A model-independent analysis of nitrogen adsorption isotherms on oxidized active carbons

A series of commercial active carbons was oxidized in the liquid-phase using various oxidizing agents such as hydrogen peroxide, perchloric acid and nitric acid. Low temperature nitrogen adsorption was used to monitor the changes in the surface properties and porous structure of active carbons occurring during oxidation process. A quantitative characterization of these changes was done on the basis of Adsorption Potential Distributions (APDs) obtained from nitrogen adsorption isotherms. The characteristic minima appearing on these distributions allowed to evaluate the specific surface area and the micropore volume, which subsequently were used to estimate the average micropore width. It was shown that the oxidation of active carbons with hydrogen peroxide, perchloric acid and nitric acid at room temperature did not cause a substantial change in the resulting APDs. This is not the case for active carbons oxidized with nitric acid at the boiling temperature. The latter oxidation caused a significant deterioration of the porous structure of active carbons in addition to the distinct changes in their surface functionality, which is evidenced by a significant difference in the structural parameters evaluated from the APDs for unmodified and strongly oxidized samples.