Photocatalysis with radical

The photochemistry of porphyrin complexes of the first-row transition metals has recently been reviewed. The photochemistry of these complexes show that they undergo energy transfer reactions, photoreduction, photooxidation, atom transfer, photocatalysis, and radical chain initiation reactions. These reactions can result from the initial photoprocess, or they can result from thermal reactions that occur with the initial photoproduct. The excited states that lead to photochemistry are CT rather than transitions. 

Since BaTi40g and Na2TieOi3 have a pentagonal prism tunnel and a rectangular tunnel structure, respectively, we have paid attention to the role of the tunnel structures in photocatalysis. A barium titanate, Ba4Tii303o, in a series of Ba-Ti titanates was chosen as a representative with non-tunnel structure, and we have compared the photocatalytic activity and the ability for the production of surface radical species with uv irradiation between the tunnel and the non-tunnel titanates. 

The ambient temperature and the possible use of solar UV are the advantages of photocatalysis moreover, Ti02 is not toxic. The reaction mechanisms of Ti02 photocatalytic oxidation of azo dyes was similar to the biodegradation process of oxidation of azo dyes with OH radical. 

The above mechanism suggests that the presence of adsorbed oxygen 02(ads) is essential for photocatalysis. It allows an increase of hole lifetime by reaction with an electron and the formation of oxidizing OH radicals. 

In early works, the reactivity associated widi TiO2 photocatalysis was assigned to free OH radicals, but more recently die oxidative species has been identified as bound HO radicals and photogenerated holes. Reaction of the substrate with these species would require either die direct adsorption of organic compounds to the semiconductor interface or Fickian diffusion of the substrate to the semiconductor surface under depletion conditions. However, these two pathways imply different mechanisms, and in some cases different reaction products or intermediates. 

In the process of photocatalysis, the electrons and holes produced on photoirradiated Ti02 powders are trapped at the particle surface to form unpaired-electron species (step (4) in Fig.D.3). Photocatalytic reactions are actually the reactions of these radicals with reactant molecules at the Ti02 surface. Electron spin resonance (ESR) spectroscopy has been used for the detection of the photoproduced radicals on Ti02 at low temperatures such as 77 K. It has been reported that photoproduced electrons are trapped at various different sites titanium atoms on the surface or inside the particles, or oxygen molecules adsorbed on the surface. On the other hand, photoproduced holes are trapped at lattice OAygen atoms near the particle surface or at surface hydroxyl groups. We analyzed these radical species for several Ti02 photocatalysts that are commercially available, and found that the differences in the photoproduced radicals resulted from different heat-treatment conditions and the reactivity with several molecules.17)... 

An interesting but under explored method for the formation of 1,2-dioxetanes employing the photocatalytic oxygenation of olefins with dioxygen via selective radical coupling using 9-mesityl-10-methylacridinium ion as an electron-transfer photocatalysis has also appeared (Equation 6) <2004JA15999>. 

A comparison of these new processes with Ti02 photocatalysis for the degradation of aniline [148] showed that the former are faster. Although all these methods are considered to proceed via OH radicals, some different intermediates were detected in the electrochemical and photocatalytic experiments. p-Benzoquinone and NH4+ appeared in all solutions tested. A recent study [149] reports 87% and 99% TOC removals after 4 hr of... 

A [2 + 2] photocycloaddition with two alkenes can also be induced by photochemical electron transfer [16,17]. In such cases, sensitizers are frequently used and the reactions therefore occur under photocatalysis [18]. Under photochemical electron transfer (PET) conditions, the diene 10 yielded in an intramolecular reaction the cyclobutane 11 (Scheme 5.2) [19], such that in this reaction a 12-membered cyclic polyether is built up. The reaction starts with excitation of the sensitizer 1,4-dicyanonaphthalene (DCN) only 0.1 equivalents of the sensitizer are added to the reaction mixture. Electron transfer occurs from the substrate 10 to the excited sensitizer, leading to the radical cation I. This intermediate then undergoes cycli-zation to the radical cation of the cyclobutane (II). Electron transfer from the radical anion of the sensitizer to the intermediate II leads to the final product 11, and regenerates the sensitizer. In some cases, for example the cydodimerization of N-vinylcarbazole, the effidency is particularly high because a chain mechanism is involved [20]. 

In the case of semiconductor assisted photocatalysis organic compounds are eventually mineralized to carbon dioxide, water, and in the case of chlorinated compounds, chloride ions. It is not unusual to encounter reports with detection of different intermediates in different laboratories have been observed. For example, in the degradation of 4-CP the most abundant intermediate detected in some reports was hydroquinone (HQ) [114,115,123], while in other studies 4-chloro-catechol, 4-CC (3,4-dihydroxychlorobenzene) was most abundant [14,116-118, 121,163]. The controversy in the reaction intermediate identification stems mainly from the surface and hydroxyl radical mediated oxidation processes. Moreover, experimental parameters such as concentration of the photocatalyst, light intensity, and concentration of oxygen also contribute in guiding the course of reaction pathway. The photocatalytic degradation of 4-CP in Ti02 slurries and thin films... 

The presence of OH radicals, H202 and 03 must cause mechanisms known from photolysis, photocatalysis or ozonation (Langlais et al. 1991). New ROS such as 03 , 02 , O-, H02, H03, HO4 and others are possible most of them are short lived. Interactions with chloride, carbonate, bicarbonate, sulphate and other ions may form longer lived radicals (Gottschalk et al. 2000 Le Truong et al. 2004). The discussion below makes it clear that there are many indications of OH radicals reacting with other ions. 

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