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 no way to estimate k(recombination), since the recombination does not produce any chemical species to be detected for estimation of its rate.