Realistic Operando‐DRIFTS Studies on Cu/ZnO Catalysts for CO2 Hydrogenation to Methanol – Direct Observation of Mono‐ionized Defect Sites and Implications for Reaction Intermediates

Abstract

AbstractOur recent study in this journal highlighted misassignments of surface intermediates of diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements of published Cu/ZnO (CZ) catalyst systems for methanol synthesis. Here we investigate a recent and very active CZ system, in part promoted by oxidative fluorination (i. e., a Cu/ZnO1–x/ZnF2 system), with realistic operando‐DRIFTS measurements of a 3H2/CO2 gas feed (30 bar, 513 K). This DRIFTS setup is linked to an online GC analysis system and the catalytic performance of the Cu/ZnO1–x catalyst showed a similar performance of the catalysis process in the DRIFTS cell as in the catalyst test station. In the DRIFTS measurements, a very broad absorption band with a maximum at about 1500 cm−1 (=I1500) is evident. This I1500 band is absent in nitrogen; its intensity increases in pure hydrogen and is particularly high during methanol synthesis. I1500 results from photoionization of an electron residing in a mono‐ionized oxygen vacancy VO+ in the ZnO1–x part of the Cu/ZnO1–x catalyst. Consequently, the I1500 band intensity provides information on the extent of the strong metal‐support interaction SMSI. Therefore, measurement of the I1500 band intensity could be a novel and efficient tool to characterize any CZ‐based catalyst systems online. Mechanistically, the maximum VO+ photoionization I1500 band intensity in a 3H2/CO2 gas stream is coupled to the reaction of CO2 giving the CO2−. radical anion intermediate that is rapidly trapped in the VO2+ site formed. This trapped intermediate may react by hydrogen migration to the well‐known surface formate (1603, 1371, and 1314 cm−1). However, the long‐lived formate with this spectroscopic signature is only a spectator. It is also visible by DRIFTS in catalyst samples that in the same setup do not produce methanol (GC). By contrast, the band intensities of the surface species at 1759, 1691, 1457 and 1398 cm−1 are directly connected to methanol production and the applied WHSV. Therefore, only these surface species are relevant for CO2 hydrogenation to methanol at higher pressure and likely represent true reaction adsorbates/intermediates.