INVESTIGATION ON ACTIVE-SITES IN PT-MO ON SILICA CATALYSTS FOR REACTIONS OF HYDROCARBONS WITH HYDROGEN

A series of bimetallic catalysts Pt-Mo deposited on silica, with the s ame (Pt + Mo) metal atom content but various Mo/Mo + Pt ratios, has be en synthesized and characterized by XPS, ESR, hydrogen chemisorption, and oxygen uptake during their reoxidation after reduction, in order t o determine the oxidation state of Pt and Mo and the catalyst surface compositions. They have been further tested for their activities in th e dehydrogenation of cyclohexane, the hydrogenation of benzene, the hy drogenolysis of butane, and the reactions of hexane, on the one hand, and of 3-methylpentane, on the other hand, with hydrogen. Finally the kinetics of butane hydrogenolysis has been extensively studied in orde r to determine the equilibrium constants of butane adsorption and the rate constants of the steps of C-C band breaking on all the Pt-Mo/SiO2 samples. The main results are that, while Pt is completely reduced in a metal state, only a fraction of Mo, which depends only on the reduc tion temperature, is reduced into metal. Mo at various oxidation state s is present in the same proportion for all the bimetallic catalysts. A Mo surface enrichment has been evidenced after reduction in hydrogen , but it has been shown that this enrichment decreases or even disappe ars in the presence of oxygen and of aromatic hydrocarbons strongly ad sorbed on the metal particle surfaces. Two kinds of reactions are dist inguished: cyclohexane dehydrogenation, alkanes isomerization, and cyc lization, with a rate-determining step occurring preferably on Pt, and alkanes hydrogenolysis, which is much faster on Pt-Mo because of pref erential adsorption on a mixed Pt-Mo site. The nature of Pt and Mo spe cies participating to this mixed site is discussed.