Unusual Redox Properties of Bismuth in Sol–Gel Bi-Mo-Ti Mixed Oxides

Abstract

Novel bismuth molybdenum titanium oxide catalysts, prepared via different sol–gel routes, were studied by X-ray photoelectron spectroscopy (XPS) at ambient temperature after evacuation and after reaction with hydrogen or oxygen. The surface Bi3+and Mo6+species of several sol–gel catalysts could be reduced and re-oxidized at room temperature, whereas the two reference materials γ-bismuth molybdate and Bi2O3were not reduced in hydrogen (10−4mbar) at temperatures below 573 K. The reduction of Bi3+to Bi0was observed to be remarkably more facile than that of Mo6+to Mo5+. Reduction of surface bismuth species in hydrogen and re-oxidation in oxygen could be followed by XPS at room temperature in the pressure range of 10−6–10−4mbar over a time scale of a few minutes to several hours. The facile reduction of surface Bi3+is probably not connected to the bulk or surface composition. It is more likely due to the unique morphology of the bismuth molybdenum oxides stabilized by titania in the sol–gel materials. It emerged from XRD and Raman investigations that the mean crystallite size of Bi- and Mo-containing phases was around 3 nm, even after calcination at 773 K. XPS and thermoanalysis revealed that the particle size of the sol–gel mixed oxides was considerably larger (>10 nm), and the reduction of Bi3+and its re-oxidation took place only in the topmost layer at ambient temperature.