An overview on dry reforming of methane: strategies to reduce carbonaceous deactivation of catalysts

Abstract

Catalytic reforming of methane (CH₄) with carbon dioxide (CO₂), known as dry reforming of methane (DRM), produces synthesis gas, which is a mixture of hydrogen (H₂) and carbon monoxide (CO). CH₄ + CO₂ → 2CO + 2H₂, ΔH° = 247.3 kJ mol⁻¹, ΔG = 61 770–67.32T. The DRM process has gained much attention recently as it reduces greenhouse gases (GHG), CO₂ and CH₄, in the atmosphere. In addition to reducing GHG, the DRM process produces valuable chemicals (CO + H₂), provides a good approach to utilizing biogas and natural gas with a significant amount of CO₂, has good capability as a chemical energy transmission system as compared to steam reforming, and finally yields the desired unity H₂/CO ratio for Fischer–Tropsch synthesis. The bimetallic Ni-based catalysts supported on Al₂O₃/TiO₂ and promoted with Ce/ZrO₂ show remarkable performances in the DRM process. But, carbonaceous deactivation of the catalysts is the major problem faced during this process. Numerous studies have been cited on various aspects of DRM, and some papers are also devoted to reviewing carbonaceous deposition problems and their remedies. However, some lacunae exist, which are highlighted in the present review paper on strategies to reduce the carbonaceous deactivation of catalysts for improved DRM efficiency by appropriate catalyst development, operating conditions, and flow reactor designs. The disposal of spent catalysts falls under the category of hazardous industrial materials and is also required to comply with stringent environmental regulations. Therefore, regeneration and reclamation techniques for spent catalysts have also been discussed.

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1. Introduction

Carbon dioxide (CO₂) is the most effective greenhouse gas arising from anthropogenic activities, and needs to be reduced in order to counteract global warming and climate change. CH₄ is 21 times more potent than CO₂ in increasing the atmospheric temperature.¹ Dry reforming of methane (DRM) has gained much attention in recent years for its remarkable performance.² It converts CH₄ and CO₂ to synthesis gas or syngas (CO + H₂), having an H₂/CO ratio close to one according to eqn (1).

CH₄ + CO₂ → 2CO + 2H₂, ΔH° = 247.3 kJ mol⁻¹, ΔG° = 61 770–67.32T kJ mol⁻¹ (1)

Thus, the DRM process not only reduces CH₄ and CO₂ emissions into the atmosphere, but also converts them to a valuable product, syngas.³ Syngas is a building block for the production of chemicals; it can be used itself as a fuel or utilized in the manufacture of useful value-added products of more interest, such as: hydrogen (H₂), methanol (CH₃OH), and liquid synthetic motor fuels {gasoline (C₅–C₁₂), naphtha (C₈–C₁₂), kerosene (C₁₁–C₁₃), and diesel (C₁₂–C₂₂)}, according to eqn (2)–(4), respectively.

CO + H₂ + H₂O → 2H₂ + CO₂, ΔH° = −41 kJ mol⁻¹ (2)

CO + 2H₂ → CH₃OH, ΔH° = −90.84 kJ mol⁻¹ (3)

nCO + (2n + 1)H₂ → C_nH_2n+2 + nH₂O, ΔH° = −ve (4)

Eqn (3) and (4) require additional H₂ in the reactant synthesis gas feed, in order to adjust the H₂/CO ratio for methanol manufacturing or the Fischer–Tropsch synthesis. Although DRM has many environmental and economic incentives, unfortunately, there are no commercial processes for the dry reforming of methane. Industrial application of the DRM process is also limited.⁴ Indeed, there are two processes of this kind that produce synthesis gas, called Calcor⁵ and Sparg.⁶

Various types of catalysts are reported in the literature for the DRM process, such as noble metals, spinels, perovskites, hydrotalcite, pyrochlore, and supported base metals.^1,2,7,8 From an industrial point of view, supported nickel is the most appropriate catalyst for the DRM process due to its availability and low cost, but Ni-based catalysts are more susceptible to deactivation in high-temperature reactions than noble metal catalysts.^4,5 This situation leads to the need to develop Ni-based catalysts with improved activity and stability. This can be done by using appropriate supports and preparation methods, and adding modifiers, such as alkaline or noble metals. A number of papers have been published on improving the activity and stability of the catalysts, using a bimetallic catalyst (Ni–Co/Al₂O₃) promoted with Sr or MgO,⁹ an Ni–Co/glass fiber spinel structure,¹⁰ Ni/(Al₂O₃/ZrO₂/Ce₂O₃/La₂O₃) modified with K and Li,¹¹ an aerogel catalyst,¹² an Rh-promoted Ni catalyst,¹³ an atomic layer dispersion method,¹⁴ and different modified Ni-based catalysts.^15–17

Noble metals (Pt, Pd, Ru, Rh, and Ir) have been found to exhibit promising catalytic activity and good coke deposition resistance, but have limited use due to their high cost and low availability. However, several studies have been reported on microwave dielectric heating over a Pt catalyst,¹⁸ Ru/Al₂O₃, and Ru/SiO₂,¹⁹ and Ru/TiO₂.²⁰ Several other approaches have been studied to resist coke deposition, such as activation techniques for the catalyst,²¹ varying operating parameters (feed composition,²² temperature, and pressure⁴), and types of reactors used,^2,23 such as combined steam-CO₂, CH₄ reforming,^24–26 and tri-reforming of CH₄.^27–31

A number of MS^32,33 and PhD theses^34–39 have been approved by different universities on various aspects of DRM, and significant activities on such subjects are also depicted in many patents.^40–42 Several review papers have been published on topics related to DRM, such as catalyst development,^43,44 influence of process parameters,² feasibility of the process,⁴ and coke management.¹ But there is still a gap in the literature for a review article solely devoted to strategies to reduce carbonaceous deactivation of the catalysts. Therefore, in an attempt to fill this gap, the present review lists all possible methods to improve the coke resistance of the catalysts.

2. Background

2.1. Sources of CO₂

Carbon dioxide (CO₂) is a primary greenhouse gas (GHG) emitted from a variety of human activities and natural sources. A huge amount of CO₂ is emitted from anthropogenic sources, such as power generation, the transportation sector, and industries. In 2012, CO₂ accounted for about 82% of all U.S. greenhouse gas emissions from human activities.⁴⁵ Carbon dioxide is naturally present in the atmosphere as part of the Earth's carbon cycle in ocean-atmosphere exchange, plant and animal respiration, soil respiration and decomposition, and volcanic eruptions.⁴⁶ Human activities are altering the carbon cycle, both by adding more CO₂ to the atmosphere and by influencing the ability of natural sinks, like forests, to remove CO₂ from the atmosphere. The government of India agreed to reduce carbon emission by 30–35% by 2030.

2.2. Sources of CH₄

Methane (CH₄) is a colorless and odorless gas produced from the digestion of organic matter via the following two ways: (1) fossil fuel-derived CH₄ is produced from thousands or millions of years old fossil remains of organic matter that lies buried deep in the ground. (2) Biomethane, on the other hand, is produced from fresh organic matter, which makes it a sustainable source of energy that can be produced worldwide. A large portion of the gas is emitted from the industrial sector, such as refineries, and petrochemical and shale gas production.^47–49 It is generally emitted to the atmosphere during production, processing, storage, and distribution, which accounts for the huge amount in the atmosphere. In addition to industries, a large proportion of CH₄ is emitted from the exhaust of vehicles and power gensets fueled by compressed natural gas (CNG). Apart from anthropogenic sources, biomethane is produced in a sizable amount from natural sources by bacterial digestion of biomass under aerobic conditions from wetland, rice fields,^48–51 terrestrial plants, and cattle and poultry farms.^52,53 Furthermore, landfill sites also emit CH₄ via anaerobic digestion by methanogenic bacteria. Biomass burning and forest fires emit additional CH₄ into the environment.⁵⁴

In recent years, air pollution has led to increased use of renewable energy sources, in place of fossil fuels. Biogas containing ∼60% biomethane is a popular source of renewable energy. It can be produced from cow dung and other animal waste, kitchen waste, municipal waste, agricultural residue, and water hyacinths. The biogas production methods and their various applications are detailed in a recent review paper.⁵⁵

Methane is a source of fuel, but if it leaks into the atmosphere, it will warm it by absorbing solar radiation, so it is called a GHG, like CO₂. The major contributor of global warming is CO₂ (∼60%), while CH₄ has a reduced contribution of ∼15%.⁵⁶ However, CH₄ has a higher global warming potential (GWP) than CO₂ but a shorter lifetime in the atmosphere of 12 years. Thus, the Intergovernmental Panel on Climate Change (IPCC) assessment report (AR) values of GWP are based on time. The GWPs for CH₄ being quoted in AR-5 are 28 on a 100-year basis and 84 over 20 years.⁴⁷ This means that one ton of CH₄ has the same effect as 84 tons of CO₂ over 20 years. One exception is the Environmental Protection Agency (EPA) GHG inventory, which uses a 100 year GWP of 21 for CH₄.

2.3. Impact of CO₂ and CH₄

Greenhouse gases transmit shorter wavelength solar radiation to the earth's surface but do not allow longer wavelengths (IR) generated by the hot earth surface to escape. They absorb the radiation then re-radiate it back to the surface as well as to the lower atmosphere. This re-emission results in an increase in the atmospheric temperature, known as the greenhouse effect. CO₂ and CH₄ are the most effective greenhouse gases. An increase in the atmospheric temperature of 0.24 °C can be visualized from 1987 to 2015 in Fig. 1. Continuation of this global warming trend until the end of the 21^st century could increase the atmospheric temperature by around 5.8 °C.^1,2 This “global warming” affects human health, climate, the ozone layer, air pollution, the sea level, and agricultural productivity. So, the survival of living beings would be very difficult in the future, if global warming continues to follow the present trend.

Fig. 1 Global warming trend from 1980 to 2015 (http://www.skepticalsscience.com).

2.3.1. Human health. A warm atmosphere can lead to more rapid development of dangerous pathogens, allowing infectious diseases like dengue fever, West Nile Virus and Lyme disease, to spread widely.^57,58 Viral dengue fever has increased globally by 30-fold during the last 50 years in America. Drinking water and food supplies have also been contaminated due to increased storms, floods, and draughts. A higher temperature is linked to a long allergenic ragweed pollen season. Ragweed is a North American weedy plant that can generate allergic pollen and cause hay fever (inflammation of the mucous membrane lining to nose), and problems associated with asthma.^46,47

2.3.2. Climate changes. The climate has become much too warm, causing more natural disasters. Precipitation patterns have also changed: arid and semi-arid regions are becoming drier, while other areas, mid-to-high latitudes, are becoming wetter. Where precipitation has increased, there has been a disproportionate increase in the frequency of the heaviest precipitation events. Warmer water in the oceans pumps more energy into tropical storms, making them stronger and potentially more destructive.^58–60 Climate change has worsened the situation over the last few years; localized record-breaking evidence of rainfall, draught, snowfall, and temperature rise has been observed within the past few years. For example, in June 2013, a cloud burst caused heavy rainfall in Uttarakhand (India), resulting in more than 6000 deaths, and thousands of people became environmental refugees; a similar situation was observed in 2014, in the Jammu & Kashmir state, while Marathwada and Vidarbha have been experiencing severe drought over the last three years due to deficient rainfall, and this has further worsened the situation, with a drastic drop in groundwater levels, acute water shortages and severe loss of crops.

2.3.3. Antarctica warming. The growing temperature of the atmosphere is breaking the Antarctica ice shelf, as the Larsen B ice shelf lost a total of 1255 square miles over 35 days in early 2002. A group of gentoo penguins nests on the icy shore of Cierva cove, Antarctica. They need cold weather to survive, and the warm atmosphere has reduced the population of penguins around from 32 000 to 11 000 in last few years.^60,61

2.3.4. Rise of sea level. The temperature rise of the atmosphere causes a rise in the sea level due to melting of icebergs. The rising sea level caused salination of coastal lands and fresh water supplies, resulting in population displacement.⁴⁷ This change in the availability and distribution of natural resources, especially water, increased the risk of draught and flooding. These types of natural disasters forced people to flee from their homes, making them environmental refugees.

2.3.5. Ozone layer depletion and photochemical smog. The atmosphere has different layers above the sea level. The first layer, just above the sea level, is the troposphere, and the next one is the stratosphere. The troposphere contains ozone, which is responsible for generating photochemical smog in the presence of sunlight, so ozone is called bad ozone here. However, tropospheric ozone production is increasing because plants absorb less ozone under the present growing conditions.^62,63 On the other hand, the stratosphere contains an ozone layer, which protects us from UV radiation, called good ozone, but this ozone layer is continuously depleting, creating a hole. This is because ozone is destroyed by free radical catalysts, such as hydroxyl radicals, nitric oxide radicals, and chlorine atoms.

3. Overview and comparison of different reforming technologies

3.1. Overview

The CH₄ reforming processes for the production of synthesis gas are as follows:⁶⁴ dry reforming of methane (DRM), steam reforming of methane (SRM), partial oxidation of methane (POM),³¹ combined reforming of methane (CRM/DRM + SRM),²⁶ auto-thermal reforming of methane (ATR/SRM + POM), and a combination of all the three previous processes called tri-reforming of methane (TRM). The heats of reaction in the standard state for the various processes are mentioned below:

Methane dry reforming is an endothermic process and requires energy input to sustain the reaction (eqn (1)):

CH₄ + CO₂ ↔ 2CO + 2H₂, ΔH° = 247.3 kJ mol⁻¹ (1)

Steam reforming of methane is also an endothermic process and requires energy input to sustain the reaction (eqn (5)):

CH₄ + H₂O ↔ CO + 3H₂, ΔH° = 206.8 kJ mol⁻¹ (5)

Alternatively, syngas can be obtained by exothermic CH₄ partial oxidative reforming, according to the following reaction (eqn (6)):

CH₄ + ½O₂ → CO + 2H₂, ΔH° = −35.6 kJ mol⁻¹ (6)

Combined dry and steam reforming of CH₄ is an endothermic process and requires energy input to sustain the reaction (eqn (7)):

3CH₄ + 2H₂O + CO₂ ↔ 4CO + 8H₂, ΔH° = 660.9 kJ mol⁻¹ (7)

Auto-thermal reforming (ATR) (eqn (8)) is mildly exothermic to maintain the sustainability of the heat losses during the process:⁶⁵

7CH₄ + 3O₂ + H₂O → 7CO + 15H₂ + CO₂, ΔH° = −6.8 kJ mol⁻¹ (8)

Tri-reforming of methane (DRM + SRM + POM)³¹ is an endothermic process that eliminates the individual disadvantages of the above three processes and increases the catalyst life as well as the process efficiency (eqn (9)):

20CH₄ + H₂O + 9O₂ + CO₂ → 21CO + 41H₂, ΔH° = 12.9 kJ mol⁻¹ (9)

Recent laboratory studies with pure gases have shown that the addition of oxygen to CO₂ reforming or the addition of oxygen to steam reforming of CH₄ can have some beneficial effects in terms of improved energy efficiency or synergistic effects in processing and in mitigation of coking.

3.2. Comparison

The steam reforming of methane (SRM) requires a high heat supply and produces products with a H₂/CO ratio of ∼3 : 1, which is relatively higher than that desired for the Fischer–Tropsch synthesis. Carbonaceous deactivation problems are also associated with this method. But if SRM and DRM are combined, this results a significant decrease in carbonaceous deactivation. In addition, POM is partial oxidation of methane, which is an exothermic process, so it would be more economical than SRM, and the process can be utilized without catalyst. The produced syngas with a H₂/CO ratio of 2 : 1 is ideal for downstream F-T processes. However, POM requires a costly air separation plant for the O₂ supply and special equipment for reaction temperatures as high as 1200 K.⁶⁵ Moreover, the ATR reactor, in comparison to that used for SRM, is moderate in cost, size, and weight requirements. The main drawback is that a relatively extensive control system is needed for ATR, to ensure proper robust operation of the fuel-processing system. Tri-reforming of methane (TRM) is a synergistic combination of endothermic CO₂ reforming (eqn (1)), steam reforming (eqn (5)), and exothermic partial oxidation of methane (eqn (6)). Chunshan et al. in 2004 (ref. 31) showed that the combination of dry reforming with steam reforming can achieve two important goals: to produce syngas with desired H₂/CO ratios and to alleviate the carbon formation problem that is significant for dry reforming. Combining steam reforming and partial oxidation with CO₂ reforming could significantly reduce or eliminate carbon formation on reforming the catalyst, thus increasing the catalyst life and process efficiency. Therefore, the proposed TRM can solve two important problems that are encountered in individual processing. But control of all the three feeds (H₂O, CO₂, & O₂) together is a big issue in TRM as an unbalanced feed can result in the opposite effect on the catalysts.

In comparison to SRM, POM, ATR, and TRM, DRM has unique advantages because of its novelty in the utilization of CO₂, which offsets the increasing GHG emissions.

4. Dry reforming of methane

DRM was first studied by Fisher and Tropsch in 1928 over Ni and Co catalysts. It has been of interest for a long time, but in recent years, the interest in DRM has escalated because of environmental concerns and commercial importance. This process has considerable advantages: (i) it is an attractive route toward CO₂ mitigation, (ii) methane conversion, and (iii) a H₂/CO ratio of approximately unity due to simultaneous occurrence of the reverse water gas reaction (RWGR), as shown by eqn (10).

CO₂ + H₂ → CO + H₂O, ΔH° = −41.2 kJ mol⁻¹, ΔG° = −8545 + 7.48T kJ mol⁻¹ (10)

4.1. Thermodynamics

The need for high external heat and a long conversion time (slow reactions) hinder the industrial application of the DRM process.⁶⁶ Furthermore, carbon deposition is also a major problem in the DRM process. Carbon deposition is significantly affected by the process thermodynamics and is likely to occur by two ways: (i) methane decomposition and (ii) Boudouard reaction. These reactions deactivate the catalyst by blocking catalyst active sites.

A reaction is thermodynamically favourable when the Gibbs free energy is negative (ΔG < 0). Studies of thermodynamic equilibrium⁶⁷ are usually done by the ΔG minimization method to understand and optimize the DRM variable combinations, such as temperature, pressure, and different feed product ratios.

The progress of a reaction is analyzed by observing the change in the extent of reaction (α) with respect to different operating parameters (temperature, pressure, and feed ratio). The equilibrium constant (in terms of partial pressure), Gibbs free energy, reaction temperature, pressure, mole fractions, stoichiometric ratio, and the gas constant are K_p, ΔG°, T, P, y, ν, and R respectively. Then, the thermodynamic relationship is defined as:

K_p = Π(y_iP)ν_i = exp(ΔG°/RT) = F(α) as fraction y_i = (n_i0 + ν_iα)/(n₀ + να), where n_i0 and n₀ are the number of moles of the i^th species and the reaction, respectively. Thus, to observe the effect of the operating parameters on the reaction, the change in α needs to be analyzed.

dα/dY = CF(α)/F′(α), (Y can be T or P)

For temperature: C = ΔH/RT² (if C is positive as for endothermic reactions, then the reaction is favored as temperature increases, and the opposite in the case of exothermic reactions).

For pressure: C = −ν/P (if C is positive, as for a reaction with decreasing moles, then the reaction is favored as pressure increases, and the opposite in the case of increasing moles).

As F(α) depends on the feed moles or feed ratios, the optimum feed ratio can be calculated, which enables the reaction to be spontaneous.⁶⁸

The DRM reaction is favored at high temperatures (endothermic reaction, positive C) and low pressures (increasing moles reaction, ν = 2). The Gibbs free energy value for the DRM reaction is positive at temperatures below 643 °C (eqn (11)) thus the reaction is not spontaneous at lower temperatures.³

CH₄ + CO₂ → 2CO + 2H₂, ΔG = 61 770–67.32T kJ mol⁻¹ (11)

at T = 643 °C, ΔG° = +104.88 kJ mol⁻¹, at T = 647 °C, ΔG° = −164.4 kJ mol⁻¹.

The mechanism accepted for coke formation on the catalyst's surface during DRM⁶⁷ involves two side reactions (methane decomposition: eqn (12); Boudouard reaction: eqn (13)).

CH₄ ↔ C + 2H₂, ΔH° = 75 kJ mol⁻¹, ΔG = 2190–26.45T kJ mol⁻¹ (12)

2CO ↔ C + CO₂, ΔH° = −171 kJ mol⁻¹, ΔG = −39 810 + 40.87T kJ mol⁻¹ (13)

The equilibrium constant for the above reactions is given as . Ginsburg et al.⁶⁶ postulated that if K₁₂ > α and K₁₃ > β, then coke formation would be thermodynamically favored, as this would establish that the reactions (eqn (12) and (13)) had not yet reached thermodynamic equilibrium from the side that promotes coke formation,⁶⁶ where α = (p_H2²/p_CH4)/P₀, β = (p_CO2/p_CO²)/P₀, P₀ = 10⁵ Pa, and p = partial pressures.

4.2. Kinetics

Reversible reactions involve two reaction rates, the forward and the reverse, r_f and r_r, respectively, with corresponding activation energies that are determined through the k₀, the reaction rate constant. The Arrhenius form relates the thermodynamics to the kinetics and defines the equilibrium constant K that correlates with the absolute temperature T and with the activation energy E_a, while k₀ is the pre-exponential factor, and R is the universal gas constant (eqn (14)–(16)).

Arrhenius form:

k = k₀ exp(−E_a/RT) (14)

Forward reaction rate constant:

r_f = k_f(C_A,C_B), k_f = k_0f exp(−E_a/RT) (15)

Reverse reaction rate constant:

r_r = k_r(C_R), k_r = k_0r exp(−E_a/RT) (16)

where C_X (X = A, B, or R) is the component concentration and k₀ is the frequency factor, as determined by the Arrhenius law.

The equilibrium constant K will be expressed as the ratio of the above rates: K = k_f/k_r.

The equilibrium constant K determines the extent to which the DRM reaction occurs.⁶⁷ The reactions cannot be shifted to the opposite side by changing the molar ratio of reactants when K is much higher than 1,⁶⁶ but for K in the vicinity of 1, varying the molar ratio of the reactants has considerable influence on the distribution of the products. Whenever, ΔE_a is negative in the DRM, a larger ln(K) indicates that a spontaneous reaction is more feasible.

4.3. Carbonaceous deactivation

Methane decomposition occurs at temperatures above 557 °C and the Boudouard reaction at temperatures below 700 °C. Thus, maximum carbon deposition is reported in the temperature range 557–700 °C, as shown in the equilibrium data for the DRM reaction (Fig. 2).

Fig. 2 Equilibrium data for DRM process.⁴⁴

Each total pressure value (shown on each line in Fig. 3) has a particular temperature, above which no carbon formation is reported with respect to the CH₄/CO₂ feed ratio, as revealed in Fig. 3.⁴⁴ At atmospheric pressure, carbon formation will be below 870 °C. Fig. 2 and 3 suggest that the temperature range for the DRM process should be 643–1027 °C and the pressure close to atmospheric.^4,44

Fig. 3 Thermodynamic analysis of total pressures, temperatures, and feed ratios for carbon formation.⁴⁴

Different types of carbon species are deposited during the DRM process, such as C_α, C_β, C_γ, C_v, and C_c (for nomenclature see Table 1). For example, CO and CH₄ dissociate at the catalyst surface and give C_α, an adsorbed atomic carbon. This reacts to give C_β, polymeric amorphous films that again react to give C_γ, C_v, and C_c.⁶⁹ These points are clearly shown in Fig. 4 and Table 1, giving details about these different forms of carbon with their respective temperature changes.

Table 1 Details of different carbon species formed on the catalyst surface

Carbon structure	Designation	Temperature range (°C)
Adsorbed, atomic carbon (surface carbide)	Cα	200–400
Polymers, amorphous films	Cβ	250–500
Ni carbide (bulk)	Cγ	150–250
Vermicular filaments or whiskers	Cv	300–1000
Graphite (crystalline) platelet films	Cc	500–550

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Fig. 4 Mechanism of carbon formation at the catalyst surface.

It should be emphasized that some forms of carbon decrease the activity of the catalyst and some do not. For example: C_β at temperatures <300–400 °C and C_c at temperatures >650 °C encapsulate the metal surface of the catalyst. At high temperatures, (500–900 °C) the carbidic form of atomic carbon mixed in Ni surface layers to a depth of more than 50–70 nm. If this accumulates at high or low temperatures, adsorbed carbon can deactivate the metal active sites.⁶⁹ The phenomenon of carbon deposition on the catalyst surface is shown in Fig. 5.

Fig. 5 Pore plugging in a supported metal catalyst owing to carbon deposition.⁶⁹

5. Strategies to reduce carbonaceous deactivation of catalysts

5.1. Development of catalysts

5.1.1. Active species of catalysts. Active metal species used in catalysts are divided into two categories: base metals and noble metals. Generally, Ni is used as the active species for DRM catalysts due to its high activity and low cost. Some other metals, such as Co or noble metals, are added to examine bimetallic effects on the catalysts. Noble metals (Ru, Rh, Ir, Pd, and Pt) have high activity and low sensitivity for carbon deposition.²
5.1.1.1. Base metal catalysts. Industrially, the metal of choice for the catalyst is nickel due to its inherent availability and lower cost in comparison to other more noble metals. However, because the reforming reaction is strongly endothermic, equilibrium yields of synthesis gas require high temperatures and, under such reaction conditions, several studies with alumina-supported metals^70,71 have shown that nickel is more susceptible to coking than noble metals; regardless of this, economic and availability constraints typically prohibit the use of noble metal catalysts on an industrial scale, and it is still worthwhile to pursue the development of supported nickel catalysts.

Recently, core–shell catalysts, especially metal@silica, have been much investigated due to their unique physico-chemical properties, which can efficiently inhibit sintering of the active metal and prevent carbon deposition. Supported Ni catalysts with core@shell structures offer lower internal diffusion resistance, since metal nanoparticles forming the shell are not buried in the support framework, such as the silica core. Metal particles in a core@shell structure usually have strong chemical and physical interactions with the support matrix and thus are highly resistant to sintering and deposition. The core@shell structure of catalysts may also allow reduction of the amount of metal used, since the core does not contain metals.⁷² Core@shell catalysts can be synthesized by a number of different techniques, which include: the deposition–precipitation method, ultrasonic treatment of the support and a nickel salt solution, sol–gel/reduction method, and reduction with a previously prepared core or by a sequential reduction process.

Lim et al. also synthesized Ni@ZrO₂ (ref. 73) with this route, and this showed a good catalytic performance and carbon resistance. Most recently, Bian and coworkers,⁷⁴ 2016, synthesized a multicore–shell of Ni–Mg phyllosilicate nanotubes (PSNTS) coated with a layer of mesoporous silica with a thickness of ∼10 nm by a modified Stöber method of hydrolysis of TEOS in an ethanol solution mixed with ammonia and cetrimonium bromide (CTAB). The authors reported that multiple small Ni particles were observed supported along the nanotube, as well as encapsulated by the silica shell on reduction by H₂ at high temperatures. They claimed that the reduced catalyst showed a high and stable conversion for DRM during a 72 h durability run at 750 °C, and much improved carbon resistance compared to the uncoated sample, which decomposed at such a high temperature.

Although the study of metal core–shell catalysts has only started a few years ago, these have exhibited great potential for application in heterogeneous catalysis, owing to their high activity, high stability, high selectivity, and other functions. Despite the great advancements in this field, several problems remain unsolved, which are listed as follows: (1) the efficient fabrication methods of the core–shell structural catalyst are still limited, and each method can only be used to prepare certain materials. (2) The core size and the shell thickness remain difficult to control precisely. (3) The yield of the core–shell catalyst remains low. (4) When a metal core is used as the active part of the catalyst, its catalytic activity is always degraded by the shell-block effect. These facts have restricted the application of the core–shell structure material; meanwhile it offers a lot of scope for the extension of the fabrication of core–shell catalysts.

Table 2 listed details of some published works based on base metal catalysts for the DRM process. Ni/CeO₂, Al₂O₃,⁷⁵ Ca, Ce-promoted Ni–Ce/SBA15,⁷⁶ and 3% Ni/γAl₂O₃ (ref. 77) result in high conversion and low carbon deposition due to the high oxygen-storing capacity of Ce. Ni–Co catalysts^9,10 also show a remarkable performance with a Sr promoter and glass fibre support.

Table 2 Base metal catalyst performances for the DRM process

Reference	Catalyst	Rxn parameter	Specific finding	Remarks
Fatesh (9)	5% Ni–5% Co/Al2O3 promoter 0–1% Sr, IM at 80 °C/5 h	CO2/CH4/N2 = 17/17/2, P = 1 atm, T = 500–700 °C, 0.6 g Cat, F/W = 60 ml min^−1 gcat^−1	Opt T = 700 °C, XCH4 = 84.9%, XCO2 = 82.3%	0.75% Sr gives best results
Talkhoncheh & Haghighi (75)	Ni nano catalyst on clinoptile, CeO2, Al2O3 IM	CH4/CO2 = 1 : 1, T = 550–850 °C, flow rate = 30 cm^3 min^−1, Cat = 0.1 g	Opt T = 850 °C, XCH4 = 93%, XCO2 = 96%, H2/CO = 0.91	Alumina gives good results
Albazari et al. (76)	Ni–Ce/SBA-15 5% Ni–5% Ce, IM first Ce then Ni	CH4/CO2 = 1 : 1, T = 650 °C, P = 1 atm, flow rate = 100 ml min^−1	XCH4 ≅ 100%, XCO2 ≅ 90% H2/CO ≅ 1	Ce/Ni < 0.04 prevents pore blocking
Ruckenstein & Hu (11)	Ni/alkaline earth metal oxides (MgO, SrO, BaO) IM	CH4/CO2 = 1 : 1, T = 790 °C, P = 1 atm, GHSV = 60 000 cm^3 h^−1 gcat^−1	Reduced (H2) XCH4 ≅ 91%, XCO2 ≅ 98%	NiO/MgO is best catalyst
Ismagilov et al. (10)	Ni–Co/glass fiber by solution combustion	CH4/CO2 = 1 : 1, T = 750 °C, flow rate = 60 cm^3 min^−1	XCH4 ≅ 90%, XCO2 ≅ 100%	0.4Co–0.4Ni giving 0.13% carbon
Al-Fatesh et al. (77)	3% Ni/γAl2O3 promoted by Ca, Ce, Imp method	CH4/CO2 = 1 : 1, Temp = 850 °C, press = 1 atm, Cat = 0.75 g, F/W = 2640 ml h^−1 g^−1	XCH4 ≅ 94.1%, XCO2 ≅ 98.3%	For 0.15% Ce, 0.05% Ca coke formation <1.5%
Mingjue Yu et al. (78)	NixMg1−xO promoted by Sn, Ce, Co, Mn	CH4/CO2 = 3 : 1, T = 760 °C, P = 1 atm, GHSV = 20 000 ml h^−1 gcat^−1	XCH4 ≅ 73%, XCO2 ≅ 90%	Ni0.10Mg0.90O catalyst gives <3.48% coke with Co

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Bimetallic catalysts. It has been also reported that the combination of nickel and cobalt catalysts reduces carbon deposition in CO methanation, partial oxidation of methane to synthesis gas, and steam or dry reforming of methane.⁹ The suppression of carbon deposition by the addition of cobalt to a nickel catalyst for CO methanation could be due to an enhancement of the hydrogenation of atomic carbon and/or inhibition of the formation of carbide species in the metal crystal.

A suitable Co : Ni ratio strongly affects the performance and stability of the catalyst. Deactivation would occur due to oxidation of the metals, if the cobalt loading is too high, whereas deactivation could be caused by coking, if the nickel loading is too high. Using a TiO₂-supported catalyst under high pressure (2 MPa), Nagaoka et al.²⁰ suggested that 90% of cobalt combined with 10% nickel in 0.05 wt% Co–Ni/TiO₂ is the best combination to provide a highly stable catalyst. A Ni–Co catalyst gives better results compared to a monometallic catalyst (Table 3) due to the synergistic effect of Ni–Co metals.¹⁶

Table 3 Comparison of conversions given by monometallic and bimetallic catalysts

Al2O3-supported catalyst	500 °C		600 °C		700 °C
	CH4%	CO2%	CH4%	CO2%	CH4%	CO2%
10 wt% Ni	18.2	26.7	50.7	56.3	81.7	83.8
5 wt% Ni + 5 wt% Co	23.8	27.8	57.2	58.8	86.1	84.5

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Zhang et al.⁷⁹ developed a bimetallic Co–Ni catalyst over Al–Mg–O supports by co-precipitation methods with a high calcination temperature, which resulted in Ni_xMg_1−x Al₂O₄ and Co_xMg_1−x Al₂O₄ spinel-like^80,81 structures and strong metal–support interactions. This method also facilitated a small metal particle size of the reduced precipitate. The combined use of Al and Mg oxides provided the catalyst with not only a relatively high surface area, but also thermal stability. Perovskite-type catalysts^82,83 have also been studied, both alone and incorporated with nickel as a transition metal, which resulted in highly coke-resistant catalysts.

5.1.1.2. Noble metal catalysts. Thermodynamic analysis shows that DRM requires reaction temperatures as high as 900 °C to attain high syngas yields. Even though not thermodynamically favored at high temperatures, the reaction is inevitably accompanied by carbon deposition. At these high temperatures, supported metal catalysts are prone to deactivation due to sintering or irreversible reaction with the support, e.g., forming inactive spinels.⁸⁰

The properties of noble metals responsible for high activity and low carbon deposition are as follows:^63,84

(1) Good dispersion ability in the form of nanoscale particles promote dissociative adsorption of H₂/O₂.

(2) More exposure of d-subshell electrons.

Souza et al.⁸⁵ showed that Pt–Al deactivated quickly during the first 20 h of CH₄–CO₂ reforming at 1073 K, while zirconia-based catalysts (with ZrO₂ content >5%) presented improved stability over the course of 60 h. In another case, a combination of Rh–Ni supported on boron nitride (BN) has shown higher DRM activity, as well as stability with respect to deactivation compared to Rh–Ni on γ-Al₂O₃ (Table 4).

Table 4 Noble metal catalysts for DRM process

Reference	Catalyst	Reaction parameters	Specific findings	Remarks
Aparicio et al. (19)	Ru/Al2O3, SiO2 IM	CH4 : CO2 : He = 10 : 10 : 80, flow rate – 100 ml min^−1, T = 823 K, 1023 K	823 K, Ru/SiO2, XCH4 ≅ 12%, Ru/Al2O3, XCH4 ≅ 14%, 1023 K, Ru/SiO2, XCH4 ≅57%, Ru/Al2O3 XCH4 ≅52%	Less activity loss for Al2O3 than SiO2
Bitter et al. (87)	Pt/ZrO2 Rh/Al2O3, ZrO2 IM	CH4 : CO2 : Ar : N2 = 42 : 42 : 75 : 10, T – 873 K, flow rate – 170 ml min^−1	Pt/ZrO2, Rh/Al2O3, Rh/ZrO2 comparable catalysts w.r.t. activity and stability	Rh/Al2O3 more active than Pt/ZrO2, Rh/ZrO2
Souza et al. (85)	Pt/ZrO2, Al2O3 IM	CH4 : CO2 : He = 1 : 1 : 18, flow rate = 200 cm^3 min^−1, T = 723–1123 K	Activity order Pt/10 Zr > Pt/5Zr > Pt/20Zr > Pt/Al at 1023 K, XCH4 ≅80%, XCO2 ≅ 91%	Pt/ZrO2 has less carbon; more stable than Pt/Al2O3
Perera et al. (88)	5% Ru/Eu2O3, IW	CH4 : CO2 : He = 1 : 1 : 6, T = 973 K	XCH4 ≅ 92%, XCO2 ≅ 90%	No sign of coke formation

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The ratio of promoter to the active metal is also as crucial as the synthesis procedure in the stable operation of the DRM catalyst. Ballarini et al.⁸⁶ studied Na, K, and Mg-promoted catalysts for 0.02% Pt/Al₂O₃, 0.1% Pt/Al₂O₃, and 0.5% Pt/Al₂O₃. They observed that 0.02% Pt/Al₂O₃ promoted with K or Na showed no observable activity, while promotion with Mg showed CH₄/CO₂ conversion greater than for K or Na-promoted 0.02% Pt/Al₂O₃. Similarly, 0.1% Pt promoted with Mg showed higher activity than the same catalyst promoted with K and Na, although all three catalysts showed deactivation.

The Ru and Rh showed the best resistance among the noble metals,⁸⁹ toward catalyst deactivation by carbon deposition. Numerous studies in the last few decades compared the activities of the noble metals in order to determine which noble metal performs best with respect to activity and resistance to carbon deposition. Hou et al. compared the stability and reactivity of Rh (5 wt%), Ru (5 wt%), and Ni (10 wt%) as supported metals. The noble metals showed higher coke resistance ability, while their activity was relatively low in comparison to Ni. In comparison to Ru, the Rh in this study presented higher dispersion on the mesoporous Al₂O₃ and exhibited higher coke resistance and higher reforming activity.

5.1.2. Catalyst particle size and metal loading. A number of published papers are devoted to clarifying the relationships between carbon deposition on Ni catalysts and metal particle size, and loading percentage. Crnivec et al.¹⁶ and Wang et al.⁹⁰ both studied the DRM process performance with respect to different metal loadings and catalyst particles. Crnivec examined this behaviour over Ni–Co/CeO₂ and ZrO₂ catalysts and Wang⁹⁰ over metal-supported catalysts. Four catalysts having metal (Ni–Co) loadings of 3, 6, 12, and 18% were prepared and examined in a CH₄–CO₂-reforming reaction.¹⁶ A considerable amount of accumulated carbon was formed with a Ni–Co loading greater than 12%. As the amount of metal loading increased, significant growth of the bimetallic particle was induced, thus indicating the existence of strong metal/support interactions at low active metal loadings. Similar observations were also reported for CH₄ conversion during the DRM process (Fig. 6).⁹⁰

Fig. 6 Effect of metal loading on CH₄ conversion at different temperatures.⁹⁰

A positive correlation between the average particle size and carbon accumulation is mentioned, in which small particles favour the initial activation of methane, whereas larger clusters enable stable long-term methane decomposition, producing carbon nano-filaments. Catalysts containing active metal particles smaller than 6 nm exhibit superior resistance to carbon accumulation on catalyst surfaces and display a certain selectivity for the RWGS reaction.¹⁶ Catalysts with a large metal particle size simulate the occurrence of the methane dehydrogenation reaction and consequently accumulate a considerable amount of carbon on their surface. Gohier et al.⁹¹ investigated the carbon nanotube growth mechanism with catalyst particle size. They observed that a small crystallite size (<5 nm) stimulates the growth of single-walled and few-walled carbon nanotubes that grow from the particle outwards, whereas larger crystallite sizes (>15 nm) are capable of forming multi-walled nanotubes that grow from beneath the particle.

5.1.3. Catalyst supports. A catalyst is composed of two individual elements: the active metal species and the support, which acts as a mesoporous substrate on which metal is dispersed. Mesoporous nanostructured materials have been studied extensively for the design of heterogeneous catalysts for the application of catalytic reforming, due to their high surface area and low density.⁹² Carbon deposition on the catalyst surface is affected by the support basicity, and it has been observed that carbon formation can be reduced by introducing Lewis basicity to the support.⁹³ The nature of the oxide support greatly affects the catalyst activity due to the varying active surface area and acid base properties. Carbon dioxide reforming involves the adsorption and dissociation of CO₂ on catalysts. Since CO₂ is well-known as an acidic gas, a highly basic support may result in a significant increase in adsorption of CO₂, resulting in high activity towards CO formation and a lower possibility of carbon formation or high stability.⁹⁴

Great efforts are made to maximize the catalyst surface area by better dispersion of active metal over the mesoporous support. The catalyst surface area gives space for the reaction to occur; while the whole area is not used for reaction, but has some specific active sites that are responsible for the reaction. A high surface area increases the probability of these active sites and results in high catalytic activity.

Table 5 describes the catalytic activity on different supports. Specifically, Al₂O₃ and TiO₂, with high basicity, provide the most stable performance in the DRM process. Yttria-stabilized zirconia (YSZ) also promotes high activity, but is experimentally verified for noble metals only. Over the Rh/YSZ catalyst, it is found that there exists a lattice oxygen species of the carrier, which intensively interacts with CO₂. A spillover of these lattice oxygen species onto the Rh surface contributes to the formation of CO and H₂O.⁹⁵ The different turnover rates are also correlated with the stabilized species of Rh atoms. It is thus concluded that the supports can influence the electronic state of Rh atoms, and that Rh metal serves as a prominent active species in the reforming reactions.^96–98

Table 5 Catalyst activity on different supports

Activity order per active metal	Reforming temp, K	Metal loading wt, %	Ref.
Rh
Al2O3 > TiO2 > SiO2	773	0.55	97
Al2O3 > TiO2 > SiO2	773	0.5	98
TiO2 > La2O3 = CeO2 > ZrO2 = MgO = SiO2 = MCM-41 > γ-Al2O3	973	0.5	94
Yttria-stabilized zirconia (YSZ) > Al2O3> TiO2 > SiO2 > La2O3 > MgO	873	0.5	95
Ru
YSZ ≫ SiO2 > TiO2 > ZrO2 > γ-Al2O3	700	0.51–0.64	99
Pt
TiO2 > Al2O3 > SiO2	723	4	100
Mo2C (molybdenum carbide)
Al2O3 > SiO2 > ZrO2 > TiO2	1220	3.8	101
Ni
γ-Al2O3 > TiO2 > ZrO2-CP > ZrO2-AS > SiO2	1073	5	102
La2O3–SiO2 > α-Al2O3–MgO > CeO2 > TiO2	973	5	94

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5.1.4. Catalyst promoters. The catalyst promoters are non-active materials that are recommended to improve the catalytic performance through surface structural effects. Promoters improve the active metal species dispersion over the support,¹⁰³ due to the existence of a strong metal–support interaction effect, which increases the basicity of the catalysts. It also supports the gasification of accumulated carbon. Ag-Promoted catalysts alter the type of coke formed on the catalyst surface from whisker to amorphous species.¹⁰⁴

Zapata et al.¹⁰⁵ studied the promoting effect of Ca, K, and Ce on the activity of Ni/SiO₂ catalysts for the methane decomposition reaction. They reported that Ce greatly increased the conversion of methane and improved the stability, while catalysts promoted with K and Ca were less active and stable than the Ni/SiO₂ catalysts.

Al Fatesh et al.¹⁰⁶ studied the effect of promoters on methane dry reforming over Ni catalysts supported on mixed α-Al₂O₃ + TiO₂-P25 (Table 6). The results for catalyst stability and activity showed that the Zr promoter with Ni provided the best catalyst activity for CH₄ and CO₂ conversions.⁷⁷

Table 6 Comparison of unpromoted and Ce, Zr-promoted catalyst performance

Promoter	Add (wt%)	CH4%	CO2%	Average H2/CO
No promoter	0.0	72.6	75.9	0.98
Ce	0.05	72.3	75.4	0.98
	0.15	71.7	73.3	0.97
	0.3	68.7	73.6	0.97
Zr	0.05	76.5	74.4	1.06
	0.15	77.7	75.9	1.04
	0.3	73.6	73.6	1.02

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5.1.4.1. Metal oxide promoters. ZrO₂ is considered as a promoter when added to the catalyst in small amounts and receives the highest interest among the other metal oxide promoters. Therdthianwong et al.¹⁰⁷ concluded that Ni/Al₂O₃ catalyst deactivation was inhibited by ZrO₂ addition. The ZrO₂ promoter enhanced the dissociation of CO₂-forming oxygen intermediates near the contact between ZrO₂ and Ni, where coke deposits were gasified afterwards. MgO and CeO₂ are also studied for the DRM process and the results improved the catalytic activity.

5.1.5. Catalyst preparation method. Catalyst preparation methods are very diverse and each catalyst may be produced via different routes. The catalytic properties of heterogeneous catalysts are strongly affected by every step of the preparation, together with the quality of the raw materials. The choice of a method for preparing a given catalyst depends on the physical and chemical characteristics desired in the final composition.¹⁰⁸ It is easily understood that the preparation methods are also dependent on the choice of the basic materials. Both unsupported and supported types of catalysts are used for the DRM process.
5.1.5.1. Sol gel method. The sol–gel process is a method used to produce mixed metal oxide catalysts. The process involves conversion of monomers into a colloidal solution or reactive state (sol) and poly-condensation of an activated molecular precursor into nanoclusters, with the inherent advantages of molecular-scale mixing of constituents and homogeneity of the sol–gel products.¹² La–Co–Ni–Sr–O-based perovskite-type¹⁰⁹ and NiAl₂O₄ spinel-type⁸¹ unsupported catalysts prepared by the sol gel resin method are reported for the DRM process. Doping quantities of Sr promote the in situ reduction at lower temperatures, producing Ni⁰ and Co⁰ active species with sizes of the order of nanometers, highly dispersed on the La₂O₂CO₃ and SrO matrix, giving rise to very active catalysts for the dry reforming of methane and inhibiting coke formation, in spite of the severe reaction conditions used.⁸¹

Zhigang Hao et al.¹² investigated DRM through a nanosized aerogel catalyst (Ni/Al₂O₃) in a fluidized bed reactor. Chen et al.¹¹⁰ described a Ni/CeO₂–Al₂O₃ catalyst prepared by two methods, using the same procedure but a different post treatment, called aerogel and xerogel catalysts. Their results showed that aerogel catalysts are more thermally stable than xerogel catalysts. The aerogel catalyst exhibited a high specific area, a low bulk density, a smaller metal particle size, a stronger metal interaction, and higher metal dispersion, compared to xerogel catalysts. All tested aerogel catalysts showed better activity and stability than impregnated catalysts. Catalytic activity depends on metal loading. Aerogel catalysts gave a low rate of carbon deposition and allowed fluidization with excellent physicochemical properties.

Thus, catalytic activity is directly proportional to metal–support interactions and surface area, and inversely proportional to the metallic catalyst particle size and bulk density. Of these, the metallic particle size and metal–support interactions affect the activity the most. The sol–gel method results in strong metal–support interactions and a smaller particle size (nanosize) as compared to other methods. Therefore, it is preferred for reforming catalysts.

5.1.5.2. Precipitation and co-precipitation method. In the precipitation method, an active metal solution is precipitated by using a precipitating agent, such as metal carbonates and hydroxides. This includes two distinct processes, namely nucleation and growth. Bimetallic catalysts for DRM are prepared by the co-precipitation method (simultaneous precipitation of more than one component) for example Ni–Co/CeO₂.¹¹¹ Wang et al.¹¹² demonstrated that precipitated catalysts (Ni/SiO₂, Ni/MgO₂) resulted in less stability and activity than impregnated or sol–gel catalysts, probably due to the lower concentration of active sites and the partial oxidation of the active Ni species by CO₂ and H₂O during the reaction. In addition, the co-precipitation process requires NaOH, and a trace of Na⁺ ions on the catalyst could decorate the nickel surface. It has been found that alkali metal ions in catalysts decreased the catalytic activity in steam reforming of methane and CO₂ reforming of methane.

As compared to sol–gel and the impregnation method, precipitation is not generally preferred for reforming catalysts because the precipitating agent (alkali metal) can result in a significant decrease in catalytic activity.

5.1.5.3. Surfactant assisted and polyol method. Naeem¹¹³ prepared Ni-based nanocatalysts by two methods: polyol (an alcohol containing multiple hydroxyl groups) and CTAB (cetyl trimethyl ammonium bromide) surfactant-assisted. Under similar reaction conditions, polyol catalysts exhibited the highest activity and selectivity, whereas the surfactant-assisted catalysts showed minimum carbon deposition. CO₂-TPD and H₂-TPR revealed that the preparation method had a significant effect on the basicity and reduction behavior of the prepared catalysts. In the case of surfactant-assisted catalysts, their high basicity was their prime characteristic that gave them better coke-resistance. The author has concluded that both preparation methods exhibited good potential for use as catalyst preparation methods for DRM. A review of this method indicated that the surfactant method increases the basicity, which results in a sufficient decrease in carbonaceous deactivation.
5.1.5.4. Impregnation method. Generally, the impregnation (incipient/wet) method is used for catalyst preparation.^76–85 Incipient impregnation, also called pore filling, is a commonly used technique for the synthesis of heterogeneous catalysts. Albarazi et al.⁷⁶ proved that the synthesis route has a marked influence on the physicochemical features of the catalyst by the preparation of Ni/SBA-15 catalysts through impregnation, as well as by co-precipitation. Pore blockage was observed in the catalyst that was prepared by co-precipitation. NiO particles with crystal sizes of 9–11 nm were observed outside the mesoporous structure of the SBA-15 support in the case of the catalysts prepared by impregnation.⁷⁶ The impregnation method has several advantages over the precipitation method: (i) filtering and washing steps are eliminated. (ii) Small metal loadings are easily prepared. (iii) It offers some control over the metal distribution on the support.
5.1.5.5. Atomic layer dispersion method. Matthew et al.¹¹⁴ studied the atomic layer dispersion (ALD) method to prepare a catalyst (Ni–Pt/Al₂O₃), using stainless steel and quartz fluidized bed reactors. The precursor is vaporized to transfer it to the fluidized reactor, so it is heated at an operating pressure less than its vapor pressure, but if this vapor pressure is not sufficient, then bubblers are used to transfer the precursor to the reactor. The reactor is connected to a mass spectrometer unit containing an ionizer to detect the molecular species pattern with time in the ALD process. Thus, a layer of precursor is formed on the Al₂O₃ bed in the reactor.¹¹⁴ Catalysts prepared by ALD showed excellent conversion and coke resistance, around double the rate of conversion compared to the impregnated catalyst. This layer dispersion technique prepared a catalyst with <3 nm sized metal particles, with high dispersion and strong metal interactions.¹¹⁴

5.1.6. Advance preparation methods.
5.1.6.1. Co-precipitation with reflux digestion. In the co-precipitation method, precipitation forms a slurry, which is filtered and washed with deionized water. For reflux digestion, the solid is refluxed in deionized water for a certain time, then mixed and filtered again at elevated temperatures. Chen¹¹⁵ demonstrated that a Ni–CaO–ZrO₂ catalyst prepared by reflux digestion benefits the formation of smaller particles in the mixed composites. Thus, a reflux-digested catalyst gives the highest surface area (245 m² g⁻¹), the highest pore volume (0.3 cm³ g⁻¹), a smaller crystallite size, and an enhanced metal–support interaction.
5.1.6.2. Non-thermal plasma treated catalysts preparation. The non-thermal plasma glow discharge technique is a novel treatment, in which the catalyst is treated in a plasma reactor for a certain time at elevated temperatures with a gas (as Ar, plasma-forming gas). Rahemi et al.¹¹⁶ synthesized Ni/Al₂O₃–ZrO₂ nanocatalysts via impregnation and non-thermal plasma treatment. Their results showed that the plasma treatment produced highly dispersed nanoparticles with a high surface area and a strong interaction between the active phase and the support. The small active metal particles over the plasma-treated nanocatalyst lead to a better catalytic activity and better coke inhibition.

Although preparation of heterogeneous DRM catalysts with different methods has been investigated extensively, new challenges still appear continuously, which demands a continuous focus on the improvement of tailored catalysts for DRM commercialization.⁶³ More broadly, despite the fact that the synthetic methodology is already well defined, obtaining the exact structure, morphology, and function of the produced products is still hard to achieve, which drives further investigation specifically focused on more controllable synthesis.

If the best choice is selected for each characteristic, then the catalyst is deemed to be the best. But sometimes economics and the availability of equipment play an important role in this decision, since if advanced preparation methods or atomic layer dispersion methods are selected, then the catalyst will give good results, but use of these methods will not be economic, so generally sol–gel, impregnation, or precipitation methods are used to prepare catalysts. Thus, each factor is important, but the economic point of view should also be considered. Still, according to literature reviews, 3–5% Ni-based catalysts (or bimetallic with Co) supported on SBA15/TiO₂/Al₂O₃ (preference wise) promoted by Ce, Zr, or Ca transition metals and prepared by sol–gel/impregnation/precipitation (preference wise) methods can be deemed the best catalysts.

5.2. Operating conditions

5.2.1. Effect of temperature with different feed ratios.
5.2.1.1. Carbon production. Nikoo et al.⁶⁷ reported an equilibrium study of the DRM process with all the possible reaction parameters. Table 7 lists all the side reactions in the DRM process. Observation of the behavior of these reactions enables the effects of operating parameters to be predicted.

Table 7 Reactions of DRM process

S.R.	Reaction	Heat of reaction
1	CH4 + CO2 ↔ 2CO + 2H2	247
2	CO2 + H2 ↔ CO + H2O	41
3	2CH4 + CO2 ↔ C2H6 + CO + H2O	106
4	2CH4 + 2CO2 ↔ C2H4 + 2CO + 2H2O	284
5	C2H6 ↔ C2H4 + H2	136
6	CO + 2H2 ↔ CH3OH	−90.6
7	CO2 + 3H2 ↔ CH3OH + H2O	−49.1
8	CH4 ↔ C + 2H2	74.9
9	2CO ↔ C + CO2	−172.4
10	CO2 + 2H2 ↔ C + 2H2O	−90
11	H2 + CO ↔ H2O + C	−131.3
12	CH3OCH3 + CO2 ↔ 3CO + 3H2	258.4
13	3H2O + CH3OCH3 ↔ 2CO2 + 6H2	136
14	CH3OCH3 + H2O ↔ 2CO + 4H2	204.8
15	2CH3OH ↔ CH3OCH3 + H2O	−37
16	CO2 + 4H2 ↔ CH4 + 2H2O	−165
17	CO + 3H2 ↔ CH4 + H2O	−206.2

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ln(K) vs. temperature is plotted for all the reactions shown in Fig. 7. The thermodynamic relation (−RT ln(K) = ΔG) states that as ln(K) increases, ΔG will be more negative, inferring a spontaneous reaction. So, these reactions, with high ln(K) values, have a high probability of occurring over the entire temperature range.⁶⁷

Fig. 7 Equilibrium data for DRM reaction.⁶⁷

Carbon may be formed via methane decomposition (reaction (8)), disproportionation (reaction (9)), hydrogenation of carbon dioxide (reaction (10)), and hydrogenation of carbon monoxide (reaction (11)). All these reactions have low ln(K) values (extremely low after 873 K) and reactions (9)–(11) are exothermic in nature, so carbon production is increased at lower temperatures, as shown in Fig. 3. In addition, carbon formation decreases with increasing CO₂/CH₄ ratio (>1) at a constant temperature, since CH₄ becomes more intensively a limiting reactant and the amount of H₂ available for reactions (10) and (11) is less, which results in a decrease in carbon formation, as shown in Fig. 8. In the case of increasing carbon formation for a CO₂/CH₄ ratio of 0.5 at low temperatures up to 873 K, while CO₂ is a limiting reactant, this is probably due to the endothermic reaction of methane decomposition (reaction (8)), which improves with increasing temperature.^4,44

Fig. 8 Carbon production with temperature and feed ratio in DRM process.⁶⁷

5.2.1.2. CH₄ and CO₂ conversion to syngas. The effect of operating temperature at atmospheric pressure on the equilibrium state is shown in Fig. 9 for methane gas. For all CO₂/CH₄ ratios, CH₄ conversion rapidly increases with increasing temperature, up to 1000 K, beyond which the conversion increases smoothly to attain unity.⁶⁷ Meanwhile, CH₄ conversion increases with the CO₂/CH₄ ratio, implying that CO₂ gas as a soft oxidant has a positive effect on CH₄ conversion in the temperature range of interest.

Fig. 9 Equilibrium CH₄ conversion to syngas with temperature and feed ratio.⁶⁷

Initially, CO₂ conversion gradually decreases with temperature, from 573 K to about 823–873 K (depending on the CO₂/CH₄ ratios). The first decreasing trend can be mainly described by reaction (10), which converts CO₂ and hydrogen to a large quantity of carbon and water, as shown in Fig. 10. CO₂ conversion begins to increase, as endothermic reactions (1) and (2) are favorable at higher temperatures. The equilibrium conversion of CO₂ reaches a maximum between 1273 K and 1473 K for all CO₂/CH₄ ratios, as similarly reported.

Fig. 10 Equilibrium CO₂ conversion with temperature and feed ratio.⁶⁷

5.2.2. Effect of pressure with different feed ratios.
5.2.2.1. CH₄ and CO₂ conversion. CO₂ and CH₄ conversions are always higher at lower pressures than at higher pressures. This suggests that at such a high temperature, greater pressures can suppress the effect of temperature on the increased reactant conversion. These decreasing trends can be expressed by endothermic CO₂ reforming of methane (reaction (1)), which tends to shift to the left (reactant) side, according to Le Chatelier's principle, as shown in Fig. 11.¹¹⁷ It is also noticed that H₂ production decreases with a lower rate, and CO production means that the H₂/CO ratio increases with pressure. This is due to methane decomposition, which is likely to occur at CO₂/CH₄ < 1.⁶⁷

Fig. 11 Effect of pressure on reactant conversion and product distribution.⁶⁷

5.2.2.2. Carbon production. The moles of solid carbon increase with increasing pressure to attain a maximum. Shamsi and Johnson et al.,¹¹⁸ performing thermodynamic calculations on the CO disproportionation and methane decomposition (as the two major reactions of carbon formation), showed that the carbon deposited by methane decomposition decreased as the pressure was increased, while it increased as a result of CO disproportionation. These results suggest that the disproportionation reaction is the most favourable contributor to carbon formation at a higher pressure, particularly when the CO₂/CH₄ ratio is higher than 1 (Fig. 12).

Fig. 12 Carbon production change with pressure.⁶⁷

5.3. Type of reactor

The regular conventional continuous fixed bed reactors are the most widely used reaction systems for DRM. A great challenge with these reactors is poor thermochemical and thermomechanical durability. A large quantity of catalysts may be required in many circumstances to achieve the desired activity. The required operational temperatures are generally high. Reactors that utilize a smaller amount of catalyst and are workable at lower reaction temperatures (but with high activity) would be very desirable. A reactor system with the possibility to couple H₂ production and separation in one step would be a better choice, because this factor would favor hydrogen selectivity and limit coke deposition.

5.3.1. Membrane reactor. Recently, Garcia-Garcia et al.¹¹⁹ showed a shift to modified membrane reactor systems as a key alternative. The authors compared the developed fiber-type membrane reactor with a tubular membrane reactor and a fixed-bed reactor (Fig. 13) under constant conditions. The designed reactor, containing a packed bed of catalyst with lead alumina coating, produced a 72% CH₄ conversion, similar to the tubular membrane system, but requiring lower amounts of Pd with negligible coke production, indicating a significant reduction in the overall dry reforming cost. This value was also much larger than the 34% conversion achieved with the regular fixed bed reactor. Three factors have been attributed to the efficiency of this reactor type: a reduction in the final reactor volume, the feasibility of increasing the number of membranes, and the high surface-area-to-volume ratio. But it is well known that Pd membranes have the drawback of intermetallic diffusion, which could also explain the low H₂ permeation observed in the tubular membrane reactors during the DRM reaction. It also has limited use due to its high cost.

Fig. 13 Different reactors compared for DRM.¹¹⁹

Haag et al.¹²⁰ showed a narrow layering of non-active Ni particles via electroless plating on non-symmetrical Al₂O₃ to allow the development of a membrane reactor with high selectivity to hydrogen separation. Thus, enhanced conversion of CH₄, and the H₂/CO ratio, thereby mitigating carbonaceous deposition, was observed. Activity studies with 33 wt% Ni-supported alumina catalyst at 550 °C showed no coke deposition, with 53% and 49% conversion of CH₄ and CO₂, respectively, and H₂/CO ratios of 1.09. Ni-Type membrane reactors are very effective systems because of a high degree of surface interaction between the deposited Ni films and hydrogen, high-temperature stability, and improved hydrogen separation properties. They have shown better performances than membrane reactors based on Pt group metals, and are much cheaper.

The use of the membrane reactor led to a significant reduction in carbon deposition, due mainly to the limitation of the RWGS and consequently the inhibition of the Boudouard reaction, but not total elimination, probably because methane decomposition is also favored by hydrogen removal.¹²⁰ Still, Ni-based membrane reactors are best economically and performance-wise.

5.3.2. Aerosol solar thermal flow reactor. Dahl et al.¹²¹ studied the DRM process in a solar thermal flow reactor in the absence of added catalyst. This study removed the problem from the base, because its not using catalyst so there will not be any problem of carbon deposition or deactivation of catalyst surface. Concentrated sunlight is used to provide the required temperature for the dry reforming reaction. Because sunlight is a renewable energy source, the cost and environmental consequences of achieving extremely high temperatures are avoided. The aerosol solar thermal reactor attains a temperature of 2000 K within a residence time of around 10 ms. Some carbon will be produced with the dry reforming reaction but it is not a problem here. In fact, the presence of carbon in the system is desirable because the fine particles enhance the radiative heat transfer during the process.

The reactor consisted of three concentric tubes (see Fig. 14). The middle tube is a porous graphite tube with two solid graphite caps on each end. The centre tube is of solid graphite and the outer is of quartz. Feed gases are introduced in the innermost tube and the reaction is completed there. The solid graphite tube is heated by concentrated sunlight, which radiates to the porous tube. Ar gas is provided to protect the innermost tube from carbon deposition. The products as well as the reactor gases exited from the bottom of the reactor. But this reactor resulted in CH₄ and CO₂ conversions of 70% and 65%, respectively, at 2100 K. It is also very difficult practically to achieve such a high temperature using concentrated sunlight.

Fig. 14 Solar thermal reactor method flowsheet.¹²¹

5.4. Strategies used in industries

5.4.1. Sparg process. The Sparg process⁶ has successfully demonstrated on a commercial scale that 1 : 1 H₂/CO synthesis gas can be produced by combined steam-CO₂/CH₄ reforming with carbon-free operation under conditions, which, without sulfur addition, would result in carbon formation. Sulfur-passivated reforming involves the continuous addition of small amounts of a sulfur compound to the reformer feed gas to provide chemisorbed sulfur on the nickel surface at amounts below the saturation level (around 40–50% saturation). This has the effect of blocking the formation and growth of carbon on the surface of the catalyst, whilst still maintaining sufficient active Ni sites to allow the reforming reactions to proceed, albeit at a reduced rate.

5.4.2. Calcor process. An economic process generating CO through CO₂ reforming has been named as the Calcor process.⁵ CO is an important feed for the production of many products or intermediates, such as organic acids, phosgene, polycarbonates, and agricultural chemicals. However, due to its toxicity, the transportation of pressurized CO is limited, so on-site production is needed. High purity CO is required, if it is used for the production of phosgene (COCl₂) as an intermediate for polycarbonate production. CH₄ and H₂ will form CCl₄ and HCl as impurities of phosgene, which would result in impaired quality of the polycarbonate or in an additional phosgene purification step. So, the preferred method of CO₂ reforming should produce a small quantity of byproducts. Steam reforming gives a high H₂/CO ratio, and H₂ is treated as a byproduct here, so the dry reforming method is used with a membrane to separate CO from H₂.

6. Regeneration and reclamation of deactivated catalyst

6.1. Regeneration

Regeneration or disposal of deactivated catalysts depends on economic and environmental issues. Catalysts can be deactivated by fouling, thermal degradation, poisoning, and sintering. Fouling means deposition of material on a catalyst surface to block active sites, such as coke deposition. Poisoning includes strong chemical interaction of feed or product components with the active sites of a catalyst.^122,123 A deactivated catalyst can be regenerated to use again in the same process or in other processes. A deactivated catalyst (carbon deposited) can also be used to make electrodes. Carbonization deposition can be reversed by gasification. Coke deposited on the surface of a catalyst is reacted with air and forms CO₂ with high energy production due to its exothermic nature. The structures of deposits and catalysts have a major effect on gasification. Gasification also depends on the temperature and amount of oxygen used. It is not always necessary to treat a deactivated catalyst; sometimes, high-molecular-weight coke is deposited, which can be removed by washing or by using particular solvents. Regeneration is always a preferred option for reusing the catalyst, but it is not always possible to use an economic method of regeneration. So catalysts are disposed of or used for landfilling. Hashemnejad et al.¹²⁴ reported a Ni-based catalyst regeneration technique to remove carbon, as well as sulfur poisoning. The regeneration is performed to redisperse agglomerated Ni particles and remove both sulfur and carbon from the Ni catalyst, using CO₂ and steam at 700 °C for 2 h.¹²⁴

6.2. Reclamation

Spent catalysts contribute a significant amount of the solid wastes generated in the chemical and allied industries. The dumping of catalysts in landfills is unacceptable, as the metals present in the catalysts can be leached into the groundwater, resulting in an environmental catastrophe. In addition to the formation of leachate, the spent catalysts, when in contact with water, can also liberate toxic gases.¹²⁵ Thus, disposal of spent catalysts is a problem as they fall under the category of hazardous industrial waste and this requires compliance with stringent environmental regulations. As a result of the stringent environmental regulations on spent catalyst handling and disposal, research on the development of processes for the recycling and reuse of waste spent catalysts has received considerable attention.

Metal recovery from catalysts is also extremely important from an economic point of view, as these metals command a significant price in the market. Oza et al. described three techniques for reclamation of Ni/Al₂O₃ catalysts:¹²⁶ (i) acid leaching, (ii) ultrasonication and (iii) chelation.

(i) Acid leaching: generally, three acids are used for this process: H₂SO₄, HCl, and HNO₃, in which Ni is recovered in the form of NiSO₄, NiCl₂, and NiNO₃, with 99%, 73%, and 95% yields, respectively. Spent catalysts contain Ni in the form of an NiO phase attached to a support (Al₂O₃). When the spent catalyst is reacted with acid (as HCl), salts of Ni and Al are formed (eqn (17))

NiO/Al₂O₃ + 8HCl → NiCl₂ + 2AlCl₃ + 4H₂O (17)

(ii) Ultrasonication: this generates alternating low-pressure and high-pressure waves in liquids, leading to the formation and violent collapse of small vacuum bubbles, known as cavitation. Each collapsing bubble can be considered as a micro-reactor, in which a temperature higher than a thousand degrees and a pressure higher than a thousand atmospheres is created instantaneously. These waves increase the rate of reaction drastically. Ultrasonication recovers Ni in 50 minutes, which is considerably less than the 5–6 hours required for acid leaching.

Acid, being highly corrosive in nature, needs expensive materials for construction, and the leaching of the support metal (e.g., aluminum) is yet another problem that requires further selective precipitation of the recovered metal. Thus, acid leaching is a hazardous process. Leaching of metals through complexation with the chelating agent, EDTA, is pursued as an alternative to acid leaching.¹²⁵

(iii) Chelation: this method uses a chelating agent e.g. ethylenediaminetetraacetic acid (EDTA), which reacts with Ni/Al₂O₃ and form a complex, as given in eqn (18).¹²⁶ This complex is reduced to NiCl₂ by reacting with HCl (eqn (19)).

Ni/Al₂O₃ + EDTA → [Ni-EDTA]²⁺ + Al₂O₃ (18)

[Ni-EDTA]²⁺ + 2HCl → NiCl₂ + EDTA (19)

After treatment, Ni is recovered in salt form by a direct crystallization method or as Ni in elemental form by an electrowinning process. A general flowsheet for the reclamation method is given in Fig. 15.¹²⁶

Fig. 15 Flow-sheet for reclamation methods.¹²⁶

Yang et al.¹²⁷ described a two-stage sulfuric acid leaching procedure, an effective acid separation and nickel enrichment technique, and an electrowinning operation at high current efficiency. An electrowinning procedure is applied to the processed solutions (leaching solution) to plate out nickel metal at about 98% current efficiency. By integrating the two-stage leaching, acid separation, and electrowinning, and by implementing the in-process recycling, the closed-loop concept can be realized to achieve near-zero discharge of liquid wastes. Fig. 16 illustrates the process concept for closed-loop nickel recovery.

Fig. 16 Closed loop for nickel recovery.¹²⁷

7. Conclusion

It can surely be concluded that dry reforming of methane (DRM) is an appropriate way to convert greenhouse gases (CH₄ and CO₂) to value-added synthesis gas, and is capable of being used in commercial applications and for resolving environmental issues. Although this concept has many environmental and economic incentives, unfortunately, there are no commercial processes for dry reforming of methane.

The endothermic nature of this process supports the carbonaceous deposition, which deactivates the catalyst and limits its usage. Nickel-based catalysts are commonly accepted for the DRM process due to their low cost, but are prone to deactivation at high temperatures, so bimetallic catalysts were introduced. The synergistic effect of two metals enhances catalyst activity. Ni–Co bimetallic catalysts are widely investigated for the DRM process and are associated with much less carbon deposition. Several supports, such as Al₂O₃, TiO₂, MgO, SiO₂, YSZ, and SBA15, are investigated under the DRM process, and it was concluded that Al₂O₃, TiO₂, and SBA15 give a good performance. Many promoters, such as Sn, Sr, Ca, Ce, K, and Zr, are used to reduce carbon accumulation, and Ce and Zr combined with a transition metal have attracted attention due to their oxygen storage capacity. This gives lattice oxygen in the Ce oxide phase under reducing conditions and generates anionic vacancies, which enhances the catalyst activity. Metal oxides, such as MgO, CeO₂, and ZrO₂, also enhance the catalytic activity in the DRM process. Perovskites (ABO₃) and spinel (AB₂O₄) are two structures formed in catalysts that have proven beneficial in the DRM process. Spinel-type catalysts have high sintering resistance and a weakly acidic nature, which increases CO₂ chemisorption, resulting in higher activity. Perovskite catalysts have high thermal stability and increase the metal dispersion on the catalyst surfaces, as well as providing small metal crystallite sizes that result in low coke production. It is concluded that catalysts with small metal crystallite sizes (≅5 nm) and optimum metal loading (3–12%) result in high activity for the DRM process.

Analysis of results obtained from various catalyst preparation methods indicated that the sol gel method was preferable to the impregnation method, and impregnation was preferable to precipitation. Atomic layer dispersion resulted in much less carbon accumulation but was slightly complicated due to the need for mass spectroscopy. Some advanced methods, such as co-precipitation with reflux digestion and non-thermal plasma treatment, were reported recently and showed good performances.

The CO₂/CH₄ reactant ratio, reaction pressure, and reaction temperature had a considerable influence on the equilibrium of the reactant conversions and solid carbon formation. Analysis of the results of all these aspects demonstrated that the optimum conditions for the DRM process are as follows: temperature range 643–1023 °C, pressure 1 atm, and feed ratio of CO₂/CH₄ 1 : 1.

The use of membrane reactors is widely accepted for the DRM process. This eliminates the difficulties faced with fixed bed reactors, such as high catalyst requirement, poor hydrogen separation, low thermochemical and thermomechanical durability, and higher conversion of reactants. Ni-Based membrane reactors are the most economic and advantageous reactors for the DRM process.

Disposal of spent catalysts is unacceptable and requires compliance with stringent environmental regulations, so the regeneration and reclamation of spent catalysts is very important. Regeneration is carried out by a gasification technique. Reclamation can be done by three techniques: acid leaching, ultrasonication, and chelation, of which ultrasonication is the fastest (50 minutes) and highest recoverability technique.

All the above highlighted conclusions of this literature review give an appropriate direction to the ongoing research in this area (DRM process) and try to facilitate its commercialization. So, future research should start from these results and proceed to achieve the goal of negligible carbonaceous deactivation, in order to achieve the industrialization of this process.

List of abbreviation

AR	Assessment reports
ATR	Auto-thermal reforming
CRM	Combined reforming of methane
CNG	Compressed natural gas
DRM	Dry reforming of methane
EPA	Environmental protection agency
GWP	Global warming potential
GHG	Greenhouse gases
IR	Infrared
IPCC	Intergovernmental panel on climate change
POM	Partial oxidation of methane
SRM	Steam reforming of methane
TRM	Tri-reforming of methane

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