Heterogeneously Catalyzed Oxidation Reactions Using ... - CHEC

CHAPTER 6
6.2.4 Catalyst characterization after reaction
Powder X‐ray diffraction patterns were recorded on a Siemens D5000 diffractometer equipped with
a Ni filter using Cu Kα‐radiation. Measurements were performed in the 2Θ range from 5 to 55° with a
step size of 0.01° and a step time of 2 s. Scanning Electron Microscopy (SEM) was measured with a
Gemini 1530 (Zeiss) instrument.
6.2.5 Flame atomic absorption spectroscopy (F-AAS)
For leaching experiments, 15 mL of the reaction solution were concentrated and the remaining solid
transferred into a crucible and treated at 700 °C in air for 4 h. The remainder was dissolved in aqua
regia and diluted to 10.0 mL with distilled water. The Co concentration was measured on a Varian
SpectrAA 220FS equipped with a cobalt SpectrAA lamp (Varian; λ = 240.7 nm). Concentrations were
determined vs. standard solutions prepared from cobalt metal dissolved in HNO3.
6.2.6 X-ray absorption spectroscopy (XAS)
Ex situ samples were measured at the ANKA XAS beamline at the ANKA synchrotron facility at the
Karlsruhe Institute of Technology (Karlsruhe, Germany). The synchrotron ring was operated at 2.5
GeV and a typical ring current of 85‐180 mA. Measurements were conducted in transmission mode
at the Co K‐edge around 7.7 keV with a Si(111) double monochromator in step scanning mode. The
beam intensity was measured by means of three ionization chambers placed before and after the
sample as well as after the reference (Co foil). In situ experiments were conducted at the X1
beamline at the Hamburger Synchrotronstrahlungslabor (HASYLAB) at the Deutsche Elektronen‐
Synchrotron (DESY, Hamburg, Germany). XAS spectra were recorded in transmission mode at the Co
K‐edge around 7.7 keV with a Si(111) double monochromator in step scanning mode in a specially
designed spectroscopic liquid phase cell applicable up to 200 bar [39, 40]. The beam intensity was
measured with ion chambers before and after the sample. Co foil for referencing was measured
extra. Prior to the experiment a thin catalyst pellet (30 mg) was pressed and adjusted to the X‐ray
path. DMF (3 mL) and (E)‐stilbene (20 mg) were added, the cell was closed, pressurized with oxygen
(2 bar) and heated to 100 °C. XANES spectra were processed by energy calibration, deglitching where
necessary, background subtraction, and normalization using the WinXAS 3.1 software [41]. EXAFS
spectra were extracted from the XAS spectra after analogous treatment and Fourier‐transformation
and fitted in R‐space. Phase shifts were calculated with the FEFF 7.0 code [42].
6.2.7 Electron paramagnetic resonance (EPR)
The spectrometer used was an X‐band Bruker EMX continuous wave instrument equipped with a TM
type cavity. The microwave power applied was 20 mW. The sample temperature in the cavity was
controlled with a Eurotherm liquid nitrogen evaporator and heater. In situ experiments were carried
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