Publication Details (including relevant citation information):
Ronald Michalsky, Peter H. Pfromm: Solar fuel production via water splitting and protonation of solid-state nitrogen ions forming ammonia, Prep. Pap.-Am. Chem. Soc., Div. Fuel Chem. 56 (2011) 2, p. 529-530.
Ammonia is the basis for modern agriculture and is used widely as base chemical for the chemical industry1. The U.S. produces about 9% of the over 100 million metric tons of NH3 produced worldwide per year2. Recently, NH3 has been proposed as sustainable transportation fuel and may enable efficient H2 storage3-5. The U.S. Department of Energy target6 of 9 wt% H2 capacity for H2-based transportation fuels in 2015 is reached readily by 18 wt% H2 in molecular NH3. Accomplished easily at 25 °C and about 10 bar, liquefaction of NH3 yields a fuel that contains by volume approximately 130-fold more H2 as H2 itself at this temperature and pressure. Modified diesel engines have been reported to combust NH3 generating mainly H2O and N2 as combustion products7. If H2 is the desired combustion fuel, NH3 may be cracked catalytically on board a vehicle to form N2 and H2 for subsequent combustion3.
The Haber-Bosch process synthesizes NH3 catalytically at high pressure and elevated temperature. This process consumes up to 5% of the natural gas produced globally and 2% of the world’s total energy production8, with significant fossil-based CO2 emissions9. Due to the significant capital expense the process is generally implemented in large facilities in the order of 1000 tons NH3 per day production and in locations near a natural gas supply10.
NH3 might be synthesized sustainably from N2 and steam via solar thermochemical processing at near atmospheric pressure. In this study, a thermodynamic rationale for the reactive synthesis of NH3 is presented along with experimental work focusing breaking the N-N triple bond on the surface of transition metals forming nitrides. Hydrolysis of various ionic, covalent, intermediate and interstitial nitrides is studied to assess the impact of the average charge of the N atom in the solid state on the yield of NH3 synthesis far from thermodynamic equilibrium. Corrosion kinetics limiting the formation of NH3 at decreased temperatures is addressed.