Decomposition mechanism of Cl-VOCs on Cu- and Fe-modified activated carbon in an O2-free atmosphere: The oxidation pathway and product distribution

Abstract

Activated carbon exhibits a high adsorption capacity for chlorinated volatile organic compounds (Cl-VOCs), while the desorption of Cl-VOCs is not desirable in the thermal regeneration progress of activated carbon. Chlorobenzene was selected as the model substance to explore whether Cl-VOCs can be oxidized as expected in the regeneration atmosphere of O2-free at the temperature range of 150–450 °C. The modification of activated carbon by Cu or Fe has been shown to considerably improve the decomposition activities. According to the breakthrough curves and detection results from XPS and in situ DRIFTS, the decomposition mechanism of C6H5Cl on Cu and Fe active sites is illustrated. The adsorbed C6H5Cl is firstly dechlorinated to form C6H6 and *Cl, and then it was found there are two pathways for the C6H6 oxidation. C6H6 is indirectly oxidized to form C6H5OH and benzoquinone species, or directly ring-opened to generate ethylene and 1,3-butadiene, and further oxidized to produce CO2, CO and H2O. The C6H6 oxidation is determined by the decomposition of acetate on Cu/AC, but the decomposition of benzoquinone and maleate on Fe/AC. The Fe3O4 and Fe2O3 active sites have a higher dechlorination activity but accelerate the deposition of FeCl2, FeCl3 and organic Cl-containing components. The CuO active center has a higher oxidation performance with a higher CO2 selectivity. About 83.0–83.5% of adsorbed chlorobenzene has been oxidized, and the main Cl-containing product is HCl accounting for approximately 77.8–81.0%, a small amount of Cl forms polychlorinated organic compounds or metal chlorides occupying 2.0–5.7%. As the regeneration temperature decreases, the total amount of Cl-containing species is reduced due the lower oxidation and dechlorination activity of catalysts. The decomposition products of Cl-VOCs in the thermal regeneration process of activated carbon were clarified and quantified, which provides a theoretical basis to control Cl-VOCs in regeneration gas.