E.M. Sadovskaya

Investigation of unsteady-state redox mechanisms over ceria based catalysts for partial oxidation of methane

Publisher Summary Within the perspective of new domains of application like domestic hydrogen production from natural gas for fuel cell, the kinetic behavior of the most performing catalytic systems has to be properly taken into account under transient conditions and at short contact time because it corresponds to frequent operating procedures such as process startup, shutdown, and fast changes in energy demand. To illustrate this new requirement, a transient kinetic approach of the partial oxidation of methane to syngas over Pt/ceria (ceria-zirconia)-based materials for which the authors have already carried out and reported various physicochemical, kinetic, and mechanistic investigations is presented. Ceria-based mixed oxides (Ce x M 1–x O y ) are versatile solid oxygen exchangers. At high temperatures (400–800oC), the redox cycle Ce 3+ ⇔Ce 4+ + e – facilitates oxygen storage and release from the bulk fluorite lattice. When combined with noble or nonnoble metal particles, this makes them ideal candidates for catalytic oxidation applications, such as the partial oxidation of methane (POM) into syngas. However, the surface redox chemistry of ceria is highly sensitive to crystal structure defects, which can be tuned by substituting some of the Ce cations with ions of different size and/or charge as illustrated in this chapter using Zr as a doping cation. This chapter describes the main role of the metal and of the support as a function of the applied transient operating conditions.

Solid Oxide Fuel Cell Cathodes: Importance of Chemical Composition and Morphology

Abstract The main aspects of the cathode materials morphology for Intermediate Temperature Solid Oxide Fuel Cells (IT SOFC) are considered in this paper. The approaches for estimation of their basic properties, e.g. oxygen mobility and surface reactivity, are described and the results of different techniques (e.g. weight and conductivity relaxation, oxygen isotope exchange) application for studies of powders and dense ceramic materials are compared. The Ruddlesden-Popper type phases (e.g. Pr2NiO4) provide enhanced oxygen mobility due to cooperative mechanism of oxygen interstitial migration. For perovskites, the oxygen mobility is increased by doping, which generates oxygen vacancies or decreases metal-oxygen bond strength. Nonadditive increasing of the oxygen diffusion coefficients found that for perovskite-fluorite nanocomposites, it can be explained by the fast oxygen migration along perovskitefluorite interfaces. Functionally graded nanocomposite cathodes provide the highest power density, the lowest area specific polarization resistance, and the best stability to degradation caused by the surface layer carbonization/ hydroxylation, thus being the most promising for thin film IT SOFC design.

On the role of oxygen in the reaction of selective NO reduction with methane over Co/ZSM-5 catalyst

In this work the dynamics of adsorbed NO species formation and their reactivity with respect to methane was studied by DRIFT and SSITKA technique. It was shown that NOΔs formed via NO interaction with adsorbed oxygen are the most active species to react with CH4. Basing on the obtained results scheme of the reaction mechanism was proposed.

Synthesis, physicochemical and catalytic properties of Ni/PrCeZrO catalysts for water-gas shift reaction

Abstract Mixed nanocrystalline ceria-zirconia oxides doped with praseodymium containing 5, 7.5, 10, and 12.5 wt. % nickel were prepared by the incipient wetness impregnation of the oxide support. Complex physicochemical characterization by X-ray diffraction analysis, ultraviolet-visible diffuse reflectance spectroscopy, high resolution transmission electron microscopy and Fourier-transform infrared spectroscopy of adsorbed CO revealed that the nickelcontaining samples are comprised of a solid solution of praseodymium, cerium and zirconium oxides with the fluorite structure as well as nickel oxide particles with a size up to 100 nm. All prepared nanocomposite catalysts show a high catalytic activity in the water-gas shift reaction. The optimum content of nickel in the catalyst providing the maximum activity was found to be 10 wt. %. A high oxygen mobility in these catalysts estimated by the temperature-programmed oxygen isotope heteroexchange with C18O2 provides required coking stability. To eliminate local overheating of the catalyst and decrease the pressure drop in the reactor, as required for further up-scaling, the active component was supported on a metal plate made of Ni-Al foam alloy. At a fixed contact time, the same level of CO conversion with a fraction of the active component was achieved with an approximately 50 wt% loading on the support.

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.

Mixed spinel-type Ni-Co-Mn oxides: synthesis, structure and catalytic properties

Abstract Mixed spinel-type oxides Co1.8Mn1.2O4, Ni0.33Co1.33Mn1.33O4 and Ni0.6Co1.2Mn1.2O4 prepared by thermal decomposition of nitrates have been studied in ethanol steam reforming reaction. Ni0.6Co1.2Mn1.2O4 demonstrated the highest activity among the oxides tested. Specificity of the cation distribution in the samples and their oxygen mobility have been studied by X-ray absorption spectroscopy and oxygen isotope heteroexchange, respectively. Doping of mixed cobalt-manganese spinel with Ni results in Mn redistribution between 3+ and 4+ oxidation states, thus increasing oxygen diffusion coefficient and the catalytic activity.