The structure–activity relationships of Rh/CeO_(2)-ZrO_(2)catalysts based on Rh metal size effect in the three-way catalytic reactions

With the continuous tightening of automotive emission regulations and the increasing promotion of energy-efficient hybrid vehicles,new challenges have arisen for the low-temperature performance of three-way catalysts(TWCs).To guide the design of next-generation TWCs,it is essential to further develop our understanding of the relationships between microstructure and catalytic performance.Here,Rh/CeO_(2)–ZrO_(2)catalysts were synthesized with different Rh metal dispersion by using a combination of the wet impregnation method and reduction treatment.These catalysts included Rh single-atom catalysts,cluster catalysts,and nanoparticle catalysts.The results showed that the Rh nanoparticle catalyst,with an average size of 1.9 nm,exhibited superior three-way catalytic performance compared to the other catalysts.Based on the catalytic activity in a series of simple reaction atmospheres such as CO+O_(2),NO+CO,and hydrocarbons(HCs)+O_(2)and operando infrared spectroscopy,we found that metallic Rh sites on Rh nanoparticles are the key factor responsible for the low-temperature catalytic performance.

Effects of loading transition metal(Mn,Cr,Fe,Cu) oxides on oxygen storage/release properties of CeO2-ZrO2 solid solution

Cerium zirconium-based(CZ) oxygen storage materials(OSMs) play a crucial role in three-way catalysts(TWCs),while CZ needs to be modified to satisfy more rigorous emission standard.In this study,transition metal(TMs=Mn,Cr,Fe,Cu) oxides modified CZ were prepared by incipient wetness impregnation method to improve the oxygen storage capacity of CZ-based materials.To clearly illustrate the influence of TM oxides,N2 adsorption-desorption,X-ray diffraction(XRD),oxygen storage capacity(OSC),temperature programmed reduction by H2(H2-TPR) and X-ray photoelectron spectroscopy(XPS) were used to characterize the physical and chemical properties of samples.It is found that,all modified CZ have higher OSC,lower reduction temperatures than those of pristine CZ.Interaction between TMOs and CZ take precedence over specific surface to influence OSC.Notably,FeOx/CZ has the highest OSC,which is about 1.9 times that of CZ and it could be attributed to synergistic effect between FeOx and CZ;CuOx/CZ has the lowest reduction temperature which is 168℃lower than that of CZ,and it can be explained by hydrogen spillover effect.

Effect of grain size on thermal stability and oxygen storage capacity of Ce_(0.17)Zr_(0.73)La_(0.02)Nd_(0.04)Y_(0.04)O_2 solid solutions

Nanostructured CeO_2-ZrO_2 materials are an irreplaceable constituent in catalytic systems for automobile exhaust purification due to their unique oxygen storage capacity(OSC). However, traditional CeO_2-ZrO_2 materials are easy to sinter at high temperature, which causes a sharp decrease of OSC. In this paper,La^(3+) , Nd^(3+) and Y^(3+) are chosen as dopants for CeO_2-ZrO_2 to improve anti-sintering and OSC properties.The Ce_(0.17) Zr_(0.73) La_(0.02) Nd_(0.04) Y_(0.04) O_2 powders(CZLNY) were prepared by co-precipitation method. The effects of grain sizes with different mixed chlorinated solution concentrations on performances were investigated. X-ray diffraction(XRD) and transmission electron micrograph(TEM) were performed to calculate the grain sizes of CZLNY. The specific surfaces, OSC and redox properties were investigated by N_2 adsorption/desorption and temperature programmed reduction(H2-TPR). The results show that introducing La^(3+) , Nd^(3+) and Y^(3+) into CeO_2-ZrO_2 lattice can improve the stability of phase structure and anti-sintering ability. Moreover, low concentration of mixed chlorinated solution remarkably improves structural and textural properties of CZLNY. Relatively large fresh grain exhibits superior thermal stability and OSC under the condition of being calcined at 800℃ for 3 h. The specific surface and OSC are42.37 m^2/g and 333.13 mmol/g after calcining at 1000℃ for 10 h, respectively. This is owing to the low sintered driving force of large grain and long-range migration energy of large pores during the sintering process, which are beneficial to the stability of structure in CZLNY materials.

The effect of hydrogen peroxide on properties of Ce0.35Zr0.55La0.055Pr0.045O2 oxides and the catalytic performance usedon Pd supported three-way catalyst

In this study,two series of cerium zirconium mixed oxides CeZrLaPrOwere 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,HTPR,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 CHand 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.

Effects of precursor moisture and inert N_2 atmosphere calcinations on structure and properties of alumina modified CeZrLaNd mixed oxides

CeO_2-ZrO_2 mixed oxides are widely used in the three-way catalysts due to their unique reversible oxygen storage and release capacity. Large surface area, high oxygen storage capacity and good thermal stability of cerium zirconium mixed oxides are the key properties for the automotive catalysts so as to meet the strict emission regulations. In this work, alumina modified CeZrLaNd mixed oxides were prepared by a co-precipitation method. The effects of moisture in precursor and inert N2 atmosphere during calcinations on the structure and properties were investigated by Brunauer-Emmett-Teller(BET) surface area measurements, X-ray diffraction(XRD), scanning electron microscopy(SEM), transmission electron microscopy(TEM), hydrogen temperature-programmed reduction(H_2-TPR), oxygen storage capacity(OSC), Raman spectroscopy, and X-ray photoelectron spectroscopy(XPS). The results show that the moisture in precursor during calcinations increases the crystal grain size of the cerium zirconium mixed oxides, improving the thermal stability. And the aged surface area of sample after being calcined at1000 ℃ for 4 h reaches 68.8 m^2/g(5.7% increase compared with the common sample). The inert N2 atmosphere endows a great pore-enlarging effect, which leads to high fresh surface area of 148.9 m2/g(13.5% increase compared with the common sample) and big pore volume of 0.5705 mL/g. The redox and oxygen storage capacity are also improved by inert N2 atmosphere with high OSC value of 241.06μmolO_2/g(41.3% increase compared with the common calcination), due to the abundant formation of the crystal defects and oxygen vacancies.

SO_(4)^(2-)-modified La,Y-doped ceria-zirconia with high oxygen storage capacity and its application in Pd-only three-way catalysts

As the oxygen redox ability shows great effects on the catalytic performances of ceria-zirconia based materials,many strategies have been utilized to improve the oxygen storage capacity.Here in this study,we report a simple and facile approach to prepare a SO_(4)^(2-)-modified La,Y-doped ceria-zirconia material(SO/CZLY-f)with high oxygen storage capacity.Due to the additional redox process between SO_(4)^(2-)and S^(2-),oxygen storage capacity of SO/CZLY-f(745.3μmol O_(2)/g)is about 1.6 times higher than that of La,Ydoped ceria-zirconia material without SO_(4)^(2-)modification.Moreover,the catalytic activities and stability of the corresponding Pd-only three-way catalyst were measured.Compared to that of Pd@CZLY-f,the operation window of CO,full conversion temperature of HC and NO over Pd@SO/CZLY-f are obviously widened and lowered,respectively.After aging treatment at 1100℃for 4 h,the superiority of aged Pdloading composite is still maintained.

Tailoring thermal stability of ceria-zirconia mixed oxide by doping of rare earth elements:From theory to experiment

Ceria-zirconia mixed oxides(CZMO)are widely used in many important catalysis fields.However,pure CZMO is known to have poor thermal stability.In this paper,a strategy was proposed to design Ce_(0.475)Zr_(0.475)M_(0.05)O_(2)(M=La,Y,Pr,Nd,Pm,Sm,Eu,Gd,Tb,Er,Lu,and,Yb)oxide surface with high thermal stability by using first-principles molecular dynamics(FPMD)simulation and experiment method.Through the structure stability analysis at different temperatures,the surface energyγas a function of R_(ion)/D_(ave)is identified as a quantitative structure descriptor for analyzing the doping effect of rare earth(RE)elements on the thermal stability of Ce_(0.475)Zr_(0.475)M_(0.05)O_(2).By doping the suitable RE,γcan be adjusted to the optimal range to enhance the thermal stability of Ce_(0.475)Zr_(0.475)M_(0.05)O_(2).With this strategy,it can be predicted that the sequence of thermal stability improvement is Y>La>Gd>Nd>Pr>Pm>Sm>Eu>Tb>Er>Yb>Lu,which was further verified by our experiment results.After thermal treatment at 1100℃for 10 h,the specific surface area(SSA)of aged Y-CZ and La-CZ samples can reach 21.34 and 19.51 m~2/g,which is 63.02%and 49.04%higher than the CZMO sample without doping because the surface doping of Y and La is in favor of inhibiting the surface atoms thermal displacement.In a word,the strategy proposed in this work can be expected to provide a viable way for designing the highly efficient CZMO materials in extensive applications and promoting the usages of the high-abundance rare-earth elements Y and La.

Effects of ammonia concentration in hydrothermal treatment on structure and redox properties of cerium zirconium solid solution

Cerium zirconium solid solution is a key washcoat material for automotive three-way catalysts(TWCs).However,improving the redox ability and high temperature thermal stability of cerium zirconium solid solution is still a challenge.In this paper,the cerium zirconium solid solution was prepared by a coprecipitation-hydrothermal method,and the effects of the ammonia concentration on their structures and redox properties were investigated.The results show that when the ammonia concentration is 0.8 mol/L,the aged sample(1100℃/10 h)of cerium zirconium solid solution has the highest specific surface area of 23.01 m^(2)/g.Additionally,the increase of ammonia concentration improves the uniformity of phase compositions and increases the oxygen vacancies.When the ammonia concentration reaches 0.4 mol/L,the cerium zirconium solid solution exhibits the best redox activity,with the lowest reduction temperature of 565℃.Therefore,increasing ammonia concentration in the hydrothermal treatment is beneficial to the thermal stability and redox performance of cerium zirconium solid solution.