Steam reforming of methane: state of the art and novel technologies

Abstract

Steam methane reforming (SMR) is one of the most promising technologies for sustainable hydrogen production. Although numerous reviews have been published on the catalysts developed for SMR, this field is advancing rapidly, prompting this review to focus on recent developments in new promoters, supports, and structural improvements for SMR catalysts. These advancements aim to reduce carbon deposition and sintering while enhancing catalyst activity and stability. This review provides a comprehensive analysis of recent research focusing on improving SMR processes using various catalyst types and operating conditions, as well as examining emerging SMR technologies. Additionally, key challenges related to SMR catalysts are discussed, along with recent progress in conventional SMR using Ni-based, transition metal, and noble metal catalysts, and advancements in SMR processes. The recent literature has highlighted the utility of supported bi-metallic catalysts and advanced SMR technologies, including chemical looping,sorption enhancement, and membrane reactors, in commercialising SMR technologies.

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1. Introduction

Increased greenhouse gas emissions, from fossil fuel usage and industrial processes, are recognised as one of the main causes of climate change.¹ Despite the adoption of the Kyoto Agreement in 1997 and the Paris Agreement in 2016, the rate of CO₂ emissions has increased steadily until 2020, when there was a 7% decrease in emissions, due to a shift from fossil fuels to renewable energy and a reduced use of energy due to COVID pandemic restrictions.²

H₂ is an energy carrier that is recognized as a potential alternative to fossil fuels, due to its high energy density³ and carbon free utilization.⁴ Considerable amounts of recent research have focused on developing sustainable H₂ production technologies,⁵ such as reforming, gasification, fermentation, and electrolysis. Today, H₂ is widely produced through steam reforming of natural gas (SRNG) due to its low cost.⁶ However, SRNG releases CO₂ into the atmosphere and is reliant on a non-renewable feedstock, making it an unsustainable process.⁷

Natural gas is predominantly composed of methane⁸ which is converted into H₂via the steam methane reforming (SMR – eqn (1)) and water gas shift (WGS – eqn (2)) reactions.⁹ A more sustainable source of methane is biogas which is produced via anaerobic digestion of biomass.¹⁰ Biomass refers to any non-fossilised organic material (food crops, wood, algae, waste etc.)¹¹ and is considered a sustainable feedstock as it is renewable and carbon neutral.¹²

As shown in Fig. 1, in a conventional NGSR process, natural gas is first desulfurized to avoid catalyst deactivation. The natural gas is then sent to the reforming unit where steam is introduced, and the reforming process takes place.

SMR CH₄ + H₂O ⇄ CO + 3H₂ ΔH = +206.1 kJ mol⁻¹ (1)

WGS CO + H₂O ⇄ CO₂ + H₂ ΔH = −41.1 kJ mol⁻¹ (2)

CH₄ + 2H₂O ⇄ CO₂ + 4H₂ ΔH = +165 kJ mol⁻¹ (3)

The SMR reaction is highly endothermic requiring a temperature of 800 °C to maximise methane conversion. The stream in the reformer output (syngas) is sent to the WGS reactor, where steam is added to syngas to convert CO into H₂.¹⁴ The H₂ produced is then purified using pressure swing adsorption.¹⁵

Fig. 1 A flow diagram of industrial H₂ production using a conventional natural gas steam reforming process.¹³

Industrial catalysis has been a crucial factor in the success of the petrochemical and fertiliser sectors, and its importance is paramount to the development of commercially viable SMR technologies.¹⁶ Ni catalysts are commonly used in SMR processes due to their high activity and lower cost relative to noble metals.¹⁷ However, a major challenge facing Ni catalysts is deactivation. The most common mechanisms of SMR catalyst deactivation are sintering, metal oxidation, thermal degradation of supports, and coke formation.¹⁸

Coking is the build-up of carbon deposits on the catalyst surface, blocking active sites and/or pores causing activity loss. Ginsburg et al.¹⁹ stated that there are five types of coke formation found on Ni catalysts: atomic carbon (C_α), polymeric films/filaments (C_β), vermicular whiskers/fibres (C_v), Ni carbide (C_γ), and graphitic platelets (C_c). Fig. 2 summarises the mechanism of coke formation. Atomic carbon (C_α) and polymeric films/filaments (C_β) are formed at low temperatures, but are then converted to graphitic platelets/films (C_c) and vermicular whiskers/fibres (C_v) at high temperatures.²⁰ Coke formation on Ni catalysts can be reduced during SMR by supporting the catalyst with metal oxides (Al₂O₃, CeO₂, MgO) and metal promoters such as noble metals (Ru, Rh, Pt).²¹

Fig. 2 Mechanism of coke deposition.¹⁸

Sintering occurs when there is growth of metal crystallites in the catalyst causing loss of active surface area of the support.²¹ There are several mechanisms of sintering including crystallite migration, atom migration, spreading/splitting, and transport of vapour. Fig. 3 depicts atom and crystallite migration mechanisms. During crystallite migration, the crystallite moves over the surface and grows to form large crystallites that combine with other crystals. The same process occurs during atomic migration but in this case, atoms migrate from crystallites and combine with larger crystals.²² Temperature, atmosphere, support type, promoters, metal and pore size all affect the sintering rate.²⁰ Sintering can be mitigated using high melting point catalyst supports^23–25 or the use of other metals as promoters.^26,27

Fig. 3 Sintering mechanism of crystallite migration (A) and atom migration (B).²⁸

Several advanced technologies have also been developed to improve the commercial viability of SMR such as sorbent-enhanced SMR, oxidative SMR, chemical looping process, photocatalytic SMR, thermo-photo hybrid SMR, solid oxide fuel cells (SOFCs), plasma SMR, and electro-catalytic SMR and membrane reactors for SMR. The aim of this review is to provide the reader with an understanding of the concepts underpinning SMR technologies, the challenges associated with the development of technologies, and an overview of how recent literature has addressed these challenges to commercialise H₂ production from SMR.

2. Ni based catalysts

Ni-based catalysts are mostly used in the SMR process because of their low cost, high activity, and H₂ selectivity, although they are prone to deactivation via coking and sintering. To prevent this, noble metal promoters (such as Ru, Rh, and Pt), non-noble metals (such as Co, Cu, and Mo), perovskite oxides, metal oxides, and supports (Al₂O₃, CeO₂, SiO₂, ZrO₂, MgO, and MgAl₂O₄) are added.^7,29

2.1. Promoters

There are different types of promoters for Ni-based catalysts in SMR such as those based on metal–metal interactions, redox metal oxide promoters, and non-metal promotors.⁵

The introduction of a second metal to a Ni-based catalyst as a promoter can improve its activity and selectivity. For example, adding Fe to Ni forms a Ni–Fe alloy, which achieves a high methane conversion (up to 97.5% at 900 °C).⁷ The most common promoters for Ni-based catalysts are noble metals (e.g. Ru, Rh, Pt, Ir, Ag, and Au) and rare earth metals (e.g. Ce and Nb).

Noble metals such as Rh, Ir, Pt and Ru have also been investigated as promoters in Ni/Al₂O₃. Morales-Cano et al.²⁶ studied and compared the results of using Rh, Ir and Ru as promoters in Ni/Al₂O₃ and concluded that the formation of Ni–Rh and Ni–Ir alloys prevented sintering as a result of the mobility of the Ni species into the metals, although at a temperature of 800 °C and pressure of 22 bar, the catalyst was subjected to deactivation. The Ni–Ru alloy exhibited poor sintering resistance due to its low mobility. The effect of Pt in a Ni/MgAl₂O₃ catalyst was investigated across a range of 0.01 to 1.0 wt%. At 1.0 wt%, the Ni–Pt species were fully dispersed on the catalyst surface, leading to the best catalytic activity and stability. However, as the Pt content decreased so did the catalyst's activity.²⁴

Ag and Au have both been tested as promotors for Ni-based catalysts to reduce coke formation.^30,31 Ag and Au were proposed as promoters for Ni catalysts in a temperature range of 450 °C to 700 °C and a C/S ratio of 1 : 4. As shown in Fig. 4, adding Au at temperatures lower than 600 °C enhanced the catalytic activity and stability of Ni/Al₂O₃. In contrast, adding Ag decreased the catalytic properties of Ni/Al₂O₃.³²

Fig. 4 Methane conversion over Au and Ag as promoters for Ni catalysts vs. temperature.³⁸

Ceria (Ce) has also been used as a promoter, improving catalytic performance and reducing coke formation, thus increasing catalyst activity and stability.³³ Ni–Ce/Mg–Al showed high activity at low temperatures (175 °C) achieving a methane conversion of 54%.³⁴ Furthermore, a Ce content of 1.02 wt% in a Ni–Ce/Al catalyst exhibited a methane conversion of 75% and high coking resistance using a S/C ratio of 3.^34,35

Niobium (Nb) is an excellent promoter that helps increase the surface area of the catalyst for methane and carbon adsorption. A Ni–Co/Al₂O₃–MgO/Nb–Zr catalyst exhibited a high methane conversion of 86.96% and CO₂ selectivity of 87.84%.³⁶ The addition of Nb in the Ni/Ce–Al₂O₃ catalyst reduced carbon deposition on the catalyst and sintering by controlling the Ni particle size dispersion and acidity of the catalyst.³⁷

Different oxides such as MgO, CaO, and La₂O₃ have been investigated as promoters to enhance Ni-based catalysts' performance and coking resistance. MgO, CaO, and La₂O₃ were applied as promoters for Ni–Ce_0.8Zr_0.2O₂ catalysts. Among these, La₂O₃ exhibited a higher catalytic activity and stability, achieving a methane conversion of 47.4%, at 600 °C and a H₂/CO ratio of 6.8.³⁹ The addition of MgO as a promoter to Ni/SBA-15 improved catalyst performance. Over 620 hours at 850 °C, methane conversion decreased from 98% to 85%, while CO₂ selectivity dropped from 92% to around 77%. MgO played a key role in enhancing the dispersion of Ni species and promoting CO₂ adsorption, which helped suppress carbon deposition.⁴⁰ When Ni catalysts were promoted by CuO, ZnO, Ga₂O₃, or Gd₂O₃, the latter proved to be the best promoter exhibiting a H₂ yield of 87% and methane conversion of 83% with a H₂/CO ratio of ∼1.0.⁴⁰ The MgO promoter for the Ni–Ce_0.8Zr_0.2O₂ catalyst exhibited excellent catalyst performance, resulting in a methane conversion of 95% at a temperature of 800 °C for 200 h as a result of its high Ni dispersion, crystallite size, and catalyst surface area.⁴¹

Boron is the only known non-metal promoter currently investigated for Ni-based catalysts.⁵ Boron as a promoter is known to make Ni-based catalysts coke resistant by covering the subsurface sites and preventing the diffusion of carbon into the bulk.⁴² Boron has been shown to improve catalyst activity and reduced coke formation by 80%. The inclusion of boron improved methane conversion from 56% to 61% in a system utilising a Ni catalyst at 800 °C and 1 atm.⁴³

2.2. Ni-based solid solution catalysts

MgO improves the H₂ concentration in the product syngas by inhibiting the desorption of CO from the catalysts surface. NiO and MgO form a solid solution which enhances the dispersion of Ni particles, improving the catalyst's activity and stability.⁴⁴ 10 wt% Ni supported on NiO–MgO (solid solution) exhibited a high catalyst activity and no carbon deposition at 700 °C.⁴⁵ The solid-solution catalyst for CO₂ reforming of methane achieved a methane conversion of 91% at an S/C ratio of 1, atmospheric pressure, and 790 °C.⁴⁴ While the NiO–MgO solid solution is not ideal for SMR, its performance can be improved by stabilizing the MgO and optimizing factors such as Ni loading and the S/C ratio. Additionally, thermally treating the NiO–MgO solid solution can enhance its effectiveness.⁴⁵

2.3. Physical–chemical characteristics of supports

2.3.1. Ni particle size. It is known that particle size reduction significantly affects the activity of SMR catalysts and hinders coke formation, due to the increased surface-to-volume ratio.^46–48 Pashchenko et al.⁴⁹ demonstrated this using a validated CFD model that calculated the effectiveness factor of Ni/Al₂O₃ catalysts at particle diameters between 0.5 and 4 mm. The effectiveness factor accounts for intraparticle diffusion of reactants into the catalyst particle. The higher the effectiveness factor, the less the catalyst's activity was being reduced by diffusion limitations. The reactions were modelled at 630 and 930 °C and a S/C ratio of 3. At both temperatures, the effectiveness factor increased exponentially as particle diameter reduced. Ni particle size can be controlled by the calcination and reduction temperature and the extent of Ni loading in the catalyst.^50,51 Bej et al.⁵¹ observed this using an alumina supported NiO–SiO₂ catalyst, where the crystallite size of NiO was in the range of 9–15 nm, Ni loading ranged between 5 and 15% and the calcination temperature was in the range of 350–500 °C. The optimum conditions of the steam reforming reaction were a temperature of 700 °C with a S/C ratio of 3.5 resulting in a methane conversion of 95.7%. The calcination reduced the size of the Ni nanoparticles and increased the Ni dispersion. 700 °C was observed to be the optimal temperature for SMR due to the formation of Ni aluminate.⁵²

2.3.2. Support morphology. The surface morphology of a catalyst greatly effects its SMR activity. Egg shell catalysts are characterised by the active components' dispersion in the outer region (shell) of the catalyst, improving the percentage of active sites used to catalyse the process.^53–55 Cho et al.⁵⁶ compared the effectiveness of Ni on tube-shaped MgAl₂O₄ pellets using a homo-type and egg shell distribution (Fig. 5). The results demonstrated that the egg-shell-type (3.5 wt% Ni) catalyst had a higher activity and coke resistance than the homo-type (5 wt% Ni) at a S/C of 0.5.

Fig. 5 Schematic diagram of egg-shell-type and homo-type catalysts with the coke formation mechanism.⁵⁶

Cho et al.⁵⁴ also tested egg-shell-type and homo-type catalysts, prepared with Ni and/or ruthenium placed on the outer (shell) and inner regions of alumina pellets. The egg-shell-type 1 wt% Ru/Al₂O₃ catalyst demonstrated higher methane conversion compared to the homo-type 1 wt% Ru/Al₂O₃ catalyst. Additionally, the egg-shell-type bimetallic catalyst (5 wt% Ni/0.7 wt% Ru) exhibited an even greater methane conversion than the egg-shell-type Ru catalyst alone. This was likely due to the enhanced accessibility of active sites on the outer surface of the catalyst and the synergistic effect between Ni and Ru.

Among the core/shell structures (homo-type catalysts), the first two exhibited the best performance at 750 °C and a H₂O/CH₄ ratio of 2, achieving a methane conversion of 97%, due to enhanced stability and active site accessibility provided by the core–shell structure. The presence of Mg contributed to improved resistance to coke formation. Additionally, the yolk–shell structure of Ni@HSS (hollow silica spheres) showed outstanding performance for 55 hours, with no detectable carbon deposition, due to the unique architecture of hollow spheres, which enables better diffusion of reactants, more effective exposure of active sites, and enhanced resistance to carbon deposition.⁵⁷

The robust bimetallic Ni–La@KCC-1 catalyst exhibited a high methane and CO₂ conversion of 92% and 88%, respectively, for over 72 h at 750 °C and a high H₂/CO ratio with a small amount of carbon deposition being observed.⁵⁸

The pore structure of a catalyst plays a crucial role in its performance, as demonstrated by the preparation of monomodal Ni–SiO₂–NH₃·H₂O, bimodal Ni–SiO₂–urea, and non-porous Ni–SiO₂–NaOH catalysts for methane reforming. The bimodal Ni catalyst, which had the largest surface area, exhibited higher catalytic activity and stability, while the non-porous Ni–SiO₂–NaOH catalyst showed lower activity due to its reduced surface area.⁵⁹ Other morphologies of Ni-based catalysts are shown in Fig. 6.

Fig. 6 (a) Nonporous,⁶⁸ (b) hollow,⁶⁹ (c) ordered mesoporous,⁷⁰ (d) multi-shell,⁷¹ (e) yolk–shell,⁵⁷ (f) hollow with hierarchical pores,⁶⁰ (g) core–shell⁷² and (h) KCC-1.⁷³

2.4. Novel metal oxide substrates

Various researchers have investigated different metal oxide substrates such as spinel ABO₄ and A₂BO₄ (Ni/La₂O₃), perovskites ABO₃ (Ni–Al₂O₃), hexaaluminates AB_yAl_12−yO_19−δ (hexaaluminates LaNi_yAl_12−yO_19−δ) and solid solutions (NiO–MgO) as supports to get excellent catalyst performance.⁶⁰

Ni catalysts are typically supported on metal oxides such as ZrO₂, CeO₂, K₂O, and MgO, which are used as promoters. Chawl et al.⁶¹ investigated the production of syngas via CO₂ reforming of methane over Ni-based perovskite catalysts. They found that when 10 wt% Ni was supported on Al₂O₃ with promoters such as ZrO₂, CeO₂, K₂O, and MgO, the catalysts exhibited higher activity, stability, and long-term performance compared to un-promoted catalysts. They concluded that ZrO₂ improved metal particle dispersion on the catalyst surface.

Perovskite-type oxides such as CaTiO₃, SrTiO₃, BaTiO₃, and LaAlO₃ are commonly used as promoters for Ni-based catalysts in SMR. While perovskite oxides enhance the performance of Ni catalysts compared to unsupported Ni/Al₂O₃, Ni/LaAlO₃ stands out as the most active catalyst, effectively suppressing carbon formation.^62,63

Other complex crystal oxide supports, such as hexaaluminates (LaNi_xAl_12−xO_19−δ), have also shown excellent catalytic performance and low coke formation at high temperatures. Among them, hexaaluminates with LaNi_xAl_12−xO_19−δ (where x = 1.0 and 0.9) exhibited methane and CO₂ conversions of 97.4% and 98%, respectively.⁶⁴

In the case of Ni/Ce_0.95M_0.05O_2−δ (M = Zr, Pr, La), Ni supported on Zr-doped ceria showed the highest surface area. When tested at three temperatures (600 °C, 750 °C, and 900 °C), increasing the vapour/CH₄ ratio at 600 °C resulted in high H₂ yield and limited coke formation. At 750 °C, the catalyst achieved the highest methane conversion.⁶⁵

2.5. Self-supported Ni catalysts

Self-supported Ni catalysts with structured carrier shapes exhibited excellent SMR catalyst performance. An example of this is a honeycomb-type Ni catalyst, which has been found to achieve 95% methane conversion at an S/C ratio of 1.362. This was linked to steam treatment and H₂ reduction at 1173 K, forming more Ni fine particles at the surface.⁶⁶ However, a Ni honeycomb catalyst with a density of 2300 cells per square inch (cpsi) exhibited a much higher catalyst activity compared to that of a Ni honeycomb catalyst with a cell density of 700 cpsi at an S/C ratio of 1.36 and in a temperature range of 873–1173 K, which shows the direct relation between catalyst activity and increased geometric surface area.⁶⁶

A Ni coil catalyst achieved a high geometric surface area of 88.1 cm² cm⁻³ at an S/C ratio of 0.62–2.48 and a temperature of 1073 K. This resulted in a methane conversion of 94.0%, CO selectivity of 91.1%, and CO₂ selectivity of 8.9%.⁶⁷ Additionally, the introduction of metallic foam improved methane conversion by 29.5% and the rate of H₂ production by 47.6%.⁶⁷

3. Other non-noble metal catalysts

3.1. Cobalt-based catalysts

The microkinetic analysis of Co based catalysts showed that the dominant reaction path is like that of Ni catalysts.

There are two dominant reaction pathways on the Co catalyst for the formation of CO, namely C + O and CH + O paths as shown in Fig. 7. In the C + O path, methane is dehydrogenated to C* which then pairs with O* to form CO₂, while in the CH + O path, CH* pairs with O* to form CHO*.⁷

Fig. 7 Dominant reaction pathways on a cobalt catalyst.⁷

Coke formation comes after the surface reaction pathway mechanism, where the C* species dissolves into the metal bulk forming carbides and then migrates to the larger particles to form a carbon deposit.⁷

Co-based catalysts show satisfactory performance at low cost. Coke formation and metal oxidation under operating conditions lead to Co deactivation, but this can be prevented using different promoters (noble metals), supporters (earth metal oxides like MgO, SrO, CaO, or BaO and other metal oxides like TiO₂ or SiO₂) or adding another metal to form a bimetallic catalyst.⁷⁴ The use of promoters like noble metals improves the performance of the Co catalyst. For example, low concentration (0.03–0.43 wt%) of Pt-promoted Co/Al₂O₃ catalysts results in high catalyst activity and selectivity in comparison to Co/Al₂O₃ without a promoter and increasing the Pt content to 0.43 wt% also slightly improves the catalytic activity.^74,75 Co can be supported on earth metal oxides like MgO, SrO, CaO, or BaO, and other metal oxides like Al₂O₃ or SiO₂. Amongst these, MgO exhibited the highest catalyst activity and stability with a methane conversion of 94%, CO yield of 93%, H₂ yield of 90% and H₂/CO ratio of 0.97.⁷⁶ Alumina-supported NiCo bimetallic catalysts achieved a methane conversion of 75% and high selectivity⁷⁷ at 700 °C and a S/C ratio of 1. In investigating the performance of Co and bimetallic Co-based SMR catalysts (Co, Co–Ni, Co–Cu, and Co–Al) supported on CeO₂, results show that Co–Ni exhibits the best catalyst performance achieving a methane conversion of 76.1%, 58.5% H₂ selectivity and 44.5% H₂ yield.⁷⁸ This is due to Ni's well-known activity in breaking C–H bonds and synergistic activity when combined as an alloy with Co (Co–Ni), with the latter improving the overall efficiency and resistance to coke formation.

3.2. Cu based catalysts

Cu as an active metal can suppress carbon formation in the syngas production step and can improve water gas shift reaction. The SMR over 5Cu/Co₆Al₂ solid shows a high methane conversion of 96% at a temperature of 650 °C with a steam to methane ratio of 3.1.⁷⁹

The introduction of Cu into a Ni-based catalyst has also been found to improve the water-gas shift (WGS) activity which may be because of the bimetallic Cu–Ni species amount.⁸⁰ This was observed for SMR when 0.01 wt% Cu was added to 0.5 wt% Ni, which increased the WGS reaction rate, although no activity was observed over the 0.5 wt% pure Cu catalyst but the addition of 0.01 wt% Ni caused an increase in methane conversion rate with temperature (methane conversion occurs over Ni and not Cu).⁸¹

Cu metal is known to be less active for SMR in comparison to metals like Ni, Rh, Pt and Pd.⁵ However, Cu-based catalysts are the most used catalysts in methanol steam reforming^82–84 and ethanol steam reforming.^85–87 The Cu/ZnO catalyst has been found to exhibit high catalyst activity for methanol reforming and the water-gas shift reaction.⁸⁸ For example, 5%Cu₅Zn₁₀Al exhibited the best catalyst performance with a methanol conversion of 98% at a temperature of 350 °C under a GHSV of 15 500 h⁻¹ and an H₂O/CH₃OH ratio of 2.⁸⁸

3.3. Molybdenum-based catalysts

Mo-based catalysts are known for their coking resistance in comparison to conventional Co and Fe Fischer–Tropsch catalysts and has been used as a Ni promoter.⁸⁹ A bimetallic Mo–Ni catalyst supported on Al₂O₃ showed a low methane conversion of 30% at a temperature of 800 °C with a high H₂/CO ratio.⁹⁰ In the case of alumina-supported Mo, the conversion is about 45–95% and the methane conversion decreases to 15–40% in the absence of the support.⁹¹ The addition of Mo as a promoter to the Ni catalyst reduced the metal surface area but increased the number of active sites. This is due to electron transfer from the Mo species to Ni, which suggests that the Mo species may block Ni active sites, ultimately decreasing the overall number of available sites.⁹² For the NiMo₂C/Al₂O₃ catalyst at a temperature of 800 °C and a pressure of 1 atm, the catalyst performance was observed and it exhibited a stable methane and CO₂ conversion of 89.9% and 94%, respectively, while maintaining an H₂/CO ratio of 0.99 in a 15 h methane reforming test.⁹³

4. Noble metal catalysts

Several noble metals, including Rh, Ru, Pt, and Pd, have been tested as catalysts for SMR when supported on Al₂O₃–MgO. Their performance follows the order: Ru > Rh > Ir > Pt > Pd, with Ru being the most active and Pt and Pd the least active.⁵ However, due to their high cost, noble metals are not commercially viable for widespread use.^5,21,94

4.1. Ru based catalysts

The reaction mechanism on Ru-based catalysts for SMR is shown in Fig. 8. It was found that methane (through activation of the C–H bond) adsorbs and dissociates, forming CH_x intermediates, with C* and CH* being the most stable surface adsorbates. This process makes the Ru catalyst surface prone to coke formation. Additionally, calculations of the activation barriers show that C–O bond formation is kinetically slower than C–H bond activation.⁹⁵

Fig. 8 Mechanism of methane reforming on a Ru catalyst.⁹⁵

Ru has been found to be one of the most active noble metals for SMR,⁹⁶ with its ability to suppress coke formation.⁹⁷ The activities of 5 wt% Ru-based catalysts supported on alumina, ceria and ceria/alumina in the temperature range 400–800 °C are reported in Fig. 9a, where it is apparent that Ru/5Ce₁₀Al exhibits a higher activity than the other catalysts, Fig. 9b. Ru/Al₂O₃ exhibited the highest methane conversion with no carbon deposited on Ru/Al₂O₃ and Ru/5Ce₁₀Al.⁹⁷

Fig. 9 a) Activity per mmol m⁻² s⁻¹ of a Ru-based catalyst between 400–800 °C. b) Methane conversion (%) of a Ru-based catalyst between 400–800 °C.⁹⁷

The addition of Ru to Ni-catalysts supported on MgO, La₂O₃, and Al₂O₃ was found to enhance the catalytic performance in the reforming reaction by improving the catalytic activity and stability and prevented coke formation.^98,99

Ru catalysts supported on MgO, Nb₂O₅·nH₂O and Nb₂O₅ for SMR showed high activity at a temperature of 700 °C and S/C ratio of 4 with complete methane conversion. However, the addition of magnesium to alumina-supported ruthenium enhanced catalyst activity by increasing the dispersion of ruthenium and decreasing the methane conversion but also prevented coke formation.¹⁰⁰ In the case of partial oxidation of methane over 1.0 wt% Ru/SiO₂ catalysts, 93.9% methane conversion was achieved with 91.6% H₂ and 94.6% CO selectivity.¹⁰¹

4.2. Rhodium based catalysts

Rhodium, as a transition metal, is one of the most active metals for SMR,¹⁰² but its high cost limits its use.¹⁰³Fig. 10 illustrates the reaction mechanism for rhodium in methane decomposition. In this process, CH_x species migrate and form ethane, which is then adsorbed on the rhodium catalyst surface and dissociated to produce H₂. Notably, methane is adsorbed on the surface, with no other products detected on the rhodium surface.¹⁰⁴

Fig. 10 Mechanism of methane decomposition over a rhodium catalyst.¹⁰⁴

Rhodium (Rh) was found to be active in methane decomposition to produce H₂ when TiO₂ and V₂O₅ were used as promoters for Rh/Al₂O₃ in methane reforming at 773 K. Both V₂O₅ and TiO₂ promoted Rh/Al₂O₃ catalysts showed increased conversion, with Rh/Al₂O₃ exhibiting the best performance. This catalyst produced the highest amount of H₂, achieving a methane conversion of about 85% in the first pulse, compared to 46% for the V₂O₅–Al₂O₃ catalyst and 5% for pure Rh.¹⁰⁵ For SMR, a 0.8% Rh/Ce_0.6Zr_0.4O₂ catalyst achieved a methane conversion of 82% at a low temperature of 550 °C and an S/C ratio of 4.¹⁰⁶ Experimental tests demonstrated that methane conversion improved from 23% to 68% under similar conditions. When comparing Rh–perovskite catalysts with Ni-based catalysts, the Rh–perovskite catalyst exhibited lower carbon deposition and higher methane conversion, improving from 23% to 68%.¹⁰⁷

4.3. Pt-based catalysts

Although Rh catalysts exhibit excellent catalyst performance for SMR, Pt has good catalytic performance for H₂ production because of its high availability and low cost compared to rhodium.¹⁰⁸ The addition of a Ce promoter to Pt/ZrO₂ catalysts improved the catalyst activity, stability and the H₂/CO ratio of the product syngas. High temperatures reduce the catalyst activity, however calcination and reduction at 500 °C caused a strong Pt–Ce surface interaction that formed Ce³⁺ sites which improved the dispersion of Pt.¹⁰⁹ In comparing a Pt (0.5%)/Al₂O₃ catalyst, Pt (0.5%)/Al₂O₃ catalyst and Pt (0.5%) Ni (10%)/Al₂O₃ bimetallic catalyst, the bimetallic catalyst (Pt (0.5%) Ni (10%)/Al₂O₃) showed the best catalyst performance during a reaction time of ∼108 h at 750 °C exhibiting methane conversion from 78% to 95%, which proves that the introduction of Pt to a Ni catalyst increases the catalyst performance and limits coke formation.^109,110

5. Steam reforming process intensification

5.1. Sorption-enhanced SMR

To address the challenges associated with SMR (increased energy consumption, coke formation on the catalyst at high temperatures, and the high cost of H₂ separation), researchers suggest sorbent-enhanced steam reforming to improve H₂ production.¹¹¹

Sorption-enhanced SMR uses a sorbent to absorb CO₂ as it is produced. This has the advantage of enhancing the rate of the WGS reaction, improving the H₂ production rate, and producing a pure H₂ stream without the need for a separate carbon capture unit, lowering process costs.^111–113 Some of the sorbents used in SE-SMR are listed in Table 1.

Table 1 Various sorbents for sorbent enhanced SMR

Natural sorbents	Synthetic sorbents
Calcium carbonate (CaCO3)	Lithium orthosilicate (Li4SiO4)
Dolomite (CaCO3 × MgCO3)	Lithium zirconate (Li2ZrO3)
Huntite (CaCO3 × 3MgCO3)	Sodium zirconate (Na2ZrO3)

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The CO₂ sorption reaction in the SESMR process includes the reforming reaction in eqn (1), water–gas shift in eqn (2) and lastly the sorption reaction in eqn (4), which takes place at the same time in one reactor.¹¹⁴

CO_2(g) + ε_(s) ⇄ CO₂ε_(s) (4)

In the equation above, ε_(s) is the sorbent for CO₂ which reacts with the carbon to form a carbonated solid. Eqn (5) shows the overall SE-SMR reaction which becomes:¹¹⁵

CH_4(g) + 2H₂O_(g) + ε_(s) ⇄ CO₂ε_(s) + 4H_2(g). (5)

CaO is an appealing sorbent due to its low cost and high adsorption capacity. It can be produced from natural minerals like limestone and dolomite which are readily available from a variety of sources.¹¹⁶

Comparing a conventional SMR to SESMR using an 18 wt% Ni/Al₂O₃ catalyst, the SESMR exhibited a methane conversion of 65% and H₂ purity of 85% at a temperature of 650 °C, pressure of 30 bar, and S/C ratio of 3, while the conventional SMR exhibited a methane conversion of 24% and H₂ purity of 49%.¹¹⁷ A NiO–CaO–Ca₁₂Al₁₄O₃₃ catalyst with 25 wt% NiO showed a methane conversion of 99.1% and H₂ production efficiency of 96.1% during 10 cycles of SESMR.¹¹⁸ The addition of NiO in Ca₁₂Al₁₄O₃₃ showed a conversion of over 80% at 650 °C with a methane-to-steam ratio of 3.4 and H₂ selectivity of 90%.¹¹⁹ A CaO–Ca₉Al₆O₁₈ sorbent and Ni/MgAl catalyst for the SESMR process showed that a high S/C ratio, low temperature, and low space velocity improved H₂ purity, although the sorbent to catalyst ratio had no effect on the H₂ purity above 90 vol%.¹²⁰

Farooqi et al.¹²¹ compared the performance of Ni/Ca₁₂Al₁₄O₃₃, Ni–Co/Ca₁₂Al₁₄O₃₃, Ni–Cu/Ca₁₂Al₁₄O₃₃, and Ni–Fe/Ca₁₂Al₁₄O₃₃ catalysts in an FBR, at 700 °C, using an S/C ratio of 3. Of these, Ni–Co/Ca₁₂Al₁₄O₃₃ performed the best, producing the highest average H₂ purity over 10 cycles (78%) which the authors attributed to the synergistic properties of Ni and Co. In the case of carbon deactivation of Ca-based sorbents, as shown in Fig. 11, the exposure of carbon and other gas mixtures to the outer layer of CaO causes a reaction between the two that forms CaCO₃. The CO₂ shell migrates through pores to the inner surface of CaO which is known as the shirking core.¹²² This causes the blockage of the Ca-based sorbents with reaction time, damaging the sorbent's pore structure.

Fig. 11 Ca-based sorbent deactivation mechanism.¹²²

Lithium zirconate (LZC) as a CO₂ sorbent has been found to offer high capacity but slow kinetics due to the enthalpy associated with the chemical bond of CO₂ on the LZC surface, although hydrotalcite (HTC) and a K-promoted HTC sorbent are the opposite showing a fast kinetics but low capacity.¹²³

A Ni/MgO catalyst and K-promoted HTC sorbent were used to analyse the performance of the SESMR process (at 500 °C, 4.47 bar and S/C ratio of 6), exhibiting a H₂ purity of 96% on a dry basis and a methane conversion of 84% which are achieved at high sorbent capacity of ≥2 mol kg⁻¹.¹²³ Comparing CaO, lithium zirconate and hydrotalcite sorbent, lithium zirconate and hydrotalcite showed better sorption at 500 °C, 5 bar and S/C ratio of 3 achieving methane conversion of 91.3% and 55.2% and H₂ purity of 94.1% and 77.8% which later increased.¹²⁴

In summary, SESMR challenges include the need for high-performance sorbents, limitations in sorbent regeneration, difficulty in maintaining long-term stability, and the complexity of integrating sorbent cycling with the reforming process.

Liu et al.¹²⁵ investigated a combining sorbent and chemical looping (section 6.5) enhanced SMR, to address the low CO₂ capture rates of sorbents at mild temperatures (500–600 °C) and low CO₂ concentrations (5–10%). In a fixed bed reactor operated at 600 °C with a S/C ratio of 4, utilising a Ni-based oxygen carrier and Li-based adsorbent achieved a methane conversion rate of 93.3%, a H₂ purity of 92.8%, and a CO₂ capture rate of 90.1%.

Furthermore, Li₄SiO₄ demonstrated sustained performance, maintaining a high CO₂ adsorption rate of 63.7% and a crushing strength exceeding 25 N even after 200 cycles.

5.2. Oxidative SMR

Oxidative SMR is an advanced technology used for an energy-efficient H₂ production process. Eqn (6) and (7) show the oxidative SMR reaction when oxygen is added to methane and steam, or methane combustion occurs.

CH₄ + H₂O ⇆ CO + 3H₂ ΔH = +205.8 kJ mol⁻¹ (6)

CH₄ + 2O₂ ⇆ CO₂ + 2H₂O ΔH = −803 kJ mol⁻¹ (7)

Ni catalysts are mostly used in conventional steam reforming but the addition of oxygen to the reaction can cause Ni to oxidise and lose its activity. Hence, the addition of small amounts of noble metals can be used to increase Ni's stability during oxidative SMR.¹²⁶ The addition of noble metals such as Rh, Pt and Pd to NiO–MgO and Ni/γ-Al₂O₃ helps to reduce coke deposition, but hot-spot formation SMR can result in the formation of noble metal atoms segregated on the metal particle surface.¹²⁷ Furthermore, this technique requires the need for a precise balance between oxygen and methane to avoid side reactions and ensure stable performance.

5.3. Plasma and electrical field enhanced SMR

Plasma SMR is similar to the conventional SMR, but plasma provides radicals and energetic species generated from electricity.¹²⁸ Cold plasma and warm plasma are examples of non-thermal plasmas that can be used to produce hydrogen on a small scale, although warm plasmas are better in terms of hydrogen efficiency and rate.¹²⁹

Choi et al.¹³⁰ compared catalytic SMR, plasma SMR, and a combination of both techniques. A microwave, set to 6 kW, was used to produce plasma, and a NiO catalyst was used for catalytic and combined SMR. A S/C ratio of 3.7 was used throughout. Plasma and catalytic SMR produced syngas with a H₂ concentration of 67%, while combined SMR's product contained 75% H₂. Combined SMR also demonstrated the highest methane conversion (92%) as compared to plasma SMR's conversion of 69% and catalytic SMR's conversion of 64%. However, catalytic SMR produced the highest energy efficiency of 55%, compared to plasma and combined SMR's efficiencies of 43% and 46%, hence the methane conversion between catalytic and combined SMR is similar from an energy perspective. It was also noted by the authors that increasing the S/C ratio to 4.7 caused the methane conversion, energy efficiency, and H₂ purity of plasma SMR to increase to 60%, 95%, and 77%, respectively. Hence the authors concluded that optimised steam plasma reforming has high potential to enhance SMR. In a comparative study by Garcia-Villalva et al.,¹³¹ plasma SMR demonstrated better performance than plasma methane reforming (MR) and plasma dry methane reforming (DMR). Plasma SMR achieved a methane conversion of 24% and an H₂ yield of 80%, whereas plasma MR and plasma DMR attained methane conversion rates of 12% and 19%, and H₂ yields of 73% and 48%, respectively. This study also identified Ni/Al₂O₃ and Ni/CeO₂ as promising catalysts for catalytically enhanced plasma SMR, a future research route.¹³² Electrical field enhanced SMR uses an electric current to enhance the activity of SMR catalysts.¹³³ Electrical field enhanced SMR over Ni–CeO₂/γ-Al₂O₃–MgO achieved a methane conversion of 96.4% and H₂ yield of 75.3% at 600 °C.¹³² At 262 °C, a Pt catalyst supported on Ce_xZr_1−xO₂ solid solution exhibited a methane conversion of 40.6%.¹³⁴ This technique is still limited by high energy consumption, limited catalyst compatibility, difficulty in achieving uniform temperature control, and challenges in scaling up the process for industrial applications.

In a study by Lu et al.,¹³⁵ the scalability of an electric field-enhanced SMR was analysed. This investigation focused on a fixed bed reactor with a height of 0.88 m and a diameter of 0.5 m. Notably, it was found that the reactor's cross-sectional temperature profile remained largely uniform, representing a significant advantage over conventional SMR when scaled.

5.4. Photocatalytic and thermo-photo hybrid SMR

Photocatalytic SMR involves the use of semiconductors and sunlight; the semiconductors cause the breaking of the C–H bond of methane and the ability for water splitting by photocatalytic oxidative activity.¹³⁶ TiO₂ is mostly used as a photocatalyst in photocatalytic SMR.¹³⁷ SrTiO₃ photocatalysts, although show moderate activity and are not expensive, have low adsorption of methane, hence there is a need for cocatalysts like noble metals Rh, Pt, and Ru in PSMR.¹³⁶ A high rate of hydrogen was produced when Pt/CaTiO₃ photocatalysts were used in PSMR.¹³⁸ Rh in K₂Ti₆O₁₃ increased the speed of the photocatalytic reaction with Rh oxide and Rh metal acting as promoters to oxidize and reduce the reaction, respectively, as shown in Fig. 12.¹³⁸

Fig. 12 Rh/Rh₂O₃/K₂Ti₆O₁₃ photocatalyst's reaction mechanism for PSMR.¹³⁸

Silver is known to improve the SrTiO₃ photocatalytic activity in photocatalysis and thermocatalysis processes by dissociating methane to form Ag–H bonds. Ag⁺ was regarded as the oxidation reaction site while adsorbing the methane molecules and Ag⁰ acted as the reduction reaction site in the process while consuming low energy.¹³⁶

The thermo-photo hybrid process was demonstrated with a Pt/black TiO₂ catalyst by filtering UV light from AM 1.5G sunlight, causing methane and steam to produce syngas without a water-gas shift.¹³⁹ The result of this was an increased H₂ yield of 185 mmol h⁻¹ g⁻¹ with a quantum efficiency of 60% achieved at 500 °C.¹³⁹ Photocatalytic and thermo-photo hybrid SMR face challenges such as low efficiency under practical conditions, limited catalyst stability and scalability, and difficulties in optimizing the balance between light absorption and heat management for sustained methane conversion.

5.5. Chemical looping SMR

Chemical SMR utilises an oxygen carrier, that often doubles as a catalyst, to oxidise methane, enhancing the production of H₂.¹³³ The metal oxide is then regenerated in an air reactor as shown in Fig. 13, where the metal oxide oxygen carrier is represented as MeO_x. MeO_x is reduced by methane to produce MeO_{x−δ1−δ2} and syngas, MeO_{x−δ1−δ2} is then oxidised by steam to recover most of the lattice oxygen to MeO_x−δ1 (the intermediate state) which produces hydrogen and finally, the intermediate state is oxidized through air oxidation to MeO_x (the original state).¹⁴⁰ The advantage of this technology is that the oxygen carrier/catalyst can be continuously regenerated, reducing tar and coke production, and that heat released from oxidation of MeO_x can be used to maintain the temperature of the reforming reactor, potentially making the process autothermal.^141,142

Fig. 13 Chemical looping SMR process for hydrogen production.¹⁴⁰

Yang et al.¹⁴³ investigated the use of a Ni loaded Fe₂O₃/Al₂O₃ oxygen carrier for methane reforming. Using a 10% NiO–Fe₂O₃/Al₂O₃ catalyst at 950 °C, a methane conversion of 96% and H₂ yield of 3.3 mmol g⁻¹ were obtained. Under these conditions, a low carbon build up (0.093 mmol g⁻¹) was observed, indicating the stability of this oxygen carrier. Hu et al.¹⁴⁴ proposed the use of Ni/Fe modified calcite as a sustainable oxygen carrier. Calcite is a natural mineral and so it's use would reduce the cost of producing the oxygen carrier. Using a temperature of 800 °C, a steam/oxygen carrier ratio of 0.67, and a methane/oxygen carrier ratio of 0.13, the average methane conversion, H₂ yield, and carbon deposition produced over a 180 min period were 96.01%, 7.04 mol kg⁻¹, and 3.28% respectively. This indicates the promise that modified natural ore oxygen carriers have for SMR.

La_1−xMnCu_xO₃ and Fe, Co and Ni in LaMnO_3+δ have been found to show excellent activity and stability while reducing coke formation during multiple cycles, exhibiting good oxygen ion mobility.^145,146 Yin et al.¹⁴⁷ investigated the use of LaMnO_3+δ, LaMn_0.7Fe_0.3O_3+δ, LaMn_0.8Co_0.2O_3+δ and LaMn_0.9Ni_0.1O_3+δ as oxygen carriers. From this, it was found that LaMn_0.7Fe_0.3O_3+δ and LaMn_0.8Co_0.2O_3+δ oxygen carriers performed the best, producing a H₂ yield of 0.6 mmol g⁻¹ and 0.8 mmol g⁻¹, respectively after twenty 10 minute cyclic tests at 850 °C.

Current chemical looping SMR challenges include the need for complex reactor designs, limited oxygen carrier stability, the potential for high material costs, and challenges in maintaining efficient cyclic operation at high temperatures.

5.6. Membrane reactors for SMR

Membrane reactors for SMR allow the removal of hydrogen by a shift in equilibrium according to Le Chatelier's principle, which improves the methane conversion and product yield of the reaction at mild temperatures.^148–150

Palladium (Pd) membranes have an incredibly high H₂ separation efficiency (>99.9999%) and excellent mechanical properties. However, their high cost and chemical instability have prevented their large-scale application.¹⁵¹ It has been noted that Pd–Ag alloys have an even higher H₂ selectivity than pure Pd membranes, hence Nemitallah et al.¹⁵² produced a model to simulate the performance of a Pd–Ag membrane reactor for SMR. This model suggests that a co-current flow configuration, high feed pressure, inlet gas temperature, and H₂ concentrations result in H₂ flux across the membrane. The recent literature has explored the use of Ni-based membranes due to their lower cost and higher stability. Wang et al.¹⁵³ investigated a Ni hollow fibre membrane and found that at 1000 °C and a S/methane ratio of 3, the H₂ production rate reached 50.84 mmol m⁻² s⁻¹, and it possessed high resistance to coke formation at temperatures exceeding 800 °C. Although membrane reactors for SMR are promising, they face limitations including susceptibility to coke formation, membrane degradation at high temperature and pressure, reduced hydrogen selectivity, high material costs, and challenges in long-term stability and integration with existing systems that need to be addressed to enable their widespread use.

5.7. Methane fuelled solid oxide fuel cells

Solid oxide fuel cells (SOFCs) directly convert stored chemical energy in fuels (natural gas, carbon monoxide or biogas) into electricity and H₂.¹⁵⁴ They have been found to offer commercial applications because of their increased energy efficiency and their fuel flexibility.¹⁵⁵ Solid oxide fuel cell devices have high conversion efficiency, provide low pollution emission and zero noise. Among the fuels used by SOFCs, methane may be the best fuel due to its abundance in natural gas and easy conversion from CO₂ and/or biomass with the use of efficient catalysts to improve the methane conversion rate.¹⁵⁶

Using 10%CH₄–5%O₂–5%CO₂ as fuel, a maximum power density of 1321.5 mW cm² was achieved at 800 °C. The addition of 5%CO₂ caused an increase in methane conversion (from 31.69% to 50.03%) and coke tolerance.¹⁵⁷ Although the interaction between Ni and yttria-stabilised zirconia (YSZ) is weak, the interaction of samarium to the Ni–YSZ (yttria-stabilised zirconia) anode cell formed smaller particles and excellent interfacial contact, which achieved a maximum power density of 1.54 W cm⁻² for methane fuel at 800 °C.¹⁵⁸ The addition of La to the Ni–YSZ anode results in increased activity for methane reforming and electrochemical oxidation of hydrogen.¹⁵⁸ An n = 2 Ruddlesden–Popper (RP) phases La_1.5Sr_1.5Mn_1.5Ni_0.5O_7±δ with 82 mol% methane, 18 mol% N₂ and a low S/C of 0.15 showed a methane conversion of 14.60 mol% and H₂ production of 24.19 mol% for more than 4 h.¹⁵⁹

In a recent study conducted by Zhang et al.,¹⁶⁰ a MgO-modified Ni₃Sn anode was investigated for applications within methane-fuelled SOFCs. The synergistic effects of MgO and Sn were found to enhance charge transfer and full gas diffusion within the anode. This modification also reduced coke buildup by promoting the preferential oxidation of carbon atoms, preventing coke accumulation. This cell demonstrated a power density of 374 mW cm⁻² and exhibited great stability over an operation period of 100 hours at 700 °C using humidified methane as a fuel.

SOFCs for SMR face challenges that can be summarized as high operating temperatures, susceptibility to carbon deposition (coking) and the need for complex fuel reforming systems to efficiently convert methane.

6. Conclusion and outlook

SMR state-of-the-art and novel technologies have been reviewed, focusing on the key issues of SMR catalysts and recent development of new promoters such as noble metals (Ru, Rh, Pt, Ir, Ag, and Au), alkali metals (e.g., K), rare earth metals (e.g., Ce and Nb), metal oxides (MgO, CaO, and La₂O₃) and non-metal like boron. Solid solution catalysts were discussed and found to enhance Ni catalyst activity when a 20 wt% NiO/MgO solid-solution catalyst exhibited a methane conversion of 91% at an S/C ratio of 1/1 at atmospheric pressure and a temperature 800 °C. Structural improvement of SMR catalysts was also highlighted in the literature review to help improve the catalyst activity and stability by Ni particle size control, surface morphology of the catalyst, metal oxide substrates and self-supporting catalysts. The review gave a comprehensive analysis of non-noble metal catalysts (Co, Cu, and Mo) and noble metal catalysts (Ru, Rh, and Pt). Moreover, a discussion of the advanced SMR process has been proposed, to improve the H₂ productivity and reduce the energy demand. The most promising advanced SMR techniques include membrane reactors, which offer efficient hydrogen separation and higher selectivity, and chemical looping, which enables CO₂ capture and improves methane conversion while reducing energy consumption. Additionally, photocatalytic and plasma-enhanced reforming methods show potential for lower energy input and cleaner processes, although challenges remain in scaling and optimizing these technologies for industrial use.

From this review current research trends tend towards the use of supported bimetallic catalysts, with Ni–Co/Al₂O₃ consistently demonstrating high activity and good stability. Further research must be conducted to enhance the performance of these catalysts, optimise process conditions, and support structure, and life cycle analyses to assess the sustainability of these catalysts. Advanced SMR technologies such as membrane reactors, sorption enhancement, and chemical looping have demonstrated their value in improving methane conversion and H₂ purity. Techno economic analyses of these technologies must be conducted to assess their commercial viability, and potential combinations (such as sorption enhanced SMR conducted in a membrane reactor) should be explored. Recent studies have also been exploring the application of SMR technologies to other feedstocks, such as glycerol, ethanol, and methanol, improving the feed flexibility of the technology and improving its viability industrially.

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.

Conflicts of interest

There are no conflicts to declare.

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