A thermodynamic study of the enthalpy of adsorption of methane on high surface area carbonaceous materials was carried out from 238 to 526 K. The absolute quantity of adsorbed methane as a function of equilibrium pressure was determined by fitting isotherms to a generalized Langmuir-type equation. Adsorption of methane on zeolite-templated carbon, an extremely high surface area material with a periodic arrangement of narrow micropores, shows an increase in isosteric enthalpy with methane occupancy; i.e., binding energies are greater as adsorption quantity increases. The heat of adsorption rises from 14 to 15 kJ/mol at near-ambient temperature and then falls to lower values at very high loading (above a relative site occupancy of 0.7), indicating that methane/ methane interactions within the adsorption layer become significant. The effect seems to be enhanced by a narrow pore-size distribution centered at 1.2 nm, approximately the width of two monolayers of methane, and reversible methane delivery increases by up to 20% over MSC-30 at temperatures and pressures near ambient.
Methane adsorption at high pressures and across a wide range of temperatures was investigated on the surface of three porous carbon adsorbents with complementary structural properties. The measured adsorption equilibria were analyzed using a method that can accurately account for nonideal fluid properties and distinguish between absolute and excess quantities of adsorption, and that also allows the direct calculation of the thermodynamic potentials relevant to adsorption. On zeolite-templated carbon (ZTC), a material that exhibits extremely high surface area with optimal pore size and homogeneous structure, methane adsorption occurs with unusual thermodynamic properties that are greatly beneficial for deliverable gas storage: an enthalpy of adsorption that increases with site occupancy, and an unusually low entropy of the adsorbed phase. The origin of these properties is elucidated by comparison of the experimental results with a statistical mechanical model. The results indicate that temperature-dependent clustering (i.e., reduced configurations) of the adsorbed phase due to enhanced lateral interactions can account for the peculiarities of methane adsorbed on ZTC.
Ethane adsorption was measured on zeolite-templated carbon (ZTC) and compared to superactivated carbon MSC-30. Isotherms measured at temperatures between 252 and 423 K were fitted using a superposition of two Langmuir isotherms, and thermodynamic properties were assessed. Unlike typical carbon adsorbents, the isosteric heat of adsorption on ZTC increases by up to 4.6 kJ/mol with surface coverage. This increase is attributed to strong adsorbate−adsorbate intermolecular interactions, a hypothesis that is shown to be consistent with fundamental estimates of intermolecular interactions. Furthermore, the molar entropy of the adsorbed phase was measured and compared to an estimate derived from statistical mechanics. While the measured and estimated entropies of the adsorbed phase of ethane on MSC-30 are in agreement, they differ significantly on ZTC at high coverage, indicative of the atypical properties of ethane adsorption on ZTC.
We have developed polymer composite inks that may be three-dimensionally (3D) printed to produce new reactor designs for CO2 capture. These inks are composed of solid sodium carbonate particles dispersed within an uncured silicone and are printed using direct ink writing (DIW). After printing, the silicone is cured, and the structures are hydrated to form aqueous sodium carbonate domains dispersed throughout the silicone. These domains enable high CO2 absorption rates by creating domains with a high surface area of the solvent per unit volume in the printed structures. These results demonstrate an order-of-magnitude improvement in CO2 absorption rates relative to a liquid pool of sodium carbonate. The results from this class of composite inks demonstrate the potential for the use of 3D printing to shape new and advanced CO2 capture reactors.
Krypton adsorption was measured at eight temperatures between 253 and 433 K on a zeolite-templated carbon and two commercial carbons. The data were fitted using a generalized Langmuir isotherm model and thermodynamic properties were extracted. Differing from that on commercial carbons, krypton adsorption on the zeolite-templated carbon is accompanied by an increasing isosteric enthalpy of adsorption, rising by up to 1.4 kJ mol(-1) as a function of coverage. This increase is a result of enhanced adsorbate-adsorbate interactions promoted by the ordered, nanostructured surface of the adsorbent. An assessment of the strength and nature of these adsorbate-adsorbate interactions is made by comparing the measured isosteric enthalpies of adsorption (and other thermodynamic quantities) to fundamental metrics of intermolecular interactions of krypton and other common gases.
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