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The photocatalytic activity under UV and visible irradiation of a series of CeO2-TiO2 composite materials prepared by microemulsion and subsequently calcined at 500 °C was evaluated using acetaldehyde as a model pollutant. The activities of three composite catalysts containing 2.5, 5 and 25 M% of ceria, as well as a nano-titania reference are compared by means of efficiency parameters; the quantum (or true) efficiency, as well as the photonic (or apparent) efficiency were evaluated. To this end, the lamp emission properties, as well as the radiation field interaction with the catalyst inside the reactor were modeled and numerically calculated. For the evaluation of the radiation absorbed by the catalysts, the optical properties of the materials were measured and a radiative transfer model that considers all potential optical events was built up. The stability of the systems under reaction conditions was also studied. Overall, the results evidence that the composite containing 2.5 Ce mol% outperforms titania by a factor of at least 2, irrespective of the illumination source used in the experiment. This shows the potential of the CeO2-TiO2 composites, a highly active and stable system, for sunlight optimum profiting. [ABSTRACT FROM AUTHOR]

Abstract: A kinetic model of the photocatalytic degradation of the azo dye Acid Orange 7 (AO7) is presented. It is based on mechanistic reaction steps and includes the modeling of the radiation absorption effects. The evaluation of the radiation field inside the reactor was achieved by solving the radiative transfer equation for the heterogeneous system. The study was carried out in a slurry batch photoreactor with recycle, employing artificial UV light as the source of radiation and TiO2 Degussa P25 as catalyst. The biodegradability and toxicity of the irradiated mixture was also assessed. The kinetic model satisfactorily represents the evolution of AO7 under different experimental conditions, with a root mean square error of the estimations of 13.9%. The biodegradability of the AO7 solution was considerably enhanced after the photocatalytic treatment. In addition, a clear reduction of the toxicity of the azo dye was achieved at the end of the experiments. [Copyright &y& Elsevier]

Abstract: This study is focused on the kinetic modeling of the photocatalytic oxidation of cyanide in slurry reactors. The developed model is based on an accepted reaction mechanism and explicitly takes into account the differences in the local volumetric rate of photon absorption (LVRPA) produced by the unavoidable radiation profiles existing in the photoreactor. The model and its correspondent procedures for the evaluation of the LVRPA distribution, and the estimation of the kinetic parameters have been successfully validated with both powder TiO2 and TiO2/SiO2 photocatalysts with improved recovery properties. In both cases, the model reproduces the influence of the catalyst loading, the initial cyanide concentration, and the inlet radiation flux on the reaction rate, with errors below 5%. The kinetic parameters estimated for the model are independent of the irradiation form, as well as the reactor size and its geometrical configuration, providing the necessary information for scaling-up and designing commercial scale photoreactors. [Copyright &y& Elsevier]

Abstract: Despite the large amount of work dealing with supported photocatalysts used to overcome the difficulties associated with the recovery of powdered titania after the photocatalytic treatments, few attempts have been made to calculate the quantum efficiency (η) of these materials. In this work the quantum efficiency for the photooxidation of cyanide with TiO2 and TiO2/SiO2 has been experimentally calculated using an approach based on the resolution of the radiative transfer equation inside the photoreactor. Due to the low absorption coefficient of the silica-supported material, a two-dimensional two-directional reactor model has been developed to describe the photon transport. The dependence of η on the incident radiation flux, the catalysts concentration and the cyanide concentration has been investigated. With both material suspensions, higher quantum efficiencies have been observed when lower radiation fluxes and higher cyanide concentrations are used. However, different trends in the quantum efficiencies are observed when increasing the catalyst concentration, leading to higher values when using powdered TiO2 and lower values for TiO2/SiO2 suspensions. [Copyright &y& Elsevier]

The solar photo-Fenton degradation of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) in aqueous solution at a natural pH (pH = 5) using ferrioxalate as iron source was investigated. The kinetic model proposed and validated in a previous contribution was used to predict the reactants concentrations during the oxidation process in a non-concentrating pilot-plant solar reactor. The effects of hydrogen peroxide to 2,4-D initial concentration ratios (R), temperature, and radiation levels were studied. Furthermore, the spectral UV/visible and broadband solar radiation fluxes incident on the reactor window were evaluated by the Simple Model for the Atmospheric Radiative Transfer of Sunshine (SMARTS2) code. The complete destruction of 2,4-D and its main intermediate 2,4-dichlorophenol (2,4-DCP) was achieved in all the experimental runs in only 90 and 120 min of reaction, respectively. In agreement with these results, a reduction of toxicity in the system (expressed as % of inhibition of Vibrio fischeri) for longer times to 90 min of reaction was attained. It is important to emphasize the good agreement between kinetic model results and experimental data obtained.

Iron pillared clay (Fe-PILC) was prepared from a montmorillonite and was characterized by scanning electron microscopy and X-ray fluorescence. Fe-PILC catalytic activity was evaluated in photo-Fenton processes applied to the degradation of 2-clorophenol. Different catalyst loads were assayed. The Fe-PILC allowed almost complete degradation of the contaminant. An increase in the contaminant degradation rate was observed, following leaching of iron during catalytic assays, which suggest the existence of a homogeneous photo-Fenton mechanism. The catalytic performance of the Fe-PILC was compared with that for goethite and zero valent iron nanoparticles. Differences were found regarding the achieved degradation levels, the efficiency in oxidant consumption, and the extension of iron leaching.