Enhanced visible-light photocatalytic activity of a TiO2 hydrosol assisted by H2O2: Surface complexation and kinetic modeling

• The surface complex (>Ti-OOH) can be easily measured in liquid TiO2 hydrosol with H2O2.
• The fast formation kinetics of >Ti-OOH were examined in a stopped-flow spectral system.
• The adsorption–desorption constant and formation rate constants are reported for the first time.
• A kinetic model composed of well-known key reactions was established.
• The rate constants for the photo-excitation of >Ti-OOH were determined.

Although the complexation of H2O2 onto TiO2 particles can form a colored surface with enhanced visible-light photocatalytic activity, the properties and behavior of this complex (>Ti-OOH) have not been thoroughly characterized. In this study, rather than using TiO2 powder, a nanoscale TiO2 hydrosol was used because it is easily measured using spectrometric methods. The results indicated that the addition of H2O2 to the TiO2 hydrosol greatly enhanced the photocatalytic degradation of methylene blue (MB) under visible light irradiation. Although the presence of O2 slightly affected the photocatalytic activity, the presence of dissolved H2O2 markedly enhanced the photocatalytic activity. Furthermore, the results demonstrated that the consumption of dissolved H2O2 may account for the decrease in MB degradation after repeated cycles. The kinetics of the formation of >Ti-OOH were examined using UV–vis spectroscopy and the stopped-flow spectral system, and the adsorption–desorption constant (714 M−1) and the complex formation rate constants ((1.01 ± 0.13) × 104 M−1 s−1 and 14.1 ± 1.2 s−1) are reported for the first time. Based on the MB degradation and >Ti-OOH formation results, a kinetic model composed of well-known key reactions was established. By fitting the experimental data to the model, the forward and backward rate constants for the photo-excitation of >Ti-OOH were determined to be (3.5 ± 0.9) × 10−5 s−1 and (6.1 ± 2.0) × 1013 M−1 s−1, respectively.

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