High activity of nickel-promoted Mn4N in chemical looping ammonia synthesis

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Chemical looping ammonia synthesis (CLAS) stands out as a promising method for the highly selective synthesis of ammonia, which allows reactions to be carried out under ambient pressure and can reduce carbon emissions. Herein a nickel-facilitated Mn4N-mediated strategy was introduced to enhance the extraction of lattice nitrogen in the CLAS process. H2 is highly mobile on nickel and reacts with lattice nitrogen on the Mn4N surface to form ammonia. Ni increases nitrogen vacancies in Mn4N, enhancing nitrogen activation. At 600 ℃, 16 wt% Ni/Mn4N achieves an ammonia yield of 826 μmol g−1h−1, which is about four times higher than that of Mn4N. Moreover, with 16 wt% Mn4N, ammonia production starts at 350 °C with a rate of 204 μmol g−1h−1, whereas with Mn4N ammonia production starts at 550 °C with an ammonia yield of 194 μmol g−1h−1, and the minimum temperature of ammonia production is reduced by 200 °C with Ni loading. Furthermore, 16 wt% Mn4N had excellent ammonia conversion rate and selectivity of 51.46 % and 91.21 %, respectively. The nitrogen carrier maintained a stable ammonia yield at a reduction temperature of 350 ℃ over 11 cycles. These findings underscore the significant potential of our proposed Ni-facilitated Mn4N-mediated CLAS, offering prospects for synthesizing ammonia at lower temperatures and enhancing ammonia synthesis efficiency. This approach holds promise for advancing towards a more environmentally friendly and efficient technology for ammonia synthesis.