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

Abstract

A number of basic sorbents based on CaO were synthesized, characterized, and tested for the sorption of CO2 and selected gas mixtures simulating flue gas. Our studies resulted in highly promising sorbents based on Cs/CaO, which demonstrated zero affinity for N2 and O2 and very low affinity for water. Moreover, we observed high CO2 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 CO2. These unique characteristics of this family of sorbents offer high promise for the development of advanced industrial sorbents for the effective CO2 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 CO2 is preferably taking place on Cs2O rather than CaO. The XPS analysis further show that the surface atomic ratio between Cs 3d5/2 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 Cs2O on the CaO support surface. CsOH- and Cs2CO3-doped CaO sorbents provided significantly higher adsorption of CO2 than CsCl mainly due to the formation of a highly dispersed Cs2O on the CaO support.