Properties of platinum-molybdenum bimetallic catalysts deposited on silica

Abstract

Bimetallic Pt-Mo catalysts deposited on silica, with an equal number of metal atoms per gram of catalyst, have been prepared by co impregnation of the support, then reduction at 500 °C under flowing hydrogen. XPS analysis indicates that the ratio I(Pt4f)/ I(Si2p) is proportional to the ratio Pt/Si of the number of platinum to silicon atoms, and that molybdenum is only partially reduced to Mo°, the ratio Mo°/Mo total being roughly the same for all catalysts. The binding energy of 4d 7 2 Pt electrons is the same whatever the catalyst composition, indicating that if any electronic modification of Pt occurs it is very slight. Hydrogen chemisorption (HC) indicates that the ratio H/Pt decreases when the molybdenum proportion increases. Oxygen uptakes at 300 °C were measured, and the degrees of oxidation of molybdenum have been estimated using two different sets of hypotheses. These results have been discussed and compared to XPS results. The catalysts were then characterized by their activity in three organic test reactions. The rate of cyclohexane dehydrogenation at 270 °C is proportional to the amount of platinum present, but rates of butane hydrogenolysis at 300 °C and of benzene hydrogenation at 50 °C indicate the occurence of synergy effects which are stronger for butane hydrogenolysis. These results are discussed, and can be accounted for if the ratio Pt s/Pt is constant throughout the series of catalysts. The rate-determining step of the cyclohexane dehydrogenation takes place only on surface platinum atoms. However butane hydrogenolysis and benzene hydrogenation probably occur on Pt-Mo at rates higher than on platinum. Mixed sites of the same composition are thought to operate on all the catalysts. Such active sites include at least one hydrogen adsorption site.