N-Sorption Capability of Al2O3-Supported Mn-/Fe-Based Nitrogen Carriers during Chemical Looping Ammonia Synthesis Technology

Chemical looping ammonia synthesis (CLAS) is proposed as a novel, milder, and environmentally friendly method for NH3 production. In the CLAS process, a looped metal nitride, with the ability to transport nitrogen via cyclic N-sorption/desorption reactions in an interconnected reactor, plays an essential role in decoupling the scaling relation between transition-state and adsorption energies of intermediate species. Nitrogen carriers based on manganese, iron, and their combinations have been previously proven to provide satisfactory CLAS behavior. However, further improvement is required on the mechanical properties of these N carriers. In the present work, the N-sorption performance of 18 nitrogen carriers supported on 2-50% (w/w) Al2O3 within the temperature range of 700-900 degrees C is tested by a thermogravimetric analyzer (TGA) and stationary bed reactor. The objective of this study is to identify feasible nitrogen carriers with high N-sorption capability and strong mechanical properties. The general observation revealed that a lower Al2O3 content (<= 5 wt %) together with a moderate bed temperature (800 degrees C) promoted the nitrogen fixation ability and physical structures of nitrogen carriers. The reaction temperature exerted a stronger effect on the performance of N carriers than the support content. Support material could only alleviate the serious agglomeration to some extent at 900 degrees C, while it could significantly improve the gas-solid contact and well solve the aggregation problem under milder conditions. Iron in the Mn-Fe-based samples was proven to enhance the dispersion of manganese and might benefit the formation of Mn nitrides. The main crystalline phases identified in the nitridized Mn- and Mn-Fe-based samples were Mn4N and Mn2N0.86, and FeN was the only N-containing phase detected for Fe-based samples after nitridation. Mn95A15-800 and Mn82Fe16Al2-800 were the most interesting candidates with excellent N-fixation performance. Considering the outstanding N-desorption capability of Fe98Al2-800, it still deserved further study, even with inferior nitridizing properties.