Influence of surface Ce/Zr ratio on formation of κ-Ce2Zr2O8 superstructure and its application in three-way catalysis

Ceria-zirconia based(CZ) materials are still a crucial component of three-way catalysts(TWCs) now.Compared with traditional cubic or tetragonal phase,CZ material with pyrochlore-type κ-Ce₂Zr₂O₈ phase shows an excellent oxygen storage capacity,and thus is examined as a promising material for developing TWCs.Considering the remarkable surface Ce/Zr ratio change during the κ-Ce₂Zr₂O₈ formation,we investigated the effects of surface C...     

Possible negative influences of increasing content of cerium on activity and hydrothermal stability of Rh/ceria-zirconia three-way catalysts

The activity and hydrothermal stability of the Rh/Ce_xZr_(1-x)O_2(x=0,0.05,0.3,0.5) model three-way catalysts for gasoline vehicle emissions control were investigated in this work.Among the Rh/Ce_xZr_(1-)_xO_2 samples with different Ce/Zr ratios,the Rh/ZrO_2 sample exhibits a significantly better activity and hydrothermal stability than the rest of the samples.The impacts of having more Ce components in the Rh/Ce_xZr_(1-x)O_2 catalysts are associated with the strong Rh-O-Ce interaction that tends to over stabilize the rhodium species.A significant amount of such rhodium atoms can be found in the bulk of the support oxides after a hydrothermal aging at 1050℃ with 10% H_2 O in air for 12 h.Differently,the sintering of rhodium on the surface of Rh/ZrO_2 catalysts is the main reason for the catalyst deactivation during the hydrothermal aging.These findings provide an example where high dispersion of supported metal induced by strong metal-support interactions does not necessarily lead to high catalytic activity.     

Construction of bimetallic Pt-Pd/CeO2-ZrO2-La2O3 catalysts with different Pt/Pd ratios and its structure-activity correlations for three-way catalytic performance

A series of Pt-Pd bimetallic catalysts supported on CeO₂-ZrO₂-La₂O₃ mixed oxides were synthesized through the conventional impregnation method.Three-way catalytic performance evaluations along with detailed physio-chemical characterizations were carried out to establish possible structure-activity correlations.Results show that on the one hand,different Pt/Pd ratios can strongly affect the TWC behaviors of Pt-Pd/CZL catalysts by modulating the synergis...     

An egg-shell bifunctional CeO2-modified NiPd/Al2O3 catalyst for petrochemical processes involving selective hydrogenation and hydroisomerization

The catalytic performance during the 1-butyne hydrogenation using two reduced Al₂O₃-supported Pd-based catalysts was carried out in a total recirculation system with an external fixed-bed reactor. The lab-prepared egg-shell NiPd/CeO₂-Al₂O₃ catalyst (NiPdCe) with Pd loading = 0.5 wt%, Ni/Pd atomic ratio = 1 and CeO₂ loading = 3 wt% was synthesized and characterized, and it was compared with an egg-shell Al₂O₃-supported Pd based commercial catalyst (PdCC). The reduced catalysts were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and high-resolution transmission electron microscopy. The textural characteristics and ammonia temperature-programmed desorption profiles of the fresh (unreduced) catalysts were also obtained. Both catalysts show high 1-butyne conversion and selectivity to 1-butene, but the catalysts also present important differences between hydroisomerizing and hydrogenating capabilities. NiPdCe catalyst shows higher capability for hydroisomerization reactions, while the PdCC catalyst exhibits higher hydrogenating capability. The observed catalytic performances can be interesting for some industrial processes and can provide a guideline for the development of a Pd-based catalyst with specific catalytic properties.     

Durability of three-way and close-coupled catalysts for Euro IV regulation

The durability of three-way catalyst （TWC） and corresponding close-coupled catalyst （CCC） for Euro Ⅳ stage regulation was investigated through Vehicle Road Running Mode tests, whereas emissions of regulated pollutants of three car fleet were investigated at every 100,000 km miles. The results showed that HC, NOx, and CO emission values could meet Euro Ⅳ regulation limits at every point. The redox properties of TWC and CCC were measured by CO reduction during each isothermal. It was obvious that both aged TWC and aged CCC behaved a good redox property at 673 and 773 K. Based on XRD and BET measurement results, TWC and CCC washcoat were characterized with good thermal stability.     

Regulating local coordination environment of rhodium single atoms in Rh/CeO2 catalysts for N2O decomposition

Rh single atom catalysts(SACs) have been insensitively investigated recently due to the maximum utilization efficiency of Rh,one of the most expensive precious metals.Although great efforts have been made in the development and application of Rh SACs,there are few reports on the precise control of the local coordination environment of Rh single sites on CeO₂ and their catalytic performance for N₂O decomposition.Herein,Rh/CeO₂ catalysts with different Rh-O coordin...     

Fabrication of a flower-like Pd/CeO2 material with improved three-way catalytic performance

Pd/CeO2 catalysts with flower-like morphology were fabricated via an ultrasonic-assisted membrane reduction (UAMR) and hydrothermal methods. The catalysts were physically characterized and evaluated fo...     

Effects of water on CO catalytic oxidation over Pd/CeO_2

The role of water in CO oxidation was investigated on Pd/CeO_2 with different morphologies(rods(R),cubes(C) and octahedrons(O)).Compared with the absence of water,CO oxidation activity increases 2 times in the presence of water on Pd/CeO_2-C;but a decrease is found on Pd/CeO_2-R.Catalyst characterization reveals that Pd is mainly in the form of solid solution(Pd_xCe_(1-x)O_(2-σ)) on Pd/CeO_2-R and a mixture of metal and Pd_xCe_(1-x)O_(2-σ) solid solution on Pd/CeO_2-C.The strong interaction between Pd and CeO_2-R results in the form of stable bidentate carbonates species;while the relatively weak interaction between Pd and CeO_2-C leads to the produce of unstable monodentate carbonates species.The effects of water on CO oxidation activity closely relate with the Pd chemical state and the types of carbonates species.Water restrains CO adsorption on Pd_xCe_(1-x)O_(2-σ) solid solution,but it has negligent effects on metallic Pd species.In the presence of water,bidentate carbonates species remains stable but the decrease in the amount of monodentate carbonates species is observed.     

Influence of Ce0.35Zr0.55Y0.10 Solid Solution on Performance of Pt-Rh Three-Way Catalysts

Ce0.35Zr0.55Y0. 10 solid solution was prepared by co-precipitation technique and characterized by specific surface area measurements （BET） and X-ray diffraction （XRD）. Ce0.35Zr0.55Y0.10 was used to prepare low Pt-Rh threeway catalyst （TWC）, and its influence on the performance of TWC was investigated. The results revealed that Ce0.35 Zr0.55Y0.10 had a cubic structure similar to Ce0.50Zr0.50O2 and its specific surface area can maintain higher than Ce0.50 Zr0.50O2 after 1000 ℃ calcination for 5 h. Being hydrothermal aged at 1000 ℃ for 5 h, the catalyst containing Ce0.35 Zr0.55Y0.10 still exhibited higher conversion of C3H8, CO and NO and lower light-off temperature in comparison with Ce0.50Zr0.50O2 TWC.     

Preparation of M/Ce1–xTixO2 (M=Pt,Rh, Ru) from sol-gel method and their catalytic oxidation activity for diesel soot

A series of Ce_1–xTi_xO₂ mixed oxide catalysts were synthesized by sol-gel method and then loading of noble metal (M = Pt, Rh, Ru) was used for soot oxidation. Ti-doped Ce_1–xTi_xO₂ catalysts (x is the molar ratio of Ti/(Ti + Ce) and ranges from 0.1 to 0.5) exhibit much better oxidation performance than CeO₂ catalyst, and the Ce_0.9Ti_0.1O₂ catalyst calcined at 500 °C has the best catalysis activity. Each noble metal (1 wt%) was supported on Ce_0.9Ti_0.1O₂ (M/C9T1) and the properties of the catalysts were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Raman, Brunauer–Emmett–Teller (BET) method, and H₂-temperature programmed reduction (H₂-TPR) results. Results show that the introduction of Ti into CeO₂ forming Ti-O-Ce structure enhances the catalytic activity and increases the number of oxygen vacancies at the catalyst surface. The noble metal is highly dispersed over Ce_0.9Ti_0.1O₂, and M/C9T1 catalysts present enhanced activity in comparison to Ce_0.9Ti_0.1O₂. It is found that noble metals can greatly increase the activity of the catalyst and the corresponding oxidation rate of soot can enhance the electron transfer capacity and oxygen adsorption capacity of the catalyst. A small amount of Ti doping in CeO₂ can significantly improve the activity of the catalyst, while a large amount of Ti reduces the performance of the catalyst because a large amount of Ti is enriched on the surface of the catalyst, which hinders the contact and reaction between the catalyst and the soot.     

Catalytic combustion of toluene over Pd-based monolithic catalysts with a novel washcoat Ce0.8Zr0.15La0.05Oδ

Two novel washcoats Ce_0.8Zr_0.15La_0.05O_δ and Ce_0.8Zr_0.2O₂ was prepared by an impregnation method, which acted as a host for the active Pd component to prepare Pd/Ce_0.8Zr_0.15La_0.05O_δ/substrate and Pd/Ce_0.8Zr_0.2O₂/substrate monolithic catalysts for toluene combustion. The washcoats was characterized by X-ray diffraction (XRD), Raman spectroscopy, Brunauner-Emmett-Teller (BET), and H₂-temperature-programmed reduction (H₂-TPR). The result indicated that both the washcoats had strong vibration-shock resistance according to ultrasonic test. Doping La³⁺ into CeO₂-ZrO₂ solid solution could generate more oxygen vacancies, and could inhibit the sinter of CeO₂-ZrO₂ solid solution when calcined at high temperatures (800, 900 and 1000 °C). The washcoat Ce_0.8Zr_0.15La_0.05O_δ had much better redox properties. The reductive temperature of Ce⁴⁺ species shifted to low temperature by 60 °C when the washcoats calcined at high temperatures (800, 900 and 1000 °C). The Pd/Ce_0.8Zr_0.15La_0.05O_δ/substrate monolithic catalyst calcination at 500 °C had the best catalytic activity and the 95% toluene conversion at a temperature as low as 190 °C. When calcined at low temperature (500 and 700 °C), the catalytic activity has little improvement, however, when calcined at high temperature, the catalytic activity of Pd/Ce_0.8Zr_0.15La_0.05O_δ/substrate monolithic catalysts had significant improvement. As catalyst washcoat, the Ce_0.8Zr_0.15La_0.05O_δ had better thermal stability than the washcoat Ce_0.8Zr_0.2O₂, the developed Pd/Ce_0.8Zr_0.15La_0.05O_δ/substrate monolithic catalyst in this work was promising for eliminating Volatile organic compounds.     

Rh/CeO2 composites prepared by combining dealloying with calcination as an efficient catalyst for CO oxidation and CH4 combustion

In this paper, a series of Rh/CeO₂ catalysts with three-dimensional porous nanorod frameworks and large specific surface area were prepared by chemical dealloying Al–Ce–Rh precursor alloys and then calcining in pure O₂. The effects of the Rh content and calcination temperature on CO oxidation and CH₄ combustion were studied, and the results reveal that the Rh/CeO₂ catalysts produced by dealloying melt-spun Al_91.3Ce₈Rh_0.7 alloy ribbons and then calcining at 500 °C exhibit the best catalytic activity, the reaction temperatures for the complete conversion of CO and CH₄ are as low as 90 and 400 °C, respectively. Furthermore, after 150 h of continuous testing at high concentrations of H₂O and CO₂, the nature of the catalyst is not irreversibly destroyed and can still return to its initial level of activity. This excellent catalytic activity is attributed to a portion of Rh being uniformly distributed on the CeO₂ nanorod surface in the form of nanoparticles, forming strong Rh–CeO₂ interfacial synergy. Another portion of Rh permeated into the CeO₂ lattice, which results in a significant increase in the number of oxygen vacancies in CeO₂, thus allowing more surface active oxygen to be adsorbed and converted from the gas phase. Moreover, the catalytic reaction can proceed even in an oxygen-free environment due to the excellent oxygen storage performance of the Rh/CeO₂ catalyst.     

贵金属/铈锆铽固溶体催化剂的制备与台架性能评价

用Ce0.6Zr0.3Tb0.1O2作为储氧材料,分别制备了实尺寸的全钯三效催化剂和双层钯、铂、铑三效催化剂。为作对比,同时制备了含商用铈锆固溶体的双层钯、铂、铑三效催化剂。在发动机台架上对所制备的催化剂进行了活性评价。3种催化剂具有较高的三效催化活性,在交叉点附近对CO和NOx的转化率接近100%,对THC的转化率接近90%。双层催化剂和单层全钯催化剂相比,不仅在富燃区具有较高的NOx转化率,而且具有较低的起燃温度。含Ce0.6Zr0.3Tb0.1O2固溶体的双层催化剂的性能优于含商用铈锆固溶体的催化剂。     

Durability of three-way and close-coupled catalysts for Euro Ⅳ regulation

The durability of three-way catalyst (TWC) and corresponding close-coupled catalyst (CCC) for Euro Ⅳ stage regulation was in-vestigated through Vehicle Road Running Mode tests, whereas emissions of regulated pollutants of three car fleet were investigated at every 100,000 km miles. The results showed that HC, NOx, and CO emission values could meet Euro Ⅳ regulation limits at every point. The redox properties of TWC and CCC were measured by CO reduction during each isothermal. It was obvious that both aged TWC and aged CCC behaved a good redox property at 673 and 773 K. Based on XRD and BET measurement results, TWC and CCC washcoat were character-ized with good thermal stability.     

Thermal stability of CeO<Subscript>2</Subscript>/ZrO<Subscript>2</Subscript> multilayer thin films prepared by pulsed laser deposition

Multilayers of CeO₂/ZrO₂ thin films were deposited on Si (100) substrates using pulsed laser deposition at an optimized oxygen partial pressure of 3×10⁻² mbar and at room temperature. The CeO₂ layer thickness was 10 nm, while the ZrO₂ layer thickness was varied as 10, 20 and 30 nm. CeO₂ and ZrO₂ layers were deposited alternately to obtain 25 bilayers. High temperature x-ray diffraction (HTXRD) results showed that the multilayer films had cubic ceria and tetragonal ZrO₂. Thermal expansion coefficients were calculated for CeO₂ and t-ZrO₂ and found to increase with the decrease of ZrO₂ layer thickness. The cross sectional transmission electron microscopy (XTEM) of CeO₂/ZrO₂ multilayer also indicated that ceria was found to be in cubic phase while zirconia contained predominantly tetragonal phase along with cubic phase in thermally annealed specimen.     

Performance of Pd/CeO2-ZrO2-Al2O3 catalyst for motorcycle

The Pd-only catalysts for motorcycle were prepared by impregnating CeO2-ZrO2-Al2O3 and CeO2-ZrO2+Al2O3 with PdCl2 aque-ous solution and characterized by X-ray diffraction (XRD), oxygen storage capacity (OSC) and H2-temperature-programmed reduction (H2-TPR) methods. The XRD result indicated that the CeO2-ZrO2-Al2O3 compound prepared by co-precipitation formed a single solid solu-tion and had good thermal stability, and Pd phase was not observed in all catalysts. The TPR results showed that the reduction temperature of Pd/CeO2-ZrO2-Al2O3 catalyst was lower than that of Pd/CeO2-ZrO2+Al2O3 catalyst whether they were fresh or aged catalysts. The Pd/CeO2-ZrO2-Al2O3 exhibited high three-way catalytic activity at low temperature, high thermal stability, and wide working window, sug-gesting a great potential for applications.     

Lewis acid-oxygen vacancy interfacial synergistic catalysis over SO42−/Ce0.84Zr0.16O2–WO3–ZrO2 for N,N-diethylation of aniline with ethanol

Herein, a new mechanism involving Lewis acid-oxygen vacancy interfacial synergistic catalysis for aniline N,N-diethylation with ethanol was proposed, and the SO₄²⁻/Ce_0.84Zr_0.16O₂–WO₃–ZrO₂ catalyst (SCWZ) with both Lewis acid sites and oxygen vacancies was synthesized by the hydrothermal method, which shows better catalytic activity than the reported solid acidic catalysts. Besides, the SO₄²⁻/ZrO₂ (SZ) and SO₄²⁻/WO₃–ZrO₂ (SWZ) catalysts were also prepared and compared with SCWZ to investigate the synergistic effect of each component. The SO₄²⁻ and WO₃ mainly generate Lewis acid by bonding with ZrO₂, which is beneficial for the fracture of the N–H bond in aniline. The Ce_0.84Zr_0.16O₂ solid solution mainly plays a vital role in generating the oxygen vacancies as the interface active species, which can participate in stripping –OH from ethanol, then the carbocation will also be released, which only needs 1.3805 kcal/mol energy, calculated by density functional theory (DFT), to be input. In comparison, the traditional reaction mechanism needs the Brønsted acidic sites to promote the protonation of ethanol, then dehydration and subsequent formation of carbocation followed, and 108.6846 kcal/mol energy needs to be input, which is far higher than that of the new mechanism. The apparent activation energy (E_a) over SCWZ was measured by experiment to be 34.09 kJ/mol, which is much lower than that of SWZ (47.10 kJ/mol) and SZ (54.37 kJ/mol), illustrating comparatively preferable kinetics for SCWZ than that of SWZ and SZ. Besides, the conversion of aniline and selectivity to N,N-diethylaniline over SCWZ reach almost 100% and 73%, respectively. The SCWZ can be renewed for 4 times without rapid deactivation, and the longevity of SCWZ is longer than that of SWZ and SZ, as the loaded SO₄²⁻ and tetragonal ZrO₂ are stabilized by Ce_0.84Zr_0.16O₂ and WO₃, respectively.     

Catalysis of Ce_xZr_(1–x)O_2 for di-iso-propyl-ether hydration

The catalytic potential of CexZr1–xO2 in isopropyl ether(DIPE) hydration was explored. While the acidic H-zeolite catalyst was favorable for propylene formation through carbenium ion mechanism, Ce-ZSM-5 and CexZr1–xO2 catalysts improved product selectivity of isopropyl alcohol(IPA) through redox mechanism. The catalytic property of CexZr1–xO2 depended on the preparation method and variable, type of cerium precursor and Ce/Zr ratio. By means of characterizations with X-ray diffraction, N2 adsorption isotherms and NH3 temperature programmed desorption, tetragonal phase of CexZr1–xO2 was proposed as the active phase in which CeO2 and ZrO2 catalyzed synergistically the DIPE hydration with IPA product selectivity. The CexZr1–xO2 prepared from cerium sulfate precursor with co-precipitation hydrothermal method exhibited maximum catalytic activity and IPA product selectivity. The precursor effect was attributed to the stabilization of SO42– species on the tetragonal phase of CexZr1–xO2 and super solid acidity.     

Preparation of Ce0.65Zr0.35O2 by co-precipitation： The role of hydrogen peroxide

The effect of H2O2 on the properties of Ce0.65Zr0.35O2 was explored by treating cerium nitrate and zirconium nitrate with a mixed aqueous solution of ammonia and ammonia-carbonate in the presence/absence of H2O2 . The resultant products were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), nitrogen adsorption/desorption, oxygen storage capacity (OSC) and H2-temperature-programmed reduction (H2-TPR). The presence of H2O2 was found to have profound effect on powder properties such as surface area, crystallite size of the samples. It was also shown that the addition of H2O2 favored the incorporation of Zr4+ into CeO2 lattice, which facilitated the formation of CeO2-ZrO2 solid solution, and enhanced the thermal stability of the samples. OSC and H2-TPR studies indicated that the use of H2O2 enhanced the OSC and redox properties. Catalytic activity tests showed that as a support, the Ce0.65Zr0.35O2 prepared in the presence of H2O2 was more suitable for three-way catalyst. The corresponding Pd-only three-way catalyst demonstrated outstanding performance: wide air to fuel operation window, low light-off and total conversion temperature for the conversion of C3H8, NO and CO.     

Mechanical properties,thermophysical properties and electronic structure of Yb3+ or Ce4+-doped La2Zr2O7-based TBCs

First-principles calculations based on density functional theory were performed to investigate the cohesive energies, elastic modulus, Debye temperatures, thermal conductivities and density of states of La_2−xYb_xZr₂O₇, La₂Zr_2−xCe_xO₇ and La_2−xYb_xZr_2−xCe_xO₇ (x = 0.00, 0.25, 0.50, 0.75, 1.00) ceramics. The results show that doping Yb³⁺ or Ce⁴⁺ into La₂Zr₂O₇ reduces its elastic modulus, thermal conductivity and Debye temperature. Compared with La_2−xYb_xZr₂O₇ (x ≠ 0.00), La₂Zr_2−xCe_xO₇ compounds have better ductility and lower Debye temperature. The Debye temperature values of La₂Zr_2−xCe_xO₇ (x ≠ 0.00) compounds are in the range of 485.0–511.5 K. Among all components, the fluorite-type La_2−xYb_xZr_2−xCe_xO₇ (x = 0.75, 1.00) compounds exhibit better mechanical and thermophysical properties, and their thermal conductivity values are only 1.213–1.246 W/(m∙K) (1073 K), which are 14.5%–16.7% lower than that of the pure La₂Zr₂O₇. Thus, our findings open an entirely new avenue for TBCs.