The dispersion of Cr V and Mo V on the surface of various oxides, as investigated by ESR spectroscopy

Abstract

Samples containing chromium ( 53Cr-enriched or not) or molybdenum ( 95Mo-enriched or not) were prepared by the equilibrium adsorption or impregnation methods at two different loadings (∼ 0.05 and 0.5%-i% by weight) using as supports ZrO 2, SiO 2, TiO 2, Y 2O 3, Al 2O 3, Ga 2O 3 or La 2O 3. After thermal activation (heating with O 2 at 773 K for the Cr samples followed by reduction with H 2 for the Mo samples). ESR signals from surface Cr V or Mo V species are observed. The spectra are satisfactorily resolved on ZrO 2, SiO 2, TiO 2, Y 2O 3, whereas they are broad (and always structureless in the case of chromium) on Al 2O 3, Ga 2O 3 or La 2O 3, whether 53Cr-enriched ( 95Mo-enriched) was used or not for the preparation of samples. Resolved spectra are assigned to mononuclear Cr V or Mo V surface species. Notably, the ESR spectrum of 95Mo V on Al 2O 3, in contrast to that of 53Cr V on the same matrix, shows partially resolved hyperfine structure which allows for the assignment of the spectrum to mononuclear Mo V species. The finding that the ESR signals of Cr V or Mo V on the surface of Al 2O 3, Ga 2O 3 or La 2O 3 are much broader than on the other matrices is attributed to unresolved interaction with neighbouring 27Al. 69.71Ga or 139La (superhyperfine interaction, shf). The shf interaction is small on Y 2O 3 in view of the low nuclear spin and magnetic moment of the 89Y nucleus and in fact resolved spectra are obtained on this matrix. Computer-calculated spectra assuming mononuclear Cr V or Mo V species interacting with two equivalent 27Al nuclei (slightly anisotropic shf interaction 6 to 8.5 gauss) are in substantial agreement with experimental spectra.