Comparative Study Of Mesoporous Catalysts For Catalytic Oxidation Of VOCs

Volatile Organic Compounds(VOCs)are the precursor of atmospheric aerosol,which can cause damage to environment and human.Catalytic oxidation is an effective method on the removal of VOCs.Meanwhile,mesoporous silica is a common supporter and template in catalytic oxidation of VOCs due to its large specific surface area and uniform pore size distribution.Mesoporous manganese oxide and cobalt oxide catalysts were synthesized successfully with mesoporous silica and commercial silica as the supporter or the template.The influence of mesoporous structure on the catalyst morphology and the physicochemical properties of catalysts was explored.Meanwhile,the influence of mesoporous structure on VOCs catalytic oxidation with different molecular size was investigated.(1)The catalytic oxidation activity of silica-supported manganese oxide catalysts depends on the structure of VOCs and the characteristic of catalysts.As for the catalytic oxidation of alkanes,Mn/SBA-15 features poor activity while catalysts with more exposed oxide surfaces perform better,suggesting that the amount of accessible active sites is a crucial factor determining the catalytic performance.In contrast,for the oxidation of aromatic hydrocarbon which occurs at high temperatures,Mn/SBA-15 performs best,suggesting that the quality rather than the quantity of active sites is crucial to the ignition of recalcitrant hydrocarbons.In comparison to ordered mesoporous silicas,the commercial silicas show better catalytic oxidation activity of large-molecule hydrocarbons because they have shorter mesopores length and more open structure.(2)The catalytic oxidation activity of silica-supported cobalt oxide catalysts also depends on the type of VOCs and the characteristic of catalysts.Co/MCM-41 has the lowest initial hydrogen reduction temperature,indicating the catalyst has the best reducibility,and the catalytic oxidation activity is better.In contrast,Co/SBA-15 which has the highest initial hydrogen reduction temperature has worse catalytic oxidation activity,suggesting the reducibility ability is the crucial factor on the catalytic oxidation activity.As for the catalytic oxidation of large-molecule hydrocarbons,Co/KIT-6 which possessing larger pore diameter and 3D pore structure has less mass transfer resistance for large-molecule hydrocarbons,and shows great catalytic oxidation activity as a result.Meanwhile,commercial silica-supported cobalt oxide catalysts show better activity because of their short aperture and small particles aggregation,suggesting the more open structure is beneficial to the catalytic oxidation activity.(3)The mesoporous manganese oxide catalysts synthesiszed by mesoporous silica templates have not completely copied the pore structure of templates comparing with commercial silica templated mesoporous manganese oxide catalysts.More open pore structure is beneficial to the transfer of manganese nitrate,therefore manganese oxides can easily copy pore structure of templates.As for catalytic oxidation of propane,the reducibility of catalysts is the most important factor.Well reducibility have positive effect on the catalytic oxidation of VOCs.The quantity and quality of active sites is crucial to larger molecule hydrocarbon catalytic oxidation.The catalytic oxidation activity is better when the access to high activity sites is easier.Manganese oxide catalysts prepared by precipitating method have more pore channel,so the transport of larger molecule hydrocarbon is easier and the catalytic oxidation activity is better.(4)The cobalt oxide catalysts synthesized with silica templates have worse catalytic oxidation activity towards VOCs comparing with the cobalt oxide catalyst synthesized by precipitation method.As for catalytic oxidation of propane,the main influence factor is the initial hydrogen reduction temperature of catalysts.But for the catalytic oxidation of larger molecule hydrocarbon,the catalytic oxidation activity of these catalysts is better with lower temperature of reduction peak and more active oxygen species supplied in the range of catalytic oxidation temperature.Besides,short pore length and 3D structure have positive effect on the transfer of larger molecule hydrocarbons,which can promote the process of VOCs catalytic oxidation.The cobalt oxide catalysts synthesized by commercial silicas show great catalytic oxidation activity on alkane,while the catalytic oxidation capacity on aromatic hydrocarbon is worse.