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.

Construction of Ni/In2O3 Integrated Nanocatalysts Based on MIL-68(In) Precursors for Efficient CO2 Hydrogenation to Methanol

The efficient and economic conversion of CO2 and renewable H2 into methanol has received intensive attention due to growing concern for anthropogenic CO2 emissions, particularly from fossil fuel combustion. Herein, we have developed a novel method for preparing Ni/In2O3 nanocatalysts by using porous MIL-68(In) and nickel(II) acetylacetonate (Ni(acac)2) as the dual precursors of In2O3 and Ni components, respectively. Combined with in-depth characterization analysis, it was revealed that the utilization of MIL-68(In) as precursors favored the good distribution of Ni nanoparticles (∼6.2 nm) on the porous In2O3 support and inhibited the metal sintering at high temperatures. The varied catalyst fabrication parameters were explored, indicating that the designed Ni/In2O3 catalyst (Ni content of 5 wt %) exhibited better catalytic performance than the compared catalyst prepared using In(OH)3 as a precursor of In2O3. The obtained Ni/In2O3 catalyst also showed excellent durability in long-term tests (120 h). However, a high Ni loading (31 wt %) would result in the formation of the Ni-In alloy phase during the CO2 hydrogenation which favored CO formation with selectivity as high as 69%. This phenomenon is more obvious if Ni and In2O3 had a strong interaction, depending on the catalyst fabrication methods. In addition, with the aid of in situ diffuse reflectance infrared Fourier transform spectroscopy and density functional theory (DFT) calculations, the Ni/In2O3 catalyst predominantly follows the formate pathway in the CO2 hydrogenation to methanol, with HCOO* and *H3CO as the major intermediates, while the small size of Ni particles is beneficial to the formation of formate species based on DFT calculation. This study suggests that the Ni/In2O3 nanocatalyst fabricated using metal-organic frameworks as precursors can effectively promote CO2 thermal hydrogenation to methanol.

Tuning Catalytic Activity of CO2 Hydrogenation to C1 Product via Metal Support Interaction Over Metal/Metal Oxide Supported Catalysts

The metal supported catalysts are emerging catalysts that are receiving a lot of attention in CO2 hydrogenation to C1 products. Numerous experiments have demonstrated that the support (usually an oxide) is crucial for the catalytic performance. The support metal oxides are used to aid in the homogeneous dispersion of metal particles, prevent agglomeration, and control morphology owing to the metal support interaction (MSI). MSI can efficiently optimize the structural and electronic properties of catalysts and tune the conversion of key reaction intermediates involved in CO2 hydrogenation, thereby enhancing the catalytic performance. There is an increasing attention is being paid to the promotion effects in the catalytic CO2 hydrogenation process. However, a systematically understanding about the effects of MSI on CO2 hydrogenation to C1 products catalytic performance has not been fully studied yet due to the diversities in catalysts and reaction conditions. Hence, the characteristics and modes of MSI in CO2 hydrogenation to C1 products are elaborated in detail in our work.

Photoinduced Precise Synthesis of Diatomic Ir1 Pd1 -In2 O3 for CO2 Hydrogenation to Methanol via Angstrom-Scale-Distance Dependent Synergistic Catalysis

The atomically dispersed metal catalysts with full atomic utilization and well-defined site structure hold great promise for various catalytic reactions. However, the single metallic site limits the comprehensive reaction performance in most reactions. Here, we demonstrated a photo-induced neighbour-deposition strategy for the precise synthesis of diatomic Ir1 Pd1 on In2 O3 applied for CO2 hydrogenation to methanol. The proximity synergism between diatomic sites enabled a striking promotion in both CO2 conversion (10.5 %) and methanol selectivity (97 %) with good stability of 100 h run. It resulted in record-breaking space-time yield to methanol (187.1 gMeOH  gmetal -1  hour-1 ). The promotional effect mainly originated from stronger CO2 adsorption on Ir site with assistance of H-spillover from Pd site, thus leading to a lower energy barrier for *HCOO pathway. It was confirmed that this synergistic effect strongly depended on the dual-site distance in an angstrom scale, which was attributed to weaker *H spillover and less electron transfer from Pd to Ir site as the Pd-to-Ir distance increased. The average dual-site distance was evaluated by our firstly proposed photoelectric model. Thus, this study introduced a pioneering strategy to precisely synthesize homonuclear/heteronuclear diatomic catalysts for facilitating the desired reaction route via diatomic synergistic catalysis.

Facile Synthesis of Iron Carbide via Pyrolysis of Ferrous Fumarate for Catalytic CO2 Hydrogenation to Lower Olefins

Hydrogenation of CO2 to olefin catalyzed by iron-based catalysts is a sustainable and important way to achieve carbon neutrality. In this study, iron-based catalysts were facilely prepared by direct pyrolysis of ferric fumarate (FF), which are applied to CO2 hydrogenation to olefin reaction to explore the effects of pyrolysis temperature and atmosphere on catalytic performance of the catalysts. Among them, NaFe-Air-400 catalyst exhibits the highest catalytic activity with 33.7 %, and light olefin selectivity reaches as high as 47.1 %. The catalytic performance of pyrolytic catalysts is better than that the impregnated NaFe catalyst on activated carbon (NaFe/AC). A series of XRD, Raman and SEM characterization results show a suitable pyrolysis temperature would promote the balance between amorphous carbon and graphene, which can affect the formation of FexCy phase, leading the distinctive activity and olefin selectivity. Hence, the presented one-step pyrolysis methodology would provide a facile and quick synthesis of highly-active iron-based catalyst design for CO2 conversion.

Nanometric Cu-ZnO Particles Supported on N-Doped Graphitic Carbon as Catalysts for the Selective CO2 Hydrogenation to Methanol

The quest for efficient catalysts based on abundant elements that can promote the selective CO2 hydrogenation to green methanol still continues. Most of the reported catalysts are based on Cu/ZnO supported in inorganic oxides, with not much progress with respect to the benchmark Cu/ZnO/Al2O3 catalyst. The use of carbon supports for Cu/ZnO particles is much less explored in spite of the favorable strong metal support interaction that these doped carbons can establish. This manuscript reports the preparation of a series of Cu-ZnO@(N)C samples consisting of Cu/ZnO particles embedded within a N-doped graphitic carbon with a wide range of Cu/Zn atomic ratio. The preparation procedure relies on the transformation of chitosan, a biomass waste, into N-doped graphitic carbon by pyrolysis, which establishes a strong interaction with Cu nanoparticles (NPs) formed simultaneously by Cu2+ salt reduction during the graphitization. Zn2+ ions are subsequently added to the Cu-graphene material by impregnation. All the Cu/ZnO@(N)C samples promote methanol formation in the CO2 hydrogenation at temperatures from 200 to 300 °C, with the temperature increasing CO2 conversion and decreasing methanol selectivity. The best performing Cu-ZnO@(N)C sample achieves at 300 °C a CO2 conversion of 23% and a methanol selectivity of 21% that is among the highest reported, particularly for a carbon-based support. DFT calculations indicate the role of pyridinic N doping atoms stabilizing the Cu/ZnO NPs and supporting the formate pathway as the most likely reaction mechanism.

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.

Zr-Based MOF-545 Metal-Organic Framework Loaded with Highly Dispersed Small Size Ni Nanoparticles for CO2 Methanation

We report the use of Zr-based metal-organic frameworks (MOFs) MOF-545 and MOF-545(Cu) as supports to prepare catalysts with uniformly and highly dispersed Ni nanoparticles (NPs) for CO2 hydrogenation into CH4. In the first step, we studied the MOF support under catalytic conditions using operando diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy, ex situ characterizations (PXRD, XPS, TEM, and EDX-element mapping), and DFT calculations. We showed that the high-temperature conditions undoubtedly confer a potential for catalytic functionality to the solids toward CH4 production, while no role of the Cu could be evidenced. The MOF was shown to be transformed into a catalytically active material, amorphized but still structured with dehydroxylated Zr-oxoclusters, in line with DFT calculations. In the second step, Ni@MOF-545 catalysts were prepared using either impregnation (IM) or double solvent (DS) methods, followed by a dry reduction (R) route under H2 to immobilize Ni NPs. The highest catalytic activity was obtained with the Ni@MOF-545 DS R catalyst (595 mmolCH4 gNi-1 h-1) with 100% CH4 selectivity and 60% CO2 conversion after ∼3 h. The higher catalytic activity of Ni@MOF-545 DS R is a result of much smaller (∼5 nm) and better dispersed Ni NPs than in the IM sample (20-40 nm), the latter exhibiting sintering. The advantages of the encapsulation of Ni NPs by the DS method and of the use of a MOF-545-based support are discussed, highlighting the interest of designing yet-unexplored Zr-based MOFs loaded with Ni NPs for CO2 hydrogenation.

Theoretical Insights into Amido Group-Mediated Enhancement of CO2 Hydrogenation to Methanol on Cobalt Catalysts

Catalytic reduction of carbon dioxide into high-value-added products, such as methanol, is an effective approach to mitigate the greenhouse effect, and improving Co-based catalysts is anticipated to yield potential catalysts with high performance and low cost. In this study, based on first-principles calculations, we elucidate the promotion effects of surface *NHx (x = 1, 2, and 3) on the carbon dioxide hydrogenation to methanol from both activity and selectivity perspectives on Co-based catalysts. The presence of *NHx reduced the energy barrier of each elementary step on Co(100) by regulating the electronic structure to alter the binding strength of intermediates or by forming a hydrogen bond between surface oxygen-containing species and *NHx to stabilize transition states. The best promotion effect for different steps corresponds to different *NHx. The energy barrier of the rate-determining step of CO2 hydrogenation to methanol is lowered from 1.55 to 0.88 eV, and the product selectivity shifts from methane to methanol with the assistance of *NHx on the Co(100) surface. A similar phenomenon is observed on the Co(111) surface. The promotion effect of *NHx on Co-based catalysts is superior to that of water, indicating that the introduction of *NHx on a Co-based catalyst is an effective strategy to enhance the catalytic performance of CO2 hydrogenation to methanol.

Efficient Thermal Management with Selective Metamaterial Absorber for Boosting Photothermal CO2 Hydrogenation under Sunlight

Photothermal catalytic CO2 hydrogenation is a prospective strategy to simultaneously reduce CO2 emission and generate value-added fuels. However, the demand of extremely intense light hinders its development in practical applications. Herein, this work reports the novel design of Ni-based selective metamaterial absorber and employs it as the photothermal catalyst for CO2 hydrogenation. The selective absorption property reduces the heat loss caused by radiation while possessing effectively solar absorption, thus substantially increasing local photothermal temperature. Notably, the enhancement of local electric field by plasmon resonance promotes the adsorption and activation of reactants. Moreover, benefiting from the ingenious morphology that Ni nanoparticles (NPs) are encapsulated by SiO2 matrix through co-sputtering, the greatly improved dispersion of Ni NPs enables enhancing the contact with reaction gas and preventing the agglomeration. Consequently, the catalyst exhibits an unprecedented CO2 conversion rate of 516.9 mmol gcat -1 h-1 under 0.8 W cm-2 irradiation, with near 90% CO selectivity and high stability. Significantly, this designed photothermal catalyst demonstrates the great potential in practical applications under sunlight. This work provides new sights for designing high-performance photothermal catalysts by thermal management.

Water and Cu+ Synergy in Selective CO2 Hydrogenation to Methanol over Cu-MgO-Al2O3 Catalysts

The CO2 hydrogenation reaction to produce methanol holds great significance as it contributes to achieving a CO2-neutral economy. Previous research identified isolated Cu+ species doping the oxide surface of a Cu-MgO-Al2O3-mixed oxide derived from a hydrotalcite precursor as the active site in CO2 hydrogenation, stabilizing monodentate formate species as a crucial intermediate in methanol synthesis. In this work, we present a molecular-level understanding of how surface water and hydroxyl groups play a crucial role in facilitating spontaneous CO2 activation at Cu+ sites and the formation of monodentate formate species. Computational evidence has been experimentally validated by comparing the catalytic performance of the Cu-MgO-Al2O3 catalyst with hydroxyl groups against that of its hydrophobic counterpart, where hydroxyl groups are blocked using an esterification method. Our work highlights the synergistic effect between doped Cu+ ions and adjacent hydroxyl groups, both of which serve as key parameters in regulating methanol production via CO2 hydrogenation. By elucidating the specific roles of these components, we contribute to advancing our understanding of the underlying mechanisms and provide valuable insights for optimizing methanol synthesis processes.

Research Progress of Non-Noble Metal Catalysts for Carbon Dioxide Methanation

The extensive utilization of fossil fuels has led to a rapid increase in atmospheric CO2 concentration, resulting in various environmental issues. To reduce reliance on fossil fuels and mitigate CO2 emissions, it is important to explore alternative methods of utilizing CO2 and H2 as raw materials to obtain high-value-added chemicals or fuels. One such method is CO2 methanation, which converts CO2 and H2 into methane (CH4), a valuable fuel and raw material for other chemicals. However, CO2 methanation faces challenges in terms of kinetics and thermodynamics. The reaction rate, CO2 conversion, and CH4 yield need to be improved to make the process more efficient. To overcome these challenges, the development of suitable catalysts is essential. Non-noble metal catalysts have gained significant attention due to their high catalytic activity and relatively low cost. In this paper, the thermodynamics and kinetics of the CO2 methanation reaction are discussed. The focus is primarily on reviewing Ni-based, Co-based, and other commonly used catalysts such as Fe-based. The effects of catalyst supports, preparation methods, and promoters on the catalytic performance of the methanation reaction are highlighted. Additionally, the paper summarizes the impact of reaction conditions such as temperature, pressure, space velocity, and H2/CO2 ratio on the catalyst performance. The mechanism of CO2 methanation is also summarized to provide a comprehensive understanding of the process. The objective of this paper is to deepen the understanding of non-noble metal catalysts in CO2 methanation reactions and provide insights for improving catalyst performance. By addressing the limitations of CO2 methanation and exploring the factors influencing catalyst effectiveness, researchers can develop more efficient and cost-effective catalysts for this reaction.

CO2 and H2 Activation on Zinc-Doped Copper Clusters

Here we systematically investigate the CO2 and H2 activation and dissociation on small Cun Zn0/+ (n=3-6) clusters using Density Functional Theory. We show that Cu6 Zn is a superatom, displaying an increased HOMO-LUMO gap and is inert towards CO2 or H2 activation or dissociation. While other neutral clusters weakly activate CO2 , the cationic clusters preferentially bind the CO2 in monodentate nonactivated way. Notably, Cu4 Zn allows for the dissociation of activated CO2 , whereas larger clusters destabilize all activated CO2 binding modes. Conversely, H2 dissociation is favored on all clusters examined, except for Cu6 Zn. Cu3 Zn+ and Cu4 Zn, favor the formation of formate through the H2 dissociation pathway rather than CO2 dissociation. These findings suggest the potential of these clusters as synthetic targets and underscore their significance in the realm of CO2 hydrogenation.

Modulation of the Structure-function Relationship of the "nano-greenhouse effect" towards Optimized Supra-photothermal Catalysis

Photothermal catalytic CO2 hydrogenation holds great promise for relieving the global environment and energy crises. The "nano-greenhouse effect" has been recognized as a crucial strategy for improving the heat management capabilities of a photothermal catalyst by ameliorating the convective and radiative heat losses. Yet it remains unclear to what degree the respective heat transfer and mass transport efficiencies depend on the specific structures. Herein, the structure-function relationship of the "nano-greenhouse effect" was investigated and optimized in a prototypical Ni@SiO2 core-shell catalyst towards photothermal CO2 catalysis. Experimental and theoretical results indicate that modulation of the thickness and porosity of the SiO2 nanoshell leads to variations in both heat preservation and mass transport properties. This work deepens the understandings on the contributing factor of the "nano-greenhouse effect" towards enhanced photothermal conversion. It also provides insights on the design principles of an ideal photothermal catalyst in balancing heat management and mass transport processes.

Sheet-Like Morphology CuO/Co3O4 Nanocomposites for Enhanced Catalysis in Hydrogenation of CO2 to Methanol

The selective hydrogenation of CO2 into high-value chemicals is an effective approach to address environmental issues. Cobalt-based catalysts have significant potential in CO2 hydrogenation reaction systems; however, there is a need to control their selectivity better. In this study, copper is introduced onto Co3O4 nanosheets using the ion exchange reverse loading method. The unique interaction of these materials significantly alters the selectivity of the cobalt-based catalyst. Results from scanning transmission electron microscopy and scanning electron microscopy indicate that this catalyst enables a more even dispersion of copper species in the Co3O4 nanosheets. Temperature-programmed reduction and X-ray photoelectron spectroscopy reveal that the catalyst facilitates the metal-metal interaction between Co and Cu. Temperature-programmed desorption experiments for CO2 and H2 demonstrate that the close interaction between Co and Cu modifies CO2 adsorption, leading to differences in catalytic activity. Moreover, the catalyst effectively suppresses CO2 methanation and promotes methanol formation by altering the alkalinity of the catalyst surface and weakening the hydrogen dissociation ability.

Direct Operando Visualization of Metal Support Interactions Induced by Hydrogen Spillover During CO2 Hydrogenation

The understanding of catalyst active sites is a fundamental challenge for the future rational design of optimized and bespoke catalysts. For instance, the partial reduction of Ce4+ surface sites to Ce3+ and the formation of oxygen vacancies are critical for CO2 hydrogenation, CO oxidation, and the water gas shift reaction. Furthermore, metal nanoparticles, the reducible support, and metal support interactions are prone to evolve under reaction conditions; therefore a catalyst structure must be characterized under operando conditions to identify active states and deduce structure-activity relationships. In the present work, temperature-induced morphological and chemical changes in Ni nanoparticle-decorated mesoporous CeO2 by means of in situ quantitative multimode electron tomography and in situ heating electron energy loss spectroscopy, respectively, are investigated. Moreover, operando electron energy loss spectroscopy is employed using a windowed gas cell and reveals the role of Ni-induced hydrogen spillover on active Ce3+ site formation and enhancement of the overall catalytic performance.

Tailoring the CO2 Hydrogenation Performance of Fe-Based Catalyst via Unique Confinement Effect of the Carbon Shell

Even though Fe-based catalysts have been widely employed for CO2 hydrogenation into hydrocarbons, oxygenates, liquid fuels, etc., the precise regulation of their physicochemical properties is needed to enhance the catalytic performance. Herein, under the guidance of the traditional concept in heterogeneous catalysis-confinement effect, a core-shell structured catalyst Na-Fe3 O4 @C is constructed to boost the CO2 hydrogenation performance. Benefiting from the carbon-chain growth limitation, tailorable H2 /CO2 ratio on the catalytic interface, and unique electronic property that all endowed by the confinement effect, the selectivity and space-time yield of light olefins (C2 = -C4 = ) are as high as 47.4 % and 15.9 g molFe -1  h-1 , respectively, which are all notably higher than that from the shell-less counterpart. The function mechanism of the confinement effect in Fe-based catalysts are clarified in detail by multiple characterization and density functional theory (DFT). This work may offer a new prospect for the rational design of CO2 hydrogenation catalyst.

Reinforcing the Efficiency of Photothermal Catalytic CO2 Methanation through Integration of Ru Nanoparticles with Photothermal MnCo2O4 Nanosheets

Carbon dioxide (CO2) hydrogenation to methane (CH4) is regarded as a promising approach for CO2 utilization, whereas achieving desirable conversion efficiency under mild conditions remains a significant challenge. Herein, we have identified ultrasmall Ru nanoparticles (∼2.5 nm) anchored on MnCo2O4 nanosheets as prospective photothermal catalysts for CO2 methanation at ambient pressure with light irradiation. Our findings revealed that MnCo2O4 nanosheets exhibit dual functionality as photothermal substrates for localized temperature enhancement and photocatalysts for electron donation. As such, the optimized Ru/MnCo2O4-2 gave a high CH4 production rate of 66.3 mmol gcat-1 h-1 (corresponding to 5.1 mol gRu-1 h-1) with 96% CH4 selectivity at 230 °C under ambient pressure and light irradiation (420-780 nm, 1.25 W cm-2), outperforming most reported plasmonic metal-based catalysts. The mechanisms behind the intriguing photothermal catalytic performance improvement were substantiated through a comprehensive investigation involving experimental characterizations, numerical simulations and density functional theory (DFT) calculations, which unveiled the synergistic effects of enhanced charge separation efficiency, improved reaction kinetics, facilitated reactant adsorption/activation and accelerated intermediate conversion under light irradiation over Ru/MnCo2O4. A comparison study showed that, with identical external input energy during the reaction, Ru/MnCo2O4-2 had a much higher catalytic efficiency compared to Ru/TiO2 and Ru/Al2O3. This study underscores the pivotal role played by photothermal supports and is believed to engender a heightened interest in plasmonic metal nanoparticles anchored on photothermal substrates for CO2 methanation under mild conditions.

Synthesis of Liquid Hydrocarbon via Direct Hydrogenation of CO2 over FeCu-Based Bifunctional Catalyst Derived from Layered Double Hydroxides

Here, we report a Na-promoted FeCu-based catalyst with excellent liquid hydrocarbon selectivity and catalytic activity. The physiochemical properties of the catalysts were comprehensively characterized by various characterization techniques. The characterization results indicate that the catalytic performance of the catalysts was closely related to the nature of the metal promoters. The Na-AlFeCu possessed the highest CO2 conversion due to enhanced CO2 adsorption of the catalysts by the introduction of Al species. The introduction of excess Mg promoter led to a strong methanation activity of the catalyst. Mn and Ga promoters exhibited high selectivity for light hydrocarbons due to their inhibition of iron carbides generation, resulting in a lack of chain growth capacity. The Na-ZnFeCu catalyst exhibited the optimal C5+ yield, owing to the fact that the Zn promoter improved the catalytic activity and liquid hydrocarbon selectivity by modulating the surface CO2 adsorption and carbide content. Carbon dioxide (CO2) hydrogenation to liquid fuel is considered a method for the utilization and conversion of CO2, whereas satisfactory activity and selectivity remains a challenge. This method provides a new idea for the catalytic hydrogenation of CO2 and from there the preparation of high-value-added products.

Direct Evidence of Dynamic Metal Support Interactions in Co/TiO2 Catalysts by Near-Ambient Pressure X-ray Photoelectron Spectroscopy

The interaction between metal particles and the oxide support, the so-called metal-support interaction, plays a critical role in the performance of heterogenous catalysts. Probing the dynamic evolution of these interactions under reactive gas atmospheres is crucial to comprehending the structure-performance relationship and eventually designing new catalysts with enhanced properties. Cobalt supported on TiO2 (Co/TiO2) is an industrially relevant catalyst applied in Fischer-Tropsch synthesis. Although it is widely acknowledged that Co/TiO2 is restructured during the reaction process, little is known about the impact of the specific gas phase environment at the material's surface. The combination of soft and hard X-ray photoemission spectroscopies are used to investigate in situ Co particles supported on pure and NaBH4-modified TiO2 under H2, O2, and CO2:H2 gas atmospheres. The combination of soft and hard X-ray photoemission methods, which allows for simultaneous probing of the chemical composition of surface and subsurface layers, is one of the study's unique features. It is shown that under H2, cobalt particles are encapsulated below a stoichiometric TiO2 layer. This arrangement is preserved under CO2 hydrogenation conditions (i.e., CO2:H2), but changes rapidly upon exposure to O2. The pretreatment of the TiO2 support with NaBH4 affects the surface mobility and prevents TiO2 spillover onto Co particles.

Carbon Dioxide Hydrogenation to Formate Catalyzed by a Neutral, Coordinatively Saturated Tris-Carbonyl Mn(I)-PNP Pincer-Type Complex

CO2 catalytic hydrogenation to formate was achieved (TONmax =ca. 3800) in the presence of the neutral, halide-free, coordinatively saturated tris(carbonyl) manganese pincer-type complex [Mn(PNP)(CO)3 ], bearing a diarylamido pincer-type PNP ligand, using DBU as base and LiOTf as Lewis acid additive, under mild reaction conditions (60 bar, 80 °C). DFT calculations suggest that the precatalyst activation key step occurs by intermolecular, base assisted dihydrogen heterolytic splitting rather than by the expected ligand-assisted intramolecular MLC-type mechanism.

Effect of Conversion, Temperature and Feed Ratio on In2 O3 /In(OH)3 Phase Transitions in Methanol Synthesis Catalysts: A Combined Experimental and Computational Study

Catalytic hydrogenation of CO2 to methanol has attracted lots of attention as it makes CO2 useable as a sustainable carbon source. This study combines theoretical calculations based on the dummy catalytic cycle model with experimental studies on the performance and degradation of indium-based model catalysts for methanol synthesis. In detail, the reversibility of phase transitions in the In2 O3 /In(OH)3 system under industrial methanol synthesis conditions are investigated depending on conversion, temperature and feed ratio. The dummy catalytic cycle model predicts a peculiar degradation behavior of In(OH)3 at 275 °C depending on the water formed either by methanol synthesis or the competing reverse water-gas-shift reaction. These results were validated by dedicated experimental studies confirming the predicted trends. Moreover, X-ray diffraction and thermogravimetric analysis proved the ensuing phase transition between the indium species. Finally, the validated model is used to predict how hydrogen drop out will affect the stability of the catalyst and derive practical strategies to prevent irreversible catalyst degradation.

Direct Observation of Ni Nanoparticle Growth in Carbon-Supported Nickel under Carbon Dioxide Hydrogenation Atmosphere

Understanding nanoparticle growth is crucial to increase the lifetime of supported metal catalysts. In this study, we employ in situ gas-phase transmission electron microscopy to visualize the movement and growth of ensembles of tens of nickel nanoparticles supported on carbon for CO2 hydrogenation at atmospheric pressure (H2:CO2 = 4:1) and relevant temperature (450 °C) in real time. We observe two modes of particle movement with an order of magnitude difference in velocity: fast, intermittent movement (vmax = 0.7 nm s-1) and slow, gradual movement (vaverage = 0.05 nm s-1). We visualize the two distinct particle growth mechanisms: diffusion and coalescence, and Ostwald ripening. The diffusion and coalescence mechanism dominates at small interparticle distances, whereas Ostwald ripening is driven by differences in particle size. Strikingly, we demonstrate an interplay between the two mechanisms, where first coalescence takes place, followed by fast Ostwald ripening due to the increased difference in particle size. Our direct visualization of the complex nanoparticle growth mechanisms highlights the relevance of studying nanoparticle growth in supported nanoparticle ensembles under reaction conditions and contributes to the fundamental understanding of the stability in supported metal catalysts.

Performance Exploration of Ni-Doped MoS2 in CO2 Hydrogenation to Methanol

The preparation of methanol chemicals through CO2 and H2 gas is a positive measure to achieve carbon neutrality. However, developing catalysts with high selectivity remains a challenge due to the irreversible side reaction of reverse water gas shift (RWGS), and the low-temperature characteristics of CO2 hydrogenation to methanol. In-plane sulfur vacancies of MoS2 can be the catalytic active sites for CH3OH formation, but the edge vacancies are more inclined to the occurrence of methane. Therefore, MoS2 and a series of MoS2/Nix and MoS2/Cox catalysts doped with different amounts are prepared by a hydrothermal method. A variety of microscopic characterizations indicate that Ni and Co doping can form NiS2 and CoS2, the existence of these substances can prevent CO2 and H2 from contacting the edge S vacancies of MoS2, and the selectivity of the main product is improved. DFT calculation illustrates that the larger range of orbital hybridization between Ni and MoS2 leads to CO2 activation and the active hydrogen is more prone to surface migration. Under optimized preparation conditions, MoS2/Ni0.2 exhibits relatively good methanol selectivity. Therefore, this strategy of improving methanol selectivity through metal doping has reference significance for the subsequent research and development of such catalysts.

The Effect of Mn, Al Doping on the CO2 Hydrogenation Performance of CaCO3 -Supported Fe-Based Catalysts

With increasingly serious environmental problems caused by the greenhouse effect, it has also become essential to reduce the concentration of CO2 in the atmosphere. In this paper, CaCO3 -supported Fe-based catalysts doped with Mn, Al, and K are prepared by a straightforward method and used for CO2 hydrogenation. The fresh and spent catalysts were characterized by SEM-EDS, BET, TG, CO2 -TPD, XRD, and XPS. The experimental results show that the highest CO2 conversion rate of Fe10Mn2Al10Ca is 35.99 %, the maximum FTY value is 293.98 μmolCO2  ⋅ g Fe - 1 ${{\rm{g}}_{{\rm{Fe}}}^{ - 1} }$  ⋅ s-1 , the maximum O/P value is 6.61, and the lowest CO selectivity is 32.21 %. At the same time, according to the characterization results, the doping of Mn and Al increased the Fe3 O4 /FeCx ratio. As the Fe3 O4 /FeCx ratio increases, the proportion of short-chain hydrocarbons (CH4 , C2-4 ) in the products increases, and the proportion of long-chain hydrocarbons (C5+ ) decrease. Therefore, the co-doping of Mn and Al promotes the conversion of CO and reduces its selectivity, and promotes the formation of light olefins. Finally, it is hoped that this study can provide a reference for further research on CaCO3 -supported Fe catalysts.

Zeolite-encaged mononuclear copper centers catalyze CO2 selective hydrogenation to methanol

The selective hydrogenation of CO2 to methanol by renewable hydrogen source represents an attractive route for CO2 recycling and is carbon neutral. Stable catalysts with high activity and methanol selectivity are being vigorously pursued, and current debates on the active site and reaction pathway need to be clarified. Here, we report a design of faujasite-encaged mononuclear Cu centers, namely Cu@FAU, for this challenging reaction. Stable methanol space-time-yield (STY) of 12.8 mmol gcat-1 h-1 and methanol selectivity of 89.5% are simultaneously achieved at a relatively low reaction temperature of 513 K, making Cu@FAU a potential methanol synthesis catalyst from CO2 hydrogenation. With zeolite-encaged mononuclear Cu centers as the destined active sites, the unique reaction pathway of stepwise CO2 hydrogenation over Cu@FAU is illustrated. This work provides a clear example of catalytic reaction with explicit structure-activity relationship and highlights the power of zeolite catalysis in complex chemical transformations.

Electrochemical Promotion of CO2 Hydrogenation Using a Pt/YSZ Fuel Cell Type Reactor

The hydrogenation of CO₂ is a reaction of key technological and environmental importance, as it contributes to the sustainable production of fuels while assisting in the reduction of a major greenhouse gas. The reaction has received substantial attention over the years within the catalysis and electrocatalysis communities. In this respect, the electrochemical promotion of catalysis (EPOC) has been applied successfully to the CO₂ hydrogenation reaction to improve the catalytic activity and selectivity of conductive films supported on solid electrolytes. However, designing an effective electrocatalytic reactor remains a challenge due to the connections required between the electrodes and the external potentiostat/galvanostat. This drawback could be alleviated if the catalytic reaction occurs in a reactor that simultaneously operates as a power generator. In this work, the Electrochemical Promotion of the CO₂ hydrogenation reaction in a low-temperature solid oxide electrolyte fuel cell (SOFC) reactor is studied using yttria-stabilized zirconia (YSZ) and a platinum (Pt) electrode catalyst. The system has been studied in two distinct operation modes: (i) when the necessary energy for the electrochemical promotion is produced through the parallel reaction of H₂ oxidation (galvanic operation) and (ii) when a galvanostat/potentiostat is used to impose the necessary potential (electrolytic operation). The performance of the fuel cell declines less than 15% in the presence of the reactant mixture (CO₂ and H₂) while producing enough current to conduct EPOC experiments. During the electrolytic operation of the electrochemical cell, the CO production rate is significantly increased by up to 50%.

One-Step Synthesis of a High Entropy Oxide-Supported Rhodium Catalyst for Highly Selective CO Production in CO2 Hydrogenation

High entropy oxide (HEO) has shown to be a new type of catalyst support with tunable composition-function properties for many chemical reactions. However, the preparation of a metal nanoparticle catalyst supported on a metal oxide support is time-consuming and takes multiple complicated steps. Herein, we used a one-step glycine-nitrate-based combustion method to synthesize highly dispersed rhodium nanoparticles on a high surface area HEO. This catalyst showed high selectivity to produce CO in CO₂ hydrogenation with 80% higher activity compared to rhodium nanoparticle-based catalysts. We also studied the effect of different metal elements in HEO and demonstrated that high CO selectivity was achieved if one of the metals in the metal oxide support favored CO production. We identified that copper and zinc were responsible for the observed high CO selectivity due to their low *CO binding strength. During hydrogenation, a strong metal-support interaction was created through charge transfer and formed an encapsulated structure between rhodium nanoparticles and the HEO support to lower the *CO binding strength, which enabled high CO selectivity in the reaction. By combining different metal oxides into HEO as a catalyst support, high activity and high selectivity can be achieved at the same time in the CO₂ hydrogenation reaction.

Copper-doped ZnO-ZrO2 solid solution catalysts for promoting methanol synthesis from CO2 hydrogenation

Copper-doped ZnO-ZrO₂ solid solution catalysts were synthesized via co-precipitation for promoting CH₃OH synthesis via hydrogenation of CO₂. Various testing methods were applied to investigate the effect of various copper contents on the catalysts. The catalytic performance was evaluated by a fixed bed reactor. XRD, HRTEM and Raman spectra collectively indicated that a ZnO-ZrO₂ solid solution catalyst with 3% Cu had a higher Cu dispersion, while the H₂-TPR results confirmed that a catalyst with 3% Cu had more Cu active sites under low temperature H₂ pretreatment. When the copper content increased to 5% and 10%, the catalyst showed a better Cu crystallinity and a worse Cu dispersion, which could have a negative effect. Therefore, the CO₂ conversion and methanol yield with a 3% CuZnO-ZrO₂ catalyst at 5 MPa, 250°C and 12 000 ml/(g h) increased by 8.6% and 7.6%, respectively. Moreover, the CH₃OH selectivity and catalytic stability of the solid solution catalyst were better than those of the traditional CZA catalyst.

Heterogeneous Catalytic Systems for Carbon Dioxide Hydrogenation to Value-Added Chemicals

Utilizing renewable energy to hydrogenate carbon dioxide into fuels eliminates massive CO2 emissions from the atmosphere and diminishes our need for using fossil fuels. This review presents the most recent developments for designing heterogeneous catalysts for the hydrogenation of CO2 to formate, methanol, and C2+ hydrocarbons. Thermodynamic challenges and mechanistic insights are discussed, providing a strong foundation to propose a suitable catalyst. The main body of this review focuses on nanostructured catalysts for constructing efficient heterogeneous systems. The most important factors affecting catalytic performance are highlighted, including active metals, supports and promoters that can potentially be used. The summary of the results and the outlook are presented in the final section.

Cu2In alloy-embedded ZrO2 catalysts for efficient CO2 hydrogenation to methanol: promotion of plasma modification

Cu₁In₂Zr₄-O-C catalysts with Cu₂In alloy structure were prepared by using the sol-gel method. Cu₁In₂Zr₄-O-PC and Cu₁In₂Zr₄-O-CP catalysts were obtained from plasma-modified Cu₁In₂Zr₄-O-C before and after calcination, respectively. Under the conditions of reaction temperature 270°C, reaction pressure 2 MPa, CO₂/H₂ = 1/3, and GHSV = 12,000 mL/(g h), Cu₁In₂Zr₄-O-PC catalyst has a high CO₂ conversion of 13.3%, methanol selectivity of 74.3%, and CH₃OH space-time yield of 3.26 mmol/gcat/h. The characterization results of X-ray diffraction (XRD), scanning electron microscopy (SEM), and temperature-programmed reduction chemisorption (H₂-TPR) showed that the plasma-modified catalyst had a low crystallinity, small particle size, good dispersion, and excellent reduction performance, leading to a better activity and selectivity. Through plasma modification, the strong interaction between Cu and In in Cu₁In₂Zr₄-O-CP catalyst, the shift of Cu 2p orbital binding energy to a lower position, and the decrease in reduction temperature all indicate that the reduction ability of Cu₁In₂Zr₄-O-CP catalyst is enhanced, and the CO₂ hydrogenation activity is improved.

Cu-bound formates are main reaction intermediates during CO2 hydrogenation to methanol over Cu/ZrO2

Cu/ZrO2 is a promising catalyst for the hydrogenation of CO2 to methanol. Reaction pathways involving formates or hydroxycarbonyls have been proposed. We show here that three different types of formates can be observed under reaction conditions at 220 °C and 3 bar, one being located on metallic Cu and two others being bound to ZrO2. The surface concentrations of formates were determined through calibration curves and their reactivity measured during chemical transient experiments. The Cu-bound formate represented only about 7% of surface formates, but exhibited a higher reactivity and was found to be the only formate that could account for all the production of methanol. Copper is thus not there only to activate H2, but also bears other crucial intermediates. This work reemphasizes that fully quantitative IR analyses and transient methods are required to unravel the role of surface species.

The Function of Hydrotalcite as a Support Material in the Alcohol- Assisted Catalytic CO2 Hydrogenation Reaction

The traditional CO2 hydrogenation reaction in gas phase always requires harsh reaction conditions to activate CO2, resulting in huge energy consumption. However, with the assistance of 1-butanol solvent, catalytic CO2 hydrogenation can be proceeded at a mild condition of 170 ᵒC and 30 bars. To further improve the catalytic performance of the widely studied Cu-ZnO-ZrO2 catalyst (CZZ), the catalysts were modified by incorporating hydrotalcite (HTC) as a support material. The addition of HTC significantly improved the copper dispersion and surface area of the catalyst. The performance of CZZ-HTC catalysts was investigated at varying weight percentages of HTC, and all showed higher space-time yield of methanol (STYMeOH) compared to the commercial catalyst. Notably, CZZ-6HTC exhibited the highest methanol selectivity, further highlighting the beneficial role of HTC as a support material.

Boosting methanol-mediated CO2 hydrogenation into aromatics by synergistically tailoring oxygen vacancy and acid site properties of multifunctional catalyst

Even though the direct hydrogenation of CO2 into aromatics has been realized via a methanol-mediated pathway and multifunctional catalyst, few works have been focused on the simultaneously rational design of each component in multifunctional catalyst to improve the performance. Also, the structure-function relationship between aromatics synthesis performance and the different catalytic components (reducible metal oxide and acidic zeolite) has been rarely investigated. Herein, we increase the oxygen vacancy (Ov) density in reducible Cr2O3 by sequential carbonization and oxidation (SCO) treatments of Cr-based metal-organic frameworks. Thanks to the enriched Ov, Cr2O3-based catalyst affords high methanol selectivity of 98.1% (without CO) at a CO2 conversion of 16.8% under high reaction temperature (350 oC). Furthermore, after combining with the acidic zeolite H-ZSM-5, the multifunctional catalyst realizes the direct conversion of CO2 into aromatics with conversion and selectivity as high as 25.4% and 80.1% (without CO), respectively. The property of acid site in H-ZSM-5, especially the Al species that located at the intersection of straight and sinusoidal channels, plays a vital role in enhancing the aromatics selectivity, which can be precisely controlled by varying the hydrothermal synthesis conditions. Our work provides a synergistic strategy to boost the aromatics synthesis performance from CO2 hydrogenation.

Solar Hydrocarbons: Single-Step, Atmospheric-Pressure Synthesis of C2-C4 Alkanes and Alkenes from CO2

Cobalt ferrite (CoFe2O4) spinel has been found to produce C2-C4 hydrocarbons in a single-step, ambient-pressure, photocatalytic hydrogenation of CO2 with a rate of 1.1 mmol g-1 h-1, selectivity of 29.8% and conversion yield of 12.9%. On stream the CoFe2O4 reconstructs to a CoFe-CoFe2O4 alloy-spinel nanocomposite which facilitates the light-assisted transformation of CO2 to CO and hydrogenation of the CO to C2-C4 hydrocarbons. Promising results obtained from a laboratory demonstrator bode well for the development of a solar hydrocarbon pilot refinery.

Recent Advances of Indium Oxide-Based Catalysts for CO2 Hydrogenation to Methanol: Experimental and Theoretical

Methanol synthesis from the hydrogenation of carbon dioxide (CO₂) with green H₂ has been proven as a promising method for CO₂ utilization. Among the various catalysts, indium oxide (In₂O₃)-based catalysts received tremendous research interest due to the excellent methanol selectivity with appreciable CO₂ conversion. Herein, the recent experimental and theoretical studies on In₂O₃-based catalysts for thermochemical CO₂ hydrogenation to methanol were systematically reviewed. It can be found that a variety of steps, such as the synthesis method and pretreatment conditions, were taken to promote the formation of oxygen vacancies on the In₂O₃ surface, which can inhibit side reactions to ensure the highly selective conversion of CO₂ into methanol. The catalytic mechanism involving the formate pathway or carboxyl pathway over In₂O₃ was comprehensively explored by kinetic studies, in situ and ex situ characterizations, and density functional theory calculations, mostly demonstrating that the formate pathway was extremely significant for methanol production. Additionally, based on the cognition of the In₂O₃ active site and the reaction path of CO₂ hydrogenation over In₂O₃, strategies were adopted to improve the catalytic performance, including (i) metal doping to enhance the adsorption and dissociation of hydrogen, improve the ability of hydrogen spillover, and form a special metal-In₂O₃ interface, and (ii) hybrid with other metal oxides to improve the dispersion of In₂O₃, enhance CO₂ adsorption capacity, and stabilize the key intermediates. Lastly, some suggestions in future research were proposed to enhance the catalytic activity of In₂O₃-based catalysts for methanol production. The present review is helpful for researchers to have an explicit version of the research status of In₂O₃-based catalysts for CO₂ hydrogenation to methanol and the design direction of next-generation catalysts.

CO2 Hydrogenation: Na Doping Promotes CO and Hydrocarbon Formation over Ru/m-ZrO2 at Elevated Pressures in Gas Phase Media

Sodium-promoted monoclinic zirconia supported ruthenium catalysts were tested for CO₂ hydrogenation at 20 bar and a H₂:CO₂ ratio of 3:1. Although increasing sodium promotion, from 2.5% to 5% by weight, slightly decreased CO₂ conversion (14% to 10%), it doubled the selectivity to both CO (~36% to ~71%) and chain growth products (~4% to ~8%) remarkably and reduced the methane selectivity by two-thirds (~60% to ~21%). For CO₂ hydrogenation during in situ DRIFTS under atmospheric pressure, it was revealed that Na increases the catalyst basicity and suppresses the reactivity of Ru sites. Higher basicity facilitates CO₂ adsorption, weakens the C-H bond of the formate intermediate promoting CO formation, and inhibits methanation occurring on ruthenium nanoparticle surfaces. The suppression of excessive hydrogenation increases the chain growth probability. Decelerated reduction during H₂-TPR/TPR-MS and H₂-TPR-EXAFS/XANES at the K-edge of ruthenium indicates that sodium is in contact with ruthenium. A comparison of the XANES spectra of unpromoted and Na-promoted catalysts after H₂ reduction showed no evidence of a promoting effect involving electron charge transfer.

Density Functional Theory Study of CO2 Hydrogenation on Transition-Metal-Doped Cu(211) Surfaces

The massive emission of CO₂ has caused a series of environmental problems, including global warming, which exacerbates natural disasters and human health. Cu-based catalysts have shown great activity in the reduction of CO₂, but the mechanism of CO₂ activation remains ambiguous. In this work, we performed density functional theory (DFT) calculations to investigate the hydrogenation of CO₂ on Cu(211)-Rh, Cu(211)-Ni, Cu(211)-Co, and Cu(211)-Ru surfaces. The doping of Rh, Ni, Co, and Ru was found to enhance CO₂ hydrogenation to produce COOH. For CO₂ hydrogenation to produce HCOO, Ru plays a positive role in promoting CO dissociation, while Rh, Ni, and Co increase the barriers. These results indicate that Ru is the most effective additive for CO₂ reduction in Cu-based catalysts. In addition, the doping of Rh, Ni, Co, and Ru alters the electronic properties of Cu, and the activity of Cu-based catalysts was subsequently affected according to differential charge analysis. The analysis of Bader charge shows good predictions for CO₂ reduction over Cu-based catalysts. This study provides some fundamental aids for the rational design of efficient and stable CO₂-reducing agents to mitigate CO₂ emission.

Ultra-thin ZrO2overcoating on CuO-ZnO-Al2O3catalyst by atomic layer deposition for improved catalytic performance of CO2hydrogenation to dimethyl ether

An ultra-thin overcoating of zirconium oxide (ZrO₂) film on CuO-ZnO-Al₂O₃(CZA) catalysts by atomic layer deposition (ALD) was proved to enhance the catalytic performance of CZA/HZSM-5 (H form of Zeolite Socony Mobil-5) bifunctional catalysts for hydrogenation of CO₂to dimethyl ether (DME). Under optimal reaction conditions (i.e. 240 °C and 2.8 MPa), the yield of product DME increased from 17.22% for the bare CZA/HZSM-5 catalysts, to 18.40% for the CZA catalyst after 5 cycles of ZrO₂ALD with HZSM-5 catalyst. All the catalysts modified by ZrO₂ALD displayed significantly improved catalytic stability of hydrogenation of CO₂to DME reaction, compared to that of CZA/HZSM-5 bifunctional catalysts. The loss of DME yield in 100 h of reaction was greatly mitigated from 6.20% (loss of absolute value) to 3.01% for the CZA catalyst with 20 cycles of ZrO₂ALD overcoating. Characterizations including hydrogen temperature programmed reduction, x-ray powder diffraction, and x-ray photoelectron spectroscopy revealed that there was strong interaction between Cu active centers and ZrO₂.

Nickel-Laden Dendritic Plasmonic Colloidosomes of Black Gold: Forced Plasmon Mediated Photocatalytic CO2 Hydrogenation

In this work, we have designed and synthesized nickel-laden dendritic plasmonic colloidosomes of Au (black gold-Ni). The photocatalytic CO₂ hydrogenation activities of black gold-Ni increased dramatically to the extent that measurable photoactivity was only observed with the black gold-Ni catalyst, with a very high photocatalytic CO production rate (2464 ± 40 mmol g_Ni^-1 h^-1) and 95% selectivity. Notably, the reaction was carried out in a flow reactor at low temperature and atmospheric pressure without external heating. The catalyst was stable for at least 100 h. Ultrafast transient absorption spectroscopy studies indicated indirect hot-electron transfer from the black gold to Ni in less than 100 fs, corroborated by a reduction in Au-plasmon electron-phonon lifetime and a bleach signal associated with Ni d-band filling. Photocatalytic reaction rates on excited black gold-Ni showed a superlinear power law dependence on the light intensity, with a power law exponent of 5.6, while photocatalytic quantum efficiencies increased with an increase in light intensity and reaction temperature, which indicated the hot-electron-mediated mechanism. The kinetic isotope effect (KIE) in light (1.91) was higher than that in the dark (∼1), which further indicated the electron-driven plasmonic CO₂ hydrogenation. Black gold-Ni catalyzed CO₂ hydrogenation in the presence of an electron-accepting molecule, methyl-p-benzoquinone, reduced the CO production rate, asserting the hot-electron-mediated mechanism. Operando diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) showed that CO₂ hydrogenation took place by a direct dissociation path via linearly bonded Ni-CO intermediates. The outstanding catalytic performance of black gold-Ni may provide a way to develop plasmonic catalysts for CO₂ reduction and other catalytic processes using black gold.

Highly Active Catalytic CO2 Hydrogenation to Lower Olefins via Spinel ZnGaOx Combined with SAPO-34

A key primary method for creating a carbon cycle and carbon neutrality is the catalytic hydrogenation of CO₂ into high value-added chemicals or fuels. In this work, ZnGaO_x oxides were prepared by parallel co-precipitation and physically mixed with SAPO-34 molecular sieves prepared by hydrothermal synthesis to produce ZnGaO_x /SAPO-34 bifunctional catalysts, which were evaluated for the catalytic synthesis of lower olefins (C₂ ⁼ -C₄ ⁼ ) from carbon dioxide hydrogenation. It was demonstrated that the reaction process requires oxygen defect activation, synergistic hydrogenation, and CO₂ alkaline adsorption of ZnGaO_x . The spinel structure of ZnGaO_x has more abundant oxygen defects and alkaline adsorption sites than the ZnGaO_x solid solution, which effectively enhances the catalytic performance. The CO₂ conversion was 28.52%, the selectivity of C₂ ⁼ -C₄ ⁼ in hydrocarbons reached 70.01%, and the single-pass yield of C₂ ⁼ -C₄ ⁼ was 10.95% at 370 °C, 3.0 MPa, and 4800 mL/g_cat /h.

Tuning CO2 Hydrogenation Selectivity through Reaction-Driven Restructuring on Cu-Ni Bimetal Catalysts

Tuning the selectivity of CO2 hydrogenation is of significant scientific interest, especially using nickel-based catalysts. Fundamental insights into CO2 hydrogenation on Ni-based catalysts demonstrate that CO is a primary intermediate, and product selectivity is strongly dependent on the oxidation state of Ni. Therefore, modifying the electronic structure of the nickel surface is a compelling strategy for tuning product selectivity. Herein, we synthesized well dispersed Cu-Ni bimetallic nanoparticles (NPs) using a simple hydrothermal method for CO selective CO2 hydrogenation. A detailed study on the monometallic (Ni and Cu) and bimetallic (CuxNi1-x) catalysts supported on γ-Al2O3 was performed to increase CO selectivity while maintaining the high reaction rate. The Cu0.5Ni0.5/γ-Al2O3 catalyst shows a high CO2 conversion and more CO product selectivity than its monometallic counterparts. The surface electronic and geometric structure of Cu0.5Ni0.5 bimetallic NPs was studied using ambient pressure X-ray photoelectron spectroscopy (AP-XPS) and in situ diffuse reflectance infrared Fourier-transform spectroscopy under reaction conditions. The Cu core atoms migrate toward the surface, resulting in the restructuring of the Cu@Ni core-shell structure to a Cu-Ni alloy during the reaction and functioning as the active site by enhancing CO desorption. A systematic correlation is obtained between catalytic activity from a continuous fixed-bed flow reactor and the surface electronic structural details derived from AP-XPS results, establishing the structure-activity relationship. This investigation contributes to providing a strategy for controlling CO2 hydrogenation selectivity by modifying the surface structure of bimetallic NP catalysts.

Architecture Design and Catalytic Activity: Non-Noble Bimetallic CoFe/fe3 O4 Core-Shell Structures for CO2 Hydrogenation

Non-noble metal catalysts now play a key role in promoting efficiently and economically catalytic reduction of CO₂ into clean energy, which is an important strategy to ameliorate global warming and resource shortage issues. Here, a non-noble bimetallic catalyst of CoFe/Fe₃ O₄ nanoparticles is successfully designed with a core-shell structure that is well dispersed on the defect-rich carbon substrate for the hydrogenation of CO₂ under mild conditions. The catalysts exhibit a high CO₂ conversion activity with the rate of 30% and CO selectivity of 99%, and extremely robust stability without performance decay over 90 h in the reverse water gas shift reaction process. Notably, it is found that the reversible exsolution/dissolution of cobalt in the Fe₃ O₄ shell will lead to a dynamic and reversible deactivation/regeneration of the catalysts, accompanying by shell thickness breathing during the repeated cycles, via atomic structure study of the catalysts at different reaction stages. Combined with density functional theory calculations, the catalytic activity reversible regeneration mechanism is proposed. This work reveals the structure-property relationship for rational structure design of the advanced non-noble metallic catalyst materials with much improved performance.

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.

Improved Itaconate Production with Ustilago cynodontis via Co-Metabolism of CO2-Derived Formate

In recent years, it was shown that itaconic acid can be produced from glucose with Ustilago strains at up to maximum theoretical yield. The use of acetate and formate as co-feedstocks can boost the efficiency of itaconate production with Ustilaginaceae wild-type strains by reducing the glucose amount and thus the agricultural land required for the biotechnological production of this chemical. Metabolically engineered strains (U. cynodontis Δfuz7 Δcyp3 ↑P_ria1 and U. cynodontis Δfuz7 Δcyp3 P_etefmttA ↑P_ria1) were applied in itaconate production, obtaining a titer of 56.1 g L^-1 and a yield of 0.55 g_itaconate per g_substrate. Both improved titer and yield (increase of 5.2 g L^-1 and 0.04 g_itaconate per g_substrate, respectively) were achieved when using sodium formate as an auxiliary substrate. By applying the design-of-experiments (DoE) methodology, cultivation parameters (glucose, sodium formate and ammonium chloride concentrations) were optimized, resulting in two empirical models predicting itaconate titer and yield for U. cynodontis Δfuz7 Δcyp3 P_etefmttA ↑P_ria1. Thereby, an almost doubled itaconate titer of 138 g L^-1 was obtained and a yield of 0.62 g_itaconate per g_substrate was reached during confirmation experiments corresponding to 86% of the theoretical maximum. In order to close the carbon cycle by production of the co-feed via a "power-to-X" route, the biphasic Ru-catalysed hydrogenation of CO₂ to formate could be integrated into the bioprocess directly using the obtained aqueous solution of formates as co-feedstock without any purification steps, demonstrating the (bio)compatibility of the two processes.

A Self-Separating Multiphasic System for Catalytic Hydrogenation of CO2 and CO2 -Derivatives to Methanol

Catalytic conversion of CO₂ and hydrogen to methanol was achieved in a self-separating multiphasic system comprising the tailor-made complex [Ru(CO)ClH(MACHO-C₁₂ )] (MACHO-C₁₂ =bis{2-[bis(4-dodecylphenyl)phosphino]ethyl}amine) in n-decane as the catalyst phase. Effective catalyst recycling was demonstrated for the carbonate and the amine-assisted pathway from CO₂ to methanol. The polar products MeOH or MeOH/H₂ O generated from the catalytic reactions spontaneously formed a separate phase, allowing product isolation and catalyst separation without the need for any additional solvent. In the amine-assisted hydrogenation of CO₂ , the catalyst phase was recycled over ten subsequent runs, reaching a total turnover number to MeOH of 19200 with an average selectivity of 96 %.

Physico-Chemical Modifications Affecting the Activity and Stability of Cu-Based Hybrid Catalysts during the Direct Hydrogenation of Carbon Dioxide into Dimethyl-Ether

The direct hydrogenation of CO₂ into dimethyl-ether (DME) has been studied in the presence of ferrierite-based CuZnZr hybrid catalysts. The samples were synthetized with three different techniques and two oxides/zeolite mass ratios. All the samples (calcined and spent) were properly characterized with different physico-chemical techniques for determining the textural and morphological nature of the catalytic surface. The experimental campaign was carried out in a fixed bed reactor at 2.5 MPa and stoichiometric H₂/CO₂ molar ratio, by varying both the reaction temperature (200-300 °C) and the spatial velocity (6.7-20.0 NL∙g_cat^-1∙h^-1). Activity tests evidenced a superior activity of catalysts at a higher oxides/zeolite weight ratio, with a maximum DME yield as high as 4.5% (58.9 mg_DME∙g_cat^-1∙h^-1) exhibited by the sample prepared by gel-oxalate coprecipitation. At lower oxide/zeolite mass ratios, the catalysts prepared by impregnation and coprecipitation exhibited comparable DME productivity, whereas the physically mixed sample showed a high activity in CO₂ hydrogenation but a low selectivity toward methanol and DME, ascribed to a minor synergy between the metal-oxide sites and the acid sites of the zeolite. Durability tests highlighted a progressive loss in activity with time on stream, mainly associated to the detrimental modifications under the adopted experimental conditions.

Recent Application of Core-Shell Nanostructured Catalysts for CO2 Thermocatalytic Conversion Processes

Carbon-intensive industries must deem carbon capture, utilization, and storage initiatives to mitigate rising CO₂ concentration by 2050. A 45% national reduction in CO₂ emissions has been projected by government to realize net zero carbon in 2030. CO₂ utilization is the prominent solution to curb not only CO₂ but other greenhouse gases, such as methane, on a large scale. For decades, thermocatalytic CO₂ conversions into clean fuels and specialty chemicals through catalytic CO₂ hydrogenation and CO₂ reforming using green hydrogen and pure methane sources have been under scrutiny. However, these processes are still immature for industrial applications because of their thermodynamic and kinetic limitations caused by rapid catalyst deactivation due to fouling, sintering, and poisoning under harsh conditions. Therefore, a key research focus on thermocatalytic CO₂ conversion is to develop high-performance and selective catalysts even at low temperatures while suppressing side reactions. Conventional catalysts suffer from a lack of precise structural control, which is detrimental toward selectivity, activity, and stability. Core-shell is a recently emerged nanomaterial that offers confinement effect to preserve multiple functionalities from sintering in CO₂ conversions. Substantial progress has been achieved to implement core-shell in direct or indirect thermocatalytic CO₂ reactions, such as methanation, methanol synthesis, Fischer-Tropsch synthesis, and dry reforming methane. However, cost-effective and simple synthesis methods and feasible mechanisms on core-shell catalysts remain to be developed. This review provides insights into recent works on core-shell catalysts for thermocatalytic CO₂ conversion into syngas and fuels.

Unraveling the Catalytic Performance of the Nonprecious Metal Single-Atom-Embedded Graphitic s-Triazine-Based C3N4 for CO2 Hydrogenation

Graphitic carbon nitride (g-C₃N₄) is regarded as a promising potent photoelectrocatalyst for CO₂ reduction. Here, extensive first-principles calculations and ab initio molecular dynamics (AIMD) simulations are performed to systematically explore the structural and electronic properties of nonprecious metal single-atom-embedded graphitic s-triazine-based C₃N₄ (M@gt-C₃N₄, M = Mn, Fe, Co, Ni, Cu, and Mo) monolayer materials and their catalytic performances as the single-atom catalysts (SACs) for CO₂ hydrogenation to HCOOH, CO, and CH₃OH. It is found that the atomically dispersed non-noble metal Mn, Fe, Co, and Mo sites anchored on gt-C₃N₄ can efficiently activate both H₂ and CO₂, and their coadsorbed state serves as a precursor to the hydrogenation of CO₂ to different C1 products. Among these SACs (M@gt-C₃N₄, M = Mn, Fe, Co, and Mo), Co@gt-C₃N₄ was predicted to have the best catalytic performance for CO₂ hydrogenation to C1 products, although their mechanistic details are somewhat different. The predicted energy barriers of the rate-determining steps for the conversion of CO₂ into HCOOH, CO, and CH₃OH on Co@gt-C₃N₄ are 0.58, 0.67, and 1.19 eV, respectively. The desorption of products is generally energy-demanding, but it can be facilitated remarkably by the subsequent adsorption of H₂, which regenerates M@gt-C₃N₄ for the next catalytic cycle. The present study demonstrates that the catalytic performance of gt-C₃N₄ can be well regulated by embedding the non-noble metal single atom, and the porous gt-C₃N₄ is nicely suited for the construction of high-performance single-atom catalysts.

Ga-Promoted CuCo-Based Catalysts for Efficient CO2 Hydrogenation to Ethanol: The Key Synergistic Role of Cu-CoGaOx Interfacial Sites

Currently, direct catalytic CO₂ hydrogenation to produce ethanol is an effective and feasible way for the resource utilization of CO₂. However, constructing non-precious metal catalysts with satisfactory activity and desirable ethanol selectivity remains a huge challenge. Herein, we reported gallium-promoted CuCo-based catalysts derived from single-source Cu-Co-Ga-Al layered double hydroxide precursors. It was manifested that the introduction of Ga species could strengthen strong interactions between Cu and Co oxide species, thereby modifying their electronic structures and thus facilitating the formation of abundant metal-oxide interfaces (i.e., Cu⁰/Cu⁺-CoGaO_x interfaces). Notably, the as-constructed Cu-CoGa catalyst with a Ga:Co molar ratio of 0.4 exhibited a high ethanol selectivity of 23.8% at a 17.8% conversion, along with a high space-time yield of 1.35 mmol_EtOH·g_cat^-1·h^-1 for ethanol under mild reaction conditions (i.e., 220 °C, 3 MPa pressure), which outperformed most non-noble metal-based catalysts previously reported. According to the comprehensive structural characterizations and in situ diffuse reflectance infrared Fourier transform spectra of CO₂/CO adsorption and CO₂ hydrogenation, it was unambiguously revealed that CH_x could be formed at oxygen vacancies of defective CoGaO_x species, while CO could be stabilized by Cu⁺ species, and thus the catalytic synergistic role of Cu⁰/Cu⁺-CoGaO_x interfacial sites promoted the generation of CH_x and CO intermediates to participate in the CH_x-CO coupling process and simultaneously inhibited alkylation reactions. The present work points out a promising new strategy for constructing CuCo-based catalysts with favorable interfacial sites for highly efficient CO₂ hydrogenation to produce ethanol.