Evgeny Kleymenov

Five-element Johann-type x-ray emission spectrometer with a single-photon-counting pixel detector

A Johann-type spectrometer with five spherically bent crystals and a pixel detector was constructed for a range of hard x-ray photon-in photon-out synchrotron techniques, covering a Bragg-angle range of 60°-88°. The spectrometer provides a sub emission line width energy resolution from sub-eV to a few eV and precise energy calibration, better than 1.5 eV for the full range of Bragg angles. The use of a pixel detector allows fast and easy optimization of the signal-to-background ratio. A concentration detection limit below 0.4 wt% was reached at the Cu Kα(1) line. The spectrometer is designed as a modular mobile device for easy integration in a multi-purpose hard x-ray synchrotron beamline, such as the SuperXAS beamline at the Swiss Light Source.

High energy resolution off-resonant spectroscopy at sub-second time resolution: (Pt(acac)2) decomposition.

We report on the decomposition of platinum acetylacetonate (Pt(acac)(2)) in hydrogen induced by flash heating. The changes in the local Pt structure were followed by high energy resolution off-resonant spectroscopy uniquely performed with sub-second time resolution. The decomposition consists of a two-step reduction process of the Pt(II) species.

Evaluation of Pt and Re oxidation state in a pressurized reactor: Difference in reduction between gas and liquid phase

Determination of metal oxidation state under relevant working conditions is crucial to understand catalytic behaviour. The reduction behaviour of Pt and Re was evaluated simultaneously as a function of support and solvent in a pressurized reactor (autoclave). The bimetallic catalysts are used in selective hydrogenation of carboxylic acids and amides. Gas phase reduction reduced the metals more efficiently, in particular Pt.

The oxidation state of copper in bimetallic (Pt–Cu, Pd–Cu) catalysts during water denitration

Catalytic denitration of water with bimetallic systems has emerged as a viable solution for removal of nitrates from drinking water. Despite the progress in process development during the last two decades, only a few studies were performed to determine catalyst structure under working conditions. Herein, we determined the relative population of Cu oxidation states in Pt–Cu and Pd–Cu bimetallic catalysts by in situ high resolution X-ray absorption spectroscopy in combination with principal component analysis. The initial state of the catalyst was a Pt–Cu or Pd–Cu alloy. Segregation of the metal components occurred under reaction conditions especially for a Pt–Cu system. The active oxidation states of copper were metallic and alloy, and their concentration was highly dependent on the amount of hydrogen in the feed. Initial alloy phase of the catalysts ensures close proximity between Cu and the noble metals after segregation, essential to maintain catalysts activity.

Structure of the methanol synthesis catalyst determined by in situ HERFD XAS and EXAFS

A Cu/ZnO/Al2O3 commercial catalyst for methanol synthesis from syngas was investigated under operational conditions. HERFD XAS and EXAFS data were recorded under different reaction gas mixtures, temperatures, and pressures. Activation of the catalyst precursor occurred via a Cu+ intermediate. The active catalyst predominantly consists of metallic Cu and ZnO. Methanol production only starts when all accessible Cu is reduced. The structure of the active catalyst did not change with temperature or pressure even though the methanol yield changed strongly. Formation of a carbon-containing layer on top of the catalyst surface was detected by TPD, which was correlated with a several-hour induction period in the methanol production after the catalyst reduction.

Fine tuning of gold electronic structure by IRMOF post-synthetic modification

In this communication, we present a strategy to fine-tune the electronic structure of gold atoms by means of post-synthetic modification of IRMOF-3 (iso-reticular metal organic framework-3) with salicylaldehyde derivatives. Changes in the electronic structure of gold were predicted by theoretical calculations and determined by resonant inelastic X-ray scattering (RIXS) at the gold L3 edge. Modification of the coordinating group affected the relative HOMO and LUMO energy levels, resulting in a change of the HOMO–LUMO gap. The ability to fine tune electronic structure may lead to tailoring of the catalytic performance and material electronics states.