Paper SS2-ThA3
RRKM Simulation of Hydrogen Dissociation on Cu(111): Addressing Dynamical Biases, Surface Temperature, and Tunneling

Thursday, October 31, 2013, 2:40 pm, Room 202 A

The effects of dynamics, surface temperature, and tunneling on the dissociative chemisorption of hydrogen on Cu(111) are explored using a dynamically-biased precursor mediated microcanonical trapping (d-PMMT) model. Transition state vibrational frequencies were taken from recent GGA-DFT electronic structure calculations and the model’s few remaining parameters were fixed by optimizing simulations to a limited number of quantum-state-resolved associative desorption experiments. The d-PMMT model reproduces a diverse variety of dissociative chemisorption and associative desorption experimental results, and, importantly, largely captures the surface temperature dependence of quantum-state-resolved dissociative sticking coefficients. Molecular translational energy parallel to the surface was treated as a spectator degree of freedom. The efficacy of molecular rotational energy to promote dissociation, relative to normal translational energy, varied monotonically from -45% to 33% as the rotational energy increased. The molecular vibrational and surface phonon efficacies were 60%. Efficacies did not vary with isotope change from H₂ to D₂. The thermal dissociative sticking coefficient for H₂/Cu(111) is predicted to vary as S(T) = S₀ exp(-E_a/RT) where S₀ = 0.075 and E_a = 49.2 kJ/mol over the 300 K ≤ T ≤ 1000 K temperature range. Dynamical effects are significant and suppress S(T) by ~2 orders of magnitude as compared to statistical expectations. For thermal dissociative chemisorption of H₂/Cu(111) at 1000 K, a temperature of catalytic interest, normal translational energy is calculated to provide 74% of the energy necessary to react, surface phonons 17%, molecular rotation 5%, and vibration 4%. Tunneling is calculated to account for 13% of S(T) at 1000 K, but more than 50% at temperatures below 400 K.