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/TiO2, Pt/TiO2, Ru/TiO2, and Rh/TiO2 was evaluated, and Ru/TiO2 contributed the highest catalytic activity and CO2 selectivity. During the oxidation, polychlorinated benzenes PhClx (x ≥ 2) were observed, and Ru/TiO2 showed apparently lower PhClx concentrations than other three samples. With the improvement of temperature, the maximum concentration appeared in the sequence of dichlorobenzene (PhCl2), trichlorobenzene (PhCl3), tetrachlorobenzene (PhCl4), and pentchlorobenzene (PhCl5), whereas the concentration of hexachlorobenzene (PhCl6) was always low and showed no apparent regularity. Besides, the dioxin-like PCBs (dl-PCBs) were collected and analyzed for Pd/TiO2 and Ru/TiO2. The ∑dl-PCBs produced by Pd/TiO2 (0.0055 ng WHO-TEQ/Nm3) was about 1.5 times that of Ru/TiO2 (0.0027 ng WHO-TEQ/Nm3). XPS analyses revealed that Ru/TiO2-used and Rh/TiO2-used gave the lowest and the highest Cl content of 0.61% and 1.87%. Ru/TiO2-used afforded the lowest (Clad+Clor)/Cl value (22.1%) and the highest Clbr/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.