High-temperature CO₂ capture by Li₄SiO₄: IR spectroscopic evidence for the double shell model

This study investigates the mechanisms of CO₂ capture by Li₄SiO₄ employing in situ Fourier transform infrared spectroscopy (FT-IR) combined with multivariate data analysis, with particular attention being paid to the influence of structural modifications derived from natural diatomite on the CO₂ sorption performance of Li₄SiO₄. Three samples were examined: a reference Li₄SiO₄ material synthesized from pure SiO₂ (SiO₂-LS), a stoichiometric mixture using calcined diatomite (ND-LS) and an over-stoichiometric sample containing 10% calcined diatomite (10% ND-LS). FTIR analysis confirmed the formation of carbonate species during CO₂ uptake. Chemometric analysis using principal component analysis (PCA) and multivariate curve resolution-alternating least squares (MCR-ALS) allowed identification of the successive formation of two distinct carbonate species (species 1 and species 2), supporting the double-shell carbonation model. In ND-derived samples, a distinct band at 1140 cm^-1, attributed to the symmetric stretching vibration (ν₁) of a carbonate species associated with magnesium carbonates, was identified. The presence of surface MgCO₃ associated with species 1 in ND-derived samples was found to enhance CO₂ capture kinetics by facilitating carbonate layer formation through interfacial diffusion pathways. This study provides valuable insights into the carbonation mechanisms of Li₄SiO₄, demonstrating that calcined diatomite improves CO₂ uptake efficiency and opening new perspectives for the optimization of lithium silicate-based CO₂ sorbents through targeted compositional modifications.