"the active species capture." Probably, you mean a "scavenger." It depends what reactive species you are trying to identify. t-BuOH are very robust to oxidation and, therefore higher concentration should be added. Commonly, you need to run a series of experiment with different concentrations of scavengers to demonstrate a concentration dependence.

Thanks for your answer, Yurii. I did mean "scavenger". I don't understand what you said "t-BuOH are very robust to oxidation", does it mean that t-BuOH are hard to oxidize？In addition, if I run a series of capture experiments, what is the criterion for determining the the optimal concentration of t-BuOH? My understanding is: as the concentration of t-BuOH increases, the organic degradation rate gradually decreases; when the concentration of t-BuOH increases to a certain level, the rate constant does not change when it reaches a certain value. This concentration is the optimal concentration we want to choose. (as shown in the attached picture)

"does it mean that t-BuOH are hard to oxidize" Yes. In addition, the good scavenger should not form a highly reactive product.

The rate of S oxidation commonly decreases with scavenger concentration as

rate = a[S]/(b[S]+c[X])

The optimal concentration does not exist. In your system active species reacts with t-BuOH and produces t-BuO(.) radical, which in its turn react with S. Therefore, you can't get a full protection of S by adding high concentration of X. The above equation does not work

By the introduction of different scavenging agents into the photocatalytic system.