Alkali carbonates promote CO₂ capture by sodium orthosilicate

Sodium orthosilicate was synthesized by a wet chemical method with further calcination at 600 °C. Mixtures of Na₄SiO₄ and alkali (Li/Na/K) carbonates were prepared by a mechanical mixing method. The CO₂ capture performance of the samples was characterized by dynamic thermogravimetric analyses and in situ XRD and Raman spectroscopy in 80 vol% CO₂ mixed with 20 vol% N₂. It was found that sodium orthosilicate could be used for CO₂ sorption, and its maximum capacity could reach up to 19.2 wt%. The addition of alkali carbonates, which serve as promoters, led to the enhancement of the CO₂ capture performance of Na₄SiO₄, especially at low temperatures, because of the formation of C₂O₅^2-. The existence of C₂O₅^2- in the mixture exposed to 80 vol% CO₂ was confirmed by in situ Raman spectra, and its geometric structure was presented by DFT calculations. The formation of C₂O₅^2- within carbonates exhibited a positive influence on the CO₂ capture at low temperatures and the enhancement of CO₂ diffusion by Grotthuss-like transport through the carbonate product shell at high temperatures besides the formation of eutectic carbonate melts.