No abstract

We study the hydrogenation of CO under ambient pressure conditions over a Co-MnO x 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 CH x –CH y 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-MnO x catalyst.

As a first step toward the rational design of Co-based catalysts with a higher activity and selectivity, we determine how one can activate a C-O bond at the interface between a metal oxide cluster and a Co(0001) support. The hypothesis here is that the metal ions in metal oxide clusters on a Co(0001) support enhance the adsorption of CO and weaken the C-O bond strength, which can then facilitate the dissociation of the CO reactant. To test this hypothesis, we developed three computational models of Ti 4 O 8 /Co(0001), Zr 4 O 8 /Co(0001), and Mn 8 O 8 /Co(0001). We quantify the CO adsorption behavior at the interface sites between an oxide cluster and the Co(0001) support as well as the corresponding IR spectra. We correlate the computed CO stretch frequencies with their CO adsorption energies, as well as the CO stretch frequency with the C-O bond length. The interface is the most favorable site for CO adsorption. Adsorption results in an increase of the C-O bond length and a decrease in its vibrational frequency.