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

Abstract

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.

Electrochemical synthesis of ammonia via Mars-van Krevelen mechanism on the (111) facets of group III–VII transition metal mononitrides

“…This step becomes the RDS for the reaction, which cannot be tuned by applying external bias and therefore is very difficult and slow at ambient conditions. So MnN is not a good catalyst at room temperature and pressure, but it could be a very good reactant for ammonia formation [6], where both theoretical [27] and experimental [28] studies show that it is thermodynamically stable in NaCl type structure, and this stoichiometry (MnN) is more stable than other phases like Mn 4 N, Mn 2 N 0.86 , Mn 3 N 2 . CrN and VN, however, seem very interesting material with relatively low onset potentials for nitrogen activation and ammonia formation.…”

“…Therefore, TMNs have evolved a potential candidate for the noble metal material catalyst and consequently represented better activity i.e. for electrochemical ammonia synthesis [3][4][5] and solar thermochemical ammonia production [6][7][8] when compared with pure metals [9]. For catalytic nitrogen reduction to ammonia, these TMNs have the extra benefit over pure transition metals since nitrogen atoms are already incorporated in their structure.…”

Aluminum Nitride Hydrolysis Enabled by Hydroxyl-Mediated Surface Proton Hopping

“…24 In recent years, a strategy analogous to redox cycling for solar thermochemical water splitting 25 has emerged for the production of NH3 using solar energy to drive a thermochemical redox cycle where a metal oxide is reduced with hydrogen, carbon monoxide, or solid carbon in the presence of nitrogen to produce a metal nitride that is subsequently hydrolyzed by steam to reform the metal oxide and produce NH3. [26][27][28][29][30][31][32] For a cycle using AlN as the metal nitride redox material, 26 the reduction step is shown as reaction 1 and the desired hydrolysis of AlN step is shown as reaction 2:…”

The Denitridation of Nitrides of Iron, Cobalt and Rhenium Under Hydrogen

“…Aluminium nitride has also been used for this purpose [18]. The reactivity of lattice nitrogen to hydrolysis has also been investigated with the aim of applying solid phase metal nitride reactants to solar ammonia production [19]. The reactivity of binary and ternary molybdenum nitrides to hydrogen has been explored [20,21].…”