ONLINE CHARACTERIZATION BY EXAFS OF TIN PROMOTED PLATINUM GRAPHITE CATALYSTS IN THE AQUEOUS-PHASE

The structure of tin promoted graphite supported platinum catalysts ha s been studied with extended X-ray absorption fine structure spectrosc opy (EXAFS). A newly developed EXAFS cell allows on-line characterizat ion 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 p erforming reactions in the liquid phase. The structures of the tin pro moted platinum catalysts were investigated directly after preparation, drying, treatments with hydrogen (363 K) and oxygen (RT) in aqueous p hase and a hydrogen gas treatment at 573 K at both the Pt L-III and th e Sn K-edge. After preparation, under aqueous hydrogen, reduced platin um can be detected with three coordinations: Pt-Pt, Pt-C and Pt-Sn. Ti n 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 oxidi zed platinum and tin. At the Pt L-III-edge only a Pt-Pt and Pt-O coord ination 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 pl atinum showing, however, different coordination numbers for the Pt-Pt and Pt-Sn coordination. Tin only shows a Sn-O coordination. A treatmen t with hydrogen at 573 K reduces both the platinum and the tin. Platin um shows a Pt-Pt, Pt-C and Pt-Sn coordination. Tin shows a Sn-Pt and S n-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 treatme nts could only be studied since the developed EXAFS cell allowed catal yst preparation and treatments avoiding contact to ambient. (C) 1998 E lsevier Science B.V.