The reaction rate for dissociative adsorption of N2 on stepped Ru(0001): Six-dimensional quantum calculations

Abstract

The reaction rate for dissociative adsorption of N 2 on stepped Ru"0001…: Six-dimensional quantum calculations Quantum-mechanical calculations of the reaction rate for dissociative adsorption of N 2 on stepped Ru͑0001͒ are presented. Converged six-dimensional quantum calculations for this heavy-atom reaction have been performed using the multiconfiguration time-dependent Hartree method. A potential-energy surface for the transition-state region is constructed from density-functional theory calculations using Shepard interpolation. The quantum results are in very good agreement with the results of the harmonic transition-state theory. In contrast to the findings of previous model calculations on similar systems, the tunneling effect is found to be small.

The effect of surface relaxation on the N2 dissociation rate on stepped Ru: A transition state theory study

“…The present results also provide an a posteriori justification of the frozen surface approximation in previous work, 5 where the contribution of the tunneling effect to the reaction rate was studied. Our justification is based on the following two arguments.…”

“…As discussed in Ref. 5, the tunneling effect is mainly determined by the imaginary frequency i and is adequately described by the Wigner tunneling theory. 12 According to this theory, the ratio between the rate constants with and without tunneling is given by the tunneling factor =1+ For example, at room temperature ͑300 K͒, the tunneling factors are 1.16 and 1.22 for the frozen and relaxed surface models, respectively.…”

“…Technical details are the same as in Ref. 5, except that the present work also includes relaxations of the Ru atoms in the topmost two layers. The transition state of N 2 dissociation is localized by constraining the N-N distance and relaxing both the other nitrogen degrees of freedom and the positions of the Ru atoms in the topmost two layers.…”

“…1-4͒. In a preceding paper, 5 we have examined the effect of tunneling on the reaction rate for N 2 on a stepped Ru surface with ͑0001͒ terraces. The positions of the surface atoms were frozen, in order to reduce the computational effort.…”

“…In a previous study we have shown that tunneling effects are negligible. 5 Thus, harmonic TST employing the relaxed/N 2 model is an accurate approach to calculate the rate constants from DFT data. The applicability of this model in firstprinciples study of the ammonia synthesis rate 2 is therefore confirmed by the present results.…”