Catalytic oxidation of chlorobenzene over noble metals (Pd, Pt, Ru, Rh) and the distributions of polychlorinated by-products

Catalytic oxidation of chlorobenzene over noble metal catalysts Pd/TiO 2 , Pt/TiO 2 , Ru/TiO 2 , and Rh/TiO 2 was evaluated, and Ru/TiO 2 contributed the highest catalytic activity and CO 2 selectivity. During the oxidation, polychlorinated benzenes PhCl x (x ≥ 2) were observed, and Ru/TiO 2 showed apparently lower PhCl x concentrations than other three samples. With the improvement of temperature, the maximum concentration appeared in the sequence of dichlorobenzene (PhCl 2 ), trichlorobenzene (PhCl 3 ), tetrachlorobenzene (PhCl 4 ), and pentchlorobenzene (PhCl 5 ), whereas the concentration of hexachlorobenzene (PhCl 6 ) was always low and showed no apparent regularity. Besides, the dioxin-like PCBs (dl-PCBs) were collected and analyzed for Pd/TiO 2 and Ru/TiO 2 . The ∑dl-PCBs produced by Pd/TiO 2 (0.0055 ng WHO-TEQ/Nm 3 ) was about 1.5 times that of Ru/TiO 2 (0.0027 ng WHO-TEQ/Nm 3 ). XPS analyses revealed that Ru/TiO 2 -used and Rh/TiO 2 -used gave the lowest and the highest Cl content of 0.61% and 1.87%. Ru/TiO 2 -used afforded the lowest (Cl ad +Cl or )/Cl value (22.1%) and the highest Cl br /Cl value (77.9%), which might be an important reason for its strongest chlorine removal ability and the lowest yields of polychlorinated by-products, whereas other three catalysts showed similarity in the Cl distributions. Additionally, systematic in-situ FTIR studies were conducted, and a reaction mechanism for the catalytic oxidation of chlorobenzene was proposed.