Molecular manipulation of the microenvironment of Au active sites on mesoporous silica for the enhanced catalytic reduction of 4-nitrophenol

In confined nanospaces and/or at confined nanoscale interfaces, the catalytic nature of active sites on the molecule level remains elusive. Herein, with the catalytic hydride reduction of 4-nitrophenol (4-NP) over a gold nanoparticle (NP) catalyst as a prototype reaction, the influence of a delicate change of the microenvironment of the catalytically active site on the kinetics of the reaction of 4-NP to 4-aminophenol (4-AP) with the introduction of various alkali-metal ion (AM⁺) salts has been investigated in detail. We demonstrate that structural water (SW) adsorbed on Au NPs in the form of {OH⁻·H₂O@Au NPs} is the real catalytically active site (not Au NPs alone), and in the presence of lithium chloride (LiCl) it shows the best catalytic performance. In addition, isotope labeling and kinetic isotope effect (KIE) experiments show that the reduction of 4-NP does not follow the classical Langmuir-Hinshelwood (L-H) bimolecular mechanism, but an interfacial SW dominated electron and proton transfer mechanism. The proposed mechanism explains why the dissociation of the O-H bond of water is the rate-determining step (RDS) of 4-NP reduction, and, counter-intuitively, the solvent water is the hydrogen source of final product 4-AP, instead of sodium borohydride (NaBH₄) reducer. Importantly, the co-existence of Li⁺ and Cl⁻ ions synergistically stabilizes the transition state of the reaction and accelerates the interfacial electron and proton transfer, consequently enhancing the reaction kinetics. The model of structural water as a bridge to transfer electrons and protons at the nanoscale interface is reminiscent of the working mechanism of photosystem two (PSII) for water splitting on Mn₄CaO₅ clusters.