STUDY OF THE CARBONACEOUS DEPOSITION ON Pt/Al_2O_3 CATALYSTS BY TEMPERATURE PROGRAMMED OXIDATION
Xu Zhu-sheng, Zang Jing-ling and Zhang Tao(Dalian Institute of Chemical Physics, Academia Sinica)
The carbonaceous deposition on the surface of Pt/Al2O3 and Pt-Sn/ A12O3 catalysts was studied by combination of temperature programmed oxidation (TPO) and hydrogen chemisorption. TPO curves showed that carbon deposited on both active metal sites and acidic sites. Two peaks which could be ascribed to metal deactivation were observed on ' both Pt/Al2O3 and Pt-Sn/Al2O3 catalysts. The Tm of two peaks on Pt-Sn/ A12O3 is higher than that on Pt/Al2O3 catalyst. The method of preparation also exerted influence upon the deposition behavior on the catalysts. On the Pt-Sn/Al2O3 prepared by complex method, not only the Tm of peaks shifted to higher temperature side, but also the range of carbonaceous oxidation temperature became more narrow than that on the Pt/ A12O3 catalyst.From hydrogen chemisorption,it was found that during carbon deposition,the amount of hydrogen chemisorption first dropped down rapidly, then reduced gradually and finally kept constant. That means some parts of platinum surface are easily poisoned by deposition and others are very difficult to be covered by carbon. The data presented show that on the platinum surface, some deposition is in the form of multilayer, some exists as monolayer, and on some sites there is no deposition. There are about 15% of the platinum surface on Pt/Al2O3 which is not covered by carbonaceous deposition. The addition of tin component into the Pt/ A12O3 reduces the amount of carbon deposition and increases the bare surface up to 30%.