CeO2 ZrO2 Research Articles

Local structure in CeO2 and CeO2–ZrO2 nanoparticles probed by Eu luminescence

We report on the local structure properties of trivalent lanthanide-doped CeO2 based nanoparticles by using Eu luminescence as a structural probe. To this aim, we investigate Eu doped CeO2 and CeO2–ZrO2 (Ce/Zr=1) by use of site-selective time-resolved luminescence spectroscopy, in situ/ex situ X-ray diffraction (XRD) and in situ/ex situ Raman spectroscopy. The CeO2 nanoparticles were synthesized by oil in water microemulsion as well as citric acid method and investigated in the as-synthesized state or after calcination in air at 500, 750 and 1000°C. For the as-synthesized Eu–CeO2 nanoparticles by oil-in-water microemusion method, the analysis of the emission/excitation spectra and emission decays of Eu suggest that only surface doping was achieved. In general, the emission of Eu in CeO2 indicates the co-existence of two main Eu centers differentiated by the mode of interaction with the oxygen vacancy. The addition of 10% Eu via wet impregnation onto pre-formed CeO2–ZrO2 followed by calcination induces a remarkable homogenous solid solution of pseudo-cubic phase. The analysis of in situ XRD, in situ Raman and in situ luminescence data evidences for the defect reconfiguration in Eu doped CeO2 during the heating/cooling cycles (30–500 at 30°C). Finally, it is suggested that the structural data inferred from luminescence complements significantly the structural information indicated by X-ray diffraction and Raman spectroscopy, such as the phase content and homogeneity and the presence of defects.

Partial oxidation of methane on Rh/ZrO2 and Rh/Ce–ZrO2 on monoliths: Catalyst restructuring at reaction conditions

0.5% Rh catalysts on ZrO2 or CeO2–ZrO2, coated on monoliths were tested for partial oxidation of methane, (GHSV=100000h−1). We ran temperature cycles up to 850°C, varying the O2/CH4 ratio from 0.5 to 0.7. The catalysts were characterized by XRD, TPR/TPO, BET and chemisorption. ZrO2 required a long conditioning step, to reach stable performance, while the CeO2–ZrO2 catalyst quickly gained steady activity. The conditioning step is explained with a restructuring of the surface, with the metallic particles becoming smaller. Ceria lowers the ignition temperature by 60°C. At low temperature, ceria allows an increase in CH4 conversion and selectivity to syngas. At high temperature, the CeO2–ZrO2 catalyst significantly increases the CH4 conversion, compared to ZrO2, approaching 100%. However, the selectivity to syngas decreases below 80%. The syngas selectivity is higher at lower O2/CH4 ratio, at any temperature, but the absolute amount of syngas can be larger with more O2.

Phase and microstructural evolutions of the CeO2–ZrO2–SiO2 system synthesized by the sol–gel process

In this paper, a sol–gel method was employed to synthesis the zircon and Ce-doped zircon in the (CeO2–)ZrO2–SiO2 system. The evolutions of their phase and microstructure with the changing composition and the sintering temperature were studied. It was found that the formation rates of zircon increase initially and then decrease with the increase of sintering temperature, and reach their maximum at 1500–1550°C, while those of Ce-doped zircon present a decrease trend from 1400°C to 1600°C. The results of XRD patterns and Raman spectrum suggest that the loadings of cerium in the zircon lattices decrease with the increase of sintering temperature from 1400°C to 1600°C. SEM images show that very compact structure of (CeO2–)ZrO2–SiO2 can be achieved by a sol–gel process, and both the increase of sintering temperature and the adding of cerium are beneficial to the growth of crystalline grains.

CeO2–MO x (M: Zr, Ti, Cu) mixed metal oxides with enhanced oxygen storage capacity

The effect of composition on the thermal stability and oxygen storage capacity (OSC) of a series of CeO2–MOx (M: Zr, Ti, Cu) mixed oxides was investigated. X-ray diffraction, N2 adsorption–desorption analysis (BET), transmission electron microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy were used to examine structural and microstructural properties. The total OSC of the fresh and aged mixed oxides were measured using thermogravimetric–differential thermal analysis at 600 °C, under alternating reductive and oxidative environment. Ceria doping was found to be effective toward an enhancement of its redox properties. The addition of Zr4+ into the CeO2 lattice was found to have the least effect in enhancing the catalytic properties. In contrast with other studies in the literature [1, 2], increasing ZrO2 content led to an increase of the OSC of the samples of up to 522 μmol O2 g−1 for 75 % Zr4+ content. On the other hand, the CeO2–CuO with (1:3) molar ratio showed an almost three times higher OSC value than CeO2–ZrO2, which is deemed in the literature as one of the most promising OSC materials. In this work, the highest OSC values (1565 μmol-O2 g−1) were obtained for the fresh sample with CeO2–CuO (25:75) composition. The CeO2–CuO (25:75) mixed oxide still showed the highest OSC values with an only 6 % drop of OSC after aging at 900 °C for 6 h among all aged catalysts. On the other hand, ZrO2 containing samples showed the best thermal resistance against aging.

Impact of cerium-based support oxides in catalytic wet air oxidation: Conflicting role of redox and acid–base properties

High surface area ceria is an excellent support of metals (Ru, Pt, etc.) for catalytic wet air oxidation reactions (CWAO). Due to the low solubility of oxygen in water, the rate of transfer of O2 from the gas phase toward the catalyst has a decisive impact on the whole oxidation process. The role of the ceria support would be to facilitate this oxygen transfer. Redox properties of ceria (generally measured by the oxygen storage capacity, OSC) have a great impact on this step. Attempts were made to modulate the redox properties of ceria by using mixed oxides: CeO2–ZrO2, CeO2–PrO2–ZrO2 or CeO2–TiO2. In doing so, acid–base properties of the supports are also modified. The role of the redox and acid–base properties on CWAO reactions (acetic acid, aniline and phenol) are reviewed. In most cases, acid–base properties can have a great impact on the catalyst stability (deactivation by carbonates or polymer deposit). Increasing the redox properties may have an adverse effect on the catalytic performances when acid–base properties bias the whole process. In some cases (phenol over CeO2–TiO2 supported catalysts), an adequate Lewis acidity is preferable to a high OSC of the catalyst.

Catalytic performance of Pt–Rh/CeZrYLa+LaAl with stoichiometric natural gas vehicles emissions

The composite support CeZrYLa+LaAl was prepared by a co-precipitation method, and Pt–Rh bimetallic catalysts were fabricated on this support using different preparation procedures. The catalytic activities of these materials were tested in a gas mixture simulating the exhaust from a stoichiometric natural gas vehicle. The as-prepared catalysts were also characterized by X-ray photoelectron spectroscopy, X-ray diffraction, N2 adsorption-desorption and H2-temperature-programmed reduction. It was found that the order of activities for CH4, CO and NO conversion was Cat3 ≈ Cat2 > Cat1, where Cat3 had the lowest light-off temperature (T50) for CO (114 °C) and NO (149 °C), the lowest complete conversion temperature (T90) for CH4 (398 °C) and CO (179 °C), and the lowest ΔT (T90–T50) for CH4 (34 °C) and CO (65 °C). Cat2 showed the lowest T50 for CH4 (342 °C), the lowest T90 for NO (174 °C), and the lowest ΔT for NO (17 °C). Cat1 had the highest T50 and T90 and the largest ΔT out of all three catalysts. Indicating that Pt–Rh bimetallic catalysts (Cat2 and Cat3) prepared by physically mixing Pt and Rh powders exhibited much better catalytic activity than those (Cat1) prepared by co-impregnation, since homogeneous Pt and Rh sites made a significant contribution to CH4/CO/NO conversions. In contrast, strong Pt–Rh interactions in the co-impregnation materials affected the oxidation states of Pt, and the Pt-enriched surface blocked active Rh sites. Moreover, Cat3 was prepared by adding additives (La3+, Zr4+ and Ba2+) into the physically mixed Pt–Rh catalysts. XRD results demonstrated that the additive cation (Zr4+) was incorporated into the CeO2–ZrO2 lattice, thus creating a higher concentration of defects and improving the O2-mobility. XPS results showed that the Cat3 had the highest Ce3+/Ce ratio, suggesting the presence of a significant quantity of oxygen vacancies and cerium in the Ce3+ state. All of these further promoted the three-way catalytic activity and widened the air-to-fuel working-window.

Novel MoO3/CeO2–ZrO2 catalyst for the selective catalytic reduction of NOx by NH3

A series of MoO3-doped CeO2–ZrO2 catalysts were investigated for the selective catalytic reduction of NOx by NH3 (NH3-SCR). It was found that the added MoO3 significantly enhanced the activity of CeO2–ZrO2 catalyst for NH3-SCR of NOx in a wide temperature range and the optimum MoO3 loading is 5%. The highly dispersed MoO3 not only resulted in more Lewis acid and Brønsted acid sites formed on the catalyst surface, but also increased the redox property of the catalyst, all of which account for the enhanced SCR activity.

Evaluation of Co, La, and Mn promoted Rh catalysts for autothermal reforming of commercial diesel: Aging and characterization

In this study, three bimetallic catalysts are evaluated for autothermal reforming (ATR) of fuels (1wt.% Rh and 6wt.% X (X=Co, La or Mn) supported on high-surface area CeO2–ZrO2). The catalysts are aged for approximately 35h and carefully characterized both as fresh and aged materials. The objective is to illuminate the changes in material properties after time on stream as well as the differences among the materials. The changes in material properties are evaluated by H2-TPR, BET surface area analysis, TEM, SEM, and STEM. The material’s tendency to coke is investigated by TPO analysis.The three materials exhibit promising initial activity. However, the Co-promoted sample decreases sharply in activity after 25h of operation. Meanwhile, the other two materials display a more stable activity throughout the evaluated time. The deactivation of the Co-promoted material could be linked to the high amount of coke deposited during operation. Based on the results from the activity evaluation and characterization, the material promoted with lanthanum displays the most promising results. The addition of lanthanum resulted in a catalyst that was both stable and had high activity, even though a low rhodium loading is used. The material also shows superior thermal resistance compared to the other two materials. In addition, the tendency to coke is significantly lower compered to the other materials, which is especially beneficial when dealing with ATR of complex fuels.

Pd catalysts for total oxidation of methane: Support effects

We prepared a series of Pd catalysts for total oxidation of methane on different supports: Al2O3, ZrO2–CeO2 and CeO2. We characterized the catalysts’ structures by XRD and their surface chemistries by temperature programmed reduction (TPR) and oxidation (TPO); and measured their activities for methane oxidation in temperature programmed reaction experiments (TPRx). By fitting a kinetic model to the TPRx results, we extracted apparent activation barriers (Eapp) and pre-exponents (Aapp) for methane oxidation over each of the catalysts. Activity correlates strongly with Aapp, which we interpret as a relative measure of the density of PdO–Pd* (* is an O-vacancy) site pairs. TPR results support this view: PdO in low activity (low Aapp) catalysts, such as Pd/CeO2, is relatively resistant to reduction, meaning that formation of the Pd* partner of the site pair is difficult—and the number of site pairs that can form is low.

Selective catalytic reduction of NO with NH3 over CeO2–ZrO2–WO3 catalysts prepared by different methods

The selective catalytic reduction (SCR) of NO by NH3 has been investigated over the CeO2–ZrO2–WO3 (CZW) catalysts prepared by hydrothermal synthesis, incipient impregnation, co-precipitation and sol–gel methods. The results indicate that the CZW catalyst prepared by hydrothermal method shows the best SCR activity, and more than 90% NO conversion is obtained at 195–450°C with a gas hourly space velocity of 50,000h−1. The samples are characterized by XRD, N2 adsorption–desorption, SEM, EDS, XPS, H2-TPR, NH3-TPD and Pyridine-IR techniques. The results imply that the superior SCR activity of CZW catalyst is contributed to the excellent redox property, strong acidity and highest content of chemisorbed oxygen species. Furthermore, the larger surface area and greater total pore volume improve the redox ability and enhance NO conversion at low temperature, while the co-existence of Lewis and Brønsted acid sites enhance the SCR activity at high temperature.

A Study on Mono and Mixed Nano‐supported Co‐based Catalysts for Dry Reforming of Methane

Effect of calcination temperature on Co‐based catalysts was studied in order to evaluate the modifications in structural and catalytic properties for catalytic reforming of CH4 with CO2 . In this regard, two sets of mono (i.e., CeO2 and ZrO2 ) and mixed nano‐supported (i.e., CeO2–ZrO2 ) cobalt‐based catalysts, calcined at two different temperatures (500 and 900 °C), were prepared. To evaluate the catalytic activity of the prepared catalysts, CO2–CH4 reforming reaction was performed in a micro tubular reactor under atmospheric conditions (1 atm) and at different reaction temperatures ranging from 450 to 800 °C. For stability evaluation, the time on stream experiments, under similar reaction conditions, was conducted at 700 °C for 9 h. Furthermore, for selected catalysts the effect of reactants feed ratio (CO2 /CH4 ) and gas space velocity (GSV) were also studied. Various techniques such as N2 adsorption–desorption isotherm, temperature‐programmed reduction (TPR), temperature‐programmed desorption (TPD), X‐ray diffraction (XRD), and thermogravimetric analysis (TGA) were used to characterize the prepared catalysts. It was found that the use of a specific calcination temperature in the pretreatment step had a strong influence on catalytic activity, stability, and amount of carbon deposition over catalyst surface in the dry reforming of methane. Additionally, the study revealed that the catalysts calcined at 500 °C had better activity and stability than those calcined at 900 °C.

Comparative study on cubic and tetragonal Cu–CeO2–ZrO2 catalysts for water gas shift reaction

Cu–Ce0.8Zr0.2O2 and Cu–Ce0.2Zr0.8O2 catalysts have been applied for water gas shift reaction to understand the effect of cubic/tetragonal structure of CeO2–ZrO2 support. Cubic Cu–Ce0.8Zr0.2O2 catalyst shows higher activity (17.4% at 320°C) than tetragonal Cu–Ce0.2Zr0.8O2 catalyst at the high gas hourly space velocity of 72,152h−1 due to the enhanced oxygen mobility and high Cu dispersion (7.8%). In addition, cubic Cu–Ce0.8Zr0.2O2 catalyst exhibits a stable activity at 320°C for 25h resulting from a strong resistance against sintering.

Kinetic Study of Hydrogen Production by the High Temperature Water Gas Shift Reaction of Reformate Gas in Conventional and Membrane Packed Bed Reactors over Ca-Promoted CeO2–ZrO2 Supported Ni–Cu Catalyst

The kinetics of the high temperature water gas shift (HTWGS) reaction was performed in a hydrogen selective membrane reactor (MR) and compared with that in a packed bed tubular reactor (PBTR) at a pressure range of 150–250 psi, temperature range of 400–500 °C and weight of catalyst/CO molar flow rate ratio of 1.1–2.7 (g-cat·hr)/mol using reformate gas over a Ni–Cu catalyst on Ca-promoted CeO2–ZrO2 support. The purpose was to estimate the effect of hydrogen removal on conversion and reaction products yield. The characteristics and hydrogen permeation properties of the MR were also studied. The activation energy obtained for the reaction in the PBTR was 190 kJ/mol, whereas the MR exhibited activation energy of ca. 50% lower than the PBTR. MR characterization results showed that hydrogen permeation and recovery increased with temperature and trans-membrane pressure, which could be correlated using Sievert’s law to yield an activation energy of 22.7 kJ/mol.

A crucial role for the CeO2–ZrO2 support for the low temperature water gas shift reaction over Cu–CeO2–ZrO2 catalysts

The catalytic activity of Cu based catalysts is correlated to its amount of defect oxygen which strongly depends on the ratio of CeO2–ZrO2.

One step hydrothermal synthesis of CeO2–ZrO2 nanocomposites and investigation of the morphological evolution

Uniform cauliflower-like and rough nano-spheres of CeO2–ZrO2 composites were synthesized by a facile hydrothermal process using only urea as a mineralizer without any surfactant or template.

Ultra-simple synthetic approach to the fabrication of CeO2–ZrO2 mixed nanoparticles into homogeneous, domain, and core–shell structures in mesoporous spherical morphologies using supercritical alcohols

CeO2–ZrO2 composite nanoparticles with mesoporous and spherical morphologies were assembled into different homogeneously dispersed, domain, and core–shell structures by changing the reaction conditions, such as the heating rate, acid, and solvent, in supercritical alcohols. CexZr1−xO2 with homogeneous structures were prepared from Ce(NO3)3 and ZrO(NO3)2 in the presence of formic acid in methanol by rapid heating (500°C/min) to 300°C, while CeO2–ZrO2 with domain structures were obtained by slow heating (5.4°C/min). Treatment of mesoporous and spherical ZrO2 nanoparticles with a Ce(NO3)3 solution in supercritical conditions yielded CeO2–ZrO2 nanocomposites with core–shell morphologies, while solvents playing a key role in controlling the thickness of the outer shell; methanol produced a very thin outer shell (<1nm) of cubic CeO2 and 2-propanol afforded a thick outer shell (100–200nm) of cubic CeO2.

Promoted RhPt bimetallic catalyst supported on δ-Al2O3 and CeO2–ZrO2 during full-scale autothermal reforming for automotive applications: Post-mortem characterization

The influence of sulfur and coke formation on the steam reforming of diesel was evaluated for two promoted RhPt bimetallic catalysts, composed of 1:1 Rh:Pt/10:10 La2O3: CeO2/ δ-Al2O3 (CAT 1) and 1:1 Rh:Pt/4:5 MgO: Y2O3/CeO2−ZrO2 (CAT 2). The intrinsic activity is related to the total Rh and Pt area observed after the exposure to sulfur. Therefore, the degree of deactivation is related to the amount of sulfur deposited on the active metal sites. Sulfur analysis on the aged catalyst washcoat showed a decreasing sulfur concentration in the axial direction of the reformer. The estimated sulfur coverage related to metal surface area after 40h on stream reached values of 0.145 in CAT 2, below the equilibrated sulfur coverage of 0.19 after tests with DIN 590. Thus, showing a partial deactivation due to sulfur poisoning. Further catalyst characterization on carbon deposits and thermal aging was performed by TPO, TGA, BET, CO chemisorption, and TEM analysis.

The hazard assessment of nanostructured CeO2-based mixed oxides on the zebrafish Danio rerio under environmentally relevant UV-A exposure

The effect of nanomaterials on biota under realistic environmental conditions is an important question. However, there is still a lack of knowledge on how different illumination conditions alter the toxicity of some photocatalytic nanomaterials. We have investigated how environmentally relevant UV-A exposure (intensity 8.50±0.61W/m2, exposure dose 9.0J/cm2) affected the toxicity of cerium oxide (CeO2)-based nanostructured materials to the early-life stages of zebrafish Danio rerio. Pure cerium oxide (CeO2), copper–cerium (CuO–CeO2) (with a nominal 10, 15 and 20mol.% CuO content), cerium–zirconium (CeO2–ZrO2) and nickel and cobalt (Ni–Co) deposited over CeO2–ZrO2 were tested. It was found that under both illumination regimes, none of the tested materials affected the normal development or induced mortality of zebrafish early-life stages up to 100mg/L. Only in the case of CuO–CeO2, the growth of larvae was decreased (96h LOEC values for CuCe10, CuCe15 and CuCe20 were 50, 50 and 10mg/L, respectively). To conclude, CeO2-based nanostructured materials are not severely toxic to zebrafish and environmentally relevant UV-A exposure does not enhance their toxicity.

Catalytic wet air oxidation of phenol over ultrasound-assisted synthesized Ni/CeO2–ZrO2 nanocatalyst used in wastewater treatment

Non-noble metal Ni with different loadings was coated on precipitated CeO2–ZrO2 support by the sonochemistry method and examined for catalytic wet air oxidation of phenol. The structure of the nanocatalysts was determined by BET, FESEM, XRD, and FTIR analyses. The results showed non-uniform morphology of the nanocatalyst at lower Ni contents changed to homogenous morphology with spherical nano particles at higher Ni contents. While the size of NiO crystals remained constant with rising Ni content, the crystallinity of NiO significantly increased. If the crystallinity of NiO was 100% in 20%wt Ni/CeO2–ZrO2, the crystallinity of NiO in 5%wt Ni was found to be 41.13%. The average particle size in Ni(15%)/CeO2–ZrO2 was 77nm in which 85.71% of particle diameters were less than 100nm. Catalytic wet air oxidation of phenol with different Ni loadings indicated improvement of phenol destruction at higher amounts of active phase. Removal of phenol increased with increasing catalyst loading from 4 to 9.0g/l but further increase to 10g/l declined the catalyst reactivity.

A search for temperature induced time-dependent structural transitions in 10 mol%Sc2O3–1 mol%CeO2–ZrO2 and 8 mol%Y2O3–ZrO2 electrolyte ceramics

Both Sc2O3–CeO2-stabilized-ZrO2 (SCSZ) and Y2O3-stabilized-ZrO2 (YSZ) show similar Young's modulus damping at 175–400 °C and 200–400 °C, respectively, by impulse excitation acoustic technique. The phase transition in SCSZ is considered responsible for the damping; however for YSZ, such a phase transition has never been reported. To clarify the relation of damping and structural transition in these two materials, the time-dependent high temperature stabilities are studied by in situ X-ray diffraction, neutron diffraction and Raman scattering during long-term annealing of SCSZ at 350 °C and YSZ at 275 °C. The cubic-to-rhombohedral transition in SCSZ is detected. However, the existence of t' phase in YSZ raw powders is confirmed without significant changes of the cubic structure during isothermal annealing. It is concluded that the phase transition contributes to the Young's modulus damping in SCSZ, while the structural transition is excluded from the reason for damping in YSZ.