Ceria-based Solid Solutions Research Articles

Perspective ceria-based solid solutions GdxBi0.2−xCe0.8O2

The series of Gd-Bi-Ce-O solid solutions were synthesized by co-precipitation method with sonication. These mesoporous nanocrystalline powders with the size of about 15nm were used as precursors to ceramics formation, sintered at 1000°C in air, for the application of materials as an electrolyte for IT-SOFC. The electrical conductivity of these materials was measured by AC impedance spectroscopy in the temperature range of 450–750°C in air. It was showed that Gd0.05Bi0.15Ce0.8O2 ceramics, prepared through acetylacetonate complex, is the perspective system for this goal and has the conductivity of 5·10−3S/cm at 600°C in air.

Effect of Er, Gd, and Nd Co-Dopants on the Properties of Sm-Doped Ceria Electrolyte for IT-SOFC

Doped ceria electrolytes of Ce0.8Sm0.20O1.90 (SDC), Ce0.8Er0.08Sm0.12O1.90 (ESDC), Ce0.8Gd0.08Sm0.12O1.90 (GSDC), and Ce0.8Nd0.08Sm0.12O1.90 (NSDC) were prepared by a citric–nitrate combustion process. The thermal decomposition of the precursor was investigated by simultaneous thermogravimetric analysis and differential thermal analysis experiments. The phase structure of the calcined and sintered powders was characterized by X-ray diffraction analysis. All of the samples were fluorite-type ceria-based solid solutions. The calcined and sintered powders were also characterized by Fourier transform infrared spectroscopy. Scanning electron microscopy was used to characterize the samples after calcination and sintering. A relative density over 95 percent of the theoretical density was achieved after the sintering process. Electrical conductivities of the sintered samples were measured using the impedance spectra method. The highest ionic conductivity value was found to be 5.28 × 10−2 S cm−1 at 1023 K (750 °C) for GSDC sintered at 1673 K (1400 °C) for 6 hours.

Influence of acetylaceton on mesoporous structure and catalytic activity of ceria-based solid solution

Gd0.1Ti0.1Zr0.1Ce0.7O2 solid solutions with a crystallite size of 3–8 nm have been prepared by the sonochemical method from inorganic salts with an acetylaceton additive. In all cases, ceria based materials exhibited a mesoporous structure with monomodal pore size distribution with diameter of 3–4 nm. It was demonstrated that such systems were more active in CO oxidation than samples, prepared without using acetylaceton (shift more than 150 °C of CO full oxidation).

Influence of preparation methods on the physicochemical properties and catalytic performance of MnOx-CeO2 catalysts for NH3-SCR at low temperature

This work examines the influence of preparation methods on the physicochemical properties and catalytic performance of MnOx-CeO2 catalysts for selective catalytic reduction of NO by NH3 (NH3-SCR) at low temperature. Five different methods, namely, mechanical mixing, impregnation, hydrothermal treatment, co-precipitation, and a sol-gel technique, were used to synthesize MnOx-CeO2 catalysts. The catalysts were characterized in detail, and an NH3-SCR model reaction was chosen to evaluate the catalytic performance. The results showed that the preparation methods affected the catalytic performance in the order: hydrothermal treatment > sol-gel > co-precipitation > impregnation > mechanical mixing. This order correlated with the surface Ce3+ and Mn4+ content, oxygen vacancies and surface adsorbed oxygen species concentration, and the amount of acidic sites and acidic strength. This trend is related to redox interactions between MnOx and CeO2. The catalyst formed by a hydrothermal treatment exhibited excellent physicochemical properties, optimal catalytic performance, and good H2O resistance in NH3-SCR reaction. This was attributed to incorporation of Mnn+ into the CeO2 lattice to form a uniform ceria-based solid solution (containing Mn-O-Ce structures). Strengthening of the electronic interactions between MnOx and CeO2, driven by the high-temperature and high-pressure conditions during the hydrothermal treatment also improved the catalyst characteristics. Thus, the hydrothermal treatment method is an efficient and environment-friendly route to synthesizing low-temperature denitrification (deNOx) catalysts.

Synthesis and application of rare-earth elements (Gd, Sm, and Nd) doped ceria-based solid solutions for methyl mercaptan catalytic decomposition

A series of rare earth elements with different ionic radii (RE=Gd, Sm and Nd) doped ceria solid solutions and pure ceria catalyst were rapidly prepared by citrate-complexation method. These catalysts were evaluated for catalytic decomposition of methyl mercaptan as a model odorous sulfur containing pollutant. According to the XRD, Raman, XPS, H2-TPR, OSC, FT-IR and CO2-TPD characterization results, the radius of RE cation played an important role in determining the structural and surface characteristics of RE doped ceria catalysts, which could affect the catalytic stability for CH3SH decomposition. Thereinto, Ce0.75Gd0.25O2-δ, with a moderate increase in the alkalinity, possessed more oxygen vacancies. More of structural defects and active sites could be provided, which should be in favor of improving catalytic stability. But for Ce0.75Nd0.25O2-δ, with too many basic sites, more of active sites would be occupied and plenty of cerium sulfide species (Ce2S3) would be accumulated on the catalyst during the reaction processes. The accumulation of Ce2S3 mainly caused the deactivation of catalyst. The catalytic stability of Ce0.75Sm0.25O2-δ was comparable to that of CeO2, which might be resulted from the combined effect of the increased oxygen vacancies and basic sites.

Enhanced catalytic performance of Ir catalysts supported on ceria-based solid solutions for methane dry reforming reaction

Methane dry reforming reaction was investigated over Ir catalysts supported on ceria-based solid solutions. The doping of Pr6O11 or ZrO2 into the matrix of ceria yields more oxygen defects, resulting in an improved redox property and enhanced metal-support interaction in Ir/Ce0.9M0.1O2 (M=Pr, Zr) catalysts compared to Ir/CeO2 catalyst. The Ir/Ce0.9M0.1O2 (M=Pr, Zr) catalysts showed higher catalytic performance and better stability than the Ir/CeO2 catalyst, which were assigned to positive effects of doping in maintaining Ir nanoparticles, stabilizing catalyst structure, and the strong metal-support interaction in reducing carbon deposition.

Cu–Mn–Ce ternary mixed-oxide catalysts for catalytic combustion of toluene

Cu–Mn, Cu–Mn–Ce, and Cu–Ce mixed-oxide catalysts were prepared by a citric acid sol–gel method and then characterized by XRD, BET, H2-TPR and XPS analyses. Their catalytic properties were investigated in the toluene combustion reaction. Results showed that the Cu–Mn–Ce ternary mixed-oxide catalyst with 1:2:4 mole ratios had the highest catalytic activity, and 99% toluene conversion was achieved at temperatures below 220°C. In the Cu–Mn–Ce catalyst, a portion of Cu and Mn species entered into the CeO2 fluorite lattice, which led to the formation of a ceria-based solid solution. Excess Cu and Mn oxides existed on the surface of the ceria-based solid solution. The coexistence of Cu–Mn mixed oxides and the ceria-based solid solution resulted in a better synergetic interaction than the Cu–Mn and Cu–Ce catalysts, which promoted catalyst reducibility, increased oxygen mobility, and enhanced the formation of abundant active oxygen species.

On Identification of Labile Oxygen in Ceria-Based Solid Solutions: Which Oxygen Leaves the Lattice?

Solid solutions of ceria with other transition metal cations have been known to exhibit enhanced reducibility and oxygen storage capacity. Formation of differential bond lengths in the lattice of solid solutions has been proposed to be responsible for the improvement in their catalytic activity. We have carried out a density functional theory analysis of ceria-based solid solutions (Ce1–xMxO2−δ (M = Zr/Mo/Pd)) to identify the role of cation substitution and vacancy formation in bond length distribution. Different anionic vacancies in close proximity of the substituted cation were identified and the effects of location of the vacancy on bond length distribution and vacancy formation energy were studied. A large improvement in the net oxygen activation was observed with the introduction of a cation in the lattice. Further, nearly 17 kcal/mol reduction in vacancy formation energy was observed for Pd-substituted compound when compared to CeO2 when the vacancy was induced at a site directly coordinated with Pd. This successfully explained reported high activity of Ce1–xPdxO2−δ for redox reactions following dual site mechanism allowing the interaction of surface species in close proximity.

Effect of metal ions doping (M = Ti4+, Sn4+) on the catalytic performance of MnOx/CeO2 catalyst for low temperature selective catalytic reduction of NO with NH3

Ceria-based solid solutions were synthesized and used as supports to prepare MnOx/Ce0.8Ti0.2O2 and MnOx/Ce0.8Sn0.2O2 catalysts (Mn/CeTi and Mn/CeSn) for low temperature selective catalytic reduction of NO by NH3. The effects of Ti or Sn doping on the catalytic performance of MnOx/CeO2 catalyst were investigated. Experimental results show that doping of Ti or Sn increases the NO removal efficiency of MnOx/CeO2. The NO conversion of Mn/CeTi catalyst is more than 90% at temperature window of 175–300°C under a gas hour space velocity of 60,000mLg−1h−1. Modified catalysts are also found to exhibit greatly improved resistance to sulfur-poisoning. Characterization results indicate that doping of Ti or Sn brings about catalysts with higher BET surface area, enhanced oxygen storage capacity and increased surface acidity. X-ray photoelectron spectroscopy (XPS) analysis of spent catalysts following SCR reaction in the presence of SO2 verify that the loss of surface Mn species was inhibited by doping of Ti, which contributes to extend the sulfur durability. The SCR activity is considered as well to be promoted by the dual redox cycles in Mn/CeTi (Mn4++Ce3+↔Mn3++Ce4+, Ce4++Ti3+↔Ce3++Ti4+) and Mn/CeSn (Mn4++Ce3+↔Mn3++Ce4+, 2Ce4++Sn2+↔2Ce3++Sn4+) catalysts.

Highly efficient nanosized Mn and Fe codoped ceria-based solid solutions for elemental mercury removal at low flue gas temperatures

Mn and Fe codoped ceria-based solid solutions are effective catalysts for Hg0 removal at low flue gas temperatures.

Gadolinium doped cerium oxide for soot oxidation: Influence of interfacial metal–support interactions

The aim of the present investigation was to ascertain the role of Al2O3, SiO2, and TiO2 supports in modulating the catalytic performance of ceria-based solid solutions. In this study, we prepared nanosized Ce-Gd/Al2O3, Ce-Gd/SiO2, and Ce-Gd/TiO2 catalysts by a deposition coprecipitation method and evaluated for soot oxidation. The synthesized catalysts were calcined at two different temperatures to assess their thermal stability and extensively characterized by various techniques, namely, XRD, Raman, BET surface area, TEM, H2-TPR, and UV–vis DRS. XRD and TEM results indicate that Ce-Gd-oxide nanoparticles are in highly dispersed form on the surface of the supports. Raman results show a prominent sharp peak and a broad peak corresponding to the F2g mode of ceria and the presence of oxygen vacancies, respectively. The presence of a significant number of oxygen vacancies in all samples is also confirmed from UV–vis DRS measurements. The H2-TPR results suggest that Gd-doping facilitates the reduction of the materials and decreases the onset temperature of reduction. Among the prepared samples, Ce-Gd/TiO2 catalyst exhibited the highest activity, suggesting the existence of strong interfacial metal support interaction between the active metal oxide and the support.

Some Structural Aspects of Ionic Conductivity in Co-Doped Ceria-Based Electrolytes

Abstract The sinters of co-doped ceria solid solutions with the formula of Ce0.85Sm0.15-x RxO1.9, where R = Y, Gd, Pr, Tb, Ox-0.15, were obtained from powders synthesised by Pechini method. The linear variation of cell parameter a vs. chemical composition was observed for Ce0.85Sm0:15-xRxO1.9, where R = Y, Gd, Tb, 0 <x<0.15 samples. However, the introduction of Pr3+ into Ce0.85Sm0.15-x PrxO1.9 caused a small deviation from linearity due to possible changes in the valence from Pr3+ to Pr4+. The determined values of oxide transference number tion for Ce0.85Sm0.15-xRxO1.9, R = Y, Gd in the temperature range 400-750°C and partial oxygen pressure from 10-6 to 1 atm were close to 1, which indicated that materials investigated exhibited practically pure ionic oxide conductivity. On the other hand, the introduction of Tb3+ or Pr3+ higher than x>0.05 into solid solution Ce0.85Sm0.15-xRxO1.9, R = Tb, Pr caused a decrease in the ionic transference number tion below 1 due to an increase in partial electronic conduction. This fact limiting investigated co-doped terbia and samaria or samaria and praseodymia ceria-based solid solutions for the further application as oxide electrolytes in solid oxide fuel cells. The analysis of bulk and grain boundary values indicated that partial substitution of Sm3+ by Y3+ or Gd3+ caused slight improvements in the ionic conductivity of Ce0.85Sm0.15-xRxO1.9. The highest ionic conductivity was found for solid solution with chemical composition Ce0.85Sm0.1Y0.05O1.9. The selected co-doped ceria samples were tested as solid electrolytes in solid oxide fuel cells operating in the intermediate temperature range 500-750°C.

Nanoscale control during synthesis of Me/La2O3, Me/CexGd1−xOy and Me/CexZr1−xOy (Me = Ni, Pt, Pd, Rh) catalysts for autothermal reforming of methane

Supported catalysts Me/La2O3, Me/CexGd1−xOy and Me/CexZr1−xOy (Me=Ni, Pt, Pd, Rh) were developed for the autothermal reforming of methane (ATR of CH4). The influence of support composition (La2O3, CexGd1−xOy, x=0.50–0.90 and CexZr1−xOy, x=0.33–0.67), type and content of the active component (5–30wt% Ni; 0.5–1.5wt% Pt, Pd or Rh) on the nanostructure of catalysts and their performance in the ATR of CH4 was investigated. The properties and structure of the catalysts in the course of their preparation and operation in the reaction were systematically characterized by means of X-ray diffraction, BET N2 adsorption/desorption, H2 temperature-programmed reduction, transmission electron microscopy and X-ray photoelectron spectroscopy techniques. The state and particle size of Ni-containing species were regulated by the support composition and Ni content. In case of the La2O3 support, the strong interaction between NiO and La2O3 led to the formation of two binary oxides LaNiO3 and La2NiO4 in the fresh samples, the composition of which was regulated by the Ni content. In case of the CexGd1−xOy and CexZr1−xOy supports, in contrast to the La2O3 support, nickel oxide and ceria-based solid solution were formed in the fresh samples. The catalyst evolution under reaction condition was studied. The conversion of methane and product (H2, CO) yields considerably increased when Ce0.8Gd0.2Oy or Ce0.5Zr0.5Oy instead of La2O3 were used as catalyst supports: at 850°C the yields of ∼35% H2 and ∼41% CO at CH4 conversion ∼76% were observed for the 10wt%Ni/La2O3, while the yields of ∼49% H2 and ∼66% CO at CH4 conversion ∼97% were observed for the 10wt% Ni/Ce0.5Zr0.5O2, which correlates with the increase of reducibility of Ni species as a result of weakening of the Ni–support interaction. The optimal value of metal content for the catalyst performance also depends on the support composition. The best ATR of CH4 performance is provided by 10wt% Ni/Ce0.5Zr0.5O2 and 1wt% Rh/Ce0.8Gd0.2O2 catalysts.

Structural characterization and catalytic evaluation of transition and rare earth metal doped ceria-based solid solutions for elemental mercury oxidation

The catalytic behavior of various CeO2-based solid solutions, namely, Ce1−xTMxO2−δ (TM = Mn, Fe, or Zr) and Ce1−xRExO2−δ (RE = Pr, La, or Sm) was studied for the removal of elemental mercury (Hg0) from coal-derived flue gas by catalytic oxidation (Hg0 → Hg2+). The investigated catalysts were synthesized by a coprecipitation method and characterized by various techniques, namely, X-ray diffraction (XRD), Raman spectroscopy (RS), high-resolution electron microscopy (HREM), Brunauer–Emmett–Teller (BET) surface area, X-ray photoelectron spectroscopy (XPS), temperature programmed reduction (TPR), and diffuse reflectance spectroscopy (UV-DRS). The XRD results confirmed the incorporation of Mn, Fe, Zr, La, Pr, and Sm cations into the CeO2 lattice and the formation of nanocrystalline solid solutions. The TEM measurements established the nanocrystalline nature of the solid solutions. The RS measurements suggested that the substitution process promotes the formation of oxygen vacancies, which hastens the diffusion rate of oxygen and improves the Hg oxidation. UV-vis DRS studies demonstrated the presence of the charge transfer transitions O2− → Ce3+ and O2− → Ce4+. The XPS and H2-TPR results suggested that the reduction of Ce4+ → Ce3+ is the foremost reason for the increase in oxygen vacancies, which are beneficial for Hg0 removal. The order of mercury oxidation activity over various doped catalysts is as follows: CM > CL > CZ > CF > CS > C > CP.

A comparative study of different doped metal cations on the reduction, adsorption and activity of CuO/Ce0.67M0.33O2 (M = Zr4+, Sn4+, Ti4+) catalysts for NO + CO reaction

A series of ceria-based solid solutions (Ce0.67Zr0.33O2, Ce0.67Sn0.33O2, Ce0.67Ti0.33O2) were synthesized by inverse co-precipitation, and then used as supports to prepare CuO/Ce0.67M0.33O2 (M=Zr4+, Sn4+, Ti4+) catalysts through wetness impregnation method. The obtained samples were investigated in detail by means of XRD, LRS, N2-physisorption, H2-TPR, XRF, XPS and in situ FT-IR techniques. The catalytic reduction of NO by CO as a model reaction was chosen to evaluate the catalytic performance of these samples. These results suggest that: (1) the reduction of CuO/Ce0.67Zr0.33O2 is easier than CuO/Ce0.67Sn0.33O2 and CuO/Ce0.67Ti0.33O2 catalysts, which may be attributed to the difference in the electronegativity of dopant; (2) the reduced state Cu+ is present in CuO/Ce0.67Zr0.33O2 at ambient temperature due to the shifting of redox equilibrium (Cu2++Ce3+ ↔ Cu++Ce4+) to right; (3) the adsorbed NO species on CuO/Ce0.67Zr0.33O2 are more liable to desorb/transform/decompose than those on CuO/Ce0.67Sn0.33O2 and CuO/Ce0.67Ti0.33O2 samples. The results of catalytic performance show that Cu+/Cu0 species play a key role in NO reduction by CO, and the activity is mainly related to the electronegativity of dopant, the reduction and adsorption behaviors of these catalysts. Furthermore, a possible reaction mechanism (schematic diagram) is tentatively proposed to understand this reaction.

Microwave-hydrothermal synthesis of gadolinium-doped nanocrystalline ceria in the presence of hexamethylenetetramine

A new method is proposed for producing nanodisperse ceriabased solid solutions, involving microwave-hydrothermal (MW-HT) treatment of aqueous solutions containing salts of cerium and dopant elements and hexamethylenetetramine (HMTA). The specifics of HMTA hydrolysis under hydrothermal conditions are studied. Xray and electron diffract ion, Raman spectroscopy, and transmission electron microscopy are used to analyze the physicochemical characteristics of Ce1 - xGdxO2 - x/2 (x = 0.10, 0.15, 0.20) nanocrystalline solid solutions produced using the new method.

PVA-assisted synthesis and characterization of nano-crystalline La3+ and Mg2+ co-doped CeO2 electrolyte for intermediate-temperature solid oxide fuel cells

A series of nano-crystalline ceria-based solid solution electrolyte, Ce0.8La0.2−x MgxO2−δ (x = 0.0, 0.05, 0.10, 0.15, and 0.2), were synthesized via the polyvinyl alcohol (PVA) assisted combustion method, and then characterized to the crystalline structure, powder morphology, sintering micro-structure, and electrical properties. Present study showed that Ce0.8La0.2−x Mg x O2−δ was exceedingly stable as a cubic phase in all temperature range and exhibited fine crystals ranging from 15 to 20 nm. After sintering at 1,400 °C, the as-prepared pellets exhibited a dense micro-structure with 96 % of theoretical density. The electrical conductivity was studied using AC impedance spectroscopy and it was observed that the composition Ce0.8La0.1 Mg0.1O2−δ showed higher electrical conductivity of 0.020 S cm−1 at 700 °C. The thermal expansion was measured using dilatometer technique in the temperature range 30–1,000 °C. The average thermal expansion coefficient of Ce0.8La0.1 Mg0.1O2−δ was 12.37 × 10−6 K−1, which was higher than that of the commonly used SOFC electrolyte YSZ (~10.8 × 10−6 K−1).

Preparation and characterization of Al and La co-doped (Ce1−x−y Al x LayO2-(x+y)/2) ceria

Effect of co-doping of lanthanum and aluminum on ionic conductivity of CeO2 for its use as a solid electrolyte in intermediate temperature solid oxide fuel cells has been studied. A few compositions in the system Ce1 − x − y Al x LayO2 − (x + y)/2 (CAL) with x = 0.05, 0.025, and 0.00 and y = 0.00, 0.025, and 0.05 such that x + y = 0.05 are prepared by auto-combustion method. X-ray diffraction patterns show that all the synthesized samples are ceria-based solid solutions. Microstructures of thermally etched samples have been studied by scanning electron microscope. To determine the contribution of grains, σ g and grain boundaries, σ gb to the total conductivity, σ t impedance is measured in the frequency range 1–1 MHz at different steady temperatures in the range 250–500 °C. It has been found that conductivity of the co-doped composition is more than singly doped with Al but less than singly doped lanthanum composition.

Processing and characterization of CoO and Sm2O3 codoped ceria solid solution electrolyte

A series of ceria-based solid solution electrolytes, CoO and Sm2O3 codoped ceria Ce0.8Sm0.2−xCoxO2−δ (denoted as CSO for x=0 and CSCx for x=0.01–0.09), are synthesized via a carbonate co-precipitate method. The phase composition of the precursor is determined by XRD, EA and TG/DSC. The oxidation valence of Co in the samples calcined at different temperatures is probed by XPS and determined to be in +2 valence state at the temperature >1000°C. The sintering investigation demonstrates that the addition of Co promotes densification to occur at lower temperatures with a uniform grain growth. Impedance spectroscopy measurements indicate an improvement in the grain and the grain boundary conductivities for the samples with appropriate Co/Sm ratio. The optimal amount of Co additive is proved to be 7mol%. It shows that CSC0.07 exhibits higher conductivity (σt=0.0538Scm−1 at 600°C) and lower activation energy (Ea=0.567eV) compared with CSO and other CSCx. The results suggest that cobalt oxide could be used not only as an effective sintering aid but as a favorable dopant to ceria as well.

Oxygen vacancy formation in CeO2 and Ce1−xZrxO2 solid solutions: electron localization, electrostatic potential and structural relaxation

Ceria (CeO(2)) and ceria-based composite materials, especially Ce(1-x)Zr(x)O(2) solid solutions, possess a wide range of applications in many important catalytic processes, such as three-way catalysts, owing to their excellent oxygen storage capacity (OSC) through the oxygen vacancy formation and refilling. Much of this activity has focused on the understanding of the electronic and structural properties of defective CeO(2) with and without doping, and comprehending the determining factor for oxygen vacancy formation and the rule to tune the formation energy by doping has constituted a central issue in material chemistry related to ceria. However, the calculation on electronic structures and the corresponding relaxation patterns in defective CeO(2-x) oxides remains at present a challenge in the DFT framework. A pragmatic approach based on density functional theory with the inclusion of on-site Coulomb correction, i.e. the so-called DFT + U technique, has been extensively applied in the majority of recent theoretical investigations. Firstly, we review briefly the latest electronic structure calculations of defective CeO(2)(111), focusing on the phenomenon of multiple configurations of the localized 4f electrons, as well as the discussions of its formation mechanism and the catalytic role in activating the O(2) molecule. Secondly, aiming at shedding light on the doping effect on tuning the oxygen vacancy formation in ceria-based solid solutions, we summarize the recent theoretical results of Ce(1-x)Zr(x)O(2) solid solutions in terms of the effect of dopant concentrations and crystal phases. A general model on O vacancy formation is also discussed; it consists of electrostatic and structural relaxation terms, and the vital role of the later is emphasized. Particularly, we discuss the crucial role of the localized structural relaxation patterns in determining the superb oxygen storage capacity in kappa-phase Ce(1-x)Zr(1-x)O(2). Thirdly, we briefly discuss some interesting findings for the oxygen vacancy formation in pure ceria nanoparticles (NPs) uncovered by DFT calculations and compare those with the bulk or extended surfaces of ceria as well as different particle sizes, emphasizing the role of the electrostatic field in determining the O vacancy formation.