Effect of activated carbon's pore size distribution on oxygen induced heel build-up

Thermal desorption of volatile organic compounds (VOCs) from activated carbon (AC) is negatively impacted by oxygen. Oxygen reacts with the adsorbed VOCs, resulting in heel buildup in the porous structure of AC and reducing its adsorption capacity. This study investigated the contribution of physical properties of AC (specific surface area, micropore volume, and total pore volume) to this type of heel. Three different ACs with different micro/mesopore structures were tested through cyclic adsorption/desorption of 1,2,4-trimethylbenzene (TMB) using ultra-pure nitrogen (& LE;5 ppmv O2) as the purge gas during thermal desorption. Each set of experiments was repeated with dry air (21% O2) to evaluate the effect of O2 on heel build-up. Pore size distribution (PSD) analysis on the adsorbents cycled in dry air suggested a correlation between the amount of heel and the microporosity of adsorbents. Differential thermogravimetric (DTG) analysis was used to distinguish between the physically adsorbed and chemically formed heel. X-ray photoelectron spectroscopy (XPS) analysis showed a noticeable increase in surface oxygen content of the ACs desorbed in air. Finally, Boehm titration confirmed the presence of different surface oxygen functional groups on these adsorbents, suggesting oxidation reactions as the main mechanism of heel build-up.