Thermodynamics and kinetics analyses of high CO₂absorption properties of Li₃NaSiO₄under various CO₂partial pressures

The properties of Li₃NaSiO₄as a CO₂absorbent were evaluated by thermogravimetry (TG) and X-ray diffraction, which revealed that the CO₂absorption and desorption reaction, Li₃NaSiO₄+ CO₂↔ LiNaCO₃+ Li₂SiO₃, is reversible. In scanning-type TG under various CO₂partial pressures (P(CO₂)), mass gain ascribed to CO₂absorption started from approximately 400 °C, irrespective ofP(CO₂). CO₂desorption was observed at a higher temperature, which was considered the approximate equilibrium temperature of the above-mentioned reaction. A pseudo-Ellingham diagram of the reaction between Li₃NaSiO₄and CO₂, constructed using the obtained approximate equilibrium temperature under eachP(CO₂), showed similar behavior to that between Li₄SiO₄and CO₂, especially between 10⁻²and 10⁻¹bar ofP(CO₂). Kinetics analysis by isothermal TG using the Jander model revealed that the reaction rate of CO₂absorption of Li₃NaSiO₄was higher than that of Li₄SiO₄. The rate-determining step of CO₂absorption by Li₃NaSiO₄was the diffusion process at the examined temperatures andP(CO₂), which was different from the rate-determining step of the reaction between Li₄SiO₄and CO₂under lowP(CO₂).