An Ionicity Rationale to Design Solid phase Metal Nitride Reactants for Solar Ammonia Production

Ammonia provides the basis of nutrition for a large portion of the human population on earth and could be used additionally as a convenient hydrogen carrier. This work studies a solar thermochemical reaction cycle that separates the reductive N₂ cleavage from the hydrogenation of nitrogen ions to NH₃ without using electricity or fossil fuel. The hydrolysis of binary metal nitrides of magnesium, aluminum, calcium, chromium, manganese, zinc, or molybdenum at 0.1 MPa and 200–1000 °C recovered up to 100 mol % of the lattice nitrogen with up to 69.9 mol % as NH₃ liberated at rates of up to 1.45 × 10^–3 mol NH₃ (mol metal)⁻¹ s^–1 for ionic nitrides. These rates and recoveries are encouraging when extrapolated to a full scale process. However, nitrides with lower ionicity are attractive due to simplified reduction conditions to recycle the oxidized reactant after NH₃ formation. For these materials diffusion in the solid limits the rate of NH₃ liberation. The nitride ionicity (9.96–68.83% relative to an ideal ionic solid) was found to correlate with the diffusion constants (6.56 × 10^–14 to 4.05 × 10^–7 cm² s^–1) suggesting that the reduction of H₂O over nitrides yielding NH₃ is governed by the activity of the lattice nitrogen or ion vacancies, respectively. The ionicity appears to be a useful rationale when developing an atomic-scale understanding of the solid-state reaction mechanism and when designing prospectively optimized ternary nitrides for producing NH₃ more sustainably and at mild conditions compared to the Haber Bosch process.