Experimental screening of metal nitrides hydrolysis for green ammonia synthesis via solar thermochemical looping

Ammonia is a fundamental chemical commodity for fertilizers and as a novel energy vector. Solar-driven ammonia synthesis is proposed as a sustainable alternative to the catalytic energy-intensive and CO2-emitting Haber-Bosch process. The considered thermochemical process aims to produce ammonia from nitrogen and water (N-2 + 3H(2)O -> 2NH(3) + 1.5O(2)) via redox cycles using a solar heat source, thus bypassing the supply of H-2 or electricity. Metal oxide/nitride redox pairs can be employed for this cyclic process. The exothermal hydrolysis reaction of nitrides produces ammonia (MxNy + 3H(2)O -> 2NH(3) + Mx'Oy'), and is followed by one or several regeneration steps (Mx'Oy'+N-2 -> MxNy+(3)/O-2(2)) requiring a heat supply from concentrated solar energy. This study aims to experimentally identify the most suitable metal nitrides in the hydrolysis step for ammonia synthesis based on solar-driven chemical-looping. As a result, FeN, CrN, BN, and Si3N4 turned out to be irrelevant candidates for NH3 production, as the hydrolysis yield was poor up to 1000 degrees C. In contrast, AlN, Li3N, Ca3N2, Mg3N2, TiN, and ZrN exhibited noteworthy reactivity depending on the temperature. The hydrolysis rate of AlN was significantly enhanced only above 1100 degrees C, TiN showed an increasing NH3 production rate with temperature (reaching 3.4 mmol/min/g at 1000 degrees C), while an optimum at 750 degrees C was unveiled for complete ZrN conversion (corresponding to the highest rate of 34.2 mmol/min/g). Hydrolysis of Li3N, Ca3N2, and Mg3N2 was complete at lower temperatures (similar to 200 degrees C), with NH3 yields of 5.9, 4.9, and 18.6 mmol/g, respectively. Solar-driven regeneration of metal nitrides at high temperature will be then necessary to demonstrate the complete feasibility of thermochemical cycles for green ammonia synthesis.