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In situ FTIR spectroscopy was used to study carbonate transformations during adsorption and desorption of CO2 in K-promoted HTlc at 450 °C just after activation. It revealed one irreversible process associated with the slow formation of polydentate carbonate during both adsorption and desorption, which explains CO2 capacity fade with cycling. It also revealed several reversible processes associated with the disappearance of free carbonate ion and the formation of unidentate, bidentate, and bridged (surface) carbonates during the adsorption of gaseous CO2 on active sites (highly basic, metal-bound unsaturated oxygen atoms) and vice versa during desorption. As the active sites became depleted during adsorption, free carbonate also transformed into bidentate, bidentate formation continued throughout adsorption, and unidentate and bridged carbonates began to disappear, possibly transforming irreversibly into polydentate. Once enough active sites became available during desorption, bidentate also began to transform back into free carbonate, unidentate and bridged carbonates continued to disappear throughout desorption, and bidentate disappearance and free carbonate formation both ceased. The slow formation of bidentate during adsorption and the slow disappearance of unidentate and bridged carbonates during desorption explains the never ending CO2 uptake and release. Changes in active site and carbonate basicity were the driving force behind K-promoted HTlc being a reversible adsorbent for CO2 at around 450 °C.
Du et al. (Thu,) studied this question.