Characterization and catalytic activity of bimetallic Pt-In/Al2O3 and Pt-Sn/Al2O3 catalysts

摘要

Bimetallic platinum-indium and platinum-tin supported on alumina catalysts were investigated by temperature programmed reduction (TPR), hydrogen chemisorption, and UV-Vis diffuse reflectance spectroscopy (DRS), DRS results indicated that the interaction between Pt and Ln or Sn takes place during the reduction step. The TPR results showed that, after reduction at 773K, about 50-80% of indium and 25-50% of tin are in a zero-valent state in the bimetallic system, depending on the preparation method. Also, there is a partial shift of the reduction of In3+ and Sn4+ to the platinum precursor temperature range. The interaction between indium and platinum was also demonstrated by the decrease of platinum adsorption capacity, when indium was present. The use of model reactions were shown to be adequate to differentiate the effects of Sn and In on Pt/Al2O3 catalysts. The turnover frequency for cyclohexane dehydrogenation, a structure insensitive reaction, was not affected by the presence of the promoter. In the case of reactions that require larger platinum ensembles to occur, the presence of the promoter caused a decrease in the turnover frequency, due to the dilution of platinum surface atoms with the promoter atoms. There was a larger decrease in the turnover frequency of the methylcyclopentane hydrogenolysis when In was used as promoter, indicating that In dilutes the Pt atoms more homogeneously than Sn. For the n-heptane conversion, the addition of In or Sn improved the stability of the catalysts, caused a decrease in the selectivity for hydrogenolysis, and an increase in the selectivity for dehydrogenation and aromatization products. The main difference between In and Sn was that Sn promoted a higher selectivity for Isomerization products. (C) 1998 Academic Press. [References: 45]