Theoretical Analysis of Energy Efficiency of Plasma-Assisted Heterogeneous Activation of Nitrogen for Ammonia Synthesis

Abstract

One of the rate limiting steps in catalytic ammonia synthesis is a nitrogen dissociation on a catalyst surface, which can be promoted by vibrational excitation of nitrogen molecules. In this work, the efficiency of plasma assisted heterogeneous nitrogen activation via vibrational excitation on the Ru surface is investigated. The analysis consists of two parts: in the first, ab initio calculations are performed for the adsorption and dissociation energy pathways of ${\text{N}}_{{\text{2}}}$ on terrace and step sites and heterogeneous vibrational energy relaxation time of adsorbed nitrogen molecule is calculated using ab initio molecular dynamics. The second part includes the solution of the chemical kinetic equations for vibrationally excited molecules both in the gas phase and on the surface, and an estimation of energy cost of heterogeneous plasma activation of nitrogen. It is shown that heterogeneous vibrational energy relaxation of nitrogen molecule on the Ru surface is rather fast, and results in relatively high energy cost of vibrational nitrogen activation, which is 16 eV/molec for atmospheric pressure and 5 eV/molec for P = 4 Torr according to our calculations.