Hydrogen storage in porous cyanometalates: Role of the exchangeable alkali metal

The hydrogen storage in zeolite-like hexacyanometalates with different exchangeable alkali metals within the cavities was studied. The H-2 adsorption isotherms were recorded at 75 and 85 K in order to estimate the involved adsorption heats using the isosteric method. The electric field gradient within the porous framework favors the hydrogen adsorption in the materials under study but also could lead to kinetic effects for the pore filling. Such effects were particularly pronounced for sodium among the studied compositions: Zn(3)A(2)[Fe(CN)(6)](2) (A = Na+, K+, Rb+, Cs+) and Zn-3[Co(CN)(6)](2). For Na+, a strong interaction with the H-2 molecule takes place, where appreciable kinetic effects even at 258 K are observed. For Zn-3[Co(CN)(6)](2) (rhombohedral phase) where the cavities are free of exchangeable metal and, in consequence, have a weak electric field gradient on their surface, the largest hydrogen storage capacity, close to 12 H-2 molecules per cavity (1.82% by weight), was observed. The hydrogen adsorption in these materials involves adsorption heats in the 6-8.5 kJ/mol range, following the order K > Rb > Cs approximate to Zn-3[Co(CN)(6)](2). The porous framework of this family of materials is formed by ellipsoidal cavities communicated by elliptical windows. The alkali metals are sited close to the windows. The pore accessibility and pore volume were evaluated from CO2 adsorption isotherms recorded at 273 K. The free volume was found to be accessible to the CO2 molecule for all of the studied compositions. According to the obtained isotherms the stabilization of the CO2 molecule within the pores is caused by the electrostatic interaction between the electric field gradient at the cavity and the adsorbate quadrupole moment. The estimated strength for the guest-host interaction and the accessible pore volume follow the order Na > K > Rb > Cs. The largest accessible pore volume was found for Zn3[Co(CN)(6)](2), close to 8 CO2 molecules per cavity (28% by weight), but with the weaker guest-host interaction. The materials under study were characterized from X-ray diffraction, thermo-gravimetric, infrared, and Mossbauer data. The obtained results shed light on the role of the electric field gradient at the cavity for the hydrogen adsorption.