CO{sub 2} capture properties of lithium silicates with different ratios of Li{sub 2}O/SiO{sub 2}: an ab initio thermodynamic and experimental approach

The lithium silicates have attracted scientific interest due to their potential use as high-temperature sorbents for CO{sub 2} capture. The electronic properties and thermodynamic stabilities of lithium silicates with different Li{sub 2}O/SiO{sub 2} ratios (Li{sub 2}O, Li{sub 8}SiO{sub 6}, Li{sub 4}SiO{sub 4}, Li{sub 6}Si{sub 2}O{sub 7}, Li{sub 2}SiO{sub 3}, Li{sub 2}Si{sub 2}O{sub 5}, Li{sub 2}Si{sub 3}O{sub 7}, and a-SiO{sub 2}) have been investigated by combining first-principles density functional theory with lattice phonon dynamics. All these lithium silicates examined are insulators with band-gaps larger than 4.5 eV. By decreasing the Li{sub 2}O/SiO{sub 2} ratio, the first valence bandwidth of the corresponding lithium silicate increases. Additionally, by decreasing the Li{sub 2}O/SiO{sub 2} ratio, the vibrational frequencies of the corresponding lithium silicates shift to higher frequencies. Based on the calculated energetic information, their CO{sub 2} absorption capabilities were extensively analyzed through thermodynamic investigations on these absorption reactions. We found that by increasing the Li{sub 2}O/SiO{sub 2} ratio when going from Li{sub 2}Si{sub 3}O{sub 7} to Li{sub 8}SiO{sub 6}, the corresponding lithium silicates have higher CO{sub 2} capture capacity, higher turnover temperatures and heats of reaction, and require higher energy inputs for regeneration. Based on our experimentally measured isotherms of the CO{sub 2} chemisorption by lithium silicates, we found that the CO{sub 2} capture reactions are two-stage processes: (1) a superficial reaction to form the external shell composed of Li{sub 2}CO{sub 3} and a metal oxide or lithium silicate secondary phase and (2) lithium diffusion from bulk to the surface with a simultaneous diffusion of CO{sub 2} into the shell to continue the CO{sub 2} chemisorption process. The second stage is the rate determining step for the capture process. By changing the mixing ratio of Li{sub 2}O and SiO{sub 2}, we can obtain different lithium silicate solids which exhibit different thermodynamic behaviors. Based on our results, three mixing scenarios are discussed to provide general guidelines for designing new CO{sub 2} sorbents to fit practical needs