Characterization of bifunctional PtSn/H[Al]ZSM5 catalysts: a comparison between two impregnation strategies

Abstract

Bifunctional monometallic Pt/H[Al]ZSM5, Sn/H[Al]ZSM5 and bimetallic PtSn/H[Al]ZSM5 catalysts were prepared and characterized by means of XPS, TEM and toluene hydrogenation. For the bimetallic samples, co-impregnation of both metals and sequential impregnation (where Sn was added to monometallic Pt catalysts) were used with two tin atomic fractions, X Sn = 0.46 and 0.28. Based on XPS data, it was found that the surface composition of the Pt–Sn bimetallic catalysts depends on the impregnation method employed: in the co-impregnated samples alloys form between both metals, while in the sequentially impregnated catalysts no alloy formation occurs but both Pt and Sn are easily reduced. Also, TEM analysis showed that the co-impregnated samples have similar particle sizes as the monometallic Pt catalysts, but that the particles in the sequentially impregnated samples are of larger sizes, closer to those seen in monometallic Sn catalysts. It is suggested here that while in the former case the particle size is not affected by the formation of Pt–Sn alloys, in the latter the tin covers the Pt species, and therefore increases the overall particle size. Finally, it was found that some hydrogenating activity could be detected with the co-impregnated catalysts but not with the sequentially impregnated samples, even though the latter had only a small fraction of atomic tin. In general, the results obtained in this study show a significant variation in catalytic properties with the method of preparation used for these bifunctional PtSn/H[Al]ZSM5 catalysts.