Chromium oxide photocatalysts

Chromium-containing mesoporous silica molecular sieves (Cr-HMS) with tetrahedrally coordinated isolated chromium oxide (chromate) moieties can operate as efficient photocatalysts for the decomposition of NO and the partial oxidation of propane with molecular oxygen under visible light irradiation (Yamashita et al., 2001). 

The effects of other transition metal ions are qualitatively similar, although their extent is decidedly smaller. Complexes of copper, chromium, and manganese with central atoms in their higher oxidation states are fairly good candidates to play the role of an environmental photocatalyst [20],... 

This activity is particularly useful for degradation of strongly hazardous substances or recalcitrant pollutants that are difficult to remove in chemical or biochemical processes. In this respect any pathway leading to abatement of chromate(VI) pollution arouses interest. One such pathway seems to be created by cooperation between iron and chromium photocatalytic cycles, which were reported as effective in conversion of chromate(VI) into Crm species [20-23,97]. A synergistic photoreduction of CrVI and Cu11 mediated by Ti02 [98], or photocatalytic reduction of Crvl and oxidation of organic matter by environmental polyoxometallates as photocatalysts [99], may constitute alternative possibilities. 

The role of photocatalysis by transition metal complexes in the environment is reviewed, and its influence on composition of the environmental compartments, transport between them, and activation of the environmental self-cleaning behavior is characterized. In description of atmospheric processes, the attention is paid to coordination compounds as photocatalysts of the transfer and redox reactions of nitrogen oxides. In the case of hydrosphere and soils, various mechanisms of organic pollutant photodegradations are presented in which the iron, copper, and chromium complexes play... 

In aspect of its toxicity, any pathway leading to abatement of chromate(VI) pollution arouse a vivid interest. One of such pathways seems to be created by cooperations between the iron and chromium photocatalytic cycles, which were reported as effectively converting chromate(Vl) into Cr(III) species. Photochemical coupling reactions between polycarboxylate Fe(III) complexes and chromate(Vl) were studied and strong collaboration between both photocatalysts was demonstrated, which was significantly affected by the oxygen concentration (16,17,95,261). On the other hand, chromium(Vl) reduction pho-toinduced by iron(lll) nitrilotriacetate accompanied by nta degradation was found to be independent of the O2 concentration, whereas the oxidation state of the chromium product depended on the pH (257). 

The effect of the addition of citric acid on the photocatalytic reduction of hazardous Cr(VI) to less hazardous Cr(III)) with titania catalysts has been studied by means of in situ EPR of chromium species by Meichtry et al. [52] using titania P25 as photocatalyst. Reduction experiments of Cr(VI) solution were performed under near-UV (366 nm) irradiation under acidic conditions (pH 2) with bubbling air. It is found that the addition of citric acid fadhtates Cr(VI) reduction with a stepwise reduction of the chromate CrO/ (V)) via formation of Cr(V) and Cr( IV) and finally Cr(III) spedes observed. In the absence of dtric add, a cycHng between the different valence states of chromium occurs because of reduction and reoxidation processes by OH radicals. The maximum rate (fivefold increase) of Cr(VI) reduction is achieved at an initial citric add/Cr(VI) molar ratio of 1.25. Citric acid is oxidized to its anionic radical by electron abstraction of surface-trapped holes Cit + h+ j, -> Cit . 

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