Competing Mechanisms in CO Hydrogenation over Co-MnOx Catalysts

We study the hydrogenation of CO under ambient pressure conditions over a Co-MnOx model catalyst using chemical transient kinetics (CTK) under calibrated molecular flow conditions. Alkanes and alkenes are shown to form with markedly differing kinetics. Quantitation of the data allows accumulating carbon and oxygen coverages to be determined at any instant of the "buildup" transients. Anderson-Schulz-Flory (ASF) chain lengthening probabilities are evaluated while approaching the steady-state of the reaction. A linear dependence of these probabilities on the transient CO gas pressure provides evidence for a CO insertion mechanism being in operation under high-coverage conditions. A detailed kinetic analysis of reactant/product formation and scavenging is in agreement with this conclusion. However, for coverages below the monolayer limit, fast CO dissociation, probably hydrogen-assisted and promoted by Mn2+, also enables significant CHx-CHy coupling to occur. Evidence was obtained from high resolution transmission electron microscopy (HRTEM) that a phase transition from Co to Co2C was triggered under atmospheric pressure conditions for the Co-MnOx catalyst.