Characterization of activated carbon surface composition by isoperibol calorimetry and x-ray photoelectron spectroscopy

The general objective of this research was to understand the nature of any specific interactions that occur between adsorbate molecules and activated carbon surfaces. The performances of these materials had been related solely to the total percentage of oxygen and not to a specific functional state of oxygen on the surface. The specific objective was to develop procedures employing isoperibol calorimetry to quantify the percentages of the oxygen-containing functional groups on activated carbon surfaces. Isoperibol calorimetry has never been employed widely for activated carbon characterization and its use has been confined to qualitative determinations only. Heats of wetting of four activated carbons were measured in several liquids. Surface areas were determined using nitrogen vapor adsorption data. Surface compositions were quantitatively determined using X-ray photoelectron spectroscopy (XPS). A good correlation was obtained between heats of wetting, normalized for surface area differences, and XPS data. The highest energy site for the adsorption of water and an alcohol on all surfaces was found to be the carbonyl group.;Using a combination of water vapor adsorption and heat of wetting studies, heats of adsorption of the first layer of water were indirectly determined. These again correlated well with the carbonyl surface concentration. The differential heats of wetting continuously decreased with increasing surface coverage, indicating that water was interacting with different types of adsorption sites with different energies (adsorption does depend on the functional state in which the oxygen exists on the surface). All the above demonstrate that specific interactions are important to the adsorption process.;To test the ability of isoperibol calorimetry to predict surface compositions, one of the activated carbons was modified using several oxidation conditions, followed by determinations of surface areas, surface compositions and heats of wetting. The four activated carbons mentioned previously were used as standards to develop equations allowing the quantification of the percentages of the oxygen-containing functional groups on the oxidized surfaces. The calculated results (calorimetry data) were in good agreement with those obtained experimentally by XPS (prediction error <4.7%). Isoperibol calorimetry was proven to be a useful technique for the quantitative analysis of activated carbons.