Carbon Dioxide Absorption Behavior and Carbonate Ion Transport of Lithium Orthosilicate / Potassium Carbonate Coexistence System Prepared By Ball Milling

Lithium Orthosilicate (Li₄SiO₄), a type of ceramic CO₂ absorbent has a large absorptive capacity (36.7 wt. %) and also has heat resistance. This material has some useful application as a separator in Molten Carbonate Fuel Cell and as a direct CO₂ absorbent in large-scale emission sources, for example, thermal power stations; because the following equilibrium reaction at the temperature around 993 K; Li₄SiO₄ + CO₂  ⇄  Li₂CO₃ + Li₂SiO_3. In this study, we focused on the absorption behaviors of CO₂ at lower temperature into Li₄SiO₄ in combination of these two methods and evaluated the physiochemical dependence of surface properties of CO₂ absorption reaction.

Li₄SiO₄ was prepared from solid state reaction. SiO₂ powder and Li₂CO₃ powder were mixed at the molar ratio Li/Si = 4.0. The mixture was calcined at 973 K for 20 h in air. The resulting powder (Li₄SiO₄) kept in an Argon atmosphere glove box and in order to confirm the effect of carbonate, was mixed with a given amount of K₂CO₃. Each mixture (2.5 g) was put in the grinding bowl with the grinding balls, it sealed up under the Ar atmosphere, and the grinding processing was done for 0-300 minutes in revolution speed 250 rpm using planetary ball mill.              Also, the grinding process was done under the condition of one cycle for 15 minutes of the grinding processing and two minutes of rest time to prevent the samples temperature from rising. Samples were characterized with X-ray diffraction (XRD), N₂-isotherm, SEM, SEM-EDX, TG-DTA and Electro spin resonance (ESR).

It was found that samples whose surface was modified with ball milling method under Ar gas flow showed a tendency to increase the surface area estimated by BET method measured by N₂-isotherm and increase of Si dangling bond measured by ESR. Also, it was found that there are two different absorption processes, one is a chemisorption at the surface (T<773 K) the other is a bulk diffusion (T>773 K) by the result of TG-DTA. As seen in isothermal analysis (Fig. 1), the CO₂ absorb reaction rate improved by the coexistence of K₂CO₃ infiltrated in Li₄SiO₄ being promoted by not only the surface area was increased and activated by the ball milling but also absorbing CO₂ of coexisting K₂CO₃. Furthermore, it was found that ΔG^‡ decreased from the result of the isothermal analysis at 873 K. We would like to consider controlling transportation of carbonate ion electrochemically according to the CO₂ absorption reaction as a separator in Molten Carbonate Fuel Cell.