Effect of titanium-doping on the properties of vanadium antimonate catalysts

Catalysts of nominal compositions VSb1-xTixO4, with x = 0, 0.1, 0.2 and 0.4, were prepared by solid state reaction of vanadium(V) oxide, antimony(UI) oxide and titanium(IV) oxide mixtures. Their structural and catalytic properties were studied by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (TPR) and catalytic heterogeneous oxidation of toluene to benzaldehyde. The samples with 10% (x = 0.1) and 20% (x = 0.2) of titanium present the highest activity and selectivity to benzaldehyde under the operating conditions used in this work. When the titanium content is 40% (x= 0.4), the activity level drops strongly below the level found for the undoped vanadium antimonate. X-ray patterns show the presence of vanadium(V) oxide as a separate phase in this sample, and this is probably the cause of the low catalytic performance. TPR results show that the replacement of antimony by titanium changes the oxidation-reduction properties of the vanadium antimonate phase. In the samples with titanium content up to 20%, there seems to be a direct relationship between the selectivity and the separation of the reduction temperatures of both reduction peaks of the TPR profile: the larger the separation, the higher is the selectivity to benzaldehyde. All the samples with x = 0, 0.1 and 0.2 show surface antimony excess despite being prepared from stoichiometric mixtures of nominal V/Sb ratios larger than or equal to 1. (C) 2000 Elsevier Science B.V. All rights reserved.