Influence of surface functionalities on ethanol adsorption characteristics in activated carbons for adsorption heat pumps

Abstract

To develop high-performance activated carbons (ACs) for adsorption heat pumps (AHPs), it is important to characterize the adsorption behaviors of the refrigerant molecules in the pores of ACs. Not only pore structures, such as pore size and shape, but also surface functionalities strongly influences the adsorption behaviors, especially for polar molecules, such as water and ethanol, which are typical refrigerants for AHP. In this study, we examined the influence of surface functional groups on the adsorption behaviors of ethanol molecules in carbon micropores using model ACs with different amounts of oxygen-containing surface functional groups but comparable porosities. For the AC with an increased amount of surface functional groups, ethanol adsorption/desorption isotherms showed significant decreases in the adsorption amounts and shortened adsorption equilibrium times compared to those with less surface functional groups throughout the entire relative pressure region. This suggests diffusional hindrance of ethanol molecules in micropores with abundant surface functional groups. To verify our hypothesis, we examined the influence of surface functional groups on the adsorption behavior of ethanol molecules using a solid-state NMR technique. The NMR results revealed that the hydroxyl group of ethanol molecules strongly interacts with the surface functional groups, giving rise to an oriented adsorption of ethanol molecules in the micropores with oxygen-containing surface functional groups. Furthermore, electrochemical analyses confirmed that diffusion resistance of electrolyte ions in the micropores increases after the introduction of oxygen-containing surface functional groups, which supports our hypothesis.

Adsorption characteristics of ethanol onto functional activated carbons with controlled oxygen content

“…Plot of ln(P) vs 1/T at different uptake range for parent Maxsorb III/ethanol pair. Tables Table 1 Elemental composition of adsorbents [23]. Table 2 Composition of ethanol used in experiment.…”

Adsorption of ethanol onto parent and surface treated activated carbon powders

“…The adsorbents used in the present study are; (i) Parent Maxsorb III which is a highly porous activated carbon powder provided by Kansai Coke & Chemicals Co. Ltd., Japan (ii) a surface treated Maxsorb III (KOH-H2 Maxsorb III), and (iii) H2-surafce treated Maxsorb III. The treatment procedure has been described in reference [20]. Thermo-physical properties and elemental composition of assorted adsorbents are given in Table 1 and 2, respectively.…”

“…Temporal histories of adsorption uptake and pressure of Maxsorb III/ethanol pair at adsorption temperature 30ºC. Table 1 Thermo-physical properties of parent and surface treated Maxsorb III [20]. Table 2 Elemental composition of parent, KOH-H2 and H2 treated Maxsorb III [20].…”

“…Table 1 Thermo-physical properties of parent and surface treated Maxsorb III [20]. Table 2 Elemental composition of parent, KOH-H2 and H2 treated Maxsorb III [20]. Table 3 Isotherm parameters of assorted adsorbent/refrigerant pairs.…”

A Review on Working Pair Used in Adsorption Cooling System

“…Kil et al 16 concluded that abundant surface functionalities decreased the adsorption of ethanol shortened the apparent adsorption equilibrium time. The solid-state NMR and electrochemical analyses conrmed that di®usional hindrance of ethanol molecules caused by surface functionalities in micropores of ACs decreases the adsorption amount and shortens the adsorption equilibrium time of ethanol.…”