Unravelling the Role of Oxygen Vacancies in the Mechanism of the Reverse Water–Gas Shift Reaction by Operando DRIFTS and Ultraviolet–Visible Spectroscopy

Solid Solution Derived Cu Clusters on Partially Reduced CuCeO2 with Abundant Oxygen Vacancies Enable Efficient Reverse Water Gas Reaction

The reverse water gas shift (RWGS) reaction provides a convenient approach to convert CO2 to CO, which facilitates to achieve the goals of carbon peaking and carbon neutrality. Herein, the Cu/CeO2 catalyst prepared by a co-precipitation method using a mixture of Na2CO3 and NaOH at pH of 10 (sample Cu/CeO2-10) achieved an intrinsic reaction rate of 428.4 mmol ⋅ gcat -1 ⋅ h-1 with 100 % CO selectivity at 400 °C and CO2/H2 ratio of 1 : 4, which is much higher than Cu/CeO2 prepared by impregnation and other methods. Various characterizations showed the highest fraction of CuCeO2 solid solution in the calcined Cu/CeO2-10, and formed highly dispersed Cu clusters (~2.5 nm) on partially reduced CuCeO2 solid solution with abundant of oxygen vacancies upon reduction. The Cu and oxygen vacancies facilitates the activation of H2 and CO2, respectively, resulting in lowered H2 and CO2 reaction orders. As a result, the synergy between the two components enhanced the overall RWGS activity with lowered activation energy. Moreover, the optimal catalyst is very stable in 24 h stability test without detectable agglomeration of Cu clusters.

Irreversible Lattice Expansion Effects in Nanoscale Indium Oxide for CO2 Hydrogenation Catalysis

Thermal energy has been considered the exclusive driving force in thermochemical catalysis, yet associated lattice expansion effects have been overlooked. To shed new light on this issue, variable temperature in situ high-resolution (scanning) transmission electron microscopy (HR-(S)TEM) and electron energy-loss spectroscopy (EELS) were employed to provide detailed information on the structural changes of an archetype nanoscale indium oxide materials and how these effects are manifest in reverse water gas shift heterogeneous catalytic reactivity. It is found that with increasing temperature and vacuum conditions, an irreversible surface lattice expansion is traced to the formation and migration of oxygen vacancies. Together, these changes are believed to be responsible for the decreased activation energy and improved reaction rate observed for the reverse water gas shift reaction. Studies of this kind provide new insight into how thermal energy affects thermochemical heterogeneous catalysis.

Light-Assisted Catalysis and the Dynamic Nature of Surface Species in the Reverse Water Gas Shift Reaction over Cu/γ-Al2O3

The reverse water gas shift (RWGS) reaction converts CO2 and H2 into CO and water. We investigated Cu/γ-Al2O3 catalysts in both thermally driven and light-assisted RWGS reactions using visible light. When driven by combined visible light and thermal energy, the CO2 conversion rates were lower than in the dark. Light-assisted reactions showed an increase in the apparent activation energy from 68 to 87 kJ/mol, indicating that light disrupts the energetically favorable pathway active in the dark. A linear correlation between irradiance and decreasing reaction rate suggests a photon-driven phenomenon. In situ diffuse reflectance infrared Fourier transform spectroscopy and TD-DFT analyses revealed that catalyst illumination causes significant, partly irreversible surface dehydroxylation, highlighting the importance of OH groups in the most favorable RWGS pathway. This study offers a novel approach to manipulate surface species and control activity in the RWGS reaction.

Rational H2 Partial Pressure over Nickel/Ceria Crystal Enables Efficient and Durable Wide-Temperature-Zone Air-Level CO2 Methanation

On the way to carbon neutrality, directly catalyzing atmospheric CO2 into high-value chemicals might be an effective approach to mitigate the negative impacts of rising airborne CO2 concentrations. Here, we pioneer the investigation of the influence of the H2/CO2 partial pressure ratio (PPR) on air-level CO2 methanation. Using Ni/CeO2 as a case catalyst, increasing H2/CO2 PPR significantly improves low-temperature CO2 conversion and high-temperature CH4 selectivity, i. e., from 10 of H2/CO2 PPR on, CO2 is completely methanized at 250 °C, and nearly 100 % CH4 selectivity is achieved at 400 °C. 100-hour stability tests demonstrate the practical application potential of Ni/CeO2 at 250 °C and 400 °C. In-situ DRIFTS reveal that reinforced formate pathway by increasing H2/CO2 PPR is responsible for the high CH4 yield. In contrast, even though the CO pathway dominated CO2 conversion on Ni is enhanced by rising H2/CO2 PPR, but at a high reaction temperature, the promoted CO desorption still leads to lower CH4 selectivity. This work offers deep insights into the direct air-level CO2 resourceization, contributing to the achievement of airborne CO2 reductions.

Light-Driven Reverse Water Gas Shift Reaction with 1000-H Stability on High-Entropy Alloy Catalysts

Highly stable and active catalysts are of significant importance and a longstanding challenge for a number of industrial chemical transformations. Here, motivated by the principle of the high entropy-stabilized structure, high-entropy alloy-loaded porous TiO2 as an efficient and sintering-resistant catalyst for the light-driven reverse water gas‒shift reaction without external heating is synthesized. The optimized CoNiCuPdRu/TiO2 catalyst exhibits a long-term stability of 1000 h (1.23 mol gmetal -1 h-1 CO production rate, >99% high selectivity). In situ characterizations confirm that the slow diffusion effect of high-entropy alloys endows the catalyst with excellent structural stability. The CO adsorption measurements and theoretical calculations consolidate that the hydrogen surface coverage weakens CO adsorption on the catalyst surface. Two major problems of catalyst deactivation - sintering and poisoning, are handled in one case, which synergistically enable unparalleled stability. This work provides new guidance for the rational design of ultradurable harsh-condition operation catalysts for industrial catalysis.

Direct cleavage of C=O double bond in CO2 by the subnano MoOx surface on Mo2N

Compared to H2-assisted activation mode, the direct dissociation of CO2 into carbonyl (*CO) with a simplified reaction route is advantageous for CO2-related synthetic processes and catalyst upgrading, while the stable C = O double bond makes it very challenging. Herein, we construct a subnano MoO3 layer on the surface of Mo2N, which provides a dynamically changing surface of MoO3↔MoOx (x < 3) for catalyzing CO2 hydrogenation. Rich oxygen vacancies on the subnano MoOx surface with a high electron donating capacity served as a scissor to directly shear the C = O double bond of CO2 to form CO at a high rate. The O atoms leached in CO2 dissociation are removed timely by H2 to regenerate active oxygen vacancies. Owing to the greatly enhanced dissociative activation of CO2, this MoOx/Mo2N catalyst without any supported active metals shows excellent performance for catalyzing CO2 hydrogenation to CO. The construction of highly disordered defective surface on heterostructures paves a new pathway for molecule activation.

Boosting photo-thermal co-catalysis CO2 methanation by tuning interface electron transfer via Mott-Schottky heterojunction effect

Photo-thermal co-catalytic reduction of CO2 to synthesize value-added chemicals presents a promising approach to addressing environmental issues. Nevertheless, traditional catalysts exhibit low light utilization efficiency, leading to the generation of a reduced number of electron-hole pairs and rapid recombination, thereby limiting catalytic performance enhancement. Herein, a Mott-Schottky heterojunction catalyst was developed by incorporating nitrogen-doped carbon coated TiO2 supported nickel (Ni) nanometallic particles (Ni/x-TiO2@NC). The optimal Ni/0.5-TiO2@NC sample displayed a conversion rate of 71.6 % and a methane (CH4) production rate of 65.3 mmol/(gcat·h) during photo-thermal co-catalytic CO2 methanation under full-spectrum illumination, with a CH4 selectivity exceeding 99.6 %. The catalyst demonstrates good stability as it shows no decay after two reaction cycles. The Mott-Schottky heterojunction catalysts display excellent efficiency in separating photo-generated electron-hole pairs and elevate the catalysts' temperature, thus accelerating the reaction rate. The in-situ experiments revealed that light-induced electron transfer effectively facilitates H2 dissociation and enhances surface defects, thereby promoting CO2 adsorption. This study introduces a novel approach for developing photo-thermal catalysts for CO2 reduction, aiming to enhance solar energy utilization and facilitate interface electron transfer.

Unmatched Redox Activity of the Palladium-Doped Indium Oxide Oxygen Carrier for Low-Temperature CO2 Splitting

The chemical conversion of CO2 into value-added products is the key technology to realize a carbon-neutral society. One representative example of such conversion is the reverse water-gas shift reaction, which produces CO from CO2. However, the activity is insufficient at ambient pressure and lower temperatures (<600 °C), making it a highly energy-intensive and impractical process. Herein, we report indium oxide nanofibers modified with palladium catalysts that exhibit significantly potent redox activities toward the reduction of CO2 splitting via chemical looping. In particular, we uncover that the doped palladium cations are selectively reduced and precipitated onto the host oxide surface as metallic nanoparticles. These catalytic gems formed operando make In2O3 lattice oxygen more redox-active in H2 and CO2 environments. As a result, the composite nanofiber catalysts demonstrate the reverse water-gas shift reaction via chemical looping at record-low temperatures (≤350 °C), while also imparting high activities (CO2 conversion: 45%). Altogether, our findings expand the viability of CO2 splitting at lower temperatures and provide design principles for indium oxide-based catalysts for CO2 conversion.

Developing a Multifunctional Cathode for Photoassisted Lithium-Sulfur Battery

Integration of solar cell and secondary battery cannot only promote solar energy application but also improve the electrochemical performance of battery. Lithium-sulfur battery (LSB) is an ideal candidate for photoassisted batteries owing to its high theoretical capacity. Unfortunately, the researches related the combination of solar energy and LSB are relatively lacking. Herein, a freestanding photoelectrode is developed for photoassisted lithium-sulfur battery (PALSB) by constructing a heterogeneous structured Au@N-TiO2 on carbon cloths (Au@N-TiO2/CC), which combines multiple advantages. The Au@N-TiO2/CC photoelectrode can produce the photoelectrons to facilitate sulfur reduction during discharge process, while generating holes to accelerate sulfur evolution during charge process, improving the kinetics of electrochemical reactions. Meanwhile, Au@N-TiO2/CC can work as an electrocatalyst to promote the conversion of intermediate polysulfides during charge/discharge process, mitigating induced side reactions. Benefiting from the synergistic effect of electrocatalysis and photocatalysis, PALSB assembled with an Au@N-TiO2/CC photoelectrode obtains ultrahigh specific capacity, excellent rate performance, and outstanding cycling performance. What is more, the Au@N-TiO2/CC assembled PALSB can be directly charged under light illumination. This work not only expands the application of solar energy but also provides a new insight to develop advanced LSBs.

In-Situ Characterization Techniques for Mechanism Studies of CO2 Hydrogenation

The paramount concerns of global warming, fossil fuel depletion, and energy crises have prompted the need of hydrocarbons productions via CO2 conversion. In order to achieve global carbon neutrality, much attention needs to be diverted towards CO2 management. Catalytic hydrogenation of CO2 is an exciting opportunity to curb the increasing CO2 and produce value-added products. However, the comprehensive understanding of CO2 hydrogenation is still a matter of discussion due to its complex reaction mechanism and involvement of various species. This review comprehensively discusses three processes: reverse water gas shift (RWGS) reaction, modified Fischer Tropsch synthesis (MFTS), and methanol-mediated route (MeOH) for CO2 hydrogenation to hydrocarbons. Along with analysing the reaction pathways, it is also very important to understand the real-time evolvement of catalytic process and reaction intermediates by employing in-situ characterization techniques under actual reaction conditions. Subsequently, in second part of this review, we provided a systematic analysis of advancements in in-situ techniques aimed to monitor the evolution of catalysts during CO2 reduction process. The section also highlights the key components of in-situ cells, their working principles, and applications in identifying reaction mechanisms for CO2 hydrogenation. Finally, by reviewing respective achievements in the field, we identify key gaps and present some future directions for CO2 hydrogenation and in-situ studies.

Hydrogenation of CO2 into Value-added Chemicals Using Solid-Supported Catalysts

Reducing CO2 emissions is an urgent global priority. In this context, several mitigation strategies, including CO2 tax and stringent legislation, have been adopted to halt the deterioration of the natural environment. Also, carbon recycling procedures undoubtedly help reduce net emissions into the atmosphere, enhancing sustainability. Utilizing Earth's abundant CO2 to produce high-potential green chemicals and light fuels opens new avenues for the chemical industry. In this context, many attempts have been devoted to converting CO2 as a feedstock into various value-added chemicals, such as CH4, lower methanol, light olefins, gasoline, and higher hydrocarbons, for numerous applications involving various catalytic reactions. Although several CO2-conversion methods have been used, including electrochemical, photochemical, and biological approaches, the hydrogenation method allows the reaction to be tuned to produce the targeted compound without significantly altering infrastructure. This review discusses the numerous hydrogenation routes and their challenges, such as catalyst design, operation, and the combined art of structure-activity relationships for the various product formations.

Polyoxometalate-HKUST-1 composite derived nanostructured Na-Cu-Mo2C catalyst for efficient reverse water gas shift reaction

Transforming CO2 to CO via reverse water-gas shift (RWGS) reaction is widely regarded as a promising technique for improving the efficiency and economics of CO2 utilization processes. Moreover, it is also considered as a pathway towards e-fuels. Cu-oxide catalysts are widely explored for low-temperature RWGS reactions; nevertheless, they tend to deactivate significantly under applied reaction conditions due to the agglomeration of copper particles at elevated temperatures. Herein, we have synthesized homogeneously distributed Cu metallic nanoparticles supported on Mo2C for the RWGS reaction by a unique approach of in situ carburization of metal-organic frameworks (MOFs) using a Cu-based MOF i.e. HKUST-1 encapsulating molybdenum-based polyoxometalates. The newly derived Na-Cu-Mo2C nanocomposite catalyst system exhibits excellent catalytic performance with a CO production rate of 3230.0 mmol gcat-1 h-1 with 100% CO selectivity. Even after 250 h of a stability test, the catalyst remained active with more than 80% of its initial activity.

Role of Atomicity and Interface on InOx-TiO2 Composites: Thermo-Photo Valorization of CO2

The synthesis, physicochemical, and functional properties of composite solids resulting from the surface spread of oxidized indium species onto nanoplatelets of anatase were investigated. Both the size and the interaction between the indium- and titanium-containing components control the functional properties. In the reduction of CO2 to CO, the best samples have an indium content between ca. 2 and 5 mol % and showed an excess rate over the photo and thermo-alone processes above 33% and an energy efficiency of 1.3%. Subnanometric (monomeric and dimeric) indium species present relatively weak thermal catalytic response but strong thermo-photo promotion of the activity. A gradual change in functional properties was observed with the growth of the indium content of the solids, leading to a progressive increase of thermal activity but lower thermo-photo promotion. The study provides a well-defined structure-activity relationship rationalizing the dual thermo-photo properties of the catalysts and establishes a guide for the development of highly active and stable composite solids for the elimination and valorization of CO2.

Highly Efficient and Selective Light-Driven Dry Reforming of Methane by a Carbon Exchange Mechanism

Dry reforming of methane (DRM) is a promising technique for converting greenhouse gases (namely, CH4 and CO2) into syngas. However, traditional thermocatalytic processes require high temperatures and suffer from low selectivity and coke-induced instability. Here, we report high-entropy alloys loaded on SrTiO3 as highly efficient and coke-resistant catalysts for light-driven DRM without a secondary source of heating. This process involves carbon exchange between reactants (i.e., CO2 and CH4) and oxygen exchange between CO2 and the lattice oxygen of supports, during which CO and H2 are gradually produced and released. Such a mechanism deeply suppresses the undesired side reactions such as reverse water-gas shift reaction and methane deep dissociation. Impressively, the optimized CoNiRuRhPd/SrTiO3 catalyst achieves ultrahigh activity (15.6/16.0 mol gmetal-1 h-1 for H2/CO production), long-term stability (∼150 h), and remarkable selectivity (∼0.96). This work opens a new avenue for future energy-efficient industrial applications.

K-guided selective regulation mechanism for CO2 hydrogenation over Ni/CeO2 catalyst

Regulating the selectivity between CO and CH4 during CO2 hydrogenation is a challenging research topic. Previous research has indicated that potassium (K) modification can adjust the product selectivity by regulating the adsorption strength of formate/CO* intermediates. Going beyond the regulation mechanism described above, this study proposes a K-guided selectivity control method based on the regulation of key intermediates HCO*/H3CO* for Ni catalysts supported on reducible carrier CeO2. By incorporating K, the CO selectivity of CO2 hydrogenation shifts from around 25.4% for Ni/CeO2 to approximately 93.8% for Ni/CeO2-K. This can be attributed to K modification causes electron aggregation in the bonding regions of HCO* and H3CO* intermediates, thus enhancing their adsorption strength. Consequently, the reaction pathway from HCO*/H3CO* to CH4 is limited, favoring the decomposition of formates to CO products. Moreover, the addition of K leads to a moderate decrease in CO2 conversion from 55.2% to 48.6%, which still surpasses values reported in most other studies. This reduction is associated with a decline in reducible Ni species and oxygen vacancy concentration in Ni/CeO2-K. As a result, the adsorption capacity for CO2 and H2 reduces, ultimately reducing CO2 hydrogenation activity.

In Situ UV-Vis-NIR Absorption Spectroscopy and Catalysis

This review highlights in situ UV-vis-NIR range absorption spectroscopy in catalysis. A variety of experimental techniques identifying reaction mechanisms, kinetics, and structural properties are discussed. Stopped flow techniques, use of laser pulses, and use of experimental perturbations are demonstrated for in situ studies of enzymatic, homogeneous, heterogeneous, and photocatalysis. They access different time scales and are applicable to different reaction systems and catalyst types. In photocatalysis, femto- and nanosecond resolved measurements through transient absorption are discussed for tracking excited states. UV-vis-NIR absorption spectroscopies for structural characterization are demonstrated especially for Cu and Fe exchanged zeolites and metalloenzymes. This requires combining different spectroscopies. Combining magnetic circular dichroism and resonance Raman spectroscopy is especially powerful. A multitude of phenomena can be tracked on transition metal catalysts on various supports, including changes in oxidation state, adsorptions, reactions, support interactions, surface plasmon resonances, and band gaps. Measurements of oxidation states, oxygen vacancies, and band gaps are shown on heterogeneous catalysts, especially for electrocatalysis. UV-vis-NIR absorption is burdened by broad absorption bands. Advanced analysis techniques enable the tracking of coking reactions on acid zeolites despite convoluted spectra. The value of UV-vis-NIR absorption spectroscopy to catalyst characterization and mechanistic investigation is clear but could be expanded.

Operando UV-vis spectroscopy for real-time monitoring of nanoparticle size in reaction conditions: a case study on rWGS over Au nanoparticles

We propose the use of surface plasmon resonance (SPR) as a distinctive marker for real-time monitoring in reaction conditions of gold nanoparticles supported on α-Al2O3. The study leverages the SPR shape-and-size dependency to monitor metal nanoparticles in reaction conditions, evidencing an influence of both dimensions and agglomerations on the SPR peak position. Operando measurements, coupling UV-vis spectroscopy and catalytic testing, allows to follow the dynamics during nanoparticle formation (Au3+ to Au0 reduction) and during the reverse water gas shift reaction (CO2 + H2 → CO + H2O). The catalyst structure and stability in reaction conditions was further confirmed by operando X-ray spectroscopy and PXRD data. Overall, this approach enables the direct acquisition of information on the structure-activity relationship of metal-based supported catalysts under actual reaction conditions.

Refining the mechanism of CO2 and H2 activation over gold-ceria catalysts by IR modulation excitation spectroscopy

The activation and utilization of the greenhouse gas CO2 is of great interest for the energy transition as a fossil-free carbon source for mitigating climate change. CO2 hydrogenation via the reverse water-gas shift reaction (RWGSR) converts CO2 to CO, a crucial component of syngas, enabling further transformation by means of the Fischer-Tropsch process. In this study, we unravel the detailed mechanism of the RWGSR on low-loaded Au/CeO2 catalysts using IR modulation excitation spectroscopy (MES), by periodically modulating the concentration of the reactants, followed by phase-sensitive detection (PSD). Applying such a MES-PSD approach to Au/CeO2 catalysts during RWGSR gives direct spectroscopic evidence for the active role of gold hydride, bidentate carbonate and hydroxyl species in the reaction mechanism, while disproving the participation of other species such as formate. Our results highlight the potential of modulation excitation spectroscopy combined with phase-sensitive detection to provide new mechanistic insight into catalytic reactions not accessible by steady-state techniques, including a profound understanding of the sequence of reaction steps.

Recent Advances in Hydrogenation of CO2 to CO with Heterogeneous Catalysts Through the RWGS Reaction

With the continuous increase in CO2 emissions, primarily from the combustion of coal and oil, the ecosystem faces a significant threat. Therefore, as an effective method to minimize the issue, the Reverse Water Gas Shift (RWGS) reaction which converts CO2 towards CO attracts much attention, is an environmentally-friendly method to mitigate climate change and lessen dependence on fossil fuels. Nevertheless, the inherent thermodynamic stability and kinetic inertness of CO2 is a big challenge under mild conditions. In addition, it remains another fundamental challenge in RWGS reaction owing to CO selectivity issue caused by CO2 further hydrogenation towards CH4 . Up till now, a series of catalysis systems have been developed for CO2 reduction reaction to produce CO. Herein, the research progress of the well-performed heterogeneous catalysts for the RWGS reaction were summarized, including the catalyst design, catalytic performance and reaction mechanism. This review will provide insights into efficient utilization of CO2 and promote the development of RWGS reaction.

Mechanism Study of Organic Sulfur Hydrogenation over Pt- and Pd-Loaded Alumina-Based Catalysts

The hydrogenation of organic sulfur (CS2) present in industrial off-gases to produce sulfur-free hydrocarbons and H2S can be achieved by using noble-metal catalysts. However, there has been a lack of comprehensive investigation into the underlying reaction mechanisms associated with this process. In this study, we have conducted an in-depth examination of the activity and selectivity of Pt- and Pd-loaded alumina-based catalysts, revealing significant disparities between them. Notably, Pd/Al2O3 catalysts exhibit an enhanced performance at low temperatures. Furthermore, we have observed that CS2 displays a higher propensity for conversion to methane when employing Pt/Al2O3 catalysts, while Pd/Al2O3 catalysts demonstrate a greater tendency for coke deposition. By combining experimental observations with theoretical calculations, we revealed that the capability of H2 spillover along with the adsorption capacity of CS2, play pivotal roles in determining the observed differences. Moreover, the key intermediate species involved in the methanation and coke pathways were identified. The intermediate CH2S* is found to be crucial in the methanation pathway, while the intermediate CSH* is identified as significant in the coke pathway.

Accelerated discovery of multi-elemental reverse water-gas shift catalysts using extrapolative machine learning approach

Designing novel catalysts is key to solving many energy and environmental challenges. Despite the promise that data science approaches, including machine learning (ML), can accelerate the development of catalysts, truly novel catalysts have rarely been discovered through ML approaches because of one of its most common limitations and criticisms-the assumed inability to extrapolate and identify extraordinary materials. Herein, we demonstrate an extrapolative ML approach to develop new multi-elemental reverse water-gas shift catalysts. Using 45 catalysts as the initial data points and performing 44 cycles of the closed loop discovery system (ML prediction + experiment), we experimentally tested a total of 300 catalysts and identified more than 100 catalysts with superior activity compared to those of the previously reported high-performance catalysts. The composition of the optimal catalyst discovered was Pt(3)/Rb(1)-Ba(1)-Mo(0.6)-Nb(0.2)/TiO2. Notably, niobium (Nb) was not included in the original dataset, and the catalyst composition identified was not predictable even by human experts.

Platinum and Frustrated Lewis Pairs on Ceria as Dual-Active Sites for Efficient Reverse Water-Gas Shift Reaction at Low Temperatures

The low-temperature reverse water-gas shift (RWGS) reaction faces the following obstacles: low activity and unsatisfactory selectivity. Herein, the dual-active sites of platinum (Pt) clusters and frustrated Lewis pair (FLP) on porous CeO2 nanorods (Ptcluster /PN-CeO2 ) provide an interface-independent pathway to boost high performance RWGS reaction at low temperatures. Mechanistic investigations illustrate that Pt clusters can effectively activate and dissociate H2 . The FLP sites, instead of the metal and support interfaces, not only enhance the strong adsorption and activation of CO2 , but also significantly weaken CO adsorption on FLP to facilitate CO release and suppress the CH4 formation. With the help of hydrogen spillover from Pt to PN-CeO2 , the Ptcluster /PN-CeO2 catalysts achieved a CO yield of 29.6 %, which is very close to the thermodynamic equilibrium yield of CO (29.8 %) at 350 °C. Meanwhile, the Ptcluster /PN-CeO2 catalysts delivered a large turnover frequency of 8720 h-1 . Moreover, Ptcluster /PN-CeO2 operated stably and continuously for at least 840 h. This finding provides a promising path toward optimizing the RWGS reaction.

Bifunctionality of Re Supported on TiO2 in Driving Methanol Formation in Low-Temperature CO2 Hydrogenation

Low temperature and high pressure are thermodynamically more favorable conditions to achieve high conversion and high methanol selectivity in CO2 hydrogenation. However, low-temperature activity is generally very poor due to the sluggish kinetics, and thus, designing highly selective catalysts active below 200 °C is a great challenge in CO2-to-methanol conversion. Recently, Re/TiO2 has been reported as a promising catalyst. We show that Re/TiO2 is indeed more active in continuous and high-pressure (56 and 331 bar) operations at 125-200 °C compared to an industrial Cu/ZnO/Al2O3 catalyst, which suffers from the formation of methyl formate and its decomposition to carbon monoxide. At lower temperatures, precise understanding and control over the active surface intermediates are crucial to boosting conversion kinetics. This work aims at elucidating the nature of active sites and active species by means of in situ/operando X-ray absorption spectroscopy, Raman spectroscopy, ambient-pressure X-ray photoelectron spectroscopy (AP-XPS), and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Transient operando DRIFTS studies uncover the activation of CO2 to form active formate intermediates leading to methanol formation and also active rhenium carbonyl intermediates leading to methane over cationic Re single atoms characterized by rhenium tricarbonyl complexes. The transient techniques enable us to differentiate the active species from the spectator one on TiO2 support, such as less reactive formate originating from spillover and methoxy from methanol adsorption. The AP-XPS supports the fact that metallic Re species act as H2 activators, leading to H-spillover and importantly to hydrogenation of the active formate intermediate present over cationic Re species. The origin of the unique reactivity of Re/TiO2 was suggested as the coexistence of cationic highly dispersed Re including single atoms, driving the formation of monodentate formate, and metallic Re clusters in the vicinity, activating the hydrogenation of the formate to methanol.

Stabilizing Co2C with H2O and K promoter for CO2 hydrogenation to C2+ hydrocarbons

The decomposition of cobalt carbide (Co₂C) to metallic cobalt in CO₂ hydrogenation results in a notable drop in the selectivity of valued C₂₊ products, and the stabilization of Co₂C remains a grand challenge. Here, we report an in situ synthesized K-Co₂C catalyst, and the selectivity of C₂₊ hydrocarbons in CO₂ hydrogenation achieves 67.3% at 300°C, 3.0 MPa. Experimental and theoretical results elucidate that CoO transforms to Co₂C in the reaction, while the stabilization of Co₂C is dependent on the reaction atmosphere and the K promoter. During the carburization, the K promoter and H₂O jointly assist in the formation of surface C^* species via the carboxylate intermediate, while the adsorption of C^* on CoO is enhanced by the K promoter. The lifetime of the K-Co₂C is further prolonged from 35 hours to over 200 hours by co-feeding H₂O. This work provides a fundamental understanding toward the role of H₂O in Co₂C chemistry, as well as the potential of extending its application in other reactions.

Defects Tune the Strong Metal-Support Interactions in Copper Supported on Defected Titanium Dioxide Catalysts for CO2 Reduction

A highly active and stable Cu-based catalyst for CO₂ to CO conversion was demonstrated by creating a strong metal-support interaction (SMSI) between Cu active sites and the TiO₂-coated dendritic fibrous nano-silica (DFNS/TiO₂) support. The DFNS/TiO₂-Cu10 catalyst showed excellent catalytic performance with a CO productivity of 5350 mmol g^-1 h^-1 (i.e., 53,506 mmol g_Cu^-1 h^-1), surpassing that of almost all copper-based thermal catalysts, with 99.8% selectivity toward CO. Even after 200 h of reaction, the catalyst remained active. Moderate initial agglomeration and high dispersion of nanoparticles (NPs) due to SMSI made the catalysts stable. Electron energy loss spectroscopy confirmed the strong interactions between copper NPs and the TiO₂ surface, supported by in situ diffuse reflectance infrared Fourier transform spectroscopy and X-ray photoelectron spectroscopy. The H₂-temperature programmed reduction (TPR) study showed α, β, and γ H₂-TPR signals, further confirming the presence of SMSI between Cu and TiO₂. In situ Raman and UV-vis diffuse reflectance spectroscopy studies provided insights into the role of oxygen vacancies and Ti³⁺ centers, which were produced by hydrogen, then consumed by CO₂, and then again regenerated by hydrogen. These continuous defect generation-regeneration processes during the progress of the reaction allowed long-term high catalytic activity and stability. The in situ studies and oxygen storage complete capacity indicated the key role of oxygen vacancies during catalysis. The in situ time-resolved Fourier transform infrared study provided an understanding of the formation of various reaction intermediates and their conversion to products with reaction time. Based on these observations, we have proposed a CO₂ reduction mechanism, which follows a redox pathway assisted by hydrogen.

Design of Cu/MoOx for CO2 Reduction via Reverse Water Gas Shift Reaction

CO2 reduction to CO as raw material for conversion to chemicals and gasoline fuels via the reverse water–gas shift (RWGS) reaction is generally acknowledged to be a promising strategy that makes the CO2 utilization process more economical and efficient. Cu-based catalysts are low-cost and have high catalytic performance but have insufficient stability due to hardening at high temperatures. In this work, a series of Cu-based catalysts supported by MoOx were synthesized for noble metal-free RWGS reactions, and the effects of MoOx support on catalyst performance were investigated. The results show that the introduction of MoOx can effectively improve the catalytic performance of RWGS reactions. The obtained Cu/MoOx (1:1) catalyst displays excellent activity with 35.85% CO2 conversion and 99% selectivity for CO at 400 °C. A combination of XRD, XPS, and HRTEM characterization results demonstrate that MoOx support enhances the metal-oxide interactions with Cu through electronic modification and geometric coverage, thus obtaining highly dispersed copper and more Cu-MoOx interfaces as well as more corresponding oxygen vacancies.

Advanced zeolite and ordered mesoporous silica-based catalysts for the conversion of CO2 to chemicals and fuels

For many years, capturing, storing or sequestering CO₂ from concentrated emission sources or from air has been a powerful technique for reducing atmospheric CO₂. Moreover, the use of CO₂ as a C1 building block to mitigate CO₂ emissions and, at the same time, produce sustainable chemicals or fuels is a challenging and promising alternative to meet global demand for chemicals and energy. Hence, the chemical incorporation and conversion of CO₂ into valuable chemicals has received much attention in the last decade, since CO₂ is an abundant, inexpensive, nontoxic, nonflammable, and renewable one-carbon building block. Nevertheless, CO₂ is the most oxidized form of carbon, thermodynamically the most stable form and kinetically inert. Consequently, the chemical conversion of CO₂ requires highly reactive, rich-energy substrates, highly stable products to be formed or harder reaction conditions. The use of catalysts constitutes an important tool in the development of sustainable chemistry, since catalysts increase the rate of the reaction without modifying the overall standard Gibbs energy in the reaction. Therefore, special attention has been paid to catalysis, and in particular to heterogeneous catalysis because of its environmentally friendly and recyclable nature attributed to simple separation and recovery, as well as its applicability to continuous reactor operations. Focusing on heterogeneous catalysts, we decided to center on zeolite and ordered mesoporous materials due to their high thermal and chemical stability and versatility, which make them good candidates for the design and development of catalysts for CO₂ conversion. In the present review, we analyze the state of the art in the last 25 years and the potential opportunities for using zeolite and OMS (ordered mesoporous silica) based materials to convert CO₂ into valuable chemicals essential for our daily lives and fuels, and to pave the way towards reducing carbon footprint. In this review, we have compiled, to the best of our knowledge, the different reactions involving catalysts based on zeolites and OMS to convert CO₂ into cyclic and dialkyl carbonates, acyclic carbamates, 2-oxazolidones, carboxylic acids, methanol, dimethylether, methane, higher alcohols (C₂₊OH), C₂₊ (gasoline, olefins and aromatics), syngas (RWGS, dry reforming of methane and alcohols), olefins (oxidative dehydrogenation of alkanes) and simple fuels by photoreduction. The use of advanced zeolite and OMS-based materials, and the development of new processes and technologies should provide a new impulse to boost the conversion of CO₂ into chemicals and fuels.

Defective Metal Oxides: Lessons from CO2 RR and Applications in NOx RR

Sluggish reaction kinetics and the undesired side reactions (hydrogen evolution reaction and self-reduction) are the main bottlenecks of electrochemical conversion reactions, such as the carbon dioxide and nitrate reduction reactions (CO₂ RR and NO₃ RR). To date, conventional strategies to overcome these challenges involve electronic structure modification and modulation of the charge-transfer behavior. Nonetheless, key aspects of surface modification, focused on boosting the intrinsic activity of active sites on the catalyst surface, are yet to be fully understood. Engingeering of oxygen vacancies (OVs) can tune surface/bulk electronic structure and improve surface active sites of electrocatalysts. The continuous breakthroughs and significant progress in the last decade position engineering of OVs as a potential technique for advancing electrocatalysis. Motivated by this, the state-of-the-art findings of the roles of OVs in both the CO₂ RR and the NO₃ RR are presented. The review starts with a description of approaches to constructing and techniques for characterizing OVs. This is followed by an overview of the mechanistic understanding of the CO₂ RR and a detailed discussion on the roles of OVs in the CO₂ RR. Then, insights into the NO₃ RR mechanism and the potential of OVs on NO₃ RR based on early findings are highlighted. Finally, the challenges in designing CO₂ RR/NO₃ RR electrocatalysts and perspectives in studying OV engineering are provided.

Evaluating metal oxide support effects on the RWGS activity of Mo2C catalysts

Mo2C supported on nonreducible metal oxides shows increased activity for the reverse water gas shift reaction compared to reducible oxides.

Enhanced CuAl2O4 Catalytic Activity via Alkalinization Treatment toward High CO2 Conversion during Reverse Water Gas Shift Reaction

CO2 catalytic conversion to CO would likely be an important part of CO2 mitigation and utilization. In this work, CuAl2O4 was developed with a spinel structure that acts as an active and stable catalyst for this reaction. Here, the fundamental characteristics of CuAl2O4 catalyst were studied to understand the catalytic mechanism for the Reverse Water Gas Shift reaction. Based on the catalytic mechanism, the CuAl2O4 catalyst was found to have exceptional catalytic activity due to the high dispersion of copper on its surface, and it could have higher catalytic activity by increasing the oxygen vacancies on the surface of the catalyst via alkalinization treatment. By combining with XPS spectra, the relationship between the Raman mode and the oxygen vacancy structure on the CuAl2O4 surface was proved. Through these studies, it was proved that alkalinization treatment can regulate the oxygen vacancies on the surface of the catalyst and thus enhance the catalytic activity.

Highly Active and Stable Ni/La-Doped Ceria Material for Catalytic CO2 Reduction by Reverse Water-Gas Shift Reaction

The design of an active, effective, and economically viable catalyst for CO₂ conversion into value-added products is crucial in the fight against global warming and energy demand. We have developed very efficient catalysts for reverse water-gas shift (rWGS) reaction. Specific conditions of the synthesis by combustion allow the obtention of macroporous materials based on nanosized Ni particles supported on a mixed oxide of high purity and crystallinity. Here, we show that Ni/La-doped CeO₂ catalysts─with the "right" Ni and La proportions─have an unprecedented catalytic performance per unit mass of catalyst for the rWGS reaction as the first step toward CO₂ valorization. Correlations between physicochemical properties and catalytic activity, obtained using a combination of different techniques such as X-ray and neutron powder diffraction, Raman spectroscopy, in situ near ambient pressure X-ray photoelectron spectroscopy, electron microscopy, and catalytic testing, point out to optimum values for the Ni loading and the La proportion. Density functional theory calculations of elementary steps of the reaction on model Ni/ceria catalysts aid toward the microscopic understanding of the nature of the active sites. This finding offers a fundamental basis for developing economical catalysts that can be effectively used for CO₂ reduction with hydrogen. A catalyst based on Ni_0.07/(Ce_0.9La_0.1O_x)_0.93 shows a CO production of 58 × 10^-5 mol_CO·g_cat^-1·s^-1 (700 °C, H₂/CO₂ = 2; selectivity to CO > 99.5), being stable for 100 h under continuous reaction.

Recent Advances in Carbon Dioxide Adsorption, Activation and Hydrogenation to Methanol using Transition Metal Carbides

The inevitable emission of carbon dioxide (CO₂ ) due to the burning of a substantial amount of fossil fuels has led to serious energy and environmental challenges. Metal-based catalytic CO₂ transformations into commodity chemicals are a favorable approach in the CO₂ mitigation strategy. Among these transformations, selective hydrogenation of CO₂ to methanol is the most promising process that not only fulfils the energy demands but also re-balances the carbon cycle. The investigation of CO₂ adsorption on the surface of heterogeneous catalyst is highly important because the formation of various intermediates which determines the selectivity of product. Transition metal carbides (TMCs) have received considerable attention in recent years because of their noble metal-like reactivity, ceramic-like properties, high chemical and thermal stability. These features make them excellent catalytic materials for a variety of transformations such as CO₂ adsorption and its conversion into value-added chemicals. Herein, the catalytic properties of TMCs are summarize along with synthetic methods, CO₂ binding modes, mechanistic studies, effects of dopant on CO₂ adsorption, and carbon/metal ratio in the CO₂ hydrogenation reaction to methanol using computational as well as experimental studies. Additionally, this Review provides an outline of the challenges and opportunities for the development of potential TMCs in CO₂ hydrogenation reactions.

Development of Power-to-X Catalytic Processes for CO2 Valorisation: From the Molecular Level to the Reactor Architecture

Nowadays, global climate change is likely the most compelling problem mankind is facing. In this scenario, decarbonisation of the chemical industry is one of the global challenges that the scientific community needs to address in the immediate future. Catalysis and catalytic processes are called to play a decisive role in the transition to a more sustainable and low-carbon future. This critical review analyses the unique advantages of structured reactors (isothermicity, a wide range of residence times availability, complex geometries) with the multifunctional design of efficient catalysts to synthesise chemicals using CO2 and renewable H2 in a Power-to-X (PTX) strategy. Fine-chemistry synthetic methods and advanced in situ/operando techniques are essential to elucidate the changes of the catalysts during the studied reaction, thus gathering fundamental information about the active species and reaction mechanisms. Such information becomes crucial to refine the catalyst’s formulation and boost the reaction’s performance. On the other hand, reactors architecture allows flow pattern and temperature control, the management of strong thermal effects and the incorporation of specifically designed materials as catalytically active phases are expected to significantly contribute to the advance in the valorisation of CO2 in the form of high added-value products. From a general perspective, this paper aims to update the state of the art in Carbon Capture and Utilisation (CCU) and PTX concepts with emphasis on processes involving the transformation of CO2 into targeted fuels and platform chemicals, combining innovation from the point of view of both structured reactor design and multifunctional catalysts development.

Cooperative Catalysis of Vibrationally Excited CO2 and Alloy Catalyst Breaks the Thermodynamic Equilibrium Limitation

Using nonthermal plasma (NTP) to promote CO₂ hydrogenation is one of the most promising approaches that overcome the limitations of conventional thermal catalysis. However, the catalytic surface reaction dynamics of NTP-activated species are still under debate. The NTP-activated CO₂ hydrogenation was investigated in Pd₂Ga/SiO₂ alloy catalysts and compared to thermal conditions. Although both thermal and NTP conditions showed close to 100% CO selectivity, it is worth emphasizing that when activated by NTP, CO₂ conversion not only improves more than 2-fold under thermal conditions but also breaks the thermodynamic equilibrium limitation. Mechanistic insights into NTP-activated species and alloy catalyst surface were investigated by using in situ transmission infrared spectroscopy, where catalyst surface species were identified during NTP irradiation. Moreover, in in situ X-ray absorption fine-structure analysis under reaction conditions, the catalyst under NTP conditions not only did not undergo restructuring affecting CO₂ hydrogenation but also could clearly rule out catalyst activation by heating. In situ characterizations of the catalysts during CO₂ hydrogenation depict that vibrationally excited CO₂ significantly enhances the catalytic reaction. The agreement of approaches combining experimental studies and density functional theory (DFT) calculations substantiates that vibrationally excited CO₂ reacts directly with hydrogen adsorbed on Pd sites while accelerating formate formation due to neighboring Ga sites. Moreover, DFT analysis deduces the key reaction pathway that the decomposition of monodentate formate is promoted by plasma-activated hydrogen species. This work enables the high designability of CO₂ hydrogenation catalysts toward value-added chemicals based on the electrification of chemical processes via NTP.

Gas-Permeable Iron-Doped Ceria Shell on Rh Nanoparticles with High Activity and Durability

Strong metal-support interaction (SMSI) is a promising strategy to control the structure of the supported metal catalyst. Especially, encapsulating metal nanoparticles through SMSI can enhance resistance against sintering but typically blocks the access of reactants onto the metal surface. Here, we report gas-permeable shells formed on Rh nanoparticles with enhanced activity and durability for the surface reaction. First, Fe species were doped into ceria, enhancing the transfer of surface oxygen species. When Rh was deposited onto the Fe-doped ceria (FC) and reduced, a shell was formed on Rh nanoparticles. Diffuse reflectance infrared Fourier-transform spectroscopy (DRIFTS) results show that the shell is formed upon reduction and removed upon oxidation reversibly. CO adsorption on the Rh surface through the shell was confirmed by cryo-DRIFTS. The reverse water gas shift (RWGS) reaction (CO₂ + H₂ → CO + H₂O) occurred on the encapsulated Rh nanoparticles effectively with selective CO formation, whereas bare Rh nanoparticles deposited on ceria produced methane as well. The CO adsorption became much weaker on the encapsulated Rh nanoparticles, and H₂-spillover occurred more on the FC, resulting in high activity for RWGS. The exposed Rh nanoparticles deposited on ceria presented degradation at 400 °C after 150 h of RWGS, whereas the encapsulated Rh nanoparticles showed no degradation with superior durability. Enhancing surface oxygen transfer can be an efficient way to form gas-permeable overlayers on metal nanoparticles with high activity and durability.

Elucidation of the reaction mechanism on dry reforming of methane in an electric field by in situ DRIFTs

With increasing expectations for carbon neutrality, dry reforming is anticipated for direct conversion of methane and carbon dioxide: the main components of biogas. We have found that dry reforming of methane in an electric field using a Pt/CeO2 catalyst proceeds with sufficient rapidity even at a low temperature of about 473 K. The effect of the electric field (EF) on dry reforming was investigated using kinetic analysis, in situ DRIFTs, XPS, and DFT calculation. In situ DRIFTs and XPS measurements indicated that the amount of carbonate, which is an adsorbed species of CO2, increased with the application of EF. XPS measurements also confirmed the reduction of CeO2 by the reaction of surface oxygen and CH4. The reaction between CH4 molecules and surface oxygen was promoted at the interface between Pt and CeO2.

Enhanced catalytic performance of atomically dispersed Pd on Pr-doped CeO2 nanorod in CO oxidation

Single-atom noble metal catalysts have been widely studied for catalytic oxidation of CO. Regulating the coordination environment of single metal atom site is an effective strategy to improve the intrinsic catalytic activity of single atom catalyst. In this work, single atom Pd catalyst supported on Pr-doped CeO₂ nanorods was prepared, and the performance and nature of Pr-coordinated atomic Pd site in CO catalytic oxidation are systematically investigated. The structure characterization using AC-HAADF-STEM, EXAFS, XRD and Raman spectroscopy demonstrate the formation of single atom Pd site and abundant surface oxygen vacancies on the surface of Pr-doped CeO₂ nanorod. With the combination of the XPS characterization and DFT calculations, the oxidation state of Pd on Pr-doped CeO₂ nanorod is determined lower than that on CeO₂ nanorod. The turnover frequency of CO oxidation is markedly increased from 8.4 × 10^-3 to 31.9 × 10^-3 s with Pr-doping at 130 ºC and GHSV of 70,000 h^-1. Combined with kinetic studies, DRIFT and DFT calculations, the doped-Pr atoms reduced the formation energy of oxygen vacancies and generate more oxygen vacancies around the atomically dispersed Pd sites on the surface of cerium oxide, which reduces the dissociation energy of oxygen, thereby accelerating the reaction rate of CO oxidation.

Partially sintered copper‒ceria as excellent catalyst for the high-temperature reverse water gas shift reaction

For high-temperature catalytic reaction, it is of significant importance and challenge to construct stable active sites in catalysts. Herein, we report the construction of sufficient and stable copper clusters in the copper‒ceria catalyst with high Cu loading (15 wt.%) for the high-temperature reverse water gas shift (RWGS) reaction. Under very harsh working conditions, the ceria nanorods suffered a partial sintering, on which the 2D and 3D copper clusters were formed. This partially sintered catalyst exhibits unmatched activity and excellent durability at high temperature. The interaction between the copper and ceria ensures the copper clusters stably anchored on the surface of ceria. Abundant in situ generated and consumed surface oxygen vacancies form synergistic effect with adjacent copper clusters to promote the reaction process. This work investigates the structure-function relation of the catalyst with sintered and inhomogeneous structure and explores the potential application of the sintered catalyst in C1 chemistry.