Journal of Electron Spectroscopy and Related Phenomena 119 (2001) 193–199
www.elsevier.nl / locate / elspec

Site-selective XAFS spectroscopy tuned to surface active sites ofcopper catalysts
Yasuo Izumi*, Fumitaka Kiyotaki, Hiroyasu Nagamori, Taketoshi Minato
Department of Environmental Chemistry and Engineering, Interdisciplinary Graduate School of Science and Engineering, Tokyo
Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226 -8502, Japan
Received 19 January 2001; received in revised form 20 February 2001; accepted 15 March 2001

Abstract
To overcome the problem of X-ray absorption fine structure (XAFS) spectroscopy in which the obtained information isthe average for all sites in sample, Rowland-type fluorescence spectrometer was designed to measure site-selective XAFS
spectra by utilizing the chemical shift of emission peak for each site. Johansson-type cylindrically-bent Ge(111) crystal andscintillation counter were mounted on the spectrometer. The improvement of energy resolution of spectrometer was
discussed when the primary beam size and two slit size in the Rowland circle were varied. The energy resolution was |1.1
eV, smaller than the core-hole lifetime width for Cu Ka 1 (2.11 eV). The site selection of the Cu 0 and Cu I site-tune spectra
was experimentally measured for the physical mixture of Cu metal1CuI and for Cu / ZnO catalyst. Obtained experimentalsite selection was compared to the theoretical estimations.  2001 Elsevier Science B.V. All rights reserved.
Keywords: XAFS; Fluorescence spectrometer; Energy resolution; Core-hole lifetime width; Chemical shift; Site selection

1. Introduction
When X-ray absorption fine structure (XAFS)
spectroscopy is applied to the sample which consistsof more than one kind of site, obtained information[i.e. bond distances and coordination numbers byextended X-ray absorption fine structure (EXAFS)
and normalized X-ray absorption near-edge structure(XANES)] is the average for all sites in sample.
Heterogeneous catalysts are typical examples thatconsist of several kinds of sites. Unique determination of local geometric and electronic structures is*Corresponding author. Fax: 181-45-924-5569.
E-mail address: yizumi@chemenv.titech.ac.jp (Y. Izumi).

impossible based on conventional XAFS spectroscopy due to this problem.
In this paper, an experimental approach to discriminate each site among co-existing sites in sample
is described by utilizing fluorescence spectrometer.
A Rowland-type fluorescence spectrometer was settled to analyze fluorescence X-rays from samples in
the XAFS measurements. XAFS spectra will be siteselective when the fluorescence spectrometer is tuned
to fluorescence peak energy of each site based on thechemical shift of fluorescence X-rays. Dysprosium
L 3 -edge XANES spectrum was reported fordysprosium nitrate by using fluorescence spectrometer [1]. The absorption edge structure was more
resolved in the spectrum compared to conventional
XANES spectrum. The reason was discussed that the0368-2048 / 01 / $ – see front matter  2001 Elsevier Science B.V. All rights reserved.
PII: S0368-2048( 01 )00292-4

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XANES spectrum obtained by using fluorescence
spectrometer was not affected by the core-hole
lifetime broadening because a narrow energy band of
the fluorescence radiation was monitored with thefluorescence spectrometer (energy resolution |0.3eV) combined with incident beam of high energyresolution (|0.7 eV) at Wiggler beamline.
Herein, the merits of experimental setup withfluorescence spectrometer are discussed for XANES
measurements and the site selection is evaluated fora model sample of both Cu 0 and Cu I sites [physicalmixture of copper metal and copper(I) iodide] andCu / ZnO catalyst in in situ condition. Cu / ZnO isheterogeneous catalyst for methanol synthesis, however, exact active site structure is still open forquestion because the Cu 0 and Cu I sites co-exist on
Cu / ZnO catalysts in in situ conditions.

2. Experimental

2.1. Design of fluorescence spectrometer
The configuration of high energy-resolution fluorescence spectrometer was illustrated in Fig. 1b incomparison with conventional setup (Fig. 1a). Detailed specification was described in Ref. [2], andimproved points for higher energy resolution measurements are summarized. Sample was placed on aplane tilted from horizontal plane, and the Rowland
circle of fluorescence spectrometer was in the vertical plane. Top surface of the sample was tilted to thedirection of incident X-ray from the horizontal plane(angle u, Fig. 1b) and also to the direction of thesample (angle x, Fig. 1b). The angles u and x were
6–138 during emission and XAFS spectra measurements.
The positions of cylindrically-bent Johansson-typeGe(111) crystal 50(H)325(V) mm 2 , R5220(61)mm] and a slit (slit 2, Fig. 1) in front of the NaI(Tl)scintillation counter (SC) were controlled by PC to
be precisely on the Rowland circle during emission
spectrum scan. Independent control of crystal anddetector enables free choice of Rowland radii in therange 127.7–240.9 mm and Bragg angles in therange 83.9–55.68. This configuration effectively
focuses the divergence originally due to the incident

Fig. 1. Configurations of conventional XAFS (a), and of siteselective XAFS by utilizing the fluorescence spectrometer (b).

synchrotron beam (polarized horizontally), and thesignal /(back ground) ratio is higher compared to thecase of 458-beam entrance and 458-beam exit. To
realize better energy resolution, the horizontal lengthof slit 0 and the vertical lengths of slits 1 and 2 areimportant. These values were optimized within thelimit to obtain minimal photon counts at SC for the
XANES measurements.

2.2. Samples
Copper powder, CuCl, CuCl 2 , CuBr, CuI, Cu 2 O,
and CuO were used as the standard compounds foremission and / or XANES spectra measurements.
Physical mixture of copper metal and CuI was also
prepared diluted in boron nitride (BN) matrix (mixedby 1:1 on the basis of Cu amount). Cu / ZnO catalysts

Y. Izumi et al. / Journal of Electron Spectroscopy and Related Phenomena 119 (2001) 193 – 199

were prepared by the co-precipitation method fromCu and Zn nitrates [3]. The Cu content was 5%(w / w) for all the samples. Obtained powder was
calcined at 623 K, followed by heating in hydrogen(53 kPa) at 523 K and in CO (27 kPa)1H 2 (27 kPa)
at 523 K. The sample was transferred in glass work
to an in situ Pyrex cell equipped with a 50-mm
Kapton film on both sides, and sealed by fire. The
sample was at room temperature during spectrum
measurements.

2.3. Emission and XANES spectrum measurements
The Cu Ka emission and the Cu K-edge XANES
spectra were measured at the Undulator beamline
10XU of SPring-8. The storage ring energy was 8.0
GeV and the ring current was 99–72 mA. Si(111)
double crystal monochromator was used. Cu Ka
emission was monitored with Ge(444) (d50.81652
˚A)
reflection. The excitation energy was
9000.0(60.1) eV. The spectrum scan step was
0.0025–0.01008 for the rotation of cylindrically-bent
crystal, corresponding to Bragg angle step of 0.12–
0.5 eV. The accumulation time of a step was 60–
120s. The spectrum energy was calibrated byequalizing the Cu metal Ka 1 peak to 8047.8 eV [4].
The peak energy positions were reproduced within
0.3 eV when several spectra were scanned at Cu Ka
by changing the angles u and x and three slit
lengths. The reproducibility was carefully checked
especially in the case of smaller length of slits (,0.5mm) where the photon counts at SC were limited. In
the case, both the emission peak shape and thealignment of Rowland-type spectrometer on a levelof |100 mm were verified.
The I0 beam was monitored by ion chamber filledwith nitrogen gas. Site-selective Cu K-edge XANES
spectra were measured by tuning the fluorescence
spectrometer at 8046.6 and 8049.1 eV for the physical mixture of Cu metal and CuI and at 8046.6 and8050.9 eV for Cu / ZnO catalyst. The accumulation
time of a step was 10–180s. The spectrum energy
was calibrated by equalizing the first inflection pointof the Cu metal K-edge to 8980.3 eV [4].
Site selection for the site-selective XANES wasevaluated by the fitting with the convolution of

195

conventional XANES spectra. The ratio of conventional XANES spectra was varied between 10:0 and
0:10, and the fit was evaluated based on the R factor(R f ) 5 e u x (exp) 2 x (fit)u 2 dE / e u x (exp)u 2 dE.

3. Results

3.1. Line widths and chemical shifts of theemission spectra
Cu Ka emission spectra were measured for Cu
metal, CuCl, CuCl 2 , the physical mixture of Cu
metal and CuI, and the Cu / ZnO catalyst. The peak
top and FWHM (full width at a half maximum) were
evaluated based on the fitting with a Gaussian and / ora Lorentzian function(s) and flat background. The
peak top energy shifted by 11.6 eV on going from
Cu metal to CuCl and by 20.6 eV on going from
CuCl to CuCl 2 . The FWHM values varied according
to the changes of horizontal length of slit 0 andvertical lengths of slits 1 and 2 (Fig. 1b). The
FWHM for Cu Ka 1 peak was as small as 2.4 eV(Fig. 2c) when the sizes of slits 0–2 were 2.0(H)3
1.0(V), 8.0(H)30.5(V), and 8.0(H)30.5(V) mm 2 ,respectively. Note that the improvement of monochromator, i.e. smaller beam size at sample, was also
important for better energy resolution of fluorescence
spectra. The FWHM value was 3.0 eV with the same
three slits sizes during the beamtime of 1999, when
old Si(111) crystals were used for monochromator.
New Si(111) crystals with better crystallinity wereused for monochromator during the beamtime of
2000, and smaller FWHM of 2.4 eV was enabled.
The Cu Ka peaks for Cu / ZnO heated in CO1H 2
at 523 K shifted by 0.6–0.9 eV compared to the Cu
IImetal (Fig. 2a,b). The shift may correspond to Cusite [5], however, wider peak width (4.0 eV for
Cu / ZnO vs. 3.5 eV for Cu metal) suggested that theCu / ZnO catalyst consisted of Cu 0 and Cu I sites andthe emission spectra were the convolution of the twocontributions. The peak energy position for thephysical mixture of Cu metal and CuI was also
between those of Cu metal (Fig. 2c) and CuI, and thepeak width for the physical mixture of Cu metal andCuI was wider than that of Cu metal or CuI.

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Y. Izumi et al. / Journal of Electron Spectroscopy and Related Phenomena 119 (2001) 193 – 199

Fig. 2. Cu Ka 1 emission spectra for Cu / ZnO catalyst (a) and Cu metal (b). The energy resolution was improved at the beamtime of 2000
for Cu metal (c). The experimental data are plotted as points and the Gaussian / Lorentzian fittings are drawn as solid lines. Two arrows
indicate tune energies for site-selective XAFS measurements.

3.2. Site-selective XANES spectra for the physicalmixture of Cu metal and CuI
The fluorescence spectrometer was tuned to8046.6 eV (tuned to Cu 0 site) and 8049.1 eV (tuned to
Cu I site) for the physical mixture of Cu metal andCuI based on the Cu Ka 1 emission spectra. Obtained
XANES spectra were shown in Fig. 3a and b,
respectively. The latter pattern was essentially thesame as that for CuI (Fig. 3e) whereas the former
pattern was not the same as that for Cu metal. The R fvalue (see its definition in Section 2) reached theminimum at the ratio of Cu metal (Fig. 3d):CuI
spectra (Fig. 3e)560(610):40(610) (minimum R f 5
1.6%) for Fig. 3a and at the ratio of Cu metal:CuI

spectra510(610):90(610) (minimum R f 50.3%)
for Fig. 3b.

3.3. Site-selective XANES spectra for the Cu /ZnO
catalyst
The conventional fluorescence XANES spectrum
for the Cu / ZnO catalyst (Fig. 4c) did not have ashoulder at 8988 eV nor a peak at 9000 eV, whichwere typical for XANES for CuO (Fig. 4h). The
possibility that the Cu II site co-exists in the Cu / ZnO
catalyst in in situ condition can be neglected, and thetargets of site-selective XANES became Cu 0 and Cu I
sites. The fluorescence spectrometer was tuned toFig. 3. Site-selective XANES spectra for the physical mixture of copper metal and copper(I) iodide. The fluorescence spectrometer was
tuned to 8046.6 eV (a) and 8049.1 eV (b). Conventional fluorescence XANES spectra for the physical mixture of Cu metal1CuI (c), Cu
metal (d), and CuI (e). All the samples were diluted in boron nitride matrix to Cu 5% (w / w).

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197

Fig. 4. Site-selective XANES spectra for Cu / ZnO catalyst. Fluorescence spectrometer was tuned to 8046.6 eV (a) and to 8050.9 eV (b).
Conventional XANES spectra for Cu / ZnO (c), Cu metal (d), Cu 2 O (e), [CuhN(CH 2 CH 2 -2-pyridyl) 3 j][BPh 4 ] (f), and CuO (h) were
measured by Lytle detector (c) or in transmission mode (d,e,h). XANES was theoretically generated by FEFF8 for Cu I atomically dispersed
on ZnO (g). Spectrum (f) was reproduced from Ref. [6].

8046.6 and 8050.9 eV, respectively, based on theemission spectrum (Fig. 2, arrows).
The site-selective XANES tuned to 8046.6 eV(Fig. 4a) resembled XANES for Cu metal (Fig. 4d).
The rising-edge energy position shifted to higher
energy for site-selective XANES tuned to 8050.9 eV(Fig. 4b), demonstrating the contribution from the
Cu I site. The peak top position at 8984 eV was thesame as that of XANES for Cu 2 O (Fig. 4e). The
region of 8993–9008 eV was intense, similar to thecase of Cu 2 O, though the peak split into two peaks.
The peak position 9028 eV was the same as that ofFig. 4d for Cu metal.
Obtained site-selective spectra were fit by standard
spectra in Fig. 4d–h. The XANES spectrum for Cu I
site atomically dispersed on wurzite ZnO(0001)
surface (Fig. 4g) was generated by FEFF8 for a
˚ from the Cu center in amodel cluster within 6 A
self-consistent field and multiple scattering modes[7,8]. The theoretical XANES was similar to
XANES of model complex [CuhN(CH 2 CH 2 -2pyridyl) 3 j][BPh 4 ] (Fig. 4f). Based on the criteria that
the R f values reached the minimum, Cu 0 and Cu I
site-selective XANES were best fit with80(610):20(610) mixture of XANES for Cu
metal1Cu 2 O and 50(610):25(610):25(610) mixture of XANES for Cu metal1Cu 2 O1Cu I atomical-

ly dispersed on ZnO surface, respectively. As theconventional XANES for Cu / ZnO catalyst (Fig. 4c)
was best fit with 70(610):30(610) mixture of Cu 0
and Cu I site-selective XANES [3], the Cu site ratio
was estimated to be Cu metal:Cu 2 O:Cu I atomicallydispersed on ZnO surface570(623):22(614):8(65)for Cu / ZnO catalyst in in situ condition.

4. Discussion

4.1. Estimation of energy resolution
The energy resolution of wave number-dispersive
spectrometer was formulated as:
DE /E 5 cot uB DQ(1)

The DQ was formulated for bent crystal as theeffects of geometrical angle width, the divergence ofbeam perpendicular to the Rowland circle plane, and
the X-ray penetration depth into the crystal [9]:DQ 5[(Ws 1 Wf )2 1 (h 2 /8R cos uB )2 1 h(2 cos uB sin uB ln 2)/m j 2 ] 1 / 2]]]]]]]]]]]]]]
8R sin uB(2)

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198

where Ws is the effective source size in the plane ofdiffraction, Wf is the slit width (vertical in Fig. 1b) on
Rowland circle in front of SC, m is the absorption
coefficient of crystal, h is the slit length (horizontal
in Fig. 1b), and R is the Rowland radius. Thus:
DE 5 E cot uB DQ
3

2

F
DG

S D
2

E (2d)h2]]](Ws 1 Wf ) 1 ]2
8R
8RC
CE 2 ln 2 2 1 / 2
1 ]]]
Kd

|

S

2

(3)

where E is the fluorescence X-ray energy, d is thelattice constant of crystal, and C is a constant˚ K is the approximation of m E 3 . The(12.398 keV A).
DE was evaluated to be 2.5 eV based on the angle x
˚ This(578) at E58.05 keV and d Ge( 444) 50.81652 A.
value was in accord with the experiments on 1999
[2]. The FWHM value during the experiments on
2000 was 2.4 eV. Assuming the Gaussian shape peak
convolution of the width due to experimental apparatus and core-hole lifetime width, improved
energy resolution of the spectrometer was estimatedto be 1.1 eV. This value includes the energy resolution of primary beam of beamline [1]. This
resolution corresponds to Ws 5Wf 50.11 mm in Eq.
(3). As the beam height was approximated to be slit
0 length(H)3tan x, the effective primary beamwidth(H) was calculated to |0.90 mm. Hence, thebeam size decrease owing to the improvement of

monochromator of beamline 10XU should be themajor reason for the better energy resolution in thecase of measurements on 2000.

4.2. Estimation of site selection based on theenergy resolution
The site selection by detecting Cu Ka 1 peak in thecase of energy resolution of spectrometer51.1 eV is
estimated, and compared to the site-selective experiments for the physical mixture of Cu metal and CuI(Fig. 3). The chemical shift on going from Cu 0 to
Cu I was relatively large (11.6 eV) for Cu metal andCuCl, however, the shifts were relatively smaller for
CuBr, CuI, or Cu 2 O (0.1–0.8 eV) compared from Cu
metal [10]. Therefore, two examples of chemical
shifts of 1.6 and 0.8 eV were plotted in Fig. 5a and b,
respectively. The two dotted peaks have the FWHM
of core-hole lifetime width (Cu K1L 3 2.11 eV [11])were separated by chemical shift of 1.6 eV (a) and
0.8 eV (b). The two solid peaks with the FWHM ofestimated energy resolution of spectrometer (1.1 eV)were separated by 2.47 eV, tune energy gap in thesite-selective measurements of Fig. 3 (8046.6 and8049.1 eV in the cases of Cu 0 and Cu I site-tune,
respectively). The ratio of overlap area (hatched)estimates site selection. The site selection was 82%(a) and 69% (b). The site selection as a function withchemical shift was plotted in (c).
The unequal site selection, i.e. 60 vs. 90% of Cu 0

Fig. 5. The estimation of site selection for Cu Ka 1 . The two dotted peaks with the FWHM of core-hole lifetime width (2.11 eV) wereseparated by chemical shift of 1.6 eV (a) and 0.8 eV (b). The two solid peaks with the FWHM of estimated energy resolution of spectrometer(1.1 eV) were separated by 2.47 eV. The ratio of overlap area (hatched) estimates site selection. The site selection as a function withchemical shift is plotted in (c).

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199

and Cu I site tunes, cannot be explained only basedon peak overlap in Fig. 5. One of the possibilities isthe energy shift due to the unsymmetric nature ofemission peak or the problem of Bragg angle reproducibility (e.g. stepping motor backlash, etc.). If thetwo tune energies (11.235 and 21.235 eV, solid
peaks in Fig. 5b) shifted by 10.8 eV, i.e. 12.035
and 20.435 eV), the site selection is estimated to be
56 (Cu 0 site tune) and 77 (Cu I site tune). These
values support experimental trends in site-selective
XANES measurements: 60(610)% for Cu 0 site tune
(Fig. 3a) and 90(610)% for Cu I site tune (Fig. 3b).

Acknowledgements

5. Conclusions

References

The site-selective XANES measurements were
performed for a model system of copper catalyst(physical mixture of Cu metal and CuI diluted inBN) by utilizing a fluorescence spectrometer. Theenergy resolution of fluorescence spectrometer was
as small as 1.1 eV, including the energy resolution of
beamline monochromator, at the Cu Ka 1 emission
energy, smaller than core-hole lifetime width (2.11
eV). The site selection was estimated to be 60(610)
and 90(610)% for Cu 0 and Cu I site-tune spectra,
respectively. Theoretical estimation was |56 and|77% based on the overlap of two peaks with thewidth of core-hole lifetime and a peak with the widthof energy resolution of spectrometer. This method
was also applied to Cu / ZnO catalyst in in situ
condition. The population of sites was estimated to
be Cu metal:Cu 2 O:Cu I atomically dispersed on ZnO
surface570(623):22(614):8(65).

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This work was supported by grants from Grant-inAid for Scientific Research from the Ministry of
Education, Science, Sports, and Culture (Monbusho)of Japan (08874066 and 12740376), Toray Science
Foundation (98-3901), Yamada Science Foundation(2000), and Kanagawa Academy of Science andTechnology (9991008). The experiments were performed under the approval of the SPring-8 Program
Review Committee (Nos. 2000B0015-CX-np and1999B0220-NX-np).