Electron spin resonance studies of isotopically labelled oxygen species adsorbed on supported molybdenum

Abstract

The molecular ion (O^–₂)_s has been observed on the surface of the molybdenum/SiO₂ and molybdenum/Al₂O₃ systems by e.s.r. Using oxygen isotopically enriched in ¹⁷O, two different hyperfine splittings were observed in the Mo/Si system corresponding to A_xx= 85 and 72 G respectively; the A_yy and A_zz values of the tensor were too small to be resolved. The observed values for A_xx are shown to arise from an unequal interaction of the unpaired electron with two oxygen nuclei in the same molecular ion. This result is contrary to the previous work for (O^–₂)_s on other oxides where the oxygen nuclei have been found to be equivalent; the difference is attributed either to a more covalent bonding with the surface ions or to adsorption at an unusual site at the surface. This is thought to be a sensitive function of the surface environment since the oxygen nuclei appear to be nearly equivalent for the Mo/Al system although the g-tensors for both systems are almost identical.