Studies Of CuO/Sn_xTi_1-xO₂ Catalysts For NO Reduction

Nitric oxide(NO_x)is one of the major atmospheric pollutants,and the abatement of NO_x has been an important task of environment protection.NO reduction by CO is known as one of the most important reactions.The selection of catalyst is crucial for the efficiency of NO and CO reaction.Up to date,the main applied catalysts for DeNO_x are noble metal catalysts,which are confronted with resource shortage. Therefore,the use of non-noble metal catalysts has attracted much attention.TiO₂ is good at resisting water and trace SO₂ along with high N₂ selectivity in the process of DeNO_x.There have been reports of NO_x abatement by TiO₂ modified catalysts.SnO₂ has plenty of lattice oxygen on its surface and interacts strongly with other active components,and has been extensively studied in the field of catalysis. Sn_xTi_1-xO₂ is more active and stable than TiO₂ and SnO₂,and thus CuO/Sn_xTi_1-xO₂ may play an important role in the NO+CO reaction.In this study,TiO₂,SnO₂ and Sn_xTi_1-xO₂ complex oxides were used as supports, and the catalysts are prepared by impregnation method.The activitis of CuO/TiO₂, CuO/SnO₂ and CuO/Sn_xTi_1-xO₂ in the NO+CO reaction were examined by means of BET,TG-DTA,H₂-TPR,XRD,Raman and FT-IR.The reaction mechanisms on 12% CuO/Sn_0.8Ti_0.2O₂ were also discussed.The main conclusions from this study are as the followings.1.The structure of TiO₂ and SnO₂ and catalytic activity of CuO/TiO₂ and CuO/SnO₂ in the NO+CO reactionCalcined temperature had a great effect on the structure of TiO₂ and SnO₂.XRD and Raman results showed that TiO₂ changed from anatase to rutile after calcination at 600℃.When calcined temperature reaches 700℃,TiO₂ existed only in the form of rutile.In comparison,SnO₂ became fully crystal when it was calcined at 600℃.After calcination at 600℃,the catalytic activity of CuO/SnO₂ was higher than that of CuO/TiO₂.NO conversion by 6%CuO/SnO₂(600℃)was 90%at reaction temperature of 300℃,compared to 83%NO conversion by 6%CuO/TiO₂(600℃)at reaction temperature of 375℃.TiO₂ and SnO₂ has low activity in NO+CO reaction,and NO conversion by TiO₂ and SnO₂ was only 14%and 13%at 500℃,respectively.After CuO was loaded,the catalytic activity was largely improved.NO conversion by 1%CuO/TiO₂ and 1% CuO/SnO₂ reached 29%and 55%at 500℃,respectively.As a whole,CuO/SnO₂ had higher activity than CuO/TiO₂ in the NO+CO reaction.As shown by H₂-TPR analysis, CuO/TiO₂ catalyst has four reduction peaks(α,β,γ,δ),theαpeak was attributed to the reduction of highly dispersed CuO on the surface of TiO₂;theβpeak to the isolated CuO;theγpeak to the CuO crystal influenced with TiO₂;and theδpeak to the CuO crystal granule.By comparison,CuO/SnO₂ catalyst had two reduction peaks(δandζ)at high temperature,indicating that the reduction of SnO₂ likely involved in two successive steps:Sn⁴⁺→Sn²⁺→Sn⁰.At low temperature, however,the reduction peaks of CuO/SnO₂ catalyst were related to the amount of CuO loading and different forms of CuO might exist on SnO₂.2.Catalytic activity and mechnisms of CuO/Sn_0.8Ti_0.2O₂ in the NO+CO reactionCuO/Sn_0.8Ti_0.2O₂ had high activity in NO+CO reaction,showing 93%NO conversion at 300℃in air and 100%NO conversion at 225℃after pretreatment with H₂.Raman analysis indicated Sn_0.8Ti_0.2O₂ was not simply compounded by TiO₂ and SnO₂ but formed new crystal.As shown by XRD analysis,there was no CuO crystal diffraction peak at 9%CuO loading,but two CuO crystal diffraction peaks at 2θ35.5°and 38.7°were present at 12%CuO loading.FT-IR detected the adsorption of NO and CO on the surface of reduced 12%CuO/Sn_0.8Ti_0.2O₂.The Cu²⁺sites and support surface adsorbed NO,and the process of NO adsorption led to the formation of N₂O and NO₃^-.In contrast,the Cu⁺,Cu⁰ sites and support surface adsorbed CO.When the mixed gas of NO and CO were adsorbed by the support surface,no NO₃^- was formed. H₂-TPR showed four reduction peaks(α,β,γandδ).Whileα,βandγpeaks were the reduction of CuO species,andδpeak was the reduction of Sn_0.8Ti_0.2O₂. 3.The study of NO and/or CO adsorption by catalystNO and CO were adsorbed by TiO₂,SnO₂ and Sn_xTi_1-xO₂.The bands at 1844～1907cm^-1were assigned as NO adsorption,the bands at 2113 and 2171cm^-1as CO adsorption,and an abvious band at 1300cm^-1to nitrate.The bands at 1600 and 1628cm^-1were assigned to NO₂ but not detected at high temperature,suggesting that chem-adsorbed NO₂ could be decompounded at high temperature.When NO+CO co-adsorbed on the surface of CuO/Sn_0.8Ti_0.2O₂ pretreated at O₂ atmosphere,bands of nitrate and carbonate were not observed and band of NO₂ disappeared.Gas-phase N₂O was present at low temperature and its adsorption become stronger with increase in CuO loading at the same temperature.After NO and CO were adsorbed by 12%CuO/Sn_0.8Ti_0.2O₂,IR band at 2235cm^-1was assigned to N₂O with high band initially and then low band as adsorption temperature increased, and finally could not be detected at 500℃.The bands at 3594～3782cm^-1were detected only at 150℃and assigned to CO₂.H₂ pretreatment greatly influenced NO and CO adsorption.When NO and CO were adsorbed by H₂-pretreated 12%CuO/Sn_0.8Ti_0.2O₂,band at 2235 cm^-1at 200℃was assigned to N₂O but not detected at 300℃,this was likely due to the high activity of H₂-pretreated catalyst which was in favor of NO conversion to its final products at low temperature.When 12%CuO/Sn_0.8Ti_0.2O₂ was pretreated at H₂ atmosphere at 400℃,band at 2235cm^-1 was small at 100℃and became bigger at 200℃and then vanished at 400℃,which may be caused by the diversification of the main active centers.