Hydrogen absorbing characteristics of R–M (R = La, Ce; M = Co, Rh, Ir, Ni, Pd, Pt) binary systems

The capacity, rate and reversibility of the hydrogen absorbing/desorbing reaction have been measured for the binary systems composed of rare earth metal, R (La or Ce), and transition metal, M (Co, Rh, Ir, Ni, Pd or Pt), by Sieverts’ method. These experimental results were discussed by comparing with the theoretical ones; density of states, cohesive energy and energy fluctuation, which were calculated by the extended Hückel method. Major results obtained are as follows. (1) The capacity of hydrogen absorption decreases almost linearly with increasing M components in R–M systems and it is explained in terms of the density of states. (2) The rate of hydrogen-absorbing reaction in La–M systems are larger than that of Ce–M systems. The larger absorption rate corresponds to the larger energy fluctuation. (3) The reversibility of the hydrogen absorbing/desorbing reaction is closely related with the cohesive energy of these systems. When the cohesive energy of the hydrogen absorbed system is large, a reverse (i.e. desorbing) reaction hardly occurs.