Temperature Dependence of Gas Physisorption Energy: Experimental and Computational Studies of Krypton on Porous Carbon

Citation

Wang, Ziyi (2023) Temperature Dependence of Gas Physisorption Energy: Experimental and Computational Studies of Krypton on Porous Carbon. Dissertation (Ph.D.), California Institute of Technology. https://resolver.caltech.edu/CaltechTHESIS:05162023-225200255

Abstract

This work comprehensively investigated the temperature dependence of physical adsorption energy, combining theoretical, computational, and experimental approaches. A thermodynamic analysis of the 2D ideal gas and the slit-pore models highlighted the role of van der Waal potentials in the adsorption energy and isotherm fitting methods, especially Henry's law. Experimental data of krypton adsorption on CNS-201 and MSC-30 porous carbon materials revealed a significant weakening in the isosteric adsorption energy with temperatures from 250 K to 330 K. Using the zero-coverage Henry's constants and Clausius–Clapeyron equation, the adsorption energies weaken for 13% and 15% for CNS-201 and MSC-30. The corresponding changing rates are 4.35k_B for CNS-201 and 3.65k_B for MSC-30.

The DFT-based computational study with the slit-pore model showed the van der Waal potentials of different-sized pores. Then it showed how the structures of the pores significantly influence the surface dynamics and the internal energies of the adsorbates at different temperatures. Gas molecules adsorbed in pores of different sizes have different heat capacities larger than the gas phase, leading to a temperature dependence of adsorption energy. Monte Carlo calculation indicated that displacements of adsorbent atoms caused by thermal vibration slightly weaken the van der Waal potentials but have a negligible effect on the temperature dependence of the adsorption energy.

The distribution of pore sizes plays a crucial role in the temperature dependence of the overall adsorption energy. With increasing temperature, the pores with higher energy states become more accessible due to the Boltzmann distribution, weakening the statistically averaged internal energy. Adsorption energy weakening of 5% and 15% for CNS-201 and MSC-30 are given by combining the computational van der Waal potentials and experimentally measured pore sizes. The changing rates are 0.62k_B and 2.03k_B.

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