Ceria Zirconia Mixed Oxides Research Articles

The pivotal role of oxygen vacancy and surface hydroxyl in the adsorption and activation of CO2 on ceria-zirconia mixed oxide

Ceria-zirconia mixed metal oxide along with pure ceria and zirconia were prepared by the co-precipitation method and characterized by N2-Physisorption, X-ray diffraction (XRD), H2-temperature programmed reduction (H2-TPR), CO-TPR with mass spectroscopy (CO-TPR/MS), oxygen storage capacity (OSC), CO and CO2-temperature programmed desorption with MS (TPD/MS), Raman spectroscopy, XPS, and diffuse reflectance infrared Fourier transform Spectroscopy (DRIFTS) techniques. Furthermore, Density Functional Theory (DFT) calculations were performed to acquire molecular insights into the role of oxygen vacancies and surface hydroxyl species in CO2 adsorption and subsequent activation. Our experimental and theoretical results corroborated the fact that the incorporation of zirconium ions into the ceria matrix enhanced the overall CO2 adsorption and activation by forming thermally stable surface species, which were eventually detected by DRIFTS. Additionally, DFT calculations revealed the oxygen vacancy mediated change in the electronic structure of ceria-zirconia mixed oxide, which had been argued to improve its catalytic activity. Furthermore, the oxygen vacancy formation energy (OVFE), oxygen storage capacity (OSC) and CO2 adsorption energies were correlated to augment the importance of these properties.

Activated carbon derived from oleander seeds supported ceria-zirconia mixed oxides for enhanced supercapacitive behaviour

Oxides of metals in combination with large surface area carbons have considered as a top contender for energy storage systems. In similar line, the present study explores the utilisation of a bi-metallic mixed oxide with activated carbon generated from biological sources as a promising electrode material for supercapacitor applications. The combination of activated carbon synthesized from the seeds of an Oleander plant that is readily available in North-east India, and the bimetallic mixed oxide (ceria-zirconia) synthesized through a conventional co-precipitation process is observed to have a specific capacitance of 270 F/g (at 1 A/g) and cycling stability with 98 % retention of specific capacitance up to 10,000 cycles. The produced electrode materials were characterised using scanning electron microscopy (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and electrochemical techniques such as cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS). All the analyses revealed that the synthesized materials have the potential to be used as supercapacitor electrodes.

Supported Ce/Zr pyrochlore monolayers as a route to single cerium atom catalysts with low temperature reducibility

The combination of structural characterization at atomic resolution, chemical data, and theoretical insights has revealed the unique nanostructures which develop in ceria supported on yttria-stabilized zirconia (YSZ) after being submitted to high-temperature reducing treatments. The results show that just a small ceria loading is needed for creating a supported Zr-rich pyrochlore (111) nanostructure, resembling the structure of single cerium atom catalysts. The specific atomic arrangement of this nanostructure allows to explain the improvement of the reducibility at low temperature. The reduction mechanism can be extrapolated to ceria-zirconia mixed oxides with pyrochlore-like cationic ordering, exposing Zr-rich (111) surfaces. The results gathered here provide key information to understand the redox behavior of these types of systems, which may contribute to improving the design of new ceria-zirconia based materials, with lower content of the lanthanide element, nearly 100% cerium atom utilization, and applications in environmental catalysis.

Composite Structured M/Ce0.75Zr0.25O2/Al2O3/FeCrAl (M = Pt, Rh, and Ru) Catalysts for Propane and n-Butane Reforming to Syngas.

Here, we report the preparation, characterization, and performance of reforming propane and n-butane into a syngas of composite structured M/Ce0.75Zr0.25O2/Al2O3/FeCrAl (M = 0.46 wt.% Pt, 0.24 wt.% Rh, and 0.24 wt.% Ru) catalysts. The catalysts are composed of a high-heat-conducting FeCrAl block with preset geometry, with a surface nearly totally covered by θ-Al2O3. Afterwards, a layer of ceria–zirconia mixed oxide was deposited. The formed oxide coating was used as a support for 2–3 nm sized Pt, Rh, or Ru nanoparticles. The performance of the catalysts in propane steam reforming decreased in the order of Rh ≈ Ru > Pt. The reformates obtained in the propane steam reforming over Rh- and Ru/Ce0.75Zr0.25O2/Al2O3/FeCrAl at 600 °C and GHSV = 8300 h−1 contained 65.2 and 62.4 vol.% of H2, respectively, and can be used as a fuel for solid oxide fuel cells. In the oxidative steam reforming of propane at 700 °C and GHSV= 17,000 h−1, the activities of the Rh- and Pt-based catalysts were similar and the compositions of the outlet gas mixtures were quite close to equilibrium in both cases. Increasing the reagent flow rate to 25,600 h−1 showed stability of the Rh/Ce0.75Zr0.25O2/Al2O3/FeCrAl performance, whereas the Pt/Ce0.75Zr0.25O2/Al2O3/FeCrAl activity decreased. A mathematical model considering the velocity field, mass balance, pressure, and temperature distribution, as well as the reaction kinetics, was suggested for the propane steam and oxidative steam reforming over the Pt- and Rh/Ce0.75Zr0.25O2/Al2O3/FeCrAl catalysts. The model well described the experimental results.

Selective hydrogenation of α,β-unsaturated aldehydes over Pt supported on cerium–zirconium mixed oxide of different composition

Cerium–zirconium mixed oxides with different Ce/Zr ratio were prepared and used as supports for Pt-containing catalysts. The study of the catalysts in the cinnamaldehyde hydrogenation reaction has shown that cinnamaldehyde conversion and cinnamyl alcohol selectivity strongly depend on the CeO2 content in the support. The highest cinnamyl alcohol yield of 81% was obtained in 105 min at room temperature and atmospheric pressure over the 1%Pt/CeO2–ZrO2 catalyst with Ce : Zr atomic ratio equal to 4 : 1.

Evaluation of cerium–zirconium mixed oxides for separation of 125Sb from radioactive liquid waste

Evaluation of cerium–zirconium mixed oxides for separation of 125Sb from radioactive liquid waste

Effects of co-precipitation temperature on structure and properties of La and Y doped cerium zirconium mixed oxides

Due to the oxygen storage and release properties, cerium zirconium mixed oxides are recognized as the key material in automotive three-way catalysts. To reveal the effects of co-precipitation temperature on structure, physical and chemical properties of multi-doped cerium zirconium mixed oxides, a series of La and Y doped cerium zirconium mixed oxides (CZLYs) were synthesized via a co-precipitation method, and the physical and chemical properties of CZLYs were systemically characterized by XRD, N2 adsorption–desorption, TEM, XPS, oxygen storage capacity (OSC) and hydrogen temperature programmed reduction (H2-TPR). The results show that co-precipitation temperature is an important parameter to influence the crystal size, oxygen storage capacity and thermal stability of CZLYs. When the co-precipitation temperature was 60 °C, the best redox properties and thermal stability of CZLYs were obtained. After thermal treatment at 1100 °C for 10 h, the specific surface area and oxygen storage capacity of the corresponding aged sample were 15.42 m2/g and 497.7 μmol/g, respectively. In addition, a mechanism was proposed to reveal the effects of co-precipitation temperature on the structure and properties of CZLYs.

Cerium–zirconium mixed oxide synthesized by sol-gel method and its effect on the oxygen vacancy and specific surface area

In this study, cerium-zirconium mixed oxides were prepared using the sol-gel methodology with cetyltrimethylammonium bromide (CTAB) for tuning the specific surface area and pore volume. The composition of Ce0.8Zr0.2O2 (CZ) was selected as being one of the most active for soot oxidation, which is a concern as an environmental pollutant. The structural and morphological properties were investigated using EDXRF, XRD, SEM/EDS, Raman and N2 physisorption at low temperature. The catalytic test was evaluated by temperature-programmed oxidation coupled with mass spectrometry (TPO/MS). Any variation in the CTAB concentration during synthesis changed the particle size and surface area, indicating that the particle formation follows a mechanism in which the CTAB acts as capping agent. Additionally, the material synthesized with CTAB was found to have higher oxygen vacancy compared to the materials from the conventional sol-gel process. The catalytic test suggested that the material with the smallest surface area had the lowest temperature of oxidation (T50% ​= ​400 ​°C), which demonstrates that the oxygen vacancy parameter is essentially more important than the specific surface area itself.

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 Ce0.475Zr0.475M0.05O2 (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 energy γ as a function of Rion/Dave is identified as a quantitative structure descriptor for analyzing the doping effect of rare earth (RE) elements on the thermal stability of Ce0.475Zr0.475M0.05O2. By doping the suitable RE, γ can be adjusted to the optimal range to enhance the thermal stability of Ce0.475Zr0.475M0.05O2. 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 °C for 10 h, the specific surface area (SSA) of aged Y-CZ and La-CZ samples can reach 21.34 and 19.51 m2/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.

A novel Ce3Zr5O16 composite with gradient distribution of elements and its application in Pd-only three-way catalyst

To equilibrate the conflict between the thermal stability and redox property of ceria-zirconia mixed oxide (CZ), a novel Ce3Zr5O16 composite with gradient distribution of elements (CZ-tcp) was prepared by two-step coprecipitation and compared with the sample (CZ-cp) obtained from conventional coprecipitation method. The distribution of elements, structural and textural properties, and redox properties of samples were investigated by HAADF-STEM equipped with EDS, XPS, XRD, Raman, SEM, N2 physisorption, oxygen storage capacity and H2-TPR systematically. The results revealed that, with respect to CZ-cp, CZ-tcp exhibits more stable phase structure, and less attenuation rates of pore volume (40.1% vs 67.9%) and oxygen storage capacity (1.0% vs 18.7%) after ageing treatment, which is on account of Zr enriched in the core of the grains and Ce enriched at the surface layer of the grains. Based on this, the catalytic performance evaluation results revealed that the catalyst supported on CZ-tcp (Pd@CZ-tcp) presents improved catalytic activity of HC and NO, and better thermal stability of CO catalytic activity compared with Pd@CZ-cp. Therefore, this designed CZ-tcp shows enhanced thermal stability and redox property at the same time and can be used as a competitive oxygen storage support for three-way catalyst.

Synthesis and studies of structured support Ce0.75Zr0.25O2/θ-Al2O3/FeCrAl

The procedure for preparing a structured composite system Ce0.75Zr0.25O2/θ-Al2O3/FeCrAl, which is used as a support for complete oxidation catalysts and catalysts for the conversion of liquid and gaseous hydrocarbons into synthesis gas, has been improved. The method for the synthesis of Ce0.75Zr0.25O2 powder has been optimized to facilitate its depositing on a FeCrAl mesh pre-coated by protective θ-Al2O3 layer. It has been shown by X-ray diffraction that the model powder sample and the cerium-zirconium mixed oxide coating on the alumina-precoated FeCrAl mesh have the same phase compositions. The protective layer of θ-Al2O3 on FeCrAl mesh possesses excellent adhesion properties that ensures high mechanical strength of the Ce0.75Zr0.25O2 coating under repeated heating–cooling cycles. The use of metal substrate as a component of the composite support provides high thermal conductivity, and thus reduces the formation of local hot spots under conditions of highly exothermic catalytic processes.

Surface effect of nano-sized cerium-zirconium oxides for the catalytic conversion of methanol and CO2 into dimethyl carbonate

The direct synthesis of Dimethyl carbonate (DMC) from methanol and CO2 is a green process which allows CO2 valorization. Among efficient catalysts, ceria, zirconia and cerium-zirconium mixed oxides are often reported as the most active catalysts. In a recent report, we discovered that cerium-zirconium mixed oxides prepared by Flame Spray Pyrolysis (FSP) show greater catalytic activities than those prepared by precipitation, although both exhibit very similar surface area and bulk features. The objective of this study was to find out the origins of the superior catalytic activities obtained by flame spray pyrolysis synthesis method by a deeper analysis of bulk and surface properties. We have opted to focus on mixed ceria-zirconia of equimolar composition (Ce0.5Zr0.5O2) as it exhibits maximum catalytic activity for both synthesis methods. Combining bulk and surface characterization as well as surface adsorption measurements using probe molecules, we propose that flame spray pyrolysis enables a surface enrichment in cerium oxide still in interaction with zirconium oxide that leads to a high concentration of adsorbed methanol at the surface, which might explain the greater activity of the catalysts prepared using this method. Beyond the application of DMC synthesis, we can anticipate that Flame Spray Pyrolysis synthesis should generate relative high surface area mixed oxides with different catalytic performances with respect to mixed oxides prepared at lower temperature owing their metastable nature.

Low temperature benzene oxidation over copper–silver catalyst: roles of copper oxide and silver on cerium–zirconium mixed oxide

The synergetic effects of the non-PGM catalytic components enabled complete benzene oxidation at low temperature, below 200 °C.

Cerium-zirconium mixed oxide nanostructures for diesel soot oxidation: synthesis and effect of structure.

Nanostructured materials have been used in several branches of science and technology. Particulate matter is one of the major air pollution concerns. In this work, nanorods and nanoparticles of Ce0.8Zr0.2O2 (CZ) mixed oxides were prepared by different routes, and the use of an organic template was evaluated in diesel soot oxidation. The catalysts were characterized by several techniques including structural analysis (XRD, TEM, N2 adsorption–desorption) and activity (TPR/MS, TPO/MS). A fast TPR/MS method is proposed to calculate hydrogen consumption that can be correlated to the oxygen storage capacity (OSC). It was demonstrated that CZ-nanorods with twice the amount of template in the syntheses (CZ-NRs-2X) were very active for soot oxidation with T50% at 351 °C, and CO2 and H2O were the only oxidation products from Printex®-U (Evonik). This catalyst, reported for the first time, was subjected to up to three cycles and it showed fair activity, proving that this morphology is one of the best mixed oxides of CZ for oxidation.

Size, nanostructure, and composition dependence of bimetallic Au–Pd supported on ceria–zirconia mixed oxide catalysts for selective oxidation of benzyl alcohol

A bimetallic Au–Pd catalyst supported on ceria–zirconia with Au:Pd molar ratio 0.8 has been synthesized using a simultaneous deposition–precipitation method and oxidized at 250, 450, and 700 °C in order to modify its particle size, nanostructure, and composition. Combined X-ray energy dispersive spectroscopy (XEDS) and X-ray photoelectron spectroscopy (XPS) analysis clearly evidence that the bimetallic Au–Pd catalyst oxidized at 250 °C is made up of a mixture of monometallic Au and Pd and bimetallic Au–Pd nanoparticles with Au:Pd ratios varying over a wide range. Increasing oxidation temperature leads to a stronger interaction between Au and Pd. Meanwhile, a slight increase of particle size and a narrowing of the Au:Pd ratio in the bimetallic nanoparticles take place. Compared with titania and activated carbon supports, the resistance against sintering at high temperatures of Au–Pd metal particles supported on ceria–zirconia is proven to be higher. A synergistic effect has been observed for selective oxidation of benzyl alcohol on these catalysts. The catalytic activity decreases only slightly after oxidation at 450 °C. However, oxidation at 700 °C results in much lower catalytic activity. Migration of Pd onto Au particles during oxidation of benzyl alcohol enhances the catalytic activity of a physical mixture of monometallic Au and Pd supported on ceria–zirconia catalysts. This fact, jointly with an analysis of the intrinsic activity, reveals the influence of the actual nature of Au–Pd interactions in the bimetallic particles, which points to higher activity of Au@Pd or AuPd@Pd nanostructures on ceria–zirconia supports.

Mechanochemical preparation of ceria-zirconia catalysts for the total oxidation of propane and naphthalene Volatile Organic Compounds

A range of ceria-zirconia mixed metal oxide catalysts were synthesised by mechanochemical milling from nitrate precursors and tested for propane and naphthalene total oxidation. The mixed CeZrOx metal oxide catalysts were more active compared to pure CeO2 and ZrO2, with the Ce0.90Zr0.10Ox catalyst the most active for both propane and naphthalene total oxidation. Catalysts were characterised by XRD, TGA, Raman spectroscopy, BET surface area, XPS, TEM and SEM-EDX techniques. Formation of CeZrOx solid solutions were observed for Zr content ranging from 5 to 25%, and phase separated materials were observed for higher Zr content of 50%. The incorporation of Zr into the CeO2 lattice increased the surface area compared to pure CeO2 and ZrO2, with evidence of a surface enrichment of Zr when concentrations were compared to the bulk. Incorporation of Zr was found to increase the surface concentration of oxygen defects in the CeZrOx mixed metal oxides, the concentration of these defect on the catalysts follows a similar trend to the propane and naphthalene total oxidation activity: The increased presence of oxygen defects and reducibility are factors responsible for the enhanced total oxidation activity of the mixed metal oxide catalysts and the mechanochemical method is an effective preparation route for these active catalysts.

Effects of precursor moisture and inert N2 atmosphere calcinations on structure and properties of alumina modified CeZrLaNd mixed oxides

CeO2-ZrO2 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 (H2-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 at 1000 °C for 4 h reaches 68.8 m2/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 241.06 μmolO2/g (41.3% increase compared with the common calcination), due to the abundant formation of the crystal defects and oxygen vacancies.

Ethylene production via catalytic ethanol dehydration by 12-tungstophosphoric acid@ceria-zirconia

The use of renewable raw materials as feedstocks for important industrial chemicals presents a challenge to the scientific community in its pursuit of sustainability. The necessary tasks include the revision of significant processes such as the ethanol-to-olefin process (ETO), and our goal was the production of ethylene. We chose 12-tungstophosphoric acid (HPW) supported on ceria-zirconia mixed oxide (Ce0.8Zr0.2O2, CZ) as a promising catalyst. The structural properties of different loadings of HPW (5–45 wt%) and different support preparation methods were examined, in addition to their activities in the gas-phase dehydration of ethanol. The maintenance of the Keggin structure was proved by infrared spectroscopy (FT-IR), 31P solid-state magic angle spinning nuclear magnetic resonance (MAS NMR) and X-ray diffraction (XRD). Scanning electron microscopy (SEM) images showed a gradual increase in the formation of agglomerates by the HPW coating on the support surface. The reaction products were followed by mass spectra in the interval of 200–400 °C. Multiple catalysts reached 100% ethanol conversion and selectivity for ethylene at low temperatures (200, 250 and 300 °C). A double effect of the support was observed: to disperse the HPW, improving its original low surface area, and to probably facilitate the coke burning process. Tests to determine the lifetime of catalysts (e.g., 26.4% HPW/CZ) performed in a pulsed microreactor coupled to a GC system showed at least 70 h of activity at 300 °C temperature, which demonstrated the potential of these catalysts.

Study of the role of chemical support and structured carrier on the CO2 methanation reaction

Power-to-Gas (PtG) technology is particularly interesting because it is based on a mature technology such as the methanation process, based on the established Sabatier reaction. The industrial applicability of such technology is severely limited by the huge exothermicity of the process, that affects the catalyst stability. The aim of this work was to investigate the effect of the chemical and physical support for Sabatier reaction. Nickel based samples were prepared, by deposing the active metal on alumina (as a commercial reference), ceria and a cerium-zirconium mixed oxide supports. The catalytic activity tests confirmed that rare heart oxides supports strongly improved the catalytic performances, both in terms of CO2 conversion and selectivity to CH4, probably due to the basic nature of such supports. The CeZr-based formulation was then deposited on structured carriers with highly thermal conductivity, like a silicon carbide monolith (SiC) and an aluminium foam. Experimental achievements highlighted the effect of conductive carriers on the temperature profile along the reactor volume. SiC monolith and aluminium foam are able to guarantee a flattest thermal profile than the powder, thus in one and ensuring a higher conversion of carbon dioxide, in the other hand strongly suppressing side-reaction contribution, therefore maximising the selectivity to methane.

Achieving efficient and inexpensive vanadium flow battery by combining CexZr1−xO2 electrocatalyst and hydrocarbon membrane

Vanadium flow battery (VFB) is one of the most promising technologies for grid scale energy storage because of its unlimited capacity and ultralong lifespan. Improving energy efficiency (EE) and reducing cost are critical to the practical application of VFB. The efficiency and cost of VFB mainly depend on its key materials such as electrode and proton-conducting membrane. Herein, we report the use of highly active ceria–zirconia mixed oxide decorated graphite felt (CexZr1−xO2/GF) electrodes in combination with a high proton-selective sulfonated poly(ether ether ketone) (SPEEK) membrane to achieve efficient and inexpensive VFB. CexZr1−xO2 electrocatalysts can improve the electrolyte wettability and electrochemical activity of the GF electrode towards VO2+/VO2+ and V2+/V3+ redox couples, leading to remarkably enhanced voltage efficiency (VE) of VFB. On the other hand, SPEEK membrane can effectively suppress the crossover of vanadium ions, resulting in greatly improved coulombic efficiency (CE) of VFB. As a result, the CexZr1−xO2/GF-SPEEK combination demonstrates superior battery efficiencies (CE, VE and EE) and cycling stability to the traditional GF-Nafion combination. Meanwhile, since hydrocarbon-based SPEEK membrane is much cheaper than commercial Nafion membrane, the cost of VFB is also significantly reduced.