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

Operando spectroscopy is a potent tool for characterizing heterogeneous catalysts under working conditions. In this study, ambient-pressure hard X-ray photoelectron spectroscopy (AP-HAXPES) of an industrial copper–zinc–alumina methanol synthesis catalyst was performed to reveal the surface chemical states of the catalyst and adsorbed intermediates during the catalytic reaction. In situ formation of metallic Cu particles was detected by operando AP-HAXPES during the reduction process of the catalyst under a hydrogen atmosphere, as well as the incorporation of Al atoms into zinc oxide (ZnO). Mass spectrometry analysis of the reaction products in the presence of near ambient-pressure CO2 and H2 gases shows the 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 function of the sample temperature between 420 and 533 K, whereas the rate of the methanol synthesis reached a maximum at 480 K and decreased at higher temperatures. The observed temperature dependence of methanol synthesis correlates well with the dynamic changes in the chemical state of the catalyst and the reaction intermediates adsorbed on the surface of the catalyst. The interfacial ZnO sites are probably responsible for the methanol synthesis under the present near-ambient pressure conditions. The present AP-HAXPES measurements clearly show the importance of operando characterization of the dynamic nature of heterogeneous catalysts.