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

Abstract

The optimization of MgO‐based adsorbents as advanced CO2‐capture materials is predominantly focused on their molten‐salt modification, for which theoretical and experimental contributions provide great insights for their high CO2‐capture performance. The underlying mechanism of the promotion effect of the molten salt on CO2 capture, however, is a topic of controversy. Herein, advanced experimental characterization techniques, including in situ environmental transmission electron microscopy (eTEM) and CO2 chemisorption by diffuse‐reflectance infrared Fourier transform spectroscopy (DRIFTS), transient 18O‐isotopic exchange, and density functional theory (DFT), are employed to elucidate the mechanism of the CO2 interaction with molten‐salt‐modified MgO in the 250–400 °C range. Herein, eTEM studies using low (2–3 mbar) and high (700 mbar) CO2 pressures illustrate the dynamic evolution of the molten NaNO3 salt promoted and unpromoted MgO carbonation with high magnification (<50 nm). The formation of 18O‐NaNO3 (use of 18O2) and C16O18O following CO2 interaction, verifies the proposed reaction path: conversion of NO3− (NO3− → NO2+ + O2–), adsorption of NO2+ on MgO with significant weakening of CO2 adsorption strength, and formation of [Mg2+… O2−] ion pairs preventing the development of an impermeable MgCO3 shell, which largely increases the rate of bulk MgO carbonation.

Uncovering the CO2 Capture Mechanism of NaNO3-Promoted MgO by 18O Isotope Labeling

“…It has been suggested that the decomposition products of molten alkali metal nitrates, specifically O 2– , NO 2 – , and NO 2 + , might play a role in the promotion mechanism of MgO-based CO 2 sorbents. ,, In molten alkali metal nitrates, the nitrate ion can self-dissociate according to normalN normalO 3 − ⇌ normalN normalO 2 + + normalO 2 − …”

“…To develop further MgO-based sorbents for CO 2 capture, it is critical to elucidate the key intermediate steps in the carbonation reaction of alkali metal nitrate-promoted MgO. In this context, isotope labeling is a useful technique to unravel reaction mechanisms as it allows to trace atoms during reactions. − Recently, Gao et al have used 18 O labeling to investigate the carbonation mechanism of NaNO 3 -promoted MgO . Based on transient 18 O isotopic exchange experiments using 18 O-labeled NaNO 3 , the authors propose that the molten salt actively participates in the carbonation reaction by dissociating into NO 2 + and O 2– .…”

Structural modification of salt-promoted MgO sorbents for intermediate temperature CO2capture

“…The carbonation causes the volume expansion of particles, resulting in more interfaces. According to Bork et al , 29 Gao et al , 30 and Lee et al , 31 the surface volume expansion from MgO to MgCO 3 was reported to be within hundreds of nanometers. However, the empty spaces in the modified morphology had a thickness of 1–2 μm.…”

Model structures of molten salt-promoted MgO to probe the mechanism of MgCO3 formation during CO2 capture at a solid–liquid interface

“…9,10,12 Although there is consensus on the promoting effect of alkali metal salts, there is an ongoing debate on the mechanism(s) through which alkali metal salt promotes carbonate formation with accelerated rates. 1,9,10,[13][14][15][16] One hypothesis considers alkali metal salts (AMS) as phase transfer catalysts that dissolve MgO yielding solvated ionic [Mg 2+ .O 2À ] pairs that have weaker bonds compared to the strong ionic bond of bulk MgO. According to density functional theory calculations by Zhang et al the dissociation of MgO into a [Mg 2+ .O 2À ] pair requires 7.1 eV (in vacuum) while only 5.3 eV is required in NaNO 3 due to solvation effects.…”