Shedding Light on Solid Sorbents: Evaluation of Supported Potassium Carbonate Particle Size and Its Effect on CO 2 Capture from Air.

Solid sorbents are essential for developing technologies that directly capture CO 2 from air. In solid sorbents, metal oxides and/or alkali metal carbonates such as potassium carbonate (K 2 CO 3 ) are promising active components owing to their high thermal stability, low cost, and ability to chemisorb the CO 2 present at low concentrations in air. However, this chemisorption process is likely limited by internal diffusion of CO 2 into the bulk of K 2 CO 3 . Therefore, the size of the K 2 CO 3 particles is expected to be an important factor in determining the kinetics of the sorption process during CO 2 capture. To date, the effects of particle size on supported K 2 CO 3 sorbents are unknown mainly because particle sizes cannot be unambiguously determined. Here, we show that by using a series of techniques, the size of supported K 2 CO 3 particles can be established. We prepared size-tuned carbon-supported K 2 CO 3 particles by tuning the K 2 CO 3 loading. We further used melting point depression of K 2 CO 3 particles to collectively estimate the average K 2 CO 3 particle sizes. Using these obtained average particle sizes, we show that the particle size critically affects the efficiency of the sorbent in CO 2 capture from air and directly affects the kinetics of CO 2 sorption as well as the energy input needed for the desorption step. By evaluating the mechanisms involved in the diffusion of CO 2 and H 2 O into K 2 CO 3 particles, we relate the microscopic characteristics of sorbents to their macroscopic performance, which is of interest for industrial-scale CO 2 capture from air.