A theoretical study on CO₂ at Li₄SiO₄ and Li₃NaSiO₄ surfaces

Lithium silicates have attracted great attention in recent years due to their potential use as high-temperature (450-700 °C) sorbents for CO₂ capture. Lithium orthosilicate (Li₄SiO₄) can theoretically adsorb CO₂ in amounts up to 0.36 g CO₂ per g Li₄SiO₄. The development of new Li₄SiO₄-based sorbents is hindered by a lack of knowledge of the mechanisms ruling CO₂ adsorption on Li₄SiO₄, especially for eutectic mixtures. In this work, the structural, electronic, thermodynamic and CO₂ capture properties of monoclinic phases of Li₄SiO₄ and a binary (Li₃NaSiO₄) eutectic mixture are investigated using density functional theory. The properties of the bulk crystal phases as well as of the relevant surfaces are analysed. Likewise, the results for CO₂-lithium silicates indicate that CO₂ is strongly adsorbed on the oxygen sites of both sorbents through chemisorption, causing an alteration not only in the chemical structure and atomic charges of the gas, as reflected by both the angles and bond distances as well as atomic charges, but also in the cell parameters of the Li₄SiO₄ and Li₃NaSiO₄ systems, especially in Li₄SiO₄(001) and Li₃NaSiO₄(010) surfaces. The results confirm strong adsorption of CO₂ molecules on all the considered surfaces and materials followed by CO₂ activation as inferred from CO₂ bending, bond elongation and surface to CO₂ charge transfer, indicating CO₂ chemisorption for all cases. The Li₄SiO₄ and Li₃NaSiO₄ surfaces may be proposed as suitable sorbents for CO₂ capture in wide temperature ranges.