Electron Paramagnetic Resonance Study on Supported Molybdenum Catalysts. Part 1: Interaction of Molybdenum Oxide with Silica and Signal Anlsotropy

The effects of reduction, evacuation and adsorbate adsorption on Mo/SiO₂ catalysts prepared by impregnation were studied by means of the electron paramagnetic resonance (EPR) technique. Pentavalent molybdenum species in the overlayer (Mo₀⁵⁺, g? = 1.941, g| = 1.895) and at the interface (Mo_i⁵⁺, g? = 1.952, g| = 1.866) between MoO₃ and SiO₂ were distinguished in the EPR spectra; they were observed from samples reduced at T < 573 K and T > 573 K respectively. The difference in their nature reflected the extent of their interaction with the support. Mo₀⁵⁺ existed in the MoO3 overlayer of the catalyst and interacted only weakly with the support. Mo_i⁵⁺ from the Mo ions connected directly to SiO₂ was affected strongly by the support. The anisotropic factor of the measured Mo_i⁵⁺ signals increased with the extent of support interaction. Superoxide ions (O₂, g₁ = 2.016, g₂ = 2.011, g₃ = 2.006) were found upon oxygen adsorption on samples evacuated at temperatures above 573 K. The observed O₂ may exist at the interface between MoO₃ and the support.