Methane photoactivation on copper molybdate. An experimental and theoretical study

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 Cu^II 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 O^2- sites relative to MoO₃, and filled band-gap orbitals stabilize homolytic adsorption on Mo^VI sites. Heterolytic adsorption on Mo^VI and O^2- sites is stable but not on Cu^II and O^2- sites. Homolytic adsorption is unstable on Cu^II sites.