Heterogeneity of Adsorption Energy of Water, Methanol and Diethyl Ether on Activated Carbons
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Heterogeneity of Adsorption Energy of Water, Methanol and Diethyl Ether on Activated Carbons VLADIMIR GUN’KO∗ Institute of Surface Chemistry, 17 General Naumov Street, 03164 Kiev, Ukraine [email protected]
TERESA J. BANDOSZ Department of Chemistry, The City College of New York, New York, NY 10031, USA
Abstract. Three samples of activated carbon of wood origin with various amounts of surface functional groups and different porous structure were used as adsorbents of water, methanol, and diethyl ether. Then the isotherms at three different temperatures were used to calculate distributions of adsorption potential, free energy of adsorption and adsorption energy. The results showed that the strength of interactions depends on both the porosity of the sample and its surface chemistry. The effect of surface chemistry is the most important in the case of water and the least important in the case of diethyl ether. The high-energy centers are related to adsorption in narrow pores containing functional groups. For molecules consisting of hydrocarbon moiety and oxygen containing groups (methanol and diethyl ether) the surface chemistry plays a role at low surface coverage. Keywords: adsorption energy, activated carbon, surface heterogeneity
Introduction Activated carbons due to the high sorption capacity are able to remove or separate traces of air and water contaminants. Such functions are linked to their high surface area, large pore volume, and the presence of pores diameter of few Angstroms. Those features play a significant role in the process of physical adsorption when only non-specific, dispersive interactions with the molecules to be adsorbed take place. For specific adsorption and chemisorption the surface chemistry of carbon becomes a crucial feature (Leon y Leon and Radovic, 1992). When the medium from which molecules to be removed are present is also able to interact in a specific way with a carbon surface competition for high energy sites exists and the feasibility of the removal process depends on the size of molecules, ∗ To
whom correspondence should be addressed.
their chemical nature and the surface features of the adsorbent (Bagreev et al., 2002). The objective of this paper to evaluate the energy of interactions of water, methanol and diethyl ether on oxidized activated carbons and the effects of carbon structure and chemistry on those interactions. Following the chemical nature of the molecules, the specific interactions via hydrogen bonding with surface functional groups should be the most pronounced in the case of water and the least pronounced in the case of diethyl ether (Salame and Bandosz, 2001). The sizes of pores should also play a role due to the various diameters ˚ of the molecules. Water molecule having d ∼ 2.9 A should be able to enter the narrowest pores of the adsorbents whereas some small pores would be excluded for diethyl ether molecule. The carbons chosen for this study were described in detail elsewhere (Salame and Bandosz, 2000). Here we only report parameters used for data interpretat
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