Armoring of MgO by a Passivation Layer Impedes Direct Air Capture of CO₂

Abstract

It has been proposed to use magnesium oxide (MgO) to separate carbon dioxide directly from the atmosphere at the gigaton level. We show experimental results on MgO single crystals reacting with the atmosphere for longer (decades) and shorter (days to months) periods with the goal of gauging reaction rates. Here, we find a substantial slowdown of an initially fast reaction as a result of mineral armoring by reaction products (surface passivation). In short-term experiments, we observe fast hydroxylation, carbonation, and formation of amorphous hydrated magnesium carbonate at early stages, leading to the formation of crystalline hydrated Mg carbonates. The preferential location of Mg carbonates along the atomic steps on the crystal surface of MgO indicates the importance of the reactive site density for carbonation kinetics. The analysis of 27-year-old single-crystal MgO samples demonstrates that the thickness of the reacted layer is limited to ∼1.5 μm on average, which is thinner than expected and indicates surface passivation. Thus, if MgO is to be employed for direct air capture of CO2, surface passivation must be circumvented.

Reaction Layer Formation on MgO in the Presence of Humidity

“…However, there is also no evidence of crystallinity in the BF-TEM images. Further research is needed to conclusively determine if the reaction layer is a crystalline phase or not, but previous studies have observed precipitation of amorphous magnesium hydroxide during reactions between magnesium sulfate and calcium hydroxide, as well as precipitation of amorphous phases during the reaction of MgO in ambient conditions for time periods shorter than 2 months. ,, …”

“…This implies the changes in density arise from reactions driven by water and its dissociation products, which may be able to continue accessing the reaction layer interface due to the roughness, nonuniformity, and/or permeability of the reaction layer observed with XRR and AFM. Since the longer-term reactions of Weber et al show evidence for crystallinity, the changes in density may be due to the reaction layer restructuring toward a more crystalline phase, presumably through the release of structural water …”

“…Recent work by Weber et al confirmed hydrated magnesium carbonate phases form on MgO surfaces on samples reacted in air for months to years, with the reaction rate slowing over time due to armoring of the surface by passivation layers . Based on diffusion modeling, the majority of the reaction occurs within the first two years . Rausis et al, had suggested that 90% carbonation of MgO will require 5–27 years, assuming little change in reaction rates over long periods of time .…”

“…Rausis et al, had suggested that 90% carbonation of MgO will require 5–27 years, assuming little change in reaction rates over long periods of time . However, Weber and coauthors observed that even after 27 years, 90% carbonation is not reached, suggesting that passivation has a significant impact . Knowledge of the formation conditions for the hydrated MgO phases is therefore critical for a successful DAC strategy.…”

“…This may be problematic for DAC as hydrated phase formation leads to less CO 2 uptake than would occur if MgCO 3 precipitates, based on the crystal volume per CO 2 molecule. Recent work by Weber et al confirmed hydrated magnesium carbonate phases form on MgO surfaces on samples reacted in air for months to years, with the reaction rate slowing over time due to armoring of the surface by passivation layers . Based on diffusion modeling, the majority of the reaction occurs within the first two years .…”