The use of electron paramagnetic resonance techniques in the molecular approach to heterogeneous catalytic processes on oxides

Abstract

This review shows that EPR spectroscopy can be a powerful tool for catalysis research. This is illustrated in the particular case of highly dispersed Mo/SiO₂ catalysts prepared by grafting. The different steps of the preparation may be identified: e.g. the EPR changes observed in the Mo⁵⁺ coordination sphere during the grafting process of MoCl₅ onto silica, showed that the grafted species is probably SiOMoCl₄(g_⊥= 1.952, g_∥= 1.968). The catalyst was also characterized after activation treatment such as thermal reduction. Three different Mo⁵⁺ species have been detected. Their coordination sphere was determined using probe molecules such as water, methanol and natural or ¹³C-enriched carbon monoxide. These Mo⁵⁺ species are molybdenyl compounds surrounded by oxygen ligands in distorted octahedral (Mo⁵⁺_6c)(g_⊥= 1.944, g_∥= 1.892), square-pyramidal (Mo⁵⁺_5c)(g_⊥= 1.957, g_∥= 1.866), and distorted tetrahedral symmetry (Mo⁵⁺_4c)(g_⊥= 1.926, g_∥= 1.755). The subscripts 6c, 5c and 4c stand for hexa-, penta- and tetra-coordinated, respectively. The EPR results obtained on UV-irradiated oxidized Mo/SiO₂ on which methanol was preadsorbed, are also consistent with the variation of selectivity in methyl formate and formaldehyde with Mo dispersion observed during methanol oxidation. They extend the reaction mechanism previously proposed on the basis of kinetic data. EPR revealed the formation of a hydroxymethyl radical ˙CH₂OH. The deactivation of this radical by an ‘external’ redox process is favoured on highly dispersed Mo catalysts such as grafted Mo/SiO₂. The process involves radical migration on silica where it can react with SiOCH₃ groups, forming methyl formate.