(382c) The Impact of Photon Excitation on Catalytic Processes at Metal Surfaces

Catalytic activity and selectivity of metallic nanoparticles are intrinsic material properties controlled by catalyst composition and geometry. The intrinsic nature of catalytic properties dictates that the outcomes of chemical reactions are relatively static in chemical reactors and cannot be manipulated on the fly. In addition, while scaling and Bronsted-Evans-Polanyi relations have facilitated the optimization of metal catalysts, they also suggest inherent limitations in metal catalysts for achieving 100% selectivity in chemical transformations of emerging importance.   

Photon excitation of operating metal catalysts has been considered as a potential approach to overcome inherent limitations of metal surfaces for driving selective reactions and to allow this functionality on demand. Over the past 30 years it has been found in a few cases that photon excitation of adsorbate covered metal surfaces induces unique reactivity channels that cannot be accessed through thermal excitation. However, the mechanism of photon energy conversion into chemical reactivity is poorly understood and there exists no generalized understanding of how the nature of the adsorbate or adsorbate-metal bonds impact their potential for being activated selectively through photon excitation. 

Photon induced activation of bonds formed between strongly chemisorbed adsorbates and metal surfaces can occur through indirect and direct photoexcitation processes. In the indirect mechanism photons are absorbed by metal states followed by the transient transfer of hot electrons into normally unpopulated antibonding orbitals, thereby depositing vibrational energy into the system through inelastic Frank Condon transitions. In the direct photoexcitation process photons induce electronic transitions localized at hybridized metal-adsorbate states, which can drive chemical reactions with resonant wavelength dependent cross-sections. In this talk we combine experimental and theoretical studies to show how selectivity can be controlled through both the indirect and direct photoexcitation of metal catalysts. We focus specifically on reactions that involve the activation of bonds formed between CO, NO and O on Pt surfaces, but will draw general conclusions on the potential of this approach for achieving high selectivity in processes where heterogeneous catalysis driven by thermal excitation has failed.