(651e) Controlling Selectivity of Catalytic Reactions on Metal Nanoparticles through Direct Photoexcitation of Targeted Metal-Adsorbate Bonds

Controlling selectivity of chemical reactions on metal surfaces is of paramount importance for the design of efficient, environmentally friendly heterogeneous catalytic processes. Governing relationships between adsorption energies and activation barriers of competing chemical pathways on metal surfaces limit the potential to control selectivity when using thermal energy to drive reactions. It has recently been shown in a few cases that visible light irradiation of metal catalysts can overcome inherent limitations of thermal-driven catalysis, inducing new chemical pathways through electronically excited states. However these photon driven reactions typically occur through a substrate-mediated transfer of photon energy to adsorbate-metal bonds, where the initial photon absorption occurs in the bulk metal states, followed by charge transfer to adsorbates. Since only the bulk metal is involved in the initial photoexcitation step, the wavelength dependent quantum yields follow the absorption spectrum of the metal, and targeted activation of adsorbate-metal bonds (which is required to control the outcome of metal catalyzed reactions) cannot be controlled in a rational manner.

In this work, we show that catalytic processes on metal surfaces can be driven and controlled through direct, resonant photoexcitation of hybridized electronic states formed due to strong chemisorption of CO on Pt surfaces. As opposed to the substrate-mediated mechanism, where photons are absorbed in the metal bulk and energy is subsequently transferred to adsorbate-metal bonds, direct photoexcitation of adsorbate-metal bonds occurs at surface Pt atoms that are directly bound to adsorbates. Pt nanoparticle size dependent studies showed that activation of Pt-CO bonds by direct photoexcitation becomes the dominant mechanism driving chemistry, rather than the substrate-mediate mechanism, on nanoparticle catalysts <5 nm in diameter due to increased surface area to volume ratios of the particles. By exploiting the resonant, adsorbate specific nature of electronic transitions formed between adsorbates and metal surfaces we show that targeted photoexcitation of catalytic 2.3 nm Pt nanoparticles with 450 nm photons enhances selectivity from 40 to 80% in the preferential oxidation of CO in an H₂ rich environment. It is expected that the development of insights into resonant photon induced electronic transitions between hybridized metal-adsorbate states should allow rational control of catalytic selectivity that cannot be achieved exclusively with thermal energy input.