ORIGINS OF LOW-TEMPERATURE 3-WAY ACTIVITY IN PT CEO2/

The ability of Pt/CeO2 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. Carbo n monoxide chemisorption measurements show that a loss of exposed meta l area occurs during the activation procedure, even though the conditi ons do not favour platinum sintering. Following activation (and subseq uent exposure to air at ambient temperature),the surface ceria is left in a highly reducible state, which is characterised by a subambient p eak during temperature-programmed reduction. We envisage that the stro ng interaction arises according to a traditional SMSI model, by the mi gration of partially reduced ceria over the surface of the platinum pa rticles (at greater than or equal to 600 degrees C). The resultant hig h degree of contact between the metal (with high work-function) and th e metal oxide (with high band gap) promotes the formation of oxygen va cancies on the ceria surface. Although the presence of this highly red ucible ceria-covering blocks conventional three-way sites on the plati num, it provides new sites that are active even at low temperatures. T herefore, unlike most previous explanations of promotion caused by a s trong interaction, we propose that the 'support' becomes the active ph ase. Re-oxidation at elevated temperatures causes the ceria covering t o coalesce, leading initially to its partial thinning, and subsequentl y to the re-exposure of the platinum particles.