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Penkala B.,Charles Gerhardt Institute | Penkala B.,Saint - Gobain | Aubert D.,Saint - Gobain | Kaper H.,Saint - Gobain | And 3 more authors.
Catalysis Science and Technology | Year: 2015

Ceria-based materials are today the most prominently used catalyst supports for CO oxidation and NOx reduction in three-way catalytic converters (TWCs) worldwide. Acting as oxygen buffer compounds, the underlying reaction mechanism and especially the distinct role of surface and lattice oxygen for catalytic reactions are still under debate. This is partially related to the complexity of the real CeO2 surface containing important amounts of water and carbonates. Combining TG-MS, Raman spectroscopic experiments and isotope labeling pulse temperature programmed oxidation reaction (ILPOR), coupled with mass spectrometric analysis of 18O-doped ceria, we explored here the oxygen uptake/release behavior under operando conditions, together with the catalytic activity related to surface and/or lattice oxygen mobility and exchange. Specific changes in the lattice dynamics induced by 18/16O isotope exchange were analyzed by Raman spectroscopy, allowing the temperature-dependent onset of lattice oxygen mobility and isotope exchange behavior to be studied selectively. For Pt-supported nano-ceria, we evidenced high catalytic performance for CO oxidation, activated slightly above ambient conditions without significant lattice oxygen participation. The distinct role of surface and lattice oxygen in the catalytic reaction of ceria catalysts is discussed as a function of temperature, grain size, Gd doping and Pt impregnation. © The Royal Society of Chemistry. Source


Pikart P.,TU Munich | Hugenschmidt C.,TU Munich | Horisberger M.,Laboratory for Developments and Methods | Matsukawa Y.,Tohoku University | And 3 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2011

Defect-sensitive and element-selective measurements on ultrathin chrome, copper, and gold layers embedded in aluminium are presented using coincident Doppler broadening spectroscopy (CDBS) with a monoenergetic positron beam. The amounts of positrons implanted in the layers of different thicknesses are calculated and compared with the experimentally gained fractions of positrons annihilating in the buried layers. A high sensitivity was already reached at an Au layer of only 2 nm thickness embedded below 200 nm Al, which was attributed to the highly efficient positron trapping in the Au layer and Au clusters. An implantation and diffusion model describes this high sensitivity for positron trapping layers. A quantum-well model of the positron wave function limits the trapping to gold clusters of a radius larger than 0.23 nm. This result was confirmed experimentally and validated with complementary TEM measurements. © 2011 American Physical Society. Source


Penkala B.,Charles Gerhardt Institute | Penkala B.,Saint - Gobain | Aubert D.,Saint - Gobain | Kaper H.,Saint - Gobain | And 3 more authors.
Catalysis Science and Technology | Year: 2015

Ceria-based materials are today the most prominently used catalyst supports for CO oxidation and NOx reduction in three-way catalytic converters (TWCs) worldwide. Acting as oxygen buffer compounds, the underlying reaction mechanism and especially the distinct role of surface and lattice oxygen for catalytic reactions are still under debate. This is partially related to the complexity of the real CeO2 surface containing important amounts of water and carbonates. Combining TG-MS, Raman spectroscopic experiments and isotope labeling pulse temperature programmed oxidation reaction (ILPOR), coupled with mass spectrometric analysis of 18O-doped ceria, we explored here the oxygen uptake/release behavior under operando conditions, together with the catalytic activity related to surface and/or lattice oxygen mobility and exchange. Specific changes in the lattice dynamics induced by 18/16O isotope exchange were analyzed by Raman spectroscopy, allowing the temperature-dependent onset of lattice oxygen mobility and isotope exchange behavior to be studied selectively. For Pt-supported nano-ceria, we evidenced high catalytic performance for CO oxidation, activated slightly above ambient conditions without significant lattice oxygen participation. The distinct role of surface and lattice oxygen in the catalytic reaction of ceria catalysts is discussed as a function of temperature, grain size, Gd doping and Pt impregnation. This journal is © The Royal Society of Chemistry. Source


Svitlyk V.,European Synchrotron Radiation Facility | Chernyshov D.,European Synchrotron Radiation Facility | Pomjakushina E.,Laboratory for Developments and Methods | Krzton-Maziopa A.,Laboratory for Developments and Methods | And 3 more authors.
Inorganic Chemistry | Year: 2011

Temperature-dependent synchrotron powder diffraction on Cs 0.83(Fe0.86Se)2 revealed first-order I4/m to I4/mmm structural transformation around 216 °C associated with a disorder of the Fe vacancies. Irreversibility observed during the transition is likely associated with a mobility of the intercalated alkali atoms. Pressure-dependent synchrotron powder diffraction on Cs0.83(Fe1-ySe) 2, Rb0.85(Fe1-ySe)2, and K 0.8(Fe1-ySe)2 (y ∼ 0.14) indicated that the I4/m superstructure reflections are present up to pressures of 120 kbar. This may indicate that the ordering of the Fe vacancies is present in both superconducting and nonsuperconductive states. © 2011 American Chemical Society. Source


Bosak A.,European Synchrotron Radiation Facility | Svitlyk V.,European Synchrotron Radiation Facility | Svitlyk V.,University of Munster | Krzton-Maziopa A.,Laboratory for Developments and Methods | And 6 more authors.
Physical Review B - Condensed Matter and Materials Physics | Year: 2012

A xFe 2-ySe 2 (A = K, Rb, Cs) superconductors are frequently complex mixtures of phases even in the form usually described as a single crystal. Phases, resulting from phase separation, metrically are very similar, so their detailed characterization is a delicate task, and the existing data are typically scarce. We characterize the room-temperature phase-separated state of Cs 0.8Fe 1.6Se 2 in reciprocal space by means of single-crystal diffraction of synchrotron radiation. Using the arguments of commensurability and detailed analysis of twinning patterns, we augment the previous findings by quantifying the intergrowth state, consisting of the tetragonal phase with ordered Fe vacancies and the minor disordered phase. Compared to the main phase, the minor one is compressed in the tetragonal a-b plane and expanded along the c direction; a set of modulated Bragg rods evidences a planar disorder. Fourfold splitting of the rods and main Bragg peaks implies a rotational twinning; close inspection of the lattice metric indicates that the symmetry of the minor phase is not higher than monoclinic, with a deviation from the orthogonal basis of ∼0.25°. © 2012 American Physical Society. Source

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