Quantification of the volume of MgCO3 formed during CO2 capture

https://doi.org/10.1039/D2TA02897B

“To visualize and quantify the volume of the magnesium carbonate formed through CO₂ absorption at the bottom of the grooves in the NaNO₃–MgO_B model system, we used 3D optical profilometry (Fig. 7). The MgCO₃ formed appeared at heights with z > 0 μm with respect to the originally flat MgO surface (z = 0 μm). Fig. 6a and b visualize a series of MgCO₃ particles at the bottom of two different grooves with depths of, respectively, 11 and 44 μm (the samples were exposed to CO₂ for 5 hours and rinsed with water prior to the measurements, NaNO₃–MgO_B-5hours-CO2). We observe that in the first 2 hours of carbonation the MgCO₃ structures grow with equal rates in all three dimensions i.e. after 2 hours of carbonation the particles have a width and thickness of 2 μm. Subsequently, MgCO₃ grows largely in a 2 dimensional fashion, i.e. parallel to the MgO plane (i.e. after 5 hours of carbonation MgCO₃ has a thickness of ca. 2 μm and lateral dimension of 10 μm) leading to the formation of “2D islands” that in some cases merge to form larger agglomerates. This observation is in agreement with a previous study that reported the formation of 2D islands of MgCO₃ (thickness in the range of 0.2–2 μm and lateral dimension in the range 10–50 μm) after 5 hours of carbonation of a NaNO₃-promoted single crystals of MgO(100).¹⁵ From our profilometry results acquired at the bottom of various grooves, it is clear that shallower grooves (e.g. depth of 11 μm) contain MgCO₃ particles that have a larger volume (when normalized by the available surface area of MgO at the bottom of the groove) than those formed in deeper grooves (e.g. of depth 44 μm). This is in agreement with our SEM results (Fig. S9†) revealing that the bottom of shallow grooves are almost fully covered by MgCO₃ whereas the bottom of deeper grooves shows a significantly lower coverage of MgCO₃. Thus, a thinner coating of NaNO₃ facilitates the further growth of MgCO₃. This is in contrast to the observations made for the nucleation step which was found to be independent of the coating thickness.

The volume (per unit area of the bottom section of the groove) of the MgCO₃ particles formed at the bottom of grooves as function of the groove depth after exposure to CO₂ for 0.5, 1, 2, and 5 hours is shown in Fig. 7c. Considering the sample that has been exposed to CO₂ for 5 hours, the MgCO₃ formed at the bottom of the shallow groove (d_groove = 6 μm) has a total volume of 0.13 μm³ μm⁻² compared to a total MgCO₃ volume of 0.013 μm³ μm⁻², i.e. 10 times lower, in the deepest groove (d_groove = 180 μm). The same trend is apparent for the samples that have been exposed to CO₂ for either 1 or 2 hours, while no apparent trend between MgCO₃ volume and groove depths is observed for a CO₂ exposure of 0.5 hour, presumably because at these short carbonation times crystal growth is still limited (instead nucleation dominates) and CO₂ diffusion has limited effects on the nucleation step. To summarize, in Fig. 7c we observe a clear trend of an accelerated growth of MgCO₃ in shallow grooves compared to deep grooves, which is attributed to a shorter diffusion path length of CO₂ to the reaction interface in such systems, yielding in turn a higher density of ionic pairs of magnesium carbonate. It is conceivable that the faster evaporation rate of NaNO₃ as observed in shallower grooves also contributes to a faster precipitation of MgCO₃.”

“Fig. 6 FIB-SEM tomography of a MgCO₃ crystal grown on the surface of MgO(100) promoted with NaNO₃ after 5 hours of exposure to CO₂i.e. NaNO₃–MgO(100)_5hours-CO2. Prior to the tomographic measurement the sample was rinsed in water. (a) SEM Image of the MgCO₃ crystal grown on a MgO(100) single crystal (the grey box shows the area that was used for the tomographic measurement; resolution in x, y, z is, respectively, 10 nm, 25 nm, and =10 nm). (b and c) Surface contours of MgCO₃ and MgO are determined by segmenting MgCO₃ and MgO through a combination of thresholding by grey-level (using the backscattered electron, BSE, signal) and a directional search for grey-level edges. (d) Segmentation of MgCO₃ and MgO in a cross-section (window size [x,z] = 22.7 × 2.7 μm) along the y-direction. Each cross section is taken at a y-spacing of 25 nm and is shown at selected positions, i.e. y = 0, 1.25, 2.5, 3.75, 5, and 6.25 μm.”

“Fig. 7 Quantification of the volume of the MgCO₃ crystals by 3D profilometry measurements after removal of NaNO₃ by rinsing with H₂O. The volume is calculated by the sum of “voxels” of MgCO₃, i.e. the voxels that are at a height z > 0 μm with respect to the flat MgO surface at the bottom of the groove (z = 0 μm, roughness subtracted) multiplied by the dimension of the pixel in the xy direction (0.69 × 0.69 μm²). As the area at the bottom of a groove is smaller in deeper grooves, the MgCO₃ volume is normalized by the area of the bottom part of the groove. Note that the optical profile measurements rely on the reflection of light at the surface. Hence, only MgCO₃ grown at the bottom of the groove can be quantified, as the steep angle of the groove wall with respect to the incoming light minimizes reflection, which hinders quantification of the volume of MgCO₃ grown on the walls of the groove. (a and b) MgCO₃ formation after 5 hours of exposure to CO₂ in grooves with a depth of 11 μm and 44 μm, respectively. (c) Total volume of MgCO₃ crystals formed at the bottom of the grooves normalized by the area of the bottom of the groove as a function of the depth of the groove (d_groove = 6, 11, 11, 22, 44, 84, 122, 155, 180 μm) and carbonation time (0.5, 1, 2, 5 hours).”

https://doi.org/10.1039/D2TA02897B