Statistical physics modeling of hydrogen absorption onto LaNi_4.6Al_0.4: Stereographic and energetic interpretations

ABSTRACT

Theoretical model expressions are used in order to adjust four absorption isotherms of hydrogen on LaNi_4.6Al_0.4 at four different temperatures (T = 288K, 293 K, 303 K and 313 K). The development of these expressions is based on statistical physics formalism and some working hypothesis. The physicochemical parameters intervening in the absorption process and involved in the model expressions could be directly deduced from the experimental absorption isotherms by numerical simulation. The best model that shows a good correlation with the experimental data has been determined. Six parameters of the model are adjusted, namely the numbers of hydrogen atoms per site n₁ and n₂, the receptor site densities N_1m and N_2m and the energetic parameters P₁ and P₂. The behaviors of these parameters are discussed in relationship with temperature of absorption isotherms. Then, a dynamic investigation of the simultaneous evolution with pressure of the two α and β phases is involved in the absorption process using the adjustment parameters. Thanks to the energetic parameters, we calculated the absorption energies, which are typically ranged between 109.871 and 133.634 kJ/mol comparable to usual chemical bond energies. The calculated thermodynamic parameters, such as entropy, Gibbs free energy and internal energy from fitted parameters values showed that the absorption of hydrogen in LaNi_4.6Al_0.4 alloy was spontaneous and exothermic in nature.

KEYWORDS:

• Hydrogen absorption
• statistical physics
• grand canonical ensemble
• absorption energy