V. Muzykantov

Pt-Supported Nanocrystalline Ceria-Zirconia Doped with La, Pr or Gd: Factors Controlling Syngas Generation in Partial Oxidation/Autothermal Reforming of Methane or Oxygenates

Nanocrystalline CeO2-ZrO2 (Ce:Zr 1:1) samples doped with La, Pr or Gd cations (containing up to 30 at.%) were prepared via the Pechini route. Pt (1.4 wt.%) was supported via impregnation with H2PtCl6 solution followed by drying and calcination. The samples’ surface features were studied by SIMS and FTIRS of adsorbed CO. The oxygen mobility was characterized by the dynamic oxygen isotope exchange and H2 TPR. Catalytic activity was studied in the flow installation using diluted feeds (0.7% CH4 +0.5% O2 or 1% C3H6O + 0.5% O2 +0.5% H2O in He). In the selective oxidation of methane (POM), the catalytic activity correlates with Pt dispersion controlled by the oxidized sample’s ability to stabilize Pt2+ cations as precursors of small reactive Pt clusters formed under reaction conditions. This is favoured by a larger doping cation (La) and a developed network of nanodomain boundaries. At comparable Pt dispersion, the highest performance was demonstrated by a La-doped system, which correlates with the highest surface/near-surface oxygen mobility controlled by the strength of Ce-O bonds in the surface layer. In the autothermal reforming of acetone, the activity trends differ from those in POM because of the more prominent role of the oxygen mobility required to prevent surface coking.

Cathodic materials for intermediate-temperature solid oxide fuel cells based on praseodymium nickelates-cobaltites

A unique combination of methods (TPD of O2, thermogravimetry, isotopic heteroexchange of oxygen in different modes) was used to carry out detailed studies of oxygen mobility and reactivity in mixed praseodymium nickelates-cobaltites (PrNi1 − xCoxO3 + δ) and their composites with doped cerium dioxide (Ce0.9Y0.1O2 − δ) as promising cathodic materials stable towards the effect of CO2 in the intermediate-temperature region. It is shown that in the case of composites of PrNi1 − xCoxO3+δ-Ce0.9Y0.1O2 − δ synthesized using the Pechini method and ultrasonic treatment, stabilization of the disordered cubic perovskite phase due to redistribution of cations between the phases provides high oxygen mobility. Preliminary results on tests of cathodic materials of this type supported on planar NiO/YSZ anodes (H.C. Starck) with a thin layer of YSZ electrolyte and a buffer Ce0.9Y0.1O2 − δ layer showed that power density of up to 0.4 W/cm2 was reached in the region of medium (600–700°C) temperatures, which was close to typical values for fuel cells of this type with cathodes based on strontium-doped perovskites and their composites with electrolytes.

Mechanism of oxygen transfer in layered lanthanide nickelates Ln2 − xNiO4 + δ (Ln = La, Pr) and their nanocomposites with Ce0.9Gd0.1O2 − δ and Y2(Ti0.8Zr0.2)1.6Mn0.4O7 − δ solid electrolytes

The mechanism of oxygen transfer in layered nickelates having a Ruddlesden-Popper structure and their nanocomposites with Ce0.9Gd0.1O2 − δ (GDC) and Y2(Ti0.8Zr0.2)1.6Mn0.4O7 − δ (YTZM) solid electrolytes having fluorite and pyrochlore structures were studied by the oxygen isotope heteroexchange method in a flow and static reactor, thermoprogrammed desorption, and semiempirical interacting bonds method. The experimental heteroexchange data were adequately described by assuming that all atoms were equivalent in exchange in the bulk of layered nickelates, which was consistent with the cooperative oxygen migration model with fast exchange between the interstitial and regular positions. Strong interaction between the domains of the nickelate phases and solid electrolytes in nanocomposites, accompanied by a redistribution of cations between the phases, hindered the cooperative oxygen migration and led to a decrease in the diffusion coefficient as the exchange rate increased.

Synthesis and properties of nanocomposites with mixed ionic-electronic conductivity on the basis of oxide phases with perovskite and fluorite structures

Nanocomposites consisting of phases with fluorite (doped CeO2) and perovskite (LaMnO3, GdMnO3) structures are synthesized using the method of ester polymeric precursors (the Pechini method) and two sources of rare-earth elements (Ln), such as pure cerium and gadolinium salts or a commercial mixture of rare-earth carbonates containing La, Ce, Pr, Nd, and Sm cations. The genesis of the nanocomposite structure as a function of the sintering temperature is investigated using X-ray diffraction and electron microscopy. It is revealed that the genesis of the nanocomposite structure is governed, in many respects, by the fact that the decomposition of the ester polymeric precursor leads to the formation of a metastable phase, namely, a fluoritelike solid solution based on ceria with an excess concentration of the cations Ln3+ (Ln3+ = La3+, Pr3+, Nd3+, Sm3+) as compared to the equilibrium concentration. As a result, the perovskite phase (identified by X-ray diffraction analysis) is formed only after the subsequent annealing at temperatures higher than 800°C, when Ln3+ cations escape from particles of the solid solution. It is demonstrated that, at annealing temperatures of up to 1100°C, particles of both phases have nanometer sizes and are characterized by a uniform spatial distribution necessary for percolation. The nanocomposites possess a high total electrical conductivity and a high mobility of lattice oxygen. The reduction rate of the nanocomposites with hydrogen or methane is higher than the reduction rate of the individual phases. The characteristics of the nanocomposites prepared from the commercial mixture of rare-earth carbonates are better than those of the samples synthesized from the pure salts.

Studies of oxygen transport mechanism in electrolytes based on doped lanthanum silicate with apatite structure using techniques of oxygen isotopic heteroexchange and impedance spectroscopy

The work presents the results of studying the mechanism of oxygen transport for a new promising class of oxygen-containing electrolytes based on lanthanum silicate with an apatite structure using impedance spectroscopy and isotopic oxygen heteroexchange. At 1000 K, in the case of samples with an optimum composition including codoped Fe and Al, σ ∼ 3 × 10−3 to 10−2 S/cm and D* reaches ∼10−8 cm2/s, which is close to the values of YSZ and Ce0.9Gd0.1O2 − δ (GDC). Lower energies of conductivity activation and oxygen diffusion for doped apatites (∼0.5–0.8 eV instead of ∼1 eV for GDC) and also equivalence as regards exchange of all oxygen atoms within apatite agree with the model, in which oxygen mobility is determined by a nonlinear cooperative migration process of oxygen atoms with fast exchange between interstitial and regular sites.

Mobility and Reactivity of the Surface and Lattice Oxygen of Some Complex Oxides with Perovskite Structure

Mobility and reactivity of the surface and bulk oxygen of perovskite-like mixed oxides including lanthanum manganite (I) and ferrite (II) systems modified by Ca (I,II) and fluorine (I), as well as some Co, Fe-containing complex perovskites were considered. Combination of thermal analysis data, oxygen isotope exchange, O2 TPD, reduction by CO, H2 and CH4 TPR, were applied to characterize the accessible surface/bulk oxygen mobility and reactivity. Comparison of these results with earlier data on the real (defect) structure of these systems by TEM, EXAFS, XRD, FTIRS, SIMS allowed to elucidate factors determining the oxygen mobility and reactivity. A quantitative description of the experimental energetic spectrum of oxygen bound with regular and defect surface sites of perovskites was obtained by using the semiempirical Interacting Bonds method with a due regard for the surface face termination and relaxation. Pronounced effect of cation vacancies on the activation barrier for the oxygen migration in the perovskite lattice has been revealed.

Advanced Sintering Techniques in Design of Planar IT SOFC and Supported Oxygen Separation Membranes

Thin film solid oxide fuel cells (SOFC) operating in the intermediate temperature (IT) range are now considered as promising for distributed, mobile, standby or auxiliary power generation. At present one of the most important scientific aims in design of solid oxide fuel cells is to lower the operating temperatures to 600-800 С. In this temperature range, majority of problems inherent to SOFC operating at high (950-1000 C) are alleviated. Thus, cations interdiffusion and solid state reactions between electrolyte and electrodes are hampered and thermal stresses are decreased which prevent degradation of the functional layers [Yamamoto, 2004 ]. Hence, design of thin film SOFC requires also elaboration of nanostructured electrodes compatible with electrolytes from chemical and thermophysical points of view and providing a developed three-phase boundary (TPB). In this respect, broad options are provided by design of nanocomposite mixed ionic-electronic conducting (MIEC) functional layers – (Sadykov et al., 2010; Sadykov et al., 2009; Sadykov et al., 2008).

Methane selective oxidation into syngas by the lattice oxygen in ceria-based solid electrolytes promoted by Pt

Abstract Nanocomposites based upon ceria doped by Zr, Zr+La or Sm with supported Pt nanoparticles efficiently convert methane into syngas by their lattice oxygen. In red-ox cycles with pure methane as reagent, the surface carbon build-up is observed, which is lower for Sm -doped samples possessing a higher surface and lattice oxygen mobility. When stoichiometric amounts of CO 2 or H 2 O are present in the feed, the catalysts efficiently operate in methane steam and dry reforming at high space velocities without deactivation.

Structural Features and Transport Properties of La(Sr)Fe1-xNixO3-δ– Ce0.9Gd0.1O2-δ Nanocomposites—Advanced Materials for IT SOFC Cathodes

Mixed ionic−electronic conducting nanocomposites comprising complex oxides - perovskite (lanthanum-strontium nickelate-ferrite [LSFN]) and gadolinium-doped ceria (GDC) have been prepared via ultrasonic dispersion of nanocrystalline powders of LSFNx and GDC in organic solvent with addition of surfactant, followed by drying and sintering up to 1300°C. Their structural and surface properties have been studied by x-ray diffraction, ultraviolet–visible (UV-vis) electron spectroscopy, transmission electron microscopy with elemental analysis, and x-ray photoelectron spectroscopy. Results of impedance spectroscopy, oxygen isotope exchange, O2 temperature-programmed desorption, weight, and conductivity relaxation experiments have revealed a strong positive effect of perovskite−fluorite nanodomain interfaces in composite on the oxygen mobility and reactivity. Testing in wet H2/air feeds for a button-size cell with functionally graded LSFN0.4–GDC cathode layer supported on a thin YSZ layer covering Ni/YSZ cermet has demonstrated high and stable performance, promising for the practical application in the intermediate temperature range.

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

ABSTRACT This 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.