Water Formation Kinetics on Co(0001) at Low and Near-Ambient Hydrogen Pressures in the Context of Fischer-Tropsch Synthesis

Understanding the kinetics of oxygen removal from catalytically active metal surfaces by hydrogen is important for several catalytic reactions such as Fischer-Tropsch synthesis, methanation of CO or CO2, and the reverse-water-gas-shift reaction. Motivated by FTS, a Co(0001) single crystal model catalyst was used to study the kinetics of oxygen removal through reaction with hydrogen. Kinetic studies in the 10-7 - 10-4 mbar H2 pressure regime show that water formation is first order in the surface hydrogen concentration while the order in oxygen concentration changes from one at low oxygen coverage to zero at high oxygen coverage. In situ XPS shows that the hydrogen surface concentration saturates around 10-1 mbar and on this basis the typical temperature of 450K needed for water formation in this pressure regime can be considered as typical for high pressures as well. The absence of OH buildup during the reaction points to O + H as the rate-limiting step, with a barrier of -120 kJ mol-1. Such a high barrier shows that slow removal of adsorbed oxygen from the surface of reactive metal catalysts such as cobalt may be rate-limiting for the overall reaction.