High-performanced porous Cr-modified N-carrier for the chemical looping synthesis of ammonia: Improved diffusion of N2 thus enhanced NH3 yield

Abstract

Chemical looping ammonia synthesis (CLAS) has attracted more attention due to low energy consumption and low CO₂ emissions. But most studies only focus on chemical kinetics, neglecting the impact of reactant diffusion, which results in limited conversion efficiencies in nitridation-ammonization reactions and low NH₃ yields. The improvement in external/interstitial diffusion during the CLAS reactions were studied in this paper to further improvement the NH₃ yield. External diffusion was studied by varying particle size, specific surface area, and pore size. The optimized nitrogen carrier with large average pore size and high specific surface area can promote the adsorption and internal transport of N₂ and H₂O molecules, so as to promote the reactions. Interstitial diffusion was studied by doping with 15 elements, Cr is found to be the promising dopant due to its ability to minimize the permeation energy needed for N₂ diffusion, and to increase the oxidative activity in the nitrogen carrier, resulting in an obvious improvement in conversion efficiency during the nitridation reaction and ammonization reaction. Finally, a porous Cr-modified nitrogen carrier with a pore size of 30.05 nm and 10 wt% of Cr doping was developed and the nitridation and the ammonization reaction efficiency was improved to 94 % and 84 % respectively. Therefore, the NH₃ yield of the advanced N-carrier was enhanced from 0.46 to 2.42 mmol g⁻¹, which is much higher than the reported N-carrier. The stability is confirmed through a fifteen-cycle test, showing minimal fluctuation in reaction conversion efficiency and NH₃ yield.