It is shown experimentally that the activity of UV-irradiated MoO3 for the partial oxidation of methane in the presence of O2 at 100°C, leading to methanol formation, is markedly enhanced when CuII is added to the catalyst. The rate of methanol formation is maximum when the concentrations of copper and molybdenum are equal. Visible light activates CuMoO4 but not MoO3. We have made a molecular orbital study of the photoactivity of CuMoO4 toward hydrogen abstraction from methane at O- centers. The activity of CuMoO4 in the visible region is assigned to O 2p → Cu 3d excitations; the Cu 3d band levels lie below the Mo 4d band and the orbitals have a large O 2p component. Mechanisms are proposed for increasing the lifetimes of the electron-hole pairs and thereby account for the synergistic effect in CuMoO4 which shows enhanced photoactivity compared with either CuO, which is photoinactive, or MoO3. Empty band-gap orbitals serve to stabilize homolytic H• and •CH3 adsorption on O2- sites relative to MoO3, and filled band-gap orbitals stabilize homolytic adsorption on MoVI sites. Heterolytic adsorption on MoVI and O2- sites is stable but not on CuII and O2- sites. Homolytic adsorption is unstable on CuII sites.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry