Study On Catalytic Oxidation Of Toluene Over MnO₂ Catalysts

Volatile Organic Compounds（VOCs）are a group of major air pollutants.They are an important precursor of haze and ozone generation.Meanwhile,they can induce teratogenicity,carcinogenicity and mutagenicity,and cause many physical discomforts and diseases.So their abatement has received widespread attention.Catalytic oxidation is a promising treatment method,which can convert VOCs into harmless H₂O and CO₂ at a lower temperature.However,the lack of efficient and economical catalysts restricts the practical application of catalytic oxidation method.Therefore,we selected toluene as the target pollutant and MnO₂ as the catalyst to carry out a series of studies.In this study,MnO₂-Ac,MnO₂-N,MnO₂-Cl and MnO₂-S samples were prepared by the redox reaction between potassium permanganate and manganese acetate,manganese nitrate,manganese chloride and manganese sulfate,and the effect of Mn（II）precursor on the toluene oxidation was studied in the simulated exhaust.The physicochemical properties of these samples were characterized by XRD,TEM,N₂ asdorption-desorption isotherms,H₂-TPR and XPS.Results showed that MnO₂-Ac showed the best activity among the four samples with T₉₀of 200°C.The catalytic activity of the four samples decreased in the order of MnO₂-Ac>MnO₂-N>MnO₂-Cl>MnO₂-S,which was highly related to the specific surface areas,oxygen vacancy concentration and the content of adsorbed oxygen.Then,the effects of the content of gas component（O₂,H₂O and toluene）and weight hourly space velocity（WHSV）on toluene oxidation over MnO₂-Ac catalyst were investigated.Gas-phase O₂ had a significant influence on toluene oxidation.H₂O had an inhibiting effect on the catalytic activity of MnO₂-Ac.The catalytic activity decreased with the increasing of H₂O content in feed gas.However,MnO₂-Ac has excellent water-resistance for toluene oxidation at 300°C.The toluene conversion was always remained at about 100%during 35 h tests.Toluene concentration has negligible effect on the catalytic activity.The activity of MnO₂-Ac decreased with increasing WHSV,but the 90%toluene conversion was also achieved at a relative low temperature under high WHSV.So it can be concluded that MnO₂-Ac is an ideal material for removing toluene.Finally,the reaction pathway of toluene oxidation over MnO₂-Ac was presented with the aid of In-situ DRIFTS.Benzoyl oxide,benzaldehyde,benzoate,maleic anhydride and long-chain carboxylate were the main intermediates.