Study On Preparation And Catalytic Performance Of Cr-based Catalysts For NO Oxidation

Nitrogen oxide(NO_x)is one of the main atmospheric pollutants,which can cause the environmental pollution phenomenon,such as acid rain,haze,et al,then leading to the undesirable severe damage to the environment and human health.Owing to very strict regulations on NO_x emissions,nitric oxide(NO),as the main source of NO_x,must be carefully controlled prior to their release into the atmosphere.The handy and inexpensive way to remove NO_x from industrial furnace is that using the rest O2 in furnace gas,NO is partly catalytic oxidized to NO2 under catalysis,then the solubility N02 is adsorbed in the alkali liquor to complete the NO_x removal process.NO catalytic oxidation technology can realize the removal of NO_x and SO2 at the same time and the cyclic utilization of N sources.In this study,several Cr-based catalysts were investigated by cerium modification,pre-modification and nonmetal doping,aiming to decrease the starting temperature with high conversion,widen the temperature window and enhance the ability to resist H2O(g)and SO2 under various furnace gas conditions.The obtained catalysts were characterized by various techniques,such as X-ray diffraction(XRD),Brunauer-Emmet-Teller(BET),Fourier transform infrared spectroscopy(FTIR),H2 temperature-programmed reduction(H2-TPR),Temperature-programmed desorption of NO(NO-TPD),X-ray photoelectron spectroscopy(XPS),in-situ diffused reflectance infrared Fourier transform spectroscopy(in-situ DRIFTS).This study is mainly focused on:(1)exploring the relationship between the physical structure and chemical properties;(2)understanding the influence of redox performances,activated ability of reaction atmosphere to confirm the active sites;(3)investigating the nature of surface-bound species as a function of reaction atmosphere via DRIFTS technique to further understand the possible reaction model of NO oxidation.After the screening of support(metal-pillared Na-montmorillonite,including TP,ZP,FP)and active component(Mn,Co,Cr,Cu),the 12%Cr/TP catalyst was selected as the research object.The characterization results showed that Cr2O3 and CrO3 scattered on the surface of Cr/TP catalyst,CrO3,as the main active site,was beneficial to adsorb and activate NO.In-situ DRIFTS results showed that the stability of surface species was closely associated with the reaction temperature.It confirmed the presence of the intermediate nitrosyl(NO+)and nitrate,and then revealed the NO oxidation process of the Cr-based catalyst,which laid the foundation theory research for follow-up study.To investigate the effect of rare earth cerium on Cr-based catalyst,the Cr1-xCex/TP(0?x III<1)catalysts were prepared by the ultrasonic assisted impregnation method.This study was mainly focused on the synergistic effect in textural properties,redox performance and surface species between them.It was found that the oxidized CrO3 and the reduced Ce2O3 could play the different roles in the NO oxidation.That is,the oxidized CrO3 was beneficial to adsorb and activate NO to form nitrosyl(NO+),which was further oxidized to nitrates species.Meanwhile,the reduced Ce2O3 was inclined to activate O2 via oxygen vacancies to generated nitrates.The kinetic fitting results indicated that the reaction order with respect to NO was 0.53,while the reaction order with respect to O2 was 0.44.To further improve the catalytic performance of NO oxidation,the Cr-HP(x)/TP catalyst was prepared by the deposition-precipitation method and H2O2 was added as the pre-modification agent.The characterization results demonstrated that more Cr(VI)species accompanied with high dispersion and low crystallinity were generated via the pre-modification,which were conductive to preferentially adsorb and activate NO to improve the catalytic performance.Nevertheless,two Cr(VI)species played the different roles in the catalytic reaction.That is,oxide state CrO3 was beneficial to form the redox reaction with Cr2O3,and chromate with anionic form was adverse to accumulate SO2 to improve their resistance.As a result,it could not only expose more active sites but also increase the contact area between the catalyst and the reactant gas.In order to retain the active ingredient and simplify the preparation procedure,the Cr/CeTi-C(x)catalysts were evaluated using a combination of sol-gel method and deposition-precipitation method.The characterization results demonstrated that the addition of cyanamide was conducive to tuning O2 activation and NO adsorption simultaneously.Moreover,the designed FTIR and Raman measurements were applied to identify the doping sites,that is,N species were inclined to substitute the O atoms around cerium to form the Ce-N-Ti and Ce-N-Ce bonds.In addition,because of the similar atomic radius between nitrogen(N)and oxygen(O),nitrogen doping led to shrinkage and distortion of lattice to form obvious defect structure.The oxygen vacancy promoted the oxygen adsorption and activation,which illustrated the function of superoxide radicals.