Chromium photoreduction

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. 

HS form thermally stable complexes with transition metals, among others with iron and chromium. On irradiation the Fe (III) complexes undergo photoreduction producing Fe(II) and organic radicals (6,14). 

Chromium(V) species, the main photoreduction products, are stable only in exceptional cases (as in the polyvinylalcohol matrix 256)). The reduction of chromium(V) to chromium(III) is a far slower process when chromium is in the presence of strong chelating agents such as polycarboxylate derivatives (257), but in general, Cr(V) undergoes a series of redox reactions and, in absence of other redox and coordinating partners, is transformed in part into a Cr(III) species and in part to Cr(VI), for example ... 

It is now realized that copper as metal next to iron and chromium participates in photoredox cycles and its role cannot be ignored. The most important part of the cycle is photoreduction of Cu(II) to Cu(I) induced by solar light and oxidation of ligands to their environmentally benign forms. Then Cu(I) is easily re-oxidized to Cu(II), which can coordinate the next ligand molecule, and thereby the Cu photocatalytic cycles contribute to continuous environmental cleaning. Besides oxida-tion/reduction, other critical processes relevant to the copper cycles are adsorption/desorption and precipitation/dissolution... 

Enzymatically active NADH has been selectively produced by visible light photoreduction of NAD using [Ru(bipy)3]S04 and [Ru(bipy)3]2(S04)3 as sensitizers and triethanolamine as electron donor (Wienkamp and Steckhan). There is continuing interest in the photogeneration of co-ordinatedly unsaturated species from metal carbonyls etc. which can act as or give rise to catalysts, e.g., for cis-trans isomerization and hydrogenation of alkenes. Ger-rity et al. have used chromium hexacarbonyl to make the first quantitative measurements of the distribution of atomic excited states produced by multiphoton dissociation of a metal carbonyl. The distribution of states turns out to be statistical rather than spin- or polarity-difierentiated. The use of perfluoromethylcyclohexane as solvent has enabled Simon and Peters to observe naked Cr(CO)s as a transient from Cr(CO)6. 

Photoreduction of chromium(III) polypyridyl complexes by Fe " allows examination of the thermal reverse reaction. For a series of complexes, reactions with Fe show very similar rate constants reflecting internal activation free energies which do not differ markedly. 

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