Electron paramagnetic resonance study of molybdenum supported catalysts labelled with 95Mo. Evidence for molybdenyl ions

Abstract

Reduction of molybdenum supported on silica leads to X-band e.p.r. spectra with g⊥= 1.940 and g‖= 1.882. When molybdenum enriched in ⁹⁵Mo is used, the hyperfine tensor can be measured as A⊥= 44 G and A‖= 98 G. The simulation of the experimental spectra has been carried out and is found to be satisfactory with the above magnetic parameters. Q-band spectra show an increase in the linewidth which prevents observation of the h.f. lines. This is due to a distribution in the components of the g tensor reflecting the presence of a number of molybdenum sites. The relative magnitude of the g-tensor components is discussed by comparing by comparing e.p.r. spectra of (NH₄)₂ MoOCl₅ in dilute form or supported on silica with other molybdenyl compounds.

It is observed that the relative magnitude of the g-tensor components of various molybdenyl compounds is very sensitive to the value of the spin–orbit coupling constant λ of the ligands and it is concluded that the present e.p.r. spectra can be assigned to pentavalent molybdenum surrounded by oxide ions and retaining one short MoO bond.