Operando high-pressure investigation of size-controlled CuZn catalysts for the methanol synthesis reaction.
TL;DR: In this article, the authors apply high-pressure operando spectroscopy methods to well-defined Cu and Cu 0.7Zn0.3 nanoparticles supported on ZnO/Al2O3, γ-Al 2O3 and SiO2 to correlate their structure, composition and catalytic performance.
Abstract: 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.
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80 citations
01 Jun 2021
TL;DR: In this paper, the state and evolution of the catalyst is defined by its environment, and the structure of the catalysts shows a strong pressure dependence, especially below 1 bar, which is a general problem in catalysis.
Abstract: 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.
57 citations
TL;DR: In this paper , the authors 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.
Abstract: 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.
36 citations
TL;DR: In this paper , the electronic metal-support interactions (EMSIs) are tuned for single-atom catalysts (SACs) in order to improve the catalytic performance of SACs.
Abstract: 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.
20 citations
TL;DR: In this paper , a series of alumina-supported NixFey nanoalloys are synthesized from NiFeAl-layered double hydroxide precursors, with the obtained catalysts used for photothermal synergistic DRM.
Abstract: Dry reforming of methane (DRM) with carbon dioxide to produce syngas is currently attracting a lot of attention for converting greenhouse gases into valuable chemicals. However, harsh reaction conditions in thermal catalysis hinder practical applications. Herein, a series of alumina‐supported NixFey nanoalloys are synthesized from NiFeAl‐layered double hydroxide precursors, with the obtained catalysts used for photothermal synergistic DRM. The Ni3Fe1 nanoalloy catalyst shows a syngas production rate of 0.96 mol g–1 h–1 under a 350 °C photothermal condition, which is about 1100 times higher than that of the same catalyst at the same temperature in the dark. Methane activation as the rate‐determining step in DRM is greatly enhanced by the localized surface plasmon resonance of the Ni3Fe1 nanoalloy in the ultraviolet region, thus accounting for the remarkably reduced reaction temperature. Meanwhile, the nanoalloy‐based photothermal synergistic DRM exhibits sunlight‐driven feasibility and 20 h of continuous operational stability.
19 citations
References
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TL;DR: Biesinger et al. as mentioned in this paper proposed a more consistent and effective approach to curve fitting based on a combination of standard spectra from quality reference samples, a survey of appropriate literature databases and/or a compilation of literature references and specific literature references where fitting procedures are available.
7,498 citations
TL;DR: This work shows how to identify the crucial atomic structure motif for the industrial Cu/ZnO/Al2O3 methanol synthesis catalyst by using a combination of experimental evidence from bulk, surface-sensitive, and imaging methods collected on real high-performance catalytic systems in combination with density functional theory calculations.
Abstract: Unlike homogeneous catalysts, heterogeneous catalysts that have been optimized through decades are typically so complex and hard to characterize that the nature of the catalytically active site is not known. This is one of the main stumbling blocks in developing rational catalyst design strategies in heterogeneous catalysis. We show here how to identify the crucial atomic structure motif for the industrial Cu/ZnO/Al{sub 2}O{sub 3} methanol synthesis catalyst. Using a combination of experimental evidence from bulk-, surface-sensitive and imaging methods collected on real high-performance catalytic systems in combination with DFT calculations. We show that the active site consists of Cu steps peppered with Zn atoms, all stabilized by a series of well defined bulk defects and surface species that need jointly to be present for the system to work.
1,888 citations
TL;DR: In situ transmission electron microscopy is used to obtain atom-resolved images of copper nanocrystals on different supports, which are catalysts for methanol synthesis and hydrocarbon conversion processes for fuel cells.
Abstract: In situ transmission electron microscopy is used to obtain atom-resolved images of copper nanocrystals on different supports. These are catalysts for methanol synthesis and hydrocarbon conversion processes for fuel cells. The nanocrystals undergo dynamic reversible shape changes in response to changes in the gaseous environment. For zinc oxide-supported samples, the changes are caused both by adsorbate-induced changes in surface energies and by changes in the interfacial energy. For copper nanocrystals supported on silica, the support has negligible influence on the structure. Nanoparticle dynamics must be included in the description of catalytic and other properties of nanomaterials. In situ microscopy offers possibilities for obtaining the relevant atomic-scale insight.
1,080 citations
TL;DR: A direct comparison between the activity of ZnCu and ZnO/Cu model catalysts for methanol synthesis is reported, highlighting a synergy of Cu andZnO at the interface that facilitates methenol synthesis via formate intermediates.
Abstract: The active sites over commercial copper/zinc oxide/aluminum oxide (Cu/ZnO/Al2O3) catalysts for carbon dioxide (CO2) hydrogenation to methanol, the Zn-Cu bimetallic sites or ZnO-Cu interfacial sites, have recently been the subject of intense debate. We report a direct comparison between the activity of ZnCu and ZnO/Cu model catalysts for methanol synthesis. By combining x-ray photoemission spectroscopy, density functional theory, and kinetic Monte Carlo simulations, we can identify and characterize the reactivity of each catalyst. Both experimental and theoretical results agree that ZnCu undergoes surface oxidation under the reaction conditions so that surface Zn transforms into ZnO and allows ZnCu to reach the activity of ZnO/Cu with the same Zn coverage. Our results highlight a synergy of Cu and ZnO at the interface that facilitates methanol synthesis via formate intermediates.
1,037 citations
TL;DR: In this article, the structure and catalytic activity of Cu/ZnO methanol synthesis catalysts have been investigated by a further developed in situ method, which combines X-ray diffraction (XRD), Xray absorption fine structure spectroscopy (XAFS), and on-line catalytic measurements by mass spectrometry.
508 citations