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.