(320e) Understanding Adsorbate-Specific Photocatalysis on Metal Surfaces

Visible photon excitation of catalytic metal surfaces has been proposed as a potential approach for inducing selective chemical reactions that are difficult to achieve using thermal energy. This approach relies on the selective deposition of photon energy into the activation of targeted adsorbate-metal bonds. Laser photo-excitation has been used for many years to induce chemical reactions on metal surfaces; however the mechanism underlying the photon-induced reactivity is still not well understood. Particularly, there is no understanding of how the efficiency of photon-induced chemical reactivity is impacted by the nature of the adsorbate-metal bond, which is critical to understand approaches to controlling selectivity.

 In this work we combine dynamical models of the processes of metal substrate photo-excitation and charge transfer induced activation of adsorbate-metal bonds to understand the impact of the adsorbate-metal bond nature, temperature, illumination intensity and wavelength on the potential for controlling selectivity through photo-excitation of metal catalysts. We have employed an extended two-temperature model (ETTM) to describe the dynamics of non-equilibrium electron distribution in photo-excited metal substrates. The coupling between energetic electrons in the metal and adsorbate electronic states is treated using a first principles inelastic electron scattering model recently developed by Schiøtz¹. By integrating the two approaches we can calculate the temporal evolution of the system after laser excitation and understand the impact of wavelength on the efficiency of chemical reactions.   

 We specifically examined the activation of metal-adsorbate bonds formed between CO, NO and O and a Pt (111) surface. Our efforts have been aimed at answering debated mechanistic questions such as the role of non-thermalized electrons (those that do not fall within the Fermi-Dirac distribution) in driving reactions and the time scale of the elementary processes involved in the chemical reactions. We have also addressed the impact of temperature in the photocatalytic processes on metal surfaces. In all of these cases, we focus on how different adsorbate-metal bonds exhibit unique behavior. We are able to explain the previously observed adsorbate specific behavior in experimental studies and provide insights into approaches that could allow for control of selectivity through photo-excitation of heterogeneous metal catalysts. 

References:

1-     Olsen, T., Gavnholt, J. & Schiøtz, J. Hot-electron-mediated desorption rates calculated from excited-state potential energy surfaces. Phys. Rev. B 79, 035403 (2009)