The Mechanism of Industrial Ammonia Synthesis Revisited: Calculations of the Role of the Associative Mechanism

Quantum chemical calculations have been used to investigate the rate constant and mechanism of ammonia synthesis on a stepped ruthenium surface at typical industrial conditions. Both the commonly accepted dissociative mechanism and an associative mechanism of ammonia formation are explicitly considered. Uncertainties on the calculated parameters have been estimated using a recently developed functional utilizing Bayesian statistics. A surprisingly stable intermediate is identified in the associative mechanism, which is reached via an accordingly low barrier. This gives rise to a much higher rate constant of ammonia synthesis for the associative mechanism than previously considered. The results confirm that at typical industrial operating conditions the dissociative mechanism is dominant, with a difference in rate constants between the two mechanisms of around 3 orders of magnitude. However, consideration of uncertainties on the calculated parameters indicates that only a small change to the activation energy of the associative mechanism may result in a large contribution to the rate. This offers an additional flexibility for catalyst design to optimize the associative mechanism that could potentially lead to faster and more efficient production of ammonia. Furthermore, it illustrates the need to consider both mechanisms when considering nanoparticle catalysts, which possess a large variety of active sites.