Effects of porous carbon additives on the CO₂ absorption performance of lithium orthosilicate

Lithium orthosilicate (Li₄SiO₄) is an attractive high-temperature CO₂ sorbent (>650 °C) because of its large theoretical absorption capacity of up to 36.7 wt%. However, slow kinetics and partial reactions with CO₂ hinder its proper operation as a sorbent under practical conditions. To allow the use of this sorbent at lower operation temperatures, the present studies explored the way to improve the CO₂ absorption kinetics and increase the degree of reaction of Li₄SiO₄. Porous carbon materials such as CMK-3 were introduced into the sorbent to provide an internal gas pathway. Upon calcination conditions, the carbon amount was controlled in the composites (Li₄SiO₄@CMK-X%, where X represents the amounts of CMK-3). In Li₄SiO₄@CMK-1.8%, CMK-3 is distributed over the whole solid; in contrast, the additive in Li₄SiO₄@CMK-0.5% is mainly observed near the surface of the solid. CO₂ gas sorption study of the composites showed that pores of CMK-3 in Li₄SiO₄ aid the diffusion of CO₂. In addition, we found that the incorporation of porous carbon provides more active sites for interactions with CO₂ through the formation of cavities between Li₄SiO₄ and CMK-3. Li₄SiO₄@CMK-1.8% had an increased CO₂ absorption capacity (35.4 wt%) and rate (15.2 wt% for the first 5 min) at 600 °C, compared to the CO₂ absorption capacity (16.3 wt%) and rate (5.1 wt% for the first 5 min) of pristine Li₄SiO₄ (p-Li₄SiO₄). To confirm the influence of porous carbon on the CO₂ absorption properties, multi-walled carbon nanotube (MWCNT) was also examined as an additive instead of CMK-3. Li₄SiO₄@CNT showed similar trends with Li₄SiO₄@CMK sorbents.