Vanadium dispersion and catalytic activity of Pd/VOx/SBA-15 catalysts in the Wacker oxidation of ethylene

Transmission electron microscopy (TEM), X-ray diffractometry (XRD), in situ diffuse reflectance ultra violet e visible (UVeVis) spectroscopy, and temperature-programmed reduction by hydrogen (H2-TPR) were used to identify the vanadia forms in vanadium-containing SBA-15 preparations (VOx/SBA-15). Wacker type supported Pd/VOx/SBA-15 catalysts were obtained by introducing Pd into VOx/SBA-15 samples using conventional impregnation method. The activity of the catalysts was tested in the gas phase partial oxidation of ethylene by O2 in the presence of H2O (Wacker oxidation). VOx/SBA-15 sample was obtained by micelle-templated synthesis using vanadium-containing synthesis gel. The vanadium became incorporated in the silica structure from the gel in near to atomic dispersion. This catalyst was quite active in ethylene oxidation to CO2 but had low Wacker activity. Isolated, polymeric and bulk vanadia species were identified in the VOx/SBA-15 prepared by wet impregnation/calcination method. The specific surface area of the sample was found to be smaller than that of the neat SBA-15 support because some pores were blocked by vanadia agglomerates. The corresponding Pd/VOx/SBA-15 catalyst showed high selectivity for acetaldehyde formation but the activity was relatively low due to low accessible active surface. A third VOx/SBA-15 sample was obtained by applying directed surface reaction between silanol groups of dehydrated SBA-15 and anhydrous solution of vanadyl acetylacetonate. Large number of accessible Pd/VOx sites were present in the corresponding Pd/VOx/SBA-15 catalyst. Latter catalyst induced ethylene oxidation to acetaldehyde with high yield at temperatures <~160 0C and with good yield to acetic acid at temperatures >~160 0C

Structure and activity of Pd/V2O5/TiO2 catalysts in Wacker oxidation of ethylene

Studies reveal that V2O5 supported on TiO2 is more active in gas phase oxidation reactions by O2 than on Al2O3 or SiO2 support. Nevertheless, the Pd/V2O5/TiO2 catalyst was hardly studied in heterogeneous Wacker-oxidation. The present study concerns preparation of Pd/V2O5/TiO2 catalysts and activity of the catalysts in selective gas phase oxidation of ethylene by O2 in presence of water at atmospheric pressure and in the temperature range of 100-200 oC. The influence of palladium and vanadia loading and the partial pressures of the reactants on the yield of oxidation products (acetaldehyde and acetic acid) were examined. The surface-bound vanadia forms were identified by X-ray diffractometry (XRD), X-ray photoelectron spectroscopy (XPS), and FT-Raman spectroscopy. The best activities were achieved with catalysts, having near to monolayer vanadia coverage of the support. Time-on-stream catalytic tests and chemical analysis of the fresh and used catalysts proved the structural and catalytic stability of the Pd/V2O5/TiO2 preparations. It was shown that under the conditions of Wacker-oxidation primary product acetaldehyde could become further oxidized to CO2, whereas no consecutive oxidation of product acetic acid proceeded

Wacker-oxidation of Ethylene over Pillared Layered Material Catalysts

This paper concerns the Wacker oxidation of ethylene by oxygen in the presence of water over supported Pd/VOx catalysts. High surface area porous supports were obtained from layer-structured materials, such as, montmorillonite (MT), laponite (LT) (smectites), and hydrotalcite (layered double hydroxide, LDH) by pillaring. Before introduction of Pd, supports MT and LDH were pillared by vanadia. The laponite was used in titania-pillared form (TiO2-LAP) as support of Pd/VOx active component. Acetaldehyde (AcH), acetic acid (AcOH) and CO2 were the products with yields and selectivities, depending on the reaction conditions and the properties of the applied catalyst. Under comparable conditions the pillared smectite catalysts gave higher AcH yield than the pillared LDH catalyst. UV vis spectroscopic examination suggested that the pillared smectites contained polymeric chains of VO4, whereas only isolated monomeric VO4 species were present in the pillared LDH. The higher catalytic activity in the Wacker oxidation was attributed to the more favorable redox property of the polymeric than of the monomeric vanadia. The V3+ ions in the polymeric species can reduce O2 to O2- ions, whereas the obtained V5+ ions are ready to pass over O to Pd0 to generate PdO whereon the oxidation of the ethylene proceeds

Cation motion in field loaded PZT ceramics

Zusammenfassung in englischer SpracheAbweichender Titel laut Übersetzung der Verfasserin/des VerfassersBlei-Zirkonat-Titanat (Pb(ZrxTi1-x)O3, kurz PZT) Keramiken wurden seit ihrer Entwicklung in den 1950er Jahren zu einer der wichtigsten ferroelektrischen Materialen. PZT findet man heutzutage in Ultraschallgeneratoren, Lautsprechern, Mikrofonen und Aktuatoren für präzise Positionierung oder in Direkteinspritzsystemen in Verbrennungsmotoren. Bei der Anwendung werden Elektrokeramiken, wie auch das PZT, oft mit hoher Spannung belastet, was zur Ermüdung und Degradation führt. Allerdings fehlt die genaue Aufklärung der Degradationsmechanismen im Fall des PZT. Zur Untersuchung der Degradation des PZT unter Feldbelastung werden Aktuator-Stacks mit Kupferinnelektroden verwendet. Die Proben werden bei 350-550 °C mit hoher DC-Spannung (100-200 V) degradiert, während sie mittels Impedanzspektroskopie und Lichtmikroskopie untersucht werden und der Zustand der Proben wird nach der Degradation mit Impedanzspektroskopie verfolgt. Mit Mikroelektroden werden die kathodischen und anodischen Prozesse getrennt gemessen bzw. die lokalen Leitfähigkeitsänderungen werden nach anodischer und kathodischer Polarisation untersucht. Zur Untersuchung der feldinduzierten (Kat)Ionenbewegung werden außer der Impedanzspektroskopie PZT-Dünnschichten synthetisiert und verwendet. Die Dünnschichten werden mit DC-Spannung polarisiert, die Änderungen der Kationkonzentrationen werden danach mit ToF-SIMS analysiert. Aufgrund der Resultate ist zu schließen, dass die Bleileerstellen bei der gegeben Temperatur unter Feld mobil sind, weiters führt vermutlich die Polarisation zum Aufbauen von Raumladungszonen an Korngrenzen.After its development in the 1950s, lead zirkonate titanate (Pb(ZrxTi1-x)O3, shortly PZT) has become one of the most important ferroelectric materials. PZT is used in ultrasonic generators, loudspeakers, microphones, actuators for precision positioning, or also for direct fuel injection systems in combustion engines. Electroceramics such as PZT are often strained by high electric field, which causes fatigue and degradation. However, degradation mechanisms of PZT are not completely understood. To investigate the degradation of PZT under high field stress, actuator stacks with copper inner electrodes are used. The degradation process is performed at elevated temperatures (350-550 °C) and high DC-voltage (100-200V). During degradation and the subsequent relaxation the sample is characterized by impedance spectroscopy and light microscopy. Using microelectrodes cathodic and anodic processes are measured separately and local conductivity changes can be investigated after cathodic and anodic polarization. For further investigation of field induced (cat)ion movement in addition to impedance spectroscopy on stack material PZT thin films are synthetized and analyzed. The thin films are polarized by DC-voltage, and then the changes of cation concentrations are analyzed by ToF-SIMS. Based on the results it can be concluded that lead vacancies are mobile at given temperatures under field, furthermore, polarization generates space charge layers at grain boundaries.9