Quantification of the Primary Processes in Aqueous Heterogeneous Photocatalysis Using Single-Stage Oxidation Reactions

Abstract

The present study aimed at the quantification of primary photocatalytic processes occurring over irradiated semiconductors. The photodegradation of HCOOH was selected as a probe reaction since it follows a single-stage oxidation mechanism (yielding CO2 and H2O), thus not relying on the formation of stable intermediate products or multiple reactions. This reaction was used to determine lumped kinetic parameters of UV-assisted photocatalysis in semiconductor slurries. A modified Fricke dosimeter (Fe2+→Fe3+) was utilized to determine the exact concentrations of reactive oxygen species in the slurry, such as •OH, HO2•, •O2−, and H2O2. These experiments yielded maxima in the concentrations of these species during the first few minutes of reaction with the subsequent attainment of a steady state. The rate of generation of reactive oxygen species characterizes the true oxidative power of photocatalysts, and it was found to increase with the catalyst concentration and reach a plateau value in the vicinity of 0.25 g/l of catalyst. This generation rate follows the pattern Ishihara ST21≈Hombikat UV100≈Degussa P25>Aldrich anatase; its value for Aldrich anatase is approximately one-third of that for the other catalysts. The effect of direct oxidation by valence-band holes was studied using the oxidation of NO−2 to NO−3 and proved to be minimal. From the kinetic data of the photodegradation of formic acid, rates of primary photocatalytic processes, such as radical generation (Rgen) and electron–hole recombination (Rrec), were determined with our methodology. In particular, Rrec was found to be inversely proportional to the BET surface area of the catalyst following the sequence Aldrich anatase>Degussa P25>Ishihara ST21≈Hombikat UV100. Using the present methodology, the quantum yield of a particular photocatalyst can be predicted satisfactorily from its radical generation rate. It should be noted that such quantum yield depends on the catalyst concentration and reactor setup to a much lesser extent than the traditionally used quantum yield for phenol photodegradation, since the latter process is accompanied by multiple secondary oxidation reactions. Thus, the method proposed provides a convenient procedure for a thorough study on semiconductor photocatalysis and can serve as a common means of photocatalyst characterization.