.6%CO2, 87.7%Ar, GHSV = 36 000 h^−1	86	143
	γ-Al2O3					32
	α-Al2O3					63
	SiO2					9
	ZrO2					12
Ni	MgO-550 cal	5	570	0.3 mL h^−1	0.45 g catalyst, t = 7 h, 14.5 mL per min CO2, 285 mL per min Ar, GHSV = 36 000 h^−1	61	145
	MgO-600 cal					78
	MgO-700 cal					64
	MgO-800 cal					7
Ni	MgO-500 red	5	570	0.3 mL h^−1	0.45 g catalyst, t = 7 h, 14.5 mL per min CO2, 285 mL per min Ar, GHSV = 36 000 h^−1	42	144
	MgO-600 red					53
	MgO-700 red					65
	MgO-800 red					49
Co	MgO	5	570	0.3 mL h^−1	0.45 g catalyst, t = 7 h, 14.5 mL per min CO2, 285 mL per min Ar, GHSV = 36 000 h^−1	2	146
		9				43
		12				77
		15				94
Ni	Palygorskite	2	650–800	5.92 mg min^−1	2 g catalyst, t = 7 h, 0–50 cm^3 per min CO2, 200–250 cm^3 per min N2	n.r.	147
		5				n.r.
		8				n.r.
MnNiRu	Al2O3	1Mn	400–800	30 g Nm^−3	Size catalyst 250–500 μm, CO2/toluene = 25, GHSV = 10 000 h^−1	85.3	148
		20Ni
		0.6Ru
CaNiRu		1Ca				95
		20Ni
		0.6Ru
CaNiRu		3Ca				98.1
		20Ni
		0.6Ru

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In a study, Chaghouri et al.,¹⁴⁹ investigated the impact of toluene during dry reforming of synthetic biogas on the performance of the Co₁Ni₁Mg₄Al₂O₄ catalyst. They found that the catalyst remained stable even at different toluene concentrations (0, 250, 400, 750, 900 and 1700 ppm), indicating no direct catalytic deactivation. However, the conversion of methane and carbon dioxide increased over time in the presence of toluene (Fig. 12a and b). After 12 hours of testing, the conversion of methane was higher at higher toluene concentrations (around 85% for 750, 900, and 1700 ppm) compared to lower concentrations (79% for 250 ppm and respectively 82% for 400 ppm). These observations were attributed to increased hydrogen production resulting from the decomposition (eqn (26)) and reforming of toluene (eqn (22)), which is highly carbonated and hydrogenated. The production of hydrogen in excess by such reactions could lead to a continuous reduction of the catalyst during the test, increasing the number of active metal sites and enhancing the conversion of the reactant. It was found out that, when toluene is present as impurity of DRM, it is promoting additionally the formation of excess carbon due to secondary reactions such as methane and toluene cracking (Fig. 12c). In addition, Tanios et al.,¹⁵⁰ investigated the durability of the mixed oxide catalyst (Co₂Ni₂Mg₂Al₂) in the absence and presence of toluene during dry reforming of methane. They observed that the catalytic activity of Co₂Ni₂Mg₂Al₂, decreases in the presence of toluene, and a higher amount of carbon forms on the catalyst surface. The deactivation of the catalyst was caused by two main factors: first, the adsorption of toluene onto the catalytic sites, which blocked these sites and reduced their activity; and second, various side reactions initiated by the presence of toluene. These side reactions were intensified by the competition between toluene and methane for reacting with carbon dioxide, further effecting the catalyst's performance.

Fig. 12 CH₄ (a) and CO₂ (b) conversions of NiCoMgAl catalyst in the presence and absence of toluene and thermos-gravimetric analysis (c) results for spent catalyst after 12 h DRM (reproduced from ref. 149 with permission from Elsevier, license number 5815910306415, copyright 2022).

3.4.3. Ethanol. On the other hand, ethanol, which is also present in biogas as an undesirable component, has received particular attention for its production from biomass-derived sources such as lignocellulosic and municipal waste.¹⁵¹ Ethanol actively participates in the reforming reactions, contributing to the overall production of synthesis gas undergoes dry reforming to generate hydrogen and carbon monoxide. However, the highly endothermic nature of the reaction requires substantial energy input.¹⁵² Thermodynamic analysis plays an essential role in determining the appropriate operating parameters for dry ethanol reforming. These parameters include reaction pressure, temperature and feed ratio, which directly influence the desired syngas yield. Dry ethanol reforming primarily involves the reaction between C₂H₅OH and CO₂ (eqn (28)). However, several undesirable side reactions also occur during the process, negatively impacting syngas production and product selectivity (eqn (29) and (30)).¹⁵³ Additionally, ethanol interacts with the catalyst by modifying surface properties of the catalyst and the reaction kinetics over dry reforming process of biogas. The specific composition of the catalyst and the operating conditions determine whether ethanol enhances or inhibits catalytic activity. Compared to other biogas components like toluene, ethanol has a higher tendency to form carbon.¹⁵⁴ Consequently, deposition of carbon on the catalyst surface is even more accented. This occurs due to the facile cleavage of C–H bonds in hydrocarbons at high reaction temperatures.¹⁵⁵

DRE: C₂H₅OH + CO₂ → 3CO + 3H₂ ΔH_298K = +296.7 kJ mol⁻¹ (28)

Ethanol dehydrogenation: C₂H₅OH → CH₃CHO + H₂ ΔH_298K = +68.5 kJ mol⁻¹ (29)

Ethanol decomposition: C₂H₅OH → CO + CH₄ + 4H₂ ΔH_298K = +49.6 kJ mol⁻¹ (30)

A study by Wang et al.¹⁵⁶ investigated the thermodynamics of dry reforming of ethanol with carbon dioxide for hydrogen production using the Gibbs free energy minimization method. The optimal conditions for hydrogen production were identified as reaction temperatures between 930 and 1000 °C, carbon dioxide to ethanol molar ratios of 1.2–1.3 at 0.1 MPa. Under these conditions, complete ethanol conversion, 94.75–94.86% hydrogen yield, and 96.77–97.04% carbon monoxide yield were achieved without carbon formation. The study also identified carbon-formed and carbon-free regions, aiding catalyst selection. Inert gases positively impacted hydrogen and carbon monoxide yields.

VOCs have a major impact on the stability and efficacy of catalysts in biogas dry reforming. In addition to secondary reactions like cracking, which result in an excessive amount of filamentous carbon, these contaminants accelerate the degradation of the catalyst. The efficiency of the reforming process is impacted by this carbon accumulation, which progressively reduces the catalyst's activity. But over time, VOCs affect the rates of CO₂ and CH₄ conversion as they enhance the production of H₂ through reforming reactions and breakdown. Catalyst reduction is facilitated by H₂ production, which raises the number of active sites and momentarily enhances catalytic activity. Long-term effects of VOCs continue to be a significant problem despite this brief improvement. Continuous exposure to these contaminants may cause irreversible deactivation of catalysts, which would lower their overall efficiency. Furthermore, the reforming process becomes even more complex due to the impact of VOCs on reaction pathways and selectivity. These modifications may impact the stability of the reaction environment. To optimize catalyst performance and develop more efficient reforming strategies, further research is needed to understand the precise mechanisms governing VOC interactions. Advanced studies can provide insights into catalyst modifications, impurity mitigation techniques, and operational adjustments to minimize deactivation. A deeper understanding of these factors will be essential for enhancing process efficiency and ensuring the long-term stability of catalytic systems in biogas dry reforming.

3.5. Water (H₂O)

Water may be present as a significant component in biogas (Table 2), with concentrations of 3 to 10% fluctuating depending on several factors such as waste composition, digester type, and methanization method employed. During dry reforming, the presence of water leads to a slight enhancement in methane conversion.¹⁵⁷ A notable study conducted by Stroud et al.,¹⁵⁸ observed a significant increase in CH₄ conversion when water was present. This improvement is associated with the concurrent occurrence of steam reforming of methane (SRM) as a parallel (eqn (31)), endothermic reaction favored at temperatures higher than 630 °C.

CH₄ + H₂O ⇌ 3H₂ + CO ΔH⁰ = +206 kJ mol⁻¹ (31)

On the other hand, the presence of water results in a decrease in CO₂ conversion. Furthermore, it leads to the production of larger quantities of hydrogen, thus explaining the increase in the H₂/CO ratio. In the literature, the addition of steam to the DRM reaction was also proved to be beneficial for catalytic performance. Itkulova et al.,¹⁵⁷ noticed an increase in the reactant conversion, which was attributed to a synergetic effect between dry and steam reforming of methane. In another study, the addition of water slightly increased methane conversion and the H₂/CO ratio, while causing a decrease in CO₂ conversion. This decrease was attributed to the occurrence of the RWGS reaction.¹⁵⁹ During the process of dry reforming of methane, the catalyst employed tends to lose its activity due to the formation of carbon. These observations can be attributed to the occurrence of the carbon gasification reaction (eqn (32) and (33)).

H₂O + C ⇌ H₂ + CO ΔH_298K = +131 kJ mol⁻¹ (32)

2H₂O + C ⇌ 2H₂ + CO₂ ΔH_298K = +90 kJ mol⁻¹ (33)

Nevertheless, the carbon generated in the presence on steam reforming is oxidized, leading to its conversion into either CO (eqn (32)) or CO₂ (eqn (33)) and finally resulting in increased H₂/CO ratio with lower CO₂ conversion. Thermodynamically, these reactions are more favorable at temperatures above 670 °C, which explains the reduced carbon production in the presence of water. Chaghouri et al.,¹⁴⁹ conducted a study to examine the impact of water presence at different concentrations (0 vol%, 2 vol%, and 5 vol%) on the process of dry reforming of methane using a NiCoMgAl catalyst. The addition of water to the inlet gas composition had a favorable effect on the catalytic performance during DRM. The primary effect of water presence was the reoxidation of the carbon produced, leading to enhanced overall stability, as supported by thermogravimetric results presented in Fig. 13a. The results revealed that the weight loss observed at temperatures below 200 °C was due to the evaporation of water. Additionally, a weight gain observed between 150 °C and 300 °C was attributed to the reoxidation of nickel and cobalt. Moreover, the weight loss observed at temperatures above 400 °C was associated with the oxidation of carbon. Comparing the samples with and without water, it was found that the carbon deposition on the sample without water was not significant (8%), while the presence of water reduced the amount of carbon formed to less than 4%. Therefore, it is highly probable that the presence of water enhances the stability of the catalyst by protecting it from excessive carbon formation and subsequent deactivation. In general, the combination of steam and carbon dioxide can be employed for methane reforming. This process, known as bi-reforming (BRM) or dual reforming, can be summarized by the following reaction (eqn (34)).¹⁶¹ Combined steam and dry reforming of methane has been extensively studied and offer significant advantages and greater interest compared to dry reforming of methane.^162,163

3CH₄ + CO₂ + 2H₂O ⇌ 8H₂ + 4CO ΔH_298K = +220 kJ mol⁻¹ (34)

Fig. 13 (a) Thermogravimetric analysis of used CoNiMgAl catalysts after 12 h stability tests at 750 °C in the absence (0% H₂O) and presence of 2% and 5% H₂O (reproduced from ref. 149 with permission from Elsevier, license number 5815910306415, copyright 2022) and (b) the effect of H₂O addition on H₂/CO ratio as a function of the reaction temperature for various CH₄/CO₂/H₂O ratios using 3wt%Pt–10 wt% Ni/Al₂O₃ catalyst (reproduced from ref. 160 with permission from American Chemical Society, copyright 2019).

H₂ production is much more enhanced, the ratio H₂/CO obtained being of 2.^164,165 For biogas upgrading, low steam content is recommended during the dry reforming process. Another study conducted by Kumar et al.,¹⁶⁶ investigated the comparison between dry reforming of methane with and without the presence of water, using 1 wt% Ni-doped with La₂Zr₂O₇ pyrochlore catalyst. They noticed that in the presence of water, no carbon formation occurred on the catalyst, unlike the process conducted without water. This observation suggests that the steam in the inlet appears to oxidize carbon or its precursors, resulting in no measurable deactivation over a 24-hour period. On the other hand, the effect of adding H₂O during the dry reforming of biogas was examined by Chein et al.,¹⁶⁰ using bi- and monometallic Ni and Pt catalysts supported on Al₂O₃. The study revealed that when H₂O was added, methane steam reforming became the predominant reaction, particularly when the biogas had a lower CO₂ content. As a result, higher yields of H₂ and CO were obtained by increasing temperature (Fig. 13b). However, as the biogas contained higher levels of CO₂, the yields of H₂ and CO decreased, indicating that steam methane reforming is occurring due to the increased CO₂ content. The presence of water in dry reforming of methane significantly influences the reaction by enhancing CH₄ conversion while decreasing CO₂ conversion. This effect is attributed to concurrent steam reforming, which increases the H₂/CO ratio and reduces carbon deposition, improving catalyst stability. Studies confirm that water addition promotes carbon gasification, thereby mitigating catalyst deactivation. Moreover, bi-reforming, which combines steam and dry reforming, enhances H₂ production and process efficiency. Optimal steam content is crucial for biogas upgrading. Overall, incorporating water into methane reforming offers advantages such as higher H₂ yield, reduced carbon formation, and improved catalyst performance, making it a promising approach.

3.6. Other impurities

In addition to the impurities discussed above, there are other impurities in biogas such as N₂, chlorine and NH₃. Chein et al.,¹²² observed that N₂ (molar ratio: N₂/CH₄ = 0 to 5), which is an inert component found in biogas, had a slightly positive impact on dry methane reforming at a pressure of 20 bar. The increase in CH₄ and CO₂ conversion was attributed to the effect of N₂ on the total amount of reaction species, leading to a decrease in pressure and favoring the dry reforming reaction. However, due to the low concentrations of N₂ typically present in biogas, their influence is considered negligible. Chlorocarbons, originating from the chlorination of organic compounds, can exert detrimental effects on catalysts, leading to their poisoning of the catalyst. CH₃Cl is one of the chlorides usually found in biogas, and its concentration in the range of 0.1 to 100 ppm.⁴⁰ Kohn et al.,¹⁶⁷ conducted an investigation into the effects of methyl chloride (CH₃Cl) at concentrations ranging from 10 to 50 ppm on the process of biogas dry reforming. They employed a Rh/γ-Al₂O₃ catalyst and conducted experiments across temperatures ranging from 350 °C to 700 °C. The study findings confirm the influence of CH₃Cl on the chemistry of Al₂O₃ support, resulting in the formation of surface chlorine. This chlorine, in turn, increased the acidity of the support material. Consequently, it caused a reversible poisoning effect on the reverse water–gas shift reaction, thereby modifying the H₂/CO ratio. The degree of chlorine poisoning increased with increasing CH₃Cl concentration and decreasing temperature. The impact of NH₃, ranging from 0 to 10 000 ppm, on the process of biogas dry reforming was investigated by Gao et al.⁹⁹ Their study revealed that NH₃ concentrations within the range of 50–10 000 ppm exerted a slight inhibitory effect on biogas dry reforming. The presence of NH₃ is resulting in a gradual decreasing of biogas conversion, starting from 10% decrease, observed within the first minutes, followed by a continuous decrease over a 12-hour period. This phenomenon can be attributed to the reduction in oxygen activity at the catalyst surface, which is caused by NH₃. These insights highlight the complex and often underestimated role of minor impurities. However, the current body of research is fragmented and limited in scope. There is a pressing need for systematic studies that evaluate these impurities under variable, realistic conditions, especially over long durations. Furthermore, the development of impurity-resistant catalysts and effective gas-cleaning methods remains an open field for innovation. As biogas emerge as a key renewable resource, understanding and mitigating the impact of such impurities is not just scientifically intriguing but also critical for commercial viability.

4. Conclusions and future outlook

This study provides a brief overview of the sources, composition, and utilization of biogas as a renewable energy resource. Biogas is considered a promising solution to the global energy crisis, but its direct use is often limited due to the presence of impurities that hinder efficiency. Dry reforming is an innovative process that converts methane (CH₄) and carbon dioxide (CO₂) into syngas, a valuable mixture of hydrogen (H₂) and carbon monoxide (CO). This process reduces greenhouse gas emissions and produces fuel useful for environmental applications. However, the presence of impurities in real biogas presents several challenges, which must be addressed to optimize the dry reforming process. The review highlights several key impurities commonly found in biogas, such as hydrogen sulfide (H₂S), siloxanes, oxygen (O₂), volatile organic compounds (VOCs), water (H₂O), nitrogen (N₂), chlorine compounds (e.g., CH₃Cl), and ammonia (NH₃), and their influence on the dry reforming process.

(a) Hydrogen Sulfide (H₂S) and Siloxanes: one of the most challenging impurities in biogas is H₂S, which can poison catalysts by binding to their active sites, leading to a significant loss in catalytic activity. Similarly, siloxanes organic compounds commonly present due to industrial processes, can also degrade catalysts, forming undesired byproducts and accelerating catalyst deactivation. Addressing these challenges requires the development of sulfur-resistant catalysts that can withstand the toxic effects of H₂S. Research into bimetallic or alloyed catalysts has shown promise in minimizing sulfur adsorption, which could extend catalyst life and improve process stability.

(b) Oxygen (O₂): while low concentrations of oxygen in biogas can be beneficial in preventing catalyst sintering and reducing carbon deposition, it can also create opposing effects. Oxygen can inhibit carbon formation, which is critical for maintaining catalyst stability. However, if oxygen levels are too high, it can reduce the effectiveness of the dry reforming process. Optimizing the concentration of oxygen is crucial to balancing these effects and improving reforming efficiency. Research into controlled oxygen injection strategies may help achieve this balance and prevent catalyst deactivation.

(c) Volatile Organic Compounds (VOCs): VOCs, such as methanol, toluene, and ethanol, can contribute to excessive carbon deposition on the catalyst. This occurs through secondary cracking reactions that generate carbon filaments, which block active sites and reduce catalytic activity. Minimizing VOC concentrations or designing catalysts that resist carbon formation is essential to improving the overall efficiency of dry reforming. Further research into VOC decomposition mechanisms and strategies to prevent carbon residues is needed to enhance process performance.

(d) Water (H₂O): water plays a beneficial role in dry reforming by facilitating the bi-reforming reaction, which reduces carbon deposition. Small amounts of water can help oxidize carbon into carbon monoxide (CO), preventing catalyst fouling. However, excessive water can dilute the biogas and reduce syngas yield. Investigating the optimal water-to-biogas ratio and the potential benefits of co-feeding water vapor during dry reforming could improve process efficiency and catalyst longevity.

(e) Other impurities: nitrogen (N₂), typically considered an inert impurity, can positively influence the process by diluting the biogas and increasing the residence time of gases on the catalyst, which can enhance the conversion of methane (CH₄) and carbon dioxide (CO₂) into syngas. In addition, chlorine compounds, such as CH₃Cl, pose a more detrimental effect by increasing catalyst acidity and forming strong bonds with metal sites, which can lead to catalyst deactivation. To address this, the development of chlorine-resistant catalysts is essential, particularly in regions where biogas contains significant chlorine levels. Ammonia (NH₃), another common impurity in biogas, inhibits reforming reactions by reducing the oxygen activity on the catalyst surface, thereby decreasing conversion rates. Research into ammonia-resistant catalysts or methods to remove ammonia from biogas before reforming could mitigate its negative impact, leading to enhanced process efficiency. Overall, managing these impurities is key to improving the performance and stability of the dry reforming process.

Future research on biogas impurities should focus on developing advanced catalysts, such as bimetallic, mesoporous, and core–shell types, to resist sulfur poisoning, carbon deposition, and enhance durability, reducing regeneration costs. Understanding the interactions of multiple impurities during dry reforming is essential for developing more effective purification strategies and predictive models. Real biogas testing, accounting for dynamic impurity fluctuations, will help bridge the gap between laboratory research and industrial applications. Improved reactor designs are needed to handle real biogas variations, reduce energy consumption, and maximize syngas output. The commercial feasibility of dry reforming depends on factors like impurity removal, catalyst regeneration, and energy consumption, with lifecycle cost analysis identifying potential scaling barriers. Additionally, the environmental impact, particularly waste management and CO₂ emissions, must be carefully evaluated to ensure the process is sustainable and contributes to a circular economy. In conclusion, addressing these challenges through advanced catalyst development, impurity management, and process optimization can improve dry reforming for large-scale application, leading to enhanced syngas production and reduced environmental impact.

Data availability

No primary research results, software or code have been included and no new data were generated or analysed as part of this review. Data citation are included in bibliographic references, as recommended by the RSC instructions.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

M. Mohamad Ali thanks University Littoral Opal Coast, Région Hauts-de-France, Séché Group, Opale Environnement and Territoire d'Innovation project for thesis funding. The authors would like to thank the project TI “Territoire d'Innovation: Dunkerque l’Energie Creative” partners for their financial support.

References

1. S. M. Vicente-Serrano, S. M. Quiring, M. Peña-Gallardo, S. Yuan, and F. Domínguez-Castro, A Review of Environmental Droughts: Increased Risk under Global Warming?, Elsevier B.V., 2020,  DOI:10.1016/j.earscirev.2019.102953.
2. N. Ahmat, S. Christopher, J. Saputra, M. N. Sukemi and M. N. Nawawi, The Impact of Energy Consumption, Economic Growth, and Non-Renewable Energy on Carbon Dioxide Emission in Malaysia, Int. J. Energy Econ. Pol., 2025, 15(1), 143–152,  DOI:10.32479/ijeep.17350.
3. M. W. Jones, G. P. Peters, T. Gasser, R. M. Andrew, C. Schwingshackl, J. Gütschow, R. A. Houghton, P. Friedlingstein, J. Pongratz and C. L. Quéré, National contributions to climate change due to historical emissions of carbon dioxide, methane, and nitrous oxide since 1850, Sci. Data, 2023, 10(1), 2052–4463,  DOI:10.1038/s41597-023-02041-1.
4. J. Ciuła, S. Kowalski, A. Generowicz, K. Barbusiński, Z. Matuszak and K. Gaska, Analysis of Energy Generation Efficiency and Reliability of a Cogeneration Unit Powered by Biogas, Energies, 2023, 16(5), 2180,  DOI:10.3390/en16052180.
5. M. P. Jerome, A. Mathai Varghese, S. Kuppireddy, G. N. Karanikolos and N. Alamoodi, Upcycling paper waste into aminosilane-functionalized cellulose-graphene oxide composite aerogel adsorbents for low-pressure CO2 capture, Sep. Purif. Technol., 2025, 360, 131089,  DOI:10.1016/j.seppur.2024.131089.
6. A. Kumar, E. Singh, R. Mishra, S. L. Lo and S. Kumar, Global trends in municipal solid waste treatment technologies through the lens of sustainable energy development opportunity, Energy, 2023, 275, 127471,  DOI:10.1016/j.energy.2023.127471.
7. A. R. Pereira, M. Simões, and I. B. Gomes, Parabens as Environmental Contaminants of Aquatic Systems Affecting Water Quality and Microbial Dynamics, Elsevier B.V., 2023,  DOI:10.1016/j.scitotenv.2023.167332.
8. F. Piadeh, O. Ikechukwu, B. Kourosh, R. P. Joseph, B. Angela, C. P. R. Jose and W. Mark, A critical review for the impact of anaerobic digestion on the sustainable development goals, J. Environ. Manage., 2024, 349, 119458,  DOI:10.1016/J.JENVMAN.2023.119458.
9. F. Sher, N. Smječanin, H. Hrnjić, A. Karadža, R. Omanović, E. Šehović and J. Sulejmanović, Emerging technologies for biogas production: A critical review on recent progress, challenges and future perspectives, Process Saf. Environ. Prot., 2024, 188, 834–859,  DOI:10.1016/J.PSEP.2024.05.138.
10. P. S. Nigam and A. Singh, Production of liquid biofuels from renewable resources, Prog. Energy Combust. Sci., 2011, 37(1), 52–68,  DOI:10.1016/J.PECS.2010.01.003.
11. R. Lin, J. Cheng, J. Zhang, J. Zhou, K. Cen and J. D. Murphy, Boosting biomethane yield and production rate with graphene: The potential of direct interspecies electron transfer in anaerobic digestion, Bioresour. Technol., 2017, 239, 345–352,  DOI:10.1016/J.BIORTECH.2017.05.017.
12. T. T. Le, P. Sharma, B. J. Bora, V. D. Tran, T. H. Truong, H. C. Le and P. Q. P. Nguyen, Fueling the future: A comprehensive review of hydrogen energy systems and their challenges, Int. J. Hydrogen Energy, 2024, 54, 791–816,  DOI:10.1016/J.IJHYDENE.2023.08.044.
13. M. M. Awad, I. Hussain, U. Mustapha, O. Ahmed Taialla, A. Musa Alhassan, E. Kotob, A. H. Shafiu Abdullahi, S. A. Ganiyu and K. Alhooshani, A critical review of recent advancements in catalytic dry reforming of methane: Physicochemical properties, current challenges, and informetric insights, Int. J. Hydrogen Energy, 2024, 76, 202–233,  DOI:10.1016/J.IJHYDENE.2024.03.319.
14. M. Alhassan, A. A. Jalil, W. Nabgan, M. Y. S. Hamid, M. B. Bahari and M. Ikram, Bibliometric studies and impediments to valorization of dry reforming of methane for hydrogen production, Fuel, 2022, 328, 125240,  DOI:10.1016/J.FUEL.2022.125240.
15. J. R. C. Rey, A. Longo, B. Rijo, C. M. Pedrero, L. A. C. Tarelho, P. S. D. Brito and C. Nobre, A review of cleaning technologies for biomass-derived syngas, Fuel, 2024, 377, 132776,  DOI:10.1016/J.FUEL.2024.132776.
16. M. Mosaad Awad, E. Kotob, O. Ahmed Taialla, I. Hussain, S. A. Ganiyu and K. Alhooshani, Recent developments and current trends on catalytic dry reforming of Methane: Hydrogen Production, thermodynamics analysis, techno feasibility, and machine learning, Energy Convers. Manag., 2024, 304, 118252,  DOI:10.1016/J.ENCONMAN.2024.118252.
17. D. Komilis, R. Barrena, R. L. Grando, V. Vogiatzi, A. Sánchez, and X. Font, A State of the Art Literature Review on Anaerobic Digestion of Food Waste: Influential Operating Parameters on Methane Yield, Springer Netherlands, 2017,  DOI:10.1007/s11157-017-9428-z.
18. A. Alengebawy, Y. Ran, A. I. Osman, K. Jin, M. Samer and P. Ai, Anaerobic digestion of agricultural waste for biogas production and sustainable bioenergy recovery: a review, Environ. Chem. Lett., 2024, 22(6), 2641–2668,  DOI:10.1007/s10311-024-01789-1.
19. N. A. S. Tjutju, J. Ammenberg, and A. Lindfors, Biogas Potential Studies: A Review of Their Scope, Approach, and Relevance, Elsevier Ltd, 2024,  DOI:10.1016/j.rser.2024.114631.
20. M. Grabovskyi, P. Kucheruk, K. Pavlichenko and H. Roubík, Influence of macronutrients and micronutrients on maize hybrids for biogas production, Environ. Sci. Pollut. Res., 2023, 30(27), 70022–70038,  DOI:10.1007/s11356-023-27235-3.
21. T. Nurgaliev, J. Müller and V. Koshelev, Biogas Potential of Agriculture, BioEnergy Res., 2022, 15(4), 2132–2144,  DOI:10.1007/s12155-022-10409-1.
22. B. Stürmer, Greening the gas grid-evaluation of the biomethane injection potential from agricultural residues in Austria, Processes, 2020, 8(5), 630,  DOI:10.3390/PR8050630.
23. F. Magnolo, H. Dekker, M. Decorte, G. Bezzi, L. Rossi, E. Meers and S. Speelman, The role of sequential cropping and biogasdoneright™ in enhancing the sustainability of agricultural systems in Europe, Agronomy, 2021, 11(11), 2102,  DOI:10.3390/agronomy11112102.
24. P. Gandhi, S. Kumar, K. Paritosh, N. Pareek and V. Vivekanand, Hotel Generated Food Waste and Its Biogas Potential: A Case Study of Jaipur City, India, Waste Biomass Valorization, 2019, 10(6), 1459–1468,  DOI:10.1007/s12649-017-0153-1.
25. R. Feiz, M. Johansson, E. Lindkvist, J. Moestedt, S. N. Påledal and F. Ometto, The biogas yield, climate impact, energy balance, nutrient recovery, and resource cost of biogas production from household food waste—A comparison of multiple cases from Sweden, J. Cleaner Prod., 2022, 378, 134536,  DOI:10.1016/j.jclepro.2022.134536.
26. R. O. dos Santos, L. de S. Santos and D. M. Prata, Simulation and optimization of a methanol synthesis process from different biogas sources, J. Cleaner Prod., 2018, 186, 821–830,  DOI:10.1016/J.JCLEPRO.2018.03.108.
27. A. Vergara-Fernández, G. Vargas, N. Alarcón and A. Velasco, Evaluation of marine algae as a source of biogas in a two-stage anaerobic reactor system, Biomass Bioenergy, 2008, 32(4), 338–344,  DOI:10.1016/j.biombioe.2007.10.005.
28. K. Archana, A. S. Visckram, P. Senthil Kumar, S. Manikandan, A. Saravanan and L. Natrayan, A review on recent technological breakthroughs in anaerobic digestion of organic biowaste for biogas generation: Challenges towards sustainable development goals, Fuel, 2024, 358, 130298,  DOI:10.1016/j.fuel.2023.130298.
29. T. Al Seadi, D. Rutz, R. Janssen, and B. Drosg, Biomass resources for biogas production, The Biogas Handbook: Science, Production and Applications, 2013, pp. 19–51, doi:  DOI:10.1533/9780857097415.1.19.
30. R. Rajagopal, D. I. Massé and G. Singh, A critical review on inhibition of anaerobic digestion process by excess ammonia, Bioresour. Technol., 2013, 143, 632–641,  DOI:10.1016/J.BIORTECH.2013.06.030.
31. Q. Feng and Y. Lin, Integrated processes of anaerobic digestion and pyrolysis for higher bioenergy recovery from lignocellulosic biomass: A brief review, Renewable Sustainable Energy Rev., 2017, 77, 1272–1287,  DOI:10.1016/J.RSER.2017.03.022.
32. S. Rasi, J. Läntelä and J. Rintala, Trace compounds affecting biogas energy utilisation – A review, Energy Convers. Manag., 2011, 52(12), 3369–3375,  DOI:10.1016/J.ENCONMAN.2011.07.005.
33. M. Madsen, J. B. Holm-Nielsen and K. H. Esbensen, Monitoring of anaerobic digestion processes: A review perspective, Renewable Sustainable Energy Rev., 2011, 15(6), 3141–3155,  DOI:10.1016/J.RSER.2011.04.026.
34. R. Baciocchi, E. Carnevale, G. Costa, R. Gavasci, L. Lombardi, T. Olivieri, L. Zanchi and D. Zingaretti, Performance of a biogas upgrading process based on alkali absorption with regeneration using air pollution control residues, Waste Manage., 2013, 33(12), 2694–2705,  DOI:10.1016/J.WASMAN.2013.08.022.
35. B. Bharathiraja, T. Sudharsana, J. Jayamuthunagai, R. Praveenkumar, S. Chozhavendhan and J. Iyyappan, RETRACTED: Biogas production–A review on composition, fuel properties, feed stock and principles of anaerobic digestion, Renewable Sustainable Energy Rev., 2018, 90, 570–582,  DOI:10.1016/J.RSER.2018.03.093.
36. G. Náthia-Neves, M. Berni, G. Dragone, S. I. Mussatto and T. Forster-Carneiro, Anaerobic digestion process: technological aspects and recent developments, Int. J. Environ. Sci. Technol., 2018, 15, 2033–2046,  DOI:10.1007/s13762-018-1682-2.
37. K. C. Surendra, D. Takara, A. G. Hashimoto and S. K. Khanal, Biogas as a sustainable energy source for developing countries: Opportunities and challenges, Renewable Sustainable Energy Rev., 2014, 31, 846–859,  DOI:10.1016/J.RSER.2013.12.015.
38. M. W. Bell, Y. S. Tang, U. Dragosits, C. R. Flechard, P. Ward and C. F. Braban, Ammonia emissions from an anaerobic digestion plant estimated using atmospheric measurements and dispersion modelling, Waste Manage., 2016, 56, 113–124,  DOI:10.1016/J.WASMAN.2016.06.002.
39. H. C. Shin, J. W. Park, K. Park and H. C. Song, Removal characteristics of trace compounds of landfill gas by activated carbon adsorption, Environ. Pollut., 2002, 119(2), 227–236,  DOI:10.1016/S0269-7491(01)00331-1.
40. A. Jaffrin, N. Bentounes, A. M. Joan and S. Makhlouf, Landfill Biogas for heating Greenhouses and providing Carbon Dioxide Supplement for Plant Growth, Biosyst. Eng., 2003, 86(1), 113–123,  DOI:10.1016/S1537-5110(03)00110-7.
41. A. Calbry-Muzyka, H. Madi, F. Rüsch-Pfund, M. Gandiglio and S. Biollaz, Biogas composition from agricultural sources and organic fraction of municipal solid waste, Renewable Energy, 2022, 181, 1000–1007,  DOI:10.1016/j.renene.2021.09.100.
42. A. Benato and A. Macor, Italian biogas plants: Trend, subsidies, cost, biogas composition and engine emissions, Energies, 2019, 12(6), 979,  DOI:10.3390/en12060979.
43. O. W. Awe, Y. Zhao, A. Nzihou, D. P. Minh, and N. Lyczko, A Review of Biogas Utilisation, Purification and Upgrading Technologies, Springer Science and Business Media B.V., 2017,  DOI:10.1007/s12649-016-9826-4.
44. L. Pera, M. Gandiglio, P. Marocco, D. Pumiglia and M. Santarelli, Trace contaminants in biogas: Biomass sources, variability and implications for technology applications, J. Environ. Chem. Eng., 2024, 12(6), 114478,  DOI:10.1016/J.JECE.2024.114478.
45. Q. Sun, H. Li, J. Yan, L. Liu, Z. Yu and X. Yu, Selection of appropriate biogas upgrading technology-a review of biogas cleaning, upgrading and utilisation, Renewable Sustainable Energy Rev., 2015, 51, 521–532,  DOI:10.1016/J.RSER.2015.06.029.
46. X. Y. Chen, H. Vinh-Thang, A. A. Ramirez, D. Rodrigue, and S. Kaliaguine, Membrane Gas Separation Technologies for Biogas Upgrading, Royal Society of Chemistry, 2015,  10.1039/c5ra00666j.
47. A. H. Bhatt and L. Tao, Economic perspectives of biogas production via anaerobic digestion, Bioengineering, 2020, 7(3), 1–19,  DOI:10.3390/bioengineering7030074.
48. M. Feroskhan and S. Ismail, Investigation of the effects of biogas composition on the performance of a biogas–diesel dual fuel CI engine, Biofuels, 2016, 7(6), 593–601,  DOI:10.1080/17597269.2016.1168025.
49. R. Muñoz, L. Meier, I. Diaz, and D. Jeison, A Review on the State-Of-The-Art of Physical/chemical and Biological Technologies for Biogas Upgrading, Springer Netherlands, 2015,  DOI:10.1007/s11157-015-9379-1.
50. M. Miltner, A. Makaruk and M. Harasek, Review on available biogas upgrading technologies and innovations towards advanced solutions, J. Cleaner Prod., 2017, 161, 1329–1337,  DOI:10.1016/j.jclepro.2017.06.045.
51. I. Ullah Khan, M. Hafiz Dzarfan Othman, H. Hashim, T. Matsuura, A. F. Ismail, M. Rezaei-DashtArzhandi and I. Wan Azelee, Biogas as a renewable energy fuel – A review of biogas upgrading, utilisation and storage, Energy Convers. Manag., 2017, 150, 277–294,  DOI:10.1016/J.ENCONMAN.2017.08.035.
52. H. Nie, H. Jiang, D. Chong, Q. Wu, C. Xu and H. Zhou, Comparison of water scrubbing and propylene carbonate absorption for biogas upgrading process, Energy Fuels, 2013, 27(6), 3239–3245,  DOI:10.1021/ef400233w.
53. K. Gaj, Applicability of Selected Methods and Sorbents to Simultaneous Removal of Siloxanes and Other Impurities from Biogas, Springer Verlag, 2017,  DOI:10.1007/s10098-017-1422-1.
54. F. Bauer, T. Persson, C. Hulteberg and D. Tamm, Biogas upgrading - technology overview, comparison and perspectives for the future, Biofuels, Bioprod. Biorefin., 2013, 7(5), 499–511,  DOI:10.1002/bbb.1423.
55. S. Jung, J. Lee, D. H. Moon, K. H. Kim and E. E. Kwon, Upgrading biogas into syngas through dry reforming, Renewable Sustainable Energy Rev., 2021, 143, 110949,  DOI:10.1016/J.RSER.2021.110949.
56. N. Martínez-Ramón, M. Romay, D. Iribarren and J. Dufour, Life-cycle assessment of hydrogen produced through chemical looping dry reforming of biogas, Int. J. Hydrogen Energy, 2024, 78, 373–381,  DOI:10.1016/J.IJHYDENE.2024.06.288.
57. F. Barrai, T. Jackson, N. Whitmore and M. J. Castaldi, The role of carbon deposition on precious metal catalyst activity during dry reforming of biogas, Catal. Today, 2007, 129(3–4), 391–396,  DOI:10.1016/j.cattod.2007.07.024.
58. M. Usman, W. M. A. Wan Daud and H. F. Abbas, Dry reforming of methane: Influence of process parameters—A review, Renewable Sustainable Energy Rev., 2015, 45, 710–744,  DOI:10.1016/J.RSER.2015.02.026.
59. K. Al-Ali, S. Kodama and H. Sekiguchi, Modeling and simulation of methane dry reforming in direct-contact bubble reactor, Sol. Energy, 2014, 102, 45–55,  DOI:10.1016/J.SOLENER.2014.01.010.
60. D. Pakhare and J. Spivey, A Review of Dry (CO2) Reforming of Methane over Noble Metal Catalysts, Royal Society of Chemistry, 2014,  10.1039/c3cs60395d.
61. I. V. Yentekakis, P. Panagiotopoulou, and G. Artemakis, A Review of Recent Efforts to Promote Dry Reforming of Methane (DRM) to Syngas Production via Bimetallic Catalyst Formulations, Elsevier B.V., 2021,  DOI:10.1016/j.apcatb.2021.120210.
62. C. Marschelke, A. Fery and A. Synytska, Janus particles: from concepts to environmentally friendly materials and sustainable applications, Colloid Polym. Sci., 2020, 298(7), 841–865,  DOI:10.1007/s00396-020-04601-y.
63. C. Palmer, D. C. Upham, S. Smart, M. J. Gordon, H. Metiu and E. W. McFarland, Dry reforming of methane catalysed by molten metal alloys, Nat. Catal., 2020, 3(1), 83–89,  DOI:10.1038/s41929-019-0416-2.
64. Z. Bian, I. Y. Suryawinata and S. Kawi, Highly carbon resistant multicore-shell catalyst derived from Ni-Mg phyllosilicate nanotubes@silica for dry reforming of methane, Appl. Catal., B, 2016, 195, 1–8,  DOI:10.1016/j.apcatb.2016.05.001.
65. O. Omoregbe, H. T. Danh, C. Nguyen-Huy, H. D. Setiabudi, S. Z. Abidin, Q. D. Truong and D. V. N. Vo, Syngas production from methane dry reforming over Ni/SBA-15 catalyst: Effect of operating parameters, Int. J. Hydrogen Energy, 2017, 42(16), 11283–11294,  DOI:10.1016/J.IJHYDENE.2017.03.146.
66. M. Chaghouri, C. Gennequin, L. H. Tidahy, F. Cazier, E. Abi-Aad, E. Veignie and C. Rafin, Low cost and renewable H2S-biofilter inoculated with Trichoderma harzianum, Environ. Technol., 2024, 45(8), 1508–1521,  DOI:10.1080/09593330.2022.2147024.
67. Y. Gao, J. Jiang, Y. Meng, F. Yan and A. Aihemaiti, A review of recent developments in hydrogen production via biogas dry reforming, Energy Convers. Manag., 2018, 171, 133–155,  DOI:10.1016/J.ENCONMAN.2018.05.083.
68. Y. Li, Z. Dai, Y. Dong, J. Xu, Q. Guo and F. Wang, Equilibrium prediction of acid gas partial oxidation with presence of CH4 and CO2 for hydrogen production, Appl. Therm. Eng., 2016, 107, 125–134,  DOI:10.1016/J.APPLTHERMALENG.2016.05.076.
69. X. Meng, W. De Jong, R. Pal and A. H. M. Verkooijen, In bed and downstream hot gas desulphurization during solid fuel gasification: A review, Fuel Process. Technol., 2010, 91(8), 964–981,  DOI:10.1016/J.FUPROC.2010.02.005.
70. T. Y. Yeo, J. Ashok and S. Kawi, Recent developments in sulphur-resilient catalytic systems for syngas production, Renewable Sustainable Energy Rev., 2019, 100, 52–70,  DOI:10.1016/J.RSER.2018.10.016.
71. D. K. Niakolas, Sulfur poisoning of Ni-based anodes for Solid Oxide Fuel Cells in H/C-based fuels, Appl. Catal., A, 2014, 486, 123–142,  DOI:10.1016/J.APCATA.2014.08.015.
72. J. Lee, R. Li, M. J. Janik and K. M. Dooley, Rare Earth/Transition Metal Oxides for Syngas Tar Reforming: A Model Compound Study, Ind. Eng. Chem. Res., 2018, 57(18), 6131–6140,  DOI:10.1021/acs.iecr.8b00682.
73. R. Y. Chein, Y. C. Chen and W. H. Chen, Experimental study on sulfur deactivation and regeneration of ni-based catalyst in dry reforming of biogas, Catalysts, 2021, 11(7), 777,  DOI:10.3390/catal11070777.
74. J. R. Rostrup-Nielsen, J. Sehested and J. K. Nørskov, Hydrogen and synthesis gas by steam- and CO2 reforming, Adv. Catal., 2002, 47, 65–139,  DOI:10.1016/S0360-0564(02)47006-X.
75. E. le Saché and T. R. Reina, Analysis of Dry Reforming as Direct Route for Gas Phase CO2 Conversion. The Past, the Present and Future of Catalytic DRM Technologies, Elsevier Ltd, 2022,  DOI:10.1016/j.pecs.2021.100970.
76. D. K. Binte Mohamed, A. Veksha, Q. L. M. Ha, W. P. Chan, T. T. Lim and G. Lisak, Advanced Ni tar reforming catalysts resistant to syngas impurities: Current knowledge, research gaps and future prospects, Fuel, 2022, 318, 123602,  DOI:10.1016/j.fuel.2022.123602.
77. I. F. Elbaba and P. T. Williams, Deactivation of nickel catalysts by sulfur and carbon for the pyrolysis-catalytic gasification/reforming of waste tires for hydrogen production, in Energy and Fuels, 2014, pp. 2104–2113,  DOI:10.1021/ef4023477.
78. I. Chen and D.-W. Shine, Resistivity to Sulfur Poisoning of Nickel-Alumina Catalysts, Ind. Eng. Chem. Res., 1988, 26, 1391–1396,  DOI:10.1021/ie00080a011.
79. J. Srinakruang, K. Sato, T. Vitidsant and K. Fujimoto, Highly efficient sulfur and coking resistance catalysts for tar gasification with steam, Fuel, 2006, 85(17–18), 2419–2426,  DOI:10.1016/j.fuel.2006.04.026.
80. C. Li, D. Hirabayashi and K. Suzuki, A crucial role of O2- and O22- on mayenite structure for biomass tar steam reforming over Ni/Ca12Al14O33, Appl. Catal., B, 2009, 88(3–4), 351–360,  DOI:10.1016/j.apcatb.2008.11.004.
81. S. L. Lakhapatri and M. A. Abraham, Analysis of catalyst deactivation during steam reforming of jet fuel on Ni-(PdRh)/γ-Al2O3 catalyst, Appl. Catal., A, 2011, 405(1–2), 149–159,  DOI:10.1016/j.apcata.2011.08.004.
82. S. A. Theofanidis, J. A. Z. Pieterse, H. Poelman, A. Longo, M. K. Sabbe, M. Virginie, C. Detavernier, G. B. Marin and V. V. Galvita, Effect of Rh in Ni-based catalysts on sulfur impurities during methane reforming, Appl. Catal., B, 2020, 267, 118691,  DOI:10.1016/J.APCATB.2020.118691.
83. L. Li, F. Hou, B. Zhang, S. Zuo, P. An, G. Li and G. Liu, Sulfur-Tolerant Ni-Pt/Al2O3Catalyst for Steam Reforming of Jet Fuel Model Compound n-Dodecane, Energy Fuels, 2020, 34(6), 7430–7438,  DOI:10.1021/acs.energyfuels.0c00769.
84. P. H. Moud, K. J. Andersson, R. Lanza and K. Engvall, Equilibrium potassium coverage and its effect on a Ni tar reforming catalyst in alkali- and sulfur-laden biomass gasification gases, Appl. Catal., B, 2016, 190, 137–146,  DOI:10.1016/j.apcatb.2016.03.007.
85. A. Hernandez, K. J. Andersson, K. Engvall and E. Kantarelis, Gas-Phase Potassium Effects and the Role of the Support on the Tar Reforming of Biomass-Derived Producer Gas over Sulfur-Equilibrated Ni/MgAl2O4, Energy Fuels, 2020, 34(9), 11103–11111,  DOI:10.1021/acs.energyfuels.0c02069.
86. S. Y. Jung, D. G. Ju, E. J. Lim, S. C. Lee, B. W. Hwang and J. C. Kim, Study of sulfur-resistant Ni-Al-based catalysts for autothermal reforming of dodecane, Int. J. Hydrogen Energy, 2015, 40(39), 13412–13422,  DOI:10.1016/j.ijhydene.2015.08.044.
87. G. Garbarino, E. Finocchio, A. Lagazzo, I. Valsamakis, P. Riani, V. S. Escribano and G. Busca, Steam reforming of ethanol–phenol mixture on Ni/Al2O3: Effect of magnesium and boron on catalytic activity in the presence and absence of sulphur, Appl. Catal., B, 2014, 147, 813–826,  DOI:10.1016/J.APCATB.2013.09.030.
88. S. A. Chattanathan, S. Adhikari, M. McVey and O. Fasina, Hydrogen production from biogas reforming and the effect of H2S on CH4 conversion, Int. J. Hydrogen Energy, 2014, 39(35), 19905–19911,  DOI:10.1016/j.ijhydene.2014.09.162.
89. V. Pawar, S. Appari, D. S. Monder and V. M. Janardhanan, Study of the Combined Deactivation Due to Sulfur Poisoning and Carbon Deposition during Biogas Dry Reforming on Supported Ni Catalyst, Ind. Eng. Chem. Res., 2017, 56(30), 8448–8455,  DOI:10.1021/acs.iecr.7b01662.
90. M. Ashrafi, C. Pfeifer, T. Pröll and H. Hofbauer, Experimental study of model biogas catalytic steam reforming: 2. Impact of sulfur on the deactivation and regeneration of Ni-based catalysts, Energy Fuels, 2008, 22(6), 4190–4195,  DOI:10.1021/ef8000828.
91. J. R. Rostrup-Nielsen, Some principles relating to the regeneration of sulfur-poisoned nickel catalyst, J. Catal., 1971, 21(2), 171–178,  DOI:10.1016/0021-9517(71)90135-7.
92. J. Blanchard, I. Achouri and N. Abatzoglou, H2S poisoning of NiAl2O4/Al2O3-YSZ catalyst during methane dry reforming, Can. J. Chem. Eng., 2016, 94(4), 650–654,  DOI:10.1002/cjce.22438.
93. X. Chen, O. Jiang, F. Yan, K. Li, S. Tian, Y. Gao and H. Zhou, Dry Reforming of Model Biogas on a Ni/SiO2 Catalyst: Overall Performance and Mechanisms of Sulfur Poisoning and Regeneration, ACS Sustain. Chem. Eng., 2017, 5(11), 10248–10257,  DOI:10.1021/acssuschemeng.7b02251.
94. B. Saha, A. Khan, H. Ibrahim and R. Idem, Evaluating the performance of non-precious metal based catalysts for sulfur-tolerance during the dry reforming of biogas, Fuel, 2014, 120, 202–217,  DOI:10.1016/j.fuel.2013.12.016.
95. M. A. Ocsachoque, J. I. Eugenio Russman, B. Irigoyen, D. Gazzoli and M. G. González, Experimental and theoretical study about sulfur deactivation of Ni/CeO2 and Rh/CeO2 catalysts, Mater. Chem. Phys., 2016, 172, 69–76,  DOI:10.1016/j.matchemphys.2015.12.062.
96. R. Chein and Z. W. Yang, H2S effect on dry reforming of biogas for syngas production, Int. J. Energy Res., 2019, 43(8), 3330–3345,  DOI:10.1002/er.4470.
97. G. Mancino, S. Cimino and L. Lisi, Sulphur poisoning of alumina supported Rh catalyst during dry reforming of methane, Catal. Today, 2016, 277, 126–132,  DOI:10.1016/j.cattod.2015.10.035.
98. W. Yin, N. Guilhaume and Y. Schuurman, Model biogas reforming over Ni-Rh/MgAl2O4 catalyst. Effect of gas impurities, Chem. Eng. J., 2020, 398, 125534–125542,  DOI:10.1016/j.cej.2020.125534.
99. Y. Gao, J. Jiang, Y. Meng, T. Ju and S. Han, Influence of H2S and NH3 on biogas dry reforming using Ni catalyst: a study on single and synergetic effect, Front. Environ. Sci. Eng., 2023, 17(3), 32,  DOI:10.1007/s11783-023-1632-1.
100. X. Liu, J. Yan, J. Mao, D. He, S. Yang, Y. Mei and Y. Luo, Inhibitor, co-catalyst, or intermetallic promoter? Probing the sulfur-tolerance of MoOx surface decoration on Ni/SiO2 during methane dry reforming, Appl. Surf. Sci., 2021, 548, 149231,  DOI:10.1016/J.APSUSC.2021.149231.
101. M. Gaillard, M. Virginie and A. Y. Khodakov, New molybdenum-based catalysts for dry reforming of methane in presence of sulfur: A promising way for biogas valorization, Catal. Today, 2017, 289, 143–150,  DOI:10.1016/j.cattod.2016.10.005.
102. S. T. Misture, K. M. McDevitt, K. C. Glass, D. D. Edwards, J. Y. Howe, K. D. Rector, H. Hec and S. C. Vogel, Sulfur-resistant and regenerable Ni/Co spinel-based catalysts for methane dry reforming, Catal. Sci. Technol., 2015, 5(9), 4565–4574,  10.1039/c5cy00800j.
103. K. Sato and K. Fujimoto, Development of new nickel based catalyst for tar reforming with superior resistance to sulfur poisoning and coking in biomass gasification, Catal. Commun., 2007, 8(11), 1697–1701,  DOI:10.1016/j.catcom.2007.01.028.
104. C. Jiang, E. Loisel, D. A. Cullen, J. A. Dorman and K. M. Dooley, On the enhanced sulfur and coking tolerance of Ni-Co-rare earth oxide catalysts for the dry reforming of methane, J. Catal., 2021, 393, 215–229,  DOI:10.1016/j.jcat.2020.11.028.
105. L. Appels, J. Baeyens and R. Dewil, Siloxane removal from biosolids by peroxidation, Energy Convers. Manag., 2008, 49(10), 2859–2864,  DOI:10.1016/j.enconman.2008.03.006.
106. G. Piechota and R. Buczkowski, Development of chromatographic methods by using direct-sampling procedure for the quantification of cyclic and linear volatile methylsiloxanes in biogas as perspective for application in online systems, Int. J. Environ. Anal. Chem., 2014, 94(8), 837–851,  DOI:10.1080/03067319.2013.879296.
107. J. A. Mueller, D. M. Di Toro and J. A. Maiello, Fate of octamethylcyclotetrasiloxane (OMCTS) in the atmosphere and in sewage treatment plants as an estimation of aquatic exposure, Environ. Toxicol. Chem., 1995, 14(10), 1657–1666,  DOI:10.1002/etc.5620141005.
108. L. Rivera-Montenegro, E. I. Valenzuela, A. González-Sánchez, R. Muñoz, and G. Quijano, Volatile Methyl Siloxanes as Key Biogas Pollutants: Occurrence, Impacts and Treatment Technologies, Springer, 2023,  DOI:10.1007/s12155-022-10525-y.
109. Y. Cheng, M. Shoeib, L. Ahrens, T. Harner and J. Ma, Wastewater treatment plants and landfills emit volatile methyl siloxanes (VMSs) to the atmosphere: Investigations using a new passive air sampler, Environ. Pollut., 2011, 159(10), 2380–2386,  DOI:10.1016/j.envpol.2011.07.002.
110. G. Ruiling, C. Shikun and L. Zifu, Research progress of siloxane removal from biogas, Int. J. Agric. Biol. Eng., 2017, 10(1), 30–39,  DOI:10.3965/j.ijabe.20171001.3043.
111. G. Piechota, Siloxanes in biogas: Approaches of sampling procedure and GC-MS method determination, Molecules, 2021, 26(7), 1953,  DOI:10.3390/molecules26071953.
112. S. Rasi, J. Lehtinen and J. Rintala, Determination of organic silicon compounds in biogas from wastewater treatments plants, landfills, and co-digestion plants, Renewable Energy, 2010, 35(12), 2666–2673,  DOI:10.1016/j.renene.2010.04.012.
113. M. Shen, Y. Zhang, D. Hu, J. Fan, and G. Zeng, A Review on Removal of Siloxanes from Biogas: with a Special Focus on Volatile Methylsiloxanes, Springer Verlag, 2018,  DOI:10.1007/s11356-018-3000-4.
114. N. H. Elsayed, A. Elwell, B. Joseph and J. N. Kuhn, Effect of silicon poisoning on catalytic dry reforming of simulated biogas, Appl. Catal., A, 2017, 538, 157–164,  DOI:10.1016/j.apcata.2017.03.024.
115. Y. Kathiraser, Z. Wang and S. Kawi, Oxidative CO2 reforming of methane in La0.6Sr 0.4Co0.8Ga0.2O3-δ (LSCG) hollow fiber membrane reactor, Environ. Sci. Technol., 2013, 47(24), 14510–14517,  DOI:10.1021/es403158k.
116. V. A. Kondratenko, C. Berger-Karin and E. V. Kondratenko, Partial oxidation of methane to syngas over γ-Al2O3-supported rh nanoparticles: Kinetic and mechanistic origins of size effect on selectivity and activity, ACS Catal., 2014, 4(9), 3136–3144,  DOI:10.1021/cs5002465.
117. V. R. Choudhary and K. C. Mondal, CO2 reforming of methane combined with steam reforming or partial oxidation of methane to syngas over NdCoO3 perovskite-type mixed metal-oxide catalyst, Appl. Energy, 2006, 83(9), 1024–1032,  DOI:10.1016/j.apenergy.2005.09.008.
118. W. H. Chen and S. C. Lin, Characterization of catalytic partial oxidation of methane with carbon dioxide utilization and excess enthalpy recovery, Appl. Energy, 2014, 162, 1141–1152,  DOI:10.1016/j.apenergy.2015.01.056.
119. L. Yang, X. Ge, C. Wan, F. Yu, and Y. Li, Progress and Perspectives in Converting Biogas to Transportation Fuels, Elsevier Ltd, 2014,  DOI:10.1016/j.rser.2014.08.008.
120. R. Chai, Z. Zhang, P. Chen, G. Zhao, Y. Liu and Y. Lu, Ni-foam-structured NiO-MOx-Al2O3 (M = Ce or Mg) nanocomposite catalyst for high throughput catalytic partial oxidation of methane to syngas, Microporous Mesoporous Mater., 2017, 253, 123–128,  DOI:10.1016/j.micromeso.2017.07.005.
121. M. A. Hassan and M. Komiyama, Catalytic Partial Oxidation of Methane in Supercritical Water: A Domain in CH ₄/H₂O–O₂/CH₄ Parameter Space Showing Significant Methane Coupling, Ind. Eng. Chem. Res., 2017, 56(23), 6618–6624,  DOI:10.1021/acs.iecr.7b00993.
122. R. Y. Chein, Y. C. Chen, C. T. Yu and J. N. Chung, Thermodynamic analysis of dry reforming of CH4 with CO2 at high pressures, J. Nat. Gas Sci. Eng., 2015, 26, 617–629,  DOI:10.1016/j.jngse.2015.07.001.
123. A. F. Lucrédio, J. M. Assaf and E. M. Assaf, Methane conversion reactions on Ni catalysts promoted with Rh: Influence of support, Appl. Catal., A, 2011, 400(1–2), 156–165,  DOI:10.1016/j.apcata.2011.04.035.
124. M. P. Kohn, M. J. Castaldi and R. J. Farrauto, Auto-thermal and dry reforming of landfill gas over a Rh/γAl2O3 monolith catalyst, Appl. Catal., B, 2010, 94(1–2), 125–133,  DOI:10.1016/j.apcatb.2009.10.029.
125. X. Chen, J. Jiang, K. Li, S. Tian and F. Yan, Energy-efficient biogas reforming process to produce syngas: The enhanced methane conversion by O2, Appl. Energy, 2017, 185, 687–697,  DOI:10.1016/j.apenergy.2016.10.114.
126. A. Jahangiri, H. Aghabozorg and H. Pahlavanzadeh, Effects of Fe substitutions by Ni in La-Ni-O perovskite-type oxides in reforming of methane with CO2 and O2, Int. J. Hydrogen Energy, 2013, 38(25), 10407–10416,  DOI:10.1016/j.ijhydene.2013.05.080.
127. P. K. Chaudhary, N. Koshta and G. Deo, Effect of O2 and temperature on the catalytic performance of Ni/Al2O3 and Ni/MgAl2O4 for the dry reforming of methane (DRM), Int. J. Hydrogen Energy, 2020, 45(7), 4490–4500,  DOI:10.1016/j.ijhydene.2019.12.053.
128. S. Y. Foo, C. K. Cheng, T. H. Nguyen and A. A. Adesina, Syngas production from CH4 dry reforming over Co-Ni/Al2O3 catalyst: Coupled reaction-deactivation kinetic analysis and the effect of O2 co-feeding on H2:CO ratio, Int. J. Hydrogen Energy, 2012, 37(22), 17019–17026,  DOI:10.1016/j.ijhydene.2012.08.136.
129. E. David and V. C. Niculescu, Volatile Organic Compounds (Vocs) as Environmental Pollutants: Occurrence and Mitigation Using Nanomaterials, MDPI, 2021,  DOI:10.3390/ijerph182413147.
130. J. I. Salazar Gómez, H. Lohmann and J. Krassowski, Determination of volatile organic compounds from biowaste and co-fermentation biogas plants by single-sorbent adsorption, Chemosphere, 2016, 153, 48–57,  DOI:10.1016/J.CHEMOSPHERE.2016.02.128.
131. A. E. Cioabla, I. Ionel, G. A. Dumitrel and F. Popescu, Comparative study on factors affecting anaerobic digestion of agricultural vegetal residues, Biotechnol. Biofuels, 2012, 5, 39,  DOI:10.1186/1754-6834-5-39.
132. A. Lakhouit, W. N. Schirmer, T. R. Johnson, H. Cabana and A. R. Cabral, Evaluation of the efficiency of an experimental biocover to reduce BTEX emissions from landfill biogas, Chemosphere, 2014, 97, 98–101,  DOI:10.1016/J.CHEMOSPHERE.2013.09.120.
133. P. F. de Sá Borba, E. M. Martins, E. Ritter and S. M. Corrêa, BTEX Emissions from the Largest Landfill in Operation in Rio de Janeiro, Brazil, Bull. Environ. Contam. Toxicol., 2017, 98(5), 624–631,  DOI:10.1007/s00128-017-2050-5.
134. S. Rasi, A. Veijanen and J. Rintala, Trace compounds of biogas from different biogas production plants, Energy, 2007, 32(8), 1375–1380,  DOI:10.1016/j.energy.2006.10.018.
135. R. Rinaldi, G. Lombardelli, M. Gatti, C. G. Visconti and M. C. Romano, Techno-economic analysis of a biogas-to-methanol process: Study of different process configurations and conditions, J. Cleaner Prod., 2023, 393, 136259,  DOI:10.1016/J.JCLEPRO.2023.136259.
136. A. Mosayebi and M. H. Eghbal Ahmadi, Combined steam and dry reforming of methanol process to syngas formation: Kinetic modeling and thermodynamic equilibrium analysis, Energy, 2022, 261, 125254,  DOI:10.1016/J.ENERGY.2022.125254.
137. A. Iulianelli, P. Ribeirinha, A. Mendes, and A. Basile, Methanol Steam Reforming for Hydrogen Generation via Conventional and Membrane Reactors: A Review, Elsevier Ltd, 2014,  DOI:10.1016/j.rser.2013.08.032.
138. Y. T. Shah and T. H. Gardner, Dry reforming of hydrocarbon feedstocks, Catal. Rev.:Sci. Eng., 2014, 56(4), 476–536,  DOI:10.1080/01614940.2014.946848.
139. H. Zhang, X. Li, F. Zhu, K. Cen, C. Du and X. Tu, Plasma assisted dry reforming of methanol for clean syngas production and high-efficiency CO2 conversion, Chem. Eng. J., 2017, 310, 114–119,  DOI:10.1016/j.cej.2016.10.104.
140. A. Mosayebi and M. H. Eghbal Ahmadi, Combined steam and dry reforming of methanol process to syngas formation: Kinetic modeling and thermodynamic equilibrium analysis, Energy, 2022, 261, 125254,  DOI:10.1016/J.ENERGY.2022.125254.
141. A. Mosayebi, Kinetic modeling and experimental investigations of dry reforming of methanol over a Cr-Mo-Mn/SiO2 catalyst, Res. Chem. Intermed., 2021, 47(7), 2951–2972,  DOI:10.1007/s11164-021-04448-0.
142. B. Christian Enger, R. Lødeng and A. Holmen, A review of catalytic partial oxidation of methane to synthesis gas with emphasis on reaction mechanisms over transition metal catalysts, Aug, 2008, 31(346), 1–27,  DOI:10.1016/j.apcata.2008.05.018.
143. M. Kong, J. Fei, S. Wang, W. Lu and X. Zheng, Influence of supports on catalytic behavior of nickel catalysts in carbon dioxide reforming of toluene as a model compound of tar from biomass gasification, Bioresour. Technol., 2011, 102(2), 2004–2008,  DOI:10.1016/j.biortech.2010.09.054.
144. M. Kong, Q. Yang, J. Fei and X. Zheng, Experimental study of Ni/MgO catalyst in carbon dioxide reforming of toluene, a model compound of tar from biomass gasification, Int. J. Hydrogen Energy, 2012, 37(18), 13355–13364,  DOI:10.1016/j.ijhydene.2012.06.108.
145. M. Kong, Q. Yang, W. Lu, Z. Fan, J. Fei, X. Zheng and T. D. Wheelock, Effect of Calcination Temperature on Characteristics and Performance of Ni/MgO Catalyst for CO2 Reforming of Toluene, Chin. J. Catal., 2012, 33(9–10), 1508–1516,  DOI:10.1016/S1872-2067(11)60424-5.
146. X. Bao, M. Kong, W. Lu, J. Fei and X. Zheng, Performance of Co/MgO catalyst for CO2 reforming of toluene as a model compound of tar derived from biomass gasification, J. Energy Chem., 2014, 23(6), 795–800,  DOI:10.1016/S2095-4956(14)60214-X.
147. T. Chen, H. Liu, P. Shi, D. Chen, L. Song, H. He and R. L. Frost, CO2 reforming of toluene as model compound of biomass tar on Ni/Palygorskite, Fuel, 2013, 107, 699–705,  DOI:10.1016/J.FUEL.2012.12.036.
148. G. Oh, S. Y. Park, M. W. Seo, H. W. Ra, T. Y. Mun, J. G. Lee and S. J. Yoon, Combined steam-dry reforming of toluene in syngas over CaNiRu/Al2O3 catalysts, Int. J. Green Energy, 2019, 16(4), 333–349,  DOI:10.1080/15435075.2019.1566729.
149. M. Chaghouri, S. Hany, F. Cazier, H. L. Tidahy, C. Gennequin and E. Abi-Aad, Impact of impurities on biogas valorization through dry reforming of methane reaction, Int. J. Hydrogen Energy, 2022, 47(95), 40415–40429,  DOI:10.1016/j.ijhydene.2022.08.248.
150. C. Tanios, C. Gennequin, M. Labaki, H. L. Tidahy, A. Aboukaïs and E. Abi-Aad, Evaluation of a catalyst durability in absence and presence of toluene impurity: Case of the material Co2NvMg2Al2 mixed oxide prepared by hydrotalcite route in methane dry reforming to produce energy, Materials, 2019, 12(9), 1362,  DOI:10.3390/ma12091362.
151. P. Champagne, Feasibility of producing bio-ethanol from waste residues: A Canadian perspective: Feasibility of producing bio-ethanol from waste residues in Canada, Resour., Conserv. Recycl., 2007, 50(3), 211–230,  DOI:10.1016/J.RESCONREC.2006.09.003.
152. B. Bej, S. Bepari, N. C. Pradhan and S. Neogi, Production of hydrogen by dry reforming of ethanol over alumina supported nano-NiO/SiO2 catalyst, Catal. Today, 2017, 291, 58–66,  DOI:10.1016/j.cattod.2016.12.010.
153. M. N. N. Shafiqah, T. J. Siang, P. S. Kumar, et al., Advanced Catalysts and Effect of Operating Parameters in Ethanol Dry Reforming for Hydrogen Generation. A Review, Springer Science and Business Media Deutschland GmbH, 2022,  DOI:10.1007/s10311-022-01394-0.
154. T. J. Siang, A. A. Jalil, A. Abdulrahman and H. U. Hambali, Enhanced carbon resistance and regenerability in methane partial oxidation to syngas using oxygen vacancy-rich fibrous Pd, Ru and Rh/KCC-1 catalysts, Environ. Chem. Lett., 2021, 19(3), 2733–2742,  DOI:10.1007/s10311-021-01192-0.
155. Z. Alipour, V. Babu Borugadda, H. Wang, and A. K. Dalai, Syngas Production through Dry Reforming: A Review on Catalysts and Their Materials, Preparation Methods and Reactor Type, Elsevier B.V., 2023,  DOI:10.1016/j.cej.2022.139416.
156. W. Wang and Y. Wang, Dry reforming of ethanol for hydrogen production: Thermodynamic investigation, Int. J. Hydrogen Energy, 2009, 34(13), 5382–5389,  DOI:10.1016/J.IJHYDENE.2009.04.054.
157. S. S. Itkulova, Y. Y. Nurmakanov, S. K. Kussanova and Y. A. Boleubayev, Production of a hydrogen-enriched syngas by combined CO2-steam reforming of methane over Co-based catalysts supported on alumina modified with zirconia, Catal. Today, 2018, 299, 272–279,  DOI:10.1016/j.cattod.2017.07.014.
158. T. Stroud, T. J. Smith, E. Le Saché, J. L. tos, M. A. Centeno, H. llano-Garcia, J. A. Odriozola and T. R. Reina, Chemical CO2 recycling via dry and bi reforming of methane using Ni-Sn/Al2O3 and Ni-Sn/CeO2-Al2O3 catalysts, Appl. Catal., B, 2018, 224, 125–135,  DOI:10.1016/J.APCATB.2017.10.047.
159. D. P. Minh, T. J. Siang, D. V. N. Vo, T. S. Phan, C. Ridart, A. Nzihou and D. Grouset, Hydrogen Production From Biogas Reforming: An Overview of Steam Reforming, Dry Reforming, Dual Reforming, and Tri-Reforming of Methane, Hydrogen Supply Chain: Design, Deployment and Operation, 2018, pp. 111–166,  DOI:10.1016/B978-0-12-811197-0.00004-X.
160. R. Chein and Z. Yang, Experimental Study on Dry Reforming of Biogas for Syngas Production over Ni-Based Catalysts, ACS Omega, 2019, 4(25), 20911–20922,  DOI:10.1021/acsomega.9b01784.
161. N. Kumar, M. Shojaee, and J. J. Spivey, Catalytic Bi-reforming of Methane: from Greenhouse Gases to Syngas, Elsevier Ltd, 2015,  DOI:10.1016/j.coche.2015.07.003.
162. Y. Khani, Z. Shariatinia and F. Bahadoran, High catalytic activity and stability of ZnLaAlO4 supported Ni, Pt and Ru nanocatalysts applied in the dry, steam and combined dry-steam reforming of methane, Chem. Eng. J., 2016, 299, 353–366,  DOI:10.1016/j.cej.2016.04.108.
163. K. Jabbour, P. Massiani, A. Davidson, S. Casale and N. El Hassan, Ordered mesoporous ‘one-pot’ synthesized Ni-Mg(Ca)-Al2O3 as effective and remarkably stable catalysts for combined steam and dry reforming of methane (CSDRM), Appl. Catal., B, 2017, 201, 527–542,  DOI:10.1016/J.APCATB.2016.08.009.
164. I. H. Son, S. J. Lee, A. Soon, H. S. Roh and H. Lee, Steam treatment on Ni/γ-Al2O3 for enhanced carbon resistance in combined steam and carbon dioxide reforming of methane, Appl. Catal., B, 2013, 134–135, 103–109,  DOI:10.1016/j.apcatb.2013.01.001.
165. G. A. Olah, A. Goeppert, M. Czaun and G. K. S. Prakash, Bi-reforming of methane from any source with steam and carbon dioxide exclusively to metgas (CO-2H2) for methanol and hydrocarbon synthesis, J. Am. Chem. Soc., 2013, 135(2), 648–650,  DOI:10.1021/ja311796n.
166. N. Kumar, Z. Wang, S. Kanitkar and J. J. Spivey, Methane reforming over Ni-based pyrochlore catalyst: deactivation studies for different reactions, Appl. Petrochem. Res., 2016, 6(3), 201–207,  DOI:10.1007/s13203-016-0166-x.
167. M. P. Kohn, M. J. Castaldi and R. J. Farrauto, Biogas reforming for syngas production: The effect of methyl chloride, Appl. Catal., B, 2014, 144, 353–361,  DOI:10.1016/j.apcatb.2013.07.031.

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