Effects of oxidation state of noble metals on the catalytic activity for total oxidation of n-hexane and toluene were investigated with 5wt% $Pd/$γ-Al_2O_3$ and 5wt% Pt/$γ-Al_2O_3$.
The catalysts were characterized with BET, XRD, XPS and TGA. TPSR(Temperature programmed surface reaction) experiments were also carried out.
The activities of n-hexane and toluene oxidation over the supported palladium catalysts were significantly increased by the hydrogen treatment at 500℃ due to the reduction of PdO species into $Pd^0$. Metallic palladium seemed to be more active than PdO for the oxidation of n-hexane and toluene at a lower temperature even if the palladium oxide was known to be an active phase for high-temperature combustion of methane. The oxidation activity of reduced palladium catalysts, however, decreased significantly during the reaction at lower temperature. This pattern of deactivation was believed to be most probably to be due to the deposition of carbon or other partially oxidized organics on the active sites.
In the case of supported platinum catalysts, the reduced platinum catalysts showed a comparable activity with oxidized form for n-hexane and toluene oxidation. The oxidation activity of reduced or oxidized platinum catalysts gradually decreased during the reaction. At lower temperature, the oxidation activity of platinum catalysts decreased significantly during the reaction at lower temperature. It was also suggested that the deactivation of reduced or oxidized platinum catalysts was due to blocking of active sites with deposition of carbon or partially oxidized organics.
