Vapor phase synthesis of cyclopentanone over nanostructured ceria–zirconia solid solution catalysts

The present study was undertaken to develop a novel and easy practical approach for synthesis of cyclopentanone, a versatile industrial ingredient. Accordingly, ceria–zirconia based nano-oxide catalysts, namely, Ce_xZr_1?xO₂ and Ce_xZr_1?xO₂/M (M = SiO₂ and Al₂O₃) were prepared and evaluated for the title reaction. The physicochemical characterization has been achieved using different techniques, namely, XRD, BET surface area, XPS, Raman, OSC, and HREM. The catalytic results revealed that Ce_xZr_1?xO₂ based nano-oxides are promising heterogeneous catalysts for synthesis of cyclopentanone. Amongst, the Ce_xZr_1?xO₂/Al₂O₃ catalyst exhibited ～100% conversion with 75% desired cyclopentanone product selectivity owing to its favorable physicochemical characteristics.     

Thermally stable composite system Al2O3-Ce0.75Zr0.25O2 for automotive three-way catalysts

Present-day three-way catalysts operate in contact with exhaust gases whose temperature is as high as >1000°C, so the problem of developing thermally stable catalytic compositions is still topical. A series of Al₂O₃-Ce_0.75Zr_0.25O₂ composites containing 0, 10, 25, and 50 wt % Al₂O₃ has been synthesized by direct precipitation. The as-prepared composites and those calcined in air at 1000 and 1100°C have been characterized by BET, X-ray diffraction, transmission electron microscopy, and temperature-programmed reduction methods. The composites aged at 1050°C in a 2% O₂ + 10% H₂O + 88% N₂ atmosphere have been used to prepare monolith catalysts, and the oxygen storage capacity (OSC) of the latter has been measured using a gas analysis setup. As the proportion of Al₂O₃ in the composite is raised, the mixing uniformity and degree of dispersion of Ce _x Zr_1?x O_2?δ particles increase, their chemical composition becomes homogeneous, and the amount of cerium involved in oxidation and reduction increases. The composite containing 50 wt % Al₂O₃ is a mixture of Ce _x Zr_1?x O_2?δ and Al₂O₃ crystallites, whose size is practically unaffected by calcination. The (Pt/Al₂O₃ + Al₂O₃-Ce_0.75Zr_0.25O₂) based on this composite has the highest OSC and is the most active. For this reason, full-scale testing of this catalyst is recommended.     

Enhancement of oxygen storage capacity by reductive treatment of Al2O3 and CeO2–ZrO2 solid solution nanocomposite

Oxygen storage capacity (OSC) of CeO₂–ZrO₂ solid solution, Ce_xZr_(1−x)O₄, is one of the most contributing factors to control the performance of an automotive catalyst. To improve the OSC, heat treatments were employed on a nanoscaled composite of Al₂O₃ and CeZrO₄ (ACZ). Reductive treatments from 700 to 1000 °C significantly improved the complete oxygen storage capacity (OSC-c) of ACZ. In particular, the OSC-c measured at 300 °C reached the theoretical maximum with a sufficient specific surface area (SSA) (35 m²/g) after reductive treatment at 1000 °C. The introduced Al₂O₃ facilitated the regular rearrangement of Ce and Zr ions in CeZrO₄ as well as helped in maintaining the sufficient SSA. Reductive treatments also enhanced the oxygen release rate (OSC-r); however, the OSC-r variation against the evaluation temperature and the reduction temperature differed from that of OSC-c. OSC-r measured below 200 °C reached its maximum against the reduction temperature at 800 °C, while those evaluated at 300 °C increased with the reduction temperature in the same manner as OSC-c.     

The Effects of P-Poisoning of CexZr1−xO2 on the Transient Oxygen Storage and Release Kinetics

The present work reports on the investigation of the effects of chemical poisoning of Ce_xZr_1?xO₂ solids by phosphorous (P) on the kinetics of oxygen storage and release (OSR) of the thus derived P-contaminated solids, as a function of Ce_xZr_1?xO₂ solid composition (x = 0.3, 0.5 and 0.7) for the first time. Phosphorous deposition on the surface of Ce_xZr_1?xO₂ particles followed by calcination in air at 850 °C forms nano-crystals of CePO₄, which lead to a drastic decrease in the population of surface and subsurface reactive and mobile oxygen species due to the formation of P–O–Ce bonding. The concentration (μmol/g) of exchangeable ¹⁶O in the solid with ¹⁸O from the gas phase was found to increase, and the degree of reduction in the Oxygen Storage Capacity (OSC) to decrease with increasing Ce content in the Ce_xZr_1?xO₂ solid after P-poisoning. The increase in the Ce content of Ce_xZr_1?xO₂ makes its OSR properties more resistant against P-poisoning due to the increasing number of poison-free Ce atoms, which are able to participate in the Ce³⁺ ? Ce⁴⁺ redox cycle.     

Preparation and microstructural characterization of (Nd1−xGdx)2(Ce1−xZrx)2O7 solid solutions

(Nd_1−xGd_x)₂(Ce_1−xZr_x)₂O₇ (0 ≤ x ≤ 1.0) powders with an average particle size of 100 nm were synthesized with chemical-coprecipitation and calcination method, and were characterized by X-ray diffractometry and scanning electron microscopy. The sintering behaviour of (Nd_1−xGd_x)₂(Ce_1−xZr_x)₂O₇ powders was studied by pressureless sintering at 1600–1700 °C for 10 h in air. The relative densities of (Nd_1−xGd_x)₂(Ce_1−xZr_x)₂O₇ solid solutions increase with increasing the sintering temperature, and gradually decrease with increasing the content of neodymium and cerium at identical temperature levels. (Nd_1−xGd_x)₂(Ce_1−xZr_x)₂O₇ solid solutions have a single phase of defect fluorite-type structure among all the composition combinations studied. The lattice parameters of (Nd_1−xGd_x)₂(Ce_1−xZr_x)₂O₇ solid solutions agree well with the Vegard's rule.     

NOx Adsorption/Desorption Processes Over Ce0.76Zr0.24O2 and Their Influence on DeSoot Activity: Effect of the Catalyst Calcination Temperature

The catalytic activity of a Ce_0.76Zr_0.24O₂ mixed oxide, calcined at different temperatures, for soot oxidation under NO _x /O₂ was correlated with the catalytic activity for NO₂ production. The Ce_0.76Zr_0.24O₂ mixed oxide samples were prepared by co-precipitation and calcination at different temperatures: 500–1000 °C. A satisfactory correlation between the total amount of NO₂ desorbed after NO + O₂ adsorption at 60 °C and the T50% for soot combustion was found. The NO _x adsorption process was also explored by in situ DRIFTS.     

Perovskite-Based Lean-Burn NOx Trap Catalysts Without Using Platinum Group Metals: K/LaCoO3/Ce1?xZrxO2

Abstract  

A series of Ce_1−x Zr _x O₂ (x = 0, 0.1, 0.2, 0.3) solid solution supported lean-burn NO _x trap (LNT) catalysts K/LaCoO₃/Ce_1−x Zr _x O₂ were prepared by successive impregnation. After sulfation the supported perovsikte LaCoO₃ was well maintained; reducing treatment partly destroyed the perovsikte, but it can be well recovered by re-oxidation treatment. Based on NO _x storage and sulfur-resisting performance of the catalysts, the optimal atomic ratio of Zr in Ce_1−x Zr _x O₂ support is x = 0.2. The catalyst K/LaCoO₃/Ce_0.8Zr_0.2O₂ exhibits much better NO _x storage capacity than the Pt-based catalyst Pt/K/Ce_0.8Zr_0.2O₂, which is highly related to its stronger capability for NO to NO₂ oxidation. During NO _x storage much larger amounts of nitrate and nitrite species were identified by in situ DRIFTS over perovskite-based catalysts than over Pt-based one. The H₂-TPR results reveal that after deep sulfation little sulfur species were deposited on the catalyst K/LaCoO₃/Ce_1−x Zr _x O₂, showing strong sulfur-resisting ability. As a result, it is thought that the full replacement of Pt by perovskite LaCoO₃ in the corresponding LNT catalysts is feasible.     

DRIFT study of the interaction of NO and O2 with the surface of Ce0.62Zr0.38O2 as deNOx catalyst

Gas–solid interactions and surface intermediates evolution after NO adsorption onto calcined Ce_0.62Zr_0.38O₂ were investigated. The results of adsorption and temperature-programmed desorption of NO were explained using diffuse reflectance Fourier transform infrared spectroscopy (DRIFTS) coupled with temperature-programmed experiments in environmental cell. Surface NO-containing species such as nitrites and nitrates were identified during evolution of NO on the surface of Ce_0.62Zr_0.38O₂ solid solution at low and high temperature. The ceria–zirconia solid solution was found to be active in deNO_x reaction in the presence of a “toluene, propene and propane” mixture.     

Influence of B-site substituent Ce on thermophysical,oxygen diffusion,and mechanical properties of La2Zr2O7

Pyrochlore-type La₂Zr₂O₇ (LZ) is a promising candidate for high-temperature thermal barrier coatings (TBCs). However, its thermal expansion coefficient and low fracture toughness are not optimal for such application and thus, need to be improved. In this study, we systematically report the effect of CeO₂ addition on phase formation, oxygen-ion diffusion, and thermophysical and mechanical properties of full compositions La₂(Zr₁?_xCe_x)₂O₇ (x = 0, 0.1, 0.3, 0.5, 0.7, 0.9, 1). La₂(Zr₁?_xCe_x)₂O₇ exhibits a pyrochlore structure at x ≤ 0.3, while a fluorite structure is observed outside this range. With the increase in CeO₂ content, thermal expansion coefficient and oxygen-ion diffusivity in La₂(Zr₁?_xCe_x)₂O₇ are increased. Oxygen-ion diffusivity of La₂(Zr₁?_xCe_x)₂O₇ is two orders of magnitude less than that of classical 8YSZ. Among La₂(Zr₁?_xCe_x)₂O₇ compounds, La₂(Zr_0.7Ce_0.3)₂O₇ and La₂(Zr_0.5Ce_0.5)₂O₇ exhibit relatively low oxygen diffusivities. The composition La₂(Zr_0.5Ce_0.5)₂O₇ presents the lowest thermal conductivity due to the strongest phonon scattering and also the highest fracture toughness due to the solid-solution toughening. The highest sintering resistance is achieved by the composition La₂(Zr_0.7Ce_0.3)₂O₇ because of its ordered pyrochlore structure and high atomic mass of Ce. Based on these results, the compositions La₂(Zr_0.5Ce_0.5)₂O₇ and La₂(Zr_0.7Ce_0.3)₂O₇ are alternatives for classical 8YSZ for TBC materials operating at ultrahigh temperatures.     

Influence of OSC Material on the Behaviour of Metallic Pd Catalysts in C2H4 and CO Oxidation as Well as in NO Reduction

The interaction of CO, C₂H₄, O₂, and NO reaction gas compounds over the metallic Pd/Al₂O₃ and Pd/OSC/Al₂O₃ monoliths was investigated in order to understand the behaviour of OSC material in the oxidation and reduction reactions. FT-IR gas analyser was used for the analysis of the product gas composition. Several activity experiments carried out with dissimilar feedstreams have revealed that the Ce _x Zr_1–x O₂ mixed oxide is an oxygen storage compound, which promotes CO and C₂H₄ oxidation as well as NO reduction in particular at low temperatures.     

The electrical properties of (Ba,Ca, Ce,Ti, Zr)O3 thermistor for wide-temperature sensor architecture

A series of new negative temperature coefficient (NTC) thermal materials based on (Ba_0.85Ca_0.15)_1-xCe_x/2(Zr_0.1Ti_0.9)O₃ (0.00 ≤ x ≤ 0.20) ceramics were synthesized by a solid-state method. X-ray diffraction, scanning electron microscope and X-ray photoelectron spectroscopy were used to demonstrate the crystal structure, morphology, and composition of the (Ba_0.85Ca_0.15)_1-xCe_x/2(Zr_0.1Ti_0.9)O₃ ceramics, which were composed of solid solution based on the BaTiO₃ phase. The average grain size of doped ceramic samples experienced the process of first decreasing and then increasing. The doping of Ce has reduced the sintering temperature. The temperature-dependent resistance analysis revealed that with the change of doping amount x, the thermal constant B_300/1200 (1.21 × 10⁴–1.13 × 10⁴ K) and the activation energy E_a300/1200 (0.9777–1.0471eV) was initially increased to maximum values at x = 0.05, followed by the decreasing when x > 0.05. It has been established that the concentration of oxygen vacancies is affected by the transition between Ce⁴⁺ and Ce³⁺ provided by high levels of Ce doping. (Ba_0.85Ca_0.15)_1-xCe_x/2(Zr_0.1Ti_0.9)O₃ ceramics exhibited excellent negative temperature characteristics in the range of 300–1200 °C. Moreover, the temperature resistance linearity was improved after samples were aged. Hence, the (Ba_0.85Ca_0.15)_1-xCe_x/2(Zr_0.1Ti_0.9)O₃ ceramics were regarded as a promising material for high-temperature NTC thermistors in a wide temperature range.     

Investigation into the phase formation in the ZrO<Subscript>2</Subscript>-CeO<Subscript>2</Subscript> system

Nanopowders of solid solutions with different compositions are prepared in the zirconia-enriched region of the ZrO₂-CeO₂ system. The crystallization of these powders and the formation of the monoclinic, cubic, and tetragonal solid solutions of the composition (Zr_{1 – x} Ce_x)O₂ are investigated. It is found that the unit cell parameters of the solid solutions increase as the cerium content increases. This confirms the fact that cerium ions [r(Ce⁴⁺) = 1.11 ] substitute for zirconium ions [r(Zr⁴⁺) = 0.98 ] in these solid solutions. The average size of crystallites of the solid solutions under investigation increases from 5 to 60 nm in the temperature range 500–1200°C.Original Russian Text Copyright © 2005 by Fizika i Khimiya Stekla, Panova, Glushkova, Nefedova.     

Mesoporous CexZr1−xO2 solid solutions supported CuO nanocatalysts for toluene total oxidation

Mesoporous Ce_xZr_1−xO₂ solid solutions were prepared by the surfactant-assisted method and used as support of CuO nanocatalysts for catalytic total oxidation of toluene. The prepared CuO/Ce_xZr_1−xO₂ catalysts have a wormhole-like mesoporous structure with high surface area and uniform pore size distribution, and the CuO nanoparticles were highly dispersed on the surface of Ce_xZr_1−xO₂. The doping of ZrO₂ in CeO₂ promotes the dispersion of active copper species and enhances the reducibility of copper species. The effect of Ce/Zr ratio, calcination temperature and CuO loading amount on the catalytic performance of CuO/Ce_xZr_1−xO₂ was investigated in detail. The 400 °C-calcined 8%CuO/Ce_0.8Zr_0.2O₂ catalyst exhibits the highest activity with the complete toluene conversion temperature of 275 °C at the condition of GHSH = 33,000 h⁻¹ and the toluene concentration of 4400 ppm. The interfacial interaction between CuO and the Ce_xZr_1−xO₂ support, highly dispersed CuO nanoparticles and the nature of the support contribute to the high catalytic activity of mesoporous CuO/Ce_xZr_1−xO₂ nanocatalysts.     

Methane combustion over Pd/ZrO2/SiC,Pd/CeO2/SiC,and Pd/Zr0.5Ce0.5O2/SiC catalysts

The performances of different promoters (CeO₂, ZrO₂ and Ce_0.5Zr_0.5O₂ solid solution) modified Pd/SiC catalysts for methane combustion are studied. XRD and XPS results showed that Zr⁴⁺ could be incorporated into the CeO₂ lattice to form Zr_0.5Ce_0.5O₂ solid solution. The catalytic activities of Pd/CeO₂/SiC and Pd/ZrO₂/SiC are lower than that of Pd/Zr_0.5Ce_0.5O₂/SiC. The Pd/Zr_0.5Ce_0.5O₂/SiC catalyst can ignite the reaction at 240 °C and obtain a methane conversion of 100% at 340 °C, and keep 100% methane conversion after 10 reaction cycles. These results indicate that active metallic nanoparticles are well stabilized on the SiC surface while the promoters serve as oxygen reservoir and retain good redox properties.     

Design of Efficient Ce<Subscript>x</Subscript>M<Subscript>1−x</Subscript>O<Subscript>2−δ</Subscript> (M = Zr,Hf, Tb and Pr) Nanosized Model Solid Solutions for CO Oxidation

Abstract  

Nanosized Ce_xM_1−xO_2−δ (M = Zr, Hf, Tb and Pr) solid solutions were prepared by a modified coprecipitation method and thermally treated at different temperatures from 773 to 1073 K in order to ascertain the thermal behavior. The structural and textural properties of the synthesized samples were investigated by means of X-ray diffraction (XRD), high resolution transmission electron microscopy (HRTEM), BET surface area, X-ray photoelectron spectroscopy (XPS) and Raman spectroscopy (RS) techniques. The catalytic efficiency has been performed towards oxygen storage/release capacity (OSC) and CO oxidation activity. The characterization results indicated that the obtained solid solutions exhibit defective cubic fluorite structure. The solid solutions of ceria–hafnia, ceria–terbia and ceria–praseodymium exhibited good thermal stability up to 1073 K. A new Ce_0.6Zr_0.4O₂ phase along with Ce_0.75Zr_0.25O₂ was observed in the case of ceria–zirconia solid solution due to more Zr⁴⁺ incorporation in the ceria lattice at higher calcination temperatures. The reducibility of ceria has been increased upon doping with Zr⁴⁺, Hf⁴⁺, Tb^3+/4+ and Pr^3+/4+ cations. This enhancement is more in case of Hf⁴⁺ doped ceria. Among various solid solutions investigated, the ceria–hafnia combination exhibited better OSC and CO oxidation activity. The high efficiency of Ce–Hf solid solution was correlated with its superior bulk oxygen mobility and other physicochemical characteristics.     

Ternary phase relations in CeO2–DyO1.5–ZrO2 system

The present work explores the sub-solidus phase relations in the CeO₂–DyO_1.5–ZrO₂ ternary system. About 80 compositions in Zr_1−xDy_xO_2−x/2, Ce_1−xDy_xO_2−x/2, (Ce_0.8Zr_0.2)_1−xDy_xO_2−x/2, Zr_1−x(Ce_0.2Dy_0.8)_xO_2−0.4x, Ce_x(Dy_0.5Zr_0.5)_1−xO_1.75+x/4 systems, were synthesized and explored to investigate the phase fields in this ternary system. Detailed XRD analysis showed the existence of a variety of phase fields viz. Fluorite-type cubic, C-type cubic, biphasic fields containing both F-type and C-type phases as well as co-existence of two different fluorite type phases. A few compositions also showed the presence of monoclinic as well the tetragonal phases. The trends observed in cell parameter are found to be governed by the competing factors of average ionic radius and the repulsion between excess anions in the lattice due to the aliovalent substitution. This ternary system showed the existence of a very wide cubic phase field. This ternary phase relation has relevance to the inert matrix fuel concept.     

Deactivation phenomena during catalytic wet air oxidation (CWAO) of phenol over platinum catalysts supported on ceria and ceria–zirconia mixed oxides

Catalytic wet air oxidation (CWAO) of aqueous solution of phenol was carried out with pure oxygen at 160 °C in a stirred batch reactor on platinum supported oxide catalysts (Pt/CeO₂^c calcined at 650 and 800 °C and Pt/Ce_xZr_1 − xO₂ with x = 0.90, 0.75 and 0.50). The catalysts were characterized before (BET, FT-IR spectroscopy, hydrogen chemisorptions, oxygen storage capacity (OSC)) and after reaction (TPO, elementary analysis, GC–MS and DTA–TGA). The results demonstrate a poisoning of the catalysts during CWAO reaction due to the formation of different forms of carbon deposit on the materials: carbonates and polymeric carbon species. This poisoning phenomenon is limited by the introduction of 50% of zirconium into ceria lattice for the catalysts presenting the lowest surface area. Polymeric deposits play a major role in the catalyst deactivation.     

PdO/Al2O3–(Ce1-X Zr X )O2 catalysts: effect of the sol-gel support composition

The contribution of (Ce_1-X Zr _X )O₂ additives to alumina supports prepared by sol-gel and the catalytic properties of PdO/Al₂O₃–(Ce_1-X Zr _X )O₂ catalysts (~0.3 wt% Pd, ~5 wt% (Ce_1-X Zr _X )O₂) in CO oxidation was herein investigated. The addition of (Ce_1-X Zr _X )O₂ to the support enhanced the surface area and decreased the size of Al₂O₃ particles. The UV–Vis bands of PdO particles and Pd²⁺ ions indicate that zirconia in (Ce_1-X Zr _X )O₂ promotes palladium-support interactions by forming highly dispersed PdO particles. Temperature-programmed reduction (TPR) in hydrogen revealed that ceria enhanced the redox capacity of the supports while zirconia lowered the reduction temperature of palladium oxide species. The comprehensive study revealed that the Ce/Zr ratio was a key factor influencing the catalytic activity of samples in CO oxidation, because palladium oxide-support interactions had a significant effect in changing of the reducibility of samples. So, the PdO/Al₂O₃–(Ce_0.5Zr_0.5)O₂ exhibited the highest catalytic activity.     

Co-doped ceria-based solid solution in the CeO2-M2O3-CaO, M = Sm, Gd system

The Pechinni method (A) as well as hydrothermal treatment (B) of co-precipitated CeO₂-based gels with NaOH solution were used to synthesise pure CeO₂, and CeO₂-based solid solutions with formula Ce_1−xM_xO₂, Ce_1−x(M_0.5Ca_0.5)_xO₂ M = Gd, Sm for 0.15 < x < 0.3 nanopowders. The thermal evolution of CeO₂-based precursors during heating them up to 1000 °C was monitored by thermal (TG, DTA) analysis and X-ray diffraction method. All nanopowders and samples sintered were found to be pure CeO₂ or ceria-based solutions with fluorite-type structure. The microstructure of CeO₂-based sintered samples at 1500 °C (A) or 1250 °C (B) was observed for 2 h under the scanning electron microscope. The electrical properties of singly Ce_1−xM_xO₂ or doubly doped CeO₂-based samples with formula Ce_1−x(M_0.5Ca_0.5)_xO₂, M = Gd, Sm, 0.15 < x < 0.30 were investigated by means of the ac impedance spectroscopy method throughout the temperature range of 600-800 °C. It has been stated that partial substitution of calcium by samarium or calcium by gadolinium in the Ce_1−x(M_0.5Ca_0.5)_xO₂, M = Gd, Sm solid solutions leads to ionic conductivity enhancement comparable with only samaria- or gadolina-doped ceria. The CeO₂-based samples with small-grained microstructures obtained from powders synthesised by hydrothermal method exhibited better ionic conductivity than samples with the same composition obtained from powders synthesised by the Pechinii method. The stability of the electrolytic properties of selected co-doped ceria sinters in fuel gases (H₂, CH₄) as well as exhaust gases from diesel engine was also investigated. The co-doped Ce_0.8(Sm_0.5Ca_0.5)_0.2O₂ or Ce_0.85(Gd_0.5Ca_0.5)_0.15O₂ dense samples would appear be to more adequate oxide electrolytes than Ce_1−xM_xO₂, M = Sm, Gd and x = 0.15 or 0.2 for electrochemical devices operating at temperatures ranging from 600 to 700 °C.     

The dehydrogenation of ethylbenzene with CO2 over CexZr1 − xO2 solid solutions

With the purpose of changing the lattice structure of CeO₂ and improving the transmission capacity of lattice oxygen, Ce_xZr_1 − xO₂ solid solutions with different Zr proportions were synthesized using a hydrothermal method and applied in oxidative dehydrogenation of ethylbenzene to styrene with CO₂ at 550 °C. The Ce_0.5Zr_0.5O₂ showed the highest activity with an ethylbenzene conversion of 55% and styrene selectivity above 86%. Analytical characterization showed that the lattice oxygen mobile capacity of Ce_xZr_1 − xO₂ solid solutions was enhanced, corresponding to the order as Ce_0.3Zr_0.7O₂ > Ce_0.5Zr_0.5O₂ > Ce_0.7Zr_0.3O₂ > CeO₂. The oxygen storage/release capacity, higher surface area and pore distribution of Ce–Zr mixed oxides play important roles in the activity of ethylbenzene dehydrogenation to styrene with CO₂.