Breaking the activity-selectivity trade-off of CO2 hydrogenation to light olefins.

Catalytic hydrogenation of CO2 to value-added fuels and chemicals is of great importance to carbon neutrality but suffers from an activity-selectivity trade-off, leading to limited catalytic performance. Herein, the ZnFeAlO4 + SAPO-34 composite catalyst was designed, which can simultaneously achieve a CO2 conversion of 42%, a CO selectivity of 50%, and a C2-C4= selectivity of 83%, resulting in a C2-C4= yield of almost 18%. This superior catalytic performance was found to be from the presence of unconventional electron-deficient tetrahedral Fe sites and electron-enriched octahedral Zn sites in the ZnFeAlO4 spinel, which were active for the CO2 deoxygenation to CO via the reverse water gas shift reaction, and CO hydrogenation to CH3OH, respectively, leading to a route for CO2 hydrogenation to C2-C4=, where the kinetics of CO2 activation can be improved, the mass transfer of CO hydrogenation can be minimized, and the C2-C4= selectivity can be enhanced via modifying the acid density of SAPO-34. Moreover, the spinel structure of ZnFeAlO4 possessed a strong ability to stabilize the active Fe and Zn sites even at elevated temperatures, resulting in long-term stability of over 450 h for this process, exhibiting great potential for large-scale applications.

Steering CO2 hydrogenation toward C-C coupling to hydrocarbons using porous organic polymer/metal interfaces.

The conversion of CO2 into fuels and chemicals is an attractive option for mitigating CO2 emissions. Controlling the selectivity of this process is beneficial to produce desirable liquid fuels, but C-C coupling is a limiting step in the reaction that requires high pressures. Here, we propose a strategy to favor C-C coupling on a supported Ru/TiO2 catalyst by encapsulating it within the polymer layers of an imine-based porous organic polymer that controls its selectivity. Such polymer confinement modifies the CO2 hydrogenation behavior of the Ru surface, significantly enhancing the C2+ production turnover frequency by 10-fold. We demonstrate that the polymer layers affect the adsorption of reactants and intermediates while being stable under the demanding reaction conditions. Our findings highlight the promising opportunity of using polymer/metal interfaces for the rational engineering of active sites and as a general tool for controlling selective transformations in supported catalyst systems.

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.

Zn promoted GaZrOx Ternary Solid Solution Oxide Combined with SAPO-34 Effectively Converts CO2 to Light Olefins with Low CO Selectivity.

We proposed a new strategy for CO2 hydrogenation to prepare light olefins by introducing Zn into GaZrOx to construct ZnGaZrOx ternary oxides, which was combined with SAPO-34 to prepare a high-performance ZnGaZrOx/SAPO-34 tandem catalyst for CO2 hydrogenation to light olefins. By optimizing the Zn doping content, the ratio and mode of the two-phase composite, and the process conditions, the 3.5 %ZnGaZrOx/SAPO-34 tandem catalyst showed excellent catalytic performance and good high-temperature inhibition of the reverse water-gas shift (RWGS) reaction. The catalyst achieved 26.6 % CO2 conversion, 82.1 % C2 =-C4 = selectivity and 11.8 % light olefins yield. The ZnGaZrOx formed by introducing an appropriate amount of Zn into GaZrOx significantly enhanced the spillover H2 effect and also induced the generation of abundant oxygen vacancies to effectively promote the activation of CO2. Importantly, the RWGS reaction was also significantly suppressed at high temperatures, with the CO selectivity being only 46.1 % at 390 °C.

Cr2 O3 Promoted In2 O3 Catalysts for CO2 Hydrogenation to Methanol.

Cr2 O3 was applied to study the modification of In2 O3 based catalysts for CO2 hydrogenation to methanol reaction. Combined with X-ray diffraction (XRD), scanning transmission electron microscopy (STEM), X-ray photoelectron spectroscopy (XPS), etc., the structure of the catalysts was characterized. The reaction performances for CO2 hydrogenation to methanol were evaluated on a stainless-steel fix-bed reactor. The results showed that solid solutions were formed for the Cr2 O3 promoted In2 O3 catalysts. The important role of electronic interaction between Cr2 O3 and In2 O3 was revealed in the hydrogenation reaction. In1.25 Cr0.75 O3 sample exhibited the highest methanol yield, which was 2.8 times higher than that of pure In2 O3 . No deactivation was observed for In1.25 Cr0.75 O3 sample during the 50 hours of reaction. The improved catalytic performance may be due to the formation of the solid solutions and the highest amount of oxygen vacancies.

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.

Size and Morphology Dependent Activity of Cu Clusters for CO2 Activation and Reduction: A First Principles Investigation.

Various Cu-based materials in diverse forms have been investigated as efficient catalysts for electrochemical reduction of CO2; however, they suffer from issues such as higher over potential and poor selectivity. The activity and selectivity of CO2 electro reduction have been shown to change significantly when the surface morphology (steps, kinks, and edges) of these catalysts is altered. In light of this, size and morphology dependent activity of selected copper clusters, Cun (n=2-20) have been evaluated for the activation and reduction of CO2 molecule. The phase-space of these copper clusters is rich in conformations of distinct morphologies starting from planar, 2D geometries to prolate-shaped geometries and also high-symmetry structures. The binding efficiency and the activation of CO2 are highest for medium sized clusters (n=9-17) with prolate-morphologies as compared to small or larger sized CunCO2 clusters that are existing mainly as planar (triangular, tetragonal etc.) or highly-symmetric geometries (icosahedron, capped-icosahedron etc.), respectively. The best performing (prolate-shaped) CunCO2 conformations are quite fluxional and also they are thermally stable, as demonstrated by the molecular dynamics simulations. Furthermore, on these CunCO2 conformations, the step-by-step hydrogenation pathways of CO2 to produce value-added products like methanol, formic acid, and methane are exceptionally favorable and energy-efficient.

Novel heterogeneous Fe-based catalysts for carbon dioxide hydrogenation to long chain α-olefins-A review.

The thermocatalytic conversion of carbon dioxide (CO2) into high value-added chemicals provides a strategy to address the environmental problems caused by excessive carbon emissions and the sustainable production of chemicals. Significant progress has been made in the CO2 hydrogenation to long chain α-olefins, but controlling C-O activation and C-C coupling remains a great challenge. This review focuses on the recent advances in catalyst design concepts for the synthesis of long chain α-olefins from CO2 hydrogenation. We have systematically summarized and analyzed the ingenious design of catalysts, reaction mechanisms, the interaction between active sites and supports, structure-activity relationship, influence of reaction process parameters on catalyst performance, and catalyst stability, as well as the regeneration methods. Meanwhile, the challenges in the development of the long chain α-olefins synthesis from CO2 hydrogenation are proposed, and the future development opportunities are prospected. The aim of this review is to provide a comprehensive perspective on long chain α-olefins synthesis from CO2 hydrogenation to inspire the invention of novel catalysts and accelerate the development of this process.

A Brief Review of Recent Theoretical Advances in Fe-Based Catalysts for CO2 Hydrogenation.

Catalytic hydrogenation presents a promising approach for converting CO2 into valuable chemicals and fuels, crucial for climate change mitigation. Iron-based catalysts have emerged as key contributors, particularly in driving the reverse water-gas shift and Fischer-Tropsch synthesis reactions. Recent research has focused on enhancing the efficiency and selectivity of these catalysts by incorporating alkali metal promoters or transition metal dopants, enabling precise adjustments to their composition and properties. This review synthesizes recent theoretical advancements in CO2 hydrogenation with iron-based catalysts, employing density functional theory and microkinetic modeling. By elucidating the underlying mechanisms involving metallic iron, iron oxides, and iron carbides, we address current challenges and provide insights for future sustainable CO2 hydrogenation developments.

Development of Multifunctional Catalysts for the Direct Hydrogenation of Carbon Dioxide to Higher Alcohols.

The direct hydrogenation of greenhouse gas CO2 to higher alcohols (C2+OH) provides a new route for the production of high-value chemicals. Due to the difficulty of C-C coupling, the formation of higher alcohols is more difficult compared to that of other compounds. In this review, we summarize recent advances in the development of multifunctional catalysts, including noble metal catalysts, Co-based catalysts, Cu-based catalysts, Fe-based catalysts, and tandem catalysts for the direct hydrogenation of CO2 to higher alcohols. Possible reaction mechanisms are discussed based on the structure-activity relationship of the catalysts. The reaction-coupling strategy holds great potential to regulate the reaction network. The effects of the reaction conditions on CO2 hydrogenation are also analyzed. Finally, we discuss the challenges and potential opportunities for the further development of direct CO2 hydrogenation to higher alcohols.

Carbon-Supported Fe-Based Catalyst for Thermal-Catalytic CO2 Hydrogenation into C2+ Alcohols: The Effect of Carbon Support Porosity on Catalytic Performance.

Carbon materials supported Fe-based catalysts possess great potential for the thermal-catalytic hydrogenation of CO2 into valuable chemicals, such as alkenes and oxygenates, due to the excellent active sites' accessibility, appropriate interaction between the active site and carbon support, as well as the excellent capacities in C-O bond activation and C-C bond coupling. Even though tremendous progress has been made to boost the CO2 hydrogenation performance of carbon-supported Fe-based catalysts, e.g., additives modification, the choice of different carbon materials (graphene or carbon nanotubes), electronic property tailoring, etc., the effect of carbon support porosity on the evolution of Fe-based active sites and the corresponding catalytic performance has been rarely investigated. Herein, a series of porous carbon samples with different porosities are obtained by the K2CO3 activation of petroleum pitch under different temperatures. Fe-based active sites and the alkali promoter Na are anchored on the porous carbon to study the effect of carbon support porosity on the physicochemical properties of Fe-based active sites and CO2 hydrogenation performance. Multiple characterizations clarify that the bigger meso/macro-pores in the carbon support are beneficial for the formation of the Fe5C2 crystal phase for C-C bond coupling, therefore boosting the synthesis of C2+ chemicals, especially C2+ alcohols (C2+OH), while the limited micro-pores are unfavorable for C2+ chemicals synthesis owing to the sluggish crystal phase evolution and reactants' inaccessibility. We wish our work could enrich the horizon for the rational design of highly efficient carbon-supported Fe-based catalysts.

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.

Applications of Ni-Based Catalysts in Photothermal CO2 Hydrogenation Reaction.

Heterogeneous CO2 hydrogenation catalytic reactions, as the strategies for CO2 emission reduction and green carbon resource recycling, play important roles in alleviating global warming and energy shortages. Among these strategies, photothermal CO2 hydrogenation technology has become one of the hot catalytic technologies by virtue of the synergistic advantages of thermal catalysis and photocatalysis. And it has attracted more and more researchers' attentions. Various kinds of effective photothermal catalysts have been gradually discovered, and nickel-based catalysts have been widely studied for their advantages of low cost, high catalytic activity, abundant reserves and thermal stability. In this review, the applications of nickel-based catalysts in photothermal CO2 hydrogenation are summarized. Finally, through a good understanding of the above applications, future modification strategies and design directions of nickel-based catalysts for improving their photothermal CO2 hydrogenation activities are proposed.

Revealing the anti-sintering phenomenon on silica-supported nickel catalysts during CO2 hydrogenation.

The CO2 catalytic hydrogenation represents a promising approach for gas-phase CO2 utilization in a direct manner. Due to its excellent hydrogenation ability, nickel has been widely studied and has shown good activities in CO2 hydrogenation reactions, in addition to its high availability and low price. However, Ni-based catalysts are prone to sintering under elevated temperatures, leading to unstable catalytic performance. In the present study, various characterization techniques were employed to study the structural evolution of Ni/SiO2 during CO2 hydrogenation. An anti-sintering phenomenon is observed for both 9% Ni/SiO2 and 1% Ni/SiO2 during CO2 hydrogenation at 400°C. Results revealed that Ni species were re-dispersed into smaller-sized nanoparticles and formed Ni0 active species. While interestingly, this anti-sintering phenomenon leads to distinct outcomes for two catalysts, with a gradual increase in both reactivity and CH4 selectivity for 9% Ni/SiO2 presumably due to the formation of abundant surface Ni° from redispersion, while an apparent decreasing trend of CH4 selectivity for 1% Ni/SiO2 sample, presumably due to the formation of ultra-small nanoparticles that diffuse and partially filled the mesoporous pores of the silica support over time. Finally, the redispersion phenomenon was found relevant to the H2 gas in the reaction environment and enhanced as the H2 concentration increased. This finding is believed to provide in-depth insights into the structural evolution of Ni-based catalysts and product selectivity control in CO2 hydrogenation reactions.

Boosting oxygen vacancies by modulating the morphology of Au decorated In2O3 with enhanced CO2 hydrogenation activity to CH3OH.

CO2 hydrogenation to methanol has become one of the most promising ways for CO2 utilization, however, the CO2 conversion rate and methanol selectivity of this reaction still need to be improved for industrial application. Here we investigated the structure-activity relationship for CO2 conversion to methanol of In2O3-based catalysts by modulating morphology and decorating Au. Three different Au/In2O3 catalysts were prepared, their activity follow the sequence of Au/In2O3-nanosphere (Au/In2O3-NS) > Au/In2O3-nanoplate (Au/In2O3-NP) > Au/In2O3-hollow microsphere (Au/In2O3-HM). Au/In2O3-NS exhibited the best performance with good CO2 conversion of 12.7%, high methanol selectivity of 59.8%, and large space time yield of 0.32 gCH3OH/(hr·gcat) at 300°C. The high performance of Au/In2O3-NS was considered as the presence of Au. It contributes to the creation of more surface oxygen vacancies, which further promoted the CO2 adsorption and facilitated CO2 activation to form the formate intermediates towards methanol. This work clearly suggests that the activity of In2O3 catalyst can be effective enhanced by structure engineering and Au decorating.

Significant Roles of Surface Hydrides in Enhancing the Performance of Cu/BaTiO2.8 H0.2 Catalyst for CO2 Hydrogenation to Methanol.

Tuning the anionic site of catalyst supports can impact reaction pathways by creating active sites on the support or influencing metal-support interactions when using supported metal nanoparticles. This study focuses on CO2 hydrogenation over supported Cu nanoparticles, revealing a 3-fold increase in methanol yield when replacing oxygen anions with hydrides in the perovskite support (Cu/BaTiO2.8 H0.2 yields ~146 mg/h/gCu vs. Cu/BaTiO3 yields ~50 mg/h/gCu). The contrast suggests that significant roles are played by the support hydrides in the reaction. Temperature programmed reaction and isotopic labelling studies indicate that BaTiO2.8 H0.2 surface hydride species follow a Mars van Krevelen mechanism in CO2 hydrogenation, promoting methanol production. High-pressure steady-state isotopic transient kinetic analysis (SSITKA) studies suggest that Cu/BaTiO2.8 H0.2 possesses both a higher density and more active and selective sites for methanol production compared to Cu/BaTiO3 . An operando high-pressure diffuse reflectance infrared spectroscopy (DRIFTS)-SSITKA study shows that formate species are the major surface intermediates over both catalysts, and the subsequent hydrogenation steps of formate are likely rate-limiting. However, the catalytic reactivity of Cu/BaTiO2.8 H0.2 towards the formate species is much higher than Cu/BaTiO3 , likely due to the altered electronic structure of interface Cu sites by the hydrides in the support as validated by density functional theory (DFT) calculations.

Designing Dual-Site Catalysts for Selectively Converting CO2 into Methanol.

The variability of CO2 hydrogenation reaction demands new potential strategies to regulate the fine structure of the catalysts for optimizing the reaction pathways. Herein, we report a dual-site strategy to boost the catalytic efficiency of CO2-to-methanol conversion. A new descriptor, τ, was initially established for screening the promising candidates with low-temperature activation capability of CO2, and sequentially a high-performance catalyst was fabricated centred with oxophilic Mo single atoms, who was further decorated with Pt nanoparticles. In CO2 hydrogenation, the obtained dual-site catalysts possess a remarkably-improved methanol generation rate (0.27 mmol gcat. -1 h-1). For comparison, the singe-site Mo and Pt-based catalysts can only produce ethanol and formate acid at a relatively low reaction rate (0.11 mmol gcat. -1 h-1 for ethanol and 0.034 mmol gcat. -1 h-1 for formate acid), respectively. Mechanism studies indicate that the introduction of Pt species could create an active hydrogen-rich environment, leading to the alterations of the adsorption configuration and conversion pathways of the *OCH2 intermediates on Mo sites. As a result, the catalytic selectivity was successfully switched.

Near-Unity Photothermal CO2 Hydrogenation to Methanol based on a Molecule/Nanocarbon Hybrid Catalyst.

Solar-driven CO2-to-methanol conversion provides an intriguing route for both solar energy storage and CO2 mitigation. For scalable applications, near-unity methanol selectivity is highly desirable to reduce the energy and cost endowed by low-value byproducts and complex separation processes, but so far has not been achieved. Here we demonstrate a molecule/nanocarbon hybrid catalyst composed of carbon nanotube-supported molecularly dispersed cobalt phthalocyanine (CoPc/CNT), which synergistically integrates high photothermal conversion capability for affording an optimal reaction temperature with homogeneous and intrinsically-efficient active sites, to achieve a catalytic activity of 2.4 mmol gcat-1 h-1 and selectivity of ~99% in direct photothermal CO2 hydrogenation to methanol reaction. Both theoretical calculations and operando characterizations consistently confirm that the unique electronic structure of CoPc and appropriate reaction temperature cooperatively enable a thermodynamic favorable reaction pathway for highly selective methanol production. This work represents an important milestone towards the development of advanced photothermal catalysts for scalable and cost-effective CO2 hydrogenation.

Selective Gas-Phase Hydrogenation of CO2 to Methanol Catalysed by Metal-Organic Frameworks.

Large scale production of green CH3 OH obtained from CO2 and green H2 is a highly wanted process due to the role of CH3 OH as H2 /energy carrier and for producing chemicals. Starting with a short summary of the advantages of metal-organic frameworks (MOFs) as catalysts in liquid-phase reactions, the present article highlights the opportunities that MOFs may offer also for some gas-phase reactions, particularly for the selective CO2 hydrogenation to CH3 OH. It is commented that there is a temperature compatibility window that combines the thermal stability of some MOFs with the temperature required in the CO2 hydrogenation to CH3 OH that frequently ranges from 250 to 300 °C. The existing literature in this area is briefly organized according to the role of MOF as providing the active sites or as support of active metal nanoparticles (NPs). Emphasis is made to show how the flexibility in design and synthesis of MOFs can be used to enhance the catalytic activity by adjusting the composition of the nodes and the structure of the linkers. The influence of structural defects and material crystallinity, as well as the role that should play theoretical calculations in models have also been highlighted.

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.