Chemistry of sulfur oxides on transition metal surfaces: a bond order conservation-Morse potential modeling perspective

Abstract

We have employed the bond order conservation-Morse potential (BOC-MP) method to analyze the chemistry of sulfur oxides on the copper and nickel group metals. Specifically, we have calculated the reaction energetics (heats of adsorption, reaction enthalpies and intrinsic activation barriers) of the decomposition and oxidation of sulfur dioxide at low coverages on fcc (111) surfaces of Cu, Ag, Au, Ni, Pd and Pt. The accuracy of the BOC-MP heats of adsorption has been corroborated by high quality ab initio calculations of the heats of SO2 adsorption on Ag and Pd surfaces. We have addressed the following issues: (1) the dissociation of SO2; (2) the stability of adsorbed SO and its likelihood of being a product of SO2 decomposition; (3) the oxidation of SO2; and, (4) the nature of adsorbed SO3 and SO4. The major model projections (obtained for low coverages and without considering diffusional effects) are: (1) the dissociation of SO2→SO + O is unfavorable on all the metals considered, but, the dissociation of SO2→S + O + O, showing distinct periodic trends, is feasible on Cu and particularly on Ni; in the presence of carbon monoxide the dissociation, SO2 + CO→S + O + CO2, may occur on all the metals examined; (2) on the Pt, Pd, Ni and Cu surfaces, SO is unstable; (3) the oxidation of SO2 to SO3 may be achieved with O, O2, H2O2 and NO as oxygen sources on Ag, Au, Pd and Pt surfaces. Although adsorbed SO3 may be readily obtained, it may be impossible to desorb SO3 intact at low coverages because SO3 will decompose to SO2 + O before desorption. (4) The most stable oxygen sulfur specie that withstands elevated temperatures should be dianion sulfate. The relevant experimental data have been discussed. Most of the model projections are in agreement with experiment, but, some suggest reconsideration of the reported experimental data or represent predictions to be verified.