Dioxin Adsorption On Oxides And Chlorobenzene Oxidation On CeO₂ And VO_x/CeO₂ Catalysts

The emission abatement of toxic chemicals, such as chlorinated aromatics and polychlorinated dioxins/furans (PCDD/F) has attracted much attention from academia, industry, government and public with the growing environmental crisis. Adsorption or catalytic oxidation of dioxin-like compounds at low temperatures is a useful strategy for the emission abatement of dioxins. In this thesis work, the adsorption/desorption behaviors of dibenzodioxin on inorganic materials have been studied. The catalytic oxidation of chlorobenzene, the possible precursors of dioxins formation, on CeO₂ and VOx/CeO₂ catalysts has been investigated.Dibenzodioxin adsorption/desorption behaviors on inorganic materials (amorphous/mesoporous silica, metal oxides, and zeolites) were investigated using in situ FT-IR spectroscopy and thermogravimetric (TG) analysis. It is proposed that the dibenzodioxin adsorption is mainly via the following three interactions: hydrogen bonding with the surface hydroxyl groups on amorphous/mesoporous silica, complexation with Lewis acid sites on metal oxides, and confinement effect of pores of mordenite and NaY with pore size close to the molecular size of dibenzodioxin. The order of dibenzodioxin desorption temperature is following: mordenite and NaY > H-ZSM-5 and Na-ZSM-5 > amorphous/mesoporous silica.A series of CeO₂ prepared via precipitation with different precipitation reagents have been used as the catalyst and catalyst support for the total oxidation of chlorobenzene. The cerium oxides and supported VOx catalysts were characterized by X-ray diffraction (XRD), temperature programmed reduction (TPR), and Raman spectroscopy. All catalysts show quite high catalytic activityfor chlorobenzene oxidation. CeO₂ catalyst prepared with urea (named as CeO₂-U) shows higher activity than those prepared with Na2CO3 and NH3-H2O₂. VOx/CeO₂-U catalyst shows the highest activity among all catalysts investigated. A total oxidation of 1000 ppm chlorobenzene on VOx/CeO₂-U can be achieved at 250 oC with space velocity of 10000 ml·g^-1·h^-1. It is proposed that the reactive crystal planes on the surface of CeO₂-U like (110) and (100) benefit to the formation of highly reactive surface V-O-Ce species. The presence of Na+ or water vapor results in the deactivation of vanadium oxide catalyst.The ethanol conversion on Au/CeO₂ and Au/SiO₂ catalysts was also investigated. It was found that both Au nanoclusters and Au3+ species are reactive for the selective oxidation of ethanol to acetaldehyde. For ethanol dehydrogenation in the absence of O₂, a significant particle size dependence effect was observed: Au nanoparticles with size below 6 nm show much higher catalytic activity than those with bigger Au particles. The yield of acetaldehyde from ethanol dehydrogenation at 400 oC is 76% on Au nanoparticles with the mean size of 5 nm.