(449f) Plasmonic Nanostructures As Platforms for Efficient Coupling of Visible Light and Thermal Energy to Drive Chemical Transformations

Phillip Christopher, Hongliang Xin, Suljo Linic

Department of Chemical Engineering, University of Michigan,

Ann Arbor, Michigan 48109-2136  

            Heterogeneous catalytic processes play a crucial role in industrial chemical synthesis, energy production and pollution mitigation.  Most commercial heterogeneous catalytic reactions are run at relatively high temperatures due to the high activation barriers associated with rate-limiting elementary steps. The high operating temperatures required to achieve reasonable product yields compromise process efficiency and catalyst stability. In this contribution we show that silver plasmonic nanostructures can concurrently use low intensity visible light (on the order of solar intensity) and thermal energy to drive industrially important catalytic oxidation (ethylene epoxidation, CO oxidation and selective NH₃ oxidation) reactions at lower temperatures than their conventional counterparts that use only thermal stimulus (P.Christopher, H.Xin, S. Linic, Nature Chemistry, In Press (2011)).

            The Ag nanoparticle catalysts exhibit a strong localized surface plasmon resonance due to the nanometer scale spatial confinement and the metals’ inherent electronic structure. The excitation of surface plasmons is characterized by the intense concentration of spatially non-homogeneous electric fields at the nanostructure surface. Based on mechanistic experimental studies and density functional calculations, we show that excited plasmons on the Ag surface act to transiently populate O₂ anti-bonding orbitals, thereby depositing vibrational energy in the O—O bond, facilitating the rate-limiting O₂ dissociation reaction and increasing the overall rate of oxidation reaction on Ag surfaces. The results suggest that the nanostructures concentrate the energy of a resonant photon flux in a very small volume at their surface rather than distributing this energy over the entire nanostructure, allowing for a direct and efficient energy channeling in the form of energetic electrons into the reaction coordinate.  This unique capacity of plasmonic nanostructure to constructively couple multiple stimuli (the energy of photon flux and the energy delivered by thermally heating the plasmonic materials) opens avenues for the development of a new class of photocatalysts for various reactions.