Enhancement of Methanol Synthesis by Oxidative Fluorination of Cu/ZnO Catalysts─Insights from Surface Analyses
Samuel M. Fehr, Karin Nguyen, Christian Njel, Ingo Krossing
Abstract
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/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 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.