Catalytic CO hydrogenation:: Mechanism and kinetics from chemical transients at low and atmospheric pressures

The hydrogenation of CO over Co model catalysts was studied using relaxation-type methods operating in situ either at atmospheric pressures or under surface science conditions. Emphasis was laid on providing information on the surface composition and on how it changes with time under catalytic reaction conditions. Using pressure forcing in chemical transient kinetics (CTK), the build-up of the steady-state was studied at 503 K and atmospheric pressure to demonstrate that the active catalyst surface is not metallic but covered with carbon, oxygen and hydrogen in excess of a monolayer equivalent. Both buildup and backward transients suggest CO to act as the monomer which probably inserts into an O-H bond to form the primary surface complex necessary for hydrocarbon and oxygenate formation. Repetitive electric field pulses (pulsed field desorption mass spectrometry, PFDMS) at low pressures have allowed the CO dissociation kinetics on a nano-sized Co 3D crystal (tip) to be monitored in the millisecond time range. No evidence for the occurrence of the Boudouard reaction was obtained in either PFDMS or CTK. Adsorbed CHx (x = 1-3) species were detected in small amounts demonstrating that CO dissociation is fast compared to carbon hydrogenation. Adsorbed Co-subcarbonyl species, Co(CO)(x) were also detected by PFDMS and possibly mediate the necessary surface mobility during the initial restructuring of the catalyst. Surface carbon seems to inhibit Co-subcarbonyl formation.