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

2012-11-08T00:00:00Z (GMT) 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 N<sub>2</sub> cleavage from the hydrogenation of nitrogen ions to NH<sub>3</sub> 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 NH<sub>3</sub> liberated at rates of up to 1.45 × 10<sup>–3</sup> mol NH<sub>3</sub> (mol metal)<sup>−1</sup> s<sup>–1</sup> 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 NH<sub>3</sub> formation. For these materials diffusion in the solid limits the rate of NH<sub>3</sub> 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<sup>–14</sup> to 4.05 × 10<sup>–7</sup> cm<sup>2</sup> s<sup>–1</sup>) suggesting that the reduction of H<sub>2</sub>O over nitrides yielding NH<sub>3</sub> 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 NH<sub>3</sub> more sustainably and at mild conditions compared to the Haber Bosch process.