Detrimental cationic doping of titania in photocatalysis: why chromium Cr3+-doping is a catastrophe for photocatalysis, both under UV- and visible irradiations

The chromium-doping of titania, initiated by electrochemists, was subsequently applied to photocatalysis to tentatively photo-sensitize titania in the visible region. The results were not conclusive and disparate. Presently, the photo-electronic behavior of pristine TiO2 and of Cr-doped (0.85 at%) TiO2 has been followed by UV-spectroscopy combined with electronic photoconductivity under vacuum and under oxygen pressure. Although Cr-doped (0.85 at%) TiO2 absorbs in the visible region, it only becomes a photoconductor through band-gap illumination and the doping causes a considerable decrease in photoconductivity by three orders of magnitude. Correspondingly, its photocatalytic activity was a disaster, confirmed by five different reactions representative of various media in which titania is active: (i) gas phase oxidations of propene and of isopropanol in propene oxide and acetone, respectively; (ii) organic liquid phase oxidations of isopropanol and of cyclohexane in acetone and cyclohexanone, respectively; and (iii) oxygen isotope exchange under partial vacuum or reduced pressure. Whereas activity was nil under visible illumination, it was strongly decreased by an inhibition factor ranging from 25 to 1000 under UV-light. This was attributed to an increase in electron–hole recombination at the Cr3+ ion sites, which play the role of acceptor centers, which, once filled, attract holes. Conversely, to be optimistic, such a doping can be suggested as a means of passivating titania and enhancing the stability of TiO2-containing materials against (solar) light, provided their inherent properties be conserved.