Total neutron scattering study of supercooled CO2 confined in an ordered mesoporous carbon

Abstract

The molecular structure and phase behavior of carbon dioxide confined in an ordered mesoporous carbon (CMK-3) are investigated by total neutron scattering with in situ adsorption and cooling/heating cycles. The neutron signal from CO2 sorbed in the micropores of CMK-3 shows clearly liquid-like properties. Close to the bulk CO2 triple point (T3), the mesopore-confined CO2 is found in a liquid yet strongly densified state with a density similar to that of solid dry ice. Upon cooling the CO2 loaded carbon sample below T3, it was observed that depletion of the confined phase occurs and CO2 molecules escape from the pores to solidify externally. It was concluded that depletion occurs only in “secondary” pores having larger sizes (>50 Å) than the well-defined uniform mesopores (47 Å) of CMK-3. These pores exist due to imperfect ordering of the hexagonal network of the carbon rods. Surprisingly, this phenomenon does not happen in the main mesopores (47 Å), implying thus that 50 Å is a critical size for depletion below T3. This in turn suggests that in larger pores a pore triple point cannot be attained. The depletion process is reversible with a temperature hysteresis; when heating the sample CO2 molecules re-fill the pores.