CexZr1-xO2 Solid Solution Research Articles

Enhanced activity of CexZr1-xO2 solid solutions supported Cu-based catalysts for hydrogenation of CO2 to methanol

Hydrogenation of CO2 to methanol is one of the promising environmentally and economically technologies which can convert CO2 to sustainable fuels. A series of CexZr1-xO2 solid solutions were synthesized by solvent evaporation self-assembly method and employed as the supports of Cu-based catalysts for CO2 hydrogenation to methanol. The obtained catalysts were systematically characterized by N2 physisorption, XRD, CO2-TPD, TEM, N2O titration, XPS and H2-TPR techniques. It is found that the reduction performance, the adsorption and activation ability for CO2 are enhanced by the formation of ceria-zirconia solid solution. The Ce/Zr ratio of the catalysts and the feature of the supports are crucial for tuning the catalytic activity. The Cu/CexZr1-xO2 catalysts display much higher methanol selectivity than Cu/ZrO2 and the improved methanol selectivity stems from inhibition of the RWGS activity.

Mn2O3-Na2WO4 doping of CexZr1-xO2 enables increased activity and selectivity for low temperature oxidative coupling of methane

Ethylene in principle can be obtained via oxidative coupling of methane (OCM), but it is urgently calling for a low-temperature active and selective catalyst. To accomplish this goal, the CexZr1-xO2 solid solutions with varied Ce/Zr molar ratio were modified with Mn2O3-Na2WO4 to fabricate OCM catalysts. The effects of Ce/Zr ratio and Mn2O3-Na2WO4 doping were investigated on the catalyst reactivity for the OCM, showing that the reaction light-off temperature, activity and selectivity are subject to the Ce/Zr molar ratio and Mn2O3-Na2WO4 doping. The promising catalyst with Ce/Zr molar ratio of 0.15/0.85 achieves 25% CH4 conversion with 67% selectivity to C2-C3 at 660 °C and is stable for at least 100 h. The catalysts were systematically studied by CH4-TPSR, O2-TPO/-TPD, CH4-pulse, in-situ FT-IR, EPR and XPS, showing that the Mn2O3-Na2WO4 doped Ce0.15Zr0.85O2 can generate more O2– species thereby leading to a markedly improved activity and selectivity at much lowered reaction temperature.

Peculiar kinetic properties of Cu-doped Pd/CexZr1-xO2 in water denitrification: Impact of Pd-Cu interaction vs structural properties of CexZr1-xO2

The performances of Pd-Cu bimetallic catalysts supported on ceria-zirconia were investigated in the reduction of nitrates by hydrogen. Particular attention was paid to the impact of structural heterogeneities in CexZr1-xO2 support materials on the catalytic properties of metallic Pd and Cu nano particles. The evaporation-induced self-assembly preparation method afforded only the tetragonal structure. Co-precipitation led to a partial segregation of ceria and/or Ce-riched mixed oxide which orients differently the interactions between palladium and copper in agreement with XPS, ToF-SIMS, TEM and EDS analysis. Kinetic measurements in batch conditions led to a significant practical issue by obtaining high intrinsic rates at low Pd loading supported on CexZr1-xO2 prepared by the self-assembly method. Ammonia as undesired by-product was usually detected. A sub-oxidation of ammonia to nitrites and nitrogen is observed with changes in the product distribution according to the preparation method of the support. Such changes have been discussed in terms intrinsic OSC properties of CexZr1-xO2 solid solution and the stability of Pd oxidation state in the course of the reaction. An optimal Pd-Cu interaction protecting Pd to extensive oxidation was observed on highly loaded Pd-Cu on CexZr1-xO2 prepared by the self-assembly method suggesting that ensemble effects is able to monitor the structure sensitive production of ammonia.

The role of defect sites and oxophilicity of the support on the phenol hydrodeoxygenation reaction

This work studies the effect of support defect sites on the performance of Pd/CexZr1-xO2 (x = 0.00; 0.25; 0.50; 0.75; 0.90) catalysts for the hydrodeoxygenation of phenol in the gas phase at 573 K. The activity and selectivity for hydrodeoxygenation of phenol depends significantly on the support used. Increasing the Zr content from x = 0.0 to 0.5, the reaction rate for hydrodeoxygenation and the selectivity to benzene remains very low. However, upon increasing the Zr content above x = 0.5 a sudden jump in reaction rate and selectivity to benzene is observed. Interestingly, this activity and selectivity boost has no direct correlation with the density of acid sites or the concentration of defects on the support. Rather, the selectivity to deoxygenated products is found to depend on the oxophilicity of the support. Increasing the Zr content enhances the strength of the interaction between the O of the carbonyl group and the oxophilic site. It is proposed that the oxophilicity of these catalysts is related to the structure of the CexZr1-xO2 solid solution formed. In addition, it is observed that the degree of deactivation during the reaction also depends on the Ce/Zr molar ratio of the support. Pd/ZrO2, Pd/Ce0.10Zr0.90O2 and Pd/Ce0.25Zr0.75O2 catalysts readily deactivate during reaction, whereas the phenol conversion only slightly decreases for Pd/CeO2, Pd/Ce0.75Zr0.25O2 and Pd/Ce0.50Zr0.50O2 catalysts. The results reveal that the density of Zr species on the surface is responsible for catalyst deactivation. The stronger adsorption between the oxygen from the phenol with Zr cations resulted in an accumulation of O-containing byproducts and catalyst deactivation.

Reinforcement of steam oxidative cracking of 1-methyl naphthalene by the synergistic combination of hydrogenation and oxidation over nickel-containing mixed oxide catalysts

Steam oxidative cracking route of 1-methyl naphthalene were investigated through the combination of oxidation and hydrogenation over nickel-containing mixed oxide catalysts. Depending on the used supports, several types of active sites for oxidation, hydrogenation, and steam decomposition were generated by various metal–support and support–support interactions. The steam reforming routes were dominant without nickel, resulting in high carbon selectivity of gas due to a full-cracking process. With nickel, steam oxidative cracking routes were strongly enhanced by a synergistic combination between hydrogenation occurring on the Ni metallic sites and oxidation over mobile lattice oxygens in a CexZr1-xO2 solid solution, resulting to the higher carbon selectivity of liquid products than that of gas products. Among the prepared catalysts, optimal interaction between Ni and ceria-zirconia-alumina mixed oxide support caused not only an increase of oxygen vacancies in solid solutions, but also the formation of smaller Ni metallic nanoparticles, resulting in reinforcement of steam oxidative cracking of 1-methylnaphthanlene.

Sol–Gel Synthesis and Distinctive Structural Features of CexZr1–xO2 Solid Solutions

We have synthesized CexZr1–xO2 solid solutions via the thermal decomposition of xerogels of different compositions prepared by drying appropriate hydrosols. The synthesized materials have been characterized by thermal analysis, X-ray diffraction, and Raman spectroscopy. The results demonstrate that the solid solutions are formed at relatively low temperatures (450–600°C). The CexZr1–xO2 samples with x = 0.5–0.9 consist of a cubic solid solution. At a lower CeO2 content (x = 0.2), the material consists of a mixture of cubic and tetragonal phases.

A novel method for the synthesis of CexZr1-xO2 solid solution with high purity of κappa phase and excellent reactive activity

Ceria-zirconia solid solution (Ce1-xZrxO2) had been widely used in environmental catalysis as catalyst and support due to its excellent oxygen storage/release capacity (OSC). Furthermore, the performance of Ce1-xZrxO2 was dependent on the phase. Notably, κappa-type CeZrO4 solid solution (κ-Ce2Zr2O8) with the highly ordered arrangement of Ce and Zr cations possessed the highest OSC. However, the synthesis of CexZr1-xO2 with high special surface area and high purity of κ-Ce2Zr2O8 was still a huge challenge by now. Herein, a new method for the synthesis of κ-Ce2Zr2O8 solid solution was proposed, based on ceria-zirconia solid solution precursor prepared by an improved solvent-thermal method. The reduction time can be shortened to 4h at 1500 ℃ to produce the CeZrO4 solid solution with pure κ-Ce2Zr2O8 and relative high special surface area. Consequently, this catalyst existed high ratio of Ce3+/Ce on the surface, a large amount of surface oxygen species and excellent OSC, leading to the high activities for CO oxidation and CH4 combustion.

ChemInform Abstract: Invited Review: Some Recent Developments in the Atomic‐Scale Characterization of Structural and Transport Properties of Ceria‐Based Catalysts and Ionic Conductors

AbstractReview: 107 refs.

Surface Stabilized Nanosized CexZr1-xO2 Solid Solutions over SiO2: Characterization by XRD, Raman, and HREM Techniques

Ce(x)Zr(1)(-)(x)O(2) solid solutions deposited over silica surface were investigated by X-ray diffraction (XRD), Raman spectroscopy (RS), and high-resolution transmission electron microscopy (HREM) techniques in order to understand the role of silica support and the temperature stability of these composite oxides. For the purpose of comparison, an unsupported Ce(x)Zr(1)(-)(x)O(2) was also synthesized and subjected to characterization by various techniques. The Ce(x)Zr(1)(-)(x)O(2)/SiO(2) (CZ/S) (1:1:2 mole ratio based on oxides) was synthesized by depositing Ce(x)Zr(1)(-)(x)O(2) solid solution over a colloidal SiO(2) support by a deposition precipitation method and unsupported Ce(x)Zr(1)(-)(x)O(2) (CZ) (1:1 mole ratio based on oxides) was prepared by a coprecipitation procedure, and the obtained catalysts were subjected to thermal treatments from 773 to 1073 K. The XRD measurements disclose the presence of cubic phases with the composition Ce(0.75)Zr(0.25)O(2) and Ce(0.6)Zr(0.4)O(2) in CZ samples, while CZ/S samples possess Ce(0.75)Zr(0.25)O(2), Ce(0.6)Zr(0.4)O(2), and Ce(0.5)Zr(0.5)O(2) in different proportions. The crystallinity of these phases increased with increasing calcination temperature. The cell a parameter estimations indicate contraction of ceria lattice due to the incorporation of zirconium cations into the CeO(2) unit cell. Raman measurements indicate the presence of oxygen vacancies, lattice defects, and displacement of oxygen ions from their normal lattice positions in both the series of samples. The HREM results reveal, in the case of CZ/S samples, a well-dispersed nanosized Ce-Zr-oxides over the surface of amorphous SiO(2). The structural features of these crystals as determined by digital diffraction analysis of experimental images reveal that the Ce-Zr-oxides are mainly in the cubic geometry and exhibit high thermal stability. Oxygen storage capacity measurements by a thermogravimetric method reveal a substantial enhancement in the oxygen vacancy concentration of CZ/S sample over the unsupported CZ sample.

Thermal Stabilization of CexZr1-xO2 Oxygen Storage Promoters by Addition of Al2O3: Effect of Thermal Aging on Textural, Structural, and Morphological Properties

Thermal stability and dispersion of CeO2 supported on Al2O3 is greatly improved by insertion of ZrO2 into the CeO2 lattice. It is shown that homogeneous nanosized CexZr1-xO2 solid solutions can be prepared on the Al2O3 surface by using a citrate complexation synthesis method. Investigation of effects of CexZr1-xO2 composition and loading of the Ce0.2Zr0.8O2 phase on thermal stability and nanostructure of the prepared materials revealed that strong interactions between the supported phase and Al2O3 are induced by the high-temperature treatment. High contents of ZrO2, choice of the CexZr1-xO2 precursors, and loading of the mixed oxide are critical factors leading to nanocomposite systems with high thermal and structural stability, consisting of particles of Ce0.2Zr0.8O2 as small as 9−20 nm in close contact with a θ-Al2O3 matrix even after calcination at 1373 K. The enhanced stability of the present materials was confirmed also under hydrothermal conditions.

Vapor phase reduction of cyclohexanone to cyclohexanol on CexZr1-xO2 solid solutions

Reduction of cyclohexanone to cyclohexanol using propane-2-ol as hydrogen donor has been carried out in vapor phase on CexZr1-xO2 solid solutions synthesized by ombustion synthesized at 302°C. The solid solutions around 0.4 mol% cerium content show better catalytic activity compared to pure ZrO2 and the selectivity to cyclohexanol is 98%. A moderate acid-base and good redox properties of CexZr1-xO2 solid solutions are seen to be responsible for the catalytic activity. A possible mechanism of hydride transfer has been proposed with cerium ions as promoters.