Study On Mechanism For Selectively Catalytic Reduction Of NO With NH₃over Mixed Oxides By In Situ FTIR And Dynamics

Nitrogen oxides （NO_x） contribute much to acid rain, ozone depletion and photochemicalsmog. In China, the pollution of acid rain is changing from sulfuric acid-type to nitric-sulfuriccomplex one. After desulfurization has achieved remarkable progress during the NationalEleventh Five-Year Plan, the total amount of NO_xemission has been controlled during theNational Twelfth Five-Year Plan. The removal of NO_xis becoming one of the most concernedindustries in the aspect of environmental protection. The commercial technology for NO_xcontrol is the selectively catalytic reduction （SCR） of NO_xwith NH₃. Therein, the SCR catalystis the most important. In addition to the composition of catalysts, the structure still possesses adecisive impact on the performance of catalysts. Previous studies have showed that a lot ofamorphous catalysts present high activity, and the working state of some crystalline catalystsis also amorphous. However, the component of the active site and the reaction mechanism stillcannot be confirmed completely.In this paper, Ce–Ti amorphous composite oxides can be obtained simply by adjustingthe molar ratios of Ce/Ti using a coprecipitation method. X-ray diffraction （XRD） resultsshow that catalysts present completely amorphous state when the Ce/Ti molar ratio iscontrolled between0.3and0.7. The activity evaluation results show that amorphous catalystsexhibit higher catalytic activities compared with crystal samples. The structure of amorphoussamples is studied by in situ XRD. Results show that amorphous state can be maintined below550℃，and CeTi₂O₆phase presents at high temperatures. The results of in situ FTIR spectra ofNO adsorption suggest a strong synergistic effect between Ce and Ti. Due to the X-rayabsorption fine-structure （XAFS） experiments show the absence of the Ce–O–Ce structure inCe–Ti amorphous composite oxide, the assignment of the activity to the crystalline andamorphous CeO₂can be excluded. However, the detected Ti–O–Ti local structure （TiO₂）shows much low activity to SCR reactions. Thus, the Ce–O–Ti short-range order structure isconfirmed as the active site for the NH₃-SCR reactions.The results of temperature programmed desorption （TPD） with NH₃show that TiO₂possesses a strong ability of NH₃adsorption, and NH₃is adsorbed on the Ti site of Ce_aTiO_x.Desorption peaks move to lower temperatures with the addition of Ce, which makes activation of NH₃become easier. NO-TPD results suggest strong interaction between Ce andTi in Ce_aTiO_x, because only one NO_xdesorption peak is observed, while CeO₂exhibit twodesorption peak.The catalytic mechanism was studied by in situ FTIR technology, which includesadsorption under different reaction atmosphere （NO_xand/or NH₃） and temperature prgramedsurface reaction （TPSR）. The adsorption experiments results showed that ammonia speciesexist in two forms: the adsorbed NH₃（combined with the Lewis acid sites） and NH₄⁺（combined with Br nsted acid sites）. Bridged nitrate and monodentate nitrate are the mainNO_xadsorbed species on the surface of Ce_aTiO_x. Furthermore, a small amount of bidentatenitrate is also observed on the surface of Ce_0.3TiO_x. When the surface of the catalyst isintroduced to NO+NH₃+O₂, the formation of NH₃and NO_xadsorbed species is observedwithin initially5min on the three samples. But when adsorption is saturated, Ce_0.3TiO_xandCe_0.5TiO_xpresent only NH₃adsorbed species, while a small amount of bridged nitrate is stillexisted on the surface of Ce_0.7TiO_x. This is due to the the lower activity of Ce_0.7TiO_xcatalystat100℃. Based on the results of TPSR after co-adsorption, we found that two kinds ofammonia adsorbed species are involved in NH₃-SCR reaction, but NH₄⁺on Br nsted acidsites is mainly functioning at lower temperature, while adsorbed NH₃on the Lewis acid sitesmainly works at higher temperature. NH₃speice can be activated easily on the surface ofcatalysts, and the adorption of NH₃is a critical step of the NH₃-SCR reaction. The wholereaction followed the E-R mechanism, in which NO_xparticipate in the SCR reaction withadsorbed NH₃species in the form of gas phase.The kinetic experiments were carried out after the exclusion of internal and externaldiffusion effects. The intrinsic activity （normalized reaction rate） and the activation energy ofthe catalysts are tested in dynamics domain. Although Ce_0.3TiO_xhas a higher apparent activity,the reaction rate of Ce_0.5TiO_xis the highest. The sequence of reaction rate at170℃isCe_0.5TiO_x> Ce_0.7TiO_x> Ce_0.3TiO_x＞Ce_0.2TiO_xCe₁TiO_x. The results of activation energymeasurement also show that Ce_0.5TiO_xhas the lowest activation energy, which equals to37.66kJ/mol.