Two-Step Chemical Looping Cycle for Renewable NH₃ Production Based on Non-Catalytic Co₃Mo₃N/Co₆Mo₆N Reactions

A two-step solar thermochemical looping cycle based on Co3Mo3N/Co6Mo6N reduction/nitridation reactions offers a pathway for green NH3 production that utilizes concentrated solar irradiation, H2O, and air as feedstocks. The NH3 production cycle steps both derive process heat from concentrated solar irradiation and encompass 1) the reduction of Co3Mo3N in H-2 to Co6Mo6N and NH3; and 2) nitridation of Co6Mo6N to Co3Mo3N with N-2. Co3Mo3N reduction/nitridation reactions are examined at different H-2 and/or N-2 partial pressures and temperatures. NH3 production is quantified in situ using liquid conductivity measurements coupled with mass spectrometry (MS). Solid-state characterization is performed to identify a surface oxygen layer that necessitates the addition of H-2 during cycling to prevent surface oxidation by trace amounts of O-2. H-2 concentrations of > 5% H-2/Ar and temperatures >500 degrees C are required to reduce Co3Mo3N to Co6Mo6N and form NH3 at 1 bar. Complete regeneration of Co3Mo3N from Co6Mo6N is achieved at conditions of 700 degrees C under 25-75% H-2/N-2. H-2 pressure-swings are observed to increase NH3 production during Co3Mo3N reduction. The results represent the first comprehensive characterization of and definitive non-catalytic production of NH3 via chemical looping with metal nitrides and provide insights for technology development.