Highly Active MnN–Li2NH Composite Catalyst for Producing COx-Free Hydrogen

Abstract

We report on the significant synergistic effect of Li2NH on MnN in catalyzing NH3 decomposition to produce COx-free hydrogen. The MnN–Li2NH exhibits catalytic activity below 623 K and reaches a hydrogen production rate of 37.5 mmolH2 gcat–1 min–1 at 773 K, which is ∼40 times the MnN and even superior to the highly active Ru-based catalysts under the same conditions. The apparent activation energy of MnN–Li2NH is similar to that of Ru/CNTs. The stability of the MnN–Li2NH composite catalyst depends on the molar ratio of MnN and Li2NH, and the catalyst with higher Mn content shows better stability. Our experimental results reveal that Li2NH reacts with MnN, giving rise to H2 and Li7MnN4; Li7MnN4, on the other hand, can be easily ammoniated to give off N2, which suggests that the catalysis is fulfilled via a redox cycle comprising (I) Li2NH + MnN → Li7MnN4 + H2 and (II) Li7MnN4 + NH3 → MnN + Li2NH + N2. The presence of Li2NH favors the formation of N-rich intermediate Li7MnN4, where Li+ executes a positive inductive effect to enhance the covalent Mn–N bonding and, therefore, leads to an altered reaction pathway and energetics. Our kinetic analyses indicate that the H2 release step encounters a relatively higher barrier than the N2 release step.