Isosteric Heats of Gas and Liquid Adsorption

Abstract

The heat of adsorption is an indicator of the strength of the interaction between an adsorbate and a solid adsorbent. This parameter can be determined from the heat released in calorimetric experiments or from the analysis of adsorption isotherms at different temperatures. The latter, called isosteric heats of adsorption, are commonly used in the characterization of materials for gas- and liquid-phase adsorption. Although the equations for the determination of isosteric heats of adsorption from the gas phase are well-known, approximate equations are frequently used for liquid-phase adsorption. We present here the rigorous equations for determining the isosteric heats of gas- and liquid-phase adsorption and their relation to the commonly used approximate equations. These equations are used to compute the isosteric heats of liquid adsorption based on the adsorption isotherms obtained from simulations for two well-defined systems, one ideal and the other nonideal. The results of using the rigorous equations are compared with those from the approximate equations. The main conclusion is that the commonly used approximate equations provide reasonable, but not perfect, estimates of the isosteric heats of liquid adsorption using only the experimental adsorption isotherms. The more accurate rigorous equations require additional information, including the heat of vaporization and, for nonideal mixtures, the heat of mixing.

Feasibility of CO2 adsorption by solid adsorbents: a review on low-temperature systems

“…This parameter can be calculated from heat released in calorimetric experiment and from the analysis of adsorption isotherm at different temperatures. Heat of adsorption determined by the latter technique is called isosteric heats of adsorption, which is generally used in the characterization of adsorbents material for gas and liquid phase adsorption (Builes et al 2013). Information about Q st is very important, particularly during gaseous phase adsorption because the heat released upon adsorption is partially adsorbed on the adsorbent, which causes a rise in the adsorbent temperature and influences the adsorption equilibrium and kinetics and thus, as a result, the overall gas adsorption capacity of the adsorbent (Martin et al 2011).…”

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“…The adsorption constant is a function of temperature, the enthalpy and entropy of adsorption, furthermore, the dependence that K ad has on temperature can be determined if the enthalpy of adsorption is known. The determination of the enthalpy or heats of adsorption from the liquid phase has been acknowledged to be more challenging than for adsorption from the gaseous phase 50 In the literature, the studies of isosteric heats of adsorption in the liquid phase have often reported low values. 50 -52 Hence, in the present work it is assumed that the heat of adsorption is relatively low, implying that for the studied temperature range the effect that temperature has on the K ad value is low enough such that it can be considered negligible.…”

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“…In addition, using the adsorption isotherms at various temperatures, the isosteric heat of adsorption can be determined. e isosteric heat of adsorption, also called enthalpy of adsorption (ΔH ads ), indicates the strength of the interaction (bond) between the adsorbate (water molecules) and the solid adsorbent [71,72]. is parameter represents the energy necessary for the heat of vaporization of moisture in a material during the adsorption process to exceed the latent heat of vaporization of pure water at a certain temperature [73].…”