In situ Raman and XRD study of CO2 sorption and desorption in air by a Na4SiO4–Na2CO3 hybrid sorbent

Abstract

Silicate–carbonate mixtures as new CO₂ capture agents have the latent application potential. CO₂ sorption or desorption processes using the Na₄SiO₄–Na₂CO₃ mixture sorbent in air were analyzed by in situ Raman spectroscopy and X-ray diffraction from 25 °C to 900 °C. The results show that the Na₄SiO₄–Na₂CO₃ mixture sorbent could continuously absorb and strip CO₂ by thermal swinging. The CO₂ sorption was produced via a two-step process depending on the temperature range. Initially, CO₂ dissolved in carbonate to produce pyrocarbonate (C₂O₅²⁻) ions, which subsequently reacted with SiO₄⁴⁻ anion to produce the polymer silicates and CO₃²⁻ anion. The C₂O₅²⁻ anion on the surface of the silicates promoted CO₂ transformation to CO₃²⁻ anion through the reaction with SiO₄⁴⁻ anions. The CO₃²⁻ anion decomposed the polymer silicates to produce orthosilicates and CO₂ gas again at high temperature. By this circulation, CO₂ could dissolve in carbonate more easily and be absorbed and stripped continuously by thermal swinging in the mixture sorbent than the pure carbonate. The processes of recovering heat and separating CO₂ from flue gas simultaneously without decreasing the temperature is an economical and attractive method for energy conservation. It offers the theoretical basis for developing new heat-storage and CO₂-capture technology.