Characteristics of adsorption and desorption of hydrogen on multi-walled carbon nanotubes

Storage capacities of H-2 in a kind of multi-walled carbon nanotubes (MWCNTs) and its corresponding system modified with K+ salt were measured using volumetric method, and characteristics of adsorption and desorption of hydrogen on MWCNTs were studied by means of temperature-programmed desorption (TPD) method. The results showed that the storage capacities of H-2 in the purified MWCNTs could reach 1.51% (mass fraction) under condition of room temperature and 9.0 MPa, with adsorption of similar to99% of the H-2 being reversible. A comparative study indicated that the modification of the K+ salt to the MWCNTs did not improve its storage capacity of hydrogen, but led to elevating of desorption temperature of adsorbed hydrogen H-2(a) to a certain extent; in addition, the K+ modification resulted also in changing of surface property of the MWCNTs from original hydrophobicity to hydrophilicity. The products desorbed on H-2/MWCNTs involved exclusively H-2 at T less than or equal to 723 K, and main part of H-2 and small amount of CH4, C2H4, and C2H2 at T greater than or equal to 773 K; whereas on H-2/K+' -MWCNTs, H2O was also involved in the desorbed products, in addition to H-2 and small amount of C, C-2-hydrocarbons. The adsorption of H-2 on the MWCNTs-based materials is in the two models: associative (molecular state) and dissociative (atomic state).