Y-doped Ceria Research Articles

Metal-insulator transition and resistive switching in Y-doped CeO2 ceramics.

Y-doped ceria, Y0.16Ce0.84O1.92, is an oxide ion conductor that shows n-type conductivity under a small applied voltage. With increasing voltage, resistive switching by 2-3 orders of magnitude occurs that is reversible with some hysteresis and is enhanced in atmospheres of reduced pO2. The switching is a bulk effect, is not associated with Schottky barriers or with a crystallographic transition, occurs rapidly after a premonitory onset period depending on conditions and shows characteristics of a Mott transition. This is the third known example of low field-induced resistive switching in a bulk ceramic and represents an emergent phenomenon in materials that are taken outside their zone of thermodynamic stability.

Oxygen Vacancy Distribution in Yttrium-Doped Ceria from 89Y-89Y Correlations via Dynamic Nuclear Polarization Solid-State NMR.

Comprehending the oxygen vacancy distribution in oxide ion conductors requires structural insights over various length scales: from the local coordination preferences to the possible formation of agglomerates comprising a large number of vacancies. In Y-doped ceria, 89Y NMR enables differentiation of yttrium sites by quantification of the oxygen vacancies in their first coordination sphere. Because of the extremely low sensitivity of 89Y, longer-range information was so far not available from NMR. Herein, we utilize metal ion-based dynamic nuclear polarization, where polarization from Gd(III) dopants provides large sensitivity enhancements homogeneously throughout the bulk of the sample. This enables following 89Y–89Y homonuclear dipolar correlations and probing the local distribution of yttrium sites, which show no evidence of the formation of oxygen vacancy rich regions. The presented approach can provide valuable structural insights for designing oxide ion conductors.

Dehydrogenation of ethane assisted by CO2 over Y-doped ceria supported Au catalysts

Dehydrogenation of ethane assisted by CO2 has been investigated over Au catalyst supported on Y-doped CeO2 nanorod. The catalysts were characterized by XRD, TEM, N2 adsorption, XPS, TG, H2-TPR and Raman spectroscopy. Enhanced activity can be obtained after the proper Y doping. The optimal catalyst Au/Ce0.9Y0.1 exhibits an ethane conversion of 20.9% and nearly 100% selectivity to ethylene without any evident tendency of deactivation in 50 h. The reducibility of surface oxygen species is found to play a key role in the dehydrogenation assisted by CO2, which can be improved by proper Y doping into CeO2 nanorod. The promoting effect of CO2 on activity is evident over Au/Ce0.9Y0.1 catalyst, due to the quick transformation of H2 product into H2O by the reverse water gas shift reaction. The catalyst stability is also improved by the addition of CO2, attributed to the elimination of coke by Bouduoard reaction.

Complete Benzene Oxidation over Mono and Bimetallic Pd—Au Catalysts on Alumina-Supported Y-Doped Ceria

The protection of environment and human health stimulates intensive research for abatement of volatile organic compounds (VOCs) in the atmosphere. Complete catalytic oxidation is an efficient, environmentally friendly and economically feasible method for elimination of VOCs. This study aims to design high performing and cost-effective catalytic formulations by exploration of appropriate and economically viable supports. Alumina-supported ceria (30 wt.%) and Y2O3 (1 wt.%)-doped ceria were prepared by mechanical mixing and were used as support of mono Au (2 wt.%) and Pd (1 wt.%) and bimetallic Pd-Au catalysts. The characterization by textural measurements, X-ray powder diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), EPR (electron paramagnetic resonance) and temperature-programmed reduction (TPR) was carried out in order to clarify the relationship between catalyst composition, textural, structural and surface properties, reducibility and catalytic performance for complete benzene oxidation. Among all studied catalysts, Pd-based catalysts exhibited the best combustion activity. In particular, monometallic Pd on alumina supported Y-doped ceria attained 100% of complete benzene conversion at 180 °C. These catalytic materials have potential to meet stringent emission regulations in an economical and effective way.

Unraveling the effect of alumina-supported Y- doped ceria composition and method of preparation on the WGS activity of gold catalysts

The demands for high-purity hydrogen required in fuel-cell applications impose new goals and challenges for design of well performing water-gas shift (WGS) catalysts. Gold-based catalysts have exhibited high activity in the WGS reaction at low temperature. Preparation of appropriate and economically viable supports with complex composition by various synthesis procedures is an attractive approach to WGS performance improvement. The effect of two different preparation methods (wet impregnation or mechanical mixing) and ceria content (10, 20 or 30 wt%) on textural, structural, surface and reductive properties and WGS activity of gold catalysts was studied. Additionally, the role of Y2O3 as a promoter of ceria was examined. Long-term stability test was carried out at 260 °C over the most active catalyst. The composition of the best performing sample (composed of about 70 wt% alumina), prepared by mechanical mixing, was considered promising in case of practical applications because of its cost efficiency. The combination of gold nanoparticles and alumina supported Y-doped ceria proved an advantageous approach for developing new catalytic formulations with high effectiveness in clean hydrogen production.

Effect of Preparation Method on the Performance for PROX of Gold Catalysts on Alumina Supported Y-Doped Ceria

Effect of Preparation Method on the Performance for PROX of Gold Catalysts on Alumina Supported Y-Doped Ceria

Structure-activity relationship in water-gas shift reaction over gold catalysts supported on Y-doped ceria

The utilization of pure hydrogen as an energy source in fuel cells gave rise to renewed interest in developing active and stable water-gas shift catalysts. Gold catalysts have proven to be very efficient for water-gas shift reaction at low temperature. The aim of the present study was to investigate the effect of: (i) different preparation methods (impregnation and coprecipitation) to obtain a modified ceria support, and (ii) the amount of Y2O3 (1.0 wt%, 2.5 wt%, 5.0 wt% and 7.5 wt%) as dopant on the water-gas shift activity of Au/CeO2 catalysts. An extended characterization by means of SBET, XRD, HRTEM/HAADF, FTIR, H2-TPR and CO-TPR measurements in combination with careful evaluation of the catalyst behavior allowed to shed light on the parameters governing the water-gas shift activity. The catalysts show very high activity (>90% CO conversion) in the temperature range 180–220 °C, with a slightly better performance of the gold catalysts on supports prepared by impregnation. The decreased activity with increasing Y2O3 concentration is related to the hindering of oxygen mobility due to ordering of surface oxygen vacancies in vicinity of segregated Y3+. The effect of catalyst pre-treatments and the stability of the best performing samples were examined as well.

Ionic conductivity of infiltrated Ln (Ln = Gd, Sm, Y)-doped ceria

This work studies the ionic conductivity of nanosized Gd-, Sm-, and Y-doped ceria prepared by the infiltration/impregnation method. The nanoparticles were deposited onto porous pure ceria substrates via infiltration-heating processes, and the conductivity was determined with the electrochemical impedance spectroscopy (EIS) using the conductive model for infiltrated phases. The conductivity of the infiltrated doped ceria changes with the doping amount, and Gd0.25Ce0.75O2−δ, Sm0.2Ce0.8O2−δ, and Y0.15Ce0.85O2−δ show the highest values of 2.56, 3.01, and 2.07 × 10−3 S·cm−1 at 600 °C, respectively. Overall, Sm-doped samples show the highest conductivity, while Y-doped samples show the lowest conductivity. In consideration of the Bruggeman factor, the intrinsic conductivity of the infiltrated doped ceria was calculated. Compared with the bulk doped ceria, the intrinsic conductivity is higher while the activation energy is lower, which may suggest different conduction mechanisms. Besides, co-doping effects on the conductivity of the infiltrated sample are less obvious than those of the bulk sample.

Alumina supported Au/Y-doped ceria catalysts for pure hydrogen production via PROX

Abstract Gold catalysts on Y-doped ceria dispersed on high surface area γ-Al2O3 were synthesized and tested in preferential CO oxidation in hydrogen rich stream (PROX). The effect of ceria loading (10, 20 or 30 wt%) was studied. The gold catalyst with the lowest ceria amount exhibited the highest PROX activity. The addition of Y2O3 (1 wt%) led to improved performance. The most favorable effect was observed in the sample with 20 wt% ceria amount. This gold catalyst showed good PROX activity and stability in the presence of CO2 and water. Catalysts characterization by XRD, HRTEM/HAADF, XPS and H2-TPR was used to elucidate the relationship between the chemical composition, state of gold, support features and catalytic properties.

An Efficient Electron-Blocking Interlayer Induced by Metal Ion Diffusion for SOFC Based on Y-Doped Ceria Electrolyte.

To suppress the internal electronic leakage at ceria-based electrolyte, a novel electron-blocking layer consisting of doped BaCe0.8Y0.2O3-δ was fabricated in situ at the interface of Ba-containing anode and Y-doped ceria electrolyte. The anode-supported full cell based on Y0.2Ce0.8O1.9 (YDC20) electrolyte presents a remarkable peak power density of 814 mW/cm2 as well as an open-circuit voltage of 1.0 V at 650 °C, which are much higher than those of the cells with Gd0.1Ce0.9O1.95 (GDC10) electrolyte (453 mW/cm2 at 650 °C) and BaCe0.8Y0.2O3-δ|Y0.2Ce0.8O1.9 (BCY|YDC20) bilayered electrolyte (419 mW/cm2 at 650 °C). The efficient promotion of the electron-blocking interlayer with high oxygen ionic conductivity is considered as the main reason for the improved performance of YDC20-based solid oxide fuel cell. The composition and the microstructure of the electron-blocking interlayer are further analyzed by scanning electron microscope and transmission electron microscope characterizations.

Cathode Functionalization Via Atomic Layer Deposition for Solid Oxide Fuel Cells

Crucial ­­processes in solid oxide fuel cell (SOFC) such as ionic transport and electrode reactions are thermally-activated processes and their kinetics are strongly dependent on the temperature. Despite intense recent efforts to reduce its operating temperature to address durability, cost and applicability issues, it is still an ongoing challenge to deal with the resulting sluggish kinetics and power losses at lower temperatures. On the other hand, while catalysts with higher surface area are preferred for maximized catalysis per mass and volume, these high-surface area catalysts are prone to agglomeration sacrificing its overall catalytic activity during operation. In this talk, we will present a couple of examples how these issues have been successfully addressed by the use of atomic layer deposition (ALD), which has emerged as an important deposition technique for various applications due to its unique atomic-scale deposition capability in a highly controllable fashion at a temperature significantly lower than conventional chemical vapor deposition techniques. We will present several examples of enhancements in cathode performance and/or durability enabled by an ultrathin coating of Y-doped zirconia or ceria (i.e. YSZ or YDC) on different nano-porous cathode materials including Pt, La(Ni,Fe)O3 and (La,Sr)(Co,Fe)O3. In addition, we will report mechanism studies mainly performed via electrochemical impedance spectroscopy and electron microscopy focusing on the shift of the rate-determining step by the ultrathin overcoat. Acknowledgement: The authors gratefully acknowledge Nissan Motor Co. Ltd. and NASA MIRO Program (Grant No. NNX15AQ01A) for their financial support.

Gold Catalysts on Y-Doped Ceria Supports for Complete Benzene Oxidation

Gold (3 wt. %) catalysts on Y-doped (1, 2.5, 5 and 7.5 wt. % Y2O3) ceria supports prepared by coprecipitation (CP) or impregnation (IM) were studied in complete benzene oxidation (CBO). A low-extent Y modification was chosen to avoid ordering of oxygen vacancies. The samples were characterized by XRD, TGA, XPS and TPR techniques. A positive role of air pretreatment at 350 °C as compared to 200 °C was established for all Y-containing catalysts and it was explained by cleaning the active sites from carbonates. The oxygen supply cannot be considered as a limiting step for benzene oxidation except for the high 7.5%-doped samples, as suggested by TGA and TPR data. On the basis of XPS results of fresh and used in CBO catalysts, the presence of cationic gold species does not seem important for high CBO activity. The gold catalyst on an IM support with 1% Y-doping exhibited the best performance. A 100% benzene conversion was achieved only over this catalyst and Au/ceria, while it was not reached even at 300 °C over all other studied catalysts. Gold and ceria particle agglomeration or coke formation should be excluded as a possible reason, and the most probable explanation could be associated with the importance of the benzene activation stage.

Sm-doped praseodymium nickelates-cobaltites and their nanocomposites with Y-doped ceria as promising cathode materials

ABSTRACTThis work aims at elucidating effects of composition and microstructure of materials based upon Pr1-ySmyNi1-xCoxO3-δ perovskites on their oxygen mobility. Pr1-ySmyNi1-xCoxO3-δ and their composites with Y-doped ceria are perspective materials for intermediate temperature solid oxide fuel cells and oxygen separation membranes. Doping PrNi1-xCoxO3-δ by Sm results in suppression of the fast channel of oxygen diffusion along extended defects due to Sm segregation in their vicinity. For nanocomposites moderate doping by Sm enhances fast oxygen diffusion in fluorite phase domains and along perovskite-fluorite interface due to a higher disordering caused by incorporation of both Pr and Sm cations into Y-doped ceria.

Ceria-supported nickel borate as a sulfur-tolerant catalyst for autothermal reforming of a proxy jet fuel

Catalytic reforming of liquid hydrocarbons to produce H2-enriched fuel in an auxiliary power unit is essential to the operation of fuel cell on board. This study explored ceria-supported nickel borate [Ni3(BO3)2] catalyst for conducting the above reaction under autothermal reforming (ATR) condition. In the present catalytic system operating at 750°C and under purge stream of feed, nickel borate undergoes partial degradation whose degree depends on the type of support used to release catalytically reactive tiny Ni0 metal crystallite and B2O3. To achieve a high fuel conversion (>95%), it is crucial to maintain a dominant ratio of nickel borate versus Ni0 nanocrystallites as the reactive site. Typically, more than 90% nickel borate remains on the Y-doped ceria (CYO) while almost all nickel borate degrade to Ni0 and B2O3 on the Gd-doped ceria (CGO) after 24h ATR of the proxy fuel comprising dodecane, naphthalene (10wt%) and thiophene (containing 100pm S). In consequence, the former catalyst shows a higher fuel conversion profile than the latter by 15–20%. It is proposed that nickel borate itself is a cocatalyst to induce cracking of fuel molecules, which provides smaller hydrocarbon species for the reforming on Ni0 crystallites, and promotes sulfur tolerance of Ni0 crystallites to a large extent on CYO according to the XPS and NMR characterizations.

Enhanced CO and Soot Oxidation Activity Over Y-Doped Ceria–Zirconia and Ceria–Lanthana Solid Solutions

Y-doped ceria–zirconia (Ce0.8Zr0.12Y0.08O2−δ, CZY) and ceria–lanthana (Ce0.8La0.12Y0.08O2−δ, CLY) ternary oxide solid solutions were synthesized by a facile coprecipitation method. Structural, textural, redox, and morphological properties of the synthesized samples were investigated by means of X-ray diffraction (XRD), inductively coupled plasma-optical emission spectroscopy (ICP–OES), Raman spectroscopy (RS), UV–visible diffuse reflectance spectroscopy (UV–vis DRS), X-ray photoelectron spectroscopy (XPS), temperature-programmed reduction by hydrogen (H2-TPR), high resolution transmission electron microscopy (HRTEM), and Brunauer–Emmett–Teller surface area (BET SA) techniques. The formation of ternary oxide solid solutions was confirmed from XRD, RS, and UV–vis DRS results. ICP–OES analysis confirmed the elemental composition in the ternary oxide solid solutions. HRTEM images revealed irregular morphology of the samples. RS, UV–vis DRS, and XPS results indicated enhanced oxygen vacancies in the Y doped samples. H2-TPR profiles confirmed a facile reduction of CZY and CLY samples at lower temperatures. BET analysis revealed an enhanced surface area for CZY and CLY samples than the respective CZ and CL undoped mixed oxides. All these factors contributed to a better CO and soot oxidation performance of CZY and CLY samples. Particularly, the CLY sample exhibited highest catalytic activity among the various samples investigated.

Oxygen mobility and surface reactivity of PrNi1−xCoxO3+δ–Ce0.9Y0.1O2−δ cathode nanocomposites

Cobalt-doped praseodymium nickelate PrNi1 − xCoxO3 − δ (PNCx) and Y-doped ceria Ce0.9Y0.1O2 − δ (YDC) oxides were synthesized via Pechini route. PNCx + YDC composites were prepared via ultrasonic dispersion of the mixture of perovskite and fluorite nanopowders in isopropanol with addition of polyvinyl butyral followed by drying, pressing and sintering at 1300 °C. The oxygen mobility and reactivity of powdered PNCx and composites obtained by crushing and milling of dense pellets were estimated by O2-TPD and oxygen isotope exchange with 18O2 and C18O2 using both static and flow (SSITKA) reactors in isothermal and temperature-programmed (TPIE) modes. For PNCx samples sintered at 1300 °C comprised of (Ni,Co)O and Ruddlesden–Popper type phases (Pr2NiO4, Pr4(Ni,Co)3O10), the oxygen mobility and reactivity tend to decrease with Co content. For composites, the oxygen mobility is much higher due to Pr transfer into YDC thus disordering perovskite-like and fluorite-like phases. TPIE C18O2 SSITKA experiments combined with SIMS analysis of the depth profiles of Pr18O and Ce18O suggest that fast oxygen diffusion in composites is provided by domains of disordered perovskite-like phases as well as Pr,Y-doped ceria. For best composites, the value of the oxygen chemical diffusion coefficient estimated by the weight relaxation technique exceeds that of well known LSFC–GDC composite.

Strain effects on the ionic conductivity of Y-doped ceria: A simulation study

In this paper we report a computational study of the effects of strain on the conductivity of Y-doped ceria (YDC). This material was chosen as it is of technological interest in the field of Solid Oxide Fuel Cells (SOFCs). The simulations were performed under realistic operational temperatures and strain (𝜖) levels. For bulk and thin film YDC, the results show that tensile strain leads to conductivity enhancements of up to 3.5 × and 1.44 × , respectively. The magnitude of these enhancements is in agreement with recent experimental and computational evidence. In addition, the methods presented herein allowed us to identify enhanced ionic conductivity in the surface regions of YDC slabs and its anisotropic character.

Defect Structure of Y-Doped Ceria on Different Length Scales

An exhaustive structural investigation of a Y-doped ceria (Ce1–xYxO2–x/2) system over different length scales was performed by combining Rietveld and Pair Distribution Function analyses of X-ray and neutron powder diffraction data. For low doping amounts, which are the most interesting for application, the local structure of Y-doped ceria can be envisaged as a set of distorted CeO2- and Y2O3-like droplets. By considering interatomic distances on a larger scale, the above droplets average out into domains resembling the crystallographic structure of Y2O3. The increasing spread and amount of the domains with doping forces them to interact with each other, leading to the formation of antiphase boundaries. Single phase systems are observed at the average ensemble level.

How to interpret current–voltage relationships of blocking grain boundaries in oxygen ionic conductors

A new model based on a linear diffusion equation is proposed to explain the current-voltage characteristics of blocking grain boundaries in Y-doped CeO2 in particular. One can also expect that the model can be applicable to the ionic conductors with blocking grain boundaries, in general. The model considers an infinitely long chain of identical grains separated by grain boundaries, which are treated as regions in which depletion layers of mobile ions are formed due to trapping of immobile charges that do not depend on the applied voltage as well as temperature. The model assumes that (1) the grain boundaries do not represent physical blocking layers, which implies that if there is a second phase at the grain boundaries, then it is too thin to impede ion diffusion and (2) the ions follow Boltzmann distribution throughout the materials. Despite its simplicity, the model successfully reproduces the "power law": current proportional to voltage power n and illustrated with the experimental example of Y-doped ceria. The model also correctly predicts that the product nT, where T is the temperature in K, is constant and is proportional to the grain boundary potential as long as the charge at the grain boundaries remains trapped. The latter allows its direct determination from the current-voltage characteristics and promises considerable simplification in the analysis of the electrical characteristics of the grain boundaries with respect to the models currently in use.

Incubational domain characterization in lightly doped ceria

Microstructures of both Gd- and Y-doped ceria with different doping level (i.e., 10at% and 25at%) have been comprehensively characterized by means of high resolution transmission electron microscopy and selected area electron diffraction. Coherent nano-sized domains can be widely observed in heavily doped ceria. Nevertheless, it was found that a large amount of dislocations actually exist in lightly doped ceria instead of heavily doped ones. Furthermore, incubational domains can be detected in lightly doped ceria, with dislocations located at the interfaces. The interactions between such linear dislocations and dopant defects have been simulated accordingly. As a consequence, the formation mechanism of incubational domains is rationalized in terms of the interaction between intrinsic dislocations of doped ceria and dopant defects. This study offers the insights into the initial state and related mechanism of the formation of nano-sized domains, which have been widely observed in heavily rare-earth-doped ceria in recent years.