THE KINETICS AND MECHANISM OF CERIUM HYDRIDE FORMATION

The kinetics of the reaction between cerium metal and gaseous hydrogen were studied within the temperature range 0-100-degrees-C and the pressure range 0.1-32 atm. Combined kinetic rate measurements and metallographic observations enabled the evaluation of the intrinsic kinetic parameters of the reaction, i.e. the velocities of the hydride front progression and their temperature-pressure dependence. Arrhenius temperature plots of the velocities yielded a pressure-independent apparent activation barrier of about 0.2 +/- 0.05 eV per H atom. For each temperature an apparent power pressure dependence of about 1/3 was displayed. Different possible mechanisms are discussed. It has been concluded that the rate-limiting step for this reaction is the penetration of hydrogen from the surface (of the hydride layer) into the subsurface region. The activation energies for this penetration process and for the desorption of chemisorbed hydrogen (from the hydride surface) were estimated to be 0.24 and 0.09 eV respectively.