High-Temperature Sorbents for CO₂ Made of Alkali Metals Doped on CaO Supports

A number of basic sorbents based on CaO were synthesized, characterized, and tested for the sorption of CO 2 and selected gas mixtures simulating flue gas. Our studies resulted in highly promising sorbents based on Cs/CaO, which demonstrated zero affinity for N 2 and O 2 and very low affinity for water. Moreover, we observed high CO 2 sorption capacities and rapid sorption/desorption characteristics in a wide temperature range reaching temperatures as high as 700 °C. The “key” feature of this family of sorbents is their basic nature, which allows for the selective chemisorption of CO 2 . These unique characteristics of this family of sorbents offer high promise for the development of advanced industrial sorbents for the effective CO 2 removal. The performance of the alkali metals as dopants on CaO follows the order Li < Na < K < Rb < Cs, which reveals a strong relationship between the sorption characteristics and the increase of the electropositivity or equivalently atomic radii of the alkali metals. From XPS investigations, including the surface atomic ratios of Cs 3d/C 1s, and Ca 2p/C 1s, it is observed that the adsorption of CO 2 is preferably taking place on Cs 2 O rather than CaO. The XPS analysis further show that the surface atomic ratio between Cs 3d 5/2 and Ca 2p core levels appreciably increase for the sorbents prepared with ∼20 wt % loading of Cs on CaO. For higher Cs loadings, the performance did not improve further due to the formation of paracrystalline Cs 2 O on the CaO support surface. CsOH- and Cs 2 CO 3 -doped CaO sorbents provided significantly higher adsorption of CO 2 than CsCl mainly due to the formation of a highly dispersed Cs 2 O on the CaO support.