Pd oxidation under UHV in a model Pd/ceria–zirconia catalyst

A model planar catalyst was prepared by depositing Pd onto a thick (few m) film of ceria–zirconia in ultrahigh vacuum (UHV), and the oxidation state of Pd and its support was determined by X-ray photoelectron spectroscopy, following thermal treatments in UHV, oxygen, or carbon monoxide. It was found that Pd could be oxidized simply by heating the catalyst in UHV, indicating that transfer of oxygen from the support to the metal is both thermodynamically favorable and facile.     

A calorimetric study of oxygen-storage in Pd/ceria and Pd/ceria–zirconia catalysts

Heats of adsorption were measured calorimetrically for O₂ adsorption on reduced Pd/alumina, Pd/ceria, and Pd/ceria–zirconia catalysts, all with 1 wt% Pd. Significantly more O₂ adsorbed on the ceria-containing catalysts due to oxidation of the support. For Pd/alumina, the heats were found to be between 180 and 220 kJ/mol, only slightly higher in magnitude than the heat of reaction for bulk oxidation of Pd. However, the heats of adsorption for both ceria and ceria–zirconia were also 200 kJ/mol, much lower than the heat of reaction for Ce₂O₃ oxidation to CeO₂, but in reasonable agreement with estimates from O₂ desorption studies on model ceria films. The implications of these results for understanding oxygen-storage properties on ceria-based catalysts are discussed.     

Selective oxidation of soot over Cu doped ceria/ceria–zirconia catalysts

Copper doped ceria and ceria–zirconia mixed oxides were prepared using the citric acid sol–gel method. The temperature-programmed oxidation (TPO) results showed that the Cu modification helped to improve the activity and selectivity of ceria and ceria–zirconia for soot catalytic oxidation. The CO-TPR results showed that Cu–Ce had a better reducibility than pure ceria at low temperatures. After ageing at 800 °C for 20 h in flow air, CuO–CeO₂ showed the maximum soot oxidation rate at 378 and 519 °C under tight and loose contact conditions, respectively, achieving a nearly 100% selectivity to CO₂ production. This effect may be attributed to the existence of well dispersed copper oxide species strongly interacting with the ceria surface, which may decrease the activation energy of soot oxidation. A conceivable mechanism of this synergetic effect was proposed.     

Mesoporous ceria–zirconia–alumina nanocomposite-supported copper as a superior catalyst for simultaneous catalytic elimination of NO–CO

A series of transition metal (Mn, Fe, Co, Ni, Cu and Ag) oxides supported on ceria–zirconia–alumina nanocomposite catalysts were prepared through wetness impregnation method. The catalytic performance of these catalysts were evaluated in the catalytic elimination of NO–CO. Activity results revealed supported copper catalyst gave the optimal catalytic activity, which was related to high dispersion of copper species (XRD and Raman), low-temperature reducibility (TPR), and more oxygen vacancies (DRS).     

Preparation of a mesoporous ceria–zirconia supported Ni–Fe catalyst for the high temperature water–gas shift reaction

A Ni–Fe/ceria–zirconia catalyst with ordered mesostructure was prepared by the hard-template method employing mesoporous silica (KIT-6) as a template to impart its highly ordered structure to the ceria–zirconia mixed oxide support. Catalytic activities of the Ni–Fe/CeO₂–ZrO₂ catalyst for the water–gas shift reaction were superior to those of a commercial Fe–Cr-based catalyst. The ordered structure of Ni–Fe/CeO₂–ZrO₂ catalyst became more stable compared to one prepared without zirconia due to structural stabilization of the mixed oxide by added zirconia in the framework. Alloying of Ni and Fe and enhanced mobility of lattice oxygen in the oxide support may promote its catalytic activity and selectivity for the water–gas shift reaction.     

Boosting intermediate temperature performance of solid oxide fuel cells via a tri-layer ceria–zirconia–ceria electrolyte

Using cost-effective fabrication methods to manufacture a high-performance solid oxide fuel cell (SOFC) is helpful to enhance the commercial viability. Here, we report an anode-supported SOFC with a three-layer Gd_0.1Ce_0.9O_1.95 (gadolinia-doped-ceria [GDC])/Y_0.148Zr_0.852O_1.926 (8YSZ)/GDC electrolyte system. The first dense GDC electrolyte is fabricated by co-sintering a thin, screen-printed GDC layer with the anode support (NiO–8YSZ substrate and NiO–GDC anode) at 1400°C for 5 h. Subsequently, two electrolyte layers are deposited via physical vapor deposition. The total electrolyte thickness is less than 5 μm in an area of 5 × 5 cm², enabling an area-specific ohmic resistance as low as 0.125 Ω cm⁻² at 500°C (under open circuit voltage), and contributing to a power density as high as 1.2 W cm⁻² at 650°C (at an operating cell voltage of 0.7 V, using humidified [10 vol.% H₂O] H₂ as fuel and air as oxidant). This work provides an effective strategy and shows the great potential of using GDC as an electrolyte for high-performance SOFC at intermediate temperature.     

Characterization of model automotive exhaust catalysts: Pd on Zr‐rich ceria–zirconia supports

Model Pd automotive three‐way catalysts were prepared with high‐surface‐area Zr‐rich ceria–zirconia powders as support materials, aged for 12 h at 1050°C under redox conditions simulating automotive exhaust gases, and characterized by a combination of techniques including oxygen storage capacity (OSC) measurements. Differences in OSC amongst aged catalysts made with materials of similar ceria–zirconia compositions, but produced by different processes, were weakly correlated with differences in support surface area. Evidence of encapsulation of Pd particles was found in most of the aged catalysts. The promotional effect of zirconia on ceria reducibility, well known from previous temperature‐programmed H₂ reduction studies, was apparent in Ce 3d core‐level spectra from X‐ray photoelectron spectroscopy measurements, which showed that both X‐ray‐induced and H₂ reduction become increasingly facile with increasing zirconia content. A slight Ce enrichment at the surface of the fresh catalysts made with the more Zr‐rich powders was found to increase upon aging. This revised version was published online in July 2006 with corrections to the Cover Date.     

Tunable ceria–zirconia support for nickel–cobalt catalyst in the enhancement of methane dry reforming with carbon dioxide

Ceria–zirconia mixed oxides (CeZr) were glycol-thermally synthesised as nano-crystalline supports with tunable ratios for the anchoring of nickel–cobalt (Ni–Co) catalyst to enhance methane dry reforming (MDR) reaction with carbon dioxide. High conversion of methane (90%) and carbon dioxide (92%), good output (H₂ = 32%; CO = 44%), and selectivity and stability of syngas prove the effectiveness of the catalyst deposited on this support. 80:20 for Ce:Zr was identified as the optimal ratio to attain active and stable catalytic performance in MDR, with a low coking content of 0.47 wt.%.     

Physical characteristics and sintering behavior of ultrafine zirconia–ceria powders

Ultrafine zirconia–12 mol% ceria powders have been prepared by the coprecipitation technique. The azeotropic distillation with n-butanol has been carried out to ensure complete elimination of the residual water in the precipitate. This procedure has proved to be quite effective in preventing the formation of agglomerates, which are responsible for inhomogeneities in the sintered microstructure, and for non-densification at low temperatures. The crystallization of the solid solution occurs at 430 °C as determined by thermal analyses. The specific surface of the calcined powder is 127.9 m² g⁻¹ and the pore size distribution exhibits only a maximum at approximately 9 nm. Total shrinkage of the compacted powder reached 30% at 1200 °C. Sintered specimens show six bands characteristics of the tetragonal phase in the Raman spectrum. Specimens with apparent densities >95% of the theoretical density and average grain size of 230–400 nm were obtained after sintering at 1200 °C.     

Tri-reforming of surrogate biogas over Ni/Mg/ceria–zirconia/alumina pellet catalysts

The performance of catalytic tri-reforming under industrially relevant situations (e.g., pellet catalysts, pressurized reactor) was investigated using surrogate biogas as the feedstock. Tri-reforming using Ni/Mg/Ce_0.6Zr_0.4O₂/Al₂O₃ pellet catalysts was studied in a bench scale fixed-bed reactor. The feed molar ratio for CH₄:CO₂:air was fixed as 1.0:0.70:0.95. The effects of temperature (800–860°C), pressure (1–6?bar), and H₂O/CH₄ molar feed ratio (0.23–0.65) were examined. Pressure has substantial impact on the reaction and transport rates and equilibrium conversions, making it a key variable. At 860°C, CO₂ conversion increased from 4 to 61% and H₂/CO molar ratio decreased from 2.0 to 1.1 as the pressure changed from 1 to 6?bar. CO₂ conversion and H₂/CO molar ratio were also influenced by the temperature and H₂O/CH₄ molar ratio. At 3?bar, CO₂ conversion varied between 4 and 43% and the H₂/CO molar ratio varied between 1.2 and 1.9 as the temperature changed from 800 to 860°C. At 3?bar and 860°C, CO₂ conversion decreased from 35 to 8% and H₂/CO molar ratio increased from 1.7 to 2.4 when the H₂O/CH₄ molar ratio was increased from 0.23 to 0.65. This work demonstrates that the tri-reforming technology is feasible for converting biogas under scaled-up conditions in a fixed-bed reactor.     

Use of a design-of-experiments approach for preparing ceria–zirconia–alumina samples by sol–gel process

In this work we successfully obtained ceria–zirconia–alumina samples by the sol–gel technique. These materials were prepared under acidic or basic conditions, using either nitric acid or ammonium hydroxide as the catalyst. A design-of-experiments approach was used in order to optimize the specific surface area, pore structure, and thermal stability of the prepared samples. It was observed that the addition of ceria and zirconia did not affect the formation of γ-Al₂O₃. The highest surface areas and smallest pore sizes were observed for specimens obtained under acidic conditions and with low to intermediate concentrations of cerium and water. The increase of the heat treatment temperature from 600 °C to 1000 °C led to both a decrease of the surface area and an increase of the mean pore size. This behavior is due to the coalescence of pores upon calcination. Samples with a high concentration of ceria showed an expressive thermal instability at high temperatures. On the other hand, the addition of zirconia increased the thermal stability of these materials. In general, samples with improved thermal stabilities were obtained under basic conditions.     

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.     

Processing and characterisation of fine crystalline ceria gadolinia–yttria stabilized zirconia powders

For low temperature SOFCs the yttria stabilized zirconia (YSZ)-coated ceria is a promising candidate for replacing YSZ-electrolyte. An important requirement for the co-firing feasibility of such a configuration is the densification of ceria at low temperatures (<1400°C). Fine crystalline gadolinia doped ceria (CGO)-powder readily sinterable at 1250°C was synthesized by co-precipitation with oxalic acid of 0·05 M and crystallization in methanol at 200°C for 6 h. The fabrication and characterisation of solid solution phases with a graded composition (CGO)_x(YSZ)_1−x, to be used as an interlayer between YSZ and CGO, in order to avoid delamination, were also studied and discussed. CGO_xYSZ_1−x powders, prepared by the glycine combustion method, required higher sintering temperatures (1500°C) to densify, while they showed significantly lower ionic conductivity than YSZ and CGO, attributed to the large lattice deformation and scattering of oxygen ions.     

Influence of the source of sulfur on the hydroconversion of 1-methylnaphthalene over a Pt–Pd/USY catalyst

Hydroconversion of 1-methylnaphthalene was performed over a Pt–Pd/USY catalyst in a batch reactor at 310 °C and 5 MPa of hydrogen pressure in cyclohexane as the solvent and in the presence of 800 ppm of sulfur resulting from different sources, hydrogen sulfide, thiophene and dibenzothiophene. In a general manner, hydrogenation of 1-methylnaphthalene into the corresponding mixture of methyltetralines is not significantly affected by the nature of the sulfur species present in the starting feed. On the contrary, going from hydrogen sulfide to thiophene and finally to dibenzothiophene, hydrogenation of methyltetralines into methyldecalines is lowered and ring-opening of methyltetralines to alkylbenzenes is enhanced. This would agree with the expected sequence of appearance of hydrogen sulfide in the feed.These results are in agreement with dissociation of hydrogen into protonic and hydride species, as already proposed in the presence of sulfided catalysts, i.e., protonic species would be involved for the hydrogenation steps and hydride species for the ring-opening steps. Hydrogen sulfide present as such or resulting from the transformation of thiophene or dibenzothiophene would then reduce the hydrogenation route, and, as a consequence, increase the hydrogenolysis route.     

Structure and thermal properties of heat treated plasma sprayed ceria–yttria co-stabilized zirconia coatings

Thick plasma sprayed thermal barrier coatings are suitable for thermal and hot corrosion protection of metal components in land-based turbine and diesel engines. In this work, ceria–yttria co-stabilized zirconia coatings were deposited by atmospheric plasma spraying in a mixture of non-transformable tetragonal t′ and cubic c zirconia phases. Free-standing coatings were isothermally annealed at 1315 °C for different times and their crystal structure was studied by XRD. No phase decomposition occurred. Columnar grains grew in the molten splats with increasing annealing time according to a preferential direction and, after 50 h of heat treatment, they were partially replaced by equiaxed grains. Both in-plane and out-of-plane thermal expansion coefficients (CTEs) were measured from coating expansion during heating. The CTE was slightly sensitive to thermal exposure in out-of-plane direction, whereas it kept almost constant in plane direction. The specific heat capacity Cp of annealed coatings, measured by differential scanning calorimetry (DSC), decreased in comparison with as-sprayed coating, due to high-temperature sintering.     

Analysis of defect mechanisms in nonstoichiometric ceria–zirconia by the microwave cavity perturbation method

In this microwave study, the defect chemistry of ceria–zirconia solid solutions (CZO, Ce_1−yZr_yO_2−δ) was investigated at high temperatures by a resonant microwave method. Specifically, the effects of temperature and Zr content on the dielectric properties and defect chemistry mechanisms in CZO were analyzed. Experiments were performed on a series of different CZO powders (y = 0.2, 0.33, 0.50, 0.67). Measurements at 600°C and different oxygen partial pressures (p_O2 = 10⁻²⁶–0.2 bar) confirm a dominant n-type conduction of small-polarons in CZO due to the preferred formation of oxygen vacancies, which is also supported by a multimodal analysis. Polarization losses were found to be negligible in the GHz range. Furthermore, an increased relative permittivity was observed in CZO, which correlates with the concentration of oxygen vacancies in CZO. Our microwave study is the first to provide a comprehensive data set for the dielectric properties of CZO powder sample in a wide range of different conditions. In addition, the connection of dielectric properties to CZO defect chemistry mechanisms is presented. The results are in good agreement with findings in the literature and may contribute to a better understanding of microwave-based state diagnosis of CZO-based materials, as it discussed for three-way catalysts.     

Thermally stable ceria–zirconia catalysts for soot oxidation by O2

Ce–Zr mixed oxides calcined at 1000 °C are more active catalysts for soot oxidation than pure CeO₂ calcined at the same temperature, both in loose and tight contact between soot and catalyst. 1000 °C sinterised-CeO₂ presents a very low surface area (2 m²/g), a large crystal size (110 nm) and a lack of surface redox properties. Ce–Zr mixed oxides present higher BET surface areas (typically 17–19 m²/g), smaller crystal sizes and enhanced redox properties. The Zr molar fraction does not affect appreciably the catalytic activity of Ce–Zr mixed oxides in the range studied (Zr molar fraction from 0.11 to 0.51).     

Benzylacetate synthesis by oxyacetoxylation of toluene on Pd–Bi binary catalyst

Benzylacetate synthesis from toluene, acetic acid and oxygen on Pd–Bi binary catalyst was studied in the liquid phase. By incorporation of Bi with Pd, both the activity and selectivity were improved. Especially better stability was obtained with the catalyst having Pd/Bi = 3. Deactivation of the catalyst was investigated in detail by XRD, XPS, TEM, elemental analysis, EPMA and so on. Comparing the used catalyst with the fresh one, it was indicated that the main cause of deactivation was the dissolution of Pd into the reaction mixture from the most outer surface of the catalyst. By adopting proper reaction conditions to prevent the Pd dissolution, the catalyst having Pd/Bi = 3 was suggested to be used as an industrial catalyst. This revised version was published online in June 2006 with corrections to the Cover Date.     

Microstructure tailoring of the nickel–yttria stabilised zirconia (Ni–YSZ) cermet hollow fibres

Nickel–yttria stabilised zirconia (Ni–YSZ) hollow fibres have been prepared by the phase inversion/sintering technique followed by a reduction process with hydrogen. This work is particularly focussed on tailoring the microstructure and the properties of hollow fibres by ethanol addition into the spinning hollow fibre suspension. Microstructure evolution change is demonstrated by increasing the amount of ethanol from 0 to 35 wt% e.g. the hollow fibre cross-section is modified from a sponge-like structure sandwiched by two thin finger-like layers to the sponge-like structure only. Higher ethanol content translates to denser hollow fibres. This trend also correlates with the shrinkage, mechanical strength and electrical conductivity of the hollow fibres. As the ethanol content is increased, shrinkage reduces, mechanical strength improves and electrical conductivity increases. The Ni–YSZ hollow fibres made from suspensions containing 15–25 wt% ethanol are considered the best option as anode supports for micro-tubular solid oxide fuel cells in terms of their median porosity values, since insufficient porosity would hinder the fuel and product transport, whereas excessive porosity would deteriorate the mechanical strength of the fibres.     

Liquid phase catalytic hydrodechlorination of aryl chlorides over Pd–Al-MCM-41 catalyst

Supported Pd catalysts were prepared by direct hydrothermal (DHT) or template-ion exchange (TIE) method on Al-substituted MCM-41 as the support and tested in the hydrodechlorination of aryl chlorides such as 4-chloroanisole in liquid phase, together with impregnated Pd catalysts as references. When Pd was loaded on Al-MCM-41 with the Si/Al ratio of 150 by the TIE method, the catalyst showed the highest activity among the Pd catalysts prepared. PdO originally formed on the catalyst surface was in situ reduced to Pd metal during the reaction as the active form for the dechlorination. The activities of the supported Pd catalysts well correlated with the Pd dispersion on the TIE or impregnated Pd catalysts, whereas the DHT catalysts showed relatively high activities independently on the Pd dispersions. It was confirmed that the dechlorination proceeded by a heterogeneous mechanism catalyzed by Pd metal particles sized less than 10 nm on the surface of the catalyst. Al substitution for Si on MCM-41 was effective for the loading of Pd metal with the high dispersion, none the less Pd was located on the surface of the TIE catalyst particles and no significant effect of mesoporous structure on the reaction was observed. Methanol was the most profitable as the solvent among the various solvents tested. Various types of arylchlorides bearing hydroxy, methoxy, methyl, nitro and phenylcarbonyl group at the p-position were efficiently dechlorinated over Pd–Al-MCM-41 catalyst at 40 °C. Electrophilic attack of arylchloride was proposed as the rate determining step, where ionic mechanism positively worked and electron-releasing substituents increased the catalytic activity.