Evaluation of the Optical Properties of Silica-Supported Titanium Dioxide Aqueous Suspensions

MARUGAN, J.; VAN GRIEKEN, R.; ALFANO, O.M.; CASSANO, A.E.

Campinas (UNICAMP), SP Brasil

Congreso; III Encontro Sobre Aplicações Ambientais de Processos Oxidativos Avançados - III EPOA; 2005

UNICAMP

The evaluation of the local volumetric rate of photon absorption constitutes an important step in the analysis and design of slurry photocatalytic reactors. This task can be achieved by solving the radiative transfer equation (RTE) inside the heterogeneous reacting system. To solve the RTE, three optical properties of the catalyst suspensions are required: the spectral specific absorption coefficient, the spectral specific scattering coefficient, and the angular distribution of the scattered photons (the phase function). In the past years a significant effort has been made to develop supported titania photocatalysts on particles of larger size to facilitate the sedimentation recovery process. This work deals with the evaluation of the optical properties of several silica-supported TiO2 photocatalysts recently developed [1]. To do this, experimental and theoretical modifications of a previously reported method [2] is proposed. This method involved extinctance and diffuse reflectance and transmittance spectrophotometric measurements of TiO2 suspensions as a function of the wavelength, the evaluation of the radiation field in the sample cell, and the application of a nonlinear multiparameter regression procedure (Levenberg-Marquardt method) to adjust theoretical values to experimental data. Two different silica materials were used as supports for the preparation of immobilized TiO2 photocatalysts: (i) an amorphous SiO2 commercially available (INEOS Silica ES70Y), with a wide pore size distribution and (ii) the so-called SBA-15 silica, a mesostructured material with a well defined pore size in the mesopore range. Three different nominal titania loading were tested for the first silica support (20, 40, and 60% TiO2) and one titania loading for the second support (20% TiO2). For comparison purposes, two different commercial titanium dioxide samples, Aldrich and Degussa P25, have been used. Both materials are well-known photocatalysts with an activity widely reported in the literature. Due to the high settling rate of the supported photocatalyst suspensions, a flow-cell system was used to perform the experimental measurements. The influence of the type of silica support and of the titania loading on the optical properties of the aqueous suspension is studied. Differences with the optical properties of unsupported TiO2 and their consequences for the photocatalytic reactor design and scale-up are also analyzed.