The use of carbon dioxide allows for rapid, high-resolution characterization of microporous materials. The Micromeritics TriStar II 3020 can be used to characterize micropores below 10 angstroms within 8 hours using CO2 as the adsorptive gas and an ice bath (273 K) for the analysis bath. Isotherms from CO2 adsorption were compared with N2 adsorption isotherms at 77.3 K. Density functional theory (DFT) models for nitrogen and carbon dioxide on slit-pore carbon were used for the pore-size-distribution calculations.
Slow diffusion (equilibration) rates—on the order of hours to days—may be observed during adsorption at cryogenic temperatures on materials with small pores. Strong adsorption potentials in confined pores result in high uptake of adsorptives, such as nitrogen, which corresponds to the filling of micropores at very low pressures that can challenge the lower sensitivity limits of one-torr pressure transducers. Analysis with carbon dioxide addresses both of these issues by having faster equilibration rates, due to increased temperature, and gas uptake at much higher pressures.
The use of carbon dioxide allows for rapid, high-resolution characterization of microporous materials. The Micromeritics TriStar II 3020 can be used to characterize micropores below 10 angstroms within 8 hours using CO2 as the adsorptive gas and an ice bath (273 K) for the analysis bath. Isotherms from CO2 adsorption were compared with N2 adsorption isotherms at 77.3 K. Density functional theory (DFT) models for nitrogen and carbon dioxide on slit-pore carbon were used for the pore-size-distribution calculations.
Slow diffusion (equilibration) rates—on the order of hours to days—may be observed during adsorption at cryogenic temperatures on materials with small pores. Strong adsorption potentials in confined pores result in high uptake of adsorptives, such as nitrogen, which corresponds to the filling of micropores at very low pressures that can challenge the lower sensitivity limits of one-torr pressure transducers. Analysis with carbon dioxide addresses both of these issues by having faster equilibration rates, due to increased temperature, and gas uptake at much higher pressures.
Two readily available carbon samples, Carboxen® 1012 from Supelco and a sample of MAST synthetic carbon, were used. A Micromeritics TriStar II 3020 was the principle instrument used for CO2 analyses up to 760 torr (relative pressure of 0.03). A Micromeritics ASAP 2050 high-pressure sorption analyzer was used to measure CO2 isotherms up to 7600 torr, and a Micromeritics ASAP 2020 was used to verify micropore sizes using DFT calculations from nitrogen isotherms at 77 K. Bone-dry carbon dioxide, prepurified nitrogen, and ultra-high purity helium were used during the analyses.
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