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 CuF 2 (XPS, thermodynamics). In the active system, the entire fluoride content transforms to ZnF 2 and the catalyst system should be formulated as Cu/ZnO 1– x /ZnF 2 (XPS). Tested for methanol production using a (1 + x ) H 2 /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 3H 2 /CO 2 mixture, and inhibit CO 2 promotion at low CO 2 concentrations. The number of surface ZnO 1– 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.