On the wavelength-dependent performance of Cr-doped silica in selective photo-oxidation

We have evaluated the hypothesis that the performance of a chromium-based photocatalyst (activity and selectivity) can be tuned by means of the wavelength to which the catalyst is exposed. To this end, we synthesized an amorphous, mesoporous silica matrix loaded with 10 at % chromium/silicon, characterized its structure, and tested its performance in selective oxidation of propane to acetone. Spectroscopy, microscopy, and X-ray diffraction show that chromium is present in this catalyst as both isolated Cr(VI) sites and crystalline Cr(III) oxide. The photochemical reaction rate depends strongly on excitation wavelength in the range of 300-700 nm, with a well-resolved maximum at 460 nm. At this wavelength, adsorbed acetone is produced with approximately 60% selectivity over carboxylates. Only very minor variations in selectivity were observed over the same range of wavelengths. By comparison of absorbance, luminescence, and reaction-excitation spectra we are able to identify the photoactivated catalytic site as an excited state of isolated Cr(VI) oxide in tetrahedral coordination. Therefore a heterogeneity of chemical structure, which is correlated with heterogeneity of an unresolved absorption spectrum, can be the basis of wavelength-dependent photocatalytic activity.