Photocatalytic cycles

Miscellaneous. Ruthenium dioxide-based thick-film resistors have been used as secondary thermometers below I K (92). Ruthenium dioxide-coated anodes ate the most widely used anode for chlorine production (93). Ruthenium(IV) oxide and other compounds ate used in the electronics industry as resistor material in apphcations where thick-film technology is used to print electrical circuits (94) (see Electronic materials). Ruthenium electroplate has similar properties to those of rhodium, but is much less expensive. Electrolytes used for mthenium electroplating (95) include [Ru2Clg(OH2)2N] Na2[Ru(N02)4(N0)0H] [13859-66-0] and (NH 2P uds(NO)] [13820-58-1], Several photocatalytic cycles that generate... 

Figure 1. Schematic of a possible photocatalytic cycle based on the photochemistry of a multielectron photoactive center (MPC) intercalated within redox active layers of a host structure.

The methane oxidation to methanal is thus realized in the catalytic cycle in which atmospheric 02 is the oxidant and the OH radicals are the catalyst, and which is coupled to photoassisted dissociation of nitrogen dioxide (Figure 9.7). The latter process yields two ozone molecules per photocatalytic cycle. 

The photocatalytic cycles may be treated as an anti-photosynthesis because they consume atmospheric oxygen and disperse energy accumulated by photosynthesis. These processes are of vital importance, because, theoretically, most atmospheric oxygen could be consumed, which would be a threat to life on Earth. Fortunately, in nature photosynthesis dominates and photocatalysis by transition metals plays only a minor, albeit useful, role of cleaning water basins of excess organic pollutants. 

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. 

Figure 12.6 (a) The structure of Pacman-type cofacial bisporphyrin and (b) simplified illustration of the Pacman effect in a photocatalytic cycle. LMCT, ligand-to-metal charge transfer... 

Keywords Environmental homogeneous photocatalysis Photoreduction of transition metal complexes NO-carriers Photodegradation of organic pollutants Photocatalytic cycles stimulated by iron, copper and chromimn complexes. 

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... 

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... 

Beside monometallic cycles, mixed metal systems are also active in nature. The cooperation between the photocatalytic cycles of two transition metals can lead to diversification of the degraded pollutants, but their efficiency can be either higher or lower compared to that of the single metal cycle. 

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). 

Figure 3. Photocatalytic cycles based on an oxidative (left) and reductive (right) quenching of an excited polypyridine complex M. (The photosensitizer M in these cycles has been called light absorption sensitizer (LAS), since it enables a photochemical reactions between chemical species which do not absorb light [74, 266].)...

The photocatalytic cycles shown in Figure 3 are based on oxidative (left) and reductive (right) quenching of electronically excited polypyridine complexes. Such cycles can operate in a homogeneous solution, provided that the excited state electron transfer is much faster than concurrent, unproductive decay to the ground state / et 1/to, where tq is the inherent excited state lifetime at given experimental conditions, but in the absence of the excited state chemical reaction 1 /tq =... 

A photocatalytic cycle based on the [Co(sep)] ". ..r system, in which substrates easily oxidized by iodine are oxidized by oxygen, is described in Ref. 374. By this process, the two [Co(sep)]3+ and I components act not only as photosensitizers but also as relays. The O2 oxidation is activated as follows ... 

An inventor extraordinaire has read that the photocatalytic cycle of nitrogen oxides (the major source of air pollution in his city) begins with the emission of NO, chiefly by automobiles. This has encouraged him to invent a simple, cheap, and efficient device that converts NO in automobile emissions to N02. Will this device help or harm local air pollution problems in his city Would it be preferable to convert the NO to N20 to HN03 to N2 ... 

For the general case of a complete photocatalytic cycle, taking the difference between eqs. 5.70 and 5.67 yields expression 5.71. 

Figure 33. Photocatalytic cycle for COj reduction to CO using/ac-Re(bpy)(CO)3X. ism for the cyclic-catalyzed formation of formate is formulated as follows ...

Turnover frequencies (TF). The speed of the photocatalytic cycle is given by the photocatalyst s TF, which is defined using (9). 

The initial step in the photocatalytic cycle has been illuminated by laser flash photolysis experiments. These experiments have shown that reductive quenching of the triplet metal-to-ligand charge transfer ( MLCT) excited state by a sacrificial donor occurs to generate a one-electron reduced (OER) species of the rhenium... 

Metal radicals formed by photolysis of metal-metal bonded dimers are catalysts for the substitution of metal hydride complexes. The proposed photocatalytic cycle is shown in Scheme 14. 

The ability of iridium photocatalytic cycle to induce concomitant reduction of cyanoaromatic and oxidation of amine was nicely illustrated further by the direct (1-arylation of saturated ketones... 

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