Photocatalytic ozonation

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). 

Wagner T, Hennemann J, Kohl C-D, Tiemann N (2011) Photocatalytic ozone sensor based on mesoporous indium oxide influence of the relative humidity on the sensing performance. Thin Sohd Films 520 918-921 Wakamura K (2005) Empirical relationships for ion conduction based on vibration amplitude in perovskite-type proton and superionic conductors. J Phys Chem Solids 66 133-142 Wang W, Virkar AV (2005) Ionic and electron-hole conduction in BaZr, Y j,03 d by 4-probe DC measurements. J Power Sources 142(1-2) 1-9... 

A similar behavior was observed for the photocatalytic ozonation of dyes on copper ferrite (CuFe204) nanoparticles. The mineralization occurred via aliphatic carboxylic acids that were deteded as dominant aliphatic intermediates where they were further oxidized slowly to CO2. Finally, inorganic anions were detected as the photocatalytic mineralization products of dyes [103]. 

Pan, Z., Cai, Q., Luo, Q., and Li, X. (2015) Photocatalytic ozonation of oxalic acid over Cu(II)-grafted Ti02 under visible light irradiation. Synth, React. Inorg. Met-Org Nano-Met Chem., 45, 447-450. 

Organic pollutants removal in wastewater by heterogeneous photocatalytic ozonation. Chemosphere, 121, 1-17. 

Mahmoodi, N.M. (2011) Photocatalytic ozonation of dyes using copper ferrite nanoparticle prepared by co-precipitation method. Desalination, 279 (1-3),... 

Catalytic ozonation Photocatalytic ozonation Catalytic wet air oxidation (CWAO)... 

Use of combination of cavitation and advanced oxidation processes such as ozonation, chemical oxidation using hydrogen peroxide and photocatalytic oxidation and use of combination of ultrasound and microwave irradiations. 

Intensification can be achieved using this approach of combination of cavitation and advanced oxidation process such as use of hydrogen peroxide, ozone and photocatalytic oxidation, only for chemical synthesis applications where free radical attack is the governing mechanism. For reactions governed by pyrolysis type mechanism, use of process intensifying parameters which result in overall increase in the cavitational intensity such as solid particles, sparging of gases etc. is recommended. 

Several articles have reviewed the ongoing work in the photocatalytic degradation of pollutants that involve oxidation or reduction processes (depending on the experimental conditions) [16,18,187,265-273], The addition of external oxidants such as ozone or hydrogen peroxide during the photocatalytic process can improve the degradation of the organic material when they are added in suitable doses [274-275],... 

These factors have led researchers to examine novel methods for tire degradation of these contaminants, including liquid-phase photocatalytic oxidation. The treatment of wastewaters contaminated with agents like chlorinated aromatics has also prompted research into the optimum methods of integrating chemical oxidation techniques with existing water treatment methods, particularly biological treatment and ozonation techniques [37],... 

MC was a popular solvent for cleaning of semiconductor and degreasing of metal parts, but due to its ozone depletion potential, it has beer phased out and its use is strictly controlled. We have studied the photocatalytic decomposition of this as a typical chlorinated saturated hydrocarbon. 

Either photocatalysis or ozonation alone achieved rapid disappearance of aromatic pesticide. In contrast, mineralization (TOC removal) was slow for both. However, photocatalytic mineralization was enhanced considerably by ozone pretreatment (Fig. 9.16).31) This effect may be explained by ozonolytic cleavage of the aromatic ring and subsequent formation of aliphatic compounds which are more degradable by photocatalysis. Simultaneous use of photocatalyst and ozonation (illuminated by 254 nm light) showed synergetic effect on TOC removal (Fig. 9.17).32) In this process scavenging of electrons by ozone is considered to play the most important role. 

Fig. 9.16 Effect of ozone pretreatment on photocatalytic degradation of the pesticide DEP. O 03, A Ti02, 03 + Ti02.

As reported by Augugliaro et al. [64] the photocatalysis can be combined with chemical or physical operations. In the first case, when the coupling is with ozonation [65, 66], ultrasonic irradiation, photo-Fenton reaction or electrochemical treatment, which influence the photocatalytic mechanism, an increase of the efficiency of the process is obtained. 

Effect of 03. Adding ozone in dioxygen or air is a very efficient means of enhancing the photocatalytic rates of the removal and, above all, the mineralization of organic pollutants both in air and in water, even if the wavelengths are intentionally selected so as not to excite ozone (39 41). This substantial effect is attributed to the difference in electron affinity between 03 (2.1 eV) and 02 (0.44 eV). Consequently, in the presence of ozone, the electrons photopromoted to the Ti02 conduction band can be captured more easily, either directly ... 

Ohtani B, Zhang SW, Nishimoto SI. Catalytic and photocatalytic decomposition of ozone at room temperature over titanium (IV) oxide. J Chem Soc Faraday Trans 1992 88 1049-1053. 

The silver-loaded titania photocatalysts possess high activity in the photocatalytic decomposition of ozone [38], photoreduction of various thiols [39], photodestruction of 1,4-dichlorobenzene [40], dehydrogenation and oxidation of alcohols (e.g. of 2-propanole [41,42]), decoloration of textile waste water [43], photokilling of bacteria [44], and others. For additional detailed information on this topic, one can use a number of review articles and books [e.g. 45-48]. 

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. 

Many have found that during the photocatalytic destruction of toluene the catalyst was susceptible to deactivation. This may be due to the accumulation of benzoic acid on the catalyst surface. Studies have also shown that the addition of TCE or ozone can either have a positive or negative effect on toluene degradation [235]. Larson and Falconer [235] observed that toluene reacts quickly on TiC to form strongly bound intermediates which react more slowly to form CO2 and water. 

Kopf P, Gilbert E, Eberle SH (2000) Ti02 Photocatalytic Oxidation of Monochloro-acetic Acid and Pyridine Influence of Ozone, J. Photochem. Photobiol. A Chem. 136 163-168. 

Gomes de Moraes S, Freire RS, Duran N (2000) Degradation and Toxicity Reduction of Textile Effluent by Combined Photocatalytic and Ozonation Processes, Chemo-sphere 40 369-373. 

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