Surface active sites in alumina-supported MoVNbTeO oxide catalysts

Methanol oxidation reaction probes the chemical nature of the active sites (formaldehyde forms on surface redox sites and dimethylether, on acidic ones) and has been used to characterize the active sites present on the surface of Mo–V–Nb–O catalysts which has been correlated with the activity for propane oxidation to acrylic acid. The effect of calcination atmosphere and the role of tellurium addition have been discussed as a function of the total coverage. The structure of Mo–V–O mixed phases strongly depends on these parameters, as determined by XRD, XPS and Raman spectroscopy. Such structures determine the catalytic behavior of catalysts during both propane and methanol oxidation. At low and medium coverages, the presence of oxidized dispersed surface vanadium structures (VOx) determines the catalytic behavior; such phases are active for methanol oxidation to formaldehyde. At high coverages, the formation of Al–Mo–O and Mo–V–O crystalline aggregates decreases the number of redox sites, whereas the presence of reduced species in rutile-like and/or MoO2 increases the redox activity for inert-calcined samples. For samples with high coverage, tellurium addition decreases the activity during methanol oxidation. This is due to a decrease in the population of surface VOx species. The interaction of tellurium with molybdenum and vanadium sites results in mixed phases with intermediate oxidation state that exhibit higher yields to acrylic acid.