(168e) Impact of Adsorbates On the Photochemistry of Metal Nanoclusters

The possibility of photon-induced selective bond-activation of adsorbates on metal surfaces has sparked an interest in utilizing metal catalysts to drive photochemical reactions in excited states, which cannot be performed using thermal energy only.  Ultrafast lasers have been used for many years to excite adsorbate covered single crystal metal surfaces and execute photo-catalytic reactions in excited states.  The low efficiencies and high laser intensities required to drive these chemistries present significant barriers that must be overcome. Recent experimental work in our group has shown that spatial confinement of catalytic active sites in metal nanoparticles results in fundamentally different photo-catalytic behavior compared to extended metal surfaces. It has been demonstrated that Pt nanoparticles, diameter < 5 nm, and plasmonic Ag nanoparticles, diameter < 100 nm, are able to efficiently drive photo-catalytic reactions through resonant energy transfer processes. The goal of this study is to explain the mechanism of these nanoparticle mediated photochemical processes.

It is well known that the addition of adsorbates to metal surfaces induces electronic structure changes in the underlying metal and introduces adsorbate-derived states near the Fermi level. We have utilized density functional theory calculations to examine adsorbate induced changes in electronic structure of Pt surfaces upon addition of molecular (CO, NO) and atomic (O, C, N) adsorbates.  The calculated densities of states were used as inputs into a model that describes the electron energy distribution induced by photon excitations of metal surfaces.  In addition, an extended two-temperature model (ETTM) is used to describe the temporal evolution of non-equilibrium electron distributions in photo-excited metal nanoparticles. Our theoretical predictions accurately capture recent wavelength dependent photocatalytic measurements on Pt nanoparticles performed in our group and allow us to postulate general mechanisms regarding the effect of molecular and atomic adsorbates on metal nanocluster photocatalysis. The results of our study provide essential insights into the possibility of state-selected bond activation of adsorbates on metal nanoparticles under low-intensity visible light illumination.