Heterogeneous photocatalysis reaction mechanisms

Sonophotocatalysis is photocatalysis with ultrasonic irradiation or the simultaneous irradiation of ultrasound and light with photocatalyst. Tnis method includes irradiation with alternating ultrasound and light. Ultrasound effects on heterogeneous photocatalytic reaction systems have been demonstrated by Mason,1 Sawada et al.,2) Kado et al.,3) and Suzuki et al.4) In these papers, not only acceleration of photocatalytic reactions but increase in product selectivity by ultrasonic irradiation has also been reported. It was postulated that ultrasound effects, such as surface cleaning, particle size reduction and increased mass transfer, were the result of the mechanical effects of ultrasound.1,5) Lindley reviewed these and other effects.5)... 

Heterogeneous photocatalysis of Pb(II) systems has received scarce attention. In our previous review (Litter, 1999), we cited a few early papers (Inoue et al., 1978, 1980a, b Kobayashi et al., 1983 Lawless et al., 1990 Maillard-Dupuy et al., 1994 Rajh et al., 1996a, b Termakone, 1984 Tennakone and Wijayantha, 1998 Thumauer et al., 1997 Torres and Cervera-March, 1992), and although some new papers appeared later (Aarthi and Madras, 2008 Chen and Ray, 2001 Chenthamarakshan et al., 1999 Kabra et al., 2007, 2008 Kobayashi et al., 1983 Mishra et al., 2007 Rajeshwar et al., 2002), information was and continues to be scant. The mechanisms of transformation of lead (II) in water by UV-Ti02 are especially attractive because they depend very much on the reaction conditions, related to the nature of the photocatalyst, the effect of oxygen, and the presence of electron donors. 

According to the expected mechanisms of heterogeneous photocatalysis (Section 5.4.1), this should provide mainly redox chemical reactions. 

The primary oxidant responsible for most advanced oxidation processes, i.e. the use of photochemical methods, is the hydroxyl radical, which is formed by the reduction reactions of electrophiles with water or hydroxide ions. The mechanism of the formation of the hydroxyl radical is well discussed in the literature. Heterogeneous photocatalytic oxidation of organic compounds in aqueous solution is achieved by the reactive hydroxyl radical. The photocatalytic process can remove a large number of organic hazardous compounds in water. Phenols, carboxylic acids and herbicides are among a large number of pollutants, which are destroyed in air and in water using photocatalysis. 

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