In kinetic analysis of general chemical reactions, a rate constant is estimated and compared. Considering that photoexcited electrons (e−) and positive holes (h+) induce a redox reaction,
it may be possible to estimate the rate constant of these active species. Since e− and h+ recombine with each other (see “Mutual recombination”), the overall photocatalytic reaction rate depends also on this recombination rate. Assuming that k(redox) and k(recombination) are rate constants of reactions by e− and h+ and their recombination, respectively, i.e., the simplest kinetic model, the ratio k(redox)/k(recombination) should be a measure of intrinsic photocatalytic activity. However, unfortunately, we have nowayto estimate k(recombination), since the recombination does not produce any chemical species to be detected for estimation of its rate.