Resolving the organization of CO2 molecules confined in silica nanopores using in situ small-angle neutron scattering and molecular dynamics simulations

Abstract

Determining the organization of CO₂ molecules confined in nanoporous environments is essential for unlocking our understanding of the fate of CO₂ stored in nanoporous materials. In this study, we investigate the organization of pressurized CO₂ molecules in silica materials, MCM-41 and SBA-15 with cylindrical pore geometries and pore diameters of 3.3 nm and 6.8 nm, respectively at pressures ranging from vacuum to about 55 bar, using in situ small angle neutron scattering (SANS) measurements and classical molecular dynamics (MD) simulations. The nanoconfined CO₂ molecules are organized into core–shell structures with the shell resulting from CO₂ adsorption on the silica surfaces. The shell thicknesses of the adsorbed CO₂ molecules in MCM-41 pores obtained by SANS measurements are 0.7 ± 0.1 Å, 2.1 ± 0.1 Å, 2.2 ± 0.1 Å, 6.7 ± 0.1 Å, 11.5 ± 0.2 Å, and 12.6 ± 0.1 Å at equilibrated pressures of about 1.0, 14.9, 24.9, 34.7, 45.0 and 54.9 bar, respectively. The shell thicknesses of the adsorbed CO₂ molecules in SBA-15 pores are 2.2 ± 0.1, 2.9 ± 0.1, 5.1 ± 0.1, 8.8 ± 0.1, 12.4 ± 0.1, and 20.0 ± 0.1 at pressures of 0.9, 15.4, 24.9, 34.9, 45.0 and 55.5 bar, respectively. Close agreement between the experimental and MD simulations results are obtained. MD simulations also suggest that the adsorption of the CO₂ molecules is primarily driven by van der Waals interactions with minor contribution from the electrostatic interactions and hydrogen bonding with the surface hydroxyl groups. These findings inform the development of novel strategies to advance low carbon energy and resource recovery and to use and store CO₂ in natural and engineered materials.