Reaction Mechanism Of Vehicle Exhausts (CO,HC And NO_x) On The Ceria-zirconia Solid Solutions

Automobile exhaust produces during the operation of automobiles.including carbon monoxide(CO),nitrogen oxides(NOx),hydrocarbons(HC)and solid suspended particulate matter(PM).which is the main mobile source of urban air pollution.When CO2 enters the atmosphere,it will result in greenhouse effect.NOx will promote the formation of acid rain,cause soil and water acidification,affect the growth of crops and forests;HC and NOx can form photochemical smog in sunlight,leading to urban air pollution.It is necessary to control and eliminate automobile exhaust emission.At present.installing catalytic converter in engine exhaust system is considered to be the most effective technology to prevent automobile exhaust pollution.Three-way catalyst can simultaneously eliminate CO,NOx and HC and convert them into H2O,CO2 and N2,the removal efficiency can reach more than 90%.Ceria-zirconia solid solutions(CexZr1-xO2)is widely used as the active component and supported material for threeway catalysis,due to its good oxygen storage/release capacity and thermal stability.These properties can broad the working window of air-fuel ratio and improve the catalytic performance of catalysts.Many scholars have studied the reaction mechanism of CO,NOx and HC on the CexZr1-xO2 surface.and it is generally believed that reaction mechanism following the Mars-van Krevelen(MvK)mechanism.However,the active intermediates in the reaction process are difficult to detect by experimental technologies,so there are still many controversies on active intermediates species,and the detailed catalytic reaction mechanism is not yet clear.In order to reveal the reaction mechanism of CO,NOx and HC on the CexZr1-xO2 surface.based on density functional theory(DFT)method,the reaction paths of NO reduction with CO(CONO),HC selective catalytic reduction of NOx(HC-SCR)and NH3 selective catalytic reduction of NOx(NH3-SCR)on Ce0.875Zr0.125O2(110)surface were systematically calculated.The role of active intermediates in the catalytic cycle was elucidated and the optimal catalytic reaction path was proposed.Moreover,the catalytic activity of Pt,Rh,Pd.Ag and Au loaded Ce0.875Zr0.125O2 single atom catalysts for CO and C3H6 oxidation and NOx reduction was evaluated,to better understand the structure-activity relationships between single stom catalysts and exhaust reactions,and guide the design of new generation of green and efficient catalysts.1.Catalytic reaction mechanism of CO-NO on the Ce0.875Zr0.125O2(110)surfaceCO-NO reaction can simultaneously convert two kinds of pollutants(CO and NO)into N2 and CO2,which is a promising heterogeneous catalytic reaction.Based on the DFT method,the reaction mechanism of CO-NO on Ce0.875Zr0.125O2(110)surface was calculated.Firstly,the surface reduction properties of CexZr1-xO2 was calculated under the theoretical calculation conditions,Ce0.875Zr0.125O2 exhibited the optimum composition to efficiently release surface lattice O among the whole composition range.Then,the adsorption behavior of CO on Ce0.875Zr0.125O2(110)surface was calculated.It was found that the most stable adsorption site was ZrT site with the adsorption energy of-0.57 eV.Two reaction paths of CO-NO reaction were explored,path 1 involving bent N2O intermediate and path 2 involving NCO intermediate.The reaction mechanism showed that CO-NO reaction following the Mars-van Krevelen(MvK)and Eley-Rideal(E-R)mechanisms.By comparing the reaction energy barrier,it was found that the reaction energy barrier of rate-determining step of path 2 was lower than that of path 1,and path 2 was more likely to occur in the thermodynamically viewpoint.The reaction energy barrier of rate-determining step was 1.34 eV,which could occur at room temperature.This study showed that both N2O and NCO species were important intermediates of CO-NO reaction on Ce0.875Zr0.125O2 surface,and Ce0.875Zr0.25O2 showed high catalytic activity for CO-NO reaction.2.Mechanistic insight into the selective catalytic reduction of NOx with propene on the Ce0.875Zr0.125O2(110)surfaceSelective catalytic reduction of NOx with hydrocarbons(HC-SCR)is a promising technology for the abatement of NOx from vehicles.The reaction mechanism was mainly investigated by in situ diffuse reflectance infrared Fourier transform spectroscopy(DRIFTS)experiments,but the detailed reaction scheme is not yet clear,especially the complex interactions between catalyst interfaces and gas molecules.In this study,by using the periodic density functional theory(DFT)method,we proposed a SCR-C3H6 scheme on the Ce0.875Zr0.125O2(110)surface.Firstly,C3H6 was partially oxidized by lattice or adsorbed oxygen to the acyl group following the Mars-van Krevelen(MVK)mechanism;then,the acyl group combined with nitrate to generate an RCOO-NO2 intermediate,followed by decarboxylation and O-shift reactions to produce R-NO2,R’CH-ONO,NCO and CN species,which were finally converted into CO2 and N2.The simulation results are in good agreement with experimental values.In addition,some catalytic characteristics were found:(1)NO3 could adsorb steadily on brid1,brid2,biden and monoden sites of the Ce0.875Zr0.125O2(110)surface;(2)the activity sequence of C3H6 by lattice O was Csp3-H bond>Csp2-H bond>C=C bond;(3)NCO was formed more easily than CN on the Ce0.875Zr0.125O2(110)surface due to the admirable activity of surface lattice oxygen.3.Theoretical and experimental insights reveal the structure-activity relationships of NbOx and MoOx modified mesoporous Ce0.67Zr0.33O2-based catalysts for NH3-SCRHighly dispersed mesoporous NbOx-Ce0.67Zr0.33O2 and MoOx-Ce0.67Zr0.33O2 catalysts exhibited good selective catalytic reduction of NOx with NH3(NH3-SCR)activity(≥ 90%NOx conversion,250～450℃)and superior H2O and SO2 tolerance and thermal stability,which were attributed to the promoted redox properties,acidity and structural characteristics.To thoroughly understand the structure-activity relationships,the model between acidic species and Ce-based oxides was constructed according to catalysts characterization results.And the theoretical calculation results based on the density functional theory(DFT)method indicated that the electrons transferred from surface Ce atoms to doped NbOx or MoOx species,resulting in the accept electrons capability of Ce were enhanced,therefore,the Lewis acidity of surface Ce atoms were improved,which is in good agreement with the results of NH3-TPD and the adsorption energies of NH3.Besides,after the addition of NbOx and MoOx,the oxygen vacancy formation energy was obviously reduced compared with Ce0.67Zr0.33O2,which can interpret the experimental phenomena that there were more oxygen vacancies on NbOx-Ce0.67Zr0.33O2 and MoOx-Ce0.67Zr0.33O2 samples.The "Standard SCR" and "Fast SCR" reaction mechanisms were proposed on NbOx-Ce0.67Zr0.33O2 surface involving redox cycle depended on the good redox properties of surface lattice O and acid site cycle depended on NbOx species offered acid sites.Additionally,the mesoporous structure not only promoted the dispersion of NbOx and MoOx but also provided sufficient exposed active sites so that the NH3-SCR performance was not effected by H2O and SO2.4.Catalytic activity of Pt,Rh,Pd,Ag and Au loaded Ce0.875Zr0.125O2(110)single atom catalysts for the eliminate of vehicle exhaustsThe single atom catalyst can maximize the utilization of active component of the catalyst and reduce the use of precious metal to achieve the highest catalytic performance.Based on the DFT method,the structure model of(Pt,Rh,Pd,Ag,Au)1/Ce0.875Zr0.125O2 single atom catalyst was constructed,and the catalytic activities of CO and C3H6 oxidation and NOx reduction on the surface were calculated.The theoretical calculation results show that Pt,Rh,Pd,Ag and Au atoms could be stably bonded on the Ce0.875Zr0.125O2(110)surface,and there were deformation electron density orbital overlap and electron transfer between single atoms and Ce0.875Zr0.125O2 surface.There was a strong interaction between single atom and Ce0.875Zr0.125O2 surface.The energy barrier of single-atom catalysts for the oxidation of CO and C3H6 shows that the loading of Pt,Rh,Pd,Ag and Au could improve the CO oxidation activity of Ce0.875Zr0.125O2,but could not improve the oxidation activity of C3H6.The activity sequence of(Pt,Rh,Pd,Ag,Au)1/Ce0.875Zr0.125O2 catalyzed H2 dissociation is Rh>Pt>Pd>Ag>Au,Rh1/Ce0.875Zr0.125O2 showed the highest catalytic activity for H2 dissociation.The subsequent catalytic cycle reaction mechanism of H2-SCR on Rh1/Ce0.875Zr0.125O2(110)surface was also calculated.The reaction energy barrier of rate-determining step was 0.92 eV,which was not high,and the reaction could proceed smoothly at room temperature.It also indicates that Rh1/Ce0.875Zr0.125O2 has strong catalytic activity for H2-SCR reaction.This study could explain the structural characteristics of single-atom catalysts at atomic-scale,and guide the design of new generation of green and efficient catalysts.