The structure of Mo-Si oxides has been investigated by XRD, FT-IR, laser Raman, and photoluminescence techniques. At low Mo content, an X-ray amorphous phase is formed, which is characterized by surface molybdates dispersed on SiOz. The surface concentration of amorphous Moo3 as well as the rate of oxidative dehydrogenation of CzH50H shows a maximum at about 1 atom % Mo. The change in the rate runs parallel with the change in the concentration of amorphous MOO,, i.e. surface polymolybdate. The concentration of tetrahedrally coordinated Mo ions in Mo-Si oxides determined by the photoluminescence technique shows a maximum again at about 1 atom % Mo content, becoming zero in the range above 3 atom % Mo content. A good parallelism between the concentration of tetrahedrally coordinated Mo ions and the rate of the metathesis reaction of propene is observed. ( 6 ) Cheng, C. P.; Schrader, G. L. J. Coral. 1979, 60, 276. ( 7 ) Jeziorowskii, H.; Knozinger, H.; Grange, P.; Gajardo, P.

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Vanadium oxide supported on titania has been studied by infrared spectra and X-ray diffraction for qualitative and quantitative analysis, together with the rate of oxidation of alcohol and reduction of oxide. The rate of ethanal formation passes through a maximum at V--Ti-1 1 (I 1 wt % V,O,) in the presence or absence of oxygen gas. At low surface concentrations of vanadium oxide below V-Ti-11 a new vanadate phase is observed in the i.r. spectra. The active oxygen species correspond to amorphous or two-dimensional vanadium oxide in the catalyst. It is concluded that the catalytic activity of V-Ti oxides originates mainly from the presence of such species.Vanadium oxide catalysts in combination with various promoters are widely used for selective oxidation of hydrocarbons. In particular, V-Ti oxide catalysts are well known because of their high selectivity and activity. The effect of the V/Ti ratio on catalyst selectivity and activity have been r e ~0 r t e d . l ~~ These studies of V-Ti oxide catalysts have suggested that the active catalytic species is likely to be a solid solution of V4+ in the TiO,. However, in the V-Ti oxide treated at lower temperatures such active species are not found. The promoting effect of V-Ti oxide has been explained as the effective disposal of V,O, on the titania surface as well as a weakening of the V=O bonds. Alternatively, the promoting effect is attributed to relative exposure of the (010) plane and stabilization of the V4+ ions. In fact, such a weakening of the V=O bond is found in Sn029 and TiO,.l0-l2 However, our understanding of the correlation between catalytic activity and surface vanadium oxide is far from complete.It has been reported from this laboratory that the promoting effect observed with V-Sn oxide in the oxidation of alkenes may partly be attributed to formation of amorphous material between the vanadium oxide and the tin oxide.', Such an amorphous material is expected to play a significant role in oxidation on V-Ti oxides. Both the acid-base properties of V-Ti oxide containing a small amount of vanadium oxide and the nature of sites active in the oxidation of ethanol have been studied by i.r. spectroscopy.14 The results suggest that oxygen in the surface V=O species acts as a basic site and plays a significant role in dehydrogenation.In this paper we report structural studies of V-Ti oxides carried out by i.r. and X.r.d. measurements, together with rate measurements of the oxidation of ethanol and the reduction of oxides. EXPERIMENTALVanadium-titanium oxide catalysts containing 2, 5, 11 and 69 wt % V,O, (V-Ti-2, V-Ti-5, V-Ti-11 and V-Ti-69, respectively) were prepared by impregnation of the TiO, support with 2929 2930

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