Electron Transfer at the Gas-Solid Interface: Reaction of Gas-Phase MoCl with Vanadium Oxide Supported on Silica

Electron transfer between two transition metals, MoCl and VO supported on silica, is studied in situ at the gas-solid interface by spectroscopy (electron paramagnetic resonance, diffuse reflectance UV-vis-near-IR, XPS, Raman), electron microscopy, and chemical analysis for a long period of time (∼9000 h). Initially, the VO/SiO (V, 3d) solid phase is composed of V O crystallites and monomeric tetrahedral vanadyl species grafted onto silica ((≡SiO)V=O). Upon reaction at 30 °C with gas-phase MoCl (Mo, 4d), the solid undergoes drastic changes in color, which define four stages. Each stage is accompanied by variations in the chemical state of the supported phase. The principal results are as follows, (i) No electron transfer and no reaction takes place between MoCl and grafted (≡SiO)V=O species, (ii) In contrast, VO crystallites undergo both electron transfer from Mo to V and chemical changes leading to a change in the speciation of V and to the formation of a new and amorphous V-O-Mo-Cl phase, (iii) The following sequence of redox potentials can be proposed for the MoCl-VO /SiO system: E(V/V) > E(Mo/Mo) > E(V /V) The difference in behavior of the two types of vanadium species toward electron transfer from MoCl is discussed in terms of the dependence of the nature of the ligands, the symmetry, and the nuclearity of the vanadium species on the redox potentials.