Deciphering the structural dynamics in molten salt–promoted MgO-based CO ₂ sorbents and their role in the CO ₂ uptake

Abstract

The development of effective CO 2 sorbents is vital to achieving net-zero CO 2 emission targets. MgO promoted with molten salts is an emerging class of CO 2 sorbents. However, the structural features that govern their performance remain elusive. Using in situ time-resolved powder x-ray diffraction, we follow the structural dynamics of a model NaNO 3 -promoted, MgO-based CO 2 sorbent. During the first few cycles of CO 2 capture and release, the sorbent deactivates owing to an increase in the sizes of the MgO crystallites, reducing in turn the abundance of available nucleation points, i.e., MgO surface defects, for MgCO 3 growth. After the third cycle, the sorbent shows a continuous reactivation, which is linked to the in situ formation of Na 2 Mg(CO 3 ) 2 crystallites that act effectively as seeds for MgCO 3 nucleation and growth. Na 2 Mg(CO 3 ) 2 forms due to the partial decomposition of NaNO 3 during regeneration at T ≥ 450°C followed by carbonation in CO 2 .

Probing the Local Structure of Na in NaNO3-Promoted, MgO-Based CO2 Sorbents via X-ray Absorption Spectroscopy

“…One hypothesis to explain the changes observed in the Na K-edge XANES spectra upon carbonation could be the decomposition of NaNO 3 , e.g., into NaNO 2 or Na 2 O which would transform further under carbonation conditions (CO 2 , 315 °C) to sodium-containing carbonates Na 2 CO 3 and/or Na 2 Mg­(CO 3 ) 2 , as previously observed . However, according to Rekhtina et al, the decomposition of NaNO 3 does not take place below 450 °C.…”

“…One hypothesis to explain the changes observed in the Na K-edge XANES spectra upon carbonation could be the decomposition of NaNO 3 , e.g., into NaNO 2 or Na 2 O which would transform further under carbonation conditions (CO 2 , 315 °C) to sodium-containing carbonates Na 2 CO 3 and/or Na 2 Mg(CO 3 ) 2 , as previously observed. 42 However, according to Rekhtina et al, the decomposition of NaNO 3 does not take place below 450 °C. Also, Na 2 CO 3 and Na 2 Mg(CO 3 ) 2 could only be formed over repeated carbonation and regeneration ( T ≥ 450 °C) cycles through the partial decomposition of NaNO 3 and the carbonation of the decomposition products, as previously observed by Rekhtina et al 42 Since the carbonation temperature was considerably below the decomposition temperature (315 °C), NaNO 3 decomposition is not expected.…”

“… 42 However, according to Rekhtina et al, the decomposition of NaNO 3 does not take place below 450 °C. Also, Na 2 CO 3 and Na 2 Mg(CO 3 ) 2 could only be formed over repeated carbonation and regeneration ( T ≥ 450 °C) cycles through the partial decomposition of NaNO 3 and the carbonation of the decomposition products, as previously observed by Rekhtina et al 42 Since the carbonation temperature was considerably below the decomposition temperature (315 °C), NaNO 3 decomposition is not expected. In addition, we compared the XANES spectra of the carbonated sorbent with the most likely NaNO 3 decomposition products (under ambient or CO 2 capture conditions), i.e., Na 2 CO 3 , Na 2 Mg(CO 3 ) 2 , NaOH, and NaNO 2 ( Figure 1 c).…”

NaNO3-Promoted MgO-Based Adsorbents Prepared from Bischofite for CO2 Capture: Experimental and Density Functional Theory Study

“…ads system substrate adsorbate (5) where E system represents the total energy of the adsorbate adsorbed on the substrate. E substrate refers to the energy of the substrate (pure MgO and NaNO 3 coadsorption with MgO).…”

“…With the continuous concern of people on global climate change, CO 2 capture, as an important means to reduce carbon emissions, has received extensive attention from countries around the world. − Compared with high-temperature CO 2 capture adsorbents such as CaO, , Na 4 SiO 4 , , Li 4 SiO 4 , , and ZrSiO 4 , MgO is considered a potential CO 2 capture material because of its wide range of sources, low application temperature (200–400 °C), and high theoretical CO 2 adsorption capacity (110 wt %). − Although MgO has broad application potential in CO 2 capture at intermedium temperature, its actual adsorption capacity (<0.5 mmol/g) is small because of its small specific surface area and slow reaction kinetics . In addressing this problem, Harada and co-workers confirmed that alkali metal nitrate/nitrite doping can significantly improve the performance of MgO for CO 2 adsorption. , Gao et al prepared a series of MgO-based adsorbent materials doped with MNO 3 /NO 2 (M = Li, Na, and K) and demonstrated that NaNO 3 was the key substance contributing to the enhanced adsorption capacity of MgO.…”