Molten Salt-Promoted MgO Adsorbents for CO₂ Capture: Transient Kinetic Studies

Abstract

Optimization of MgO adsorbents is predominantly focused on the regulation of appropriate adsorption sites for CO 2 associated with Mg 2+ -O 2− sites of low coordination. Here, for the first time, we conducted transient kinetic experiments to identify and characterize changes of the CO 2 molecular path in MgO-based CO 2 adsorbents upon the addition of molten salt modifiers. Among the optimized samples, addition of 10 mol % NaNO 2 on the surface of MgO exhibited the highest CO 2 uptake (15.7 mmol g −1 ) at 350 °C compared to less than 0.1 mmol g −1 for the unpromoted MgO. Kinetic modeling showed that the interaction of molten salt-promoted MgO with CO 2 at 300 °C involves three different processes, namely, fast surface adsorption associated with surface-active basic sites, chemical reaction associated with MgCO 3 formation, and a slow diffusion step being the rate-limiting step of the carbonation process. Furthermore, transient kinetic studies coupled with mass spectrometry under low CO 2 partial pressure agreed well with the kinetic simulation results based on TGA measurements, demonstrating an in-depth understanding of the CO 2 -capturing performance gained and its considerable significance for future practical designs of precombustion CO 2 capture.

Unravelling the Mechanism of Intermediate‐Temperature CO2 Interaction with Molten‐NaNO3‐Salt‐Promoted MgO

“…This indicated the conversion of sodium nitrite to sodium nitrate, while MgO still remained in its pure bulk crystalline structure as also reported. [ 23 ] Detailed X‐ray photoelectron spectra of N 1s region confirmed that the surface was enriched with NaNO 2 /NaNO 3 species (Figure 1b). [ 26 ] The X‐ray photoelectron spectroscopy (XPS) spectrum of fresh NaNO 2 ‐promoted MgO sample depicted in Figure 1b shows a peak at 403.8 eV, which is attributed to the N 1s core level of NO 2 − .…”

“…The latter is due to the lower diffraction intensities derived from the various planes of NaNO 2 and its smaller amount (wt%) present in the sample. [23,28] Scanning electron microscopy (SEM) studies were further conducted on NaNO 2 -promoted MgO after activation, and typical adsorbent agglomerated particles are shown in Figure S1a, Supporting Information. Furthermore, energy-dispersive X-ray spectroscopy (EDS) analysis (N, O, Na, and Mg) indicated that the molten salt was evenly dispersed on the MgO surface (Figure S1b, Supporting Information).…”

“…A recent work (also mentioned above) proved that MgCO 3 formation is highly oriented at the NaNO 3 /MgO interface as opposed to a precipitation with random orientation in the molten salt. [ 16 ] Transient kinetic studies on the present molten NaNO 3 ‐salt‐promoted MgO were recently reported, [ 23 ] where results of the effect of promoter loading and CO 2 adsorption temperature in the 275–375 °C range were presented and discussed.…”

CO2 Capture at Medium to High Temperature Using Solid Oxide-Based Sorbents: Fundamental Aspects, Mechanistic Insights, and Recent Advances

“…Given the lower sorption temperatures of MgO compared to CaO, a popular approach to enhance the kinetics of CO 2 uptake of MgO is its promotion with alkali metal nitrates such as LiNO 3 , NaNO 3 , and KNO 3 . ,− Webley and co-workers , studied adsorbents based on double salts of MgCO 3 with either K 2 CO 3 or Na 2 CO 3 prepared by wet mixing; others investigated also Cs 2 CO 3 or Rb 2 CO 3 . , K–Mg double salts reached a CO 2 uptake of 8 wt % at 375 °C in pure CO 2 , while Na–Mg double salts showed higher CO 2 uptakes reaching between 15 wt % and ∼20.7 wt % . Zhang et al .…”

Capture and electrochemical conversion of CO2 in molten alkali metal borate–carbonate blends

“…The overall kinetics of the CO 2 uptake by the molten salts with the nucleation and growth of the carbonate crystalline particles, which can eventually form diffusional barriers for further uptake can be qualitatively described by considering the time intervals over which the uptake rate changes separately. 12,45,46 The initial CO 2 uptake can be described by the pseudo firstorder reaction kinetics: 12,45…”