DETERMINATION OF THE ACTUAL PHOTOCATALYTIC RATE OF H2O2 DECOMPOSITION OVER SUSPENDED TIO2 - FITTING TO THE LANGMUIR HINSHELWOOD FORM

The kinetics of oxygen evolution from UV-illuminated acidic aqueous suspensions of a powder TiO2 sample containing nominal initial H2O2 concentrations, c0, in the range 10(-5)-10(-1) M has been studied by use of a gas chromatograph with a noise-optimized preamplifier. Steady-state rates were reached except at the lowest c0. The dark O2 evolution was negligible. To determine the contribution of the photochemical decomposition of H2O2, the use of evolution rates measured in the absence of TiO2 was shown to be irrelevant. Therefore this contribution was derived, in an original manner, from the rates obtained with a Cr3+-doped TiO2 sample that has the same properties as the undoped specimen for the absorption and scattering of UV light but a negligible photocatalytic activity as evidenced by other reactions. The steady-state photocatalytic rates thus calculated were numerically fitted to the Langmuir-Hinshelwood mechanism by iterative variations of the constants. According to this mechanism, an optimal surface coverage by H2O2 of ca. 0.4 was found to correspond to the maximal photocatalytic effect to background ratio q observed for c0 = ca. 1 mM. The value of q obviously depends on the wavelength range and type of TiO2; it was found equal to 25 in the region 300-400 nm for TiO2 Degussa P25, which illustrates the importance of the photocatalytic decomposition. The total O2 evolution, smaller than that expected from H2O2 --> H2O + 1/2O2, was thought consistent with the previously suggested internal hydroxylation of TiO2 by H2O2.