Cycles of carbon capture using sol–gel Li4SiO4 particles

https://doi.org/10.1039/C6TA06133H

“The CO₂ absorption capacity values of the particles in the temperature range of 673–973 K are compared in Fig. 2e. For collecting the absorption data, the samples were heated to the absorption temperature at 10 °C min⁻¹ in 100% N₂ gas and kept for 2 h under a 100% CO₂ gas flow. As expected, the amount of CO₂ absorbed was found to increase with increase in the absorption temperature. This relationship between temperature and absorption capacity as well as the mechanism of absorption are better expressed using kinetic constants and activation energy values calculated from the Arrhenius plots of kinetic constants as shown in Fig. 2f. Further details on the calculation of kinetic constants are included in section S5 of the ESI.† It should be noted that, contrary to the general trend, Fig. 2f indicates that the K₂ values are approximately 10 times larger than those of K₁. The larger K₂ values obtained here signifies faster lithium ion diffusion to the reaction interface compared to the chemisorption reaction.²⁹ The activation energy values calculated from the plots were 22.70 kJ mol⁻¹ for the chemisorption process (corresponding to K₁) and 61.10 kJ mol⁻¹ for the diffusion process (corresponding to K₂). The higher activation energy value for the diffusion process substantiated the extremely fast diffusion of lithium ions from the core of the material to the reactive interface at absorption temperatures.

Absorption rates were also calculated from the first two minutes of absorption curves at different temperatures. The particles synthesised through a microwave sol–gel method displayed an enhanced absorption rate of 0.093 wt% s⁻¹ at 973 K. This value is much higher than the values reported in the recent literature (see table in ESI S1† for a comparison of some of the reported values). This enhanced absorption rate should mainly be attributed to the nano-rod morphology characterized by a very small thickness/width of the particle, facilitating a rapid surface carbonate layer formation over the entire length during the first stage of absorption. Moreover, the rod morphology should also have enabled easier surface reaction providing shorter diffusion pathways for lithium from the bulk to the surface of the particle.

The cyclic stability and regenerability of the powder samples were evaluated through cyclic absorption–desorption measurements and the results are shown in Fig. 2g [absorption with 100% CO₂ and desorption with 100% N₂ gases]. The primary aim of this cyclic loading experiment was to examine whether the sintering of the particles or the segregation of the carbonate phase due to continuous use of the absorbent at high temperatures induced any decay in absorption performance. The initial absorption run was done at 973 K; thereafter the temperature of absorption was switched to 873 K and cyclic absorption–desorption performance for 9 consecutive cycles was recorded. The initial run at high temperature leading to more or less full conversion of the material to Li₂CO₃ and further to the Li₄SiO₄ phase helped to realize higher absorption values at 873 K (Fig. 2e and g). As shown in Fig. 2g the samples displayed consistent absorption–desorption performance for all the 10 cycles measured indicating high durability and cyclic stability of the materials. Furthermore, it should be noted that the desorption rate was better than the absorption rate through all the measurements. The large desorption rate obtained without thermal cycling would allow the application of the materials for CO₂ capture in pressure swing mode. Although, further cycling studies of thousands of cycles may be necessary before considering the material for real life applications, initial results as reported in Fig. 2g indicated that the microwave sol–gel powders may be considered as promising materials for industrial absorption applications.”

“Fig. 2 TEM images (a and b) of microwave sol–gel Li₄SiO₄ particles after the CO₂ absorption process; (c and d) particles after desorption process. (e) CO₂ absorption curves at various temperatures. (f) Graph of ln K versus 1/T for the two different processes of chemisorption (K₁) and diffusion (K₂) observed in the microwave sol–gel sample; (g) absorption–desorption performance of microwave sol–gel Li₄SiO₄ powders for 10 cycles. The first cycle of the run was done at 973 K and further runs at 873 K. In all cases, desorption was carried out by switching 100% CO₂ gas to 100% N₂ gas.”