Oxygen species, photocatalytic oxidation reaction

Most photocatalytic studies conducted at low aromatic concentrations report no detectable concentrations of gas-phase intermediates [12,17,18]. Traces of intermediates may be present in the gas phase, but at levels below the detection limits of the analytical instruments employed in these studies. There is evidence, however, for either reaction intermediates or reaction by-products on the catalyst surface, even at these low concentrations. Catalyst discoloration, typically a yellowish or brownish color, is often reported following the photocatalytic oxidation of aromatic contaminants at low to moderate gas-phase concentrations [3,4,7,17,52]. These intermediates or reaction by-products may be largely trapped on the catalyst surface by the higher affinity of oxygenated species, like alcohols and aldehydes, for TiO, surfaces when compared to the aromatic parent compounds. 

A varying and much more complex mechanistic situation exists in heterogeneous photocatalysis (Fig. 5-13). With respect to the transient oxygen species, comparable overall oxidation reactions are usually observed, but the set of primary reactive oxygen species is slightly different. It is commonly assumed, that superoxide radical anions and hydroxyl radicals are the primary species formed after photogeneration of the electron-hole pair of a semiconductor catalyst in the presence of water and air (Serpone, 1996). In the presence of ozone, ozonide radical anions or are formed by fast electron transfer reaction of superoxide radical anions with O3 molecules. The combination Ti02-03-UV/VIS is called photocatalytic ozonation (Kopf et al., 2000). For example, it was applied for the decomposition of tri-chloroethene in the gas phase (Shen and Kub, 2002). 

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 hydroxyl radicals formed may abstract hydrogen atoms from polymer molecules and thus initiate the auto-oxidation reaction. ZnO accelerates the process of nylon photo-oxidation [199]. Kuriacose and Markham [358] suggested, that several species of oxygen, 02(l g, 1 Jg), Of, O" and O, can be formed at the photocatalytic ZnO surface. 

The mechanism(s) by which these photocatalyzed oxidations are initiated remain uncertain. Early proposals have included involvement of either the photo-produced holes (h+) arising directly from semiconductor photo-excitation, or the (presumed) derivative hydroxyl radical (OH) which was argued to arise from the hole oxidation of adsorbed hydroxyls (h+ + OH-—> OH ). Recent subambient studies [4] with physisorbed chloromethane and oxygen suggest the dioxygen anion (02 ) as a key active species, and the photocatalytic high efficiency chain destruction of TCE is argued to be initiated by chlorine radicals (Cl) [5]. The chlorine-enhanced photocatalytic destruction of air contaminants has been proposed [1, 2, 6] to depend upon reactions initiated by chlorine radicals. 

Typically sunlight absorption can excite Fe(III) complexes to ligand-to-metal charge transfer (LMCT) excited states, which decay via photoinduced electron transfer (PET) to the Fe-center from the inner (ligand) or external electron donor. The photo-chemically generated Fe(ll) species is then reoxidized to the initial Fe(lll) compound or its derivative (e.g., aqua complex) closing the photocatalytic cycle. As result many environmental pollutants are oxidized by molecular oxygen in reactions driven... 

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