(545f) Nanosize-Induced Resonant Photocatalysis On Metal Clusters

Photon-induced electron transfer between solid-state materials and molecules is of fundamental importance for the design of many solar energy conversion processes including dye sensitized solar cells and photocatalysis. Heterogeneous photocatalysis is typically assumed to be a two-step process where formation of electron-hole pairs occurs in the solid-state photocatalyst (semiconductors or large metal particles/single crystals) followed by a subsequent charge transfer to adsorbates. This process of energetic charge carrier generation and transfer results in wavelength dependent quantum efficiencies that strictly follow the absorption spectrum of the solid state photocatalysts.

In this work, we show evidence for direct photon-induced metal-to-adsorbate electron transitions that induce efficient photocatalytic processes on Pt nanoclusters. This process is fundamentally different than what occurs on large metal particles or semiconductors as it is a single step process. As a result of the single step nature of the process the wavelength dependence of photocatalytic CO oxidation on small Pt clusters show a resonant behavior, unique to this excitation mechanism.  Particle size dependent and in-situ diffuse reflectance UV-Vis Spectroscopy experiments are utilized to examine the nanosize-induced direct electron transfer process. We also compare the results for photocatalytic carbon monoxide, ammonia and hydrogen oxidation reactions on Pt clusters to develop a general understanding of how the nature of the chemical transformation dictates the excitation mechanism (two-step vs. direct process). Our studies show that the adsorbate-induced electronic structure of the metal-adsorbate complex is an indicator of which excitation mechanisms will be prominent under visible light illumination on small metal nanoclusters. Our results will be discussed in the context of utilizing solar energy to enhance important chemical reaction rates efficiently and with state-selectivity.