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 same (Pt + Mo) metal atom content but various Mo/Mo + Pt ratios, has been synthesized and characterized by XPS, ESR, hydrogen chemisorption, and oxygen uptake during their reoxidation after reduction, in order to determine the oxidation state of Pt and Mo and the catalyst surface compositions. They have been further tested for their activities in the dehydrogenation of cyclohexane, the hydrogenation of benzene, the hydrogenolysis 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 order 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 reduction temperature, is reduced into metal. Mo at various oxidation states 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 disappears in the presence of oxygen and of aromatic hydrocarbons strongly adsorbed on the metal particle surfaces. Two kinds of reactions are distinguished: cyclohexane dehydrogenation, alkanes isomerization, and cyclization, with a rate-determining step occurring preferably on Pt, and alkanes hydrogenolysis, which is much faster on Pt-Mo because of preferential adsorption on a mixed Pt-Mo site. The nature of Pt and Mo species participating to this mixed site is discussed.