Geometric and electronic structures of Pt/V2O5/TiO2 diesel engine exhaust gas purification catalysts before and after aging

Abstract

In spite of their technical use and relevance the structural properties of diesel engine exhaust gas catalysts are not yet understood on an atomic scale. This study investigates structural changes during aging of Pt/V₂O₅/TiO₂ based catalysts in order to get insight into the mutual interaction of the active constituents. X-ray photoelectron spectrometry, secondary ion mass spectrometry and X-ray absorption fine structure measurements were used to characterize the chemical composition and the electronic and geometric structure of the catalyst's surface and bulk. In the freshly prepared catalyst the interaction among the active species turns out to be moderate, i.e. vanadium is present in a distorted 5+ state. TiO₂, as expected, is present as in the precursor and Pt is found to form monometallic clusters of about 16 Å diameter without any significant interaction with the other metal species. Particularly, no alloying is observed. Upon aging of the catalyst vanadium is reduced to 4+ and preferentially orders in a VO₂ structure although other minor phases may also be present. This is accompanied by a partial transformation of TiO₂ from the initial anatase to rutile. Like V, Pt undergoes a strong modification upon aging: Pt is partly oxidized but still forms metal particles with a slightly smaller size.