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

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journal contribution
posted on 08.11.2012, 00:00 by Ronald Michalsky, Peter H. Pfromm
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.