Electron spin resonance and electron spin-echo spectroscopic studies of supported-molybdenum catalysts. Interaction between molybdenum, adsorbate and oxygen molecules

Abstract

The interaction of various molybdenum species, obtained by high-temperature reduction of the supported catalyst, Mo/SiO₂, with various adsorbates and molecular oxygen has been examined by the techniques of electron spin resonance (e.s.r.) and electron spin-echo modulation (e.s.e.m.). Two species observable by e.s.r. after such reduction, Mo(A) with g_∥= 1.865 and Mo(B) with g_∥= 1.895, seem to be insensitive to oxygen adsorption, indicating that the electron-donating centre causing the formation of O^–₂ is not observable by e.s.r. at room temperature, 77 K or 4 K. If polar adsorbates such as H₂O, CH₃OH, NH₃ or C₅H₅N are adsorbed prior to exposing the reduced Mo/SiO₂ surface to oxygen, O^–₂ is not formed. If a non-polar adsorbate such as C₂H₄ is adsorbed before or after O₂ adsorption, O^–₂ is readily formed. O^–₂ can also be formed if the adsorbate is CH₃CN. Mo(A) is sensitive to all adsorbates, leading to the formation of a new Mo⁵⁺ species, Mo(C), presumably through the completion of the coordination sphere. It is suggested that Mo⁵⁺ or Mo⁴⁺ in symmetric tetrahedral coordination is a likely centre for the formation of an O^–₂ species. Both e.s.r. and e.s.e.m. indicate that when small olefins are adsorbed after the formation of O^–₂ a reaction takes place, even at room temperature.