Effect of praseodymium additive on CeO_2(ZrO_2)/TiO_2 for selective catalytic reduction of NO by NH_3

A series of praseodymium added CeO_2(ZrO_2)/TiO_2 catalysts separately prepared by methods of sol-gel and impregnation were tested for selective catalytic reduction of NO, and characterized by X-ray diffraction(XRD), N_2-brumauer-emmett-teller(N_2-BET), NH_3-temperature programmed desorption(NH_3-TPD), H_2-temperature programmed reduction(H_2-TPR), PL spectra, Raman spectra, electron paramagnetic resonance(EPR) and X-ray photoelectron spectroscopy(XPS), respectively. Influence of preparation method on catalytic performance was studied. Results showed that the influence of Pr addition on catalytic performance of the CeO_2(ZrO_2)/TiO_2 catalysts was different between the sol-gel method and the impregnation method. The Pr addition tended to interact with TiO_2 and formed the structure of Ti-O-Pr in the sol-gel method while it was more likely to interact with CeO_2 forming the structure of Ce-O-Pr in the impregnation method. The total acid amount and redox properties of the catalysts prepared by sol-gel method decreased with the addition of Pr element, which resulted in decrease of catalytic activity. In contrast, the Pr-added catalyst prepared by impregnation method was found to possess easier reducibility, more total acid amount and higher proportion of Ce~(3+) species, which was favourable for higher catalytic activity.     

High activity of CeO_2-TiO_2 composites for deep oxidation of 1,2-dichloroethane

CeO_2-TiO_2 catalysts prepared by different methods were investigated for deep oxidation of 1,2-dichloroethane(DCE),as a typical representative of the chlorinated volatile organic compounds(CVOCs).Characterization analysis reveals that CeO_2-TiO_2 catalysts prepared by sol-gel and coprecipitation methods exhibit higher specific area,CeO_2 and TiO_2 particles are highly dispersed into each other and the reducibility and mobility of active oxygen species are obviously promoted due to the strong interaction between the two catalysts CeO_2 and TiO_2,resulting in higher catalytic activity for DCE oxidation to and less chlorinated byproduct.The high calcination temperature would lead to the formation of a new monoclinic phase Ce_(0.3)Ti_(0.7)O_2 and sintering,which is the main reason for the catalytic activity for DCE oxidation markedly decreases.     

Influence of CeO_2 loading on structure and catalytic activity for NH_3-SCR over TiO_2-supported CeO_2

A series of supported CeO_2/TiO_2 catalysts were prepared to explore the influence of CeO_2 loading on these catalysts for the selective catalytic reduction of NO_3 by NH_3(NH_3-SCR).The catalysts were investigated in detail by means of XRD,Raman,H_2-TPR,NH_3-TPD,XPS,in situ DRIFTS,and NH3-SCR reaction.The activity of the catalyst is closely related to the content of CeO_2.When the loading of CeO_2 is near the dispersion capacity(1.16 mmol Ce~(4+)/100 m~2 TiO_2),the catalytic activity is better.This may be because that the dispersed CeO_2 is the active species and the catalyst has appropriate redox property,along with the larger amounts of surface Ce content and surface adsorbed oxygen species.Finally,a possible reaction mechanism via the Langmuir-Hinshelwood(L-H) mechanism is tentatively proposed to further understand the NH_3-SCR reaction.     

Modification of Cu/ZSM-5 catalyst with CeO_2 for selective catalytic reduction of NO_x with ammonia

Cu/ZSM-5 and CeO_2-modified Cu/ZSM-5 catalysts were prepared by a wetness impregnation method. The addition of CeO_2 was found to enhance the NO_x selective catalytic reduction(SCR) activity of the catalyst at low temperatures, but the high-temperature activity was weakened. The catalysts were characterized by X-ray diffraction(XRD), nitrogen physisorption, inductively coupled plasma optical emission spectrometry(ICP-OES), X-ray photoelectron spectroscopy(XPS), electron paramagnetic resonance(EPR), H_2 temperature-programmed reduction(TPR) and NH_3 temperature-programmed desorption(TPD). The results showed that more CuO clusters instead of isolated Cu~(2+) species were obtained on the modified catalyst. These active CuO clusters, as well as the Cu-Ce synergistic effect, improved the redox property of the catalyst and low-temperatures SCR activity via promoting the oxidation of NO to NO_2 and fast SCR reaction. The loss in high-temperatures activity was attributed to the enhanced competitive oxidation of NH_3 by O_2 and decreased surface acidity of the catalyst.     

Effects of MO_x(M=Mn,Cu,Sb,La)on V-Mo-Ce/Ti selective catalytic reduction catalysts

A series of MO_x-V_2O_5-MoO_3-CeO_2/TiO_2(M=Mn,Cu,Sb,and La) catalysts were prepared via an impregnation method.The physico-chemical properties of the catalysts were characterized and their NH_3-SCR of NO performance was compared.The Mn-loaded catalyst(Mn5V1Mo3Ce7/Ti) exhibits a large number of acid sites of varying strength,and together with good reducibility of the catalyst,contributes to the optimal SCR performance.The sulphate species formed in the presence of SO_2 significantly enhance the H_2O and SO_2 tolerance of Mn5V1Mo3Ce7/Ti.The Cu-loaded catalyst(Cu5V1Mo3Ce7/Ti)demonstrates potential in flue gas applications in the absence of SO_2 at low temperatures because of the excellent redox ability observed and the high degree of weak acid sites.The Sb and La loaded catalysts(Sb5V1Mo3Ce7/Ti and La5V1Mo3Ce7/Ti),especially La5V1Mo3 Ce7/Ti,exhibit the largest number of acid sites and the lowest reducibility,and therefore,may be suitable for use in high temperature denitrification applications.     

CeO_2/CuO catalysts using different template agent for preferential CO oxidation in H_2-rich stream

The CeO_2/CuO catalysts using different template agent(F68 L64, F127 and P123) were synthesized by the simple template and impregnation method. They were characterized by FESEM, XRD, N2 physisorption and H2-TPR techniques. It is found that the CeO_2/CuO catalysts are double pore distribution, and CeO_2 can enter into the gap of CuO supports and form the contact interface of copper and cerium. Among the asprepared catalysts, the CeO_2/CuO-F127 catalyst displays better activity at lower temperature and the CeO_2/CuO-P123 catalyst presents higher activity at higher temperature. The CeO_2/CuO-P123 catalyst has the smallest crystallite sizes of CuO and CeO_2 as well as the largest size of cubes, which may improve the interaction of copper and cerium and enhance the performance of CO oxidation.     

Catalytic oxidation of formaldehyde over CeO_2-Co_3O_4 catalysts

A series of CeO_2-Co_3O_4 mixed oxide catalysts with different Co/(Co+Ce) atomic ratios were synthesized by citric acid sol-gel method and used for catalytic oxidation of formaldehyde(HCHO). Many techniques such as Brumauer-Emmett-Teller(BET), X-ray diffraction(XRD), scanning electron microscopy(FE-SEM), temperature programmed reduction(H_2-TPR), temperature-programmed desorption(O_2-TPD) and X-ray photoelectron spectroscopy(XPS) were used to characterize catalysts. The results of catalytic performance tests showed that the catalyst CeO_2-Co_3O_4 with Co/(Co+Ce) ratio of 0.95 exhibited the best performance, and the temperature of complete oxidation of HCHO was 80 oC. The analytical results indicated that the addition of CeO_2 enhanced the specific surface area of Co_3O_4 and the fine dispersion of both of them. Moreover, the strong interaction between CeO_2 and Co_3O_4 resulted in the unique redox properties, which enhanced the available surface active oxygen and led to high valence state of cobalt oxide species. All those effects played crucial roles in the excellent performance of CeO_2-Co_3O_4 for the HCHO oxidation.     

Synergistic catalytic removals of NO,CO and HC over CeO2 modified Mn-Mo-W-Ox/TiO2-SiO2 catalyst

A series of Mn-Mo-W-O_x/TiO_2-SiO_2 catalysts was modified with CeO_2 using an extrusion molding method. The catalytic activities of the obtained catalysts were tested for the synergistic catalytic removals of CO, NO and C_3H_8. The ratio of catalyst composition on catalytic activities for NH_3-SCR was optimized, which reveals that the molar ratio of Ti/Si was 9:1 and the catalyst containing 1.5 wt% CeO_2 and 12 wt% Mn-Mo-W-O_x exhibits the best catalytic performances. These samples were characterized by XRD, N_2-BET, Py-IR, NH_3-TPD, SEM/element mapping, H_2-TPR and XPS, respectively. Results show that the optimal catalyst exhibits more than 99% NO conversion, 86% CO conversion and 100% C_3H_8 conversion under GHSV of 5000 h~(-1). In addition, the GHSV has little influence on removal of NO when it is less than 15,000 h~(-1). Furthermore, the addition of CeO_2 will enhance the surface acidity, increase Mn~(4+)concentration and inhibit the grain growth, which are favorable for the excellent catalytic performance.Anyway,the 1.5 wt% CeO_2-12 wt% Mn-Mo-W-O_x/TiO_2-SiO_2 possesses outstanding redox properties,abundant acid sites and high Mn~(4+) concentration, which provide a guarantee for synergistic catalytic removal of CO, NO and HC.     

Cerium,manganese and cerium/manganese ceramic monolithic catalysts.Study of VOCs and PM removal

Ceramic supported cerium, manganese and cerium-manganese catalysts were prepared by direct impregnation of aqueous precursor, and characterized by scanning electron microscopy(SEM), X-ray diffraction(XRD), Brunauer-Emmett-Teller method(BET), temperature programmed reduction(H_2-TPR), X-ray photoelectron spectroscopy(XPS) acidity measurements and electrical conductivity. The catalytic activity was evaluated for volatile organic compounds(VOC)(ethanol, methyl ethyl ketone and toluene) oxidation. Additionally, catalysts were tested in particulate matter(PM) combustion. The characterization results indicated that Ce was in the form of Ce~(4+) and Ce~(3+), and Mn existed in the form of Mn~(4+) and Mn~(3+) on the surface of the Mn/AC sample and in the form of Mn~(4+) in the Ce/Mn/AC monolith. VOC oxidation results revealed that the Ce/Mn/AC sample showed an excellent performance compared with ceramic supported CeO_2(Ce/AC) and MnO_x(Mn/AC) samples. The PM combustion was also higher on Ce/Mn/AC monoliths. The enhanced catalytic activity was mainly attributed to the Ce and Mn interaction which enhanced the acidity, conductivity and the reducibility of the oxides.     

New insights into MnCe(Ba)O_x/TiO_2 composite oxide catalyst:Barium additive accelerated ammonia conversion

MnCeO_x/TiO_2 has been widely used in selective catalytic reduction(SCR) of NO_x at low temperature.However,it is often poisoned in the presence of water vapor and sulfur dioxide.In this work,the promotion mechanism of Ba modification was investigated.Results show that the doped BaO reacts with CeO_2 and forms BaCeO_3.This unique perovskite structure of BaCeO_3 significantly enhances NO oxidation and NH_3 activation of MnCeO_x/TiO_2 catalyst so that the NO conversion and the resistances to SO_2 improve.It is found that Ba species obviously promotes the NO adsorption ability and improve the redox properties of MnCeO_x/TiO_2 catalyst.While the acid properties of the catalyst are inhibited by Ba modification and among which Lewis acid sites are dominant for both MnCeO_x/TiO_2 and MnCe(Ba)O_x/TiO_2 catalysts.Furthermore,in situ DRIFT experiments reveal that the NO reduction upon MnCeO_x/TiO_2 and MnCe(Ba)O_x/TiO_2 catalysts follows both E-R and L-H mechanisms,in which L-H is preferred.Ba species enhances the formation of active nitrate species,which accelerates the NO reduction through L-H mechanism.It is interesting that although Ba species weakens the NH_3 adsorption,it induces the ammonia conversion to coordination ammonia,which in turn accelerates the catalytic reaction.     

La_2O_3掺杂对V_2O_5-WO_3/TiO_2催化剂SCR性能的影响

通过共沉淀法将La组分掺入到TiO_2载体中,采用浸渍法将V和W组分负载到La_2O_3-TiO_2复合氧化物上,制备出V2O5-WO3/La_2O_3-TiO_2催化剂。考察不同La_2O_3掺杂量对其NH3催化还原NO性能的影响,同时通过多种物理化学手段进行表征分析。XRD,NH_3-in situ DRIFTS,H_2-TPR,XPS和UV-vis DRS测试结果表明,元素La与Ti以La-O-Ti键相互作用,生成高度分散的镧物种,也使得TiO_2具有更好的热稳定性,但由于LaO_x物种与VO_x物种和WO_x物种之间的相互作用,降低了催化剂的氧化还原性和表面Brnsted酸数量,从而降低了其催化活性。     

Effects of support property on the catalytic performance of CeO2-ZrO2-CrOx for 1,2-dichloroethane oxidation

HZSM-5, Al₂O₃, TiO₂ and SiO₂ supported CeO₂-ZrO₂-CrO_x catalysts were prepared by deposition-precipitation method and tested for deep catalytic oxidation of 1,2-dichloroethane（DCE）, as one of the common chlorinated organic pollutants. All the catalysts were characterized by means of N₂ adsorption-desorption, X-ray photoelectron spectroscopy（XPS）, ammonia-temperatureprogrammed desorption（NH₃-TPD） and hydrogen temperature-programmed reduction（H2-TPR）. The characterization results revealed that there was strongly synergistic effect between the oxidizability of CZCr species and the acidity of supports, which obviously promoted the catalytic activity for DCE degradation. 20% CZCr/HZSM-5 showed the highest activity and good durability during the long-term continuous test. The catalytic activity decreased in the order: 20%CZCr/HZSM-5>CZCr>20%CZCr/TiO₂>20%CZCr/Al₂O₃>20%CZCr/SiO₂.     

Rare earth oxide modified Cu-Mn compounds supported on TiO₂ catalysts for low temperature selective catalytic oxidation of ammonia and in lean oxygen

Selective catalytic oxidation(SCO) of ammonia was carried out over Cu-Mn compounds catalysts modified with trivalent rare earth oxide Ce2O3 and La2O3 respectively.TiO2 was used as support and different ratio of O2 were tested in order to find an appropriate O2 concentration(vol.%),and the results showed that 1%O2(vol.%) was propitious to SCO of ammonia.The effects of the two rare earth oxides modified catalysts Ce2O3-Cu-Mn/TiO2 and La2O3-Cu-Mn/TiO2 on the catalytic activity and selectivity of ammonia oxidation were investigated under the reaction condition of 500 ppm ammonia,1%O2(vol.%),at the temperature from 125 to 250 oC.The results revealed the beneficial role of Ce2O3 and La2O3 in catalytic activity at low temperature and lean oxygen concentration,while the modification with Ce2O3 and La2O3 led to the negative influence on N2 selectivity.For the catalysts modified with Ce showed lower NO and N2O selectivity than the catalysts modified with La,then the effects of different Ce loadings on catalytic activity and selectivity were also considered,in combination with catalysts preparation methods,which include incipient wet impregnation,sol-gel method and co-precipitation.Results revealed that the catalysts prepared by sol-gel method obtained preferable catalytic activity compared with the others,reaching 99% ammonia at 200 oC,whereas 96% NO was detected.It also indicated that different catalyst preparation method significantly determined production distribution.     

Catalytic performance of a Pt-Rh/CeO_2-ZrO_2-La_2O_3-Nd_2O_3 three-way compress nature gas catalyst prepared by a modified double-solvent method

A Pt-Rh three-way catalyst(M-DS) supported on CeO_2-ZrO_2-La_2O_3-Nd_2O_3 and its analogous supported catalyst(DS) were developed via a modified double-solvent method and conventional double-solvent method, respectively. The as-prepared catalysts were characterized by N_2 adsorption-desorption, X-ray diffraction(XRD), CO-chemisorption, X-ray photoelectron spectroscopy(XPS) and hydrogen temperature-programmed reduction(H_2-TPR). The preformed Pt nanoparticles generated using ethanol as a reducing agent on M-DS presented enhanced Pt dispersion regardless of aging treatment as confirmed by XRD and CO-chemisorption measurements. The textural properties and reduction ability of M-DS were maintained to a large extent after aging treatment. This result was consistent with those of the N_2 adsorption-desorption and H_2-TPR, respectively. Meanwhile, the XPS analysis demonstrated that higher Pt~0 species and larger Ce~(3+) concentration could be obtained for M-DS. In the conversion of a simulated compressed natural gas(CNG) vehicle exhaust, both fresh and aged M-DS showed a significant enhancement in the activity and N_2-selectivity. Particularly, the complete conversion temperature(T_(90)) of CH_4 over the aged M-DS catalyst was 65 oC lower than that over the aged catalyst by conventional double-solvent method.     

Influence of cobalt on performance of Cu-CeO_2 catalysts for preferential oxidation of CO

Copper and cobalt oxides supported on CeO_2 were investigated for preferential oxidation of carbon monoxide(CO-PROX) in the presence of excess hydrogen and CO_2.(Cuo)_(1-x)(Co_3 O_4)_(x/3)-(CeO_2)_(2.5)(x=0,0.25,0.50,0.75,0.85 and 1) catalysts were prepared by coprecipitation method.These mixed oxide catalysts were characterized by several physicochemical techniques,such as BET surface area(S_(BET)),X-ray diffraction(XRD),high resolution transmission electron microscopy(HRTEM),temperature programmed reduction(TPR) and X-ray photoelectron spectroscopy(XPS).XRD studies show the peaks related to CuO and Co_3 O_4 phases in copper and cobalt containing CeO_2 catalysts.The average particle size of the CeO_2 crystallites is in the range of 8-10 nm as evaluated from HRTEM studies.XPS studies demonstrate that Cu,Co and Ce in(cuO)_(1-x)(Co_3O_4)_(x/3)-(CeO_2)_(2.5) catalysts are presented in+2 and +1,+3 and +2 and +4 and +3 oxidation states,respectively.The catalyst with x=0.75 shows better activity and selectivity towards CO-PROX.Though the catalyst with only copper(CuO-CeO_2,x=0) shows good activity but reverse water gas shift(RWGS) reaction is noticed at high temperature.On the other hand,RWGS reaction is suppressed on the cobalt containing CuO-ceO_2 catalyst.Cobalt on CeO_2 with x=1 shows hardly any activity for PRoX reaction at low temperatures.No methanation activity is observed on CuO-CeO_2 or Co_3O_4-CeO_2 catalysts.In contrast,combination of copper and cobalt on CeO_2 shows methanation of CO where enhanced activity is observed with increasing in cobalt content.     

New insights into the role of vanadia species as active sites for selective catalytic reduction of NO with ammonia over VO_x/CeO_2 catalysts

A series of VO_x/CeO_2 catalysts we re synthesized via vanadia supported on ceria with different BET surface areas.The catalysts were employed to investigate the active sites for the selective catalytic reduction of NO with NH_3(NH3-SCR).The kinetic results show that VO_x/CeO_2 catalysts exhibit nearly constant apparent activation energies(E_a),indicating the same SCR reaction mechanism.The V-O-Ce bridging modes and oligomeric VO_x were identified and quantified by Raman,FT-IR and H_2-TPR.The amounts of the V-O-Ce bridging modes calculated by H_2-TPR are correlated to the NH_3-SCR intrinsic reaction rates.The turnover frequencies(TOFs) show a constant value at the same temperature,which were calculated based on the number of V-O-Ce bridging modes of VO_x/CeO_2 catalysts.Therefore,it can be concluded that the V-O-Ce bridging modes are the active sites of VO_x/CeO_2 catalysts for the NH3-SCR reaction.     

Promotion effect of niobium on ceria catalyst for selective catalytic reduction of NO with NH3

The CeO₂, Ce–Nb–O_x and Nb₂O₅ catalysts were synthesized by citric acid method and the promotion effect of Nb on ceria for selective catalytic reduction (SCR) of NO with NH₃ was investigated. The catalytic activity measurements indicate that the mixed oxide Ce–Nb–O_x presents a higher SCR activity than the single oxide CeO₂ or Nb₂O₅ catalyst. In addition, the Ce–Nb–O_x catalyst shows high resistance towards H₂O and SO₂ at 280 °C. The Raman, X-ray photoelectron spectra and temperature programmed reduction with H₂ results indicate that the incorporation of Nb provides abundant oxygen vacancies for capturing more surface adsorbed oxygen, which provides a superior redox capability and accelerates the renewal of active sites. Furthermore, the Fourier transform infrared spectra and temperature programmed desorption of NH₃ results suggest that niobium pentoxide shows high surface acidity, which is partly retained in the Ce–Nb–O_x catalyst possessing a high content of Lewis and Brønsted acid sites. Therefore, the incorporation of Nb improves both the redox and acidic capacities of Ce–Nb–O_x catalyst for the SCR reaction. Here, the redox behavior is primarily taken on Ce and the acidity is well improved by Nb, so the synergistic effect should exist between Ce and Nb. In terms of the reaction mechanism, in situ DRIFT experiments suggest that both NH₃ on Lewis acid sites and NH₄⁺ on Brønsted acid sites can react with NO species, and adsorbed NO and NO₂ species can both be reduced by NH₃. In the SCR process, O₂ primarily acts as the accelerant to improve the redox and acid cycles and plays an important role. This work proves that the combination of redox and acidic properties of different constituents can be feasible for catalyst design to obtain a superior SCR performance.     

The effect of hydrogen peroxide on properties of Ce_(0.35)Zr_(0.55)La_(0.055)Pr_(0.045)O_2 oxides and the catalytic performance used on Pd supported three-way catalyst

In this study,two series of cerium zirconium mixed oxides Ce_(0.35)Zr_(0.55)La_(0.055)Pr_(0.045)O_2 were prepared under traditional co-precipitation and oxidation co-precipitation methods respectively. The physicochemical properties of the samples were compared under these two methods and assessed by XRD,Raman,BET,TEM,H_2 TPR,OSC,XPS and catalysts measurements. The formation of homogeneity phase structure can be facilitated by changing the precipitating properties of Ce3+ under oxidation coprecipitation method, which is helpful to enhance the homogeneity of Ce and Zr at atomic level.What's more, it is conducive to remove impurities Na~+ and Cl~-by oxidation co-precipitation with hydrogen peroxide. The catalysts activities are related to both the redox properties and the textural properties of mixed oxides. The Pd-only TWCs supported on the CZLP-H-F exhibits better catalytic performance and thermal stability with wider air/fuel ratio operation window, lower light-off and full conversion temperatures of C_3 H_8 and NO. The homogeneity of phase structure for cerium zirconium mixed oxide can be predicted and deduced from detecting the atomic distribution uniformity of its precursor. So this work not only provides insights into the mechanisms for phase segregation of cerium zirconium mixed oxide, but also provides a guidance to improve homogeneity of cerium zirconium mixed oxide by adding additives.     

Preparation and characterization of Ce_(1-x)Pr_xO_2 supports and their catalytic activities

In this work, the addition of praseodymium(Pr) into ceria as a mixed oxide support in a form of Ce_(1-x)Pr_xO_2(x = 0.01,0.025, 0.050, 0.075 and 0.10) was prepared using a co-precipitation method. The structural and textural properties of the synthesized supports were characterized by X-ray diffraction(XRD), N_2 adsorption-desorption, Raman spectroscopy, H_2-temperature programmed reduction(H_2-TPR) and H_2-chemisorption. Upon addition of Pr, XRD patterns and Raman spectra indicated an enlargement of ceria unit cell and the characteristics Raman broad peak at 570 cm~(-1) which was attributed to the existence of oxygen vacancies in the ceria lattice. This indicated that some Ce~(4+) ions in ceria were replaced by larger Pr~(3+) cations. To evidence the incorporation of Pr~(3+) cations into ceria lattice,X-ray absorption near edge structure(XANES) was employed. The results showed that the oxidation states of Ce in mixed oxide supports were slightly lower than 4+ while those of Pr were still the same as a precursor salt. Therefore, the incorporation of Pr~(3+) into ceria lattice would lead to strain and unbalanced charge and result in oxygen vacancies. The reducibility of Ce_(1-x)Pr_xO_2 mixed oxide supports was investigated by H_2-TPR and temperature-resolved X-ray absorption spectroscopy experiment under reduction conditions. XANES spectra of Ce L_3 edges showed a lower surface reduction temperature(Ce~(4+)to Ce~(3+)) of Ce_(0.925)Pr_(0.075)O_2 than that of CeO_2 which agreed with H_2-TPR results. H_2-chemisorption indicated that Pr promoted the dispersion of the metal catalyst on the mixed oxide support and increased the adsorption site for CO. For WGS reaction, 1% Pd/mixed oxide support had higher WGS activity than 1%Pd/ceria. The increase of WGS activity was due to the increase of Pd dispersion on the support and the existence of oxygen vacancies produced from incorporation of Pr into the ceria lattice.     

Rare earths (Ce,Y, Pr) modified Pd/La_2O_3-ZrO_2-Al_2O_3 catalysts used in lean-burn natural gas fueled vehicles

14%REO_x-2.5%La_2 O_3-33.5%ZrO_2-50%Al_2 O_3(RE = Ce, Y, Pr) composites were prepared by a coprecipitation method. The Pd catalysts were obtained by an aqueous solution of Pd(NO_3)_2 loaded on the rare earths modified composites with an initial wet impregnated method. The experiment results show that catalytic activity of the rare earths modified Pd/La_2 O_3-ZrO_2-Al_2 O_3 catalysts is better than bare sample for methane oxidation. The structural characterization results reveal that the rare earths modified Pd catalysts increase amounts of surface active oxygen species by X-ray photoelectron spectroscopy(XPS) analysis and improve the dispersion of active component from H2-temperature programmed reduction(H2-TPR) measurement compared with bare sample. Especially,Pd/14%Y_2 O_3-2.5%La_2 O_3-33.5%ZrO_2-50%Al_2 O_3 sample exhibits highly active stability, it is related to the Pd particles highly dispersion,which was observed by transmission electron microscope(TEM) images.