Surface co-ordination of oxygen on oxygen-deficient TiO2 and MoO3 as revealed by e.s.r.-measurements

E.s.r. spectra were obtained from TiO2(anatase) and MoO3 after evacuation at 500°C and subsequent addition of O2 at room temperature. The spectra observed for TiO2 are quite complex: by varying the degree of reduction, the O2-pressure, the microwave power and the temperature for e.s.r. recording they were separated in a number of components. On degassed TiO2 two of the components are assigned respectively to an F-center and to Ti3+, both at the surface. A third signal remained unidentified. Addition of O2 causes the emergence of at least four components. Three of these were assigned to O2-surface co-ordination complexes, viz., the L-form (similar to CN–co-ordination), the P-form (similar to C2H2-co-ordination) and the A-form (similar to H2O2). The fourth signal is believed to be connected with O– formed by H-abstraction from OH– presumably by the peroxo-complexes.On MoO3 after evacuation four component signals were observed, one of which is assigned to an F-centre, a broader and more intense one to surface Mo5+, the remaining two signals remaining unidentified. On O2-addition none of the signals observed for TiO2 were seen to occur here, the interaction with O2 being almost solely restricted to a decrease in intensity of the F-center and Mo5+-signals. The Bi2O3+MoO3 system after evacuation is similar to MoO3 except for the absence of the Mo5+-signal. On NiO no signals could be found either in the evacuated or in the “reoxidized” form. The possible significance of the presence of absence of the co-ordinated O2-species for selective oxidation catalysis is discussed.