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Axial changes of catalyst structure and temperature in a fixed-bed microreactor during
noble metal catalysed partial oxidation of methane
Stefan Hannemann1, Jan-Dierk Grunwaldt1,2,*, Bertram Kimmerle1, Alfons Baiker1
Pit Boye3, and Christian Schroer3
1
Institute for Chemical and Bioengineering, Department of Chemistry and Applied Biosciences,
ETH Zurich, Hönggerberg – HCI, CH-8093 Zurich, Switzerland
2
Department of Chemical and Biochemical Engineering, Technical University of Denmark, DK2800 Kgs. Lyngby, Denmark
3
Institute for Structural Physics, Technical University of Dresden, D-01062 Dresden, Germany
The CPO reaction has been studied using 2D-XAS and IR temperature mapping combined with
MS tracing. In this ESI some additional details on the experimental setups, the on-line gas
analysis and the analysis of the XAS data are presented.
1. Catalyst preparation
2.5%Pt-2.5%Rh/Al2O3 and 2.5%Rh/Al2O3 catalysts were prepared by flame spray pyrolysis using
a solution of Pt(II) acetylacetonate (purum, Fluka), Rh(III) acetylacetonate (99%+, Acros) and
Al(III) acetylacetonate (99%, Abcr) in a fresh mixed solution of 50/50 (v/v) methanol and acetic
acid. The precursor solutions were sprayed into a methane oxygen flame via a nozzle [1, 2]. In
each case, 40 ml of the corresponding solution was fed by a syringe pump (Inotech, 50 ml syringe,
3 ml min-1) into the centre of a methane/oxygen flame ring. The liquid was dispersed by oxygen
(3 l min-1) forming a fine spray. The total gas flow rate through the premixed methane/oxygensupporting flame ring was 3.5 l min-1 with a CH4/O2 ratio of 0.92. Ca. 50 cm above the flame a
cylindric steel vessel was situated with a filter inside. Product particles were collected on a glass
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fiber filter (Whatman GF/A, 26 cm in diameter) with the help of a vacuum pump. The inner 22
cm of the filter were scraped off using a spatula, and for catalytic tests, the powder was pressed,
crushed and sieved to fractions between 100 and 200 μm for the spectroscopic/catalytic
experiments (more details in ref. [3])
2. Details on the acquisition and analysis of the XAS data by a “macrobeam” and with an
X-ray camera
The experimental setup with the top of the oven and the capillary microreactor is shown in Figure
1 of the publication. The arrangement was both used for in situ XAS studies with ionization
chambers (0.5 mm x 0.5 mm large beam) and the X-ray camera as well as measurements with the
infrared camera. In both experimental XAS setups the energy was scanned in the XANES region
around the Pt L3-edge and Rh K-edge in steps of 1 eV (from 11530 to 11660 eV and from 23190
to 23350 eV, respectively).
Figure S1: Spectra of flame made 2.5% Pt-2.5% Rh/Al2O3 (1.5 mm capillary, 0.05-0.1 mm
sieved fraction, 16.5 mg catalyst, 6% CH4, 3% O2 in He, 5 ml·min–1) at the Rh-K edges (left) and
Pt-L3 (right) recorded by a 0.5 mm × 1.8 mm beam. Increasing distances from the entrance of the
catalytic bed result in more reduced species as demonstrated for Rh (at 307°C, Scan A): 3 mm,
3.2 mm, 3.3 mm and 3.4 mm, and Pt (at 316°C, Scan B): 1 mm, 3.25 mm, 3.4 mm, 4 mm.
Positions refer to the centre of the beam. Hydrogen signals amount to 0.19 (Scan A) and 0.25
(Scan B).
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Conducting the experiment with the camera at each energy step, an X-ray image was recorded
with and without the capillary, effectively measuring the transmitted and incident intensities (i.e.
flat field scans) in each pixel. The exposure time for each image was 10 s using a lead shutter
(Festo). To remove the influence of dark currents and read-out noise, dark field scans were
recorded, averaged and subtracted from each image. For each pixel the absorption was ascribed
to the logarithm of the ratio of the incident and transmittance intensities resulting in entire
XANES spectra. To visualize zones of oxidized and reduced elements, the spectra were fitted by
linear combination of one spectrum from the oxidized and one from the reduced zone as well as
the featureless background. Further details on the setup and data processing are given in Ref. [3].
3. XANES at the Rh K- and Pt L3-edge
Figure S1 gives the results during scanning with a 0.5 mm × 0.5 mm beam at the Rh K-edge and
the Pt L3-edge over the catalyst bed. Obviously, the spectra change from oxidized form on the
inlet side of the microreactor to reduced species on the outlet side of the reactor. The XANES
data were compared with the literature [5, 6]. Although the whiteline clearly appears the noble
metals are only partially oxidized as discussed previously in Ref. [4].
4. Examples of extracted XANES data from scanning with the X-ray camera at the Pt L3and the Rh K-edge
Figures S2a and S2b give examples of the extracted XANES spectra taken with the X-ray camera.
The difference in the features (pre-edge, whiteline, post-edge) is much more pronounced at the
Rh K-edge; therefore, the S/N-ratio is also better at the Rh K-edge than at the Pt L3-edge. The
spectra served as basis for the linear combination in Figures 3 and 4.
5. Stability of positions of the transition zone
A certain movement with time accompanied with change in catalytic activity was always found.
For example, at 23 ml·min–1 Scan D (215 °C) roughly corresponds to Scan G (212 °C), but in the
latter scan the gradient was shifted more towards the inlet. Note that during the experiments,
particularly below 320°C, a small temperature decrease at the thermocouple with time was
observed. Scan G was taken before Scan D with a higher temperature at the thermocouple. This
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was also reflected by a higher hydrogen signal during Scan G. At 322 °C, the transition zone was
more stable and hydrogen formation was similar (Scans F and I).
Figure S2: X-ray absorption spectra extracted from the X-ray images (recorded as function of
energy) serving as reference spectra for Figure 3 of the paper (Rh K-edge, left, blue are oxidized
Rh-species and red are reduced Rh-species) and Figure 4 (Pt L3-edge, gray are more oxidized Pt
species and blue are more reduced Pt-species).
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6. Catalytic performance during the catalytic partial oxidation of methane
In Table 2 the catalytic performance was described only in terms of hydrogen evolution. In
previous studies it was shown that the disappearance of oxygen was coupled to the appearance of
hydrogen and carbon monoxide [1]. In order to show the strong correlation between furnace
temperature, the oxygen concentration and the hydrogen formation, Figure S3 is shown: At the
moment when hydrogen disappears, oxygen appears at the outlet. The extinction is only observed
after 10 min cooling below 315°C. Note that the ignition occurs at higher temperature than the
extinction temperature.
Figure S3: Different mass spectrometric signals (not calibrated) during the partial oxidation of
methane over 2.5%Pt-2.5%Rh/Al2O3 (9 mg, 1 mm capillary); conditions: 6%CH4/3%O2/He, 12
ml·min-1, thick line represents the temperature just below the capillary.
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