α-Al2O3-loaded promoted the N-desorption reaction and inhibited NH3 decomposition in chemical looping ammonia generation: Experiments and DFT simulations

Abstract

Chemical looping ammonia generation is a promising, efficient, and environmentally friendly synthesis of ammonia by cycling N-absorption and N-desorption reactions. A portion of the generated NH3 decomposes at the reaction temperature of the N-desorption reaction. In order to promote the N-desorption reaction and inhibit the decomposition of NH3, the effect of α-Al2O3 loading on the N-desorption reaction was investigated in this paper by fixed-bed experiments. The mechanism was revealed using Density functional theory (DFT) calculations. The results showed that when the reaction temperature increased, the conversion of AlN and the yield of NH3 increased, but the actual NH3 conversion decreased. When the steam concentration increased, the conversion of AlN, the yield of NH3, and the actual NH3 conversion increased. The loading of α-Al2O3 could facilitate the N-desorption reaction and inhibit the decomposition of NH3. The 40 wt% α-Al2O3 had the highest conversion of AlN. The 80 wt% α-Al2O3 had the highest yield of NH3. The actual efficiency of ammonia production at 1075 °C with 80 wt% load increased from 45.5% without load to 58.5%. The DFT calculations revealed the mechanism: α-Al2O3 surface promotes the dissociation of H2O molecules to make more hydroxyl groups in the reaction system, which promotes the transfer of H+ and the N-desorption reaction. Meanwhile, the adsorption of NH3 by the α-Al2O3 surface protected NH3 and inhibited its decomposition.