CHEMISTRY AT CATALYTIC SURFACES - SO2 OXIDATION ON NOBLE-METALS

The high temperature solid electrolyte cell air, M|ZrO2(8%Y2O3)|M, air-SO2SO3, where M is Pt, Ag, or Au, was used to monitor the oxygen activity on Pt, Ag, or Au catalyst films exposed to mixtures of O2, SO2, and SO3 at temperatures above 400 °C and 1 atm total pressure. One electrode functioned simultaneously as both electrode and catalyst. The opencircuit EMF of the cell reflected the oxygen activity on the working catalyst and simultaneously the steady-state chemical kinetics were observed. For Pt it was found that the surface oxygen activity generally does not equal the gas phase PO, but is determined by PSO or the P SO PSO ratio, depending on the temperature and gas phase composition. These observations suggest that the rate-limiting step in the SO2 oxidation on Pt is not generally the reaction between chemisorbed oxygen and gaseous SO2, except at high temperatures and very low Pso, but rather the desorption of a chemisorbed phase of SO3 or the adsorption of oxygen, depending upon the temperature, PSO, PSO, and PO. The results with Au and Ag films show that chemisorbed SO3 is also formed on these metals exposed to SO2, O2 mixtures. The values of surface oxygen activity compared to those obtained for Pt, provide an explanation for the weakly catalytic and noncatalytic properties of Au and Ag, respectively, for the SO2 oxidation. © 1979.