Dynamic surface-coverage alteration based on microkinetic analysis for enhanced ammonia synthesis over ruthenium catalysts at low temperatures
William J. Movick, Fuminao Kishimoto, Kazuhiro Takanabe
Abstract
The ammonia (NH3) synthesis reaction has been well established via Harbor-Bosch process, but recent demands of NH3 as a hydrogen carrier necessitates the synthesis operations at much milder conditions for renewable energy utilization. Catalytic ammonia synthesis is among the most investigated heterogeneous catalytic reactions but the detailed kinetic studies especially at low temperatures are still missing. This study investigated thorough microkinetic analysis at a wide range of reaction temperatures from 200 to 400 °C for NH3 synthesis. Although the united kinetic expression for the NH3 synthesis was established at all temperature range, the rate at low temperatures was found to be strongly reduced not only by hydrogen but also NH3 (NHx) surface intermediates that become more significant as the conversion increases. Knowing these strong inhibitions by both the reactant (H2) and product (NH3), transient perturbation of the surface intermediates by alternating between pure H2 and N2 gas flow was tested, which may lead to rate jumps by controlling the most abundant surface intermediates. The results show dramatic enhancement of the overall NH3 synthesis rate of as high as > 1 mmol g−1 h−1 over 5 wt% Ru/CeO2 catalysts at 200 °C and 0.1 MPa. The results not only give insight into improving existing catalysts and catalytic reaction operations, but also cast doubts on the consistency of how to report the rates of ammonia synthesis due to the strong dependence of the catalysts on ammonia yield, especially at low temperatures.