Thermodynamic prediction of the nonstoichiometric phase Zr1–zCezO2–x in the ZrO2–CeO1.5–CeO2 system

A thermodynamic estimation of the ZrO₂–CeO₂ and ZrO₂–CeO_1.5 systems, as well as the cubic phase in the CeO_1.5–CeO₂ system has been developed and the complex relation between the nonstoichiometry, y, in Ce_zO_2–y and the oxygen partial pressure at different temperatures is evaluated. The behavior of the nonstoichiometry phase Zr_1–zCe_zO_2–x is described based on the thermodynamic estimation in the ZrO₂–CeO₂, CeO_1.5–CeO₂ and ZrO₂–CeO_1.5 systems. Additionally, the interdependence among miscellaneous factors, which can be used to describe the change in oxidation states of cerium such as the oxygen partial pressure, the CeO_1.5 fraction in CeO_1.5–CeO₂ in the quasi-ternary system, the nonstoichiometry y and the difference between the activity of CeO₂ and CeO_1.5 are predicted. The calculated results are found to be very useful to explain the influence of pressureless sintering at different O₂ partial pressures on the mechanical properties of CeO₂-stabilised ZrO₂ ceramics     

Chemical Removal of CeO2 Segregated on the Surface of CeO2–ZrO2 Binary Oxides for Improvement of OSC

A chemical treatment to remove residual CeO₂ phase on CeO₂–ZrO₂ (CZ) solid solution was carried out. A CZ was treated by H₂O₂ for the reduction of Ce⁴⁺ to Ce³⁺ and then HNO₃ for the dissolution of Ce³⁺ compounds (H–CZ). H₂-TPR, TEM-EDX and XPS analyses revealed the removal of CeO₂ phase and the homogeneous distribution of Ce species. About 20% improvement in oxygen storage capacity (OSC) of H–CZ was confirmed at 773 K by the weight measurements under H₂/N₂ and air atmospheres, indicating that the HNO₃/H₂O₂ treatment was effective to avoid the deterioration of the OSC by segregated CeO₂ on the CZ binary oxides.     

Structural studies of a ZrO2–CeO2 doped system

The local structure around Zr, Ce and dopant atoms (Fe and Ni) in the ZrO₂–CeO₂ system investigated by X-ray absorption spectroscopy (XAS) is reported to better understand the tetragonal phase stabilization process of zirconia. The first coordination shell around Zr atoms is not sensitive to the introduction of dopants or to an increase in the ceria content (from 12 to 20 mol%). Ce ions maintain the eight-fold coordination as in CeO₂, but with an altered bond distance. The formation of vacancies resulting from reduction of Ce atoms can be discarded, because XANES spectra clearly show that Ce ions are preferentially in a tetravalent state. XANES and EXAFS experiments at the Fe K-edge evidence that the local order around Fe is quite different from that of the Fe₂O₃ oxide. On the one hand, ab initio EXAFS calculations show that iron atoms form a solid solution with tetragonal ZrO₂. The EXAFS simulation of the first coordination shell around iron evidences that the substitution of zirconium atoms by iron ones generates oxygen vacancies into the tetragonal network. This is a driven force for the tetragonal phase stabilization process. For Ni doped samples, EXAFS results show that Ni–O mean bond length is similar to the distance found in the oxide material, i.e., NiO compound. Besides this result, no evidence of similar solid solution formation for Ni-doped systems has emerged from the EXAFS analysis.     

Morphological and Rheological Characteristics of PVP–CeO2 Blends

This work focuses on the flow behavior of the blend comprising polyvinyl pyrrolidone and cerium (IV) oxide (CeO₂) particles in submicron size, under low shear rates. The polyvinyl pyrrolidone–CeO₂ blends have been prepared and characterized by scanning electron microscopy, X-ray diffraction, and viscometry. The generation of core–shell morphology was verified from the scanning electron micrographs. Scanning electron microscopy shows that the blend formed is of porous nature. The particle size of CeO₂ increases with the concentration of both CeO₂ and polymer due to aggregation. The blend containing as high as 35?wt% of CeO₂ was found to exhibit pseudo-plastic response under low shear rate. The reasons for the observed morphology and other properties along with mechanism were explained. The main factor, which governs the properties of the end product, was van der Waals attractive forces that exist among the constituents of the system prepared.     

Formation of anisaldehyde via hydroxymethylation of anisole over SnO2–CeO2 catalysts

Hydroxymethylation of anisole has been carried out over SnO₂–CeO₂ catalysts in the temperature range 623–723 K. Methoxybenzaldehyde (anisaldehyde) and condensation products were formed along with minor quantities of methoxybenzyl alcohol, o‐cresol, phenol and 2,6‐xylenol. A maximum anisaldehyde selectivity of 64% was obtained at 623 K at an anisole conversion of 46% under optimized conditions. Catalytic activity of these systems in the formation of aldehyde is ascribed to the presence of weak acid sites and redox metal sites. This revised version was published online in July 2006 with corrections to the Cover Date.     

Vapor-Phase Dehydration of 1,3-butanediol over CeO2–ZrO2 Catalysts

The dehydration of 1,3-butanediol was investigated over CeO₂–ZrO₂ catalysts prepared by impregnation at temperatures of 325–375 °C. Pure CeO₂ selectively catalyzed the dehydration of 1,3-butanediol to form 3-buten-2-ol and 2-buten-1-ol, while pure ZrO₂, which was less active than pure CeO₂, catalyzed the dehydration to 3-buten-1-ol. In the CeO₂/ZrO₂ catalyst in which CeO₂ was supported on zirconia, the presence of a small amount of CeO₂ suppressed the formation of 3-buten-1-ol and induced the dehydration of 1,3-butanediol to form 3-buten-2-ol and 2-buten-1-ol and the subsequent dehydrogenation of 3-buten-2-ol to form 3-buten-2-one and butanone. The activity would be related to the redox features of CeO₂. The monoclinic phase of zirconia support decreased while the cubic CeO₂ phase increased as CeO₂ content was increased. In contrast, in the ZrO₂/CeO₂ catalyst in which ZrO₂ was supported on cubic CeO₂, only the cubic CeO₂ phase was observed and ZrO₂ species appeared in the form of a solid solution of CeO₂–ZrO₂ with fluorite structure. Regardless of zirconia loading, ZrO₂ species did not affect the catalytic activity of ZrO₂/CeO₂, which was controlled by CeO₂ species.     

Thermodynamic calculation of Ms in ZrO2–CeO2–Y2O3 system

The difference of Gibbs free energy between tetragonal and monoclinic phases in ZrO₂–CeO₂–Y₂O₃ as a function of composition and temperature is thermodynamically calculated from the three related binary systems. In 8 mol% CeO₂–0.5 mol% Y₂O₃–ZrO₂, the equilibrium temperature between tetragonal and monoclinic phases, T₀, is obtained as 832.5 K and the M_s temperature of this alloy with a mean grain size of 0.90 μm is calculated as 249.9 K using the approach derived by Hsu et al. [J. Mater. Sci., 18(1983)3206; 20(1985)23; Acta Metall., 37(1989)3091; Acta Metall. Mater., 39(1991)1045; Mater. Trans. JIM, 37(1996)1284], which is in good agreement with the experimental one of 253 K.     

Autothermal Reforming of Methane Over CeO2–ZrO2–La2O3 Supported Rh Catalyst

Autothermal reforming of methane was studied over La-doped ceria–zirconia-supported Rh catalysts. The CH₄ conversion increased from 49 to 60% on increasing the content of La³⁺ from 5 to 15%, while further increase in the La³⁺ content led to a slight decrease on both CH₄ conversion and H₂ selectivity. H₂-TPR and UV–vis DRS spectrum showed that the interaction between Rh and the support was enhanced by increasing the content of La. We speculated that a so-called “Rh–La interfacial species” was formed on the surface of the support, which played an important role in catalytic activity. The balance between exposed Rh and the “Rh–La interfacial species” was necessary to improve the catalytic activity. Upon increasing the Rh loading on 15% La-doped ceria–zirconia support, the balance was built, i.e., CH₄ conversion increased from ~60 to 69% by increasing Rh loading from 0.1 to 0.5 wt% and only 2% conversion was elevated by doubling the Rh loading from 0.5 to 1.0 wt%.     

The CO2–CeO2 interaction and its role in the CeO2 reactivity

The interaction between CO₂ and CeO₂ and its role in the surface reactivity of alumina-supported cerium oxide has been studied by programmed thermodesorption (TPD) of CO₂ and FTIR spectroscopy. The performance of Ce/Al₂O₃ systems was then analyzed for the propane oxidation in presence of CO₂. The results have shown that the catalytic activity decreased when carbonate species are formed at the surface of CeO₂. This behavior was attributed to the presence of CO₂ from three different sources: contamination before use, during the handling of the samples, contamination proceeding from the reactants and from CO₂ produced by the reaction itself. This revised version was published online in July 2006 with corrections to the Cover Date.     

Intramolecular selective hydrogenation of cinnamaldehyde over CeO2–ZrO2-supported Pt catalysts

Selective liquid phase hydrogenation of cinnamaldehyde is reported, for the first time, over CeO₂, ZrO₂, and CeO₂–ZrO₂-supported Pt catalysts. Cinnamyl alcohol is the selective product. These catalysts are highly active and selective even at 25 °C and found to be superior to most of the hitherto known supported Pt catalysts. Alkali addition (NaOH) has enhanced the performance of these catalysts. At an optimized reaction condition, 95.8% conversion of cinnamaldehyde and 93.4% selectivity of cinnamyl alcohol have been obtained. Acidity of the support (due to the presence of ZrO₂ component) and higher electron density at Pt (due to CeO₂ component) are attributed to be responsible for the superior catalytic activity of Pt supported on CeO₂–ZrO₂ composite material.     

Production of hydrogen via combined steam reforming of methanol over CuO–CeO2 catalysts

Hydrogen production by (combined) steam reforming of methanol (CSRM) was investigated over CuO–CeO₂ catalysts prepared via the urea-combustion method. The characteristics of the resulting oxides were strongly influenced by the autoignition time during synthesis and the sample prepared with near stoichiometric quantity of urea had less favorable catalytic properties. Catalysts prepared from urea-rich or lean mixtures were more active and selective and an optimum behavior was obtained with 75% excess of urea and Cu/(Cu + Ce) ratio equal to 0.15. The higher methanol conversion in the CSRM process may be attributed to more efficient heat transfer in the bed due to combustion of part of methanol.     

Structure-activity Relation of Fe2O3–CeO2 Composite Catalysts in CO Oxidation

A series of Fe₂O₃–CeO₂ composite catalysts were synthesized by coprecipitation and characterized by X-ray diffraction (XRD), BET surface area measurement, Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). Their catalytic activities in CO oxidation were also tested. The Fe₂O₃–CeO₂ composites with an Fe molar percentage below 0.3 form solid solutions with the CeO₂ cubic fluorite structure, in which the doped Fe³⁺ initially substitutes Ce⁴⁺ in fluorite cubic CeO₂, but then mostly locate in the interstitial sites after a critical concentration of doped Fe³⁺. With an Fe molar percentage between 0.3 and 0.95, the Fe₂O₃–CeO₂ composites are mixed oxides of the cubic fluorite CeO₂ solid solution and the hematite Fe₂O₃. XPS results indicate that CeO₂ is enriched in the surface region of Fe₂O₃–CeO₂ composites. The Fe₂O₃–CeO₂ composites have much higher catalytic activities in CO oxidation than the individual pure CeO₂ and Fe₂O₃, and the Fe_0.1Ce_0.9 composite shows the best catalytic performance. The structure-activity relation of the Fe₂O₃–CeO₂ composites in CO oxidation is discussed in terms of the formation of solid solution and surface oxygen vacancies. Our results demonstrate a proportional relation between the catalytic activity of cubic CeO₂-like solid solutions and their density of oxygen vacancies, which directly proves the formation of oxygen vacancies as the key step in CO oxidation over oxide catalysts.     

Microwave Dielectric Properties of SrO–2CeO2–nTiO2 (n=4 and 5) Ceramics

SrO–2CeO₂–nTiO₂ ceramics with n=4 and 5 have been prepared through conventional solid-state ceramic route. Structural and microstructural characterizations of the sintered ceramics were carried out using X-ray diffraction, laser Raman, and scanning electron microscopic studies. Microwave dielectric properties have been measured using postresonator and resonant cavity techniques. SrO–2CeO₂–4TiO₂ and SrO–2CeO₂–5TiO₂ ceramics showed dielectric constants of 71.3 and 71.7, respectively, around 4 GHz together with a relatively high unloaded quality factor.     

Abatement of Carbon Monoxide over CeO2–CoO x Catalysts: Effect of Preparation Method

Nanocrystalline TiO₂ with average crystallite sizes 7 and 15 nm were synthesized by the solvothermal method using titanium n-butoxide and 1,4-butanediol as titanium precursor and solvent, respectively. Four transition metals (Ag, Co, Ni, Pt) were supported on the TiO₂ supports by incipient wetness impregnation with metal loadings 1 and 25 atomic%. For any metal type, metal dispersion and CO oxidation activities were higher when the catalysts were prepared on the larger crystallite size TiO₂ for both high and low metal loadings, despite their lower surface areas. The presence of higher amount of Ti³⁺ on the larger crystallite size TiO₂ probably stabilized small metal particles during impregnation, calcination, and reduction steps via stronger metal-support interaction; hence higher CO oxidation activities and lower light-off temperature were obtained.     

The catalytic performance of Ti-PILC supported CrOx–CeO2 catalysts for n-butylamine oxidation

Journal of Porous Materials - A series of titanium pillared clays (Ti-PILC) materials are synthesized by different preparation conditions and the influence of the texture/structure nature of...     

Oxidative Dehydrogenation of Ethylbenzene to Styrene with CO2 Over V2O5–Sb2O5–CeO2/TiO2–ZrO2 Catalysts

Utilization of CO₂ as a soft oxidant in the oxidative dehydrogenation of ethylbenzene to styrene, a remarkable V₂O₅–Sb₂O₅–CeO₂/TiO₂–ZrO₂ catalyst has been accomplished. Preparation of TiO₂–ZrO₂ support by co-precipitation followed by a single step deposition of V and Ce and Sb as stabilizers yielded a highly active and selective catalyst. Acid–base and redox properties including sustained stability of active species (V⁵⁺) are responsible for high activity of V₂O₅–Sb₂O₅–CeO₂/TiO₂–ZrO₂ catalyst. The redox cycle is related to the dispersed V⁵⁺ and lattice reduced vanadium site in the VSbO₄ phase. The antimony oxide inhibits the easy redox cycle between different vanadia species.     

Preparation and characterization of CeO2–TiO2 support for Ru catalysts: Application in CWAO of p-hydroxybenzoic acid

The catalytic wet air oxidation of aqueous solutions of p-hydroxybenzoic acid has been carried out over CeO₂–TiO₂ supported ruthenium catalysts (Ru/Ce–Ti) at 140 °C and 50 bar of air. High activity of ruthenium supported catalysts was observed. It was found that the decrease of the molar ratio Ce/Ti from 3 to 1/3, improves the activity of Ru catalysts. The activity of the samples decreases in the following order: Ru/Ce–Ti (1/3) > Ru/CeO₂ ≈ Ru/TiO₂ > Ru/TiO₂DT51. Characterization of samples was performed by means of N₂ adsorption–desorption, XRD, UV–visible, TPR, SEM and TEM.     

Catalytic Performance of Aged Rh/CeO2–ZrO2 for NO–C3H6–O2 Reaction Under a Stoichiometric Condition

The activity of Rh/CeO₂ for NO reduction by C₃H₆ was gradually deceased by mixing with ZrO₂ until 68 mol%. Rh supported on CeO₂–ZrO₂ with higher OSC was found to show lower catalytic activity. High OSC of CeO₂–ZrO₂ would probably stabilize the surface of Rh in oxidized state, resulting in low activity and low efficiency of C₃H₆ utilization for NO reduction. In situ FT-IR spectroscopy suggested that mononitrosyl species such as Rh(NO)^δ? and Rh(NO)^δ+ are reaction intermediates in the NO–C₃H₆–O₂ reaction over Rh/CeO₂–ZrO₂ catalysts.     

Photocatalytic reduction of Hg using core–shell Fe/CeO2 hollow sphere nanocomposites

A sol–gel method was used to prepare Fe/CeO₂ hollow sphere nanocomposites. For comparison, a direct calcination of cerium nitrate was used to prepare CeO₂ nanoparticles and Fe/CeO₂ nanoparticles. The photocatalytic reduction of Hg was used to study the photocatalytic performance of the prepared nanocomposite photocatalysts using visible-light irradiation. The BET surface areas of the CeO₂ nanoparticles and CeO₂ hollow spheres were 76 and 160 m²/g, respectively. The BET surface area of the hollow sphere CeO₂ and CeO₂ nanoparticles decreased to 145 and 57 m²/g, respectively, by adding iron nanometal. The TEM results revealed that the shapes of the CeO₂ nanoparticle and hollow sphere materials are spherical nanoparticles and uniform nanospheres, respectively. The Fe/CeO₂ nanoparticles and Fe/CeO₂ hollow spheres are spherical nanoparticles and core–shell, respectively. The photocatalytic performance by the Fe/CeO₂ hollow spheres was 50, 3.9, and 1.4 times more efficient than that observed from the CeO₂ nanoparticles, Fe/CeO₂ nanoparticles, and CeO₂ hollow spheres, respectively.