Effect of calcium aluminates on the structure evolution of CaO during the calcium looping process: A DFT study

Calcium looping has been recognized as a key technology for both CO2 capture and concentrated solar energy storage. The inert support is significant in improving the sintering-resistance of CaO-based materials. This paper provided a comprehensive study of structural, thermodynamic and electronic insights into calcium aluminates as supports for CaO by density functional theory (DFT) calculations. Thermodynamic stabilities of three phases including Ca3Al2O6, Ca12All4O33 and Ca4Al6O13 were investigated and the adsorption parameters (energies and electronic properties) of CaO binding with calcium aluminates were determined. The results show that Al-O bonds play dominant roles in determining the crystal strength of CaxAlyOz. The phonon dispersions show that three CaxAlyOz are dynamically stable. Ca4Al6O13 has the more negative cohesive energy and formation enthalpy than Ca3Al2O6 and Ca12All4O33. O and Ca atoms of CaxAlyOz contribute mostly to the reactivity for CaO adsorption, which is related to the degree of freedom of O atoms in CaxAlyOz. The adsorption energies of CaO on Ca3Al2O6, Ca12All4O33 and Ca4Al6O13 are-5.12 eV,-4.27 eV and-3.83 eV, which are more than 2 times higher than that of pure CaO. The corresponding charge transfers (0.34 e, 0.34 e and 0.15 e) from CaO to CaxAlyOz surfaces are larger than that in pure CaO. The interaction of CaO and CaxAlyOz results in electron redistribution around O sites, which may slightly affect further CO2 adsorption reactivity. The higher average bond populations of AlO4 skeleton with adsorbed CaO indicate that CaxAlyOz with superior refractory properties are good supports for CaO. The study furnishes information for calcium aluminates-supported CaO and provides evidence for further research on composite CaO-based materials with high reactivity and novel properties.