On-line characterization by EXAFS of tin promoted platinum graphite catalysts in the aqueous phase

The structure of tin promoted graphite supported platinum catalysts has been studied with extended X-ray absorption fine structure spectroscopy (EXAFS). A newly developed EXAFS cell allows on-line characterization avoiding contact to ambient or drying. Hereto catalyst samples are transferred from a slurry reactor to the EXAFS cell forming a "bed" of catalyst particles in the EXAFS cell. The cell design was based on considerations concerning possible mass transport limitations while performing reactions in the liquid phase. The structures of the tin promoted platinum catalysts were investigated directly after preparation, drying, treatments with hydrogen (363 K) and oxygen (RT) in aqueous phase and a hydrogen gas treatment at 573 K at both the Pt L-III and the Sn K-edge. After preparation, under aqueous hydrogen, reduced platinum can be detected with three coordinations: Pt-Pt, Pt-C and Pt-Sn. Tin appears to be partly oxidic showing a Sn-O and a Sn-Pt coordination. A treatment with aqueous oxygen or exposure to ambient leads to oxidized platinum and tin. At the Pt L-III-edge only a Pt-Pt and Pt-O coordination for platinum are detected. At the Sn K-edge tin has only a Sn-O coordination. An aqueous treatment with hydrogen at 363 K reduces platinum showing, however, different coordination numbers for the Pt-Pt and Pt-Sn coordination. Tin only shows a Sn-O coordination. A treatment with hydrogen at 573 K reduces both the platinum and the tin. Platinum shows a Pt-Pt, Pt-C and Pt-Sn coordination. Tin shows a Sn-Pt and Sn-O coordination indicating tin deposition on the platinum, tin being bonded via oxygen to the graphite support. Reductive treatments in the aqueous phase appear to reduce platinum and only the tin deposited on the platinum. The effects of drying and consecutive reductive treatments could only be studied since the developed EXAFS cell allowed catalyst preparation and treatments avoiding contact to ambient. (C) 1998 Elsevier Science B.V.