Interaction of water with stepped Ni(760): associative versus dissociative adsorption and autocatalytic decomposition

Abstract

The irreversible adsorption of water on the stepped Ni(760) surface (which is vicinal to the (110) surface) at low temperature and the interaction of water with this surface at temperatures between 398 and 873 K have been studied by work function (Δф), thermal desorption spectroscopy (TDS), Fourier transform infrared reflection-absorption spectroscopy (FTIR-RAS), low energy electron diffraction (LEED) and nuclear reaction analysis (NRA). The low temperature adsorption behaviour of this system is very similar to that of D2ONi(110): The saturation coverage for the first chemisorbed layer is 0.5 ML as measured by NRA (ML = monolayer = 1.15 × 1015 water molecules cm−2 and is expressed in terms of the density of atoms in the Ni(110) surface). No FTIR activity (1000 to 4000 cm−1) is observed until the coverage exceeds 0.5 ML, the coverage at which the bilayer starts to form. Some of the adsorbed water dissociates into OH(ads) + H(ads) at ∼ 200 K. However, the TDS of D2O from Ni(760) apparently shows much less water in the so-called A1 desorption state (i.e. water molecules stabilized by OH(ads)) compared with D2ONi(110). At temperatures above 398 K, water is observed to dissociate on clean Ni(760), forming H2(g) and O(ads). The O(ads) initially promotes further water dissociation; a process which has been termed “autocatalytic”. The Ni(760) surface appears to be much more reactive in this regard than Ni(110) resulting in a significantly higher probability of reaction. A (2 × 1)-O phase is formed upon completion of the reaction, corresponding to an absolute oxygen coverage of 0.3–0.4 ML depending on the temperature of reaction. The autocatalytic growth kinetics are modelled in terms of the nucleation of islands along step edges, using a statistical theory and computer simulations. The model we propose for the stepped surface is a slightly modified version of one we have previously proposed for Ni(110).