SURFACE AND CATALYTIC CHEMISTRY OF SMALL HYDROCARBONS ON OXIDIZED MOLYBDENUM

It is shown that model oxides grown on metallic substrates catalyze pr opylene metathesis to form ethylene and butene with an activity that m imics that of supported catalysts for reaction below similar to 650 K. Another reaction regime is found for olefin metathesis above similar to 650 K, where the reaction proceeds with a much higher activation en ergy of similar to 60 kcal/mol. Unfortunately, alkenes do not react to any detecable extent on the model oxide surfaces in ultrahigh vacuum. However, the high-temperature(> 650 K) metathesis rate is found to be affected by the presence of oxygen overlayers, which also modify the chemistry of alkenes on Mo(100) in ultrahigh vacuum. It is found that methane is formed by reaction of alkenes on O/Mo(100). The only other products detected are ascribed to either hydrogenation or total therma l decomposition into carbon and hydrogen. It is shown, using iodine-co ntaining molecules to graft hydrocarbon fragments onto the surface, th at alkenes can dissociate, forming carbenes which react to yield metha ne. This chemistry is in accord with that found catalytically at high temperatures, where the product distribution from the reaction of ethy lene is well described by a Schulz-Florey distribution and the product distribution from propylene is well described by copolymerization of carbenes and methyl carbenes.