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Effect of Photocatalyst Aggregation on Photocatalytic Reaction Rate Concentration Dependence
Vanya Lilova1, Emil Lilov1, Svetlozar Nedev1
1Department of Physics, University of Chemical Technology and Metallurgy, 8 Kliment Ohridski Blvd., 1797 Sofia, Bulgaria.
Abstract:
The influence of suspension depth and pollutant concentration on the rate and efficiency of photocatalytic degradation was investigated in aqueous TiO2 suspensions using methyl orange (MO) as a model pollutant. Both the reaction rate and the efficiency increased by more than one order of magnitude upon a relatively small decrease in suspension layer thickness. The reaction rate exhibited a complex N-shaped dependence on dye concentration, deviating from the monotonic behavior predicted by the Eley-Rideal and Langmuir-Hinshelwood mechanisms, as well as from the relationship derived in our previous study. To elucidate the origin of this behavior, nanoparticle aggregation was examined by sedimentation kinetics, low-acceleration centrifugation, and scanning electron microscopy (SEM). The results suggest that the interplay between enhanced dye adsorption and the reduction in the available photocatalyst surface area due to aggregation leads to the appearance of a maximum in the concentration dependence of the reaction rate. The relationship between reaction rate and photocatalytic efficiency was also analyzed. Although both parameters are correlated, efficiency values strongly depend on the selected reaction time interval, which complicates direct comparison between studies employing different experimental protocols. Consequently, the reaction rate appears to be a more reliable parameter for describing photocatalytic kinetics.
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