A Cu/ZnO catalyst system for methanol synthesis promoted by oxidative fluorination was studied. Gaseous F2 reacts in a first step mainly with CuO to give CuF2 (XPS, thermodynamics). In the active system, the entire fluoride content transforms to ZnF2 and the catalyst system should be formulated as Cu/ZnO1–x /ZnF2 (XPS). Tested for methanol production using a (1 + x) H2/CO x syngas (x = 1, ..., 2, eight steps@40 bar, 473 and 513 K), the fluorinated systems have optimal performance at x = 2, that is, a 3H2/CO2 mixture, and inhibit CO2 promotion at low CO2 concentrations. The number of surface ZnO1–x oxygen defect sites and the number of active sites for methanol synthesis increased in the fluorinated systems after H2 reduction (refined chemisorption measurements, XPS, and BET analysis). Concomitantly, the number of active sites for the (reverse) water–gas shift reaction decreased. Both account for the increased methanol activity and selectivity of the fluorinated catalyst systems and imply negligible water inhibition for the fluorinated case.

Our 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/ZnO 1-x /ZnF 2 system), with realistic operando-DRIFTS measurements of a 3H 2 / CO 2 gas feed (30 bar, 513 K). This DRIFTS setup is linked to an online GC analysis system and the catalytic performance of the Cu/ZnO 1-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 À 1 (= I 1500 ) is evident. This I 1500 band is absent in nitrogen; its intensity increases in pure hydrogen and is particularly high during methanol synthesis. I 1500 results from photoionization of an electron residing in a mono-ionized oxygen vacancy V O + in the ZnO 1-x part of the Cu/ ZnO 1-x catalyst. Consequently, the I 1500 band intensity provides information on the extent of the strong metal-support interaction SMSI. Therefore, measurement of the I 1500 band intensity could be a novel and efficient tool to characterize any CZ-based catalyst systems online. Mechanistically, the maximum V O + photoionization I 1500 band intensity in a 3H 2 /CO 2 gas stream is coupled to the reaction of CO 2 giving the CO 2 À * radical anion intermediate that is rapidly trapped in the V O 2 + site formed. This trapped intermediate may react by hydrogen migration to the well-known surface formate (1603, 1371, and 1314 cm À 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 À 1 are directly connected to methanol production and the applied WHSV. Therefore, only these surface species are relevant for CO 2 hydrogenation to methanol at higher pressure and likely represent true reaction adsorbates/intermediates.

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