High Ammonia Adsorption in MFM-300 Materials: Dynamics and Charge Transfer in Host–Guest Binding

Abstract

Ammonia (NH₃) is a promising energy resource owing to its high hydrogen density. However, its widespread application is restricted by the lack of efficient and corrosion-resistant storage materials. Here, we report high NH₃ adsorption in a series of robust metal-organic framework (MOF) materials, MFM-300(M) (M = Fe, V, Cr, In). MFM-300(M) (M = Fe, V^III, Cr) show fully reversible capacity for >20 cycles, reaching capacities of 16.1, 15.6, and 14.0 mmol g^-1, respectively, at 273 K and 1 bar. Under the same conditions, MFM-300(V^IV) exhibits the highest uptake among this series of MOFs of 17.3 mmol g^-1. <i>In situ</i> neutron powder diffraction, single-crystal X-ray diffraction, and electron paramagnetic resonance spectroscopy confirm that the redox-active V center enables host-guest charge transfer, with V^IV being reduced to V^III and NH₃ being oxidized to hydrazine (N₂H₄). A combination of <i>in situ</i> inelastic neutron scattering and DFT modeling has revealed the binding dynamics of adsorbed NH₃ within these MOFs to afford a comprehensive insight into the application of MOF materials to the adsorption and conversion of NH₃.