Enhancement of Methanol Synthesis by Oxidative Fluorination of Cu/ZnO Catalysts-Insights from Surface Analyses

A Cu/ZnO catalyst system for methanol synthesis promoted by oxidative fluorination was studied. Gaseous F-2 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) H-2/COx syngas (x = 1, ..., 2, eight steps@40 bar, 473 and 513 K), the fluorinated systems have optimal performance at x = 2, that is, a 3H(2)/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 H-2 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.