No abstract

The effect of the size of gold particles deposited on CeO 2 and TiO 2 supports on their catalytic behavior in the aerobic oxidation of benzyl alcohol in different solvents (mesitylene, toluene, and supercritical carbon dioxide) has been investigated. The size of supported gold particles deposited via a colloidal route was in the range 1.3-11.3 nm, as determined by means of EXAFS and HAADF-STEM measurements. The catalytic performance of the supported gold catalysts in the different solvents revealed a significant effect of the gold particle size. Optimal activity was observed for catalysts with medium particle size (ca. 6.9 nm) whereas smaller and bigger particles showed inferior activity. Identical trends for the activity-particle size relationship were found using Au/CeO 2 and Au/TiO 2 for the reaction at atmospheric pressure in conventional solvents (mesitylene, toluene) as well as under supercritical conditions (scCO 2 ). Selectivity to benzaldehyde was only weakly affected by the gold particle size and mainly depended on reaction conditions. In supercritical CO 2 (scCO 2 ) selectivity was higher than in the conventional solvents under atmospheric pressure. All catalysts tested with particle sizes ranging from 1.3 to 11.3 nm showed excellent selectivity of 99% or higher under supercritical conditions.

No abstract

We present a spatiotemporal operando X-ray absorption study of a highly dynamic process, the ignition of the noble metal catalyzed partial oxidation of methane. Evolvement and propagation of the platinum component's structural changes are investigated with a high-speed X-ray camera, which in combination with temperature profiling by IR-thermography and catalytic activity measurements by online mass spectrometry gives insight into the first stages of the ignition of the reaction toward hydrogen and carbon monoxide.Ignitions or light-offs of heterogeneously catalyzed oxidation reactions are a special challenge for operando studies on catalytic reactors, which have gained increasing attention for the identification of active sites of catalysts and optimal reactor design. 1-6 Their understanding is important both for the verification of reaction models and for the optimization of reaction parameters. 7,8 This requires spatiotemporal studies in the subsecond time scale. Crucial aspects are possible changes in structure or oxidation state, correlated to product compositions as well as mass and heat transfer.One of these reactions is the catalytic partial oxidation (CPO) of hydrocarbons to carbon monoxide and hydrogen, which is considered an important alternative to presently utilized processes in natural gas and biomass conversion such as steam and autothermal reforming. 9 Besides pure simulation 10 or spectroscopic and kinetic studies, 11-14 temperature profiling 15 and mass spectrometry 16 were undertaken to gain a clearer picture of the processes occurring during ignition and light-off of catalytic partial oxidation reactions.Herein we investigate the ignition process of the noble-metal catalyzed CPO of methane in a fixed-bed capillary microreactor with focus on the oxidation state of the noble metal and the temperature of the catalyst bed. Starting with an untreated 2.5 wt % Rh-2.5 wt % Pt/Al 2 O 3 catalyst prepared by flame spray pyrolysis 17 and monitoring by XAS and online mass spectrometry during heat-up in the reaction gas mixture (CH 4 /O 2 ) 2/1; here, 6% CH 4 /3% O 2 /He), the only detectable products below the ignition temperature are CO 2 and H 2 O and the whole catalyst bed stays oxidized. Above the ignition temperature both scanning X-ray absorption spectroscopy and full field X-ray absorption spectroscopic microscopy have shown that the catalyst bed consists of oxidized noble metal species in the inlet zone and reduced noble metal species in the end zone of the catalyst bed. 18,19 This is indicated schematically in Figure 1, which also gives an overview of the setup.To study the structural changes in a dynamic manner, a different approach had to be used, the principle of which is explained with the help of Figure 1. The strongest difference in the XANES spectra of oxidized and metallic platinum can be found at the whiteline at 11596 eV. Therefore, X-ray absorption images were recorded only at this specific energy. To obtain a sufficiently high time-resolution a FReLoN (Fast Readout Low Noise) camera (d...

Reducing the irreducible: Incorporation of a lanthanide cation into the vacant coordination pocket of a uranyl Pacman complex results in single electron reduction to form stable pentavalent uranyl–rare‐earth complexes with uranyl–oxo–rare‐earth bonds (see scheme; py=pyridine, R=SiMe3).

The dynamics of the ignition and extinction of the catalytic partial oxidation (CPO) of methane to hydrogen and carbon monoxide over Pt-Rh/Al(2)O(3) and Pt/Al(2)O(3) were studied in the subsecond timescale using quick-EXAFS with a novel cam-driven X-ray monochromator employing Si(111) and Si(311) crystals. The experiments were performed under reaction conditions in a small fixed-bed capillary reactor. For the first time XAS data were taken with this QEXAFS technique with a Si(311) crystal that opens the energy range up to 35 keV. In addition, both XANES and EXAFS data are shown at the Pt L(3)-edge, allowing to discuss the potential and limitation of this technique in catalysis and related areas. With respect to the noble metal catalysed partial oxidation of methane, several interesting observations were made: structural changes during ignition were-independent of the chosen reaction conditions-significantly faster than during the extinction of the reaction. The dynamic behavior of the catalysts was dependent on the flow conditions and the respective noble metal component(s). Higher reaction gas flow led to a faster ignition process. While the ignition over Pt-Rh/Al(2)O(3) occurred at lower temperature than over Pt/Al(2)O(3), the structural changes during ignition were significantly faster in the latter case. The rate of reduction of the catalyst during ignition was also dependent on the axial position in the fixed-bed. The spectroscopic results provide important insight into the ignition and extinction behavior of the CPO of methane and are complementing results from time-resolved infrared thermography and full field X-ray microscopy studies.

 Users may download and print one copy of any publication from the public portal for the purpose of private study or research.  You may not further distribute the material or use it for any profit-making activity or commercial gain  You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.

Pd/Al(2)O(3) catalysts showed an oscillatory behaviour during the catalytic partial oxidation (CPO) of methane, which was investigated simultaneously by IR-thermography, X-ray absorption spectroscopy, and online mass-spectrometry to correlate the temperature, state of the catalyst and catalytic performance. The following stages were observed: (i) build-up of a temperature maximum in the first half of the catalyst bed, (ii) reduction of palladium in the end zone of the catalyst bed with a front moving toward the entrance zone, (iii) strong hot spot formation accompanied by reduction of palladium due to self-reduction leading to extinction of the process. The latter was the key driver for the oscillations and thus gave additional insight into the mechanism of partial methane oxidation.