Thermal Gradual Dehydrogenation of Calcium Alanate Ca(AlH₄)₂. The System Phase Composition During the Process of Dehydrogenation

Abstract

The statistical theory of thermal stage-by-stage decomposition of calcium alanate with the release of hydrogen is developed. The calculation of free energies of all constituent phases of chemical reactions is performed on the basis of molecular-kinetic theory. Their dependences on temperature, hydrogen-atoms’ concentration and activity, values of energy parameters of the system are determined. The conditions of thermodynamic equilibrium of all phases determining the temperature dependence of hydrogen concentration are found. The numerical values of energy constants are evaluated, using the literature experimental data. The plots of temperature dependence of hydrogen solubility in the studied phases are constructed with the curve breaks and bends at the points of transitions between phases. The plot of temperature dependence of hydrogen extraction from the system with the temperature rise is obtained. The comparison of numerical values of free energies of phases in the points of phase transformations is performed that confirms the experimentally revealed phase composition of the system, i.e. the possibility of each phase realization in the certain temperature and hydrogen-concentration ranges.