The effect of ionization and CH_3 ligand for hydrogen storage in Co- and Ni-based organometallic compounds

摘要

Maximum capacities of the hydrogen storage in organometallic compounds consisting of Co and Ni atoms bound to C_mH_m ring (m = 4, 5; capped type) were, respectively, found as 3.48 and 3.49 wt % (Guo et al., Struct Chem, 2009, 20, 1107). Here, we extend this study to structures having a transition metal (TM) inserted in C_mH_m ring (inserted type), having TM located on either a C_4H 4+ or a C_5H 5+ molecule, and the CH_3 ligand bound to the organometallic compounds. We find that for the CoC_4H_4 and NiC_4H _4 complexes, the capped types are 1.39 eV and 1.41 eV higher in energy than the inserted types, respectively, while the ground states for CoC_5H_5 and NiC_5H_5 complexes are found to be the capped type, which are lower than the inserted types, respectively, by 1.27 eV and 1.31 eV. The maximum capacity of hydrogen storage reached 5.13 wt % for both of CoC_4H 4+(H_2)_3 complex and the inserted-type CoC_4H_4(H_2) _3 complex with a reasonable binding energy (0.3-1.0 eV per H _2). The positively charged C_4H_4 and C _5H_5 molecules do not only improve the capacity of hydrogen storage but also make all H_2 adsorbing in molecular form and keep the adsorption energy in an ideal range. After adding the CH_3 ligand to the compounds, the average adsorption energy of H_2 decreased to an ideal range 0.61-0.94 eV per H_2 and the stability of the compounds is also improved. Finally, we analyze the HOMO-LUMO gaps and display the kinetic stability when H_2 was added to organometallic compounds.