Strong Evidence of the Role of H2O in Affecting Methanol Selectivity from CO2 Hydrogenation over Cu-ZnO-ZrO2

Thermochemical CO2 Reduction to Methanol over Metal-Based Single-Atom Catalysts (SACs): Outlook and Challenges for Developments

The conversion of thermodynamically inert CO2 into methanol holds immense promise for addressing the pressing environmental and energy challenges of our time. This article offers a succinct overview of the development of single-atom catalysts (SACs) for thermochemical hydrogenation of CO2 to methanol, encompassing research advancements, advantages, potential hurdles, and other essential aspects related to these catalysts. Our aim of this work is to provide a deeper understanding of the intricacies of the catalytic structures of the single-atom sites and their unique structure-activity relationships in catalyzing the conversion of CO2 to methanol. We also present insights into the optimal design of SACs, drawing from our own research and those of fellow scientists. This research thrust is poised to contribute significantly to the development of next-generation SACs, which are crucial in advancing the sustainable production of methanol from CO2.

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.

Sustainable methanol production from carbon dioxide: advances, challenges, and future prospects

The urgent need to address global carbon emissions and promote sustainable energy solutions has led to a growing interest in carbon dioxide (CO2) conversion technologies. Among these, the transformation of CO2 into methanol (MeOH) has gained prominence as an effective mitigation strategy. This review paper provides a comprehensive exploration of recent advances and applications in the direct utilization of CO2 for the synthesis of MeOH, encompassing various aspects from catalysts to market analysis, environmental impact, and future prospects. We begin by introducing the current state of CO2 mitigation strategies, highlighting the significance of carbon recycling through MeOH production. The paper delves into the chemistry and technology behind the conversion of CO2 into MeOH, encompassing key themes such as feedstock selection, material and energy supply, and the various conversion processes, including chemical, electrochemical, photochemical, and photoelectrochemical pathways. An in-depth analysis of heterogeneous and homogeneous catalysts for MeOH synthesis is provided, shedding light on the advantages and drawbacks of each. Furthermore, we explore diverse routes for CO2 hydrogenation into MeOH, emphasizing the technological advances and production processes associated with this sustainable transformation. As MeOH holds a pivotal role in a wide range of chemical applications and emerges as a promising transportation fuel, the paper explores its various chemical uses, transportation, storage, and distribution, as well as the evolving MeOH market. The environmental and energy implications of CO2 conversion to MeOH are discussed, including a thermodynamic analysis of the process and cost and energy evaluations for large-scale catalytic hydrogenation.

Heterogeneous Oxidase-Type Catalysis for H2 Generation at Low Temperatures

The long-term objective in the field of heterogeneous catalysis is to develop an enzyme-like catalytic pathway that can achieve exceptional catalytic performance even at low temperatures. Herein, we have demonstrated a heterogeneous oxidase-type catalysis on the ZnO-supported Ru clusters (Ru/ZnO) for efficient H2 generation from an aqueous solution of formaldehyde (HCHO) at low temperatures. Due to its unique reaction pathway, the Ru/ZnO catalysts exhibited a temperature-insensitive activity for H2 generation at the temperature of 15 to 45 °C. Remarkably, even at a low temperature of 5 °C, the Ru/ZnO catalysts still enabled an H2 generation rate of 13.8 mmol gcat-1 h-1 with a turnover frequency (TOF) of 1678 h-1. Additionally, instead of producing a CO2/CO molecule, the HCHO molecule underwent a transformation into formic acid and/or formate as the byproduct. This finding presents a novel class of heterogeneous catalysts to expand the potential application scenarios of liquid hydrogen storage and transportation systems.

Unveiling the Role of Water in Heterogeneous Photocatalysis of Methanol Conversion for Efficient Hydrogen Production

Water molecules, which act as both solvent and reactant, play critical roles in photocatalytic reactions for methanol conversion. However, the influence of water on the adsorption of methanol and desorption of liquid products, which are two essential steps that control the performance in photocatalysis, has been well under-explored. Herein, we reveal the role of water in heterogeneous photocatalytic processes of methanol conversion on the platinized carbon nitride (Pt/C3N4) model photocatalyst. In situ spectroscopy techniques, isotope effects, and computational calculations demonstrate that water shows adverse effects on the adsorption of methanol molecules and desorption processes of methanol oxidation products on the surface of Pt/C3N4, significantly altering the reaction pathways in photocatalytic methanol conversion process. Guided by these discoveries, a photothermal-assisted photocatalytic system is designed to achieve a high solar-to-hydrogen (STH) conversion efficiency of 2.3 %, which is among the highest values reported. This work highlights the important roles of solvents in controlling the adsorption/desorption behaviours of liquid-phase heterogeneous catalysis.

Low-nuclearity CuZn ensembles on ZnZrOx catalyze methanol synthesis from CO2

Metal promotion could unlock high performance in zinc-zirconium catalysts, ZnZrOx, for CO2 hydrogenation to methanol. Still, with most efforts devoted to costly palladium, the optimal metal choice and necessary atomic-level architecture remain unclear. Herein, we investigate the promotion of ZnZrOx catalysts with small amounts (0.5 mol%) of diverse hydrogenation metals (Re, Co, Au, Ni, Rh, Ag, Ir, Ru, Pt, Pd, and Cu) prepared via a standardized flame spray pyrolysis approach. Cu emerges as the most effective promoter, doubling methanol productivity. Operando X-ray absorption, infrared, and electron paramagnetic resonance spectroscopic analyses and density functional theory simulations reveal that Cu0 species form Zn-rich low-nuclearity CuZn clusters on the ZrO2 surface during reaction, which correlates with the generation of oxygen vacancies in their vicinity. Mechanistic studies demonstrate that this catalytic ensemble promotes the rapid hydrogenation of intermediate formate into methanol while effectively suppressing CO production, showcasing the potential of low-nuclearity metal ensembles in CO2-based methanol synthesis.

A Ce-CuZn catalyst with abundant Cu/Zn-OV-Ce active sites for CO2 hydrogenation to methanol

CO2 hydrogenation to chemicals and fuels is a significant approach for achieving carbon neutrality. It is essential to rationally design the chemical structure and catalytic active sites towards the development of efficient catalysts. Here we show a Ce-CuZn catalyst with enriched Cu/Zn-OV-Ce active sites fabricated through the atomic-level substitution of Cu and Zn into Ce-MOF precursor. The Ce-CuZn catalyst exhibits a high methanol selectivity of 71.1% and a space-time yield of methanol up to 400.3 g·kgcat-1·h-1 with excellent stability for 170 h at 260 °C, comparable to that of the state-of-the-art CuZnAl catalysts. Controlled experiments and DFT calculations confirm that the incorporation of Cu and Zn into CeO2 with abundant oxygen vacancies can facilitate H2 dissociation energetically and thus improve CO2 hydrogenation over the Ce-CuZn catalyst via formate intermediates. This work offers an atomic-level design strategy for constructing efficient multi-metal catalysts for methanol synthesis through precise control of active sites.

Role of H2O in Catalytic Conversion of C1 Molecules

Due to their role in controlling global climate change, the selective conversion of C1 molecules such as CH4, CO, and CO2 has attracted widespread attention. Typically, H2O competes with the reactant molecules to adsorb on the active sites and therefore inhibits the reaction or causes catalyst deactivation. However, H2O can also participate in the catalytic conversion of C1 molecules as a reactant or a promoter. Herein, we provide a perspective on recent progress in the mechanistic studies of H2O-mediated conversion of C1 molecules. We aim to provide an in-depth and systematic understanding of H2O as a promoter, a proton-transfer agent, an oxidant, a direct source of hydrogen or oxygen, and its influence on the catalytic activity, selectivity, and stability. We also summarize strategies for modifying catalysts or catalytic microenvironments by chemical or physical means to optimize the positive effects and minimize the negative effects of H2O on the reactions of C1 molecules. Finally, we discuss challenges and opportunities in catalyst design, characterization techniques, and theoretical modeling of the H2O-mediated catalytic conversion of C1 molecules.

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.

Unlocking a Dual-Channel Pathway in CO2 Hydrogenation to Methanol over Single-Site Zirconium on Amorphous Silica

Converting CO2 into methanol on a large scale is of great significance in the sustainable methanol economy. Zirconia species are considered to be an essential support in Cu-based catalysts due to their excellent properties for CO2 adsorption and activation. However, the evolution of Zr species during the reaction and the effect of their structure on the reaction pathways remain unclear. Herein, single-site Zr species in an amorphous SiO2 matrix are created by enhancing the Zr-Si interaction in Cu/ZrO2 -SiO2 catalysts. In situ X-ray absorption spectroscopy (XAS) reveals that the coordination environment of single-site Zr is sensitive to the atmosphere and reaction conditions. We demonstrate that the CO2 adsorption occurs preferably on the interface of Cu and single-site Zr rather than on ZrO2 nanoparticles. Methanol synthesis in reverse water-gas-shift (RWGS)+CO-hydro pathway is verified only over single-dispersed Zr sites, whereas the ordinary formate pathway occurs on ZrO2 nanoparticles. Thus, it expands a non-competitive parallel pathway as a supplement to the dominant formate pathway, resulting in the enhancement of Cu activity sixfold and twofold based on Cu/SiO2 and Cu/ZrO2 catalysts, respectively. The establishment of this dual-channel pathway by single-site Zr species in this work opens new horizons for understanding the role of atomically dispersed oxides in catalysis science.

Direct Conversion of CO2 into Alcohols Using Cu-Based Zeolite Catalysts

The direct hydrogenation of CO2 into alcohols is an attractive but challenging catalytic reaction. Herein, it was shown that Cu nanoparticles supported on MFI and BEA zeolites have high catalytic activity and selectivity for converting CO2 into ethanol and isopropanol. Furthermore, we investigated the effect of introducing mesopores via carbon templating and encapsulating the Cu nanoparticles via subsequent recrystallization. All the catalysts were characterized by N2 physisorption, XRD, SEM, TEM, NH3 TPD, XPS, and XRF, before we tested them in a high-pressure water-filled autoclave with a constant partial pressure of CO2 (1 MPa) and an increasing partial pressure of H2 (3-5 MPa). In general, the mesoporous zeolite catalysts resulted in a higher CO2 conversion and selectivity toward ethanol than their non-mesoporous equivalents, while the recrystallized catalyst with encapsulated Cu nanoparticles had a higher selectivity towards isopropanol. For example, Cu@m-S1 showed the highest isopropanol productivity among the recrystallized mesoporous zeolites, corresponding to 20.51 mmol g-1  h-1 under the given reaction conditions. These findings highlight the importance of mesopores in zeolite catalysts for CO2 hydrogenation to alcohols and point a new direction for further research and development.

Light-Induced Dynamic Restructuring of Cu Active Sites on TiO2 for Low-Temperature H2 Production from Methanol and Water

The structure and configuration of reaction centers, which dominantly govern the catalytic behaviors, often undergo dynamic transformations under reaction conditions, yet little is known about how to exploit these features to favor the catalytic functions. Here, we demonstrate a facile light activation strategy over a TiO2-supported Cu catalyst to regulate the dynamic restructuring of Cu active sites during low-temperature methanol steam reforming. Under illumination, the thermally deactivated Cu/TiO2 undergoes structural restoration from inoperative Cu2O to the originally active metallic Cu caused by photoexcited charge carriers from TiO2, thereby leading to substantially enhanced activity and stability. Given the low-intensity solar irradiation, the optimized Cu/TiO2 displays a H2 production rate of 1724.1 μmol g-1 min-1, outperforming most of the conventional photocatalytic and thermocatalytic processes. Taking advantages of the strong light-matter-reactant interaction, we achieve in situ manipulation of the Cu active sites, suggesting the feasibility for real-time functionalization of catalysts.

Dual active sites over Cu-ZnO-ZrO2 catalysts for carbon dioxide hydrogenation to methanol

CO₂ hydrogenation to methanol is a significant approach to tackle the problem of global warming and simultaneously meet the demand for the portable fuel. Cu-ZnO catalysts with various kinds of promoters have received wide attention. However, the role of promoter and the form of active sites in CO₂ hydrogenation are still in debate. Here, various molar ratios of ZrO₂ were added into the Cu-ZnO catalysts to tune the distributions of Cu⁰ and Cu⁺ species. A volcano-like trend between the ratio of Cu⁺/ (Cu⁺ + Cu⁰) and the amount of ZrO₂ is presented, among which the CuZn10Zr (the molar ratio of ZrO₂ is 10%) catalyst reaches the highest value. Correspondingly, the maximum value of space-time yield to methanol with 0.65 g_MeOH/(g_cat·hr) is obtained on CuZn10Zr at reaction conditions of 220°C and 3 MPa. Detailed characterizations demonstrate that dual active sites are proposed during CO₂ hydrogenation over CuZn10Zr catalyst. The exposed Cu⁰ takes participate in the activation of H₂, while on the Cu⁺ species, the intermediate of formate from the co-adsorption of CO₂ and H₂ prefers to be further hydrogenated to CH₃OH than decomposing into the by-product of CO, yielding a high selectivity of methanol.

Theoretical Perspective of Promoting Direct Methane-to-Methanol Conversion at Complex Metal Oxide-Metal Interfaces

Direct methane conversion to methanol has been considered as an effective and economic way to address greenhouse effects and the current high demand for methanol in industry. However, the process has long been challenging due to lack of viable catalysts to compromise the activation of methane that typically occurs at high temperatures and retaining of produced methanol that requires mild conditions. This Perspective demonstrates an effective strategy to promote direct methane to methanol conversion by engineering the active sites and chemical environments at complex metal oxide - copper oxide - copper interfaces. Such effort strongly depends on extensive theoretical studies by combining density functional theory (DFT) calculations and kinetic Monte Carlo (KMC) simulations to provide in-depth understanding of reaction mechanism and active sites, which build a strong basis to enable the identification of design principles and advance the catalyst optimization for selective CH₄-to-CH₃OH conversion.

Photocatalytic Reduction of Carbon Dioxide to Methanol: Carbonaceous Materials, Kinetics, Industrial Feasibility, and Future Directions

Photocatalytic carbon dioxide reduction (PCCR) for methanol synthesis (CH₃OH) targeting renewable energy resources is an attractive way to create a sustainable environment and also balance the carbon-neutral series. The application of PCCR to methanol enables the generation of solar energy while reducing CO₂, killing two birds with one stone in terms of energy and the environment. In recent years, research on CO₂ utilization has focused on hydrogenation of CO₂ to methanol due to global warming. This article mainly focuses on selective carbonaceous materials such as graphene, mesoporous carbon, and carbon nanotubes (CNTs) as catalysts for heterogeneous photocatalytic CO₂ reduction to methanol. In addition, special emphasis will be placed on the state of the art of PCCR catalysts as this type of research will be of great benefit for further development in this field. The main features of the reaction kinetics, techno-economic study, and current technological developments in PCCR are covered in detail.

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.

Probing the Nature of Zinc in Copper-Zinc-Zirconium Catalysts by Operando Spectroscopies for CO2 Hydrogenation to Methanol

Active Zn species in Cu-based methanol synthesis catalysts have not been clearly identified yet due to their complex nature and dynamic structural changes during reactions. Herein, atomically dispersed Zn on ZrO₂ support is established in Cu-based catalysts by separating Zn and Zr components from Cu (Cu-ZnZr) via the double-nozzle flame spray pyrolysis (DFSP) method. It exhibits superiority in methanol selectivity and yield compared to those with Cu-ZnO interface and isolated ZnO nanoparticles. Operando X-ray absorption spectroscopy (XAS) reveals that the atomically dispersed Zn species are induced during the reaction due to the strengthened Zn-Zr interaction. They can suppress formate decomposition to CO and decrease the H₂ dissociation energy, shifting the reaction to methanol production. This work enlightens the rational design of unique Zn species by regulating coordination environments and offers a new perspective for exploring complex interactions in multi-component catalysts.

Tailoring the Dispersion of Metals on ZnO with Preadsorbed Water

The dispersity of metal particles over oxide surfaces is generally critical for the applications of the metal/oxide hybridized systems. In this work, we have experimentally investigated the hydration effect of preadsorbed water species over the Cu and Pd particles deposited on the ZnO(10-10) surface. Using scanning tunneling microscopy (STM), we clearly saw that both Cu and Pd grow as three-dimensional particles on the clean ZnO(10-10) surface but disperse into single atoms and few-atom clusters on the water-covered surfaces. Moreover, X-ray photoelectron spectroscopy (XPS) measurements revealed that Cu is readily oxidized by interacting with the molecular water while Pd tends to bind the surface hydroxyls and keep neutral status. Our work has demonstrated the effective role of the surface water in tuning the morphologies as well as electronic states of the supported metals, which may bring new insights to a number of important surface processes with water in presence.

Flame-made ternary Pd-In2O3-ZrO2 catalyst with enhanced oxygen vacancy generation for CO2 hydrogenation to methanol

Palladium promotion and deposition on monoclinic zirconia are effective strategies to boost the performance of bulk In₂O₃ in CO₂-to-methanol and could unlock superior reactivity if well integrated into a single catalytic system. However, harnessing synergic effects of the individual components is crucial and very challenging as it requires precise control over their assembly. Herein, we present ternary Pd-In₂O₃-ZrO₂ catalysts prepared by flame spray pyrolysis (FSP) with remarkable methanol productivity and improved metal utilization, surpassing their binary counterparts. Unlike established impregnation and co-precipitation methods, FSP produces materials combining low-nuclearity palladium species associated with In₂O₃ monolayers highly dispersed on the ZrO₂ carrier, whose surface partially transforms from a tetragonal into a monoclinic-like structure upon reaction. A pioneering protocol developed to quantify oxygen vacancies using in situ electron paramagnetic resonance spectroscopy reveals their enhanced generation because of this unique catalyst architecture, thereby rationalizing its high and sustained methanol productivity.

Assembling multicomponent catalysts to harness synergic effects is challenging. Now, flame spray pyrolysis permits the synthesis of ternary Pd-In₂O₃-ZrO₂ catalysts with an optimal architecture and an enriched density of oxygen vacancies for maximal performance in CO₂-based methanol synthesis.

Catalytic Hydrogenation of CO2 to Methanol: A Review

High-efficiency utilization of CO2 facilitates the reduction of CO2 concentration in the global atmosphere and hence the alleviation of the greenhouse effect. The catalytic hydrogenation of CO2 to produce value-added chemicals exhibits attractive prospects by potentially building energy recycling loops. Particularly, methanol is one of the practically important objective products, and the catalytic hydrogenation of CO2 to synthesize methanol has been extensively studied. In this review, we focus on some basic concepts on CO2 activation, the recent research advances in the catalytic hydrogenation of CO2 to methanol, the development of high-performance catalysts, and microscopic insight into the reaction mechanisms. Finally, some thinking on the present research and possible future trend is presented.

Unveiling the Atomic Structure and Growth Dynamics of One-Dimensional Water on ZnO(10-10)

The adsorption and organization state of water on the metal oxide surface is of critical importance for wide fields where interface chemistry dominates. On the technically important ZnO(10-10) surface, we found water assembles into an one-dimensional (1D) chain structure at submonolayer coverage instead of the well-known half-dissociated two-dimensional (2D) island. With a combination of high resolution scanning tunneling microscopy (STM) and density functional theory (DFT) calculations, we clearly distinguished the single and double water chains, which are composed of dissociated monomers and half-dissociated dimers, respectively. Moreover, we unambiguously determined that single water molecules dissociate spontaneously before agglomerating into ordered phase, which is contrary to the proposition of previous studies. These results have deepened our understandings of the adsorbed water species on the ZnO surface, which may bring new insights into the mechanisms of water-stimulated surface reactions.

Dielectric Barrier Discharge Plasma-Assisted Catalytic CO2 Hydrogenation: Synergy of Catalyst and Plasma

CO2 hydrogenation is an effective way to convert CO2 to value-added chemicals (e.g., CH4 and CH3OH). As a thermal catalytic process, it suffers from dissatisfactory catalytic performances (low conversion/selectivity and poor stability) and high energy input. By utilizing the dielectric barrier discharge (DBD) technology, the catalyst and plasma could generate a synergy, activating the whole process in a mild condition, and enhancing the conversion efficiency of CO2 and selectivity of targeted product. In this review, a comprehensive summary of the applications of DBD plasma in catalytic CO2 hydrogenation is provided in detail. Moreover, the state-of-the-art design of the reactor and optimization of reaction parameters are discussed. Furthermore, several mechanisms based on simulations and experiments are provided. In the end, the existing challenges of this hybrid system and corresponding solutions are proposed.

Photo-assisted effective and selective reduction of CO2 to methanol on a Cu–ZnO–ZrO2 catalyst

Highly selective catalysis of CO2 hydrogenation to methanol with photo-assistance on Cu–ZnO–ZrO2, a photothermal synergistic catalyst.

Coalescence and shape oscillation of Au nanoparticles in CO2 hydrogenation to methanol

Recently, there has been renewed interest in Au nanoparticle (Au NP) catalysts owing to their high selectivity for CO₂ hydrogenation to methanol. However, there is still limited knowledge on the main factors of the catalytic activity and product selectivity of Au NPs. To address this issue, we utilized in situ transmission electron microscopy to observe the evolution of Au NP catalysts during CO₂ hydrogenation to methanol at 260 °C under ambient pressure. During the reaction, Au NPs sized ≤5 nm coalesced rapidly, forming stable Au NPs sized 5-10 nm with oscillating shapes. The first-principles calculations demonstrated that the adsorption of the reactant gas CO₂ is the main factor in inducing the coalescence of Au NPs, and CO and/or H₂O adsorption generated by the reaction caused the oscillation of the Au NP shape. Furthermore, the adsorption of various gas molecules resulted in continuous changes in the structure of the catalyst active center. In this study, the in situ observation of the dynamic evolution of the Au NP morphology is important in understanding the structural transformation of Au NP catalysts at the nanometer scale and determining the active site motifs under the reaction conditions. Moreover, this would allow us to further understand the size effect and the dynamic evolution behavior of the active center of Au NP catalysts, thereby providing a new idea for the development and application of new catalysts and strong theoretical support for heterogeneous catalytic reaction mechanisms.

Neighboring Zn-Zr Sites in a Metal-Organic Framework for CO2 Hydrogenation

ZrZnO_x is active in catalyzing carbon dioxide (CO₂) hydrogenation to methanol (MeOH) via a synergy between ZnO_x and ZrO_x. Here we report the construction of Zn²⁺-O-Zr⁴⁺ sites in a metal-organic framework (MOF) to reveal insights into the structural requirement for MeOH production. The Zn²⁺-O-Zr⁴⁺ sites are obtained by postsynthetic treatment of Zr₆(μ₃-O)₄(μ₃-OH)₄ nodes of MOF-808 by ZnEt₂ and a mild thermal treatment to remove capping ligands and afford exposed metal sites for catalysis. The resultant MOF-808-Zn catalyst exhibits >99% MeOH selectivity in CO₂ hydrogenation at 250 °C and a high space-time yield of up to 190.7 mg_MeOH g_Zn^-1 h^-1. The catalytic activity is stable for at least 100 h. X-ray absorption spectroscopy (XAS) analyses indicate the presence of Zn²⁺-O-Zr⁴⁺ centers instead of Zn_mO_n clusters. Temperature-programmed desorption (TPD) of hydrogen and H/D exchange tests show the activation of H₂ by Zn²⁺ centers. Open Zr⁴⁺ sites are also critical, as Zn²⁺ centers supported on Zr-based nodes of other MOFs without open Zr⁴⁺ sites fail to produce MeOH. TPD of CO₂ reveals the importance of bicarbonate decomposition under reaction conditions in generating open Zr⁴⁺ sites for CO₂ activation. The well-defined local structures of metal-oxo nodes in MOFs provide a unique opportunity to elucidate structural details of bifunctional catalytic centers.