Effect of surface orientation and sulfur coverage on the oxidation of carbon on Ni surfaces

Abstract

The reaction between adsorbed oxygen and segregated carbon on a cylindrical nickel single-crystal has been examined with Auger electron spectroscopy (AES) and temperature programmed desorption (TPD), for a range of surface orientation, oxygen exposure, and sulfur coverage. It was found that for small oxygen exposures, surface carbon and surface oxygen react during TPD to form a CO desorption peak, labeled β 1. The β 1 CO peak temperature and peak shape vary with orientation. At higher oxygen coverages, the CO desorption peak split into low-temperature and high-temperature peaks. The behavior of the β 1 CO desorption peak for large oxygen exposures is consistent with a model of the carbon-oxygen recombination reaction in which the morphologies of the initial carbon and oxygen phases change during oxygen exposure as a result of repulsive lateral interactions. High oxygen exposures result in the formation of large regions of contact between the two phases; this is believed to produce the low-temperature β 1 CO desorption peak. Small segregated-sulfur coverages, and low oxygen exposures, caused the β 1 CO peak to shift to lower temperatures for all orientations. Sulfur is believed to cause more frequent contact between carbon and oxygen for small oxygen exposures because it compresses the adsorbed oxygen and segregated carbon into the sulfur-free areas of the surface. Large coverages of segregated sulfur inhibited carbon segregation on some, and oxygen adsorption on most, orientations. The absence of reactant species explains the disappearance of the β 1 CO peak during TPD from orientations which had a high sulfur coverage.