Electronic structural and lattice thermodynamic properties of MAlO2 and M5AlO4 (M=Li, Na, K) sorbents for CO2 capture applications

Abstract

The electronic properties and thermal stabilities of MAlO2 and M5AlO4 (M=Li, Na, K) are investigated by the density functional theory and lattice phonon dynamics. Based on the calculated electronic and lattice thermodynamic properties, their abilities to capture CO2 as solid sorbents are analyzed. The calculated electronic structural properties of MAlO2 and M5AlO4 indicate that all of these alkali aluminates are semiconductors with bandgap range of 2.4~6.4 eV. The 1st valence bands of these alkali aluminates are located in 0~-6 eV under Fermi levels and are mainly contributed by p orbitals of O, s and p orbitals of Al and M. The phonon vibrational frequencies of M5AlO4 spread at lower frequency range compared to their MAlO2 phases. With increasing temperature, the calculated phonon free energies of M5AlO4 decrease faster than their corresponding MAlO2 while their entropies have opposite trends. The reaction 2MAlO2 + CO2 = M2CO3 + Al2O3 has higher reaction heat and Gibbs free energy change than those of corresponding reaction 2/5M5AlO4 + CO2 = M2CO3 + 1/5Al2O3, which shows the former reaction possesses lower turnover temperature. Among these alkali aluminates studied, the β-NaAlO2, lt-KAlO2, and γ-LiAlO2 are better candidates that could be applied for CO2 capture technologies.