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 Mo[double bond, length as m-dash]O bond.