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 (H ads ): (i) chemical adsorption with H ads > 12 kcal/mol, (ii) condensation with H ads approximately 10 kcal/mol, and (iii) physical adsorption with H ads < 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 N 2 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 N 2 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 N 2 -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 H 2 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.
Significant interest has grown in
the development of earth-abundant
and efficient catalytic materials for hydrogen generation. Layered
transition metal dichalcogenides present opportunities for efficient
electrocatalytic systems. Here, we report the modification of 1D MoO
x
/MoS2 core–shell nanostructures
by lithium intercalation and the corresponding changes in morphology,
structure, and mechanism of H2 evolution. The 1D nanowires
exhibit significant improvement in H2 evolution properties
after lithiation, reducing the hydrogen evolution reaction (HER) onset
potential by ∼50 mV and increasing the generated current density
by ∼600%. The high electrochemical activity in the nanowires
results from disruption of MoS2 layers in the outer shell,
leading to increased activity and concentration of defect sites. This
is in contrast to the typical mechanism of improved catalysis following
lithium exfoliation, i.e., crystal phase transformation. These structural
changes are verified by a combination of Raman and X-ray photoelectron
spectroscopy (XPS).
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