(748d) Metal Nitride Redox Materials for Solar-Driven Low-Pressure Ammonia Synthesis

The activity of many heterogeneous catalysts is limited by strong correlations between activation energies and adsorption energies of reaction intermediates at the surface. Although the reaction is thermodynamically favorable at ambient conditions, the catalytic synthesis of ammonia (NH₃) from N₂ and H₂ requires some of the most extreme conditions of the chemical industry. To synthesize NH₃ fertilizer and chemical fuel sustainably from air, water, and concentrated sunlight as renewable source of process heat, we demonstrate how NH₃ can be produced at ambient pressure via nitrogen reduction with a looped metal nitride, followed by separate hydrogenation of the lattice nitrogen into NH₃. Separating NH₃ synthesis into two reaction steps introduces an additional degree of freedom when designing catalysts with desirable activation and adsorption energies. We discuss thermochemical calculations for the hydrogenation of alkali and alkaline earth metal nitrides and the reduction of transition metal nitrides to outline a promoting role of lattice hydrogen in ammonia evolution. Employing electronic structure theory, this is understood due to the activity of nitrogen vacancies controlling the redox-intercalation of hydrogen and the formation and hydrogenation of adsorbed nitrogen species. Finally, we experimentally confirm the predicted NH₃ formation trends with 56.3, 80.7, and 128 μmol NH₃ evolved mol^-1 metal min^-1 at 1 bar and above 550°C via reduction of Mn₆N_2.58 to Mn₄N and hydrogenation of Ca₃N₂ and Sr₂N to Ca₂NH and SrH₂, respectively.