(284b) Energetic Electron Induced Chemical Reactions On Metal Surfaces: First-Principles Based Electron Scattering Model

Understanding of the interaction between energetic electrons and molecules adsorbed on metal surfaces is of great significance in fundamental surface science and heterogeneous catalysis. In this study, we have employed an electron scattering model with first-principles calculations to investigate the energetic electron induced activation of adsorbed diatomic molecules over metal surfaces. In this model, the electronic system of the adsorbate and substrate is described by a Newns-Anderson-type Hamiltonian^[1] with the linear coupling of unoccupied resonant electronic states and vibrational modes of the adsorbate. When the resonant electronic state of the adsorbate becomes transiently occupied due to electron scattering, the adsorbate confined within the Born-Oppenheimer ground state potential well undergoes a Franck-Condon transition onto the excited state potential energy surface (PES) calculated using linear expansion ΔSCF-DFT method implemented in GPAW.^[2] The reaction probability of the energetic electron induced surface reaction can be calculated based on the model using several parameters from density functional theory calculations. The critical feature of the model compared with previous ones^[1][3] is that the effect of substrate temperature has been explicitly included, and the energy transfer between excited adsorbate vibrational state and photon modes of metal substrates has been taken into account using Friction model.

We have applied the model to study energetic electron mediated oxygen activation on plasmonic silver (Ag) nanostructures. We found that the energetic electrons, generated due to surface plasmon resonance (SPR) excitation of Ag nanostructures, can induce a significant enhancement in the rate of O-O bond activation by transient occupation of the O2 2π* anti-bonding orbital. The electron scattering induces nuclear motion along the reaction coordinate of O-O bond, thereby facilitating the dissociation step. The driving force for nuclear motion is characterized by the coupling constant obtained from the excited state PES and the normal mode frequency of O-O bond. We have shown that the model can quantitatively capture the phenomenon of temperature and wavelength dependent rate enhancement of ethylene epoxidation reaction over Ag due to visible light illumination.^[4]

[1]        J. W. Gadzuk, Phys. Rev. B 1991, 44, 13466.

[2]        J. Enkovaara, C. Rostgaard, J. J. Mortensen, et al., J. Phys.: Condens. Matter 2010, 22, 253202.

[3]        T. Olsen, J. Gavnholt, J. Schiotz, Phys. Rev. B 2009, 79, 035403.

[4]        P. Christopher, H. Xin, S. Linic, Nat Chem 2011, advance online publication, DOI 10.1038/nchem.1032.