ISSN 1203-8407 © 2010 Science & Technology Network, Inc. J. Adv. Oxid. Technol. Vol. 13, No. 3, 2010 262 Considerations of Particle Size in Aqueous Phase Photocatalysis kinetics with TiO 2 Catherine Almquist* Paper and Chemical Engineering Department, Miami University, Oxford, Ohio 45056, USA Abstract: There are inconsistencies in the literature regarding the optimum particle size for TiO 2 in photocatalytic oxidation reactions in aqueous slurries. The apparent optimum particle size has been reported as low as 3 nm and ranges to greater than 30 nm. It was hypothesized that the inconsistencies reported in the literature stem from differences in the photocatalyst, the reactor system and the operating parameters used, among other factors. In this study, a mathematical model was developed to elucidate the effects of selected operating parameters on the apparent optimum particle size in aqueous-phase TiO 2 photocatalysis. The model utilizes five parameters, which were varied to best fit one set of experimental data: the photocatalytic oxidation of dimethyl methylphosphonate (DMMP) in water. For this set of experimental data, the apparent optimum particle size was approximately 25 nm, and the model parameters were set to fit this set of data. The sensitivity of the model to the model parameters and operating conditions were assessed, and the theoretical trends based upon model calculations were supported by observed trends of experimental data reported in the literature. It was found that the apparent optimum particle size can change significantly with TiO 2 concentration and depth of slurry perpendicular to the incident light. This supports that the light scattering model parameter, k scat , is the most significant model parameter that determines the apparent optimum particle size. Introduction There have been many published manuscripts reporting on the role of particle size in photocatalysis over the past 25 years (1-12). In brief, authors state that competing effects of increasing surface area, decreasing light absorption efficiencies, and increasing rates of electron-hole recombination as particle size decreases results in an optimum particle size for aqueous-phase TiO 2 photocatalysis. However, the reported “optimum” particle sizes in the literature vary widely for TiO 2 photocatalysis, from < 5 nm to over 30 nm. Table 1 provides a partial list of authors that have investigated the role of particle size in TiO 2 photocatalysis and their key findings. Noted is that the role of particle size in TiO 2 photocatalytic slurry reactors cannot be isolated easily in theory or in practice. Subtle differences in surface sites, defects, and structural properties of the photo- catalysts result from differences in synthesis methods and can affect the efficiency at which electron-hole pairs are formed and recombined upon absorption of light energy. In addition, differences in reactor design and experimental operating parameters can affect the apparent optimum particle size by altering secondary (agglomerate) particle formation, adsorption coeffi- *E-mail address: almquic@muohio.edu ; Tel.: 513-529-0767; Fax: 513-529-0761 cients, and the light propagation into the reaction slurry. Therefore, the isolation of the effect of particle size is difficult, at best. However, in this paper, a model was developed to consider the role of particle size in the observed kinetics of photocatalytic oxidation reactions in aqueous slurry reactors. Theory Although several reactions occur in photocatalysis, as presented in the published literature (13-17), the reactions used in the development of this model, and a list of assumptions used to formulate this model are provided in Tables 2 and 3, respectively. As shown in Reaction 1 of Table 2, the absorption of light by TiO 2 generates electron-hole (e - -h + ) pairs in the reaction slurry. The rate at which electron-hole pairs are generated per unit volume of reaction slurry at depth z into the reactor is represented as follows: ) ( ) ( 1 z I k z g   (1) where g(z) represents the rate of electron-hole generation on the surface of the TiO 2 particles (moles s -1 m -3 ) in the slurry reactor at a distance z into the reaction slurry,  is the light absorption coefficient (m -1 ) (Eq 2), I(z) is the light intensity (W/m 2 ) at a depth z (m) into the reactor, and k 1 is the rate constant (moles e - -h + at surface of the TiO 2 particles W -1 s -1 ). Brought to you by | UCL - University College London Authenticated Download Date | 5/4/18 3:50 PM