Determination of Photoadsorption Capacity of Polychrystalline TiO2 Catalyst in Irradiated Slurry

Abstract

Abstract In the field of heterogeneous catalysis all reactions involve the chemical adsorption of one or more of the reactants onto the catalyst surface as an intermediate step in the overall reaction. The quantification, identification, and knowledge of the behavior of chemisorbed species are the object of extensive investigation being this information used for understanding the actual catalytic mechanisms. In principle heterogeneous photocatalysis follows the same pattern with the difference that the chemical adsorption is initiated by light absorbed by the photocatalyst surface (photoadsorption). In the practice, however, while in heterogeneous catalysis the investigation of chemisorption does not exhibit insurmountable difficulties, in photocatalysis this investigation is hindered by the fact that photoadsorption and photoreaction simultaneously start under irradiation of the catalyst then preventing the investigation of photoadsorption phenomenon per se. This chapter presents a quantitative method able to determine the amount of substrate photoadsorbed onto the catalyst surface in liquid–solid systems under reaction conditions. The method is applied to the following photocatalytic processes carried out in aqueous suspensions: (i) oxidation of phenol in the presence of a commercial TiO 2 catalyst (Degussa P25) and (ii) oxidation of benzyl alcohol in the presence of a home-prepared TiO 2 catalyst. The influence of substrate concentration, catalyst amount, and irradiation power on photoadsorption is investigated. The equilibrium data of photoadsorption capacity is fitted vs. the liquid-phase substrate concentration by using the following adsorption isotherms: Langmuir, Freundlich, and Redlich–Peterson. By taking into account the photoadsorption phenomena, the kinetic modeling of the photooxidation processes are carried out for each of the adsorption isotherms and the parameters of the models are determined. In the case of benzyl alcohol, the results indicate that the Langmuir adsorption isotherm best fits the photoadsorption phenomenon while for phenol the Freundlich adsorption isotherm gives a better fit. In both cases all the parameters of kinetic model are favorably affected by an increase of the photon flow absorbed per unit mass of catalyst.