2, University of Technology Sydney, Sydney, New South Wales, Australia
3, Imperial College London, London, , United Kingdom
4, University of Sydney, Sydney, New South Wales, Australia
5, Calix, Ltd., Sydney, New South Wales, Australia
In recent years, sorbent-based CO2 capture technologies using metal oxides have been extensively studied. Among various metal oxides which can capture CO2 via adsorption, magnesium oxide (MgO) is among the most widely investigated because of its potential for pre-combustion CO2 capture applications such as coal gasification and natural gas reforming . While MgO has a theoretical CO2 capture capacity of ~24 mmol.g-1, its reaction with CO2 gas molecules is usually restricted by the surface product layer which limits its actual capacity to < 1 mmol.g-1 . In the last decade, much attention has been paid to improving the properties of MgO to realize its potential as a CO2 sorbent. Herein, we report a significant promoting effect of eutectic (binary, ternary and quaternary) alkali salt mixtures on the CO2 capture performance of MgO. The sorbents were prepared by wet impregnation of MgO with varying loadings of the eutectic mixtures. The physicochemical properties were characterized by various techniques such as BET, ICP-OES, XRD, FT-IR, CO2-TPD, SEM and TEM, and the capture capacity was evaluated using thermogravimetric analysis (TGA). The results suggest that the nature of the salt and its melting temperature (Tm) are crucial factors in the performance of the sorbent. The sorbent modified with NaNO3 (Tm ≈ 302 °C) exhibited the highest CO2 uptake of 11.9 mmol.g-1 (523.7 mg CO2 per 1 g of sorbent) after carbonation for one hour at 300 °C under ambient pressure while those modified with LiNO3 and KNO3 did not perform well. Interestingly, the use of eutectic mixtures of these salts not only improved the CO2 uptake but also broadened the operating temperature window. In the case of the (LiNaK)NO3- and (LiNaK)NO3-NaNO2-modified sorbents, the CO2 uptakes were better than that of the NaNO3-modified sorbent at temperatures below and above 300 °C. The results of the FT-IR spectroscopy reveal the presence of two types of carbonates (surface and bulk) while the XRD confirms the formation of well-crystallized MgCO3 after CO2 sorption. Based on these findings, a mechanism that involves a gas-liquid-solid interface as an alternative pathway for the reaction of gaseous CO2 and the solvated [Mg2+...O2-] ionic pairs will be discussed.
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