Adsorbate interactions on a Pt(110) surface. II. Effect of sulfur on the catalytic CO oxidation

Abstract

The interaction between adsorbed sulfur and oxygen on Pt(110) was investigated and found to be strong resulting in a Langmuir-Hinshelwood-type surface reaction with SO2 as the reaction product. The kinetics of this surface reaction were studied as a function of initial sulfur coverage, partial pressure of oxygen, and temperature. For low coverage of sulfur, θ < 0.28, the reaction was quite fast while for higher sulfur coverages, 0.28 < θ < 0.75, the reaction became very slow. In this latter range, the reaction proceeded inhomogeneously and gave rise to a nonuniform distribution of adsorbed sulfur. The distribution of sulfur was scanned in two dimensions across the surface by Auger electron spectroscopy with a lateral resolution of about 0.02 cm. The same technique permitted an estimate of the surface diffusion coefficient of sulfur on Pt(110) at 450°C of 3 × 10−7 cm2/sec. In a steady state experiment the rate of CO oxidation was measured by mass spectrometry as a function of the sulfur coverage. In the region 0 ≤ θ < 0.28, this rate decreased to near zero as (1 − κθ)2, with κ = 3.5 as the surface coordination of sulfur. For θ > 0.28 the CO2 formation was indistinguishable from the background CO2 level. It appears that within the region θ < 0.28, the CO2 formation rate is solely governed by the rate of dissociative adsorption of oxygen.