Structure of Na Species in Promoted CaO‐Based Sorbents and Their Effect on the Rate and Extent of the CO<sub>2</sub> Uptake
Maximilian Krödel, Lorenz Abduly, Manouchehr Nadjafi, Agnieszka Kierzkowska, Alexander V. Yakimov, Alexander H. Bork, Felix Donat, Christophe Copéret, Paula M. Abdala, Christoph R. Müller
Abstract
Abstract To advance CaO‐based CO 2 sorbents it is crucial to understand how their structural parameters control the cyclic CO 2 uptake. Here, CaO‐based sorbents with varying ratios of Na 2 CO 3 :CaCO 3 are synthesized via mechanochemical activation of a mixture of Na 2 CO 3 and CaCO 3 to investigate the effect of sodium species on the structure, morphology, carbonation rate and cyclic CO 2 uptake of the CO 2 sorbents. The addition of Na 2 CO 3 in the range of 0.1–0.2 mol% improves the CO 2 uptake by up to 80% after 10 cycles when compared to ball‐milled bare CaCO 3 , while for Na 2 CO 3 loadings >0.3 mol% the cyclic CO 2 uptake decreases by more than 40%. Energy dispersive X‐ray spectroscopy (EDX), transmission electron microscopy, X‐ray absorption spectroscopy (XAS), and 23 Na MAS NMR, reveal that in sorbents with Na 2 CO 3 contents <0.3 mol% Na exists in highly distributed, noncrystalline [Na 2 Ca(CO 3 ) 2 ] units. These species stabilize the surface area of the sorbent in pores of diameters >100 nm, and enhance the diffusion of CO 2 through CaCO 3 . For Na 2 CO 3 contents >0.3 mol%, the accelerated deactivation of the sorbents via sintering is related to the formation of crystalline Na 2 Ca(CO 3 ) 2 and the high mobility of Na.