Partial Substitution Of Ce Research Articles

The influence of partial substitution of Ce with K in CeMO3 (M = Mn, Fe, Co, Ni, Cu) perovskite catalysts on soot combustion performance

A series of Ce1−xKxMO3 (M = Mn, Fe, Co, Ni, Cu, and x = 0, 0.1, 0.2, 0.3, 0.4) perovskite catalysts were synthesized by citrate sol-gel method, in which A-site element Ce was partially substituted with K. The results showed that the catalytic performance could be significantly improved by partially substituting of Ce with K in CeCoO3 catalyst for soot combustion. Ce0.8K0.2CoO3 perovskite catalyst exhibited a superior catalytic activity, and its corresponding T10, T50, T90 and SmCO2 were 326, 383, 426 °C and 98.6%, respectively. The characterization results indicated that K+ had been successfully incorporated in the perovskite crystal structure. Partially substituting of Ce with K resulted in an increase in specific surface area. It also resulted in the formation of Co4+ in the perovskite catalysts together with abundant presence of oxygen vacancies on the surface. Moreover, it was of great significance to facilitate the activation of lattice oxygen species. In situ DRIFTS result showed that NO2 produced by thermal decomposition of monodentate nitrate species played a key role in soot combustion process.

Development of cost-effective NdFeB permanent magnets by partial substitution of Ce and La

Development of cost-effective NdFeB permanent magnets by partial substitution of Ce and La

Clarifying the basic phase structure and magnetic behavior of directly quenched (Ce,La)2Fe14B alloys with various Ce/La ratios

To improve the performance/cost ratio of NdFeB based permanent magnets, Ce or/and La substitutions for Nd have been suggested. To better understand the effects of these substitutions, the fundamental behavior of (Ce,La)-Fe-B alloys has to be clearly understood. Here, we reported a systematic investigation on the phase structure, microstructure, and magnetic properties of melt-spun (Ce,La)2Fe14B alloys. The results confirm that partial substitution of Ce by La can effectively enhance the hard magnetism and thermal stability of (Ce1-xLax)2Fe14B alloys, while over 80% La substitution leads to the decomposition of 2:14:1 phase. The lattice parameters a, c and the Curie temperature Tc of 2:14:1 phase increase linearly with the increasing La content. La substitution can effectively refine the grain, resulting in the enhancement of inter-grain exchange coupling. The (Ce0.7La0.3)2Fe14B alloy with a mean grain size of 25 nm exhibits high remanence, maximum energy product and intrinsic coercivity up to 0.69 T, 6.2 MGOe and 217 kA/m, respectively. The present work provides a good understanding on the melt-spun (Ce,La)-Fe-B system for further developing low cost rare earth permanent magnets.

Magnetic hardening of Nd-Ce-Fe-B films with high Ce concentration

Partial substitution of Ce in Nd-Fe-B magnets is a feasible way to cope with the crisis of Nd and Dy in Nd-Fe-B production and reduce the cost of Nd-Fe-B magnets. In the present paper, the Nd-Ce-Fe-B films with high performance have been successfully fabricated by using an ultra-high vacuum (UHV) magnetron sputtering system. High magnetic performance with a ceorcivity of 13.3 kOe, a remanence of 11.4 kGs and a maximum energy product of 29.4 GMOe is obtained with the Ce substitution for more than 50 wt.% Nd without Dy addition. The high coercivity and (BH)max achieved in this work are much larger than those of previously reported Nd-Ce-Fe-B magnets with the same Ce concentration. The phase structure, microstructure and coercivity mechanism are analyzed. The coercivity mechanism is determined to be mainly dominated by nucleation. Based on the microstructure observation and coercivity mechanism analysis, the fine and well separated grains, smooth grain surface, small and less inhomogeneities should be responsible for the high coercivity. Our results encourage the further improvement of magnetic properties in Ce magnets including the bulk material with high Ce concentration.

Effect of addition of elements on two-glass-forming ability of Zr–Ce–Co–Cu immiscible alloys

ABSTRACTThe effects of Al element addition and partial substitution of Ce with La, Pr, and Nd on the two-glass-forming ability of the phase-separating Zr–Ce–Co–Cu alloy system have been studied. The distribution of the additions in the two coexistent immiscible liquids was analysed thermodynamically. The results indicate that Al almost equally distributes in the two liquids, whereas the elements La, Pr, and Nd are predominantly found in the Ce–Cu-rich liquid. X-ray diffraction analysis reveals that these additions obviously enhance the two-glass-forming ability of the coexisting liquids. This work presents a strategy for improving the two-glass-forming ability of immiscible alloys to obtain phase-separated bulk metallic glasses.

Ionic/Electronic Conductivity, Thermal/Chemical Expansion and Oxygen Permeation in Pr and Gd Co-Doped Ceria PrxGd0.1Ce0.9-xO1.95-δ

The oxygen permeation flux of Ce0.9Gd0.1O1.95-δ (CGO)-based oxygen transport membranes under oxidizing conditions is limited by the electronic conductivity of the material. This work aims to enhance the bulk ambipolar conductivity of CGO by partial substitution of Ce with the redox active element Pr. A series of compositions of PrxGd0.1Ce0.9-xO1.95-δ (x = 0, 0.02, 0.05, 0.08, 0.15, 0.25, 0.3 and 0.4) was prepared by solid state reaction. X-ray powder diffraction (XPD) indicates that Pr is completely dissolved in the fluorite structure up to 40 at.%. Pronounced nonlinear thermal expansion behavior was observed as a function of temperature, due to the simultaneous contributions of both thermal and chemical expansion. The electronic and ionic conductivities were measured as a function of temperature and oxygen partial pressure. Within the range from 10 to 15 at.% Pr, a drastic drop of the activation energy of the hole mobility and an abrupt increase of the hole conductivity at low temperature was observed. The behavior could be rationalized by a simple percolation model. Oxygen permeation fluxes through disk shaped samples fed with air on one side and N2 on the other side were also measured. The oxygen flux through Pr0.05Gd0.1Ce0.85O1.95-δ was higher than that for CGO by one order of magnitude owing to the enhanced electronic conductivity albeit the flux is still limited by the electronic conductivity. In terms of the electronic and ionic conductivity, the estimated maximum oxygen permeation flux of a 10 μm Pr0.4Gd0.1Ce0.9O1.95-δ -based membrane exceeds 10 Nml cm−2 min−1 at 900°C under a small oxygen potential gradient (0.21/10−3 bar) which is promising for use in oxygen production and in oxy-fuel combustion. Also the material may be well applicable to SOFC/SOEC composite electrodes where mixed conductivity is also desirable.

Hydrogen permeability and stability of BaCe0.85Tb0.05Zr0.1O3−δ asymmetric membranes

A mixed proton and electron conductor BaCe0.85Tb0.05Zr0.1O3−δ (BCTZ) has been developed by the partial substitution of Ce with Zr in BaCe0.95Tb0.05O3−δ (BCT) to improve the phase structure stability of BCT. The BCTZ membranes with asymmetric structure have been successfully prepared and evaluated for hydrogen separation. A stable hydrogen permeation flux has been found over 370h operation. The improved stability recommends BCTZ for a potential application in the hydrogen separation.

Anomalous specific heat, magnetic, and electrical transport properties around magnetic instability in Ce3−xLaxCo4Sn13

Ce3Co4Sn13 exhibits interesting physical properties including a large increase in the magnetic specific heat, ΔC(T)/T, at low-temperatures with a maximum value of about 4JK−2molCe−1 and a cross-over from heavy fermion behavior with antiferromagnetic correlations at low magnetic field to single impurity Kondo behavior by applying the field larger than ∼3 T. In order to study the expected quantum criticality of Ce3Co4Sn13 we investigated electrical resistivity, magnetoresistivity, magnetic susceptibility and specific heat of Ce3−xLaxCo4Sn13 and showed that partial substitution of Ce by La does not lead to long range magnetic order, and the low-temperature properties are not typical of the quantum critical point (QCP). The magnetoresistivity of Ce3−xLaxCo4Sn13 has a conventional Kondo character. With increasing of the magnetic field resistivity follows power-law behavior over one decade in temperature for the samples with x ≠ 0 of the series, and exponent n in ρ ∼ Tn is strongly field dependent. The low-temperature specific heat of Ce3−xLaxCo4Sn13 exhibits evolution from a magnetically correlated heavy fermion state to a single impurity state at the critical concentration x = xC = 0.6, however, for xC the C(T)/T behavior is not typical for QCP. The low-temperature resistivity and magnetic susceptibility data give evidence for changeover from Kondo lattice to a single-ion Kondo behaviour at xC, when the La content in Ce3−xLaxCo4Sn13 increases.

Rare Earth‐doped Ceria Catalysts for ODHE Reaction in a Catalytic Modified MIEC Membrane Reactor

AbstractAn intensification process for the selective oxidation of hydrocarbons integrates a catalytic reactor and an oxygen separation membrane. This work presents the study of oxidative dehydrogenation of ethane at 1123 K in a catalytic membrane reactor based on mixed ionic‐electronic conducting (MIEC) membranes. The surface of a membrane made of Ba0.5Sr0.5Co0.8Fe0.2O3−δ has been activated using different porous catalytic layers based on rare earth‐doped cerias (fluorite structure) and the porous catalytic coating was deposited by screen printing (coating around 15 μm). The different catalyst formulations were developed by partial substitution of Ce and were synthesized by co‐precipitation followed by cobalt impregnation when required. Specifically, seven different catalysts based on the system Ce1−xLnxO2−δ (x=0.1 or 0.2; Ln=Tb, Pr, Er, Gd, and Tb+Er), including the effect of cobalt addition (2 % molar) in Ce0.8Tb0.2O2−δ, were studied. The ceria catalysts were studied by XRD, SEM, DC‐conductivity as a function of oxygen partial pressure, and the high‐temperature stability in a CO2 environment was assessed using thermogravimetry. Then, the influence of the ceria catalytic coating on the oxygen permeation flux through the MIEC membrane was studied using argon and methane as the sweep gas in the permeate side. Finally, oxidative dehydrogenation of ethane reaction tests were performed at 1123 K, as a function of the ethane concentration in the feed. The use of a disk‐shaped membrane in the reactor made it possible to prevent the direct contact of gaseous oxygen and hydrocarbons and thus to increase the ethylene yield. High ethylene yields (up to ≈84 %) were obtained using a catalytic coating based on 20 % Tb‐doped ceria including macropores produced by the addition of graphite platelets in the screen printing ink. The high yields obtained in this kind of catalytic membrane are attributed to the combination of: the high catalytic activity; the control of the oxygen concentration in the gas phase (reaction chamber); and the appropriate fluid dynamics, enabling the fast ethylene evacuation.

Effects of electron doping on the structure and magnetic properties in Ca2−xCexFeMoO6 double perovskites

A sol–gel method has been used to prepare a series of polycrystalline Ca 2− x Ce x FeMoO 6 ( x = 0–0.4) double perovskites. Rietveld refinement of X-ray diffraction data indicates that although cell parameters increase slightly, the symmetry of the crystalline structure is preserved upon Ce doping. The partial substitution of Ce 3+ for Ca 2+ reduces the saturation magnetization, however it considerably enhances the Curie temperature ( T C ) from 365 K for x = 0 to 393 K for x = 0.4. Transport measurement shows that Ca 2− x Ce x FeMoO 6 exhibits metallic behavior, and the resistivity of Ca 2− x Ce x FeMoO 6 increases first, and then decreases with the increasing of x . The behavior of the increasing of T C and the decreasing of resistivity upon Ce doping was explained by the electron doping effects.

BaCe1-x-yZrxYyO3-d Proton Conductors: The Role of the Synthetic Route on their Properties

Proton conducting electrolyte materials are promising candidates for the development of low temperature solid oxide fuel cells. Among them, various doped barium cerate compounds, such as BaCe1-xYxO3-δ, have been intensively investigated because of their high proton conductivity. As a drawback, cerates are chemically unstable in CO2-containing atmospheres, such as in case of hydrocarbon or syngas fuelled fuel cells. Conversely, doped barium zirconates electrolyte materials exhibit high chemical stability but lower proton conductivity. A goal of this work was to obtain full-density proton conducting materials having both high conductivity and good chemical stability. In particular, the compositions investigated in this work were BaCe1-x-yZrxYyO3-δ (x=0, 0.1, 0.2, 0.3, 0.4; y=0.15, 0.20). The partial substitution of Ce by Zr in doped barium cerate considerably increased their chemical stability in CO2 atmosphere without sensibly affecting the conductivity.

Investigation of electrical conductivity as a function of dopant-ion radius in the systems Zr 0.75Ce 0.08M 0.17O 1.92 (M=Nd, Sm, Gd, Dy, Ho, Y, Er, Yb, Sc)

Electrical conductivity versus dopant ionic radius studies in zirconia- and ceria-based, solid oxide fuel cell (SOFC) electrolyte systems have shown that oxygen-ion conductivity is highest when the host and dopant ions are similar in size [J. Am. Ceram. Soc. 48 (1965) 286; Solid State Ionics 37 (1989) 67; Solid State Ionics 5 (1981) 547]. Under these conditions, it is thought that the conduction paths within the crystal lattice become less distorted [Solid State Ionics 8 (1983) 201]. In this study, binary ZrO 2–M 2O 3 unit cells were expanded, via the partial substitution of Ce +4 for Zr +4 into the lattice, in an attempt to identify new, ternary, zirconia/ceria-based electrolyte systems with enhanced electrical conductivity. The compositions Zr 0.75Ce 0.08M 0.17O 1.92 (M=Nd, Sm, Gd, Dy, Ho, Y, Yb, Sc) were prepared using traditional solid state techniques. Bulk phase characterisation and precise lattice parameter measurements were performed with X-ray diffraction techniques. Four-probe DC conductivity measurements between 400 and 900 °C showed that the dopant-ion radius influenced electrical conductivity. The conductivity versus dopant-ion radius trends previously observed in zirconia-based, binary systems are clearly apparent in the ternary systems investigated in this study. The addition of ceria was found to have a negative influence on the electrical conductivity over the temperature range 400–900 °C. It is suggested that distortion of the oxygen-ion conduction path by the presence of the larger M +3 and Ce +4 species (relative to Zr +4) is the reason for the decreasing electrical conductivity as a function of increasing dopant size and ceria addition, respectively.

Thermodynamic particularities of some CeNi5-based metal hydride systems with high dissociation pressure

The thermodynamic behaviour of the CeNi5H2, Ce0.8La0.2Ni5H2 and Ce0.8La0.2Ni4.7Cu0.3-H2 systems during a few activation absorption-desorption cycles was studied by means of high gaseous pressure technique. P-C isotherms in the temperature range from 273 to 353 K were measured and thermodynamic parameters for hydride formation and decomposition reactions were calculated. For all studied systems the same hydrogen capacity (6.5–6.8 H atoms per AB5 unit at 1000 atm and 296 K) and the same isotherm shape with a long nearly horizontal plateau and very large hysteresis were obtained. The change in alloy composition in the range CeNi5 →Ce0.8La0.2Ni5 →Ce0.8La0.2Ni4.7Cu0.3 led to an increase in plateau length and a decrease in hysteresis factor Pabs/Pdes. The hysteresis factor also decreased considerably after two or three activation cycles, by 4–8 times as compared with the first cycle. This phenomenon was connected not only with an absorption pressure decrease but also with a significant increase in desorption pressure. The relative change P1/Pact in equilibrium pressure during the activation did not depend on alloy composition for the absorption reaction, but in the case of desorption it decreased with partial substitution of Ce and Ni by La and Cu correspondingly.

CeO2-based solid solutions with the fluorite structure as novel and effective catalysts for methane combustion

The partial substitution of Ce in CeO2 with isovalent elements like Hf and Zr greatly increases the overall activity of methane combustion measured as light-off and ignition temperature; the presence of a defective fluorite-structured oxide is recognized as a key factor in the activity enhancement.

Superconductivity in compound Y1-2x Ce x Ba2+x Cu3O6·5+δ system

Partial substitution of Ce in place of Y has been achieved by taking care of charge compensation and compounds with the general formula Y1-2 xCexBa2+ xCu3O6·5+δ have been successfully prepared. X-ray powder diffraction analysis reveals that this substitution gives orthorhombic single phase for 0⩽x⩽0·3. TheT c remains nearly unchanged between 94-90 K.