Platinum-tin bimetallic catalysts supported on L-zeolite: Synthesis and characterization

The main goal of this research is to improve the sulfur resistance of platinum catalysts supported on L-zeolite by the addition of tin. The catalysts were prepared by both sequential impregnation and coimpregnation of the platinum and tin precursors and characterized by hydrogen and carbon monoxide chemisorptions, X-ray absorption spectroscopy, benzene/toluene competitive hydrogenation, cyclohexane dehydrogenation, and n-hexane reforming reactions with or without the addition of thiophene. The results of the chemisorption studies and the analyses of the X-ray fine structure spectra of platinum and tin showed increasing segregation of platinum ensembles with increasing tin loadings. They also demonstrated that the coimpregnation preparation resulted in smaller platinum ensembles. The electronic effects on the platinum by the addition of tin were observed in the studies of the X-ray near-edge structure spectra of both metals and the benzene/toluene competitive hydrogenation as well. However, detailed analyses of the results of the cyclohexane dehydrogenation and n-hexane reforming showed that the catalytic performance of the bimetallic catalysts were mainly influenced by the ensemble effect instead of the electronic effect. The sulfur resistance of the catalysts was much improved by the addition of Sn, which is attributed to the dramatically reduced mobility of platinum atoms by alloying with tin or anchoring of the platinum and platinum-tin alloy on top of the tin complex formed between partially reduced tin species and the L-zeolite support.