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, Co1−x T x [Fe(CN)5NO] with T = Mn, Fe, Ni, Zn, and Cd; and in Cu0.55Ni0.45[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.