EFFECT OF COKING-BURNING OFF CYCLE ON THE PROPERTIES OF Pt/Al_2O_3 AND PtSn/Al_2O_3 CATALYSTS
Fan Yi-ning, Zang Jing-ling and Lin Li-wu (Dalian Institute of Chemical Physics, Academia Sinica)
Effect of coking-burning off cycle on the properties of two PtSn/Al2O3 catalysts prepared by impregnation with Sn prior to Pt [PtSn(s)] and by complex method [PtSn(c)], and of Pt/Al2O3 catalyst was studied by hydrogen chemisorption, electron microscope and selective area electron diffraction technique. TEM results show that the order of sintering is PtPtSn(s)PtSn(c), i.e. the addition of Sn into Pt/Al2O3 increases its thermal stability, and the method of Sn addition exerts influence upon the sintering behavior. From the diffraction patterns, it is found that coking-burning off cycle results in the destruction of bimetallic clusters on PtSn(c) and the formation of metallic Pt and Sn, whereas the bimetallic components on PtSn(s) remains in most area. As segregation occurred, the Sn deposited on Pt surface and the amount of hydrogen chemisorbed by Pt decrease. For comparison, the oxidation-reduction' cycle on the catalysts was also performed. It is shown that the sintering and' segregation are mainly caused by carbonaceous on the surface of the catalysts, therefore, the stability of catalyst during regeneration depends upon the characteristics of metal-support-chemical environment interaction.