Roles for K2CO3 doping on elevated temperature CO2 adsorption of potassium promoted layered double oxides

Abstract

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·zH2O) exhibited the highest working CO2 capacity of 0.320 mmol/g at 400 °C and 1 atm. After doping with 20 wt% K2CO3, K20-MG70 (Mg/Al ratio: 2.98) gave highest CO2 capacity of 0.722 mmol/g. At low CO2 partial pressures, however, K20-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 K20-MG30, providing a superior performance for the adsorption of low concentration CO2.