Reaction mechanism heterogeneous photocatalyst

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)... 

This complexity determines that investigations on heterogeneous photo-catalytic processes sometimes report information only on dark adsorption and use this information for discussing the results obtained under irradiation. This extrapolation is not adequate as the characteristics of photocatalyst surface change under irradiation and, moreover, active photoadsorption centers are generated. Nowadays very effective methods allow a soimd characterization of bulk properties of catalysts, and powerful spectroscopies give valuable information on surface properties. Unfortunately information on the photoadsorption extent under real reaction conditions, that is, at the same operative conditions at which the photoreactivity tests are performed, are not available. For the cases in which photoreaction events only occur on the catalyst surface, a critical step to affect the effectiveness of the transformation of a given compound is to understand the adsorption process of that compound on the catalyst surface. The study of the adsorbability of the substrate allows one to predict the mechanism and kinetics that promote its photoreaction and also to correctly compare the performance of different photocatalytic systems. 

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. 

---