THE DISPERSION OF CRV AND MOV ON THE SURFACE OF VARIOUS OXIDES, AS INVESTIGATED BY ESR SPECTROSCOPY

Samples containing chromium (Cr-53-enriched or not) or molybdenum (Mo-95-enriched or not) were prepared by the equilibrium adsorption or impregnation methods at two different loadings (approximately 0.05 and 0.5%-1% by weight) using as supports ZrO2, SiO2, TiO2, Y2O3, Al2O3, Ga2O3 or La2O3. After thermal activation (heating with O2 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 ZrO2, SiO2, TiO2, Y2O3, whereas they are broad (and always structureless in the case of chromium) on Al2O3, Ga2O3 or La2O3, whether Cr-53-enriched (Mo-95-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 Mo-95(v) on Al2O3, in contrast to that of Cr-53(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 Al2O3Ga2O3 or La2O3 are much broader than on the other matrices is attributed to unresolved interaction with neighbouring Al-27, Ga-69, Ga-71 or La-139 (superhyperfine interaction, shf). The shf interaction is small on Y2O3 in view of the low nuclear spin and magnetic moment of the Y-89 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 Al-27 nuclei (slightly anisotropic shf interaction 6 to 8.5 gauss) are in substantial agreement with experimental spectra.