AbstractPt‐basierte Materialien sind weitverbreitet in der heterogenen Katalyse für die katalytische Entfernung von Schadstoffen. Oft wird der Zustand des aktiven Platins als abhängig von den experimentellen Bedingungen beschrieben: bei niedrigen Temperaturen unterhalb der Light‐off‐Temperatur liegt mit CO vergiftetes metallisches Pt vor, bei höheren Temperaturen oberhalb der Light‐off‐Temperatur oxidiertes Pt. Im Gegensatz zu vorherigen Studien zeigen wir hier, dass metallisches wie auch oxidiertes Pt in ähnlichen Proportionen an der Oberfläche von hoch aktiven, ca. 1 nm großen Nanopartikeln unter Reaktionsbedingungen bei 30 °C vorliegen. Die simultane Gegenwart von metallischem und oxidiertem Pt ermöglicht eine Synergie zwischen den beiden Phasen. Dabei bietet das metallische Pt Adsorptionsstellen für CO, während das oxidierte Pt vermutlich reaktiven Sauerstoff zuführt. Unsere Ergebnisse zeigen die Komplexität von dualen oxidischen‐metallischen Pt‐Katalysatoren und ihre leicht veränderliche Natur unter den jeweiligen Reaktionsbedingungen.

Pt-based materials are widely used as heterogeneous catalysts, in particular for pollutant removal applications. The state of Pt has often been proposed to differ depending on experimental conditions, for example, metallic Pt poisoned with CO being present at lower temperature before light-off, while an oxidized Pt surface prevails above light-off temperature. In stark contrast to all previous reports, we show herein that both metallic and oxidized Pt are present in similar proportions under reaction conditions at the surface of ca. 1 nm nanoparticles showing high activity at 30 °C. The simultaneous presence of metallic and oxidized Pt enables a synergy between these phases. The main role of the metallic Pt phase is to provide strong adsorption sites for CO, while that of oxidized Pt supposedly supplies reactive oxygen. Our results emphasize the complex dual oxidic-metallic nature of supported Pt catalysts and platinum's evolving nature under reaction conditions.

The design of nanoporous perovskite oxides is considered an efficient strategy to develop performing, sustainable catalysts for the conversion of methane. The dependency of nanoporosity on the oxygen defect chemistry and the catalytic activity of perovskite oxides toward CH4 and CO oxidation was studied here. A novel colloidal synthesis route for nanoporous, high-temperature stable SrTi0.65Fe0.35O3-δ with specific surface areas (SSA) ranging from 45 to 80 m2/g and pore sizes from 10 to 100 nm was developed. High-temperature investigations by in situ synchrotron X-ray diffraction (XRD) and TG-MS combined with H2-TPR and Mössbauer spectroscopy showed that the porosity improved the release of surface oxygen and the oxygen diffusion, whereas the release of lattice oxygen depended more on the state of the iron species and strain effects in the materials. Regarding catalysis, light-off tests showed that low-temperature CO oxidation significantly benefitted from the enhancement of the SSA, whereas high-temperature CH4 oxidation is influenced more by the dioxygen release. During isothermal long-term catalysis tests, however, the continuous oxygen release from large SSA materials promoted both CO and CH4 conversion. Hence, if SSA maximization turned out to efficiently improve low-temperature and long-term catalysis applications, the role of both reducible metal center concentration and crystal structure cannot be completely ignored, as they also contribute to the perovskite oxygen release properties.

Two sets of copper oxide–brownmillerite Ca2Fe2O5catalysts are prepared and studied for CO oxidation.

Using a simple slow decomposition method of nitrate precursors, high-surface area platinum-doped ceria with a crystallite size of 9 nm can be prepared. The catalytic performance of the compound can be tuned by changing the reduction temperature under hydrogen (300 degrees C, 500 degrees C and 700 degrees C). The catalyst treated at 300 degrees C shows the best catalytic performance, being active at room temperature. The materials were analysed using a combination of structural characterization methods (X-ray diffraction (XRD), nitrogen physisorption, high angle annular dark field scanning transmission electron microscopy (HAADF-STEM)), surface sensitive methods (X-ray photoelectron spectroscopy (XPS), H-2-chemisorption and H-2-temperature-programmed reduction (TPR)) and X-ray absorption fluorescence spectroscopy (XAFS). HAADF-STEM and XAFS analysis suggests successful doping of platinum in the ceria lattice. After pretreatment at 300 degrees C, the situation is slightly different. While no defined platinum nanoparticles can be identified on the surface, some platinum is in a reduced state (XPS, H-2-chemisorption).

Platinum nanoparticles dispersed on nanosized ceria are active for CO oxidation at room temperature after hydrogen pretreatment. High angular annular dark field scanning transmission electron microscopy (HAADF-STEM) analysis of the reduced catalyst shows spreading of the 1 nm sized platinum particles under the electron beam, characteristic for a two-dimensional strong metal support interaction. In situ X-ray absorption fluorescence spectroscopy (XAFS) reveals a Pt-O distance of 2.1 angstrom, which is significantly longer than the Pt-O distance in PtO2 (2.0 angstrom). This elongated Pt-O distance can be related to interaction of the platinum species with cerium oxide in the form of a low-temperature active species support interaction. These findings contribute to the general understanding of catalytic systems operating at low temperature.

Understanding the role of lattice oxygen in CO oxidation over ceria-based compounds, revealed by the combination of novel isotope-based techniques, ILARS and ILPOR.

Toluene oxidation was measured over Pt nanoparticles synthesized using a modified polyol reduction method and deposited on ionically conductive yttria-stabilized zirconia (Pt/YSZ) for three different loadings (1.1, 0.8, 0.4 %), and non-ionically conductive γ-alumina (Pt/γ-Al2O3) as a comparison (metal loading 0.7 %). It was found that nanoparticles supported on YSZ, tested as a support for the first time for toluene oxidation, have greater catalytic activity compared to a conventional γ-Al2O3 support in spite of a lower specific area and Pt dispersion. This could be explained by the stronger metal-support interactions between Pt and YSZ due to the ionic conductivity of YSZ and presence of oxygen vacancies.