Chemical Engineering Journal, Vol.366, 181-191, 2019
Roles for K2CO3 doping on elevated temperature CO2 adsorption of potassium promoted layered double oxides
Despite the great attraction of using potassium promoted magnesium-aluminum layered double oxides (K-LDOs) as elevated temperature CO2 adsorbents, the understanding of CO2 adsorption mechanism of K-LDOs is still confusing and controversial due to the complexity of adsorbent compositions. In this work, in situ techniques were adopted to verify the synergistic mechanism of K2CO3 doping (0-40 wt%) and Mg/Al mole ratio (0.55, 2.17, and 2.98) on the CO2 capture of K-LDOs. Before K2CO3 doping, the commercially available MG63 ([Mg0.69Al0.31(OH)(2)] (CO3) 0.16 center dot zH(2)O) exhibited the highest working CO2 capacity of 0.320 mmol/g at 400 degrees C and 1 atm. After doping with 20 wt% K2CO3, K-20-MG70 (Mg/Al ratio: 2.98) gave highest CO2 capacity of 0.722 mmol/g. At low CO2 partial pressures, however, K-20-MG30 (Mg/Al ratio: 0.55) with the lowest Mg/Al ratio owned the best capture performance. Results from in situ Fourier transform infrared spectroscopy indicate that the changeable CO2 adsorption performance of K-LDOs was controlled by two mechanisms. For K-LDOs with high Mg/Al ratios, the K2CO3 doping is mainly localized in the bulk phase, and acts as a reactant to form high stable K-Mg double carbonates after adsorbing CO2. With increasing the Al content, surface modification occurs and becomes the dominant enhancement mechanism via the interaction between K+ and unsaturated oxygen sites, which are generated by the partial substitution of Mg2+ with Al3+. The reversible formation of bidentate carbonates are the main CO2 species on K-Al2O3, K-LDOs, and K-MgO, whereas unidentate carbonates with a stronger binding affinity are only formed on K-20-MG30, providing a superior performance for the adsorption of low concentration CO2.