Despite extensive research on the extent and rate of MgO's reaction with CO₂ under various conditions, little is known about whether impurities incorporated into the solid, such as iron, enhance or impede hydroxylation and carbonation reactions. This is particularly important because Fe is a common impurity in MgO, and when assessing the economics of MgO looping as a direct air capture (DAC) technique, additional expenses and logistics need to be considered if pure material is required. Here, we used synchrotron x-ray scattering, grazing incidence x-ray diffraction, and electron microscopy to understand how iron doping impacts MgO reactivity and passivation layer formation under ambient conditions, varying humidity, and CO₂ exposure.
We categorized our findings into three conditions to determine surface structures and reactivity: effects of relative humidity over short periods of time (0-15 minutes in N₂), long periods (33% R.H and 75% R.H for 8 and 30 days in N₂) and in CO₂ (0-90 minutes at >90% R.H. and either 33% R.H or 75% R.H. for 30 days). Our short-term in-situ atomic force microscopy results suggest similar mechanisms for both the MgO and (Mg,Fe)O samples, with an increase in precipitate size over time. However, based on our transmission electron microscopy (TEM) and x-ray reflectivity (XRR) results, the reaction layer thickness and density are smaller and carbonates slower in the (Mg,Fe)O samples compared to pure MgO. Our long-term results indicate crystalline secondary phase formation on both the MgO and (Mg,Fe)O samples, with crystallinity of phases on (Mg,Fe)O decreasing at higher R.H. Our XRR results showed a thicker reaction layer at 75% R.H compared to 33% R.H. At both relative humidities, the pure MgO samples had a thicker reaction layer than the (Mg,Fe)O samples. Consistent with both our short term and long term finding, XRR analysis show that the reaction layer formed more rapidly and was thicker in pure MgO samples compared to (Mg,Fe)O samples.
Overall, our results show that iron impurities in the MgO disrupt hydrated phase formation and inhibit film growth during both hydroxylation and carbonation of the surface. The smaller reaction layer thicknesses for the (Mg,Fe)O samples suggests that naturally occurring impurities in MgO could reduce CO₂ uptake during direct air capture. Therefore, using MgO with minimal iron incorporated is likely preferable for mineral looping applications.

Gabriela Camacho

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