Beamline practice at BL28B2 (DXAFS)
Ver. 2013/08/26 K. Kato, K. Nitta, T. Uruga (JASRI)
1Introduction
The bending magnet beamline, BL28B2, is used for various applications oftime-resolved energy dispersive XAFS (DXAFS). In the practical training course, weplan to show how to measure and analysis DXAFS spectra. We will also demonstratein-situ time-resolved DXAFS measurement of catalyst samples under reaction
condition.

2Plan of practice
9:30 – Introduction of beamline, alignment of optics, major scientific activities atBL28B2.
10:00 – Setup of the DXAFS measurement system.
12:00 – Introduction of preliminary process of DXAFS spectra using software
ATHENA.
12:30 – Lunch
13:30 – Introduction of in-situ DXAFS experiment and sample preparation.
14:00 – Setup of in-situ DXAFS equipment system.
15:00 – Measurement of in-situ DXAFS spectra of catalysts during reduction process.
16:00 – Data treatment and preliminary analysis of DXAFS spectra.
17:00 – Close.

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Outline of DXAFS measurement at BL28B2
3.1

Measurement principle of DXAFS
Figure1 shows schematic of the DXAFS measurement system. White x-rayscoming from the bending magnet or undulator light source are diffracted by abent crystal in the polychromator and focused on the sample. The diffractedx-rays have different energies, depending on the diffraction angle. The x-rays
are measured with a position-sensitive detector (PSD) at the position
corresponding to their energy. X-ray intensity profiles measured with andwithout the sample are numerically converted into an absorption spectrum.

Top View
White X-rays

θMinBent Crystal

θMaxSample

E HighE Low
Position Sensitive DetectorFig.1 Layout of DXAFS measurement system
3.2

DXAFS station at BL28B2
Figure 2 shows schematic layout of DXAFS spectrometers at BL28B2. The
station consists of polychromator, a mirror for cutting a higher harmonics x-rays
and position sensitive detector (PSD). There are two types of polychromator; one
is for the Laue configuration and another for the Bragg configuration. In theLaue configuration (Fig. 2(b)), the polychromator is a bender with a fixed radiuscurvature, R=2000mm. The polychromator crystal has Si (111) surface, withdimensions of 90(w) x 20(h) x 0.2(t) mm. In the Bragg configuration, the bender
is a four-point bender, which can load an independent bending force on each endof the crystal with dimensions of 200(w) x 40(h) x 1(t) mm. In the energy region
from 5 to 12 keV, the crystal is positioned using Bragg geometry (Fig. 2(a)). A
Laue configuration is used for the net plane Si (hkk), such as Si (422) or Si (511),
in the energy region above 12 keV because the energy resolution decreases due

to the depth of x-ray penetration into the crystal.
The PSD used is CCD (spatial resolution, 9.9 micrometer; minimum readouttime, 6 msec) coupled with a fluorescent screen and a lens system (Fig. 3). The
fluorescent materials and their thickness were optimized for measurement inthe x-ray range.

Fig.2 Schematic layout of DXAFS spectrometer at BL28B2 for (a) Bragg geometryand (b) Laue geometry.

Fig.3 Schematic view of PSD.

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Alignment of DXAFS spectrometer
Set the polychromator for the Laue configuration.

Roughly adjust the diffraction angle of the polychromator crystal to cover theenergy range for the DXAFS measurement.

Adjust the position of the 2θarm and PSD on the diffracted x-ray beam.

Finely adjust the diffraction angle of the polychromator crystal by measuring thetransmission image of the standard sample to set the absorption edge on theadequate position in the PSD image.

Adjust the rotation angle around axis normal to the polychromator crystal surface
to remove unwanted diffractions (glitches) observed in the PSD image.

Adjust the tilt angle of the PSD for the vertical line of the PSD to be parallel to theabsorption line.

Find the focal point of the diffracted x-ray beam by slit scan in the horizontal
direction, and set sample on the focal point.

5

Adjust the PSD position to detect the DXAFS image.

DXAFS measurement of the standard sample
Figure 4 shows the setup of the DXAFS measurement system in the Laueconfiguration.



Set the standard sample at the focal point.

Measure the I0 and I1 images by the PSD.
The I0 images are measured by removing the sample form the x-ray beamusing a stepping motor stage.

Set the measurement time of the PSD.
Select exposure time of the CCD not to saturate the CCD pixels.

Set the number of measurement images to achieve data quality enough for
the XANES or EXAFS analysis, which decide the time resolution of themeasurements.

Measure the dark images.
The dark images are measured by closing the beam shutter.



The dark images are subtracted from the I0 and I1 images as the dark noiseof the PSD.

The exposure time and number of the dark images are set to the same as
those of I0 and I1.

Calculate absorption spectrum

After subtracting the dark images, I0 and I1 images are summed each other.

The 2-dimensional I0 and I1 data are summed in the vertical direction to get
1-dimensional I0 and I1 data.

The absorption spectra are calculated by dividing I1 by I0 and calculatinglogarithm.

Calibrate the x-ray energyThe horizontal axis of the PSD in the absorption spectra is converted to thex-ray energy by comparison with the conventional XAFS spectra of the same
standard sample.

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Preliminary process of XAFS spectra using software ATHENA

The row data of DXAFS spectra are processed to the ATHENA-readable dataformat.

ATHENA processes the measured XAFS spectra, such as background removal
and Fourier transforms.
ATHENA is one of most popular free software for processing XAFS data.
http://cars9.uchicago.edu/~ravel/software/Model fitting of XAFS data is done by another software ARTEMIS includinginterfaces to ATOMS and FEFF.

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Measurement of in-situ DXAFS spectra
7.1

Sample preparation
The sample for the in-situ DXAFS measurement is packed into the cylindricalsample holder by hand with very weak pressing force.（Fig.5）.

Fig.5 Pack of sample in sample holder

7.2 Setup of in-situ DXAFS equipment
 Experimental condition in this in-situ measurement.
 Sample: Pd/USY catalyst.
 Measurement energy: around Pd K– edge (24.3keV)
 Sample temperature: room temperature.
 Reaction gases: H2 and He and their mixture.


Set the sample cell including the sample.

Connect q-mass outlet of the sample cell to monitor the concentration ofeach outlet gases (Figs. 6 and 7).

Fig.6 Control system of in-situ DXAFS measurement.

Fig.7 Setup of in-situ DXAFS equipment

7.3In-situ DXAFS measurement of catalyst during reduction process.
 DXAFS measurement of the catalyst in initial state.
 Flow 100% - He 90ccm into the sample cell.
 Measure DXAFS spectra.
 DXAFS measurement of the sample during reduction process.
 Flow 8% - H2/He 90ccm (H2: 7 ccm, He: 83 ccm) into the sample cell for 15 20 min.

 Continually measure DXAFS spectra every 5 min during gas flow.
 Check changes of Fourier transform–DXAFS spectra during reduction
process using ATHENA.
 Switch 8%-H2/He to 100% - He flow.
 Flow 100% - He for 10 – 15 min until no residual H2 in the sample cell(checking by Q-mass).
 Measure DXAFS spectra.
 Check the sample color.