Autocatalytic decomposition of water on nickel (110)

Abstract

The interaction of water with clean Ni(110) at temperatures between 425 and 770 K has been studied by work function ([Delta][phi]), desorption mass analysis, and low-energy electron diffraction (LEED). Water is observed to dissociate on clean Ni(110), forming adsorbed oxygen and H[sub 2](g) in a process which is itself catalyzed (and hence termed autocatalytic) by the adsorbed oxygen product. Initial dissociation of water is believed to originate on defects where adsorbed oxygen then promotes growth of O (2 [times] 1) islands in a process which becomes self-poisoning in the limit of saturation of the (2 [times] 1) phase (0.5 ML (ML = monolayer)). The growth kinetics indicate the autocatalysis process is most active when the oxygen coverage is 0.1 ML. A statistical theory supported by computer simulation is developed which models the autocatalytic growth kinetics in terms of nucleation of islands along step edges. The temperature dependence of autocatalysis exhibits a maximum at approx. 600 K. This complex temperature dependence is explained in terms of transition-state theory. 25 refs., 10 figs.