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 NH(3) 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 NH(3) 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 N(2) to Cr nitride, hydrolyzing Cr nitride powder with steam to NH(3) and Cr(2)O(3), and finally reducing Cr(2)O(3) powder back to Cr with mixtures of 1-17, CO, and N(2). At about 1000 degrees C it was found that Cr readily fixes N(2) from the gas phase as Cr nitride (4.13 x 10(-2) mol N(2)/mol Cr/min, 85 +/- 4 mol% of hexagonal Cr(2)N after 5.6 min). Cr(2)N converts over time to a cubic CrN phase. Corrosion of Cr nitride with steam at 1000 C and about 1 bar forms Cr(2)O(3) 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 NH(3). This results in a yield of 0.15 +/- 0.02 mol% NH3 relative to the N in the nitride (1.07 x 10(-4) mol NH(3)/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 NH(3) only slightly (0.24 +/- 0.01 or 0.39 +/- 0.03 mol% NH(3) relative to the N in the nitride respectively). The thermochemical cycle is closed by heating Cr(2)O(3) to 1200-1600 C with a reduction yield near the surface of the particles of approximately 82.85 mol% (40 min at 1600 degrees C) in a gas stream of H(2) and CO (2.7 x 10(-3) mol Cr/mol Cr(2)O(3)/min). An unreacted core model was applied to estimate the activation energy of Cr(2)O(3) 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 NH(3) production from N(2) and H(2)O at ambient pressure without natural gas consumption. (C) 2011 Elsevier Ltd. All rights reserved.