Chromium as reactant for solar thermochemical synthesis of ammonia from steam, nitrogen, and biomass at atmospheric pressure

Ammonia for fertilization plays a crucial role in agriculture. It is an important commodity chemical, and it can serve as a fuel for combustion engines or as a carrier molecule for hydrogen. Global NH3 production of over 100 million metric tons per year relies almost entirely on natural gas for energy and hydrogen. About 2% of the world’s energy budget is spent to produce NH3. Experiments towards a solar thermochemical cycle for NH3 synthesis at near atmospheric pressure using a transition metal reactant and a Fresnel-lens solar furnace are reported here: reacting Cr metal powder with gaseous N2 to Cr nitride, hydrolyzing Cr nitride powder with steam to NH3 and Cr2O3, and finally reducing Cr2O3 powder back to Cr with mixtures of H2, CO, and N2. At about 1000 °C it was found that Cr readily fixes N2 from the gas phase as Cr nitride (4.13 × 10−2 mol N2/mol Cr/min, 85 ± 4 mol% of hexagonal Cr2N after 5.6 min). Cr2N converts over time to a cubic CrN phase. Corrosion of Cr nitride with steam at 1000 °C and about 1 bar forms Cr2O3 and CrO while liberating 53 ± 11 mol% of the nitrogen contained in the solid Cr nitride in 60 min. Of the N liberated, 0.28 ± 0.07 mol% forms the desired NH3. This results in a yield of 0.15 ± 0.02 mol% NH3 relative to the N in the nitride (1.07 × 10−4 mol NH3/mol Cr/min). Addition of CaO/Ca(OH)2 powder or quartz wool to provide more reactive sites and promote protonation of N increased the yield of NH3 only slightly (0.24 ± 0.01 or 0.39 ± 0.03 mol% NH3 relative to the N in the nitride respectively). The thermochemical cycle is closed by heating Cr2O3 to 1200–1600 °C with a reduction yield near the surface of the particles of approximately 82.85 mol% (40 min at 1600 °C) in a gas stream of H2 and CO (2.7 × 10−3 mol Cr/mol Cr2O3/min). An unreacted core model was applied to estimate the activation energy of Cr2O3 reduction with 128 ± 4 kJ/mol. Cr appears promising to promote nitridation and oxide reduction as a basis for a future custom-designed reactant with high specific surface area enabling sustainable and more scalable NH3 production from N2 and H2O at ambient pressure without natural gas consumption.

► Conceptual reactant and process improvements of solar-driven NH3 synthesis at 1 bar.
► Cr readily fixes N2 forming nitrides.
► Corrosion kinetics limits Cr nitride hydrolysis yielding Cr2O3 and traces NH3.
► Cr2O3 is reduced to Cr by simulated gasified biomass at above 1200 °C.
► Cr-composite reactants may allow reduced reaction temperatures and use of a gaseous reducing agent.