Azo dyes and other visible light sensitive materials are sometimes used as model compounds in photocatalytic degradation studies. Their degradation pathways are relatively easy to follow and this makes it possible to evaluate photocatalytic degradation data and determine photocatalytic efficiency. In this study, Langmuir Hinshelwood kinetic models have been applied to photocatalytic data obtained on the degradation of methyl orange in a TiO2 foam. Analysis of degradation data indicate that while photocatalytic data are amenable to Langmuir Hinshelwood treatment at the early stages of reaction, the model is insufficient to explain observed photocatalytic kinetic data at the later stages of the degradation where observed degradation rates are slower because of light scattering and absorption effects. It is suggested that absorption and desorption processes taking place on the catalyst surfaces could account for the differences in the observed rates of the photodegradation. As a result, the first order kinetic rate constant and the Langmuir Hinshelwood rate constant obtained from analysis of experimental data were found to be markedly different, indicating limited applicability of the Langmuir Hinshelwood model.
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