Inhibition of Reaction Layer Formation on MgO(100) by Doping with Trace Amounts of Iron

Abstract

Despite extensive research on MgO's reactivity in the presence of CO₂ under various conditions, little is known about whether impurities incorporated into the solid, such as iron, enhance or impede hydroxylation and carbonation reactions. The purity of the MgO required for the successful implementation of MgO looping as a direct air capture technology affects the deployment costs. With this motivation, we tested how incorporated iron impacts MgO (100) reactivity and passivation layer formation under ambient conditions by using atomic force microscopy, electron microscopy, and synchrotron-based X-ray scattering. Based on electron microprobe analysis, our MgO samples were 0.5 wt % iron, and Mössbauer spectroscopy results indicated that 70% of the iron is present as Fe^(II). We find that even these low levels of iron dopants impeded both the hydroxylation at various relative humidities (10%, 33%, 75%, and >95%) and carbonation in CO₂ (33%, 75%, and >95%) on the (100) surface. Crystalline reaction products were formed. Reaction layers on the sample were easily removed by exposing the sample to deionized water for 2 min. Overall, our findings demonstrate that the presence of iron dopants slows the reaction rate of MgO, indicating that MgO without incorporated iron is preferable for mineral looping applications.

Figure 1

In situ AFM images of (Mg,Fe)O in contact with MgO-saturated solution, adjusted pH = 12.46. (a) Height mode AFM image of a freshly cleaved (Mg,Fe)O sample. (b) Small precipitates are visible after 3 min reaction with MgO-saturated solution. (c) After 28 min of reaction, the precipitates are larger. (d) After 152 min of reaction, large precipitates are visible.

Figure 2

STEM-EELS maps of (Mg,Fe)O reacted overnight at 11% RH (sample name: (Mg,Fe)O 11p-2). (a), (c), and (e) show high angle annular dark field (HAADF) images of interface reaction layer. Spectraintegrated over the colored rectanglesare given by the graphs of respective colors in the figures (b), d), and (f). Red rectangles are carbon coating, yellow/light blue are interface, and dark blue are bulk (Mg,Fe)O).

Figure 3

STEM-EELS maps of (Mg,Fe)O reacted at 11% RH for 4 h and a total of 15 min at >95% N₂ (sample name: (Mg,Fe)O 11p-4h). (a), (c), and (e) show high angle annular dark field (HAADF) images of interface reaction layer. Spectra integrated over the colored rectangles are given by the graphs of respective colors in the figures(b), (d), and (f). Red rectangles are carbon coating, light blue are interface, and dark blue are bulk (Mg,Fe)O).

Figure 4

XRR profile (a) of MgO and (Mg,Fe)O samples reacted in humid N₂ from 0 to 15 min and the scattering length density (SLD) profiles (b) from the fits of the data. XRR profile (c) for MgO and (Mg,Fe)O samples reacted in air at 33% RH and 75% RH for 8 days and their scattering length density (SLD) profiles (d) from the fits of the data. MgO data reproduced from reference. Copyright 2024 American Chemical Society.

Figure 5

Electron microscopy characterization of (Mg,Fe)O reacted for 8 days in air at 33% RH (a–c) and at 75% RH (d–f).

Figure 6

Electron microscopy results of MgO (a–c) and (Mg,Fe)O (d–f) ex situ carbonation experiments. Samples were reacted at 75% RH for 30 days in the presence of CO₂. Image (a)–(c) are from Yang et al., 2025. MgO data reproduced from reference. Copyright 2025 American Chemical Society.

Figure 7

XRR profile (a) of MgO and (Mg,Fe)O samples reacted in humid CO₂ for 0–90 min and their scattering length density (SLD) profiles (b) from the fits of the data. XRR profile (c) of MgO and (Mg,Fe)O samples reacted under 75% RH in air or CO₂ and their scattering length density (SLD) profiles (d) from the fits of the data. MgO data reproduced from reference. Copyright 2025 American Chemical Society.