Preparation of molten salt-promoted MgO

https://doi.org/10.1039/D2TA02897B

“The fabrication of the NaNO₃–MgO model structure is illustrated in Fig. 1. We used ultra-short pulse (USP) laser ablation to microstructure MgO(100) single crystals with grooves of fixed width and varying depths, which are subsequently filled with NaNO₃, as shown in Fig. 1a (see also ESI Fig. S1† for illustration of the USP laser setup and ablation process). Fig. 1 shows grooves of different depths, d_groove, and arrows point to the location of the (buried) NaNO₃/MgO interface and the triple phase boundary (CO₂/NaNO₃/MgO) line in such model sorbent structures. Two sets of MgO model grooves were fabricated and are referred here to as MgO_A and MgO_B. The sample MgO_A, shown in Fig. 1, was designed to contain groves with a large difference between the most shallow (d_groove = 5 μm) and the deepest groove (d_groove = 238 μm) to explore also the effect of the diffusion length of CO₂ on the formation of MgCO₃. Optical profilometry measurements rely on the reflection of incoming light. However, the steep walls of the groove in MgO_A limited reflection and consequently prevented the measurement of this sample by optical profilometry. Therefore, an additional sample referred to as MgO_B, with groove of depths between d_groove = 6–180 μm was designed. The grooves had a wide and flat bottom to enable (top view) characterization of MgCO₃ formation by SEM and optical profilometry measurements (Fig. S2 and S3†). The shape and roughness of the microstructured grooves with and without NaNO₃ are visualized by SEM and are given in Fig. 1b–g). A top view of six grooves in MgO_A at different magnifications is shown in Fig. 1b and e, while cross-sections (with and without NaNO₃) are imaged in Fig. 1c–g. For MgO_A, all the grooves have the same length (and width of 3000 μm and 100 μm respectively, while the groove depths vary between d_groove = 5–238 μm. In sample MgO_B all grooves have an identical length (3000 μm) and the depths varied between d_groove = 6–180 μm. In addition, in sample MgO_B the width of the grooves was wider at the top (200 μm) and the grooves had a smaller taper angle than the ones in sample MgO_A allowing for a wide flat bottom to enable optical profilometry measurements as described earlier, Table S1.† The average roughness of the most shallow groove was ca. 0.41 μm, and increased to ca. 1.1 μm for the deepest groove (ESI Fig. S3c†). Here, the average roughness is calculated as the standard deviation from the mean value along the length of the groove measured over a 600 μm wide window in the y-direction. A cross-sectional view of the alkali salt coated sample, i.e. NaNO₃–MgO_A, shows an intimate contact between the surface of MgO and the promoter NaNO₃ confirming an excellent wetting of the surface of MgO by NaNO₃ (Fig. 1g).”

“Fig. 1 Fabrication and SEM characterization of the NaNO₃–MgO_A model structure to determine where MgCO₃ nucleates and grows most favorably during CO₂ capture. (a) Schematic of the fabrication process of the NaNO₃–MgO_A model system using USP laser ablation to microstructure grooves in a MgO(100) single crystal of depth, d_groove and top width w_groove, which are subsequently filled with NaNO₃. The black arrows point to the triple phase boundary (TPB) line (CO₂/NaNO₃/MgO) and the (buried) NaNO₃/MgO interface. (b–g) SEM images of grooves without and with NaNO₃. (b) Top view of six laser ablated grooves in a MgO(100) single crystal. (c) Mechanically cleaved sample exposing cross-sections of three grooves without a NaNO₃ promoter and (d) shows the same three grooves with the NaNO₃ promoter. (e–g) high magnification images of the respective structures shown in (b–d).”