Insights into the Nonthermal Effects of Light in Dry Reforming of Methane to Enhance the H2/CO Ratio Near Unity over Ni/Ga2O3

Sustaining syngas production at a near-unity H2/CO ratio in the photo-induced dry reforming of methane independent of the reactant gas composition

A syngas was generated at a near-unity H2/CO ratio independent of the reactant gas composition over Rh-loaded strontium titanate in the photo-induced dry reforming of methane. Moreover, light irradiation inhibited side reactions, such as the reverse water gas shift reaction and the precipitation of the solid-state carbon. These results were not explained by thermodynamic equilibrium, but were presumably owing to the unique effect of the photogenerated electron-hole pairs.

In situ DRIFTs-based comprehensive reaction mechanism of photo-thermal synergetic catalysis for dry reforming of methane over Ru-CeO2 catalyst

Solar-driven photo-thermal dry reforming of methane (DRM) is an environmentally friendly production route for high-value-added chemicals. However, the lack of thorough understanding of the mechanism for photo-thermal reaction has limited its further development. Here, we systematically investigated the mechanism of photo-thermal DRM reaction with the representative of Ru/CeO2 catalyst. Through in situ DRIFTs and transient experiments, comprehensive investigation into the reaction steps and their reactive sites in the process of DRM reaction were conducted. Besides, the excitation and migration direction of photo-electron was determined by ISI-XPS experiments, and the change of surface defect structure induced by light was characterized by ISI-EPR experiments. Based on the above results, the photo-enhancement effect on each micro-reaction step was determined. This study provides a theoretical basis for the industrialization of photo-thermal DRM reaction and its development of catalysts.

Photo-Thermal Dry Reforming of Methane with PGM-Free and PGM-Based Catalysts: A Review

Dry reforming of methane (DRM) is considered one of the most promising technologies for efficient greenhouse gas management thanks to the fact that through this reaction, it is possible to reduce CO2 and CH4 to obtain syngas, a mixture of H2 and CO, with a suitable ratio for the Fischer-Tropsch production of long-chain hydrocarbons. Two other main processes can yield H2 from CH4, i.e., Steam Reforming of Methane (SRM) and Partial Oxidation of Methane (POM), even though, not having CO2 as a reagent, they are considered less green. Recently, scientists' challenge is to overcome the many drawbacks of DRM reactions, i.e., the use of precious metal-based catalysts, the high temperatures of the process, metal particle sintering and carbon deposition on the catalysts' surfaces. To overcome these issues, one proposed solution is to implement photo-thermal dry reforming of methane in which irradiation with light is used in combination with heating to improve the efficiency of the process. In this paper, we review the work of several groups aiming to investigate the pivotal promoting role of light radiation in DRM. Focus is also placed on the catalysts' design and the progress needed for bringing DRM to an industrial scale.

Efficient dry reforming of methane realized by photoinduced acceleration of oxygen migration rate

Methane dry reforming (DRM) can consume greenhouse gases (CH4 and CO2) to produce valuable Fischer-Tropsch syngas (CO and H2). However, conventional thermally driven DRM consume large amounts of energy and face problems such as catalyst sintering and carbon deposition leading to insufficient catalytic activity. In this study, a photothermal synergistic TiO2/CeO2/Ru catalyst with high efficiency was designed. Under the light condition, the yields of H2 and CO reached 496.3 mmol g-1 h-1 and 522.4 mmol g-1 h-1, respectively. In addition, the catalyst demonstrated excellent stability after 100 h cyclic stability test. In-situ X-ray photoelectron spectroscopy (IS-XPS) and density functional theory (DFT) calculations revealed that the heterojunction interface formed by TiO2/CeO2/Ru is favourable for capturing photogenerated electrons and suppressing the recombination rate of photons and holes, thus improving the photocatalytic performance. Furthermore, light-induced metal-to-metal charge transfer (MMCT) accelerated oxygen migration, which not only improved the catalytic activity, but also suppressed the formation of carbon deposits on the catalyst surface, thereby enhancing the cycling stability. This study explores the mechanism of photothermally synergistic DRM, which provides a new pathway for the efficient use of solar energy.

Non-Photochemical Origin of Selectivity Difference between Light and Dark Catalytic Conditions

The interest in introducing light into heterogeneous catalysis is driven not only by the urgent need of replacing fossil energy but also by the promise of controlling product selectivity by light. The product selectivity differences observed in recent studies between light and dark reactions are often attributed to photochemical effects. Here, we report the discovery of a non-photochemical origin of selectivity difference, at essentially the same CO2 conversion rate, between photothermal and thermal CO2 hydrogenation reactions over a Ru/TiO2-x catalyst. While the presence of the photochemical effect from ultraviolet light is confirmed, it merely enhances the catalytic activity. Systematic investigation reveals that the gradual formation of an adsorbate-mediated strong metal-support interaction under catalytic conditions is responsible for the variation in the catalytic selectivity. We demonstrate that differences in product selectivity under light/dark reactions do not necessarily originate from photochemical effects. Our study refines the basis for determining photochemical effects and highlights the importance of excluding non-photochemical effects in mechanistic studies of light-controlled product selectivity.

Light-driven thermocatalytic CO2 reduction by CH4 on alumina-cluster-modified Ni nanoparticles with excellent durability and high light-to-fuel efficiency promoted by the photoactivation effect

Using inexhaustible solar energy to drive efficient light-driven thermocatalytic CO2 reduction by CH4 (DRM) is an attractive approach that can synchronously reduce the greenhouse effect and convert solar energy into fuels. However, it is often limited by the intense light intensity required to produce high fuel production rates, and the catalyst deactivation due to severe carbon deposition generated from side reactions. Herein, a nanostructure of alumina-cluster-modified Ni nanoparticles supported on Al2O3 nanorods (ACM-Ni/Al2O3) was synthesized, displaying good catalytic performance under focused UV-vis-IR illumination. By light-driven thermocatalytic DRM on ACM-Ni/Al2O3 at a reduced light intensity of 76.9 kW m-2, the high fuel production rates of H2 (rH2, 65.7 mmol g-1 min-1) and CO (rCO, 78.8 mmol g-1 min-1), as well as an efficient light-to-fuel efficiency (η, 26.3 %) are achieved without additional heating. The rH2 and rCO of light-driven thermocatalysis are 2.9 and 1.9 times higher, respectively, compared to conventional thermocatalysis at the same temperature. We have discovered that high light-driven thermocatalytic activity originates from the photoactivation effect, significantly reducing the apparent activation energy and facilitating C* oxidation as a decisive step in DRM. ACM-Ni/Al2O3 possesses excellent durability and exhibits an extremely low coking rate of 4.40 × 10-3 gc gcatalyst-1 h-1, which is 26.8 times lower than that of the reference sample without Al2O3 cluster modification (R-Ni/Al2O3). This is owing to a decrease in activation energies (Ea) of C* oxidation and an increase in Ea of C* polymerization by the surface modification of Ni nanoparticles with Al2O3 clusters, effectively inhibiting carbon deposition.

Product Peroxidation Inhibition in Methane Photooxidation into Methanol

Methane photooxidation into methanol offers a practical approach for the generation of high-value chemicals and the efficient storage of solar energy. However, the propensity for C─H bonds in the desired products to cleave more easily than those in methane molecules results in a continuous dehydrogenation process, inevitably leading to methanol peroxidation. Consequently, inhibiting methanol peroxidation is perceived as one of the most formidable challenges in the field of direct conversion of methane to methanol. This review offers a thorough overview of the typical mechanisms involved radical mechanism and active site mechanism and the regulatory methods employed to inhibit product peroxidation in methane photooxidation. Additionally, several perspectives on the future research direction of this crucial field are proposed.

Supra-Photothermal CO2 Methanation over Greenhouse-Like Plasmonic Superstructures of Ultrasmall Cobalt Nanoparticles

Improving the solar-to-thermal energy conversion efficiency of photothermal nanomaterials at no expense of other physicochemical properties, e.g., the catalytic reactivity of metal nanoparticles, is highly desired for diverse applications but remains a big challenge. Herein, a synergistic strategy is developed for enhanced photothermal conversion by a greenhouse-like plasmonic superstructure of 4 nm cobalt nanoparticles while maintaining their intrinsic catalytic reactivity. The silica shell plays a key role in retaining the plasmonic superstructures for efficient use of the full solar spectrum, and reducing the heat loss of cobalt nanoparticles via the nano-greenhouse effect. The optimized plasmonic superstructure catalyst exhibits supra-photothermal CO2 methanation performance with a record-high rate of 2.3 mol gCo -1 h-1 , close to 100% CH4 selectivity, and desirable catalytic stability. This work reveals the great potential of nanoscale greenhouse effect in enhancing photothermal conversions through the combination with conventional promoting strategies, shedding light on the design of efficient photothermal nanomaterials for demanding applications.

Light-Reinforced Key Intermediate for Anticoking To Boost Highly Durable Methane Dry Reforming over Single Atom Ni Active Sites on CeO2

Dry reforming of methane (DRM) has been investigated for more than a century; the paramount stumbling block in its industrial application is the inevitable sintering of catalysts and excessive carbon emissions at high temperatures. However, the low-temperature DRM process still suffered from poor reactivity and severe catalyst deactivation from coking. Herein, we proposed a concept that highly durable DRM could be achieved at low temperatures via fabricating the active site integration with light irradiation. The active sites with Ni-O coordination (NiSA/CeO2) and Ni-Ni coordination (NiNP/CeO2) on CeO2, respectively, were successfully constructed to obtain two targeted reaction paths that produced the key intermediate (CH3O*) for anticoking during DRM. In particular, the operando diffuse reflectance infrared Fourier transform spectroscopy coupling with steady-state isotopic transient kinetic analysis (operando DRIFTS-SSITKA) was utilized and successfully tracked the anticoking paths during the DRM process. It was found that the path from CH3* to CH3O* over NiSA/CeO2 was the key path for anticoking. Furthermore, the targeted reaction path from CH3* to CH3O* was reinforced by light irradiation during the DRM process. Hence, the NiSA/CeO2 catalyst exhibits excellent stability with negligible carbon deposition for 230 h under thermo-photo catalytic DRM at a low temperature of 472 °C, while NiNP/CeO2 shows apparent coke deposition behavior after 0.5 h in solely thermal-driven DRM. The findings are vital as they provide critical insights into the simultaneous achievement of low-temperature and anticoking DRM process through distinguishing and directionally regulating the key intermediate species.

Highly Efficient Solar-Driven Dry Reforming of Methane on a Rh/LaNiO3 Catalyst through a Light-induced Metal-To-Metal Charge Transfer Process

As an energy-saving and green method, solar-driven dry reforming of methane (DRM) is expected to introduce new activation processes and prevent sintering and coking of the catalysts. However, it still lacks an efficient way to coordinate the regulation of activation of reactants and lattice oxygen migration. In this study, Rh/LaNiO3 is designed as a highly efficient photothermal catalyst for solar-driven DRM, which performs production rates of 452.3 mmol h-1  gRh -1 for H2 and 527.6 mmol h-1  gRh -1 for CO2 under a light intensity of 1.5 W cm-2 , with an excellent stability. Moreover, a remarkable light-to-chemical energy efficiency (LTCEE) of 10.72% is achieved under a light intensity of 3.5 W cm-2 . The characterizations of surface electronic and chemical properties and theoretical analysis demonstrate that strong adsorption for CH4 and CO2 , light-induced metal-to-metal charge transfer (MMCT) process and high oxygen mobility together bring Rh/LaNiO3 excellent performance for solar-driven DRM.

Fast-Response Nickel-Promoted Indium Oxide Catalysts for Carbon Dioxide Hydrogenation from Intermittent Solar Hydrogen

Construction of a "net-zero-emission" system through CO2 hydrogenation to methanol with solar energy is an eco-friendly way to mitigate the greenhouse effect. Traditional CO2 hydrogenation demands centralized mass production for cost reduction with mass water electrolysis for hydrogen supply. To achieve continuous reaction with intermittent and fluctuating flow of H2 on a small-scale for distributed application scenarios, modulating the catalyst interface environment and chemical adsorption capacity to adapt fluctuating reaction conditions is highly desired. This paper describes a distributed clean CO2 utilization system in which the surface structure of catalysts is carefully regulated. The Ni catalyst with unsaturated electrons loaded on In2 O3 can reduce the dissociation energy of H2 to overcome the slow response of intermittent H2 supply, exhibiting a faster response (12 min) than bare oxide catalysts (42 min). Moreover, the introduction of Ni enhances the sensitivity of the catalyst to hydrogen, yielding a Ni/In2 O3 catalyst with a good performance at lower H2 concentrations with a 15 times adaptability for wider hydrogen fluctuation range than In2 O3 , greatly reducing the negative impact of unstable H2 supplies derived from renewable energies.

Interfacial C-S Bonds of g-C3 N4 /Bi19 Br3 S27 S-Scheme Heterojunction for Enhanced Photocatalytic CO2 Reduction

Step-scheme (S-scheme) heterojunctions have been extensively studied in photocatalytic carbon dioxide (CO₂ ) reduction due to their excellent charge separation and high redox ability. The built-in electric field at the interface of a S-scheme heterojunction serves as the driving force for charge transfer, however, the poor interfacial contact greatly restricts the carrier migration rate. Herein, we synthesized the g-C₃ N₄ /Bi₁₉ Br₃ S₂₇ S-scheme heterostructure through in situ deposition of Bi₁₉ Br₃ S₂₇ (BBS) on porous g-C₃ N₄ (P-CN) nanosheets. The C-S bonds formed at the interface help to enhance the built-in electric field, thereby promoting the charge transfer and separation. As a result, the CO₂ reduction reaction performance of 10 %Bi₁₉ Br₃ S₂₇ /g-C₃ N₄ (BBS/P-CN) reaches 32.78 μmol g^-1 h^-1 , which is 341.4 and 18.7 times higher than that of pure BBS and P-CN, respectively. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared spectroscopy (FTIR) prove the presence of chemical bonds (C-S) between the P-CN and BBS. The S-scheme charge-transfer mechanism was analyzed via XPS and density functional theory (DFT) calculations. This work provides a new idea for designing heterojunction photocatalysts with interfacial chemical bonds to achieve high charge-transfer and catalytic activity.

Mo2TiC2 MXene-Supported Ru Clusters for Efficient Photothermal Reverse Water-Gas Shift

Driving metal-cluster-catalyzed high-temperature chemical reactions by sunlight holds promise for the development of negative-carbon-footprint industrial catalysis, which has yet often been hindered by the poor ability of metal clusters to harvest and utilize the full spectrum of solar energy. Here, we report the preparation of Mo2TiC2 MXene-supported Ru clusters (Ru/Mo2TiC2) with pronounced broadband sunlight absorption ability and high sintering resistance. Under illumination of focused sunlight, Ru/Mo2TiC2 can catalyze the reverse water-gas shift (RWGS) reaction to produce carbon monoxide from the greenhouse gas carbon dioxide and renewable hydrogen with enhanced activity, selectivity, and stability compared to their nanoparticle counterparts. Notably, the CO production rate of MXene-supported Ru clusters reached 4.0 mol·gRu-1·h-1, which is among the best reported so far for photothermal RWGS catalysts. Detailed studies suggest that the production of methane is kinetically inhibited by the rapid desorption of CO from the surface of the Ru clusters.

Highly Active Ni-Ru Bimetallic Catalyst Integrated with MFI Zeolite-Loaded Cerium Zirconium Oxide for Dry Reforming of Methane

The dry reforming of methane (DRM) is a new potential technology that converts two major greenhouse gases into useful chemical feedstocks. The main challenge faced by this process is maintaining the catalyst with high catalytic activity and long-term stability. Here, a simple and effective preparation route for the synthesis of functional nanomolecular sieve catalysts (NiRu_xCZZ5) from kaolinite tailings was developed for dry reforming of methane with CO₂. The silica monoliths with flower-like spherical and micropore structures (ZSM-5) were prepared by crystal growth method, and the metal components were loaded by ultrasonic-assisted impregnation method. The NiRu_0.5CZZ5 catalyst exhibited excellent catalytic performance (maxmium CO₂ and CH₄ conversions up to 100 and 95.6%, respectively) and very good stability (up to 100h). The interfacial confinement and the strong support interaction are principally responsible for the excellent catalytic activity of the catalyst. The in situ DRIFTS was used to elucidate the possible carbon conversion steps, and stable surface intermediates were also identified.

Sustainable routes for acetic acid production: Traditional processes vs a low-carbon, biogas-based strategy

The conversion of biogas, mainly formed of CO₂ and CH₄, into high-value platform chemicals is increasing attention in a context of low-carbon societies. In this new paradigm, acetic acid (AA) is deemed as an interesting product for the chemical industry. Herein we present a fresh overview of the current manufacturing approaches, compared to potential low-carbon alternatives. The use of biogas as primary feedstock to produce acetic acid is an auspicious alternative, representing a step-ahead on carbon-neutral industrial processes. Within the spirit of a circular economy, we propose and analyse a new BIO-strategy with two noteworthy pathways to potentially lower the environmental impact. The generation of syngas via dry reforming (DRM) combined with CO₂ utilisation offers a way to produce acetic acid in a two-step approach (BIO-Indirect route), replacing the conventional, petroleum-derived steam reforming process. The most recent advances on catalyst design and technology are discussed. On the other hand, the BIO-Direct route offers a ground-breaking, atom-efficient way to directly generate acetic acid from biogas. Nevertheless, due to thermodynamic restrictions, the use of plasma technology is needed to directly produce acetic acid. This very promising approach is still in an early stage. Particularly, progress in catalyst design is mandatory to enable low-carbon routes for acetic acid production.

Quo Vadis Dry Reforming of Methane?—A Review on Its Chemical, Environmental, and Industrial Prospects

In recent years, the catalytic dry reforming of methane (DRM) has increasingly come into academic focus. The interesting aspect of this reaction is seemingly the conversion of CO2 and methane, two greenhouse gases, into a valuable synthesis gas (syngas) mixture with an otherwise unachievable but industrially relevant H2/CO ratio of one. In a possible scenario, the chemical conversion of CO2 and CH4 to syngas could be used in consecutive reactions to produce synthetic fuels, with combustion to harness the stored energy. Although the educts of DRM suggest a superior impact of this reaction to mitigate global warming, its potential as a chemical energy converter and greenhouse gas absorber has still to be elucidated. In this review article, we will provide insights into the industrial maturity of this reaction and critically discuss its applicability as a cornerstone in the energy transition. We derive these insights from assessing the current state of research and knowledge on DRM. We conclude that the entire industrial process of syngas production from two greenhouse gases, including heating with current technologies, releases at least 1.23 moles of CO2 per mol of CO2 converted in the catalytic reaction. Furthermore, we show that synthetic fuels derived from this reaction exhibit a negative carbon dioxide capturing efficiency which is similar to burning methane directly in the air. We also outline potential applications and introduce prospective technologies toward a net-zero CO2 strategy based on DRM.

Thermo-photo catalysis: a whole greater than the sum of its parts

Thermo-photo catalysis, which is the catalysis with the participation of both thermal and photo energies, not only reduces the large energy consumption of thermal catalysis but also addresses the low efficiency of photocatalysis. As a whole greater than the sum of its parts, thermo-photo catalysis has been proven as an effective and promising technology to drive chemical reactions. In this review, we first clarify the definition (beyond photo-thermal catalysis and plasmonic catalysis), classification, and principles of thermo-photo catalysis and then reveal its superiority over individual thermal catalysis and photocatalysis. After elucidating the design principles and strategies toward highly efficient thermo-photo catalytic systems, an ample discussion on the synergetic effects of thermal and photo energies is provided from two perspectives, namely, the promotion of photocatalysis by thermal energy and the promotion of thermal catalysis by photo energy. Subsequently, state-of-the-art techniques applied to explore thermo-photo catalytic mechanisms are reviewed, followed by a summary on the broad applications of thermo-photo catalysis and its energy management toward industrialization. In the end, current challenges and potential research directions related to thermo-photo catalysis are outlined.

Regulating the Spin State of Single Noble Metal Atoms by Hydroxyl for Selective Dehydrogenation of CH4 Direct Conversion to CH3OH

The key scientific challenge for methane (CH₄) direct conversion to methanol (CH₃OH) is considered to be the prevention of overoxidation of target products, which is restrained by the difficulty in the well-controlled process of selective dehydrogenation. Herein, we take single noble metal atom-anchored hexagonal boron nitride nanosheets with B vacancies (M_SA/B_1-xN) as the model materials and first propose that the dehydrogenation in the direct conversion of CH₄ to CH₃OH is highly dependent on the spin state of the noble metal. The results reveal that the noble metal with a higher spin magnetic moment is beneficial to the formation of the spin channels for electron transfer, which boosts the dissociation of C-H bonds. The promoted process of dehydrogenation will lead not only to the effective activation of CH₄ but also to the easy overoxidation of CH₃OH. More importantly, it is found that the spin state of noble metals can be regulated by the introduction of hydroxyl (OH), which realizes the selective dehydrogenation in the process of CH₄ direct conversion to CH₃OH. Among them, Ag_SA/B_1-xN exhibits the best performance owing to the dynamic regulation spin state of a single Ag atom by OH. On the one hand, the introduction of OH significantly reduces the energy barrier of C-H bond dissociation by the increase in the spin magnetic moment. On the other hand, the high spin magnetic moment of a single Ag atom during the process of subsequent dehydrogenation can be modulated to nearly zero. As a result, the spin channel for electron transfer between the adsorbed CH₃OH and reactive sites is broken, which hinders its overoxidation. This work opens a new path to designing catalysts for selective dehydrogenation by tuning the spin state of local electronic structures.

DFT-Based Microkinetic Model Analysis of Dry Reforming of Methane over Ru7/CeO2(111) and Ru7/CeO2(110): Key Role of Surface Lattice Oxygen Vacancy

CeO2 supported metal cluster catalysts play the vital roles on dry reforming (DRM) reaction which convert greenhouse gases (CH4 and CO2) to syngas, but the mechanism of surface lattice oxygen...

The selective production of CH4via photocatalytic CO2 reduction over Pd-modified BiOCl

The selective production of CH4via photocatalytic CO2 reduction was achieved over Pd-modified BiOCl.

Photothermal synergy for efficient dry reforming of CH4 by Ag/AgBr/CsPbBr3 composite

Dry reforming of CH4 by photothermal catalysis is considered to be a promising approach to produce syngas. It can not only store useful chemical energy, but also consume CO2 and...

Dual-Function Reaction Center for Simultaneous Activation of CH4 and O2 via Oxygen Vacancies during Direct Selective Oxidation of CH4 into CH3OH

The direct oxidation of methane (CH₄) to methanol (CH₃OH) has been a focus of global concern and is quite challenging due to the thermodynamically stable CH₄ and uncontrolled overoxidation of the products. Here, we provided a new viewpoint on the role of oxygen vacancies that induce a dual-function center in enhancing the adsorption and activation of both CH₄ and O₂ reactants for the subsequent selective formation of a CH₃OH liquid fuel on two-dimensional BiOCl photocatalysts at atmospheric pressure. The key for the favorable activity lies in the simultaneous ability of the electron-trapped Bi atom in activating CH₄ and the formation of ^•O₂^- radicals due to the activation of O₂ at the adjacent oxygen vacancy site, which immediately attack the activated CH₄ to directly produce CH₃OH, in initiating the oxidation reaction. What is more, the relatively low reaction barriers and the easy desorption of CH₃OH concertedly facilitate the highly selective conversion of CH₄ up to 85 μmol of CH₃OH, with a small amount of CO₂ and CO as the byproducts over the BiOCl nanosheets with an oxygen vacancy concentration of 2.4%. This work could broaden the avenue toward the application of oxygen-defective metal oxides in the photocatalytic selective conversion of CH₄ to CH₃OH and offer a disparate perspective on the role of oxygen vacancy acting as the surface electron transfer channel in enhancing the photocatalytic performance.