Formation of PtSiO overlayers on polycrystalline Pt, and its influence on the catalytic activity

Abstract

The H 2 + O 2 reaction to H 2O over pure polycrystalline Pt at 1 atm causes the formation of a several ångström thick overlayer composed of Si, O and Pt, at temperatures as low as 320 K. This low temperature of formation is considerably lower than that previously observed in the segregation of bulk impurities or SiO x formation on Pt single crystals in ultra-high vacuum. The formation of the PtSiO layer, which was continuously followed by AES, was accompanied by a decrease in the catalytic activity for the H 2 + O 2 and H 2D 2 reactions with a reproducible limiting activity characteristic for the PtSiO overlayer. This limiting activity is lower than for pure Pt but still quite high on an absolute scale. The Si + O saturated surface does not adsorb O 2 and CO irreversibly at 300 K. The PtSiO overlayer is thermally very stable, but can be slowly decomposed at 1070 K. In a H 2/O 2 mixture the overlayer readily decomposes at temperatures > 900 K, i.e. the catalytic reaction enhances the decomposition. The mechanisms for formation and decomposition of the PtSiO overlayer are discussed. It is tentatively proposed that dissolved hydrogen and grain boundary diffusion of Si are the main reasons for the low formation temperature. The possible role of the PtSiO overlayer in establishing chemical oscillations (which are readily established on both the pure and the overlayer covered surface) is shortly discussed.