Induced activation of the commercial Cu/ZnO/Al2O3 catalyst for the steam reforming of methanol

Copper-zinc-alumina catalysts are used industrially for methanol synthesis from feedstock containing carbon monoxide and carbon dioxide. The high performance of the catalyst stems from synergies that develop between its components. This important catalytic system has been investigated with a myriad of approaches, however, no comprehensive agreement on the fundamental source of its high activity has been reached. One potential source of disagreement is the considerable variation in pressure used in studies to understand a process that is performed industrially at pressures above 20 bar. Here, by systematically studying the catalyst state during temperature-programmed reduction and under carbon dioxide hydrogenation with in situ and operando X-ray absorption spectroscopy over four orders of magnitude in pressure, we show how the state and evolution of the catalyst is defined by its environment. The structure of the catalyst shows a strong pressure dependence, especially below 1 bar. As pressure gaps are a general problem in catalysis, these observations have wide-ranging ramifications. Copper-zinc-alumina is used in industry to catalyse the synthesis of methanol from CO2, but many aspects of its high performance remain elusive. Now, by using in situ and operando techniques over four orders of magnitude in pressure, the authors show how the catalyst structure and kinetics change with the applied conditions.

Following the structure of copper-zinc-alumina across the pressure gap in carbon dioxide hydrogenation

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. Description Zinc’s state in methanol synthesis Methanol can be synthesized from carbon monoxide (CO), carbon dioxide (CO2), and molecular hydrogen (H2) over copper–zinc (Cu–Zn) catalysts, but studies have disagreed about the chemical state of Zn. Although x-ray photoelectron spectroscopy (XPS) can determine its oxidation state, many studies have been limited to reaction pressures of a few millibars, where the rates are low. Amann et al. performed XPS at 180 to 500 millibars for CO2 and CO hydrogenation over a Zn/ZnO/Cu(211) surface at high turnover rates. Stoichiometric mixtures of CO2 and H2 formed ZnO, but for CO and H2, Zn became more metallic and formed Cu alloys. In industrial synthesis, CO2 and H2 are mixed with CO, and the presence of CO would generate Cu–Zn alloy sites active for CO2 reduction to methanol. —PDS CO induces Zn–Cu surface alloy sites that are active for CO2 hydrogenation in catalytic methanol synthesis.

https://www.science.org/doi/pdf/10.1126/science.abj7747?download=true

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

To address the ever-increasing CO 2 concentration problem in the atmospheric air arisen by massive consumption of fossil fuels, electrocatalytic technologies that reduce CO 2 to generate high value-added products have been gaining increasing interest. Cu-based CO 2 reduction catalysts have attracted widespread attention owing to their capability of generating C 1 and C 2+ products. However, Cu-based catalysts are highly challenged by their low product selectivity. Recently, Cu-based bimetallic catalysts have been found the unique catalytical activity and selectivity in CO 2 reduction reactions (CO 2 RR). The incorporation of other metals can change the electronic circumstance of Cu-based catalysts, promoting the adsorption ability of the intermediate products and consequently leading to high selectivity. In this minireview, we intend to summarize recent advances of Cu-based bimetallic catalysts in producing C 1 and C 2+ products, involving designing heterostructure, alloy, defects and surface modification engineering. We pay special attention to the regulation of electronic structure of the composite catalysts, as well as insights into the relationship between electronic property and catalytical performance for Cu-based bimetallic catalysts. This article analyzed the influence of the electronic structure of materials on the catalytic performance. Focusing on the application of four methods of Heterostructure Engineering, Alloy Engineering, Defect design Engineering and Surface modification Engineering to Cu-based bimetallic materials, the advantages of electronic regulation of materials and catalyzing CO 2 to generate high-value chemicals. 

Progress in Regulating Electronic Structure Strategies on Cu-Based Bimetallic Catalysts for CO2 Reduction Reaction

The development of highly active single-atom catalysts (SACs) and identifying their intrinsic active sites in oxidizing industrial hazardous hydrocarbons are challenging prospects. Tuning the electronic metal-support interactions (EMSIs) is valid for modulating the catalytic performance of SACs. We propose that the modulation of the EMSIs in a Pt1 -CuO SAC significantly promotes the activity of the catalyst in acetone oxidation. The EMSIs promote charge redistribution through the unified Pt-O-Cu moieties, which modulates the d-band structure of atomic Pt sites, and strengthens the adsorption and activation of reactants. The positively charged Pt atoms are superior for activating acetone at low temperatures, and the stretched Cu-O bonds facilitate the activation of lattice oxygen atoms to participate in subsequent oxidation. We believe that this work will guide researchers to engineer efficient SACs for application in hydrocarbon oxidation reactions. 

Modulating the Electronic Metal‐Support Interactions in Single‐Atom Pt <sub>1</sub> −CuO Catalyst for Boosting Acetone Oxidation

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 Cu0.7Zn0.3 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. The nature of the active species over Cu/ZnO catalysts for methanol synthesis remains elusive. Here, the authors shed light on the evolution of the nanoparticle/support interface and correlate its structural and chemical transformations with changes in the catalytic performance.

/pdf/operando-high-pressure-investigation-of-size-controlled-cuzn-3fvfu1ofh7.pdf

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

The influence of the crystallographic orientation on surface segregation and alloy formation in model PdCu methanol synthesis catalysts was investigated in situ using near-ambient pressure X-ray photoelectron spectroscopy under CO2 hydrogenation conditions. Combined with scanning tunneling microscopy and density functional theory calculations, the study showed that submonolayers of Pd undergo spontaneous alloy formation on Cu(110) and Cu(100) surfaces in vacuum, whereas they do not form an alloy on Cu(111). Upon heating in H2, inward diffusion of Pd into the Cu lattice is favored, facilitating alloying on all Cu surfaces. Under CO2 hydrogenation reaction conditions, the alloying trend becomes stronger, promoted by the reaction intermediate HCOO*, especially on Pd/Cu(110). This work demonstrates that surface alloying may be a key factor in the enhancement of the catalytic activity of PdCu catalysts as compared to their monometallic counterparts. Furthermore, it sheds light on the hydrogen activation mechanism during catalytic hydrogenation on copper-based catalysts.

/pdf/crystallographic-orientation-dependence-of-surface-2h3sktamvy.pdf

Osman Karslıoğlu

Papers

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

Crystallographic Orientation Dependence of Surface Segregation and Alloying on PdCu Catalysts for CO2 Hydrogenation.