Structure and reaction condition dependent mechanism for ammonia synthesis on Ru-based catalyst

Abstract

The ammonia synthesis by Haber–Bosch process is an important catalytic process for the production of fertilizers. Despite extensive research, the exact reaction mechanism remains unsettled. To date, only dissociative mechanism, in which the adsorbed N2 decomposes directly, is investigated on the archetypical Ru-based catalyst. By density functional theory calculation and micro-kinetic modeling, we show that the associative mechanism, which is initiated by N2 hydrogenation to NNH, is dominant on Ru terrace, vacancy and adatom sites regardless of temperatures (523.15–723.15 K) and sticking coefficients (or partial pressures) of N2 and H2 (SN2 = 1.0 × 10−12-1.0 and SH2 = 0.001–1.0). The associative mechanism also dominates on Ru step (A4) and kink sites at a high SN2 or SH2 (SN2 = 1.0 or SH2 = 1.0), which exhibit higher reaction rate than Ru B5 site. The preferable associative mechanism originates from the high θH⁎/θ⁎ ratio or the significant backward process of N2* dissociation induced by high θN⁎/θ⁎ ratio, and has markedly different rate-determining step from the traditional dissociative mechanism (N2 hydrogenation vs dissociation). We also show that the equilibrium approximation is more convenient and efficient than the micro-kinetic modeling, which can streamline the fundamental understanding and accelerate the high-throughput screening of ammonia synthesis materials.