A theoretical study on CO at LiSiO and LiNaSiO surfaces.

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