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

Clay minerals are abundant in caprock formations for anthropogenic storage sites for CO2, and they are potential capture materials for CO2 postcombustion sequestration. We investigate the response to CO2 exposure of dried fluorohectorite clay intercalated with Li+, Na+, Cs+, Ca2+, and Ba2+. By in situ powder X-ray diffraction, we demonstrate that fluorohectorite with Na+, Cs+, Ca2+, or Ba2+ does not swell in response to CO2 and that Li-fluorohectorite does swell. A linear uptake response is observed for Li-fluorohectorite by gravimetric adsorption, and we relate the adsorption to tightly bound residual water, which exposes adsorption sites within the interlayer. The experimental results are supported by DFT calculations.

CO2 Adsorption Enhanced by Tuning the Layer Charge in a Clay Mineral.

Due to the compact two-dimensional interlayer pore space and the high density of interlayer molecular adsorption sites, clay minerals are competitive adsorption materials for carbon dioxide capture. We demonstrate that with a decreasing interlayer surface charge in a clay mineral, the adsorption capacity for CO2 increases, while the pressure threshold for adsorption and swelling in response to CO2 decreases. Synthetic nickel-exchanged fluorohectorite was investigated with three different layer charges varying from 0.3 to 0.7 per formula unit of Si4O10F2. We associate the mechanism for the higher CO2 adsorption with more accessible space and adsorption sites for CO2 within the interlayers. The low onset pressure for the lower-charge clay is attributed to weaker cohesion due to the attractive electrostatic forces between the layers. The excess adsorption capacity of the clay is measured to be 8.6, 6.5, and 4.5 wt % for the lowest, intermediate, and highest layer charges, respectively. Upon release of CO2, the highest-layer charge clay retains significantly more CO2. This pressure hysteresis is related to the same cohesion mechanism, where CO2 is first released from the edges of the particles thereby closing exit paths and trapping the molecules in the center of the clay particles.

Small-Angle Neutron Scattering Characterization of SrCl2-ENG Composites for Thermochemical Heat Storage.

This work presents an in situ nanoscale structural characterization of a SrCl2-expanded natural graphite (ENG) composite during ammonia absorption and desorption using small-angle neutron scattering (SANS) together with X-ray powder diffraction and sorption measurements. For the processing of the composite material SANS patterns, we developed and implemented two methods, which showed comparable results. The study allowed following the evolution of the SrCl2 particles and the nanopores inside the particles during five sorption cycles. The structural changes were compared to the absorption and desorption kinetic measurements, allowing us to make qualitative analysis of the impact of the structural changes on the material properties, such as thermal conductivity and permeability. It was shown that the structural evolution of the composite material did not affect the desorption rate but significantly influenced the absorption rate after the first cycle. We also observed a significant improvement of the absorption kinetics due to the formation of nanopores in the fully deammoniated sample. In addition, the ENG matrix was shown to hinder the agglomeration of the SrCl2 particles during sorption processes, which is in contrast to literature findings reported for a nonsupported metal halide. The findings presented in this study can be of great interest in the research areas where SrCl2-ENG composites are widely studied, i.e., heat storage, heat pumps/refrigerators, deNOx removal, and solid-state ammonia storage.