Adsorption of oxygen by heat-treated granular and fibrous activated carbons

Three granular and one fibrous activated carbon with different physicochemical characteristics were subjected to heat treatment at 900°C under vacuum or hydrogen flow. Subsequently, oxygen chemi- and physisorption isotherms were volumetrically obtained at 34°C. Oxygen sorption experiments showed lower amounts of oxygen uptake by the H₂-treated than by the vacuum-treated carbons, indicating that H₂ treatment effectively stabilized the surfaces of various carbons tested in this study. At low pressures, from ∼0.001 to ∼5 mmHg, adsorption of oxygen was governed by irreversible chemisorption, which was well described by the Langmuir equation. At higher pressures oxygen uptake occurred as a result of physisorption, which was in agreement with Henry's law. Kinetic studies showed that oxygen chemisorption was affected by both carbon surface chemistry and porosity. The results indicated that oxygen chemisorption initially started in the mesopore region from the high energetic sites without any mass transfer limitation; thus a constant oxygen uptake rate was observed. Once the majority of these sites were utilized, chemisorption proceeded toward the less energetic sites in mesopores as well as all the sites located in micropores. As a result, an exponential decrease in the oxygen uptake rate was observed.