Mechanistic crossover in photocatalytic transfer hydrogenation of nitroaromatics upon switching from static to periodic illumination

Numerous studies have demonstrated the potential of controlled periodic illumination in improving the quantum efficiency of heterogeneous photocatalytic reactions. However, the nature of the enhancement effect remains debated. Here, we report the promotion in photocatalytic activity of HCOOH-mediated 4-nitrophenol hydrogenation over Pd/TiO₂ via periodic illumination. Periodic illumination at 12 Hz led to a non-monotonic relationship between 4-nitrophenol conversion as a function of duty cycle, with 2.4 times enhancement in aminophenol yield at 10 % duty cycle. However, the improvement in photocatalytic activity was progressively diminished with increasing duty cycles and eventually discontinued at duty cycles above 50 %. Further exploring the operating limits of the current setup, we observed a maximum enhancement in 4-nitrophenol hydrogenation activity of 3.4-fold equivalent to 40-fold increase in quantum yield. Evaluation of simultaneous H₂ evolution rates and the effects of pH on aminophenol yield suggest that reduced amount of H* on the Pd nanoparticles contributes to 4-nitrophenol adsorption. We hypothesize that this difference in surface population leads to a crossover between two different kinetically dominant reaction mechanisms. Substantiated by kinetic and Pd poisoning studies, high H* under static and periodic illumination at duty cycles above 50 % leads to a dominant ionic pathway. In contrast, periodic illumination at lower duty cycles favors 4-nitrophenol adsorption and subsequent H* addition.