In order to obtain a useful expression for the kinetics of the photocatalyzed total oxidation reaction of alkanes with oxygen and TiO 2 as the catalyst, the dependence of the reaction rate on the concentration of the starting material species, the absorbed irradiation intensity, the wavelength of the light and the temperature was investigated. The experiments were performed in a CSTR system. The rate law found for alkane oxidation differs from that obtained for olefin oxidation due to a different reaction mechanism. While a reversible catalyst deactivation process takes place in the latter case, there is no hint of such a process during alkane oxidation. The kinetic data lead to the conclusion that the formation of an alkane and a hydrogen-radical at a defect site of the catalyst is the first step in the total oxidation reaction of these species followed by the formation of a stronger adsorbed hydroperoxide species. The subsequent transformation of the hydroperoxide into further oxidized species mediated by the electron-hole pairs generated by the absorbed light on the semiconductor's surface was recognized to be the rate determining step. The reaction behavior for the investigated alkanes could be modeled using this concept with the exception of methane. Its rate law differs strongly from that found for ethane, propane as well as iso-butane.

In order to obtain a useful expression for the kinetics of the photocatalyzed total oxidation reaction of olefines with TiO 2 as the catalyst, the dependence of reaction rate on the concentration of the educt species, the absorbed irradiation intensity, the wavelength of the light and the temperature was investigated. The experiments were performed in a CSTR system. The results obtained for steady-state conditions showed that besides the photocatalytic activity, the TiO 2 powder fixed on one wall of the reactor has, in addition, a thermal activity for higher temperature. Although the reaction behavior of the system is relatively complex, it can be modeled straightforward by a concept proposing a cooperative coadsorption of oxygen and the olefine species mediated by the electron/hole pairs generated by the absorbed light on the semiconductor's surface.

n order to obey the different regulations that governments of many countries have issued to protect our atmosphere, a large number of I effluent gas cleaning processes have been developed. Most of them make use of classical catalyst material like metal or metal oxides which become catalytically active only at higher temperature values, e.g. 200 to 400°C. From an economical point of view, the use of this type of catalyst is less profitable in situations where the concentration of the hazardous components in the gas stream is of some hundreds of ppm only and the temperature of the gas is near room temperature. Since under these conditions autothermic operation is not possible, an external heat source is needed to heat the effluent gas stream to reaction temperature.One way to circumvent this disadvantage is the use of photocatalysis. In this case, a semiconductor material like TiO, or ZnO is the catalytically active component in which electron/hole pairs are generated under irradiation with UV light. These charge pairs can facilitate the adsorption of oxygen and some hazardous species, activate them and, thus, open redox reaction pathways leading to the total destruction of the waste compounds. In practical applications, often, different types of hydrocarbon species, e.g. alkanes, alkenes, aromatics, have to be removed from effluent gas streams containing these components in relatively low concentration. Since such a problem often occurs in smaller companies, the use of photocatalysis in gas cleaning processes can be of wide interest, especially since the power of the UV lamps can be relatively low and the oxidation process proceeds already at room temperature.The setup of a useful photocatalytic reactor, however, presupposes a useful expression for the kinetics of the oxidation reactions in order to make clear the main parameters influencing the process in question and to make sure that the legal prescriptions are fulfilled. Although a relatively large number of papers on the photocatalytic oxidation of hydrocarbons have been published in the open literature (e.g. Courbon et al., 1973;Lyashenko et al ., 1977;Pichat et al., 1979; Formenti et al., 1971a, b Pichat et al., 1982Herrmann et al., 1979; Formenti et al., 1972; Formenti et al., 1976a, b;and Disdier et al., 1977), nearly all of them have been interested in mechanistic aspects. The development of kinetic expressions to describe reaction rate as a function of the influencing parameters are not so numerous (e.g. Peral et al.

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