Hydrogen storage in porous transition metals nitroprussides

Transition metals nitroprussides form a family of porous molecular materials with relative wide diversity of crystalline structures and also of porous network topologies. These features make nitroprussides interesting cyanometallates-based materials where the role of structural factors on the hydrogen storage can be evaluated. The hydrogen adsorption was studied in T[Fe(CN)₅NO] with T = Mn, Fe, Co, Ni, Cu, Zn, and Cd; in a series of mixed compositions, Co_1-X[Fe(CN)₅NO] with T=Mn, Fe, Ni, Zn, and Cd; and in Cu_0.55Ni_0.55[Fe(CN)₅NO]. The largest hydrogen storage capacity was found for Ni[Fe(CN)₅NO], 2.54 mol/mol (1.85 wt %) at 75 K and 850 Torr. The hydrogen adsorption in nitroprussides shows a marked dependence on the properties of the metal (T) situated at the cavity surface. The electrostatic interaction between the hydrogen molecule quadrupole moment and the electric field gradient at the cavity surface appears to be the main driving force for the hydrogen adsorption, without discarding a possible direct interaction of H₂ with the metal (T). In structures with narrow channels (Mn, Cd), pronounced kinetic effects for the H₂ adsorption isotherms are observed, which were ascribed to a strong and localized interaction between the H₂ molecule and the metal at the cavity surface. The pore accessibility and the pore volume were evaluated from CO₂ adsorption isotherms. The free volume for all the compositions are accessible to the CO₂ molecule. The CO₂ stabilization within the cavities is also dominated by the electrostatic interaction. All the samples were previously characterized using X-ray energy-dispersed spectroscopy, X-ray diffraction, thermogravimetry, and infrared and Mössbauer spectroscopies.