An X-ray photoelectron spectroscopy and reaction study of Pt$z.sbnd;Sn catalysts

Platinum-tin catalysts prepared by different methods, and supported on γ-Al2O3 have been studied with X-ray Photoelectron Spectroscopy (XPS), temperature-programmed reduction, and reaction measurements. Two major conclusions are drawn from the results. First, tin is present primarily in the oxidized form (Sn(II)) after high-temperature reduction (500 °C) of Pt-Sn γ- Al 2 O 3 catalysts, whereas alloy formation can occur on silica under the same conditions. No evidence for substantial amounts of Sn(0) is found on reduced γ-Al2O3-based catalysts. The spectroscopic reduction results are in agreement with TPR measurements in the present (and previous) work which show an average Sn reduction of 50%. Even PtSn complexes, which are readily reducible to Sn(0), are not reduced below Sn(II) on γ-Al2O3. Second, quantitative XPS measurements of surface tin concentration versus bulk loading reveal an inhomogeneous distribution of tin in the impregnated γ-Al2O3 samples, with a large excess of tin on the external surface of Al2O3 at loadings below 1 wt% Sn. The results show that impregnation of tin followed by platinum can result in separation of the two components due to the strong adsorption of tin ions in the outer surface of the γ-Al2O3 particles. The best distribution of tin is found in the coprecipitated Sn-γ-Al2O3 (Patent) preparations where the tin is uniformly distributed throughout the matrix. The reaction measurements of catalysts prepared via the different methods are compared and it was concluded that the activities and selectivities in methylcyclopentane and cyclohexane conversion are sensitive to the method of tin introduction. A catalyst prepared via the coprecipitation of Sn with γ-Al2O3 followed by impregnation of platinum gave the highest sensitivities to activity and selectivity changes in the latter reactions. Whereas it is important to have platinum and Sn(II) in the vicinity of each other on the support, the mechanism of interaction is not well understood at this stage.