Adsorption of CO2 at 273 K up to 4 MPa has been studied in activated carbons and carbon molecular sieves of different origins and pore size distribution. The materials selected for this study include carbon molecular sieves with a pore size (i.e., pore width) between 0.3 and 0.5 nm, activated carbons with supermicroporosity (pore size between 0.7 and 2 nm), and mesoporous and macroporous activated carbons. The relative fugacities covered in the experiments ranges from 10 -4 to nearly 1. Additionally, N2 adsorption at 77 K at subatmospheric pressures has been also done. The experimental conditions used allow us to compare both measurements at similar adsorption potentials, in which both gases adsorb in the different ranges of porosity. The results obtained show that CO2 adsorbs at 273 K in the different ranges of porosity following a mechanism similar to that of N2 at 77 K. CO2 is sensitive to narrow micropores not accessible to N2 at 77 K, and hence, it is an adequate complement to N2 at 77 K. This is especially important for the characterization of the narrow micropores of carbonaceous solids and, especially, carbon molecular sieves. CO2 adsorbs in mesopores according to the capillary condensation mechanism.
The adsorption of CO2 up to pressures of 4 MPa has
been studied using two series of activated carbon
fibers (ACFs) covering a wide range of burn-off. The relative
fugacities covered in these experiments range
from 3 × 10-4 to 0.76. Additionally,
N2 adsorption at 77 K and CO2 adsorption at 273
and 298 K at
subatmospheric pressures have been carried out. The experiments
performed at high pressures allow us
to compare both adsorptives at similar ranges of adsorption potential.
The results obtained led to the
following conclusions: (i) CO2 adsorption at 273 K at
subatmospheric pressures is a suitable technique
to characterize the narrow microporosity of the ACF. (ii) The use
of N2 to characterize the narrow
microporosity is not appropriate because its adsorption is limited by
the existence of diffusional restrictions
in this type of porosity. (iii) CO2 at 273 K (or 298
K) is an adsorptive that behaves quite similarly to
N2
at 77 K at comparable relative pressure ranges; thus, CO2
adsorbs in the super-microporosity range (pore
size: 0.7−2 nm) at 298 K if pressures of about 4 MPa are
used.
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