Water and Hydroxyl Reactivity on Flat and Stepped Cobalt Surfaces

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Hydroxyl adsorbates generally appear as transient species during water formation from adsorbed oxygen and hydrogen atoms on a metal surface, a reaction that is part of the catalytic cycle in various important surface-catalyzed reactions such as Fischer–Tropsch synthesis. In the present work, temperature-programmed desorption and in situ synchrotron XPS were used to study water adsorption and OH reactivity on a flat and a stepped cobalt single crystal surface. Water adsorbs intact on the flat Co(0001) surface and desorbs around 160 K. Electrons induce dissociation of water and produce OH species at low temperature. Hydroxyl species can also be formed by the reaction between O_ad and H₂O, but only for high initial oxygen coverage while low coverage O_ad appears largely unreactive. Reactive hydrogen species (H atoms) produced by a hot tungsten filament hydrogenate adsorbed oxygen atoms at low temperature already and both OH_ad and H₂O are formed. In all cases, hydroxyl adsorbates react around 190 K to form water via 2 OH_ad → H₂O (g) + O_ad associated with an activation barrier of 40–50 kJ mol^–1. Water readily dissociates on the step sites exposed by vicinal Co(10–19). A part of the OH_ad species recombine to form water and oxygen between 200 and 300 K, while decomposition of OH_ad into O_ad and H_ad dominates above 370 K. For catalysis, the high reactivity of step sites for water dissociation and the high stability of OH_ad at these sites implies that O removal from these sites may be difficult and may limit the overall rate of Fischer–Tropsch synthesis on cobalt catalysts.