Pd@SnO2 and SnO2@Pd Core@Shell Nanocomposite Sensors

Pd@SnO₂ and SnO₂@Pd core@shell nanocomposites are prepared via a microemulsion approach. Both nanocomposites exhibit high‐surface, porous matrices of SnO₂ shells (>150 m² g^?1) with very small SnO₂ crystallites (<10 nm) and palladium (Pd) nanoparticles (<10 nm) that are uniformly distributed in the porous SnO₂ matrix. Although similar by first sight, Pd@SnO₂ and SnO₂@Pd are significantly different in view of their structure with Pd inside or outside the SnO₂ shell and in view of their sensor performance. As SMOX‐based sensors (SMOX: semiconducting metal oxide), both nanocomposites show a very good sensor performance for the detection of CO and H₂. Especially, the Pd@SnO₂ core@shell nanocomposite is unique and shows a fast response time (τ₉₀ < 30 s) and a very good response at low temperature (<250 °C), especially under humid‐air conditions. Extraordinarily high sensor signals are observed when exposing the Pd@SnO₂ nanocomposite to CO in humid air. Under these conditions, even commercial sensors (Figaro TGS 2442, Applied Sensor MLC, E2V MICS 5521) are outperformed.     

In situ extended X-ray absorption fine structure study during selective alcohol oxidation over Pd/Al2O3 in supercritical carbon dioxide

High-pressure in situ X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) data are reported during the selective oxidation of benzyl alcohol to benzaldehyde in supercritical carbon dioxide over a Pd/Al(2)O(3) catalyst (shell impregnated). For this purpose, a continuous-flow system with a spectroscopic cell suitable for in situ X-ray absorption studies on heterogeneous catalysts up to 200 degrees C and 200 bar has been developed. Due to the high contribution of the dense fluid to the overall X-ray absorption, high stability of the process pressure is mandatory, particularly when recording EXAFS spectra. According to EXAFS and XANES results, the palladium particles were fully reduced after exposure to benzyl alcohol in scCO(2). In contrast to Pd-catalyzed liquid-phase oxidation, a higher oxygen tolerance of the catalyst was observed. Palladium was partially oxidized on the surface under typical reaction conditions (0.9 mol % benzyl alcohol/0.5 mol % O(2) in carbon dioxide), which gradually increased when the concentration of oxygen in the feed was raised. Both XANES and EXAFS data uncovered that palladium is mainly oxidized on the surface or within the outermost layers. These results are in accordance with simulations of the XANES data using the FEFF8.20 code (program for ab initio calculations on multiple scattering XAS) and EXAFS data fitting/simulation.     

XAFS16 at KIT: Five Days of Cutting-Edge X-ray Science in Karlsruhe

The sixteenth international conference on X-ray Absorption Fine Structure, XAFS16, was held August 23–28, 2015, at the Karlsruhe Institute of Technology (KIT) in Germany. Since the first such meeting in 1981, the XAFS conference series has become the foremost international meeting covering developments and applications in X-ray absorption spectroscopy. Jointly organized by KIT, DESY (Hamburg), HZB (Berlin), and the European XFEL (Hamburg), under the auspices of the International X-ray Absorption Society (IXAS), the XAFS16 conference hosted over 550 participants (including more than 160 young scientists) from 37 different countries, establishing a new record for the number of participants. The local organization was provided by the KIT institutes INE, IKFT, and ITCP and the ANKA synchrotron facility, with Jan-Dierk Grunwaldt leading the local organizing committee.     

In situ spectroscopic investigation of heterogeneous catalysts and reaction media at high pressure

In situ characterization of catalysts by means of complementary spectroscopic techniques can be regarded as the first step towards rational catalyst design. Spurred by the growing interest of catalytic reactions in supercritical fluids and by several industrial reactions traditionally performed at high pressure (>10 bar), new demands and challenges are put to in situ spectroscopic characterization of heterogeneous catalytic reactions. In this article, we discuss the development and the use of spectroscopic and related techniques suitable for elucidating such high-pressure reactions. Selected examples from phase behaviour studies with a view cell, investigations with transmission and attenuated total reflection (ATR) infrared spectroscopy as well as X-ray absorption spectroscopy (EXAFS, XANES), are presented to show the strategies, opportunities and limitations of such high pressure in situ studies. Different facets appear to be important to gain insight into catalytic reactions in supercritical fluids: the identification of the phase behaviour of the reaction mixture, the behaviour of the fluid inside the porous catalyst, the processes occurring at the solid-fluid interface, the possible dissolution of active species and, similar as in gas-solid reactions, the establishment of structure-activity relationships.     

Liquid phase oxidation of alcohols with oxygen: in situ monitoring of the oxidation state of Bi-promoted Pd/Al2O3

In situ, time-resolved XAS studies on a Bi-Pd/Al2O3 catalyst indicate that Pd, and Bi located on the Pd surface, are in a reduced, metallic state during the oxidation of 1-phenylethanol with molecular oxygen--a key for understanding the role of promoter in the reaction mechanism.     

Synthesis and Characterisation of Hierarchically Structured Titanium Silicalite-1 Zeolites with Large Intracrystalline Macropores

The successful synthesis of hierarchically structured titanium silicalite-1 (TS-1) with large intracrystalline macropores by steam-assisted crystallisation of mesoporous silica particles is reported. The macropore topology was imaged in 3D by using electron tomography and synchrotron radiation-based ptychographic X-ray computed tomography, revealing interconnected macropores within the crystals accounting for about 30 % of the particle volume. The study of the macropore formation mechanism revealed that the mesoporous silica particles act as a sacrificial macropore template during the synthesis. Silicon-to-titanium ratio of the macroporous TS-1 samples was successfully tuned from 100 to 44. The hierarchically structured TS-1 exhibited high activity in the liquid phase epoxidation of 2-octene with hydrogen peroxide. The hierarchically structured TS-1 surpassed a conventional nano-sized TS-1 sample in terms of alkene conversion and showed comparable selectivity to the epoxide. The flexible synthesis route described here can be used to prepare hierarchical zeolites with improved mass transport properties for other selective oxidation reactions.