Helen Anne Hatcher

Origins of low-temperature three-way activity in Pt/CeO2

Abstract The ability of Pt/CeO 2 to exhibit three-way catalytic activity at low temperatures results from a strong metal-support interaction, which is induced by activating the catalyst under a reducing atmosphere. Carbon monoxide chemisorption measurements show that a loss of exposed metal area occurs during the activation procedure, even though the conditions do not favour platinum sintering. Following activation (and subsequent exposure to air at ambient temperature), the surface ceria is left in a highly reducible state, which is characterised by a subambient peak during temperature-programmed reduction. We envisage that the strong interaction arises according to a traditional SMSI model, by the migration of partially reduced ceria over the surface of the platinum particles (at ⩾ 600°C). The resultant high degree of contact between the metal (with high work-function) and the metal oxide (with high band gap) promotes the formation of oxygen vacancies on the ceria surface. Although the presence of this highly reducible ceria-covering blocks conventional three-way sites on the platinum, it provides new sites that are active even at low temperatures. Therefore, unlike most previous explanations of promotion caused by a strong interaction, we propose that the ‘support’ becomes the active phase. Re-oxidation at elevated temperatures causes the ceria covering to coalesce, leading initially to its partial thinning, and subsequently to the re-exposure of the platinum particles.

Effects of SO2 on the alkane activity of three-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, ispromoted by the adsorption of SOx species by the support. Under fuel-rich atmospheres, the presence of SO2 severelyinhibits steam reforming. The poisoning is associated with the formation of S2− on the platinum and of SO42− 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 SOx species.