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

Abstract

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.