(532c) Temperature Dependence of CO2 Sorption in Micro-Mesoporous Carbons

Temperature
Dependence of CO₂ Sorption in Micro-Mesoporous Carbons

F. Silvio P. Dantas¹, Richard Cimino¹, Katie
A. Cychosz², Matthias Thommes², Alexander V. Neimark¹

¹Rutgers University, Department of
Chemical and Biochemical Engineering, 98 Brett Road, Piscataway, NJ 08854, USA

²Quantachrome Instruments, 1900
Corporate Dr., Boynton Beach, FL 33426, USA

Carbon dioxide has proven
to be a suitable probe molecule to characterize microporous carbons due to its
ability to be adsorbed at ambient temperatures, allowing for faster diffusion
rates and penetration into narrowest micropores not normally accessible by
cryogenic adsorbates.  Until recently, the limit in resolution of CO₂
adsorption @ 273 K was ~ 2nm due to the high pressures (~35 bar) necessary at
273 K to fill mesopores.  However, recent advances in instrumentation have
allowed CO₂ isotherms to be measured up to the saturation pressure. At
this temperature, high pressure CO₂ experiments in micro-mesoporous
carbons have shown that CO₂ isotherms are reversible and do not show
hysteresis in the mesopore region. However, upon temperature reduction, the
hysteresis phenomenon does occur. This feature has important implications for
the application of theoretical methods for determining the pore size
distribution (PSD) by CO₂ adsorption. In this work, we investigate
the influence of temperature on the adsorption of CO₂ as it pertains
to the characterization of micro-mesoporous carbons, with the goal of
developing a reliable method of assessing the PSD for these systems. Using
Grand Canonical Monte Carlo (GCMC) and Quenched Solid Density Functional Theory
(QSDFT), theoretical adsorption isotherms were calculated in model carbon pores
in the temperature range of 205 K to 273 K. The adsorbent was modeled using simple
homogeneous slit and cylindrical shaped pores while the adsorbate was
represented either as a 3-center molecule (GCMC) or its LJ representation (QSDFT).
The theoretical isotherms were combined into adsorption “kernels”, which were
then used to solve the integral adsorption equation to obtain the surface area,
pore volume and pore size distribution of several micro-mesoporous carbon
samples. The results were compared to the conventional QSDFT methods for both argon
and nitrogen adsorption. The theoretical results are compared to experimental adsorption
isotherms on reference micro-mesoporous carbons.