OXYGEN CHEMISORPTION AND OXIDE FILM GROWTH ON NI(100), (110), AND (111) - STICKING PROBABILITIES AND MICROCALORIMETRIC ADSORPTION HEATS

摘要

Using single-crystal adsorption calorimetry, heat data have been measured for the adsorption of oxygen on the three low-index planes of Ni at 300 K along with corresponding sticking probabilities. New data are presented with coadsorbed potassium on each plane, and temperature-dependent data for O-2/Ni{100}. The initial heats of adsorption of oxygen on Ni{100}, {110}, and {111} are 550, 475, and 440 kJ (mol O-2)(-1), respectively, at 300 K, and the heat is found to drop rapidly with coverage in the chemisorption regime, indicating strong interadsorbate interactions. However, this rapid decline is not seen with coadsorbed potassium, a difference discussed both in terms of electron availability and coadsorbate attractions. The integral heats of adsorption for oxide film formation are 220, 290, and 320 kJ mol(-1), respectively. Corresponding sticking probability measurements show initial values, all less than unity, of 0.63, 0.78, and just 0.23, again for the {100}, {110}, and {111} surfaces in that order. The coverage dependence of the sticking probability is consistent in each case with a passivating oxide film four layers thick. Comparable data for Ni{100} obtained using a pyroelectric detector gave good agreement with the conventional results at 300 K. At 410 K, however, the heat-coverage curve was flat up to 0.25 monolayers. Data were also obtained at 90 K. Analysis and Monte Carlo simulation of the temperature-dependent adsorption heat curves indicates that the large drop in adsorption heat with coverage seen at room temperature is consistent with a local second-nearest neighbor adatom-adatom repulsion rather than a long-range electronic effect. (C) 1997 American Institute of Physics. [References: 85]