Possible effects of site isolation in antimony oxide-modified vanadia/titania catalysts for selective oxidation of o-xylene

Titania-supported vanadia catalysts were modified by addition of antimony oxide for application in o-xylene selective oxidation to phthalic anhydride. It was shown that active and selective catalysts can be prepared by ball-milling mixtures of powders of TiO2, V2O5 and Sb2O3 followed by calcination. X-ray photoelectron spectroscopy proves the formation of highly dispersed overlayers of vanadium oxide and antimony oxide, in which V5+ is partially reduced to lower oxidation states and Sb3+ is partially oxidized to Sb5+. Antimony oxide segregated into the outermost surface layers. It is therefore inferred that the presence of the antimony oxide modifier spatially separates V-O species and leads to "site isolation" which may be responsible for the positive effect of the modifier for the catalyst's selectivity.