A comparative study of the photocatalytic oxidation of propane on anatase, rutile, and mixed-phase anatase-rutile TiO2 nanoparticles:: Role of surface intermediates

The photocatalytic oxidation of propane was investigated by simultaneous in situ diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) and quadrupole mass spectrometry (QMS) on anatase, rutile, and mixed anatase-rutile TiO2 nanoparticles prepared by hydrothermal treatments of microemulsions. The mixed anatase-rutile sample was compared with a commercial sample (Degussa P25) with similar anatase to rutile mol% ratio. The measured carbon mass balance for the different TiO2 materials reveals that the total oxidation rate (propane --> CO2) over the first 25-min illumination period is highest for samples containing large anatase nanoparticles and that mixed anatase-rutile nanoparticles are superior to single-phase anatase nanoparticles. These findings are correlated with the main intermediate surface species observed on the different nanoparticle systems by DRIFTS. In particular, eta(1)-acetone and bridging bidentate formate (mu-formate) is detected. On both anatase and rutile, it-formate is the final hydrocarbon surface species. Further oxidation yields bicarbonate, carbonate, CO2, and H2O. On all TiO2 samples, the concentration of surface intermediates is found to be proportional to the accumulated carbon concentration, as deduced from the gas-phase carbon mass balance measurements. This shows that the rate-determining step is the oxidation of strongly bound surface intermediates. Furthermore, it is found that the rate-determining step is structure-sensitive. On anatase, photo-oxidation of acetone limits the total oxidation, whereas on rutile, formate does so. The latter is attributed to a combination of thermal dissociation of acetone on defect sites, which aids acetone oxidation, and the strong bonding of mu-formate to the (110) surface facets on rutile. A synergetic effect between anatase and rutile particles is observed, where the measured photo-oxidation rate from either QMS or DRIFT exceeds that for the individual constituent particles. (c) 2007 Elsevier Inc. All rights reserved.