Fabrication and Electrochemical Properties of Epitaxial Samarium‐Doped Ceria Films on SrTiO3‐Buffered MgO Substrates

Thin films of samarium‐oxide‐doped (20 mol%) ceria (SDC) are grown by pulsed‐laser deposition (PLD) on (001) MgO single‐crystal substrates. SrTiO₃ (STO) prepared by PLD is used as a buffer layer on the MgO substrates to enable epitaxial growth of the fluorite‐structured SDC film; the STO layer provides a proper crystalline match between SDC and MgO, resulting in highly crystalline, epitaxial SDC films grown in the (001) orientation. Film conductivity is evaluated by electrochemical impedance spectroscopy measurements, which are performed at various temperatures (400–775 °C) in a wide range of oxygen partial pressure (pO₂) values (10^?25?1 atm) in order to separate ionic and electronic conductivity contributions. At 700 °C, SDC/STO films on (100) MgO exhibit a dominant ionic conductivity of about 7 × 10^?2 S cm^?1, down to pO₂ values of about 10^?15 atm. The absence of grain boundaries make the SDC/STO/MgO heterostructures stable to oxidation‐reduction cycles at high temperatures, in contrast to that observed for the more disordered SDC/STO films, which degraded after hydrogen exposure.     

Highly homogeneous multicomponent mesoporous catalysts: Defective amorphous vs. nanocrystalline CeO2 structure towards CO-PROX reaction

Mesoporous multicomponent materials with uniform pore sizes and well-defined network geometries are viewed as highly attractive candidates in the catalysis field. However, their synthesis at a high homogeneity level is considered quite challenging. Herein, alumina based mixed oxides (Al/Ce/Cu or Fe) are produced through a facile evaporation-induced-self-assembly route and tested towards preferential oxidation of CO in H₂-rich gas. The effect of several synthetic parameters is investigated, with citric acid addition identified as a key factor in view of obtaining high mesoscopic order and notable homogeneity, particularly at high dopant amounts. Following thermal aging at 900 °C, metal oxides distribution and nanoporous nature are well-preserved with a parallel nucleation of ceria nanoparticles into the semi-crystalline inorganic framework. CO-PROX assessment of the aged samples reveals a drastic enhancement in catalytic activity, especially for the ternary Cu–Ce–Al system, associated with material's structural reconstruction strongly affecting metal–support interaction.     

Hybrid palladium-ceria nanorod electrocatalysts applications in oxygen reduction and ethanol oxidation reactions in alkaline media

Fossil fuel alternatives are being increasingly studied, and alkaline direct ethanol fuel cells (ADEFC) have acquired importance, as to ethanol is a renewable fuel. In this context, the aims of the present study were to synthesize, characterize and evaluate electrocatalytic activity in oxygen reduction reaction (ORR) and ethanol oxidation reaction (EOR) using hybrid electrocatalysts based on Pd nanoparticles and CeO₂ nanorods supported on carbon black for application in ADEFC. The highest OCV, maximum current and power densities obtained using Pd₁₅(CeO₂ NR)₁₀(Vn)₇₅ as the cathode and Pd₁₀(CeO₂ NR)₂₀(Vn)₇₀ as the anode were 1270 mV, 190 mA cm^?2 and 65 mW cm^?2, respectively. These interesting results are justified by the highest I_D/I_G ratio and ECSA, which suggest a high number of oxygenated species, defects and vacancies in these electrocatalysts and by the synergistic effect between CeO₂ NR and Pd nanoparticles. Therefore, these hybrid electrocatalysts are promising for ADEFC applications.     

Effect of Y-doping on the catalytic properties of CuO/CeO2 catalysts for water-gas shift reaction

CuO/ceria and CuO/Y-doped ceria catalysts were synthesized. The Y-modified supports (1.0, 2.5 and 5.0 wt% Y₂O₃) were prepared by coprecipitation. CuO (3 wt% Cu) was loaded by deposition-precipitation. Having in mind the known effect of Y³⁺ modification for the generation of oxygen vacancies in ceria, its positive role on the water-gas shift (WGS) performance was expected. However, the catalytic test showed a trend of decreased WGS activity by increasing the Y-dopant amount, nevertheless that the differences were not very substantial. On the basis of XRD, XPS, EPR, Raman spectroscopy and H₂-TPR results the explanation related to the key role of the oxygen mobility influenced by Y-doping could be proposed. The reason of the inferior WGS performance with increasing Y-content would be the higher amount of surface oxygen vacancies around Y³⁺ ions which disturbed the Cu–O_vac–Ce active sites for WGS reaction. Though, during the long run catalytic tests in WGS reaction a positive effect of Y-doping for improved stability of CuO/ceria catalysts was evidenced.     

Pd–Cu alloy membrane deposited on CeO2 modified porous nickel support for hydrogen separation

In this study, the hydrogen permeation behavior of a Pd₉₃–Cu₇ alloy membrane deposited on ceria-modified porous nickel support (PNS) was evaluated. PNS, which has an average pore size of 600 nm, was modified by alumina sol. Alumina sol was prepared using precursors that had a mean particle size of 300 nm. Alumina-modified PNS was further treated with ceria sol modification to produce a smoother surface morphology and narrow surface pores. A 7 μm thick Pd₉₃–Cu₇ alloy membrane was made on an alumina-modified PNS and a ceria-finished membrane was fabricated by magnetron sputtering followed by Cu-reflow at 700 °C for 2 h. SEM analysis showed that the membrane deposited on a ceria-finished PNS contained more clear grain boundaries than the membrane deposited on the alumina-modified PNS. The membrane was mounted in a stainless steel permeation cell with a gold-plated stainless steel O-ring. Permeation tests were then conducted using pure hydrogen and helium at temperatures ranging from 673 to 773 K and feed side pressures ranging from 100 to 400 kPa. These tests showed that the membrane had a hydrogen permeation flux of 2.8 × 10⁻¹ mol m⁻² s⁻¹ with H₂/He selectivity of >50,000 at a temperature of 773 K and pressure difference of 400 kPa.     

The role of SiO2 and sintering temperature on the grain boundary properties of Ce0.8Sm0.2O2−δ

The solid solution Ce_0.8Sm_0.2O_2−δ (20CSO) was synthesized by freeze-drying precursor procedure. Well-crystallized powders with nanometric grain sizes were obtained after calcining the precursor at 375 °C for 4 h. The effect of SiO₂-addition and sintering temperature on the properties of the bulk and grain boundary processes were studied. For this purpose, 20CSO-SiO₂ samples were prepared by the addition of 0.05 or 0.5 mol% SiO₂ to Ce_0.8Sm_0.2O_2−δ, in the form of tetraethyl orthosilicate (TEOS). Also, 2 mol% Co was added to some of the precalcined compositions with and without silica-addition. Cobalt free samples were sintered at 1400, 1500 and 1600 °C and cobalt-added samples were sintered 1150 °C, for 10 h to obtain dense pellets. The electrical behaviour of the bulk was revealed to be nearly independent on sintering temperature and/or on the addition of impurities of SiO₂ and Co to the grain boundaries. This was explained by the low solubility of impurities in the grain fluorite structure. However, the grain boundary resistance showed important differences as function of sintering temperature and with the presence of impurities. The analysis of grain boundary properties suggests that segregated impurities affect the microstructure and also segregation of Sm at the space charge layer, thus changing both the specific grain boundary conductivity and microstructural parameters.     

Effect of process modification and presence of H2O2 in the synthesis of samaria-doped ceria powders for fuel cell applications

A fundamental step for a sustainable industrial development based on “H₂ Economy” is the implementation of fuel cell technology, in terms of new devices, materials and convenient processes for their production. Rare earth doped ceria oxides are suitable materials for the new generations of cells and their cost effective production becomes fundamental as the price of rare earths is increasing. In this view, our study investigates a modified method of co-precipitation of Ce_0.8Sm_0.2O_1.9−x (SDC) evaluating the effects of adding of H₂O₂ in the process. The parameters controlled were the molar ratio [H₂O₂]/[M³⁺], (M³⁺ = Ce³⁺, Sm³⁺ present in starting nitrate salts solutions) and the pH of precipitation; in some cases the precipitates were also treated under reflux at 373 K overnight. The powder catalysts, both as fresh precipitates and calcined oxides were analyzed via N₂ adsorption (BET), X-Ray diffraction (XRD) and temperature programmed reduction (TPR) techniques and their morphological, structural and redox properties were correlated with the synthesis parameters used. The electrical conductivity properties of these materials have also been investigated via electrochemical impedance spectroscopy (EIS) and the results compared with those of a commercial oxide. The synthesis approach was shown to be very versatile in the development of materials with properties exploitable for applications in catalysis and in intermediate temperature Solid Oxide Fuel Cell (IT-SOFC) systems.     

Dual-templating fabrication of three-dimensionally ordered macroporous ceria with hierarchical pores and its use as a support for enhanced catalytic performance of preferential CO oxidation

In this study, several three-dimensionally ordered macroporous (3DOM) CeO₂ having hierarchical pore structure were successfully prepared via a dual ‘hard-soft’ templating strategy using Ce(NO₃)₃·6H₂O containing phenol-formaldehyde (PF) resol or/and sucrose as the ceria precursor. The resulting CeO₂ samples were characterized by N₂ adsorption-desorption analysis, scanning electron microscopy, transmission electron microscopy and X-ray diffraction, which showed that the hierarchical 3DOM CeO₂ possessed interconnected networks of the ordered macropore structures with large mesopores, and both the BET surface area and pore volume increased significantly compared with the conventional 3DOM CeO₂. The improved textural parameters should be attributed to the emergence of mesopores in the interconnected three-dimensional skeleton, which were formed by oxidative removal of carbon produced from carbonization of PF resol or sucrose. The hierarchical 3DOM CeO₂ exhibited a superior performance to the conventional 3DOM or bulk CeO₂ when used as supports for Ir catalysts in preferential CO oxidation.     

Formation of ordered nanocrystalline CeO2 structures during thermal decomposition of cerium formate Ce(HCOO)3

In this article, we report the formation of ordered porous nanocrystalline ceria CeO₂ during oxidative thermolysis of cerium formate Ce(HCOO)₃. Ordering of the reaction product occurs due to the presence of similar structural elements in the CeO₂ and Ce(HCOO)₃ crystalline lattices. It was shown that the morphology and the structure of the products of thermal decomposition of Ce(HCOO)₃ are greatly influenced by the composition of the gaseous atmosphere, in which the decomposition takes place. During thermal decomposition of Ce(HCOO)₃ in an atmosphere of argon, no ordering of the synthesized CeO₂ nanocrystals is observed. It was concluded that the presence of oxygen in the gaseous atmosphere is crucial for obtaining ordered nanocrystalline CeO₂ from the Ce(HCOO)₃ precursor.     

Comparison of low and high pressure infiltration regimes on the density and highly porous microstructure of ceria ecoceramics made from sustainable cork templates

Cork templates were used to produce lightweight bulk biomimetic ecoceramic (environmentally conscious ceramic) monoliths. Bulk/monolithic ceramics are vital for many applications, i.e. energy materials and fuel cells. Using simple and flexible, aqueous green-chemistry procedures, for the first time the influence of infiltration regime, number of infiltration cycles and sintering temperature on ecoceramic density and microstructure was studied. This lightweight three-dimensionally ordered macroporous (3DOM) CeO₂ preserved the hexagonal cellular structure of cork, but unlike the wood, the rear cell walls were open, greatly increasing open porosity. Higher sintering temperatures (1600 instead of 1000 °C) were required to produce cm size monolithic ecoceramics mechanically strong enough to be handled. The infiltration regime and number of infiltration cycles affected density and porosity. Lower infiltration pressure led to higher porosity ecoceramics (3.3–5.7%), which may favour catalytic performance, showing the possibility of tailoring porosity and specific surface area by modifying the number of infiltration cycles.