Influence of CO₂ on Nanoconfined Water in a Clay Mineral

Abstract

Developing new technologies for carbon sequestration and long-term carbon storage is important. Clay minerals are interesting in this context as they are low-cost, naturally abundant, can adsorb considerable amounts of CO 2 , and are present in storage sites for anthropogenic carbon. Here, to better understand the intercalation mechanisms of CO 2 in dehydrated and hydrated synthetic Na-fluorohectorite clay, we have combined powder X-ray diffraction, inelastic and quasi-elastic neutron scattering, and density functional theory calculations. For dehydrated Nafluorohectorite, we observe no crystalline swelling or spectroscopic changes in response to CO 2 , whereas for the hydrated case, damping of the librational modes related to the intercalated water was clearly observed. These findings suggest the formation of a more disordered water coordination in the interlayer associated with highly confined water molecules. From the simulations, we conclude that intercalated water molecules decrease the layer−layer cohesion energy and create physical space for CO 2 intercalation. Furthermore, we confirm that interlayer confinement reduces the Na + hydration number when compared to that in bulk aqueous water, which may allow for proton transfer and hydroxide formation followed by CO 2 adsorption in the form of carbonates. The experimental results are discussed in the context of previous and present observations on, a similar smectite, Ni-fluorohectorite, for which it is established that CO 2 attaches to the edge of nickel hydroxide islands present in the interlayer.

Intercalation of CO2 Selected by Type of Interlayer Cation in Dried Synthetic Hectorite

“…Li-Hec shows a much more sluggish dehydration behavior compared to Na-Hec. For Na-Hec, only a very limited residual water population has been observed by INS (inelastic neutron spectroscopy) to be left in the sample after dehydration to 145 °C . Recent experiments on a Li-Hec from Corning found that dried Li-Hec contains tightly bound water molecules that have a significantly higher adsorption energy, 225 kJ/mol compared to 31 kJ/mol for the more loosely bound water population in the sample.…”

“…Na-Hec ( Figure 1 c), which is observed to have a basal spacing of 9.6 Å, in line with previous observations of dehydrated Na-Hec, 17 does not swell in response to CO 2 , and there are no changes in the peak observed after CO 2 exposure, again confirming our previous reports. 30 …”

“…Na-Hec (Figure 1c), which is observed to have a basal spacing of 9.6 Å, in line with previous observations of dehydrated Na-Hec, 17 does not swell in response to CO 2 , and there are no changes in the peak observed after CO 2 exposure, again confirming our previous reports. 30 Upon drying of Ca-Hec, the Bragg reflection forms a doublepeak structure with seemingly two phases (Figure 1d) at 10 and 10.3 Å, with a combined peak width significantly broader than that of the other ions investigated. After exposure to CO 2 , the Bragg reflection forms a single symmetric peak at 10.3 Å.…”

“…Following the same methodological protocol described in our recent studies, ,, we perform first-principles calculations within the framework of density functional theory (DFT) , as implemented in the SIESTA package . We considered a mesh cutoff of 400 Ry and a double-ζ plus polarization (DZP) basis set to represent the atomic charge in real space.…”

Two Forms of Ice Identified in Mars-like Clay Using Neutron Spectroscopy

“…The interaction of water with clay mineral surfaces has been subject to intense study for decades, − but has received heightened interest in the past 20 years. − Much of the recent interest stems from an incomplete understanding of the complementary, but often competing, roles played by hydrating counter ions and the clay mineral interlayer surface itself, various pore sizes present within clay gels, and how each of these influence reactivity in various contexts, e.g., as catalysts, adsorbents, or carriers . Indeed, water interaction with minerals in general is perhaps a defining feature of the evolution of Earth , and has proven to be an important aspect of current interplanetary exploration and study. − Samples returned to Earth from the Ryugu asteroid by the Hayabusa spacecraft, for example, have already been analyzed finding CO 2 -bearing water inclusions to be present within a pyrrhotite crystal …”