Computational design of catalysts for the sustainable reduction of nitrogen into ammonia

Abstract

Nitrogen fixation into ammonia supports the food supply we rely on, along with other industries, and it can become the vector to an inexpensive renewable energy storage system. However, the usage of fossil feedstock and centralized Haber-Bosch synthesis conditions causes >2% of the global anthropogenic CO2 emissions. The demand for sustainable ammonia production drives immense research interest in developing catalysts that can work under mild conditions as it can potentially replace the existing centralized fossil fuel-based Haber-Bosch process with small, decentralized units. Even the most active bulk inorganic catalysts fail due to low activity and parasitic hydrogen evolution at low temperatures. A completely new class of catalysts, ternary ruthenium hydrides (Li4RuH6 and Ba2RuH6 ) composed of the electron- and H-rich [RuH6] anionic centers is shown to follow non-dissociative dinitrogen reduction, where hydridic H transports electron and proton between the centers, and the Li(Ba) cations stabilize NxHy (x: 0 to 2, y: 0 to 3) intermediates. The synergistic effect facilitates a very complex reaction mechanism with a narrow energy span and small kinetic barriers leading to functional ammonia production kinetics under mild conditions. It also observed that the [RuH6] catalytic center activate N2 preferentially and avoid hydrogen over-saturation at low temperatures and near ambient pressure, by delicately balancing H2 chemisorption and N2 activation. The unprecedented yield at low temperatures occurs due to a shift in the rate-determining reaction intermediates and transition states, where the reaction orders in hydrogen and ammonia change dramatically. Vanadium oxynitride, a member of the unexplored class of catalysts for ammonia evolution containing multi anionic species, is also studied thoroughly with computational studies. For the first time, the nitrogen reduction pathway in oxynitrides and the role of the mixed anions is established. The coexistence of oxygen and nitrogen leads to improved stability of the active surface-states, activity, and selectivity over hydrogen evolution. Moderate to low oxygen concentration is suggested to best impact catalytic properties due to optimal balance between consecutive protonation preference at N-sites over V-sites, low onset potential (0.4V-RHE), and facile N2 adsorption at N-vacancy sites. The critical N-vacancy active sites that support a Mars van Krevelen mechanism for ammonia evolution are protected from self-annihilation by the mixedvalence anions, large kinetic barriers, and site blocking by O*/OH*/H* due to highly favorable N2 absorption.