Microcalorimetric Study of the Influence of Surface Chemistry on the Adsorption of Water by High Surface Area Carbons

Abstract

Microcalorimetric techniques have been employed to investigate the adsorption of water on chemically modified, high surface area carbons. A matrix of treatments and carbons were selected to test some hypotheses. Heats of adsorption of water indicate that adsorption is a strong function of surface chemistry. Three mechanisms of water adsorption are delineated according to measured differential heats of adsorption (Hads): (i) chemical adsorption with Hads > 12 kcal/mol, (ii) condensation with Hads approximately 10 kcal/mol, and (iii) physical adsorption with Hads < 10 kcal/mol. The absolute and relative amounts of water adsorption arising from each mechanism are a function of surface chemistry. Adsorption of water on carbons dried at 175 °C in N2 generates typical Type V adsorption isotherms. Heat of adsorption data for water adsorbed on dried carbon indicates that condensation accounts for the sharp rise in adsorption at a relative humidity of approximately 0.5. Treating carbons with N2 at 950 °C generates surfaces that initially adsorb water through a mechanism of chemical adsorption, followed by condensation, and finally physical adsorption. On high-temperature N2-treated carbons heats of adsorption exceed 100 kcal/mol, suggesting chemisorption of water at unsaturated carbon surface sites produced during the high-temperature reduction of oxygen species. Carbons reduced with H2 at 950 °C are hydrophobic, and microcalorimetric data reveals that the small amount of adsorption observed arises from either chemical or physical adsorption rather than condensation. Hydrophobic carbon surfaces subsequently oxygenated at 150 °C showed significant increases in the amount of water adsorbed through physical adsorption. These results demonstrate that microcalorimetric techniques complement standard isotherm measurements in describing the nature of water adsorption on carbon surfaces.