Molecular Manipulation of Micro-Environment of Au Active Sites on Mesoporous Silica for Enhanced Catalytic Reduction of 4-Nitrophenol

At the confined nanospace and/or nanoscale interface, the catalytic nature of active sites on the molecule level still remains elusive. Herein, with the catalytic hydride reduction of 4-nitrophenol (4-NP) over gold nanoparticles (NPs) catalysts as a prototype reaction, the influence of delicate change of microenvironment of catalytic active site on the reaction kinetics of 4-NP to 4-aminophenol (4-AP) with the introduction of varied alkali-metal ion (AM+) salt, have been deeply investigated. We demonstrate that structural water (SW) adsorbed on Au NPs in the form of {OH-·H2O@Au NPs} is the real catalytic active sites (not alone Au NPs), and in the presence of lithium chloride (LiCl), it shows the best catalytic performance. In addition, the isotope labeling and kinetic isotope effect (KIE) experiments evidences 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 answers why the dissociation of 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 reaction and accelerates the interfacial electron and proton transfer, consequently enhancing the reaction kinetics. The model of structural water as a bridge to transfer electron and proton at nanoscale interface is the reminiscent of working mechanism of photosystem two (PSII) for water splitting on Mn4CaO5 cluster.