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

In spite of their technical use and relevance the structural properties of diesel engine exhaust gas catalysts are not yet understood on an atomic sca le. 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 stru cture 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. van adium 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 Angstrom 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 alth ough other minor phases may also be present. This is accompanied by a parti al transformation of TiO2 from the initial anatase to rutile. Like V, Pt un dergoes a strong modification upon aging: Pt is partly oxidized but still f orms metal particles with a slightly smaller size.