Operando Characterization of Copper-Zinc-Alumina Catalyst for Methanol Synthesis from Carbon Dioxide and Hydrogen by Ambient-Pressure Hard X-ray Photoelectron Spectroscopy

Operandospectroscopy is a potent tool for characterizing heterogeneouscatalysts under working conditions. In this study, ambient-pressurehard X-ray photoelectron spectroscopy (AP-HAXPES) of an industrialcopper-zinc-alumina methanol synthesis catalyst wasperformed to reveal the surface chemical states of the catalyst andadsorbed intermediates during the catalytic reaction. In situ formationof metallic Cu particles was detected by operando AP-HAXPES duringthe reduction process of the catalyst under a hydrogen atmosphere,as well as the incorporation of Al atoms into zinc oxide (ZnO). Massspectrometry analysis of the reaction products in the presence ofnear ambient-pressure CO2 and H-2 gases showsthe formation of methanol and CO via the reverse water-gas shift (rWGS).Temperature dependence differs between rWGS and methanol synthesis;the reaction products of rWGS monotonically increased as a functionof the sample temperature between 420 and 533 K, whereas the rateof the methanol synthesis reached a maximum at 480 K and decreasedat higher temperatures. The observed temperature dependence of methanolsynthesis correlates well with the dynamic changes in the chemicalstate of the catalyst and the reaction intermediates adsorbed on thesurface of the catalyst. The interfacial ZnO sites are probably responsiblefor the methanol synthesis under the present near-ambient pressureconditions. The present AP-HAXPES measurements clearly show the importanceof operando characterization of the dynamic nature of heterogeneouscatalysts.