A theoretical study on CO 2 at Li 4 SiO 4 and Li 3 NaSiO 4 surfaces.

Lithium silicates have attracted great attention in recent years due to their potential use as high-temperature (450-700 °C) sorbents for CO ₂ capture. Lithium orthosilicate (Li ₄ SiO ₄ ) can theoretically adsorb CO ₂ in amounts up to 0.36 g CO ₂ per g Li ₄ SiO ₄ . The development of new Li ₄ SiO ₄ -based sorbents is hindered by a lack of knowledge of the mechanisms ruling CO ₂ adsorption on Li ₄ SiO ₄ , especially for eutectic mixtures. In this work, the structural, electronic, thermodynamic and CO ₂ capture properties of monoclinic phases of Li ₄ SiO ₄ and a binary (Li ₃ NaSiO ₄ ) 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 CO ₂ -lithium silicates indicate that CO ₂ 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 Li ₄ SiO ₄ and Li ₃ NaSiO ₄ systems, especially in Li ₄ SiO ₄ (001) and Li ₃ NaSiO ₄ (010) surfaces. The results confirm strong adsorption of CO ₂ molecules on all the considered surfaces and materials followed by CO ₂ activation as inferred from CO ₂ bending, bond elongation and surface to CO ₂ charge transfer, indicating CO ₂ chemisorption for all cases. The Li ₄ SiO ₄ and Li ₃ NaSiO ₄ surfaces may be proposed as suitable sorbents for CO ₂ capture in wide temperature ranges.