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.     

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.     

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.     

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).     

Water–gas shift activity of ceria supported Au–Re catalysts

Au–Re/ceria, Au/ceria and Re/ceria catalysts were prepared using deposition precipitation and impregnation techniques for Au and Re addition, respectively, except the sample prepared by sequential impregnation. Catalysts were characterized by HRTEM-EDS, SEM-EDS, XPS and XRD. WGS activity tests on the samples were performed in the temperature range 200–450 °C. The effects of Re incorporation, metal addition sequence, space velocity and H₂O/CO ratio on the catalytic performance were investigated. The novel Au–Re/ceria catalysts showed high activity in WGS reaction, especially at high H₂O/CO ratios, led by the presence of catalytically active and steam tolerant sites formed on the bimetallic catalysts.     

Construction of titania–ceria nanostructured composites with tailored heterojunction for photocurrent enhancement

Titania–ceria (TiO₂–CeO₂) nanostructured composites based on the design of coating the surfaces of anodized TiO₂ nanotube arrays with small band gap CeO₂ nanoparticles have been constructed and characterized to demonstrate the effectiveness of the TiO₂–CeO₂ semiconductor heterojunction in enhancing the photocurrent response of TiO₂-based photoelectrodes. The TiO₂–CeO₂ heterojunction was confirmed to possess conduction and valence band offsets (0.81 and 1.59 eV, respectively) which promote the separation of photoinduced electron–hole pairs. The photocurrent densities of the TiO₂–CeO₂ composites prepared with low annealing temperatures were about 25–40% larger than that of the anatase TiO₂ nanotube arrays. When the nanoparticle-on-nanotube architecture of the TiO₂–CeO₂ heterostructure was maintained under specific processing conditions, the benefits of having a high specific surface area, a small band gap component capable of absorbing visible light, and a favorable heterojunction were achieved together for photocurrent enhancement.     

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.     

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.     

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.     

Synthesis of nanocrystalline gadolinium doped ceria via sol–gel combustion and sol–gel synthesis routes

Gadolinium doped ceria (GDC) has been investigated as a promising material for application as an electrolyte in intermediate temperature solid oxide fuel cells (IT-SOFC). In this work, 10GDC powders (Gd_0.1Ce_0.9O_1.95) were synthesized by sol–gel combustion and sol–gel synthesis routes using the same complexing agents in both procedures. The thermal behavior of Gd–Ce–O precursor gels was investigated by TG–DSC measurements. X-ray diffraction (XRD) analysis was used for the characterization of phase purity and crystallinity of synthesized samples. Scanning electron microscopy (SEM) was employed for the estimation of surface morphological features. Nitrogen adsorption–desorption (BET model) was used for evaluation of specific surface area. The surface composition was determined by X-ray photoelectron spectroscopy (XPS). Electrical properties of synthesized ceramic samples were studied by means of impedance spectroscopy.     

Catalytic combustion of methane over M (Ni,Co, Cu) supported on ceria–magnesia

The catalytic activity of a series of M(= Ni, Co, Cu)/(CeO₂)_x–(MgO)_1 − x catalysts for methane combustion was investigated. (CeO₂)_x–(MgO)_1 − x supports were prepared by a sol-gel method. The influence of CeO₂ content and active components such as Ni, Co and Cu are discussed. The results indicate that the activity of the catalysts depends strongly on CeO₂ content. The Ni/(CeO₂)_0.1 − (MgO)_0.9 catalyst showed the highest catalytic activity and good thermal stability for methane combustion. The highly dispersed NiO is the main active site for methane combustion. Fresh M (Ni, Co and Cu)/(CeO₂)_0.1–(MgO)_0.9 catalysts showed that the activity of CuO for methane combustion was slightly higher than that of NiO and CoO, while the thermal stability increased in the order Cu < Co < Ni. Cu/(CeO₂)_0.1–(MgO)_0.9 catalyst was sintered after a second evaluation. Consequently, (CeO₂)_0.1–(MgO)_0.9 is deemed to be a good support for Ni.     

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.     

Ionic–gelation synthesis of gadolinium doped ceria (Ce0.8Gd0.2O1.90) nanocomposite powder using sodium-alginate

Nanocomposite powders of gadolinium-doped ceria (GDC, Ce_0.8Gd_0.2O_1.9) were synthesized via thermal treatment of the gel formed by contacting ionic solutions of sodium alginate as the jelling template and metal (gadolinium/cerium) nitrates as the starting material. The influence of calcination temperature and sodium alginate loading fraction on the properties of the synthesized GDC nanocomposite powders was investigated. Characterization was performed by energy dispersive X-ray spectroscopy, powder X-ray diffraction, thermogravimetric analysis, Field Emission Scanning Electron Microscopy, Fourier transformed infrared spectroscopy and nitrogen adsorption/desorption analysis. It was observed that the particle size and the surface area of the produced GDC nanocomposite powders are dominantly controlled by the calcination temperature, while the effect of sodium alginate loading fraction is limited by the range of the calcination temperature. In this study, the smallest mesoporous GDC nanocomposite powder with cubic fluorite structure (8 nm crystallite size and 3.05±0.005 m²/g surface area) was synthesized using 2 wt% of sodium alginate at a calcination temperature of 550 °C (for 4 h). The results of this study could help to perceive the influence of the basic processing variables on the particle size and the other physiochemical properties of GDC nanocomposite powders produced by the ionic-gelation method.     

Investigations on the properties of ceria–zirconia-supported Ni and Rh catalysts and their performance in acetic acid steam reforming

The production of hydrogen via steam reforming of acetic acid was examined over Ni and Rh supported on a CeO₂–ZrO₂-mixed oxide. The catalysts were tested at 550–650–750 °C using steam/carbon = 3. Steam reforming, water gas shift, and decarboxylation are the main reactions taking place over the support alone. In parallel, dehydrogenation leads to the formation of carbon deposits on the surface of the mixed oxide. The addition of the metals enables the reforming reactions to proceed with high rates producing hydrogen yields close to thermodynamic equilibrium even at 650 °C. The oxygen exchange reactions are enhanced leading to much lower coke deposition. The nature of the metal affects not only the quantity but also the quality and the location of the carbon deposits, as evidenced from temperature-programing oxidation tests. The synergy of the support and metal is the key factor for the low coke deposition, which is even lower for the Rh catalyst.     

Low Temperature Water–Gas Shift/Methanol Steam Reforming: Alkali Doping to Facilitate the Scission of Formate and Methoxy C–H Bonds over Pt/ceria Catalyst

Doping Pt/ceria catalysts with the Group 1 alkali metals was found to lead to an important weakening of the C–H bond of formate and methoxy species. This was demonstrated by a shift to lower wavenumbers of the formate and methoxy ν(CH) vibrational modes by DRIFTS spectroscopy. Li and Na-doped Pt/ceria catalysts were tested relative to the undoped catalyst for low temperature water–gas shift and methanol steam reforming using a fixed bed reactor and exhibited higher catalytic activity. Steaming of formate and methoxy species pre-adsorbed on the catalyst surface during in-situ DRIFTS spectroscopy suggested that the species were more reactive for dehydrogenation steps in the catalytic cycle for the Li and Na-doped catalysts relative to undoped Pt/ceria. However, with increasing atomic number over the series of alkali-doped catalysts, the stability of a fraction of the carbonate species was found to increase. This was observed during TPD-MS measurements of the adsorbed CO₂ probe molecule by a systematic increase of a high temperature peak for a fraction of the CO₂ desorbed. This result indicates that alkali-doping is an optimization problem—that is, while improving the dehydrogenation rates of methoxy and formate species, the carbonate intermediate stability increases, making it difficult to liberate the CO₂. Infrared spectroscopy results of CO adsorbed on Pt and ceria suggest that the alkali dopant is located on, and electronically modifies, both the Pt and ceria components. The results not only lend further support to the role that methoxy and formate species play as intermediates in the catalytic mechanisms, but also provide a path forward for improving rates by means other than resorting to higher noble metal loadings.     

La0.6Sr0.4Co0.2Fe0.8O3-δ oxygen electrodes for solid oxide cells prepared by polymer precursor and nitrates solution infiltration into gadolinium doped ceria backbone

Infiltration is a method, which can be applied for the electrode preparation. In this paper oxygen electrode is prepared solely by the infiltration of La_0.6Sr_0.4Co_0.2Fe_0.8O_3‐δ (LSCF) into Ce_0.8Gd_0.2O_2-δ (CGO) backbone. The use a polymer precursor as an infiltrating medium, instead of an aqueous nitrate salts solution is presented. It is shown that the polymer forms the single-phase perovskite at 600 °C, contrary to the nitrates solution. As a result, obtained area specific resistance (ASR) is lowered from 0.21 Ω cm² to 0.16 Ω cm² at 600 °C. More than 35% of LSCF in the oxygen electrode decreases the performance.     

Development of electrochemical cell with layered composite of the Gd-doped ceria/electronic conductor system for generation of H2–CO fuel through oxidation–reduction of CH4–CO2 mixed gases

This paper shows the recent results on the development of layered composite promoting two types of electrochemical reactions (oxidation and reduction) in one cell. This cell consisted of porous Ni–Gd-doped (GDC) ceria cathode/thin porous GDC electrolyte (50 μm)/porous SrRuO₃–GDC anode. The external electric current was flowed in this cell at the electric field strength of 1.25 and 6.25 V/cm. The mixed gases of CH₄ (30–70%) and CO₂ (70–30%) were fed at the rate of 50 ml/min to the cell heated at 400–800 °C under the electric field. In the cathode, CO₂ was reduced to CO (CO₂ + 2e^? → CO + O^2?) and the formed CO and O^2? ions were transported to the anode through the pores and surface and interior of grains of GDC film. On the other hand, CH₄ was oxidized in the anode to form CO and H₂ through the reaction with diffusing O^2? ions (CH₄ + O^2? → CO + 2H₂ + 2e^?). As a result, H₂–CO mixed fuel was produced from the CH₄–CO₂ mixed gases (CH₄ + CO₂ → 2H₂ + 2CO). This electrochemical reaction proceeded completely at 800 °C and no blockage of gases was measured for long time (>10 h). Only H₂–CO fuel was generated in the wide gas compositions of starting CH₄–CO₂ gases.     

Effect of Sm content on the properties and electrochemical performance of SmxSr1 − xCoO3 − δ (0.2 ≤ x ≤ 0.8) as an oxygen reduction electrodes on doped ceria electrolytes

Sm_xSr_{1 − x}CoO_{3 − δ} (SSCx) materials are promising cathodes for IT-SOFCs. The influence of Sm content in SSCx (0.2 ≤ x ≤ 0.8) oxides on their oxygen nonstoichiometry, oxygen desorption, thermal expansion behavior, electrical conductivity and electrochemical activity for oxygen reduction is systematically studied by iodometric titration, oxygen-temperature programmed desorption (O₂-TPD), dilatometer, four-probe DC conductivity, electrochemical impedance spectroscopy (EIS) and three-electrode polarization test, respectively. Iodometric titration experiments demonstrate that the electrical charge neutrality compensation in SSCx proceeds preferably through the oxidation of cobalt ion for high Sm³⁺ contents (x ≥ 0.6). However, it proceeds mainly through the creation of oxygen vacancies at x ≤ 0.5. O₂-TPD shows SSC5 possesses the highest oxygen desorption ability among the range of SSCx materials tested. The thermal expansion coefficients (TECs) are high between the transition temperature and 900 °C, showing values typically larger than 20 × 10⁻⁶ K⁻¹. All dense materials show high electrical conductivity with a maximum value of ∼1885 S cm⁻¹ for SSC6 in air, while SSC5 has the highest electrical conductivity in nitrogen. EIS analysis of porous electrodes demonstrates that SSC5 has the lowest area specific resistance (ASR) value (0.42 Ω cm²) at 600 °C. Cathodic overpotential testing demonstrates that SSC5 also has the largest exchange current density of 60 mA cm⁻² at 600 °C in air.