CHARACTERIZATION AND CATALYTIC ACTIVITY OF BIMETALLIC PT-IN AL2O3 ANDPT-SN/AL2O3 CATALYSTS/

Bimetallic platinum-indium and platinum-tin supported on alumina catal ysts were investigated by temperature programmed reduction (TPR), hydr ogen chemisorption, and UV-Vis diffuse reflectance spectroscopy (DRS), DRS results indicated that the interaction between Pt and Ln or Sn ta kes place during the reduction step. The TPR results showed that, afte r 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 preparat ion method. Also, there is a partial shift of the reduction of In3+ an d Sn4+ to the platinum precursor temperature range. The interaction be tween indium and platinum was also demonstrated by the decrease of pla tinum adsorption capacity, when indium was present. The use of model r eactions were shown to be adequate to differentiate the effects of Sn and In on Pt/Al2O3 catalysts. The turnover frequency for cyclohexane d ehydrogenation, a structure insensitive reaction, was not affected by the presence of the promoter. In the case of reactions that require la rger 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 I n was used as promoter, indicating that In dilutes the Pt atoms more h omogeneously than Sn. For the n-heptane conversion, the addition of In or Sn improved the stability of the catalysts, caused a decrease in t he selectivity for hydrogenolysis, and an increase in the selectivity for dehydrogenation and aromatization products. The main difference be tween In and Sn was that Sn promoted a higher selectivity for Isomeriz ation products. (C) 1998 Academic Press.