The influence of strongly reducing conditions on strong metal–support interactions in Cu/ZnO catalysts used for methanol synthesis

The need for robust model systems in the study of hybrid interfaces for photocatalysis and photoelectrocatalysis

Small molecule conversion to value-added products using renewable energy sources has emerged as a promising strategy to mitigate our reliance on fossil fuels. Hybrid materials that integrate the strengths of photoabsorbers and co-catalysts (electrocatalysts) are essential for maximizing the efficiency of photochemical (PC) and photoelectrochemical (PEC) systems. In this perspective, we will focus on the need for fundamental studies with a strong emphasis on the importance of beginning with well-defined hybrid interfaces. A particular focus is given to small molecule adsorption studies that correlate surface structure and chemistry to reactivity, highlighting its potential in characterizing complex interfaces. We also make the case for understanding how light and electrochemical environments influence surface structure, adsorption, and reactivity and should be considered in model hybrid system design. Finally, we provide a framework to connect the theory and experiment of model hybrid surfaces to provide a molecular understanding of PC and PEC at these interfaces and accelerate our integration of these materials into real systems capable of meeting our renewable energy needs.

Controlling Metal-Support Interactions to Engineer Highly Active and Stable Catalysts for COx Hydrogenation

This perspective focuses on the modulation of metal-support interaction (MSI) in catalysts for COx hydrogenation, highlighting their profound impact on catalytic performance. Firstly, it outlines different strategies, including the use of highly reducible oxides and moderate reduction treatments, which induce the classical strong metal-support interaction (SMSI) effect and the electronic metal-support interaction (EMSI) effect. Morphology engineering and crystalline phase manipulation of oxides presented as effective methods to control EMSI are also discussed. The discrimination of SMSI and EMSI can be achieved using oxides with low encapsulation tendencies, such as ZrO2, which supports electronic modifications without or minimizing the overgrowth issues, optimizing the catalytic performance for methanation. Then, the synergy between Cu and ZnO in methanol synthesis, enhanced by SMSI, is emphasized inside. Optimizing support oxides to control oxygen vacancies enhances the catalytic performance of CO2 hydrogenation to methanol. Perspectives for the future research on the fundamental understanding of structure-MSI-performance relationship for catalyst design is discussed.

Metal-support interactions in metal oxide-supported atomic, cluster, and nanoparticle catalysis

Supported metal catalysts are essential to a plethora of processes in the chemical industry. The overall performance of these catalysts depends strongly on the interaction of adsorbates at the atomic level, which can be manipulated and controlled by the different constituents of the active material (i.e., support and active metal). The description of catalyst activity and the relationship between active constituent and the support, or metal-support interactions (MSI), in heterogeneous (thermo)catalysts is a complex phenomenon with multivariate (dependent and independent) contributions that are difficult to disentangle, both experimentally and theoretically. So-called "strong metal-support interactions" have been reported for several decades and summarized in excellent review articles. However, in recent years, there has been a proliferation of new findings related to atomically dispersed metal sites, metal oxide defects, and, for example, the generation and evolution of MSI under reaction conditions, which has led to the designation of (sub)classifications of MSI deserving to be critically and systematically evaluated. These include dynamic restructuring under alternating redox and reaction conditions, adsorbate-induced MSI, and evidence of strong interactions in oxide-supported metal oxide catalysts. Here, we review recent literature on MSI in oxide-supported metal particles to provide an up-to-date understanding of the underlying physicochemical principles that dominate the observed effects in supported metal atomic, cluster, and nanoparticle catalysts. Critical evaluation of different subclassifications of MSI is provided, along with discussions on the formation mechanisms, theoretical and characterization advances, and tuning strategies to manipulate catalytic reaction performance. We also provide a perspective on the future of the field, and we discuss the analysis of different MSI effects on catalysis quantitatively.

The Enigma of Methanol Synthesis by Cu/ZnO/Al2O3-Based Catalysts

The activity and durability of the Cu/ZnO/Al2O3 (CZA) catalyst formulation for methanol synthesis from CO/CO2/H2 feeds far exceed the sum of its individual components. As such, this ternary catalytic system is a prime example of synergy in catalysis, one that has been employed for the large scale commercial production of methanol since its inception in the mid 1960s with precious little alteration to its original formulation. Methanol is a key building block of the chemical industry. It is also an attractive energy storage molecule, which can also be produced from CO2 and H2 alone, making efficient use of sequestered CO2. As such, this somewhat unusual catalyst formulation has an enormous role to play in the modern chemical industry and the world of global economics, to which the correspondingly voluminous and ongoing research, which began in the 1920s, attests. Yet, despite this commercial success, and while research aimed at understanding how this formulation functions has continued throughout the decades, a comprehensive and universally agreed upon understanding of how this material achieves what it does has yet to be realized. After nigh on a century of research into CZA catalysts, the purpose of this Review is to appraise what has been achieved to date, and to show how, and how far, the field has evolved. To do so, this Review evaluates the research regarding this catalyst formulation in a chronological order and critically assesses the validity and novelty of various hypotheses and claims that have been made over the years. Ultimately, the Review attempts to derive a holistic summary of what the current body of literature tells us about the fundamental sources of the synergies at work within the CZA catalyst and, from this, suggest ways in which the field may yet be further advanced.

Metal-Support Interactions in Heterogeneous Catalysis: DFT Calculations on the Interaction of Copper Nanoparticles with Magnesium Oxide

Oxide supports play an important role in enhancing the catalytic properties of transition metal nanoparticles in heterogeneous catalysis. How extensively interactions between the oxide support and the nanoparticles impact the electronic structure as well as the surface properties of the nanoparticles is hence of high interest. In this study, the influence of a magnesium oxide support on the properties of copper nanoparticles with different size, shape, and adsorption sites is investigated using density functional theory (DFT) calculations. By proposing simple models to reduce the cost of the calculations while maintaining the accuracy of the results, we show using the nonreducible oxide support MgO as an example that there is no significant influence of the MgO support on the electronic structure of the copper nanoparticles, with the exception of adsorption directly at the Cu-MgO interface. We also propose a simplified methodology that allows us to reduce the cost of the calculations, while the accuracy of the results is maintained. We demonstrate in addition that the Cu nanowire model corresponds well to the nanoparticle model, which reduces the computational cost even further.

Gas-Dependent Active Sites on Cu/ZnO Clusters for CH3OH Synthesis

This study describes an instantaneously gas-induced dynamic transition of an industrial Cu/ZnO/Al₂O₃ catalyst. Cu/ZnO clusters become "alive" and lead to a promotion in reaction rate by almost one magnitude, in response to the variation of the reactant components. The promotional changes are functions of either CO₂-to-CO or H₂O-to-H₂ ratio which determines the oxygen chemical potential thus drives Cu/ZnO clusters to undergo reconstruction and allows the maximum formation of Cu-Zn²⁺ sites for CH₃OH synthesis.

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.

Manganese Oxide as a Promoter for Copper Catalysts in CO2 and CO Hydrogenation

In this work, we discuss the role of manganese oxide as a promoter in Cu catalysts supported on graphitic carbon during hydrogenation of CO2 and CO. MnOx is a selectivity modifier in an H2/CO2 feed and is a highly effective activity promoter in an H2/CO feed. Interestingly, the presence of MnOx suppresses the methanol formation from CO2 (TOF of 0.7 ⋅ 10-3 s-1 at 533 K and 40 bar) and enhances the low-temperature reverse water-gas shift reaction (TOF of 5.7 ⋅ 10-3 s-1) with a selectivity to CO of 87 %C. Using time-resolved XAS at high temperatures and pressures, we find significant absorption of CO2 to the MnO, which is reversed if CO2 is removed from the feed. This work reveals fundamental differences in the promoting effect of MnOx and ZnOx and contributes to a better understanding of the role of reducible oxide promoters in Cu-based hydrogenation catalysts.

Insight into the Nature of the ZnO x Promoter during Methanol Synthesis

Despite the great commercial relevance of zinc-promoted copper catalysts for methanol synthesis, the nature of the Cu-ZnO x synergy and the nature of the active Zn-based promoter species under industrially relevant conditions are still a topic of vivid debate. Detailed characterization of the chemical speciation of any promoter under high-pressure working conditions is challenging but specifically hampered by the large fraction of Zn spectator species bound to the oxidic catalyst support. We present the use of weakly interacting graphitic carbon supports as a tool to study the active speciation of the Zn promoter phase that is in close contact with the Cu nanoparticles using time-resolved X-ray absorption spectroscopy under working conditions. Without an oxidic support, much fewer Zn species need to be added for maximum catalyst activity. A 5-15 min exposure to 1 bar H2 at 543 K only slightly reduces the Zn(II), but exposure for several hours to 20 bar H2/CO and/or H2/CO/CO2 leads to an average Zn oxidation number of +(0.5-0.6), only slightly increasing to +0.8 in a 20 bar H2/CO2 feed. This means that most of the added Zn is in a zerovalent oxidation state during methanol synthesis conditions. The Zn average coordination number is 8, showing that this phase is not at the surface but surrounded by other metal atoms (whether Zn or Cu), and indicating that the Zn diffuses into the Cu nanoparticles under reaction conditions. The time scale of this process corresponds to that of the generally observed activation period for these catalysts. These results reveal the speciation of the relevant Zn promoter species under methanol synthesis conditions and, more generally, present the use of weakly interacting graphitic supports as an important strategy to avoid excessive spectator species, thereby allowing us to study the nature of relevant promoter species.

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.

Efficient CO2 catalytic hydrogenation over CuOx–ZnO/silicalite-1 with stable Cu+ species

Efficient and stable CuOx–ZnO/S-1 catalysts for CO2 hydrogenation were inexpensively prepared, in which the ZnO–Cu2O interface and silanol nests play key roles.

Methanol Reforming Processes for Fuel Cell Applications

Hydrogen production through methanol reforming processes has been stimulated over the years due to increasing interest in fuel cell technology and clean energy production. Among different types of methanol reforming, the steam reforming of methanol has attracted great interest as reformate gas stream where high concentration of hydrogen is produced with a negligible amount of carbon monoxide. In this review, recent progress of the main reforming processes of methanol towards hydrogen production is summarized. Different catalytic systems are reviewed for the steam reforming of methanol: mainly copper- and group 8–10-based catalysts, highlighting the catalytic key properties, while the promoting effect of the latter group in copper activity and selectivity is also discussed. The effect of different preparation methods, different promoters/stabilizers, and the formation mechanism is analyzed. Moreover, the integration of methanol steam reforming process and the high temperature–polymer electrolyte membrane fuel cells (HT-PEMFCs) for the development of clean energy production is discussed.

Structural Evolution of Cu/ZnO Catalysts during Water-Gas Shift Reaction: An In Situ Transmission Electron Microscopy Study

Supported metal catalysts experience significant structural evolution during the activation process and reaction conditions, which is critical to achieve a desired active surface and interface enabling efficient catalytic processes. However, such dynamic structural information and related mechanistic understandings remain largely elusive owing to the limitation of real-time capturing dynamic information under reaction conditions. Here, using in situ environment transmission electron microscopy, we demonstrate the atomic-scale structural evolution of the model Cu/ZnO catalyst under relevant water-gas shift reaction (WGSR) conditions. Under a CO gas environment, Cu nanoparticles decompose into smaller Cu species and redistribute on ZnO supports with either the crystalline Cu₂O or amorphous CuO_x phase due to a strong CO-Cu interaction. In addition, we visualize various metal-support interactions between Cu and ZnO under reaction conditions, e.g., ZnO clusters precipitating on Cu nanoparticles, which are critical to understand active sites of Cu/ZnO as catalysts for WGSR. These in situ atomic-scale observations highlight the dynamic interplays between Cu and ZnO that can be extended to other supported metal catalysts.

Structural and Chemical Transformations of Zinc Oxide Ultrathin Films on Pd(111) Surfaces

Structural and chemical transformations of ultrathin oxide films on transition metals lie at the heart of many complex phenomena in heterogeneous catalysis, such as the strong metal-support interaction (SMSI). However, there is limited atomic-scale understanding of these transformations, especially for irreducible oxides such as ZnO. Here, by combining density functional theory calculations and surface science techniques, including scanning tunneling microscopy, X-ray photoelectron spectroscopy, high-resolution electron energy loss spectroscopy, and low-energy electron diffraction, we investigated the interfacial interaction of well-defined ultrathin ZnO_xH_y films on Pd(111) under varying gas-phase conditions [ultrahigh vacuum (UHV), 5 × 10^-7 mbar of O₂, and a D₂/O₂ mixture] to shed light on the SMSI effect of irreducible oxides. Sequential treatment of submonolayer zinc oxide films in a D₂/O₂ mixture (1:4) at 550 K evoked reversible structural transformations from a bilayer to a monolayer and further to a Pd-Zn near-surface alloy, demonstrating that zinc oxide, as an irreducible oxide, can spread on metal surfaces and show an SMSI-like behavior in the presence of hydrogen. A mixed canonical-grand canonical phase diagram was developed to bridge the gap between UHV conditions and true SMSI environments, revealing that, in addition to surface alloy formation, certain ZnO_xH_y films with stoichiometries that do not exist in bulk are stabilized by Pd in the presence of hydrogen. Based on the combined theoretical and experimental observations, we propose that SMSI metal nanoparticle encapsulation for irreducible oxide supports such as ZnO involves both surface (hydroxy)oxide and surface alloy formation, depending on the environmental conditions.

Flame Synthesis of Cu/ZnO-CeO2 Catalysts: Synergistic Metal-Support Interactions Promote CH3OH Selectivity in CO2 Hydrogenation

The hydrogenation of CO₂ to CH₃OH is an important reaction for future renewable energy scenarios. Herein, we compare Cu/ZnO, Cu/CeO₂, and Cu/ZnO–CeO₂ catalysts prepared by flame spray pyrolysis. The Cu loading and support composition were varied to understand the role of Cu–ZnO and Cu–CeO₂ interactions. CeO₂ addition improves Cu dispersion with respect to ZnO, owing to stronger Cu–CeO₂ interactions. The ternary Cu/ZnO–CeO₂ catalysts displayed a substantially higher CH₃OH selectivity than binary Cu/CeO₂ and Cu/ZnO catalysts. The high CH₃OH selectivity in comparison with a commercial Cu–ZnO catalyst is also confirmed for Cu/ZnO–CeO₂ catalyst prepared with high Cu loading (∼40 wt %). In situ IR spectroscopy was used to probe metal–support interactions in the reduced catalysts and to gain insight into CO₂ hydrogenation over the Cu–Zn–Ce oxide catalysts. The higher CH₃OH selectivity can be explained by synergistic Cu–CeO₂ and Cu–ZnO interactions. Cu–ZnO interactions promote CO₂ hydrogenation to CH₃OH by Zn-decorated Cu active sites. Cu–CeO₂ interactions inhibit the reverse water–gas shift reaction due to a high formate coverage of Cu and a high rate of hydrogenation of the CO intermediate to CH₃OH. These insights emphasize the potential of fine-tuning metal–support interactions to develop improved Cu-based catalysts for CO₂ hydrogenation to CH₃OH.

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.

Identifying the nature of the active sites in methanol synthesis over Cu/ZnO/Al2O3 catalysts

The heterogeneously catalysed reaction of hydrogen with carbon monoxide and carbon dioxide (syngas) to methanol is nearly 100 years old, and the standard methanol catalyst Cu/ZnO/Al2O3 has been applied for more than 50 years. Still, the nature of the Zn species on the metallic Cu0 particles (interface sites) is heavily debated. Here, we show that these Zn species are not metallic, but have a positively charged nature under industrial methanol synthesis conditions. Our kinetic results are based on a self-built high-pressure pulse unit, which allows us to inject selective reversible poisons into the syngas feed passing through a fixed-bed reactor containing an industrial Cu/ZnO/Al2O3 catalyst under high-pressure conditions. This method allows us to perform surface-sensitive operando investigations as a function of the reaction conditions, demonstrating that the rate of methanol formation is only decreased in CO2-containing syngas mixtures when pulsing NH3 or methylamines as basic probe molecules.

Atomic Layer Deposition of ZnO on Mesoporous Silica: Insights into Growth Behavior of ZnO via In-Situ Thermogravimetric Analysis

ZnO is a remarkable material with many applications in electronics and catalysis. Atomic layer deposition (ALD) of ZnO on flat substrates is an industrially applied and well-known process. Various studies describe the growth of ZnO layers on flat substrates. However, the growth characteristics and reaction mechanisms of atomic layer deposition of ZnO on mesoporous powders have not been well studied. This study investigates the ZnO ALD process based on diethylzinc (DEZn) and water with silica powder as substrate. In-situ thermogravimetric analysis gives direct access to the growth rates and reaction mechanisms of this process. Ex-situ analytics, e.g., N₂ sorption analysis, XRD, XRF, HRTEM, and STEM-EDX mapping, confirm deposition of homogenous and thin films of ZnO on SiO₂. In summary, this study offers new insights into the fundamentals of an ALD process on high surface area powders.

State of the art and perspectives in heterogeneous catalysis of CO2 hydrogenation to methanol

The ever-increasing amount of anthropogenic carbon dioxide (CO₂) emissions has resulted in great environmental impacts, the heterogeneous catalysis of CO₂ hydrogenation to methanol is of great significance.

Cu@ZIF-8 derived inverse ZnO/Cu catalyst with sub-5 nm ZnO for efficient CO2 hydrogenation to methanol

Cu@ZIF-8 derived inverse ZnO/Cu with sub-5 nm ZnO acts as an efficient catalyst for CO₂ hydrogenation to methanol.

Superior Stability of Au/SiO2 Compared to Au/TiO2 Catalysts for the Selective Hydrogenation of Butadiene

Supported gold nanoparticles are highly selective catalysts for a range of both liquid-phase and gas-phase hydrogenation reactions. However, little is known about their stability during gas-phase catalysis and the influence of the support thereon. We report on the activity, selectivity, and stability of 2-4 nm Au nanoparticulate catalysts, supported on either TiO₂ or SiO₂, for the hydrogenation of 0.3% butadiene in the presence of 30% propene. Direct comparison of the stability of the Au catalysts was possible as they were prepared via the same method but on different supports. At full conversion of butadiene, only 0.1% of the propene was converted for both supported catalysts, demonstrating their high selectivity. The TiO₂-supported catalysts showed a steady loss of activity, which was recovered by heating in air. We demonstrated that the deactivation was not caused by significant metal particle growth or strong metal-support interaction, but rather, it is related to the deposition of carbonaceous species under reaction conditions. In contrast, all the SiO₂-supported catalysts were highly stable, with very limited formation of carbonaceous deposits. It shows that SiO₂-supported catalysts, despite their 2-3 times lower initial activities, clearly outperform TiO₂-supported catalysts within a day of run time.

New Insights into the Role of Al2 O3 in the Promotion of CuZnAl Catalysts: A Model Study

The Cu/Al₂ O₃ /ZnO(0001)-Zn ternary model catalysts and their binary analogues were prepared and characterized. It was found that Al₂ O₃ grew on the ZnO(0001)-Zn surface by a layer-by-layer model, whereas Cu grew on the ZnO(0001)-Zn surface as two-dimensional clusters up to 0.2 monolayers (ML), and thereafter formed three-dimensional clusters. Because of the layer-by-layer growth of Al₂ O₃ on the ZnO(0001)-Zn, Cu/Al₂ O₃ can be considered without the effect of ZnO. Ternary model catalyst Cu/Al₂ O₃ /ZnO(0001)-Zn, which has all three parts on the surface, was prepared by deposition of Cu on the surface of Al₂ O₃ /ZnO(0001)-Zn. Low-energy ion scattering spectra showed that Cu preferred to locate at the Al₂ O₃ /ZnO interfaces. Compared with Cu/ZnO, the addition of Al₂ O₃ obviously suppressed the reduction of copper oxides and led to a higher concentration of Cu⁺ . The Cu clusters were found to be covered by thin ZnO_x overlayers after reduction of Cu/Al₂ O₃ /ZnO(0001)-Zn by using H₂ . Therefore, the high activity of industrial Cu/ZnO/Al₂ O₃ catalysts may origin from Cu⁺ -rich clusters at the Al₂ O₃ /ZnO interface that are covered by thin ZnO_x overlayers.

IR spectroscopic investigations of chemical and photochemical reactions on metal oxides: bridging the materials gap

In this review, we highlight recent progress (2008–2016) in infrared reflection absorption spectroscopy (IRRAS) studies on oxide powders achieved by using different types of metal oxide single crystals as reference systems.

Structure sensitivity of Cu and CuZn catalysts relevant to industrial methanol synthesis

For decades it has been debated whether the conversion of synthesis gas to methanol over copper catalysts is sensitive or insensitive to the structure of the copper surface. Here we have systematically investigated the effect of the copper particle size in the range where changes in surface structure occur, that is, below 10 nm, for catalysts with and without zinc promotor at industrially relevant conditions for methanol synthesis. Regardless of the presence or absence of a zinc promotor in the form of zinc oxide or zinc silicate, the surface-specific activity decreases significantly for copper particles smaller than 8 nm, thus revealing structure sensitivity. In view of recent theoretical studies we propose that the methanol synthesis reaction takes place at copper surface sites with a unique configuration of atoms such as step-edge sites, which smaller particles cannot accommodate.

The dependence of the Cu-catalysed methanol synthesis on the structure of the Cu surface is a matter of debate. Here the authors show that activity falls for Cu and Cu-Zn particles below 8 nm and propose this is due to the absence of certain atomic configurations on the smaller particle surfaces.