EFFECTS OF SO2 ON THE ALKANE ACTIVITY OF 3-WAY CATALYSTS

Over current Pt-Rh/CeO2-Al2O3 catalysts, the conversion of alkanes occurs by two principal mechanisms: direct oxidation (HC + O2) and steam reforming (HC + H2O). Sulphur dioxide can influence both these mechanisms. Direct oxidation, which predominates when the exhaust-gas is fuel-lean, is promoted by the adsorption of SO(x) species by the support. Under fuel-rich atmospheres, the presence of SO2 severely inhibits steam reforming. The poisoning is associated with the formation of S2- on the platinum and of SO4(2-) on the support, but there is no indication of S-species being retained by the rhodium. It is proposed that each of the two mechanisms is sensitive to a different type of interaction at the metal-support interface. Direct oxidation is enhanced by the transfer of electrons from the precious metal to the support; steam reforming occurs at interfacial sites, which can be blocked by adsorbed SO(x) species.