Hydrogenation of CO2 on ZnO/Cu(100) and ZnO/Cu(111) Catalysts: Role of Copper Structure and Metal–Oxide Interface in Methanol Synthesis

Interfacial Catalysis at Atomic Level

Heterogeneous catalysts are pivotal to the chemical and energy industries, which are central to a multitude of industrial processes. Large-scale industrial catalytic processes rely on special structures at the nano- or atomic level, where reactions proceed on the so-called active sites of heterogeneous catalysts. The complexity of these catalysts and active sites often lies in the interfacial regions where different components in the catalysts come into contact. Recent advances in synthetic methods, characterization technologies, and reaction kinetics studies have provided atomic-scale insights into these critical interfaces. Achieving atomic precision in interfacial engineering allows for the manipulation of electronic profiles, adsorption patterns, and surface motifs, deepening our understanding of reaction mechanisms at the atomic or molecular level. This mechanistic understanding is indispensable not only for fundamental scientific inquiry but also for the design of the next generation of highly efficient industrial catalysts. This review examines the latest developments in atomic-scale interfacial engineering, covering fundamental concepts, catalyst design, mechanistic insights, and characterization techniques, and shares our perspective on the future trajectory of this dynamic research field.

Highly efficient hydrogenation of furfural to furfuryl alcohol over Cu-Al2O3-ZnO catalyst

The development of simple, efficient and economical catalysts for the hydrogenation of biomass to produce high value-added chemicals is of great significance in solving the energy crisis. In this work, a series of non-precious metal catalysts (Cu-Al2O3-ZnO) with different defect sites were prepared by etching Devarda's alloy. Under optimized mild reaction conditions, the furfural conversion and furfuryl alcohol selectivity are both greater than 99.0%, and the catalyst has good reusability. Characterisation and experiments were used to investigate the activate species for hydrogenation reaction. It can be proved that the low-valent Cu species in the Cu-Al2O3-ZnO catalysts play an important role as adsorption and dissociation sites for H2. Different etching degrees and sample reduction temperatures of the alloy can be used to adjust the content of acidic sites such as Al2O3 and CuO, which have appropriate adsorption properties for furfural. ZnO promotes the dispersion of the Cu species and enhances the accessibility of the active sites. The etching method achieves the interaction between species to further enhance the stability and activity of the catalyst. The catalytic performance of the catalyst is very competitive and this study provides a new method for the efficient hydrogenation of furfural to furfuryl alcohol.

Tuning the electronic structure and SMSI by integrating trimetallic sites with defective ceria for the CO2 reduction reaction

Heterogeneous catalysts have emerged as a potential key for closing the carbon cycle by converting carbon dioxide (CO2) into value-added chemicals. In this work, we report a highly active and stable ceria (CeO2)-based electronically tuned trimetallic catalyst for CO2 to CO conversion. A unique distribution of electron density between the defective ceria support and the trimetallic nanoparticles (of Ni, Cu, Zn) was established by creating the strong metal support interaction (SMSI) between them. The catalyst showed CO productivity of 49,279 mmol g-1 h-1 at 650 °C. CO selectivity up to 99% and excellent stability (rate remained unchanged even after 100 h) stemmed from the synergistic interactions among Ni-Cu-Zn sites and their SMSI with the defective ceria support. High-energy-resolution fluorescence-detection X-ray absorption spectroscopy (HERFD-XAS) confirmed this SMSI, further corroborated by in situ electron energy loss spectroscopy (EELS) and density functional theory (DFT) simulations. The in situ studies (HERFD-XAS & EELS) indicated the key role of oxygen vacancies of defective CeO2 during catalysis. The in situ transmission electron microscopy (TEM) imaging under catalytic conditions visualized the movement and growth of active trimetallic sites, which completely stopped once SMSI was established. In situ FTIR (supported by DFT) provided a molecular-level understanding of the formation of various reaction intermediates and their conversion into products, which followed a complex coupling of direct dissociation and redox pathway assisted by hydrogen, simultaneously on different active sites. Thus, sophisticated manipulation of electronic properties of trimetallic sites and defect dynamics significantly enhanced catalytic performance during CO2 to CO conversion.

Synthesis of defect-rich La2O2CO3 supports for enhanced CO2-to-methanol conversion efficiency

Converting CO2 to methanol is crucial for addressing fuel scarcity and mitigating the greenhouse effect. Cu-based catalysts, with their diverse surface states, offer the potential to control reaction pathways and generate reactive H* species. However, a major challenge lies in oxidizing active Cu0 species by water generated during the catalytic process. While nonreducible metal oxides are beneficial in stabilizing metallic states, their limited capability to generate surface oxygen vacancies (OV) hinders CO2 activation. Herein, we present a strategy by doping Nd into a La2O2CO3 (LOC) support, enhancing OV formation by disrupting its lattice dyadicity. This leads to higher Cu0 concentration and improved CO2 activation. The resulting Cu/LOC:Nd catalyst notably outperforms Cu/LOC and CuZnAl catalysts, achieving a methanol yield of 9.9 moles of methanol per hour per mole of Cu. Our approach opens up possibilities for enhancing Cu-based catalysts in CO2 conversion.

Cu-Supported ZnO under Conditions of CO2 Reduction to Methanol: Why 0.2 ML Coverage?

By hydrogenating carbon dioxide to value-added products such as methanol, heterogeneous catalysts can lower greenhouse gas emissions and generate alternative liquid fuels. The most common commercial catalyst for the reduction of CO2 to methanol is Cu/ZnO/Al2O3, where ZnO improves conversion and selectivity toward methanol. The structure of this catalyst is thought to be Zn oxy(hydroxyl) overlayers on the nanometer scale on Cu. In the presence of CO2 and H2 under reaction conditions, the Cu substrate itself can be restructured and/or partially oxidized at its interface with ZnO, or the Zn might be reduced, possibly completely to a CuZn alloy, making the exact structure and stoichiometry of the active site a topic of active debate. In this study, we examine Zn3 clusters on Cu(100) and Cu(111), as a subnano model of the catalyst. We use a grand canonical genetic algorithm to sample the system structure and stoichiometry under catalytic conditions: T of 550 K, initial partial pressures of H2 of 4.5 atm and CO2 of 0.5 atm, and 1% conversion. We uncover a strong dependence of the catalyst stoichiometry on the surface coverage. At the optimal 0.2 ML surface coverage, chains of Zn(OH) form on both Cu surfaces. On Cu(100), the catalyst has many thermally accessible metastable minima, whereas on Cu(111), it does not. No oxidation or reconstruction of the Cu is found. However, at a lower coverage of Zn, Zn3 clusters take on a metallic form on Cu(100), and slightly oxidized Zn3O on Cu(111), while the surface uptakes H to form a variety of low hydrides of Cu. We thus hypothesize that the 0.2 ML Zn coverage is optimal, as found experimentally, because of the stronger yet incomplete oxidation afforded by Zn at this coverage.

Metal substrate engineering to modulate CO2 hydrogenation to methanol on inverse Zr3O6/CuPd catalysts

It is well known that the performance of some key catalytic reactions has a strong dependence on metal catalyst surfaces. In the current work, this concept is further extended to the CuPd alloy-supported zirconium oxide inverse catalyst for CO2 hydrogenation to methanol. A combined DFT and microkinetic simulation study reveal that both the metal substrate surface and the precise exposed Cu or Pd metal atoms on the substrate have a pivotal influence on the catalytic mechanism and performance of the inverse catalyst for CO2 hydrogenation to methanol. Herein, CuPd(100), (111), and (110) surfaces with either Cu and Pd terminations have been examined, which provided five metal substrates as support for the inverse catalyst. Three different mechanisms, including the formate pathway, RWGS + CO-hydro pathway, and CO2 direct activation pathway, are explored under the same conditions; they take place at the interfacial sites between the metal alloy and oxide. The calculations indicated that the inverse catalyst with the CuPd(100) substrate demonstrates better performance than those with CuPd(110) and (111) for both formate and RWGS + CO-hydro mechanisms. Conversely, the reaction pathway is more sensitive to exposed atoms on the metal substrate. The best inverse catalyst, Zr3O6/CuPd(100) with either Cu or Pd terminations, demonstrated a methanol formation TOF above 0.30 site-1 s-1 and the selectivity was above 90% at 573 K, as evaluated from microkinetic simulation. The coverage analysis indicates the most populated species is HCOO*, which is consistent with experimental reports. Both kinetic and thermodynamics control steps are identified from DRC analysis for the best performing catalysts. Overall, the current study confirms the catalytic performance of the inverse Zr3O6/CuPd catalyst and demonstrates the tunable effects of the metal alloy substrate, which can facilitate effective optimization.

Enhanced Methanol Synthesis from CO2 Hydrogenation Achieved by Tuning the Cu-ZnO Interaction in ZnO/Cu2O Nanocube Catalysts Supported on ZrO2 and SiO2

The nature of the Cu-Zn interaction and especially the role of Zn in Cu/ZnO catalysts used for methanol synthesis from CO2 hydrogenation are still debated. Migration of Zn onto the Cu surface during reaction results in a Cu-ZnO interface, which is crucial for the catalytic activity. However, whether a Cu-Zn alloy or a Cu-ZnO structure is formed and the transformation of this interface under working conditions demand further investigation. Here, ZnO/Cu2O core-shell cubic nanoparticles with various ZnO shell thicknesses, supported on SiO2 or ZrO2 were prepared to create an intimate contact between Cu and ZnO. The evolution of the catalyst's structure and composition during and after the CO2 hydrogenation reaction were investigated by means of operando spectroscopy, diffraction, and ex situ microscopy methods. The Zn loading has a direct effect on the oxidation state of Zn, which, in turn, affects the catalytic performance. High Zn loadings, resulting in a stable ZnO catalyst shell, lead to increased methanol production when compared to Zn-free particles. Low Zn loadings, in contrast, leading to the presence of metallic Zn species during reaction, showed no significant improvement over the bare Cu particles. Therefore, our work highlights that there is a minimum content of Zn (or optimum ZnO shell thickness) needed to activate the Cu catalyst. Furthermore, in order to minimize catalyst deactivation, the Zn species must be present as ZnOx and not metallic Zn or Cu-Zn alloy, which is undesirably formed during the reaction when the precatalyst ZnO overlayer is too thin.

Demonstrating Pressure Jumping as a Tool to Address the Pressure Gap in High Pressure Photoelectron Spectroscopy of CO and CO2 Hydrogenation on Rh(211)

Operando probing by x-ray photoelectron spectroscopy (XPS) of certain hydrogenation reactions are often limited by the scattering of photoelectrons in the gas phase. This work describes a method designed to partially circumvent this so called pressure gap. By performing a rapid switch from a high pressure (where acquisition is impossible) to a lower pressure we can for a short while probe a "remnant" of the high pressure surface as well as the time dynamics during the re-equilibration to the new pressure. This methodology is demonstrated using the CO2 and the CO hydrogenation reaction over Rh(211). In the CO2 hydrogenation reaction, the remnant surface of a 2 bar pressure shows an adsorbate distribution which favors chemisorbed CHx adsorbates over chemisorbed CO. This contrasts against previous static operando spectra acquired at lower pressures. Furthermore, the pressure jumping method yields a faster acquisition and more detailed spectra than static operando measurements above 1 bar. In the CO hydrogenation reaction, we observe that CHx accumulated faster during the 275 mbar low pressure regime, and different hypotheses are presented regarding this observation.

Single-atom alloys of Cu(211) with earth-abundant metals for enhanced activity towards CO2 dissociation

CO2, a byproduct from various industrial reactions, must not be released into the atmosphere and should be managed through capture, conversion, and utilization. The first step in converting CO2 into valuable products is to break the C-O bond. This work focuses on designing Single Atom Catalysts (SACs) by doping Cu(211) surface with 13 different s, p, and d block elements with an aim to minimize the activation barrier for C-O bond cleavage. Our work demonstrates that SACs of Mg/Al/Pt@Cu(211) favor CO2 chemisorption compared to Cu(211) where CO2 physisorbs. The barrier for CO2 dissociation is lowest for Mg@Cu(211) and it increases in the order Mg@Cu(211) < Al@Cu(211) < Pt@Cu(211) < Zn@Cu(211) < Ga@Cu(211) < Cu@Cu(211) < Pd@Cu(211). These findings suggest that doping Cu(211) with earth-abundant metal like Mg can potentially be a viable catalyst for CO2 conversion, providing a promising solution to reduce carbon footprint and mitigate climate change.

Characterization of CO Adsorbed to Clean and Partially Oxidized Cu(211) and Cu(111)

Copper-based catalysts gain activity through the presence of poorly coordinated Cu atoms and incomplete oxidation at the surface. The catalytic mechanisms can in principle be observed by controlled dosing of reactants to single-crystal substrates. However, the interconnected influences of surface defects, partial oxidation, and adsorbate coverage present a large matrix of conditions that have not been fully explored in the literature. We recently characterized oxygen and carbon monoxide coadsorption on Cu(111), a nominally defect-free surface, and now extend our study to the stepped surface Cu(211). Temperature-programmed desorption of CO adsorbed to bare metal surfaces confirms that two sites dominate desorption from a saturated layer: atop terrace atoms of local (111) character and atop step edge atoms with CO bound more strongly to the latter. At low coverage, discrete CO resonances in reflection adsorption infrared spectra can be assigned to these sites: 2077 cm-1 for extended (111) terraces, 2093 cm-1 for step sites, and additional kink-adsorbed molecules at 2110 cm-1. With increasing coverage, in contrast to Cu(111), the infrared spectral features on Cu(211) evolve and shift as a consequence of dipole-dipole coupling between differentially occupied types of sites. Auger electron spectroscopy shows that exposure to background O2 oxidizes the (211) surface at a rate nearly 1 order of magnitude greater than (111); we argue that the resulting surface is stoichiometric Cu2O, as previously found for Cu(111). This oxide binds CO less strongly than the bare metal and the underlying crystal cut continues to influence the adsorption sites available to CO. On oxidized (111) terraces, broad absorption peaks at 2115-2120 cm-1; on oxidized Cu(211), CO adsorbed to step sites appears as a resolved secondary peak at 2144 cm-1. This suite of spectroscopic signatures, obtained under carefully controlled conditions, will help to determine the origin and fate of adsorbed species in future studies of reaction mechanisms on copper.

CO2 and CO Capture on the ZnO Surface: A GCMC and Electronic Structure Study

The amount of polluting gases released into the atmosphere has grown drastically. Among them, it is possible to cite the release of CO2 and CO gases on a large scale as one of the products of the complete and incomplete combustion of petroleum-derived fuels. It is worth noting that the production of energy by burning fossil fuels supplies the energy demand but causes environmental damage, and several studies have addressed the reduction. One of them is using materials with the potential to capture these gases. The experimental and theoretical studies have significant contributions that promote advances in this area. Among the materials investigated, ZnO has emerged, demonstrating the considerable potential for capturing various gases, including CO2 and CO. This work used density functional theory (DFT) and Grand Canonical Monte Carlo Method (GCMC) to investigate the adsorption of CO2 and CO on the surface of Zinc oxide (ZnO) to obtain adsorption isotherms and interaction energy and the interaction nature. The results suggest that CO2 adsorption slightly changed the angle of the O-C-O to values less than 180°. For the CO, its carbon atom interacts simultaneously with Zn and O of the ZnO surface. However, CO interactions have an ionic character with a lower binding energy value than the CO2 interaction. The energies calculated using the PM6 and DFT methods generated results compatible with the experimental values. In applications involving a mixture of these two gases, the adsorption of CO2 should be favored, and there may be inhibition of the adsorption of CO for high CO2 concentrations.

Atomic-Scale Visualization of Heterolytic H2 Dissociation and COx Hydrogenation on ZnO under Ambient Conditions

Studying catalytic hydrogenation reactions on oxide surfaces at the atomic scale has been challenging because of the typical occurrence of these processes at ambient or elevated pressures, rendering them less accessible to atomic-scale techniques. Here, we report an atomic-scale study on H2 dissociation and the hydrogenation of CO and CO2 on ZnO using ambient pressure scanning tunneling microscopy, ambient pressure X-ray photoelectron spectroscopy, and density functional theory (DFT) calculations. We directly visualized the heterolytic dissociation of H2 on ZnO(101̅0) under ambient pressure and found that dissociation reaction does not require the assistance of surface defects. The presence of CO or CO2 on ZnO at 300 K does not impede the availability of surface sites for H2 dissociation; instead, CO can even enhance the stability of coadsorbed hydride species, thereby facilitating their dissociative adsorption. Our results show that hydride is the active species for hydrogenation, while hydroxyl cannot hydrogenate CO/CO2 on ZnO. Both AP studies and DFT calculations showed that the hydrogenation of CO2 on ZnO is thermodynamically and kinetically more favorable compared to that of CO hydrogenation. Our results point toward a two-step mechanism for CO hydrogenation, involving initial oxidation to CO2 at step sites on ZnO followed by reaction with hydride to form formate. These findings provide molecular insights into the hydrogenation of CO/CO2 on ZnO and deepen our understanding of syngas conversion and oxide catalysis in general.

Unraveling surface structures of gallium promoted transition metal catalysts in CO2 hydrogenation

Gallium-containing alloys have recently been reported to hydrogenate CO2 to methanol at ambient pressures. However, a full understanding of the Ga-promoted catalysts is still missing due to the lack of information about the surface structures formed under reaction conditions. Here, we employed near ambient pressure scanning tunneling microscopy and x-ray photoelectron spectroscopy to monitor the evolution of well-defined Cu-Ga surfaces during CO2 hydrogenation. We show the formation of two-dimensional Ga(III) oxide islands embedded into the Cu surface in the reaction atmosphere. The islands are a few atomic layers in thickness and considerably differ from bulk Ga2O3 polymorphs. Such a complex structure, which could not be determined with conventional characterization methods on powder catalysts, should be used for elucidating the reaction mechanism on the Ga-promoted metal catalysts.

Confinement Effects in Well-Defined Metal-Organic Frameworks (MOFs) for Selective CO2 Hydrogenation: A Review

Decarbonization has become an urgent affair to restrain global warming. CO₂ hydrogenation coupled with H₂ derived from water electrolysis is considered a promising route to mitigate the negative impact of carbon emission and also promote the application of hydrogen. It is of great significance to develop catalysts with excellent performance and large-scale implementation. In the past decades, metal-organic frameworks (MOFs) have been widely involved in the rational design of catalysts for CO₂ hydrogenation due to their high surface areas, tunable porosities, well-ordered pore structures, and diversities in metals and functional groups. Confinement effects in MOFs or MOF-derived materials have been reported to promote the stability of CO₂ hydrogenation catalysts, such as molecular complexes of immobilization effect, active sites in size effect, stabilization in the encapsulation effect, and electron transfer and interfacial catalysis in the synergistic effect. This review attempts to summarize the progress of MOF-based CO₂ hydrogenation catalysts up to now, and demonstrate the synthetic strategies, unique features, and enhancement mechanisms compared with traditionally supported catalysts. Great emphasis will be placed on various confinement effects in CO₂ hydrogenation. The challenges and opportunities in precise design, synthesis, and applications of MOF-confined catalysis for CO₂ hydrogenation are also summarized.

Engineering nanoscale H supply chain to accelerate methanol synthesis on ZnZrOx

Metal promotion is the most widely adopted strategy for enhancing the hydrogenation functionality of an oxide catalyst. Typically, metal nanoparticles or dopants are located directly on the catalyst surface to create interfacial synergy with active sites on the oxide, but the enhancement effect may be compromised by insufficient hydrogen delivery to these sites. Here, we introduce a strategy to promote a ZnZrO_x methanol synthesis catalyst by incorporating hydrogen activation and delivery functions through optimized integration of ZnZrO_x and Pd supported on carbon nanotube (Pd/CNT). The CNT in the Pd/CNT + ZnZrO_x system delivers hydrogen activated on Pd to a broad area on the ZnZrO_x surface, with an enhancement factor of 10 compared to the conventional Pd-promoted ZnZrO_x catalyst, which only transfers hydrogen to Pd-adjacent sites. In CO₂ hydrogenation to methanol, Pd/CNT + ZnZrO_x exhibits drastically boosted activity-the highest among reported ZnZrO_x-based catalysts-and excellent stability over 600 h on stream test, showing potential for practical implementation.

Shape-Dependent CO2 Hydrogenation to Methanol over Cu2O Nanocubes Supported on ZnO

The hydrogenation of CO₂ to methanol over Cu/ZnO-based catalysts is highly sensitive to the surface composition and catalyst structure. Thus, its optimization requires a deep understanding of the influence of the pre-catalyst structure on its evolution under realistic reaction conditions, including the formation and stabilization of the most active sites. Here, the role of the pre-catalyst shape (cubic vs spherical) in the activity and selectivity of ZnO-supported Cu nanoparticles was investigated during methanol synthesis. A combination of ex situ, in situ, and operando microscopy, spectroscopy, and diffraction methods revealed drastic changes in the morphology and composition of the shaped pre-catalysts under reaction conditions. In particular, the rounding of the cubes and partial loss of the (100) facets were observed, although such motifs remained in smaller domains. Nonetheless, the initial pre-catalyst structure was found to strongly affect its subsequent transformation in the course of the CO₂ hydrogenation reaction and activity/selectivity trends. In particular, the cubic Cu particles displayed an increased activity for methanol production, although at the cost of a slightly reduced selectivity when compared to similarly sized spherical particles. These findings were rationalized with the help of density functional theory calculations.

Ga and Zn increase the oxygen affinity of Cu-based catalysts for the CO x hydrogenation according to ab initio atomistic thermodynamics

The direct hydrogenation of CO or CO2 to methanol, a highly vivid research area in the context of sustainable development, is typically carried out with Cu-based catalysts. Specific elements (so-called promoters) improve the catalytic performance of these systems under a broad range of reaction conditions (from pure CO to pure CO2). Some of these promoters, such as Ga and Zn, can alloy with Cu and their role remains a matter of debate. In that context, we used periodic DFT calculations on slab models and ab initio thermodynamics to evaluate both metal alloying and surface formation by considering multiple surface facets, different promoter concentrations and spatial distributions as well as adsorption of several species (O*, H*, CO* and ) for different gas phase compositions. Both Ga and Zn form an fcc-alloy with Cu due to the stronger interaction of the promoters with Cu than with themselves. While the Cu-Ga-alloy is more stable than the Cu-Zn-alloy at low promoter concentrations (<25%), further increasing the promoter concentration reverses this trend, due to the unfavoured Ga-Ga-interactions. Under CO2 hydrogenation conditions, a substantial amount of O* can adsorb onto the alloy surfaces, resulting in partial dealloying and oxidation of the promoters. Therefore, the CO2 hydrogenation conditions are actually rather oxidising for both Ga and Zn despite the large amount of H2 present in the feedstock. Thus, the growth of a GaO x /ZnO x overlayer is thermodynamically preferred under reaction conditions, enhancing CO2 adsorption, and this effect is more pronounced for the Cu-Ga-system than for the Cu-Zn-system. In contrast, under CO hydrogenation conditions, fully reduced and alloyed surfaces partially covered with H* and CO* are expected, with mixed CO/CO2 hydrogenation conditions resulting in a mixture of reduced and oxidised states. This shows that the active atmosphere tunes the preferred state of the catalyst, influencing the catalytic activity and stability, indicating that the still widespread image of a static catalyst under reaction conditions is insufficient to understand the complex interplay of processes taking place on a catalyst surface under reaction conditions, and that dynamic effects must be considered.

Mechanism of Catalytic CO2 Hydrogenation to Methane and Methanol Using a Bimetallic Cu3Pd Cluster at a Zirconia Support

For very small nanocluster-based catalysts, the exploration of the influence of the particle size, composition, and support offers precisely variable parameters in a wide material search space to control catalysts' performance. We present the mechanism of the CO₂ methanation reaction on the oxidized bimetallic Cu₃Pd tetramer (Cu₃PdO₂) supported on a zirconia model support represented by Zr₁₂O₂₄ based on the energy profile obtained from density functional theory calculations on the reaction of CO₂ and H₂. In order to determine the role of the Pd atom, the performance of Cu₃PdO₂ with monometallic Cu₄O₂ at the same support has been compared. Parallel to methane formation, the alternative path of methanol formation at this catalyst has also been investigated. The results show that the exchange of a single atom in Cu₄ with a single Pd atom improves catalyst/s performance via lowering the barriers associated with hydrogen dissociation steps that occur on the Pd atom. The above-mentioned results suggest that the doping strategy at the level of single atoms can offer a precise control knob for designing new catalysts with desired performance.

RAIRS Characterization of CO and O Coadsorption on Cu(111)

In a study preliminary to investigating CO₂ dissociation, we report our results on oxygen and carbon monoxide coadsorption on Cu(111). We use reflection adsorption infrared spectroscopy and Auger electron spectroscopy to characterize and quantify adsorbed species. On clean Cu(111), the CO internal stretch mode appears initially at 2077 cm^-1 for a surface temperature of ∼80 K. We accurately reproduce the previously determined redshift of the absorption band with increasing CO coverage. We subsequently oxidize the surface by exposure to O₂ at 300 K to ensure O₂ dissociation. The band's frequency and line shape of subsequently adsorbed CO at ∼80 K are not affected. However, the maximum absorbance and integrated peak intensities drop with increasing O coverage. The data suggest that CO is not adsorbed near O, likely as a consequence of the mechanism of Cu(111) surface oxidation by O₂ at 300 K. We discuss whether our RAIRS results may be used to quantify CO₂ dissociation in the zero-coverage limit.

Ensemble representation of catalytic interfaces: soloists, orchestras, and everything in-between

Catalytic systems are complex and dynamic, exploring vast chemical spaces on multiple timescales. In this perspective, we discuss the dynamic behavior of fluxional, heterogeneous thermal and electrocatalysts and the ensembles of many isomers which govern their behavior. We develop a new paradigm in catalysis theory in which highly fluxional systems, namely sub-nano clusters, isomerize on a much shorter timescale than that of the catalyzed reaction, so macroscopic properties arise from the thermal ensemble of isomers, not just the ground state. Accurate chemical predictions can only be reached through a many-structure picture of the catalyst, and we explain the breakdown of conventional methods such as linear scaling relations and size-selected prevention of sintering. We capitalize on the forward-looking discussion of the means of controlling the size of these dynamic ensembles. This control, such that the most effective or selective isomers can dominate the system, is essential for the fluxional catalyst to be practicable, and their targeted synthesis to be possible. It will also provide a fundamental lever of catalyst design. Finally, we discuss computational tools and experimental methods for probing ensembles and the role of specific isomers. We hope that catalyst optimization using chemically informed descriptors of ensemble nature and size will become a new norm in the field of catalysis and have broad impacts in sustainable energy, efficient chemical production, and more.

Who Does the Job? How Copper Can Replace Noble Metals in Sustainable Catalysis by the Formation of Copper–Mixed Oxide Interfaces

Following the need for an innovative catalyst and material design in catalysis, we provide a comparative approach using pure and Pd-doped LaCu_xMn_1–xO₃ (x = 0.3 and 0.5) perovskite catalysts to elucidate the beneficial role of the Cu/perovskite and the promoting effect of Cu_yPd_x/perovskite interfaces developing in situ under model NO + CO reaction conditions. The observed bifunctional synergism in terms of activity and N₂ selectivity is essentially attributed to an oxygen-deficient perovskite interface, which provides efficient NO activation sites in contact with in situ exsolved surface-bound monometallic Cu and bimetallic CuPd nanoparticles. The latter promotes the decomposition of the intermediate N₂O at low temperatures, enhancing the selectivity toward N₂. We show that the intelligent Cu/perovskite interfacial design is the prerequisite to effectively replace noble metals by catalytically equally potent metal–mixed-oxide interfaces. We have provided the proof of principle for the NO + CO test reaction but anticipate the extension to a universal concept applicable to similar materials and reactions.

In Situ Spectroscopic Characterization and Theoretical Calculations Identify Partially Reduced ZnO1-x /Cu Interfaces for Methanol Synthesis from CO2

The active site of the industrial Cu/ZnO/Al₂ O₃ catalyst used in CO₂ hydrogenation to methanol has been debated for decades. Grand challenges remain in the characterization of structure, composition, and chemical state, both microscopically and spectroscopically, and complete theoretical calculations are limited when it comes to describing the intrinsic activity of the catalyst over the diverse range of structures that emerge under realistic conditions. Here a series of inverse model catalysts of ZnO on copper hydroxide were prepared where the size of ZnO was precisely tuned from atomically dispersed species to nanoparticles using atomic layer deposition. ZnO decoration boosted methanol formation to a rate of 877 g_MeOH  kg_cat ^-1  h^-1 with ≈80 % selectivity at 493 K. High pressure in situ X-ray absorption spectroscopy demonstrated that the atomically dispersed ZnO species are prone to aggregate at oxygen-deficient ZnO ensembles instead of forming CuZn metal alloys. By modeling various potential active structures, density functional theory calculations and microkinetic simulations revealed that ZnO/Cu interfaces with oxygen vacancies, rather than stoichiometric interfaces, Cu and CuZn alloys were essential to catalytic activation.

The state of zinc in methanol synthesis over a Zn/ZnO/Cu(211) model catalyst

The active chemical state of zinc (Zn) in a zinc-copper (Zn-Cu) catalyst during carbon dioxide/carbon monoxide (CO₂/CO) hydrogenation has been debated to be Zn oxide (ZnO) nanoparticles, metallic Zn, or a Zn-Cu surface alloy. We used x-ray photoelectron spectroscopy at 180 to 500 millibar to probe the nature of Zn and reaction intermediates during CO₂/CO hydrogenation over Zn/ZnO/Cu(211), where the temperature is sufficiently high for the reaction to rapidly turn over, thus creating an almost adsorbate-free surface. Tuning of the grazing incidence angle makes it possible to achieve either surface or bulk sensitivity. Hydrogenation of CO₂ gives preference to ZnO in the form of clusters or nanoparticles, whereas in pure CO a surface Zn-Cu alloy becomes more prominent. The results reveal a specific role of CO in the formation of the Zn-Cu surface alloy as an active phase that facilitates efficient CO₂ 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.

Chemical Batteries with CO2

Efforts to obtain raw materials from CO2 by catalytic reduction as a means of combating greenhouse gas emissions are pushing the boundaries of the chemical industry. The dimensions of modern energy regimes, on the one hand, and the necessary transport and trade of globally produced renewable energy, on the other, will require the use of chemical batteries in conjunction with the local production of renewable electricity. The synthesis of methanol is an important option for chemical batteries and will, for that reason, be described here in detail. It is also shown that the necessary, robust, and fundamental understanding of processes and the material science of catalysts for the hydrogenation of CO2 does not yet exist.

Synergetic effect of metal–support for enhanced performance of the Cu–ZnO–ZrO2/UGSO catalyst for CO2 hydrogenation to methanol

Novel Cu–ZnO–ZrO2/UGSO catalysts for CO2 hydrogenation to methanol were developed using a metallurgical residue as catalytic support, focusing on (i) the synergy of Cu/Zn/Zr and UGSO composition and (ii) UGSO modification, on catalytic activity and stability.

Copper–zinc oxide interface as a methanol-selective structure in Cu–ZnO catalyst during catalytic hydrogenation of carbon dioxide to methanol

The proper contact of zinc oxide and copper phases is essential achieving high activity/selectivity toward methanol in the Cu–ZnO system.

Techniques for the characterization of single atom catalysts

Single atom catalysts (SACs) are a hot research area recently. Over most of the SACs, the singly dispersed atoms are the active sites, which contribute to the catalytic activities significantly compared with a catalyst with continuously packed active sites. It is essential to determine whether SACs have been successfully synthesized. Several techniques have been applied for the characterization of the dispersion states of the active sites over SACs, such as Energy Dispersive X-ray spectroscopy (EDX), Electron Energy Loss Spectroscopy (EELS), etc. In this review, the techniques for the identification of the singly dispersed sites over SACs are introduced, the advantages and limitations of each technique are pointed out, and the future research directions have been discussed. It is hoped that this review will be helpful for a more comprehensive understanding of the characterization and detection methods involved in SACs, and stimulate and promote the further development of this emerging research field.

Cesium-Induced Active Sites for C-C Coupling and Ethanol Synthesis from CO2 Hydrogenation on Cu/ZnO(0001̅) Surfaces

The efficient conversion of carbon dioxide, a major air pollutant, into ethanol or higher alcohols is a big challenge in heterogeneous catalysis, generating great interest in both basic scientific research and commercial applications. Here, we report the facilitated methanol synthesis and the enabled ethanol synthesis from carbon dioxide hydrogenation on a catalyst generated by codepositing Cs and Cu on a ZnO(0001̅) substrate. A combination of catalytic testing, X-ray photoelectron spectroscopy (XPS) measurements, and calculations based on density functional theory (DFT) and kinetic Monte Carlo (KMC) simulation was used. The results of XPS showed a clear change in the reaction mechanism when going from Cs/Cu(111) to a Cs/Cu/ZnO(0001̅) catalyst. The Cs-promoting effect on C-C coupling is a result of a synergy among Cs, Cu, and ZnO components that leads to the presence of CH_x and CH_yO species on the surface. According to the DFT-based KMC simulations, the deposition of Cs introduces multifunctional sites with a unique structure at the Cu-Cs-ZnO interface, particularly being able to promote the interaction with CO₂ and thus the methanol synthesis predominantly via the formate pathway. More importantly, it tunes the CHO binding strongly enough to facilitate the HCOOH decomposition to CHO via the formate pathway, but weakly enough to allow further hydrogenation to methanol. The fine-tuning of CHO binding also enables a close alignment of a CHO pair to facilitate the C-C coupling and eventually ethanol synthesis. Our study opens new possibilities to allow the highly active and selective conversion of carbon dioxide to higher alcohols on widely used and low-cost Cu-based catalysts.

Operando high-pressure investigation of size-controlled CuZn catalysts for the methanol synthesis reaction

Although Cu/ZnO-based catalysts have been long used for the hydrogenation of CO₂ to methanol, open questions still remain regarding the role and the dynamic nature of the active sites formed at the metal-oxide interface. Here, we apply high-pressure operando spectroscopy methods to well-defined Cu and Cu_0.7Zn_0.3 nanoparticles supported on ZnO/Al₂O₃, γ-Al₂O₃ and SiO₂ to correlate their structure, composition and catalytic performance. We obtain similar activity and methanol selectivity for Cu/ZnO/Al₂O₃ and CuZn/SiO₂, but the methanol yield decreases with time on stream for the latter sample. Operando X-ray absorption spectroscopy data reveal the formation of reduced Zn species coexisting with ZnO on CuZn/SiO₂. Near-ambient pressure X-ray photoelectron spectroscopy shows Zn surface segregation and the formation of a ZnO-rich shell on CuZn/SiO₂. In this work we demonstrate the beneficial effect of Zn, even in diluted form, and highlight the influence of the oxide support and the Cu-Zn interface in the reactivity.

Combustion Products of Calcium Carbide Reused by Cu-Based Catalysts for Acetylene Carbonylation

Sustainable development is a worldwide concern. This work mainly focuses on the reuse of the combustion products of calcium carbide and the influence of different kinds of copper on the acetylene carbonylation reaction. A series of catalysts were prepared by heating the precursors under various atmospheres (air, hydrogen, and nitrogen). The X-ray diffraction and the X-ray photoelectron spectroscopy have been analyzed regarding copper species composition and content in catalysts. The result of the Cu⁺-promoted reaction was in good agreement with the conducted density functional theory analysis, and we speculate that Cu⁺ promotes the transfer of electrons in the reaction. Transmission electron microscopy and elemental mapping evaluation confirmed the difference in Cu dispersion. Characterization of catalysts using temperature programmed desorption and pyridine Fourier-transform infrared revealed differences in their acidity. Acidity was found to be favorable for acetylene carbonylation. Selectivity and yield of the CuAlZn-LDO(N) catalyst at 225 °C were 73 and 70%, respectively, and the catalyst showed good stability over two consecutive cycles of reuse.

Optimized Microwave-Based Synthesis of Thermally Stable Inverse Catalytic Core-shell Motifs for CO2 Hydrogenation

The rational synthesis of Cu@TiO₂ core@shell nanowire (NW) structures was thoroughly explored using a microwave-assisted method through the tuning of experimental parameters such as but not limited to (i) controlled variation in molar ratios, (ii) the effect of discrete Ti precursors, (iii) the method of addition of the precursors themselves, and (iv) time of irradiation. Uniform coatings were obtained using Cu/Ti molar ratios of 1:2, 1:1, 2:1, and 4:1, respectively. It should be noted that although relative molar precursor concentrations primarily determined the magnitude of the resulting shell size, the dependence was nonlinear. Moreover, additionally important reaction parameters, such as precursor identity, the means of addition of precursors, and the reaction time, were individually explored with the objective of creating a series of optimized reaction conditions. As compared with Cu NWs alone, it is evident that both of the Cu@TiO₂ core-shell NW samples, regardless of pretreatment conditions, evinced much better catalytic performance, up to as much as 20 times greater activity as compared with standard Cu NWs. These results imply the significance of the Cu/TiO₂ interface in terms of promoting CO₂ hydrogenation, because TiO₂ alone is known to be inert for this reaction. Furthermore, it is additionally notable that the N₂ annealing pretreatment is crucial in terms of preserving the overall Cu@TiO₂ core@shell structure. We also systematically analyzed and tracked the structural and chemical evolution of our catalysts before and after the CO₂ reduction experiments. Indeed, we discovered that the core@shell wire motif was essentially maintained and conserved after this high-temperature reaction process, thereby accentuating the thermal stability and physical robustness of our as-prepared hierarchical motifs.

Present and new frontiers in materials research by ambient pressure x-ray photoelectron spectroscopy

In this topical review we catagorise all ambient pressure x-ray photoelectron spectroscopy publications that have appeared between the 1970s and the end of 2018 according to their scientific field. We find that catalysis, surface science and materials science are predominant, while, for example, electrocatalysis and thin film growth are emerging. All catalysis publications that we could identify are cited, and selected case stories with increasing complexity in terms of surface structure or chemical reaction are discussed. For thin film growth we discuss recent examples from chemical vapour deposition and atomic layer deposition. Finally, we also discuss current frontiers of ambient pressure x-ray photoelectron spectroscopy research, indicating some directions of future development of the field.

Synergy between a Silver-Copper Surface Alloy Composition and Carbon Dioxide Adsorption and Activation

Bimetallic electrocatalysts provide a promising strategy for improving performance, especially in the enhancement of selectivity of CO₂ reduction reactions. However, the first step of CO₂ activation on bimetallic materials remains obscure. Considering bimetallic silver-copper (AgCu) as an example, we coupled ambient pressure X-ray photoelectron spectroscopy (APXPS) and quantum mechanics (QM) to examine CO₂ adsorption and activation on AgCu exposed to CO₂ with and without H₂O at 298 K. The interplay between adsorbed species and the surface alloy composition of Cu and Ag is studied in atomic details. The APXPS experiment and density functional theory (DFT) calculations indicate that the clean sample has a Ag-rich surface layer. Upon adsorption of CO₂ and surface O, we found that it is thermodynamically more favorable to induce subsurface Cu atoms substitution for some surface Ag atoms, modifying the stability and activation of CO₂-related chemisorbed species. We further characterized this substitution effect by correlating the new adsorption species with the observed binding energy (BE) shift and intensity change in APXPS.

The Study of Reverse Water Gas Shift Reaction Activity over Different Interfaces: The Design of Cu-Plate ZnO Model Catalysts

CO2 hydrogenation to methanol is one of the main and valuable catalytic reactions applied on Cu/ZnO-based catalysts; the interface formed through Zn migration from ZnO support to the surface of Cu nanoparticle (ZnOx-Cu NP-ZnO) has been reported to account for methanol synthesis from CO2 hydrogenation. However, the accompanied reverse water gas shift (RWGS) reaction significantly decreases methanol selectivity and deactivates catalysts soon. Inhibition of RWGS is thus of great importance to afford high yield of methanol. The clear understanding of the reactivity of RWGS reaction on both the direct contact Cu-ZnO interface and ZnOx-Cu NP-ZnO interface is essential to reveal the low methanol selectivity in CO2 hydrogenation to methanol and look for efficient catalysts for RWGS reaction. Cu doped plate ZnO (ZnO:XCu) model catalysts were prepared through a hydrothermal method to simulate direct contact Cu-ZnO interface and plate ZnO supported 1 wt % Cu (1Cu/ZnO) catalyst was prepared by wet impregnation for comparison in RWGS reaction. Electron paramagnetic resonance (EPR), XRD, SEM, Raman, hydrogen temperature-programmed reduction (H2-TPR) and CO2 temperature-programmed desorption (CO2-TPD) were employed to characterize these catalysts. The characterization results confirmed that Cu incorporated into ZnO lattice and finally formed direct contact Cu-ZnO interface after H2 reduction. The catalytic performance revealed that direct contact Cu-ZnO interface displays inferior RWGS reaction reactivity at reaction temperature lower than 500 °C, compared with the ZnOx-Cu NP-ZnO interface; however, it is more stable at reaction temperature higher than 500 °C, enables ZnO:XCu model catalysts superior catalytic activity to that of 1Cu/ZnO. This finding will facilitate the designing of robust and efficient catalysts for both CO2 hydrogenation to methanol and RWGS reactions.