Photoinduced reactions of methane with molybdena supported on silica

Photoinduced reactions of methane on the surface of molybdena–silica have been studied using u.v. irradiation in the temperature range 293–773 K. During irradiation, photoadsorption of methane (up to 300 mmol of CH4 per mol of Mo) is found to be the predominant process with barely detectable formation of gaseous products. During heating of the irradiated samples from 293 to 473 K, in addition to thermodesorption of methane, which reaches 40–50% of photoadsorbed CH4, the desorption of considerable amounts of ethylene, ethane, hydrogen and smaller amounts of C3 and C4 alkenes and alkanes is observed. E.s.r., u.v.–visible and i.r. measurements of the irradiated samples show the presence of Mo5+ and M4+ ions as well as complexes of Mo4+ with olefins. The effects of O2, N2O and H2O on the photoinduced reactions of methane and on thermodesorption of the products have also been studied. Possible reaction mechanisms for the photoadsorption of methane and for the formation of C2 and higher hydrocarbons are discussed.