ELECTRON MICROSCOPIC STUDY OF Pt-Sn/AI_2O_3 CATALYSTS
Zang Jing-ling, Du Hong-zhang, Cai Hai-lin, Lin Li-wu, (Dalian Institute of Chemical Physics, Academia Sinica)and Qiao Gui-wen (Shenyang Institute of Metal Research, Acadcmiu Sinica)
The surface structure of four kinds of Pt-Sn catalysts containing 0.38%(wt) of Pt with Pt/Sn atom ratio equal to 1/2 prepared via different methods has been studied by selective area electron diffraction technique and electron microscope. The diffraction patterns corresponding to metallic Pt, Sn as well as, SnO·SnO2 solid solution were found from tht selective area of the catalysts prepared by co-impregnation method. PtSn, PtSn2 alloys were found instead of metallic Pt and Sn on the catalysts prepared by separate impregnation with Pt prior to Sn or its reverse. In contrast to these catalysts, neither Pt, Sn nor their alloy was found on catalysts prepared by complex method. The electron diffraction pattern indicated that, besides lines of SnO·SnO2, some of the -unknown Pt-Sn species also existed on the complex impregnating catalysts which implies that some PtxSny cluster which has been mentioned in other papers was shown in this case. As the atom ratio of Pt/Sn decreased to 1/5, the lines of metallic Sn appeared while the unknown species remained.Further investigation of these unknown species was carried out by studying the morphology and structure of the complex impregnating catalysts by means of a. high resolution electron microscope. Some crystallites with pentagonal and hexagonal shapes and with one or two dimensional lattice images were recorded. The crystallites with twin andmulti-twin structure were also observed. The lattice fringe of metallic tinappeared on the image of the catalyst with the atom ratio of Pt/Sn = 1/5. The chemisorption results showed that those kinds of active sites which are favorable to high temperature, adsorption of H2 were mainly originated from these unknown Pt-Sn species.