Unravelling the kinetics of NH3 decomposition over highly active Ru/CeO2 catalysts: Effect of the support on H-poisoning and rate determining steps

Ru/CeO2 catalysts have recently gained attention for low temperature NH3 decomposition: the combination of a highly active metal as Ru with a support as CeO2, known to enhance several catalytic processes thanks to the ability to create oxygen vacancies, makes this formulation highly promising. In this study, the performances and kinetics of NH3 decomposition over Ru/CeO2 were investigated and compared with those of a reference Ru/Al2O3 catalyst by applying multiple techniques. Steady-state activity tests under diluted and concentrated conditions were performed to study the main kinetic dependences, develop rate laws and identify the nature of the rate determining steps. Dynamic experiments of adsorption/desorption and DRIFT experiments under flow conditions and with H2/D2 switch were conducted to identify and quantify the adsorbed species as well as the storage capacity and mobility of H-species, thus providing a support to the kinetic interpretation. The remarkable intrinsic activity exhibited by Ru/CeO2 catalyst can be associated to the important mitigation of the H-poisoning effect that, instead, limits NH3 decomposition over Ru/Al2O3. On Ru/CeO2, intermediate H* species would easily migrate from the Ru-sites to the support, thereby hindering the formation of Ru-hydrides, which instead were visible on Ru/Al2O3. The higher catalytic activity of Ru/CeO2 allowed a wider operating temperature range, which revealed that with decreasing temperature below 300 °C the rate-determining step of the reaction shifts from ammonia activation to N* associative desorption.