4 research outputs found
Role of Proton Diffusion in the Nonexponential Kinetics of Proton-Coupled Electron Transfer from Photoreduced ZnO Nanocrystals
Experiments have suggested that photoreduced ZnO nanocrystals transfer an electron and a proton to organic radicals through a concerted proton-coupled electron transfer (PCET) mechanism. The kinetics of this process was studied by monitoring the decay of the absorbance that reflects the concentration of electrons in the conduction bands of the nanocrystals. Interestingly, this absorbance exhibited nonexponential decay kinetics that could not be explained by heterogeneities of the nanoparticles or electron content. To determine if proton diffusion from inside the nanocrystal to reactive sites on the surface could lead to such nonexponential kinetics, herein this process is modeled using kinetic Monte Carlo simulations. These simulations provide the survival probability of a proton hopping among bulk, subsurface, and surface sites within the nanocrystal until it reaches a reactive surface site where it transfers to an organic radical. Using activation barriers predominantly obtained from periodic density functional theory, the simulations reproduce the nonexponential decay kinetics. This nonexponential behavior is found to arise from the broad distribution of lifetimes caused by different types of subsurface and surface sites. The longer lifetimes are associated with the proton becoming temporarily trapped in a subsurface site that does not have direct access to a reactive surface site due to capping ligands. These calculations suggest that movement of the protons rather than the electrons dominate the nonexponential kinetics of the PCET reaction. Thus, the impact of both bulk and surface properties of metal-oxide nanoparticles on proton conductivity should be considered when designing heterogeneous catalysts
Theoretical Insights into Proton-Coupled Electron Transfer from a Photoreduced ZnO Nanocrystal to an Organic Radical
Proton-coupled electron transfer (PCET) at metal-oxide
nanoparticle interfaces plays a critical role in many photocatalytic reactions
and energy conversion processes. Recent experimental studies have shown
that photoreduced ZnO nanocrystals react by PCET with organic
hydrogen atom acceptors such as the nitroxyl radical TEMPO. Herein,
the interfacial PCET rate constant is calculated in the framework of
vibronically nonadiabatic PCET theory, which treats the electrons and
transferring proton quantum mechanically. The input quantities to the
PCET rate constant, including the electronic couplings, are calculated with
density functional theory. The computed interfacial PCET rate constant is
consistent with the experimentally measured value for this system,
providing validation for this PCET theory. In this model, the electron
transfers from the conduction band of the ZnO nanocrystal to TEMPO concertedly with proton transfer from a surface oxygen
of the ZnO nanocrystal to the oxygen of TEMPO. Moreover, the proton tunneling at the interface is gated by the relatively lowfrequency
proton donor−acceptor motion between the TEMPO radical and the ZnO nanocrystal. The ZnO nanocrystal and
TEMPO are found to contribute similar amounts to the inner-sphere reorganization energy, implicating structural reorganization
at the nanocrystal surface. These fundamental mechanistic insights may guide the design of metal-oxide nanocatalysts for a wide
range of energy conversion processes