State Key Laboratory of Advanced Refractories， Key Laboratory of Hubei Province for Coal Conversion and New Carbon Materials， Wuhan University of Science and Technology， Wuhan 430081， China
Against the backdrop of the global energy structure’s heavy reliance on fossil fuels， massive emissions of greenhouse gases including CO2 and CH4 have triggered severe environmental issues represented by the greenhouse effect. Dry reforming of methane（DRM）technology has attracted much attention because it enables the simultaneous conversion of two major greenhouse gases（CO2 and CH4）into high-value-added syngas（an H2/CO mixture）. Herein， we systematically review the recent research advances and key findings of DRM in recent years， and elaborates on its fundamental reaction mechanisms. Within thermocatalytic systems， this review summarizes the influences of catalyst active components， promoters and supports on the reactivity， selectivity and stability of the DRM reaction. Furthermore， it discusses the development potential and trend of emerging catalytic systems， including Joule heating catalysis， photocatalysis and photo-thermal catalysis， for further boosting DRM performance. Finally， the core challenges restricting the industrial application of DRM are concluded， and future research directions are prospected.
Contents
1 Introduction
2 Reaction mechanism of dry reforming of methane
3 Research progress of thermocatalytic dry reforming of methane
3.1 The active component of the catalyst
3.2 Catalyst additives
3.3 Catalyst carrier
3.4 Formation and Elimination of Carbon Deposits in DRM
4 Trend of dry reforming of methane
4.1 Joule heating catalytic dry reforming of methane
4.2 Photocatalytic dry reforming of methane
4.3 Photothermal synergistic catalytic dry reforming of methane
With the development of the global economy, human society's high dependence on non-renewable fossil energy sources such as coal, oil, and natural gas has led to CO2 and CH4 and other greenhouse gases being emitted in large quantities, triggering a series of ecological and environmental problems[1]. Achieving efficient conversion and utilization of greenhouse gases through catalytic technology innovation[2-5] has become an important issue urgently needing resolution in the current energy field, possessing low-carbon economic value and practical significance for sustainable development[6].
Among greenhouse gases, CO2has the highest proportion and the greatest impact; excessive emissions cause serious environmental problems[7], and its resource utilization has always been a research hotspot. For example, converting CO2into lactic acid, pyruvic acid[8]etc. via microbial or enzymatic catalysis processes; converting CO2into carbon nanotubes, graphene, etc. via molten salt electrolysis. In addition, catalytic reactions can be used to convert CO2into organic compounds such as methanol, formic acid, and acetic acid[9-12]. Although CH4content in the atmosphere is lower than that of CO2, its greenhouse effect is 25~30 times that of CO2, contributing up to 20% to global warming[13]. Meanwhile, CH4is the main component of natural gas, biogas, and shale gas, and also a primary raw material for producing chemical products. Furthermore, CH4plays a core role in syngas production, which is achieved through technologies such as reforming[14]. Reforming, as a conventional method for producing syngas, includes SRM (steam reforming of methane, CH4+H2O↔CO+3H2)[15-18], POM (partial oxidation of methane, CH4+1/2O2↔CO+2H2)[19-21]and DRM (dry reforming of methane, CH4+CO2↔2CO+2H2). Among these, DRM simultaneously consumes CH4and CO2, two greenhouse gases, to generate H2/CO syngas at a 1:1 ratio[22]; the produced syngas can be used to produce methanol, dimethyl ether, and liquid hydrocarbons via Fischer-Tropsch synthesis[23]etc.
DRM is a strongly endothermic reaction that requires high temperatures to proceed[24], while catalysts can effectively lower the activation energy of the reaction, improving the reaction rate and syngas production efficiency[25], thus becoming an indispensable role in thermocatalytic reactions. However, traditional catalysts still face issues in practical applications such as catalyst deactivation, insufficient selectivity, and mismatched product ratios[26]. Therefore, enhancing the catalytic selectivity of catalysts and reducing their deactivation rate remain key challenges in the current field of catalysis.
In recent years, Joule heating catalysis, photocatalysis, and photothermal synergistic catalysis have attracted significant attention in the field of DRM. Joule heating directly heats the catalyst bulk via electric current, greatly improving energy efficiency; photocatalysis utilizes photogenerated electron-hole pairs to drive reactions, enabling the activation of CH4 and CO2, thereby substantially reducing reaction energy consumption[27]; photothermal synergistic catalysis leverages the synergistic effects of thermal and light energy[28], which not only significantly enhances the rate and selectivity of catalytic reactions but also effectively suppresses carbon deposition on the catalyst surface, thereby extending the catalyst's service life[29]. However, these catalytic methods still face numerous challenges in practical applications[30], such as reactor design in Joule heating catalysis, low light absorption efficiency, high recombination rates of photogenerated carriers, and unclear photothermal conversion mechanisms in photocatalysis and photothermal catalysis[31] and other issues. Therefore, developing efficient and stable catalytic materials, optimizing reaction systems, and deeply investigating their reaction mechanisms are important directions for future research.
Based on the above issues, this paper reviews the latest research progress on DRM in recent years, covering the screening of active components, promoters, and supports for catalysts, as well as catalyst preparation methods and emerging catalytic systems.
2 Mechanism of methane dry reforming reaction
The mechanism of dry reforming of methane is complex and involves multiple intermediate steps. Currently, the surface reaction mechanism is generally accepted by researchers, which is mainly divided into three steps[32]: (1) adsorption and activation of reactants; (2) surface reaction; (3) product desorption.
CH4As a highly symmetric nonpolar molecule, its C—H bond energy is high, requiring adsorption and dissociation at active sites on the catalyst surface (such as metal Ni, Ru, etc.). Its activation process involves stepwise dehydrogenation, where the first dehydrogenation step CH4*→CH3*+H requires the highest energy and is also the rate-determining step of the reaction[33]. CH4's sp3hybridized structure's high symmetry and nonpolar characteristics make it difficult to adsorb onto the catalyst surface via electrostatic interactions; it requires activation through electron transfer from transition metal d orbitals, resulting in a dissociation energy for the first C—H bond as high as 439.3 kJ/mol[34]. Subsequently, CH3* undergoes further dehydrogenation to generate species such as CH2* and CH*, etc. Excessive dehydrogenation of CHx* will generate C*, forming carbon deposits that adhere to the catalyst surface, leading to catalyst deactivation. CHx* can also couple with O species (O* or OH*) produced by CO2activation to produce CHxO*, which further dehydrogenates to generate CO, effectively inhibiting carbon deposit formation. Therefore, an excellent catalyst must not only lower the reaction energy barrier for the first dehydrogenation step but also suppress the problem of excessive dehydrogenation of CHx* forming carbon deposits[35].
CO2 is a linear molecule, and its activation requires electrons provided by the catalyst. For supported catalysts, researchers generally believe that CO2 adsorbs on the support or at the interface between the support and the active metal[36]. CO2 adsorption follows two pathways: one is dissociative adsorption, where CO2 adsorbs at the interface position, causing the O＝C＝O bond angle to bend, thereby breaking to generate CO* and O*; the other is non-dissociative adsorption, which typically occurs on basic supports (such as CaO, MgO, etc.). That is, CO2 adsorbs in molecular form, first forming carbonates, and then decomposing into O* and CO[37]. Furthermore, on reducible supports, oxygen vacancies (Ov) are considered the active sites for CO2 adsorption and dissociation. The metal cations of reducible supports can interconvert between two oxidation states, and the generated Ov enhances CO2 adsorption[38]. For example, Luan et al.[39] prepared a series of Co-In/C catalysts rich in Ov by introducing In. Appropriate In2O3 doping can significantly increase the Ov concentration on the catalyst surface, promoting CO2 adsorption and activation capability, thereby accelerating the DRM reaction rate.
CH4During the dry reforming process, the following reactions may occur[40]:
in the above reactions, the reaction heat ΔHfor the main reaction (Equation (1)) is 247 kJ/mol[41]. Equations (2)~(4), as side reactions of DRM, may lead to carbon deposition[42], among which the excessive cracking of CH4(Equation (2)) and the CO disproportionation reaction (Equation (3)) are the main factors causing carbon deposition[43]. In addition, Equations (5)~(6) are side reaction equations affecting the composition ratio of syngas; the reverse water-gas shift reaction (RWGS, Equation (5)) occurs easily at high temperatures[44], and is the main reason why the H2/CO ratio in syngas is less than 1; steam reforming (Equation (6)) may occur when H2O is generated by RWGS, leading to an increase in the H2proportion, deviating from the target product ratio. Asshown in Figure 1, DRM has complex reaction pathways[45], including direct dehydrogenation and oxidative dehydrogenation of CH4, as well as direct dissociation and hydrogenative dissociation of CO2. Therefore, it is necessary to develop high-performance catalysts to reduce the activation energy, promote the main reaction, and suppress side reactions, thereby improving the yield and selectivity of the products.
3 Research on Thermal Catalytic Dry Reforming of Methane
Currently, thermal catalysis is widely used in DRM due to its advantages such as high conversion rates and good selectivity. In the thermal catalytic process, the rational design of the catalyst is key to determining catalytic performance. Among them, the catalyst mainly consists of three parts: active components, supports, and promoters.
3.1 Catalyst active component
The active component is the core part capable of directly participating in chemical reactions and playing a key role in reaction activity and selectivity. Precious metals such as ruthenium (Ru), iridium (Ir), and rhodium (Rh), as well as transition metals like nickel (Ni), cobalt (Co), and iron (Fe), are common active components. They possess unique electronic structures and surface properties that enable them to adsorb and activate reactant molecules. Among these, transition metals are frequently used as active centers widely applied in DRM due to their high catalytic activity and low cost.[46].
3.1.1 Transition metal catalyst
Among transition metal catalysts, Ni-based catalysts exhibit the most superior catalytic activity, with their performance primarily depending on the dispersion of Ni on the support and its resistance to sintering. In recent years, extensive research has been conducted on the structural regulation and performance optimization of Ni-based catalysts. Among these, preparation methods serve as the fundamental means to control the valence state distribution of Ni active sites, interfacial structures, and metal-support interactions, directly determining the intrinsic activity and reaction stability of the catalysts. Liu et al.[47]systematically compared the effects of hydrothermal, co-precipitation, and impregnation methods on the structure and DRM performance of Ni/MgO catalysts. The results revealed that the Ni-MgO-Imp catalyst prepared by the impregnation method is rich in structurally stable Ni0/Ni2+active sites. Through electron transfer between Ni0/Ni2+, these sites significantly enhance the catalyst's resistance to sintering and carbon deposition. Furthermore, characterization via in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) indicates that the DRM reaction on this catalyst follows the formate pathway (Figure 2), wherein Ni0/Ni2+sites promote the conversion of HCO3-to HCOO-, subsequently generating H2and CO. This study elucidates the synergistic mechanism by which the regulation of active site valence states influences both the reaction pathway and catalytic stability.
Zhang et al.[48]Ultra-small and highly dispersed sub-nanometer Ni/SBA15-OH (SBA15: mesoporous SiO2molecular sieve) catalysts were prepared at high temperatures via in situ hydroxyl trapping technology. During a 150-hour DRM reaction, the Ni particle size of the catalyst grew from 1.94 nm to 6.31 nm. These Ni sites exhibit high mobility and moderate interaction on the hydroxyl-rich SBA15 support, effectively suppressing the agglomeration and growth of Ni particles during long-term reactions, thereby significantly enhancing the high-temperature stability of the catalyst.
Traditional studies often attribute the catalytic activity of Ni-based catalysts to metallic Ni sites, while Ni2+'s catalytic role is frequently overlooked. To further investigate the role of Ni2+, Cheng et al.[49]employed a ligand-protected hydrothermal synthesis method to highly disperse Ni atoms within the pure silica MFI zeolite framework, preparing a single-site cationic nickel catalyst (Ni-MFI-HT). Compared with metallic Ni catalysts having the same loading, it exhibited higher catalytic activity and resistance to coking stability. The reason lies in the Ni2+within the zeolite framework forming Ni2+-O2-active sites with surrounding oxygen atoms, which promoted the cleavage of C—H bonds in CH4. Moreover, the Ni-MFI-HT catalyst follows a partial oxidation pathway (CHx*+CO2→CO+H2), effectively suppressing coke formation. Furthermore, the atomic-level dispersion of Ni2+within the zeolite framework ensures complete exposure of its active sites, further enhancing catalytic performance.
Under harsh reaction conditions such as high temperatures, catalyst deactivation can result from carbon deposition, catalyst phase transformation, and sintering. To address the difficulty of catalyst regeneration, Jo et al.[50]prepared a Ni-doped CaTiO3/CaO multifunctional material (MFM) for integrated CO2capture and dry reforming of methane (ICCDRM). Asshown in Figure 3, under a reducing atmosphere, Ni nanoparticles uniformly exsolve from CaTiO3and redisperse back into the CaTiO3lattice under an oxidizing atmosphere, achieving in-situ cyclic exsolution and re-solid solution of the Ni catalyst. During 30 cycles at 700 °C, the Ni consistently maintained a nanoparticle size (approximately 7 nm), owing to CaTiO3acting as a physical barrier between CaO grains, effectively suppressing the sintering of CaO/CaCO3during the carbon reaction; furthermore, the strong interaction between Ni and CaTiO3reduces carbon deposition, further enhancing the stability of the catalyst.
The performance of different Ni-based catalysts is asTable 1shown. Although Ni-based catalysts are considered the most promising industrial candidates for DRM due to their high activity and low cost, carbon deposition and high-temperature sintering hinder their commercial application. Based on this, researchers have investigated transition metals such as Fe and Co.
Compared to Ni, Co possesses a unique electronic structure; although its C—H bond activation capability is slightly weaker at low temperatures, the introduction of Co species is crucial for constructing oxygen vacancies and regulating surface basic sites. Liu et al.[51]confirmed through the construction of a Co/MnO catalyst that: the electronic effect between Co and the MnO support forms a structure with Mn3+-Ov-Co0structured Co0-Ovsynergistic active centers, which not only promote the adsorption and activation of CO2, but Ovas secondary active sites can effectively migrate active oxygen species to gasify carbon precursor deposits. Furthermore, Winterstein et al.[52]further investigated the interfacial reaction process between Co and Ga2O3supports; during the pre-reduction process in H2, the CoGa intermetallic compound formed by the catalyst dynamically decomposes under DRM atmosphere, reconstructing into a Co-Ga2O3interface, which exhibits higher activity, stability, and resistance to coking behavior, benefiting from the redox properties of Ga2O3, significantly suppressing the formation of graphitic carbon.
Fe is typically not used alone as an active center for DRM reactions but rather acts synergistically with Ni, focusing on modifying the oxygen affinity of Ni-based catalyst surfaces to suppress carbon deposition. Zhang et al.[44]conducted kinetic studies on Ni-Fe alloy catalysts and found that adding a small amount of Fe to Ni can dilute the surface Ni active sites, forming small-sized Ni clusters and slowing down the growth of carbon nuclei; additionally, Fe increases the oxygen affinity of the alloy surface, promoting CO2adsorption of O* species generated from dissociation, where more surface O* further promotes the gasification and removal of carbon.
Al-Fatesh et al.[53]Supported Ni-Fe bimetallic catalysts on ZrO2-Al2O3composite supports exhibited excellent DRM catalytic performance. This is attributed to the formation of a Ni3Fe alloy phase after reduction, which effectively suppressed the sintering of metallic Ni at 800°C. Furthermore, Fe acts as an "oxygen carrier" therein, and its redox cycle (Fe0↔FeOx) serves as an oxygen transport channel, ensuring the oxidative removal of coke precursors.
3.1.2 Precious metal catalysts
Compared to transition metal catalysts, noble metal catalysts such as Ru, Rh, and Ir, although expensive, offer advantages including high activity, high stability, and excellent resistance to carbon deposition.[71-73].
Wang et al.[74]prepared Ir/CeO with different Ir loadings (0.2%~3%) using the impregnation-reduction method2-xcatalysts and found that Ir loading is the core factor regulating the metal-support interface structure: low Ir loading achieves high dispersion of active species, constructing abundant Irδ+-Ov-Ce3+active interfaces; high loading easily triggers Ir particle agglomeration, causing a significant decrease in the number of active interfaces. Further revealed through in-situ IR and DFT, the Irδ+-O-Ce3+interface structure can promote the formation of CHxO* species and inhibit carbon deposition caused by excessive decomposition of CH2*, which is the reason for the high catalytic activity and excellent carbon resistance of the Irδ+-O-Ce3+interface in DRM. This work indicates that forming an interface structure between the metal and the support is beneficial for inhibiting carbon deposition, providing a new approach for catalyst structural design.
Strong metal-support interaction (SMSI) can effectively enhance catalyst performance[55,75-76]. Studies have shown that CO2 pretreatment can in situ induce the formation of SMSI, thereby significantly enhancing the catalyst's resistance to coking, metal dispersion, and reaction activity. Gangarajula et al.[77] prepared Rh/CeO with a Rh loading of 0.1 wt.% via the impregnation method2 catalyst and subjected it to CO2 pretreatment at 850 °C, forming a special strong metal-support interaction (CO2-SMSI): At high temperatures, CO2 decomposition produces CO and active O; CO further interacts with CeO2, promoting the reduction of Ce4+ to Ce3+ and its migration to the surface of Rh nanoparticles, forming an encapsulation structure; meanwhile, active O participates in the partial oxidation of Rh, forming a mixed-valence Rh+/Rh0 interface. This special SMSI effect suppresses the high-temperature sintering of Rh particles, which is key to the catalyst's high catalytic stability; while the Rh+/Rh0 interface can effectively promote C—H bond activation, which is key to the catalyst's high DRM catalytic activity. This study breaks through the limitations of traditional SMSI, which tends to form dense encapsulation layers that shield active sites, revealing the dual regulation mechanism of CO2-SMSI on the activity and stability of DRM catalysts.
Traditional catalysts (such as nanoparticles) suffer from deficiencies in atom utilization efficiency, uniformity of active sites, and reaction selectivity; in contrast, single-atom catalysts (SACs) exhibit significant advantages, possessing abundant active sites, highly uniform active centers, and near 100% atom utilization, thereby demonstrating superior catalytic activity and resistance to coking. Zhang et al.[78]synthesized Ru-based single-atom catalysts with different Ru-O coordination numbers (high-coordination Ru-HC and low-coordination Ru-LC) using one-pot and two-step methods, respectively, to investigate the impact of different coordination environments on the DRM performance of Ru SACs. In Ru-LC, the Ru 4d orbitals are richly filled with electrons, which is more favorable for C—H bond activation and promotes H2generation, but also leads to an accelerated rate of C species formation; after 20 h of testing, the catalyst deactivation rate was 12.3%, with an H2/CO product ratio of 0.57. In Ru-HC, abundant Ru-O-Ce interfacial sites and higher interfacial electron density effectively enhance the adsorption capacity for CO2, while the lower H spillover energy barrier facilitates the CO2* + H* → COOH* → CO* + OH* reaction pathway; the catalyst showed almost no deactivation after 20 h of testing; however, the OH species generated during this process reduced the product H2/CO ratio. Investigating the influence of the coordination environment provides a theoretical basis for the subsequent optimized design of SACs in DRM reactions.
Tang et al.[79]prepared Rh/CeO with a Rh loading of 0.15 wt.% using an improved deposition-precipitation method2 SACs, and further investigated the catalyst structural evolution in the DRM reaction as well as the influence of product molecules on the dispersion and coordination environment of SACs. Asshown in Figure 4, after reduction by H2 at 350 °C, Rh1 single atoms agglomerated to form nanoparticles of 0.8~1.0 nm; when the temperature rose to 400 °C, the CO produced by the reaction broke the Rh nanoparticles into small clusters (6~10 atoms), and the chemical bond state correspondingly shifted from being dominated by Rh-Rh bonds to an increase in Rh-O bonds; as the temperature continued to rise to 600~700 °C, the higher CO concentration ultimately caused the complete dissociation of Rh clusters, finally forming single-atom Rh1O5 active sites. Notably, after the reaction ended and the temperature decreased, Rh1O5 would again agglomerate into Rh2/Rh3 clusters. This study breaks the static perception of catalyst structures, indicating that the active sites of SACs are not immutable but undergo dynamic evolution with reaction conditions (such as temperature and products), which also provides new strategies for SAC design.
The catalytic performance of some noble metal catalysts is asshown in Table 2. In summary, transition metal catalysts exhibit high catalytic activity and low cost, showing promising application prospects in DRM; however, they face numerous issues such as easy sintering and coking. By optimizing preparation methods, such as in-situ hydroxyl trapping and regulating the Ni0/Ni2+ratio, their dispersibility, anti-sintering capability, and anti-coking ability can be effectively enhanced. In contrast, noble metal catalysts possess advantages such as high activity, resistance to coking, and high stability, but their high cost and the tendency for particle agglomeration at high loadings limit their large-scale application. Developing SACs is one of the ideal choices to address the aforementioned problems; however, current understanding of the coordination environment, structural dynamic reconstruction, and reaction characteristics of single-atom catalysts remains relatively limited. Notably, combining small amounts of noble metals with transition metals to construct bimetallic or multimetallic catalytic systems is considered an effective strategy to balance cost and performance while further improving the comprehensive performance of DRM catalysts.
3.2 Catalyst promoter
In the field of catalysis, promoters are key components introduced into catalyst systems to optimize their performance. Although promoters sometimes lack independent catalytic activity, their synergistic interaction with the main active component can significantly enhance the catalyst's activity, stability, resistance to coking, and product selectivity.[88-90]. Therefore, in the design of DRM catalytic systems, the purposeful introduction of a second component (such as a metal or metal oxide) as a promoter to adjust the electronic structure and surface spatial structure of the catalyst has become one of the effective approaches to improve its comprehensive performance.[91-92].
Zhao et al.[93]prepared highly dispersed Ni-Pt/Al2O3bimetallic catalysts using atomic layer deposition. The Ni-Pt catalyst achieved a CH4conversion of approximately 74% and a CO2conversion of approximately 82% at 650 °C, maintaining high activity after 48 hours of reaction, whereas the monometallic Ni catalyst rapidly deactivated due to carbon deposition. Although both catalysts generated large amounts of carbon nanotubes (CNTs), there were significant differences in how their structural characteristics affected catalytic activity. The CNTs formed on the surface of the Ni-Pt catalyst had more structural defects; after depositing on the Al2O3support surface, they did not completely cover the active sites, thus preserving catalytic activity. In contrast, the CNTs formed on the Ni catalyst surface had high crystallinity and few defects, leading to the encapsulation of active sites and deactivation. Further confirmed by CH4-TPD and in situ DRIFTS, the Ni-Pt catalyst surface retained strong CH4adsorption sites (Figure 5) and formed formate (HCOO*) intermediates, indicating that the bimetallic catalyst altered the reaction pathway and reduced carbon deposition.
Liu et al.[94]prepared a series of NixGa100-x/Al2O3(x=0, 50, 60, 70, 80, and 100) catalysts using mechanochemical methods. Among them, Ni60Ga40/Al2O3exhibited excellent catalytic performance in the DRM reaction, with CH4and CO2conversions of 75% and 85%, respectively, and an H2/CO ratio of 0.93 within 120 h at 750 °C. Experimental characterization revealed that Ni and Ga formed a Ni3Ga intermetallic compound (IMC) in the catalyst, which effectively suppressed high-temperature sintering of Ni particles and excessive cracking of CH4. Meanwhile, Ga existed in the form of a Ga2O3-Al2O3solid solution, rich in Ov, which promoted the activation of CO2, thereby reducing carbon deposition.
Hussien et al.[95]prepared Ni/SBA-15 catalysts using CeLaCuO ternary oxides as promoters via the deposition-precipitation method. DRM experimental results showed that at 750 °C and 30,000 mL/gca/h space velocity, the prepared 5%Ni/30%CeLa10Cu-SBA-15 catalyst (with a Ce:La:Cu atomic ratio of 45:45:10) exhibited excellent catalytic performance, with CH4and CO2conversion rates both exceeding 80%. Long-term stability tests revealed that the activity of this catalyst remained stable after 40 h of reaction, whereas the Ni/SBA-15 catalyst without promoters gradually deactivated. Characterization results from X-ray diffraction (XRD), thermogravimetric analysis (TGA), and Raman spectroscopy indicated that a cubic fluorite-structured CeO2phase was formed in the 30%CeLa10Cu catalyst, which facilitated the formation of a stable Ce-Cu-O solid solution and enhanced the interaction between the support and promoter through Ce—O—Si bonds. This catalyst exhibited less carbon deposition during the reaction, with the deposited carbon primarily having an amorphous structure, demonstrating excellent resistance to coking. High-resolution transmission electron microscopy (HRTEM) and X-ray photoelectron spectroscopy (XPS) analyses further confirmed the presence of a Ni-Cu bimetallic alloy. This alloy structure effectively regulated the electronic structure of the catalyst, thereby significantly improving its long-term stability.
It can be seen that the introduction of promoters can effectively regulate the electronic structure characteristics and structural stability of the catalyst. By forming intermetallic compounds with the active components or strengthening the metal-support interface interactions, promoters alter the electronic structure of active sites, optimizing the adsorption and activation processes of reactants. Meanwhile, this structural regulation also helps suppress the excessive cracking of CH4, reducing the formation and accumulation of carbon deposits, thereby significantly improving the long-term stability of the catalyst.
3.3 Catalyst support
In addition to active components and auxiliaries, the selection and design of catalyst supports also have a decisive impact on overall catalytic performance. Supports not only provide highly dispersed anchoring sites for active metals but can also further influence the activity, selectivity, and stability of catalytic reactions by regulating the electronic and spatial structures of the active metals.[96-99]. Currently, catalyst supports commonly used in DRM reactions mainly include metal or non-metal oxides, composite oxides, and high-entropy oxides.[58,100-104].
Precise regulation of the catalyst support by constructing a functional shell structure can significantly improve the dispersion and stability of the active components. Yang et al.[105]adopted a two-step hydrothermal method (Figure 6), by adjusting the amount of Ce(NO3)3 solution, synthesized a series of core-shell structured catalysts Ni-MCM-41@2 with different CeOxCeO2 shell thicknesses (where MCM-41 is a SiO2 mesoporous molecular sieve, x = 0.1, 1, 2, 3, and 4, corresponding to the addition amounts of Ce(NO3)3 solution of 1, 10, 20, 30, and 40 mL, respectively). Among them, Ni-MCM-41@2CeO2 exhibited the optimal catalytic performance: under conditions of 700 °C, the conversion rates of CH4 and CO2 reached as high as 80.47% and 76.66%, respectively, and the activity decreased by only about 3% after 10 h of reaction. In addition, the carbon deposition amount of this catalyst was only 0.28%, far lower than that of the shell-free Ni-Ce/MCM-41 catalyst. Characterization results indicated that Ni nanoparticles were highly dispersed and confined within the MCM-41 framework and the CeO2 shell, thereby effectively suppressing the sintering of Ni nanoparticles and carbon deposition phenomena during high-temperature reactions. Density functional theory (DFT) calculations further confirmed that the CeO2 shell significantly reduced the dissociation energy barriers of CH4 and CO2, thereby enhancing catalytic activity.
Traditional support preparation methods have limitations in regulating the spacing of active centers and suppressing the sintering of active metal particles. However, this novel structural design of oxide nano-islands breaks the structural singularity of traditional supports, providing a new pathway to suppress the sintering of active metal particles. Li et al.[106]Using spherical SiO2with an average particle size of approximately 270 nm as the support, a series of ultra-small CeO2nano-island modified SiO2were first prepared via electrostatic adsorption, followed by impregnation to load Ni2+precursors to prepare Ni/Ce-SiO2catalysts. In DRM reaction tests conducted at 800 °C for 2000 h, this catalyst maintained stable CH4and CO2conversion rates of 92% and 96%, respectively, with almost no side reactions or significant catalyst deactivation observed. Asshown in Figure 7, the appropriate introduction of CeO2leads to partial electron transfer from Ni to CeO2, thereby reducing the antibonding state of Ni sites, moderately weakening the dissociation ability for CH4, while enhancing the adsorption and activation of CO2and suppressing carbon deposition; this is key to the high stability of the catalyst. Furthermore, the unique oxygen migration and reduction characteristics possessed by CeO2not only protect metal particles from sintering but also enable rapid transport of oxygen atoms. Meanwhile, the nanoscale size and tunable surface structure of the CeO2nano-islands can further optimize local surface active sites, thereby enhancing catalytic efficiency.
Zhou et al.[107]Based on the principle of electrostatic adsorption, by precisely controlling the pH in the solution, LaO2was introduced onto the surface of the SiOxnano-islands to load Ru nanoparticles (Ru NPs) on the LaOxnano-islands, preparing the Ru/LaOx-SiO2catalyst. HAADF-STEM images show that LaOxnano-islands are anchored on the amorphous SiO2support surface, while Ru NPs are fixed at the edges of the LaOxnano-islands. DRM results indicate that under conditions of 800 °C and a total space velocity of 250,000 mL/g/h, the catalyst achieves a CH4conversion rate of 83.4%. During a 400 h stability test, the CH4conversion rate remained stable at around 83%, the H2/CO ratio was approximately 0.94, and the average size of Ru NPs in the catalyst remained around 1.4 nm, with no significant deactivation observed. This is mainly attributed to the strong adhesion between LaOxnano-islands and Ru NPs, which causes Ru to preferentially anchor on the nano-islands, forming a "potential well" that inhibits the migration and aggregation of Ru NPs. Furthermore, the presence of LaOxnano-islands also alters the electronic state of Ru, promoting electron transfer from Ru to LaOx, thereby increasing the proportion of Ruδ+, which further promotes the activation of CO2, ultimately significantly enhancing catalytic performance.
High-entropy oxides (HEOs), by virtue of their multi-component characteristics, possess flexibly tunable lattice oxygen (OL) mobility and surface basicity. Meanwhile, the entropy stabilization effect can effectively anchor and highly disperse active metal nanoparticles, suppressing their high-temperature agglomeration. Thus, they are regarded as a highly promising candidate material for developing high-performance DRM catalysts. Zhang et al.[108]successfully prepared a series of HEO nanofibers with diameters of 50~100 nm via electrospinning. These HEO nanofibers exhibit high specific surface areas (up to 197 m2/g) and abundant Ov. In the DRM reaction, the spinel-type (Ni3MoCoZn)Al12O24nanofibers demonstrated the best catalytic performance. After H2reduction, part of the Ni precipitated from the lattice to form NPs with an average size of about 6 nm. At 700 °C, this catalyst achieved a CH4conversion rate exceeding 96%, a CO2conversion rate exceeding 99%, and an H2/CO ratio of approximately 0.98. Furthermore, it exhibited excellent stability during a 100-hour long-term reaction, with no significant carbon deposition observed. This is mainly attributed to the high dispersion of Ni active sites by the high-entropy nanofibers, as well as the abundant Ovwhich promoted the adsorption and activation of CO2, forming more bidentate carbonate species on the catalyst surface, thereby accelerating the reaction between CO2and carbonaceous intermediates.
The above results indicate that the support plays a key role in DRM reaction activity, stability, and coking resistance by regulating the dispersion of active metals and the interaction between the support and the metal. Through controllable preparation processes, functional support structures such as shell modification and nano-islands have significantly improved the dispersion and stability of active components, suppressing the sintering and coking of metal active sites. Compared with traditional oxide supports, high-entropy oxides possess high specific surface area, abundant Ovand high stability, which enhance reactant conversion rates and coking resistance. Typically, the synthesis processes of these supports are complex, and their electronic structures and action mechanisms are difficult to fully elucidate through conventional characterization techniques. Therefore, combining in-situ characterization techniques with DFT calculations can deeply establish the structure-activity relationship between "structure" and "performance," thereby guiding catalyst design and revealing catalytic mechanisms.
3.4 Formation and Removal of Carbon Deposits in DRM
The synergistic regulation strategies of active components, promoters, and supports in catalysts essentially point to an unavoidable core issue in DRM catalyst design: carbon deposition. Therefore, clarifying the formation characteristics and elimination mechanisms of carbon species in the DRM reaction is crucial for designing long-life catalysts.
The carbon species deposited in the DRM reaction are mainly divided into three categories, asFigure 8shows, their morphology is closely related to the reaction temperature[109-110]. The first is amorphous carbon, formed via CO disproportionation at low temperatures below 500°C, which is easily oxidized and removed by CO2; however, excessive accumulation can still cover active sites and cause deactivation. The second is graphitic carbon, formed by deep cracking of CH4at high temperatures above 600°C, which is difficult to oxidize and remove by CO2and is a major factor in catalyst deactivation. The third is filamentous carbon or carbon nanotubes, formed via carbon dissolution and precipitation at medium-to-high temperatures (500~800°C); excessive growth leads to catalyst deactivation.
However, recent studies have found that the amount of carbon deposition is not equivalent to the degree of deactivation. Wang et al.[110]using Ni/CexZr1-xO2as a catalyst, systematically investigated the relationship between carbon deposition and catalyst performance in the DRM reaction, finding that 0.6Ni/CeO, which exhibited the highest catalytic activity,2produced the largest total amount of carbon deposition. HRTEM observations revealed that deposited carbon could migrate from the surface of Ni nanoparticles to the periphery of the catalyst, exposing active sites and maintaining catalytic activity. This study demonstrates that carbon deposition does not necessarily lead to reduced catalytic performance; the key lies in whether the carbon possesses the ability to migrate and avoid covering active centers.
In response to different carbon species in the DRM reaction, researchers have developed various strategies for coke elimination, with the core lying in providing active O* to oxidize coke into CO. Based on the source of active O*, the coke elimination mechanisms can be categorized as follows.
First is CO2dissociation-mediated coke elimination by O*, asshown in Figure 9a[111]. Its coke elimination mechanism lies in the dual function of Ov: adsorption activation sites for CO2and support redox enabling dynamic regeneration of Ov, forming a cycle of "CO2adsorption → O* formation → coke elimination → Ovregeneration". Xia et al.[112]prepared Ni/CeO catalysts with a pinecone-like spherical layered cross-linked structure via hydrothermal method, finding that layered CeO2is rich in O2v, capable of continuously releasing active oxygen species to eliminate coke via in-situ gasification; within 50 h, coke accumulation barely increased, achieving a dynamic balance between carbon generation and consumption. Gao et al.[62]employed ultrasound-assisted impregnation to introduce CeO2O4as a promoter into Ni/MgAl2, finding that the addition of CeO2significantly increased the Ovconcentration of the catalyst, thereby enhancing the adsorption and dissociation of CO2and promoting coke elimination.
Second, alkaline supports enhance CO2 adsorption for coke elimination. The core mechanism lies in utilizing basic sites and acidic CO2 molecules' strong interactions to efficiently adsorb and activate CO2, generating active oxygen species, thereby achieving real-time oxidation and elimination of coke. Al-Fatesh et al.[113] prepared Ni/Ba-YSZ catalysts doped with the alkaline promoter Ba via impregnation, finding that alkaline BaO promoted reaction with CO2 to form BaCO3, which subsequently reacted with coke to generate CO and regenerate BaO (BaCO3 + C → BaO + 2CO), suppressing coke accumulation at catalyst active sites through a cyclic process of "BaO adsorbing CO2→BaCO3 formation→coke elimination→BaO regeneration". Zeng et al.[114] prepared Ni/2O3) composite Ni/m-SiO2 catalysts, finding that adding alkaline promoters significantly improved DRM catalytic performance and coke suppression effects. Among them, MgO addition greatly enhanced high-temperature adsorption and activation of CO2, asFigure 9b shows[114], achieving rapid in-situ elimination of coke during the reaction process, endowing the catalyst with excellent coke resistance and long-term operational stability.
Third is the lattice oxygen OL-involved coke elimination, asshown in Figure 9c[115], which typically follows the Mars-Van Krevelen (MvK) mechanism[116-119]. CH4decomposes at active sites to produce carbon species C*, which migrate to the metal-support interface and react with surface OLto generate CO and create an Ov; CO2is activated and dissociated at Ovsites to produce an O atom that refills the Ov, completing the regeneration of OL. For instance, Hussien et al.[120]prepared a La3+-doped Ce-Cu-O composite support-loaded Ni catalyst with excellent anti-coking stability. The study found that the rate at which OLin the support oxidizes coke to CO is far higher than that of CO2dissociation producing active O, thanks to La3+doping significantly enhancing the OLmigration rate of the Ce-Cu-O support, promoting rapid coke elimination. Das et al.[121]found that small amounts of Zr-doped CeO2can cause lattice contraction, forming a Ce-Zr solid solution, increasing OLmigration rate, significantly reducing coking behavior in DRM reactions, confirming that OLmobility is the core regulatory factor for DRM activity.
In summary, the common feature of these three carbon deposit elimination mechanisms lies in the continuous supply capability of active O*, with their essential difference being the source of O*: CO2dissociation, carbonate decomposition, and lattice oxygen OLmigration. From the perspective of catalyst design, the regulation of Ovand the optimization of OLmigration rate are the two core strategies for improving carbon deposit elimination efficiency, providing a clear direction for developing anti-coking catalysts.
4 Development Trends of Dry Reforming of Methane
Although significant progress has been made in suppressing carbon deposition through support engineering and promoter optimization in the aforementioned thermal catalytic systems, traditional thermal catalytic DRM still faces two inherent limitations. The first is that the thermal catalytic DRM process requires a large amount of fossil energy to provide heat, which implies huge energy consumption and CO2 secondary emissions, contradicting the concept of sustainable development[122]; the second is the thermodynamic equilibrium limitation, where even at higher reaction temperatures, the single-pass conversion ratio of CH4 and CO2 is difficult to further improve.
In recent years, Joule heating catalysis, photocatalysis, and photothermal synergistic catalysis technologies have received widespread attention. Joule heating catalysis utilizes the catalyst bed or structural elements integrated with it as resistive heating bodies; when electric current passes directly through them, heat is generated uniformly inside or on the surface of the catalyst particles, avoiding heat conduction losses from the reactor walls and the bed, thereby achieving higher overall energy efficiency. Photocatalytic DRM reactions operate under mild conditions, capable of driving reactions using solar energy at lower temperatures, offering advantages such as reduced greenhouse gas emissions, environmental friendliness, and energy conservation. As an emerging catalytic method, photothermal synergistic catalysis effectively combines the strengths of photocatalysis and thermal catalysis, maximizing the overcoming of limitations inherent in single catalytic approaches and significantly improving the utilization efficiency of both thermal and light energy. The following section will review recent research progress on DRM reactions from three aspects: Joule heating catalysis, photocatalysis, and photothermal catalysis.
4.1 Joule heating catalyzed dry reforming of methane
In recent years, Joule heating catalysis technology has developed rapidly. Researchers have integrated catalytically active components with conductive substrate materials (such as metal foams, carbon materials, and conductive ceramics) to construct monolithic catalytic heating elements, achieving rapid temperature rise. This strategy has brought revolutionary changes to heat supply for catalytic reactions, effectively reducing the energy consumption of DRM reactions and improving overall energy utilization efficiency.
Zheng et al.[123]filled the Ni/Al prepared by impregnation method into2O3catalyst within SiSiC open-cell foam for Joule heating catalytic DRM reaction. The SiSiC foam acts as an internal Joule heating element, achieving selective heating of the catalyst bed; only 65 W of power is required to reach a reaction temperature of 800 °C with uniform temperature distribution. DRM reaction results indicate that CH4and CO2conversion rates reached 94% and 64% respectively, an improvement of 5%~10% compared to traditional heating methods. Notably, no carbon deposition was observed throughout the entire reaction process, primarily due to the uniform temperature distribution of the internal Joule heating. Meanwhile, the Ni in the catalyst and the Al2O3support formed a NiAl2O4phase, which enhanced metal-support interaction, improving its resistance to sintering and carbon deposition. This study demonstrates that Joule heating catalysis holds great potential for improving DRM efficiency, suppressing carbon deposition, and reducing energy consumption.
Rapid Pulse Joule Heating (RPH) replaces "steady-state heating" with "dynamic pulses," enabling efficient temperature control and flexible regulation of reaction performance. Yu et al.[124]developed an RPH reactor equipped with an in-situ Raman spectrometer, using a bimetallic PtNi/SiO2catalyst for Joule heat-catalyzed DRM reactions, achieving an average temperature of 675 °C at a power of 42 W. CH4conversion reached 67%, with a product H2/CO ratio of 0.95, while the catalyst showed no significant deactivation under pulse heating. Raman spectroscopy and XRD confirmed that the periodic pulse heating of RPH effectively suppressed coke formation. In the high-temperature range (>900 °C), RPH can eliminate coke via the reverse Boudouard reaction (C+CO2→2CO), whereas in the medium-temperature range (500~900 °C), it can suppress the transformation of amorphous carbon to graphitic carbon through rapid heating. Furthermore, when employing RPH for catalytic DRM reactions, the catalyst metal nanoparticles did not grow significantly, indicating a relatively stable structure.
Traditional Joule heating serves only as a heat source to meet the high-temperature requirements of the DRM reaction, without attention to the reaction mechanism and intermediate conversion pathways. Zhao et al.[125]proposed an electrically driven dry reforming of methane (e-DRM) technology based on conductive catalysts. They loaded Ni-La2O3/AC (activated carbon) catalysts prepared by equal-volume impregnation into a self-made reactor (Figure 10); passing an electric current heats and drives the reaction, requiring only 16 W of power to reach 700 °C in the catalyst bed with uniform temperature distribution. e-DRM results show that at 700 °C, the conversions of CO2and CH4are approximately 90% and 50%, respectively, with a product H2/CO ratio of 0.95. After 120 h of continuous operation, a CO2conversion of 77% and an H2/CO ratio of 0.9 were maintained. Experimental characterization results indicate that in traditional thermally driven DRM, the catalyst generates large amounts of La2O2CO3intermediates; Ni is oxidized to NiO, and composite phases such as La3Ni2O7are formed. In contrast, in e-DRM, electrical driving promotes OLrelease, accelerating the decomposition of La2O2CO3and the reduction of NiOx, suppressing La2O3carbonation and excessive Ni oxidation; no significant carbon deposition or AC gasification was observed. The energy efficiency of e-DRM reaches as high as 2.976 mmol/kJ, which is 6–12 times that of thermal catalytic DRM, providing a feasible pathway for its industrial application.
Huang et al.[126]embedded Ni nanoparticles into the zeolite framework to construct an encapsulated catalyst, and combined it with internal Joule heating, achieving operation at 800 °C for 330 h continuously without deactivation, far surpassing traditional catalysts. Mechanistic studies revealed that silanol groups in the zeolite framework formed a unique synergistic effect with the embedded Ni nanoparticles, promoting a bidentate carbonate-mediated CO2activation pathway under current induction, significantly improving CO2conversion efficiency and facilitating coke elimination.
Joule heating catalyzed DRM utilizes electric current to directly convert into heat within the catalyst, achieving a fundamental shift from indirect heat transfer to direct heating, which means that Joule heating offers extremely high energy efficiency. Notably, Joule heating catalysis features a unique anti-coking mechanism: first, uniform temperature distribution eliminates local low-temperature zones ("cold spots") present in traditional heating methods; these "cold spots" are precisely the main regions where CO disproportionation reactions generate coke.[123]; second, replacing steady-state heating with Joule pulse heating enables rapid temperature rise in the medium-temperature range to suppress C* species deposition, while maintaining temperature in the high-temperature range to promote coke removal.[124]. Furthermore, some studies indicate that electric current can alter the electronic state of catalysts to drive special reaction pathways, promoting reactant conversion and coke removal. However, while Joule heating catalyzed DRM demonstrates the aforementioned significant advantages, it also faces numerous challenges and issues. First is the issue of reactor design and engineering scale-up; current research mostly focuses on laboratory scale, and how to achieve stable current and temperature in large-scale reactors remains a core problem. Second is the long-term stability of catalysts and reactors; under the dual effects of high temperature and electric current, operational lifespan is affected, with current testing concentrated at the hundred-hour level, lacking industrial-grade data beyond thousands of hours. Third, the mechanism of Joule heating catalysis remains unclear; how electric current influences the dynamic reconstruction of active sites and the adsorption behavior of surface intermediates still requires investigation combined with in-situ characterization.
4.2 Photocatalytic dry reforming of methane
Photocatalytic dry reforming of methane (photoDRM) is a novel catalytic method that utilizes photogenerated carriers to participate in the reaction. Under light irradiation, this reaction can achieve CH4 and CO2efficient conversion. This process not only has significant value for renewable energy utilization but also reduces greenhouse gas emissions and enables the circular use of carbon resources.
Shoji et al.[127]reported a nanostructured catalyst prepared using nanophase separation technology, composed of interwoven Rh metal and CeO2 (Rh#CeO2). Characterization revealed that Rh and CeO2 in Rh#CeO2 are interwoven in the form of a nanoscale fibrous network, forming an alternating striped structure of Rh and CeO2, which effectively promotes the separation of photogenerated electrons and holes. Photogenerated electrons transfer from CeO2 to Rh, driving CO2 reduction, while holes remain in CeO2, promoting CH4 oxidation. This catalyst achieves CH4 and CO2 conversion rates of 64% and 62%, respectively, under UV irradiation alone, and exhibits excellent stability during 100 hours of continuous reaction.
Recent studies have found that some metal-free covalent organic frameworks (COFs) can also exhibit excellent catalytic performance in photo-DRM reaction systems. Yin et al.[128]designed a novel COF material (TPE-COP), which promotes the separation of photogenerated electron-hole pairs through the "push-pull effect" between an electron donor (TAPA: tris(4-aminophenyl)amine) and an electron acceptor (TPE-CHO: a tetraphenylethylene derivative with high photosensitivity), significantly enhancing photocatalytic performance. After 20 h of irradiation, the CO and H2yields in TPE-COP reached 1123.6 and 30.8 μmol/g, respectively, far exceeding those of the TPE-CHO small-molecule precursor. DFT calculations indicate that its narrower bandgap favors light absorption and charge separation, further confirming the promoting effect of the donor-acceptor structure within the polymer backbone on the separation of photogenerated electron-hole pairs. This study provides new insights for the application of metal-free catalysts in the field of photo-DRM.
Strong metal-support interaction (SMSI) has attracted significant attention in heterogeneous catalysis, but its influence mechanism in photocatalytic systems remains unclear. Existing studies indicate that electron-deficient sites on metal surfaces play a key role in methane C—H bond dissociation, yet research on regulating photoDRM catalytic activity by modulating the valence state of metal nanoparticles is scarce. Li et al.[129]compared H2-pretreated support Ru/TiO2-H2with untreated Ru/TiO2catalysts and found that under full-spectrum light irradiation of 12.0 W/cm2, Ru/TiO2-H2exhibited higher activity, with H2and CO yields reaching 645.5 and 708.4 mmol/g/h, respectively. This performance enhancement is attributed to H2pretreatment weakening the electron back-spillover induced by SMSI: under illumination, photo-excited hot electrons from Ru NPs in the Ru/TiO2-H2catalyst migrate to Ov, promoting CO2activation; meanwhile, Ruδ+remains in an electron-deficient state, accelerating CH4decomposition; whereas in Ru/TiO2, the SMSI effect is stronger, causing electrons from the TiO2conduction band to inject into Ru, leading to increased Ru electron density, which is unfavorable for CH4activation and CO desorption. Thus, it can be seen that strong SMSI effects are detrimental to the photoDRM reaction over Ru/TiO2, and suppressing SMSI via H2pretreatment of TiO2can significantly enhance photoDRM performance.
Increasingly, researchers believe that during the photo-DRM catalytic process, Ovis not a static structural defect, but a dynamic active site that is continuously generated, participates in reactions, and regenerates under light excitation; its dynamic evolution plays a key role in the catalytic reaction. From the perspective of carrier separation, Ovcan promote the separation of photogenerated charges under illumination. On one hand, Ovcan act as an electron trapping center to suppress the recombination of photogenerated electron-hole pairs; on the other hand, photogenerated holes can migrate to OLsites, promoting the activation and migration of OL. Li et al.[119]partially substituted Fe in LaFeO3with Mn, finding that Mn substitution leads to charge transfer from La to Mn. Synergizing with light irradiation, this reconstructs the electronic structure of LaFeMnO3, weakens the La-O-Mn chemical bonds, and promotes the migration of OL; meanwhile, photogenerated holes migrate to OLsites, further enhancing the OL-mediated DRM activity (Figure 11a). This achieved HRu/h production rates of 42.89 and 54.92 mol/g for H2and CO, respectively, with stable operation for over 150 h. Xue et al.[130]introduced surface Ovinto hematite nanobelts, promoting the formation of electron-rich Fe sites and the breaking of Fe—O bonds via photogenerated electrons, while also enhancing the photo-activation of OLthrough photogenerated holes; the activated O* reacts with CH4to generate the key intermediate CHxO*, efficiently promoting the oxidative dehydrogenation of CH4at low temperatures (Figure 11b). These works further reveal the intrinsic mechanism by which Ovsynergistically regulates OLactivity with photogenerated carriers under light irradiation.
In the reaction pathway of photoDRM, Ov, OLand the reactant molecules constitute a tightly coupled dynamic oxygen cycle network. Shoji et al.[131]prepared SrTiO3supported Rh catalysts (Rh/STO) using a hydrothermal method, finding that under light irradiation, OLin STO acts as a key oxygen transfer mediator: CO2is reduced by photogenerated electrons on Rh nanoparticles to generate O2-which embeds into the STO lattice, migrates to the interface, and reacts with carbon species derived from CH4to produce CO, while simultaneously promoting the oxidation of CH4, completing a full lattice oxygen OL-mediated cycle, asshown in Figure 12. In this process, Ov serves as a key intermediate, formed after OLmigration, and then filled by combination with O* generated from CO2. This process achieves conversion rates of both CH4and CO2exceeding 50% solely under UV irradiation. Zhang et al.[132]proposed a Ni0-NiO-Sr2Nb2O7ternary heterojunction catalyst, where photoexcitation effectively promotes the activation of O2Nb2O7in SrL, and the generated active O* efficiently eliminates carbon deposition, while Ovis regenerated via the MvK mechanism through the CO2reduction reaction, ensuring a continuous supply of active O*. This dynamic oxygen cycle mechanism achieves the oxidative dehydrogenation of CH4and the reduction conversion of CO2, further enhancing catalytic activity and stability.
Furthermore, Ov's dynamic characteristics are also manifested in the synergistic reconstruction between Ov-metal active sites and reactants. Xiong et al.[133]constructed dynamic Rh3lattice to build dynamic Rhδ+-Ov-Ti interfacial sites, exhibiting stability for up to 100 h under light irradiation. CH4dissociates at Rhδ+sites into CH3, which is then oxidized by OLto CH3O and further dehydrogenated; CO2dissociates at Ovinto CO and replenishes OL, completing the catalytic cycle. Both dynamic interfaces reflect the synergistic reconstruction between Ovand metal active centers. Furthermore, the generation and filling of Ovdepend not only on the consumption and conversion of CH4and CO2, but are also influenced by the transfer of photogenerated electrons to the metal.
In photocatalytic DRM, numerous strategies such as the regulation of nanophase separation structures, the "push-pull effect" between donors and acceptors in COF materials, and the influence of SMSI on the electronic state of metals, all point to a core issue—the efficient separation and utilization of photogenerated carriers, which is essential for achieving CH4, CO2efficient conversion under mild conditions. On one hand, separated electrons and holes migrate directionally to CO2and CH4, constructing an oxidation-reduction cycle; on the other hand, efficient carrier separation implies improved efficiency, enabling reactants to achieve single-pass conversion rates exceeding thermodynamic equilibrium. Furthermore, recent studies indicate that Ovis no longer a static defect, but a dynamic active site that is continuously generated under illumination, participates in reactions, and undergoes cyclic regeneration. Together with OLand active metals, it constitutes a dynamic oxygen circulation network that dominates the oxidative dehydrogenation of CH4and the reductive regeneration of CO2. However, purely light-driven DRM processes still face challenges such as insufficient visible light utilization and limited reaction rates. To overcome these limitations, photothermal synergistic catalysis strategies, which couple light energy with thermal energy, have emerged.
4.3 Photothermal synergistic catalysis for dry reforming of methane
As an innovative pathway, photothermal catalysis integrates the dual advantages of light and heat. In terms of energy supply and mechanism, photothermal catalytic DRM can be categorized into two types: light-assisted thermal catalysis and light-driven thermal catalysis. The former relies primarily on external heating as the driving force, with photogenerated carriers assisting in the catalytic process; the latter uses light energy as the sole or primary energy source, directly providing the required reaction heat through efficient photothermal effects to achieve truly solar-driven operation. Through the synergistic action of light and heat, photothermal catalytic DRM not only overcomes the limitations of high energy consumption and easy coking in traditional thermal catalysis but also addresses the bottlenecks of slow reaction rates and low efficiency in standalone photocatalysis. The photothermal synergy mechanism offers a technical pathway for DRM that combines high activity, high selectivity, and sustainability.
Tang et al.[134]prepared Ru/SrTiO with a 1 wt.% Ru loading using the hydrothermal method.3 catalyst. Under photothermal synergistic conditions, the DRM catalytic activity and stability of the Ru/SrTiO3 catalyst were significantly improved. Under external heating at 600 °C and irradiation with a 300 W xenon lamp, the H2 and CO yields reached 320.29 and 388.99 mmol/g/h, respectively, which is 1.4–1.5 times that of thermal catalysis. In a 30 h stability test at 600 °C, the decreases in H2 and CO yields were both less than 10%, demonstrating good stability. The enhancement in catalytic performance mainly stems from photo-induced electron transfer and the photothermal effect within the photothermal synergy, where charge transfer plays a key role. In situ XPS results show that under light irradiation, the binding energies of Sr 3d, Ti 2p, and O 1s shift positively, while those of Ru 3p3/2 and Ru 3d shift negatively, indicating electron transfer from SrTiO3 to Ru. As shown in Figure 13, light irradiation induces electron transfer from SrTiO3 to Ru, participating in the DRM reaction via interband transitions, effectively reducing the activation energy barriers for CH4 dissociation and H2 generation, and suppressing the RWGS reaction. Meanwhile, Ru loading promotes the formation of Ov, which not only serves as an effective electron trapping center to suppress photo-generated electron-hole recombination but also acts as an activation site for CO2, facilitating C=O bond cleavage and CO production, thereby effectively enhancing the performance of the DRM reaction.
In photothermal catalytic reactions, element doping can optimize the properties of catalyst supports, electronic structures, and reaction pathways to enhance reactant adsorption and improve catalyst stability. Zhang et al.[135]prepared atomically dispersed Ru1/Mg-CeO2-NR single-atom nanorod catalysts using an electrostatic adsorption method. Under 500 °C and 2.9 W/cm2irradiation, the generation rates of H2and CO reached 0.56 and 0.85 mol/gcat/h, respectively, which are 1.88 and 1.65 times higher than those under non-illuminated conditions. By regulating the morphology of the CeO2support combined with optimization via Mg doping, the UV absorption edge of the catalyst underwent a red shift, significantly enhancing absorbance across the full spectrum. Meanwhile, electron enrichment at Ru sites and increased Ovconcentration further improved the separation efficiency of photogenerated charges and surface reaction activity of the catalyst. In situ XPS and DFT calculations demonstrated that Ru-O-Ce and Ru-O-Mg structures facilitate the transfer of photogenerated electrons from the support to Ru active sites, lowering the dissociation energy barrier of CH4. Simultaneously, Mg doping effectively reduced the formation energy of Ovin the support, enhancing the adsorption and conversion of CO2. The oxygen circulation pathway (Ru-O-Ce → Ru-Ov-Ce) effectively suppressed carbon deposition, improving catalyst stability.
Zhang et al.[136]prepared Ni-doped MgO-supported Ni NPs catalysts (Ni/Ni-MgO) via a two-step method. Under UV-vis-IR light irradiation at 660 °C and 78.8 kW/m2, the H2and CO production rates reached 82.78 and 98.05 mmol/g/min, respectively. After a 93 h durability test, the H2and CO production rates decreased only slightly, and the catalyst exhibited an extremely low carbon deposition rate; compared to the undoped Ni/MgO catalyst, its carbon deposition rate was reduced by approximately 11.6 times. Following Ni2+doping into MgO, its 3d electron orbitals enhanced the catalyst's light absorption capability across the full spectrum. Meanwhile, the doped MgO, acting as a dielectric substrate, strengthened the localized surface plasmon resonance (LSPR) effect of the supported Ni NPs. The LSPR effect not only excites hot electrons to promote reactant activation but also generates a local thermal field via the photothermal effect, significantly increasing the catalyst surface temperature to achieve synergistic photothermal catalysis. Furthermore, CO-TPD and isotopic labeling experiments indicated that active oxygen in the Ni-doped MgO support can migrate to the interface between Ni NPs and MgO, participating in the oxidation of carbon species and thereby improving the catalytic performance of Ni/Ni-MgO.
Semimetals, with their unique electronic structures, high atomic utilization, and photothermal synergistic mechanisms, achieve efficient and stable catalytic reactions, providing a highly promising technical pathway for converting greenhouse gases into syngas using solar energy. Hu et al.[137]By mixing Ni(NO3)2 with Al2O3 and calcining, followed by reduction under UV-vis-IR irradiation in an H2/Ar gas flow, a nickel semimetal catalyst (hm-Ni/Al2O3) was successfully prepared, exhibiting a BET specific surface area of 132.1 m2/g and a Ni loading of 0.52 wt.%. Under an light intensity of 345.6 kW/m2, the catalyst reached an equilibrium temperature of 706 ℃, with specific generation rates for H2 and CO reaching as high as 8572.96 and 9614.26 mmol/min/gNi, respectively, representing an order-of-magnitude improvement compared to traditional Ni nanoparticle catalysts. Furthermore, no carbon deposition was detected during the reaction; after 70 hours of prolonged operation, the catalytic activity remained essentially unchanged, demonstrating excellent coking resistance and long-term stability. Experimental characterization and DFT calculations revealed that Ni is distributed on the Al2O3 support surface in the form of single-atomic-layer clusters. The band structure, magnetic properties, and electrical characteristics all confirm the semimetallic nature of this catalyst; this feature alters the d-electron state of Ni, causing a shift in the d-band center, thereby modifying the DRM kinetics such that the energy barrier for C oxidation is lower than that for polymerization, thus inhibiting coke deposition. Additionally, light irradiation promotes CO desorption and the oxidation of C species, while simultaneously modulating the catalyst's electronic structure to reduce the reaction activation energy.
Defects and Ov in catalytic materials not only provide active sites for reactions but also reduce electron-hole recombination, extend the lifetime of photogenerated carriers, and thereby enhance catalytic activity. Kushida et al.[138]loaded Rh nanoparticles onto protonated titanate nanobelts via photodeposition to prepare Rh/TiO2-B nanobelt photocatalysts. The results showed that under UV irradiation at 150 °C, the conversion rates of CH4and CO2were significantly improved, with H2and CO generation rates reaching 21.5 and 21.2 mmol/h/g, respectively, demonstrating excellent catalytic activity. Furthermore, the Rh/TiO2-B nanobelts maintained stable catalytic performance over a prolonged reaction period of 30 h. Asshown in Figure 14, TiO2-B nanobelts contain Ti3+defects and Ovwhich play a critical role in the catalytic reaction. The Ti3+defects act as electron trapping sites, reducing the probability of recombination between photogenerated electrons and holes, making the conduction band potential of TiO2-B more negative and enhancing the ability of electrons to transfer to Rh nanoparticles; Ovcan provide "channels" for charge separation and adsorb and store O ions generated from the reduction of CO2. The synergy between these two factors promotes efficient separation of photogenerated electrons and holes, thereby improving catalytic efficiency.
The synergistic effect between the active component and the support, as well as the active oxygen species in the support and Ovsignificantly promote the photothermal DRM reaction. To further investigate the role of support morphology and Ovin photothermal DRM, Pu et al.[139]prepared Ni catalysts supported on CeO2with different morphologies for photothermal DRM reactions. The study found that compared to Ni/CeO2-NS, Ni/CeO2-NR has surface OLand Ovthat are more easily activated, exhibiting stronger light absorption capacity and a narrower bandgap. Under an irradiation intensity of 1.61 W/cm2and a reaction temperature of 700 °C, the CH2-NR catalyst achieved CH4and CO2conversions of 67.9% and 81.7%, respectively, with corresponding reaction rates of 204 and 245 μmol/g/s, and coke deposition less than 0.5 wt.%. Analysis via various characterization techniques reveals that the superior performance of the Ni/CeO2-NR catalyst in photothermal catalytic DRM benefits from its unique structural properties and photothermal synergy. On one hand, photogenerated carriers produced by light irradiation promote the reaction process, and surface OLand Oveffectively suppress coke formation; on the other hand, strong metal-support interactions effectively restrict Ni particle sintering. This work provides new insights for addressing common issues of coke deposition and metal sintering in traditional DRM.
High-entropy oxides, by virtue of their unique metal precipitation-redissolution dynamic mechanism, can achieve light-induced Ovchemical cyclic regeneration, thereby effectively promoting the DRM reaction. Guo et al.[140]prepared a Rh-loaded CoNiFeZnCr-HEO nanocomposite catalyst via co-precipitation for photothermal catalytic DRM reactions. Studies show that under illumination, this HEO carrier can induce a dynamic "precipitation-redissolution" process in the CoNi alloy (Figure 15), triggering Ovchemical cyclic regeneration, thereby significantly enhancing the activation capability of CH4and CO2. Without additional heating and under a light intensity of 4.0 W/cm2, the surface temperature of the CoNiFeZnCr-HEO/Rh catalyst can reach approximately 480 °C, demonstrating good catalytic performance. The conversion rates of CH4and CO2were 59% and 53%, respectively, remaining stable during a 200-hour long-term test. The yields of H2and CO were 0.242 and 0.246 mol/g/h, respectively, with an H2/CO ratio of approximately 0.98. Results from in-situ XPS, DRIFTS, and DFT calculations indicate that the metal dynamic migration and Ovchemical cycling mechanism promoted by the high-entropy effect can effectively suppress carbon deposition formation and sintering of metal active centers, improving catalytic performance. This opens new pathways for the research and application of high-entropy catalysts in the field of photothermal catalytic DRM.
To further advance experimental research results toward practical applications, Rao et al.[141]based on dish concentrator technology, built an outdoor solar catalytic reaction system with autonomous sun-tracking capabilities for photothermal catalytic DRM. First, Ni1/CeO2(100) catalysts were prepared via hydrothermal and equal-volume impregnation methods. Characterizations such as HAADF-STEM and XAFS indicated that Ni was dispersed in single-atom form and formed a unique Ni-O2(100) coordination structure on the surface. Under conditions of no additional heating and an irradiance of 6.8 W/m4, the reaction temperature of the catalytic system could rise to 585 °C, with conversion rates of CH2 and CO4 both reaching over 93%, and the system operated continuously and stably for 800 h without catalyst deactivation. The excellent catalytic performance benefits from the bifunctional active Ni-O2 sites. On one hand, utilizing abundant coordinated oxygen to promptly capture CH4 species generated from methane dissociation, converting them into coke-resistant intermediate CH3O, thereby effectively blocking the carbon deposition pathway; on the other hand, under focused illumination, these sites can efficiently generate O3, which not only greatly promotes the adsorption and activation of COv to produce CO, but also enables the cyclic regeneration of coordinated oxygen, constructing an efficient, stable, and coke-resistant catalytic cycle system (2Figure 16a). In an undiluted reactant gas atmosphere (CH/CO4=1), during the optimal noon sunlight period, conversion rates of CH2 and CO4 reached approximately 16% (2Figure 16b, c). This result demonstrates the feasibility of this catalytic system for photothermal catalytic DRM under real solar light conditions.
A comparison of the performance of different catalysts in photothermal DRM is asshown in Table 3. Photothermal catalysis relies on the synergy between photochemical and thermochemical reaction pathways. Compared with traditional photocatalysis and thermocatalysis, it reduces energy consumption during the DRM process while significantly improving catalytic activity, selectivity, and stability. However, the mechanism by which solar energy alters reaction pathways in photothermal catalytic systems remains to be explored. Developing appropriate in situ characterization techniques to observe the evolution of reactant molecules on the catalyst surface during photothermal reactions is of great value for investigating the mechanisms of photothermal catalysis. Future efforts still require a deeper understanding of photothermal synergistic catalysis and the rational design of catalysts to enhance catalytic performance, thereby advancing toward industrial applications.
5 Conclusion and Outlook
DRM simultaneously converts CH4 and CO2, two greenhouse gases, into syngas, holding significant research value and application potential in the fields of energy conversion and carbon emission reduction. This paper systematically reviews the research progress of thermal catalysis, Joule heating catalysis, photocatalysis, and photothermal synergistic catalysis in DRM, deeply analyzing the influence patterns and reaction mechanisms of active components, promoters, and supports on catalytic performance and anti-coking performance. Based on the current research status and challenges, clear viewpoints and future development trends are proposed from three perspectives: activity enhancement strategies, anti-coking strategies, and the development direction of multi-energy driven systems.
(1) Strategies to enhance catalytic activity and stability: To address the issues of high-temperature sintering and activity decay in transition metal catalysts, technologies such as single-atom catalysis and spatial confinement by supports should be vigorously developed. Controlling the size of active metal particles to below 2 nm and achieving their high dispersion on the support can significantly improve the catalytic activity and stability of DRM. Meanwhile, bimetallic alloying and trace doping with noble metals are effective approaches to balance cost and performance. Introducing a second metal such as Fe, Co, or Cu to regulate the electronic structure of Ni can enhance surface oxygen affinity and promote CO2dissociation for oxygen supply; alternatively, adding trace amounts of noble metals such as Ru, Rh, or Pt to construct an "electron-rich active interface" can lower the activation energy barrier for C—H bond activation, thereby efficiently promoting the dissociation and conversion of reactants.
(2) Anti-coking catalyst design strategies: Coking is the core issue of DRM catalyst deactivation, and its suppression strategy should shift from static defect regulation to dynamic oxygen cycle management. Constructing high-concentration Ovand OLmigration channels is a powerful means to eliminate coking. By doping CeO2, ZrO2, La2O3and other variable-valence metal oxides, the migration rate of OLand the regeneration capacity of surface Ovare enhanced, forming a closed loop of "CO2adsorption → O* formation → coking elimination → Ovregeneration", which can effectively suppress coking generation. In addition, surface modification with alkali or alkaline earth metal promoters (such as MgO, CaO, and BaO) can enhance CO2adsorption capacity, generate carbonate species in situ, and eliminate coking through synergistic "adsorption-activation". Meanwhile, utilizing strong metal-support interactions or intermetallic compound-confined active centers reduces the formation of large-sized metal clusters, and regulating the metal d-band center via electron transfer can further slow down the nucleation and aggregation of carbon species.
(3) Development directions for multi-energy driven catalytic systems: Joule heating catalysis, photocatalysis, and photothermal synergistic catalysis represent the frontier development directions of DRM technology, yet each faces bottlenecks. Although Joule heating catalysis features high energy efficiency, rapid temperature rise, and a unique heating mode that suppresses carbon deposition, the mechanism by which current influences reaction pathways remains unclear. It is necessary to develop in-situ characterization techniques to further reveal the regulatory patterns of current on active centers and intermediate adsorption behaviors. Photocatalytic systems are limited by poor visible light response and low photon utilization efficiency; future efforts should focus on developing efficient catalysts driven by visible light. This can be achieved by expanding the light absorption range through band engineering (such as doping and solid solution construction) and designing heterojunctions or Schottky junctions to promote efficient spatial charge separation. The photothermal synergistic effect can effectively reduce reaction activation energy and suppress side reactions, thereby further improving the single-pass conversion rate of reactants and catalytic performance. However, it is necessary to clarify the contribution weights and action mechanisms of light-driven thermal catalysis versus light-assisted thermal catalysis. In addition to developing efficient light-thermal coupled reactors, future research could further construct multi-field coupled reaction systems, such as electric-light-thermal and magnetic-light-thermal coupled reaction systems, to enhance the catalytic performance and conversion efficiency of DRM reactions.
In summary, future breakthroughs in DRM catalysts depend on the deep integration of atomic-scale active center structure analysis, the evolution of intermediates and active sites under dynamic reaction conditions, coke elimination, and the design of multi-energy field coupled reaction systems. In recent years, artificial intelligence has adopted a "data-driven machine learning high-throughput experimentation" approach to replace the traditional experimental method of "looking for a needle in a haystack," further promoting catalyst design and optimization, and enabling the accurate screening of efficient catalysts[155-159]. Through the combination of artificial intelligence and multi-energy coupling-driven catalysis, it is expected to promote the transition of DRM technology from the laboratory to pilot-scale demonstration within 5 to 10 years, providing a green and efficient industrial pathway for the resource utilization of greenhouse gases and the supply of clean energy.
MalyshevS A, ShlyakhtinO A, LoktevA S, MazoG N, TimofeevG M, MukhinI E, KaplinI Yu, SvetogorovR D, ValeevR G, DedovA G. J. Solid State Chem., 2022, 312: 123267.
HussienA G S, DamaskinosC M, DabbawalaA A, AnjumD H, VasiliadesM A, KhaleelM T A, WehbeN, EfstathiouA M, PolychronopoulouK. Appl. Catal., B, 2022, 304: 121015.
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