Removal of toluene using ozone at room temperature over mesoporous Mn/Al₂O₃ catalysts

The catalytic oxidation of toluene with ozone at room temperature was carried out over hierarchically ordered mesoporous catalysts (CeO₂ (meso), Mn₂O₃ (meso), ZrO₂ (meso), and γ-Al₂O₃ (meso)) and Al₂O₃ with various textural properties and phases (γ-Al₂O₃ (meso), γ-Al₂O₃ (13 nm), and α-Al₂O₃) to examine the effects of the nature of the catalyst on the catalytic activity. The catalysts were characterized by N₂-physisorption measurements, powder X-ray diffraction, temperature programmed reduction, X-ray photoelectron spectroscopy and scanning transmission electron microscopy with energy dispersive spectroscopy. Among the ordered mesoporous catalysts, γ-Al₂O₃ (meso) had the highest toluene removal efficiency because of its highest surface area and pore volume, which in turn was selected for further investigation. Manganese (Mn) was introduced to various Al₂O₃ to improve the toluene removal efficiency. Comparing the Mn-loaded catalysts supported on various Al₂O₃ with different crystalline phases or pore structures, Mn/γ-Al₂O₃ (meso), had the highest catalytic activity as well as the highest CO₂/CO ratio. The higher activity was attributed to the larger surface area, weaker interaction between Mn and Al₂O₃, and larger portion of Mn₂O₃ phase. The increase in ozone concentration led to an improvement in the carbon balance but this enhancement was insufficient due to the deposition of by-products on the catalyst. After long term tests at room temperature, the reaction intermediates and carbonaceous deposits of the used catalysts were identified.