In situ EXAFS studies of copper on ZrO2 during catalytic hydrogenation of CO2

Article abstract of DOI:10.1016/j.elspec.2005.01.281

Speciation of copper on ZrO₂ during the catalytic hydrogenation of CO₂ has been studied by in situ extended X-ray absorption fine structural (EXAFS) spectroscopy in the present work. The EXAFS data indicated that about 3.1 nearest oxygen atoms surrounded the Cu atoms with a CuO bond distance of 1.95 ? in the calcined Cu/ZrO₂ catalyst. Reduction of the catalyst in hydrogen at 573 K led to a formation of CuCu (2.56 ?) bonds with a coordination number (CN) of 5.6. Mainly CO and CH₃OH were generated in the hydrogenation of CO₂ catalyzed by the reduced Cu/ZrO₂ catalyst at 673 K. About 76% of the Cu(0) species was oxidized to Cu(I) (27%) and Cu(II) (49%) on ZrO₂ during the catalytic hydrogenation process.

Full text of DOI:10.1016/j.elspec.2005.01.281

Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 373–376
In situ EXAFS studies of copper on ZrO2 during catalytic
hydrogenation of CO2
S.-H. Liua, H. Paul Wanga, ∗, H.-C. Wangb, Y.W. Yangc
a
Department of Environmental Engineering, National Cheng Kung University, Tainan City, Taiwan
b
Department of Medicine, Veterans General Hospital-Kaoshiung, Kaoshiung, Taiwan
c
National Synchrotron Radiation Research Center, Hsinchu, Taiwan
Available online 11 March 2005
Abstract
Speciation of copper on ZrO2 during the catalytic hydrogenation of CO2 has been studied by in situ extended X-ray absorption fine structural
(EXAFS) spectroscopy in the present work. The EXAFS data indicated that about 3.1 nearest oxygen atoms surrounded the Cu atoms with a
˚
Cu O bond distance of 1.95 A in the calcined Cu/ZrO2 catalyst. Reduction of the catalyst in hydrogen at 573 K led to a formation of Cu Cu
˚
(2.56 A) bonds with a coordination number (CN) of 5.6. Mainly CO and CH3OH were generated in the hydrogenation of CO2 catalyzed by
the reduced Cu/ZrO2 catalyst at 673 K. About 76% of the Cu(0) species was oxidized to Cu(I) (27%) and Cu(II) (49%) on ZrO2 during the
catalytic hydrogenation process.
© 2005 Elsevier B.V. All rights reserved.
Keywords: CO2; Hydrogenation; ZrO2; XANES; EXAFS
1. Introduction
process [3]. However, they have been suffered a very poor
activity for the hydrogenation of CO2 [4].
Hydrogenation of CO2 to generate valuable chemicals
has been widely investigated from the viewpoint of reducing
the global warming effect caused by the CO2 emission. It
is also of great interest from the aspect of use of carbon
source from CO2 for the synthesis of oxygenates and
hydrocarbons [1]. In the near future, due to the rapid decline
of petroleum reserves, the use of CO2 as C1 building blocks
in the organic synthesis would be environmentally and
economically attractive. The amount of CO2 to be fixed that
way is very small compared to the huge amount of CO2
produced by burning fossil fuels. However, the industrial use
of CO2 as a feedstock has so far been limited by its inherent
thermodynamic stability. In the past decade, considerable
researches have been investigated to develop methods of
activating CO2 for subsequent chemical transformation [2].
Processes for synthesis of methanol from syngas catalyzed
by Cu/ZnO/Al2O3 have been well established in an industrial
Generally, catalyst preparation procedures such as im-
pregnation, ion exchange, deposition precipitation and co-
precipitation may influence the catalytic behavior [3]. Var-
ious metal oxides have been used as support materials for
hydrogenation of CO2. ZrO2 is of great interest in catalysis
because of its mechanical and thermal stability, high specific
surface area, and semiconductor properties. ZrO2 has been
found to maintain a good long-term stability in catalytic hy-
drogenation of CO2 [5]. ZrO2-supported copper catalysts are
very active with a high selectivity for methanol synthesis [6].
However, nature of the catalyst in the CO2 hydrogenation
process has not been extensively studied.
X-ray absorption near edge structural (XANES) and ex-
tended X-ray absorption fine structural (EXAFS) spectro-
scopies are very useful in identification of elements with a
different environment, degree of aggregation, or location [7].
By EXAFS, we found that copper oxides (in ZSM-5 or ZSM-
48) involved in the catalytic decomposition of NO [8] and
oxidation of chlorophenols in supercritical water [9]. These
speciation data was very useful in revealing the structure of
∗
Corresponding author. Tel.: +886 6 276 3608; fax: +886 6 275 2790.
E-mail address: wanghp@mail.ncku.edu.tw (H.P. Wang).
0368-2048/$ – see front matter © 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.elspec.2005.01.281

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S.-H. Liu et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 373–376
active species in the catalysis process. Thus, the main ob-
jective of the present work was to investigate the speciation
of copper on ZrO2 in the catalytic hydrogenation of CO2 by
EXAFS. An in situ EXAFS cell was used in the experiments.
2. Experimental
The 5 wt.% Cu/ZrO2 catalyst was prepared by impregna-
tion of copper (Cu(CH3COO)2) onto ZrO2 (Prochemistry Co.
Ltd. (GR, 99.9%)). The Cu/ZrO2 catalyst was dried at 378 K
for 16 h and calcined at 823 K for 8 h. About 0.5 g of the
Cu/ZrO2 catalyst was activated by reduction in 10% H2/He
gas (flow rate = 40 mL/min) at 573 K for 4 h. Hydrogenation
of CO2 was conducted at 423–923 K in the flowing H2/CO2
gas with a molar ratio of 4/1. Product gases were analyzed
by gas chromatography and on-line FTIR spectroscopy.
Infrared spectra were recorded on a Diglab FT-IR spectrom-
eter (FTS-40) with fully computerized data storage and data
handling capability. For all spectra reported, a 64-scan data
Fig. 1. Temperature programmed reduction of the calcined Cu/ZrO2 catalyst
in 10% H2/He at a heating rate of 10 K min−1
.
3. Results and discussion
accumulation was carried out at a resolution of 4 cm−1
.
Fig. 1 shows the temperature programed reduction of
the calcined Cu/ZrO2 catalyst in the flowing H2/He gas at
323–837 K. A fast reduction of CuO on ZrO2 was observed
at 573 K. In addition, the observation of essentially little re-
duction of CuO on ZrO2 at >600 K suggests a rather little
interaction between CuO and the support ZrO2.
Catalytic hydrogenation of CO2 was conducted at
423–923 K. The temperature dependence for hydrogenation
of CO2 on the reduced Cu/ZrO2 catalyst is shown in Fig. 2. It
is clear that hydrogenation of CO2 was enhanced by Cu/ZrO2
at >673 K. Yields of CO and CH3OH from the hydrogenation
of CO2 also increased as the reaction temperature increased
and subsequently reached 28 and 22%, respectively, at 923 K.
For further understanding the nature of the active copper
speciesonZrO2, X-rayabsorptionspectraofthecatalystwere
In the temperature programmed reduction (TPR) exper-
iments, the calcined Cu/ZrO2 catalyst was reduced in 10%
H2/He with a heating rate of 10 K min−1. A thermogravi-
metric analyzer (SDT 2960 simultaneous TGA-DTA) was
used to monitor the weight change of the catalyst in the
reduction process.
The X-ray absorption spectroscopic study for the catalytic
hydrogenation of CO2 on Cu/ZrO2 was conducted in an in
situ EXAFS cell [8]. The EXAFS spectra of the catalyst were
collected on the Wiggler beamline at the Taiwan Synchrotron
Radiation Research Center. The electron storage ring pro-
vided energy of 1.3 GeV (current of 80–200 mA). A Si(1 1 1)
double-crystal monochromator was used for selection of en-
ergy with an energy resolution of 1.9 × 10−4 (eV/eV). The
absorption spectra were collected in ion chambers that filled
with helium gas. Beam energy was calibrated by the adsorp-
tion edge of Cu foil at an energy of 8980 eV. The standard
deviation calculated from the averaged spectra was used to
estimate the statistical noise and error associated with each
structural parameter. Samples were measured in the trans-
mission mode. Each EXAFS spectrum was recorded at least
twice.
The EXAFS data were analyzed using the UWXAFS 3.0
and FEFF 8.0 programs [7]. The isolated EXAFS data was
normalized to the edge jump and converted to the wavenum-
ber scale. The Fourier transform was performed on k2-
−1
˚
weighted EXAFS oscillations in the range of 3.5–12.5 A
.
XANES spectra of standard samples such as Cu2O, CuO, and
Cu foil were also measured on the Wiggler beamline. Semi-
quantitative analyses of the edge spectra were conducted by
the least-square fitting of linear combinations of standard
spectra to the spectrum of the sample. The height and area of
the near-edge band in a copper spectrum were quantitatively
proportional to the amount of copper species.
Fig. 2. Temperature dependence for the composition of product gases: (a)
CO2, (b) CO, and (c) CH3OH in the catalytic hydrogenation of CO2 on
Cu/ZrO2.

S.-H. Liu et al. / Journal of Electron Spectroscopy and Related Phenomena 144–147 (2005) 373–376
375
Table 1
EXAFS data of the Cu/ZrO2 catalysts
σ2 (A )
2
˚
˚
Shell
R (A)
CN
3.1
5.6
10.7
1.9
Oxidized Cu/ZrO2
Reduced Cu/ZrO2
Cu
O
1.95
2.56
3.57
1.91
2.51
0.003
0.004
0.006
0.004
0.003
Cu Cu
Cu Cu
a
Cu/ZrO2
Cu
O
Cu Cu
5.4
R: bond distance; CN: coordination number; σ2: Debye–Waller factor.
a
The in situ EXAFS spectra of the catalyst were measured during the
catalytic hydrogenation of CO2 at 673 K.
of the EXAFS data fitting for copper on ZrO2 was obtained.
In Table 1, the calcined Cu/ZrO2 catalyst has a Cu O bond
˚
distance of 1.95 A with a coordination number (CN) of 3.1.
In the second shells, the bond distance of Cu (O) Cu was
˚
2.90 A with a CN of 12.6. Reduction of the catalyst led to a
formation of mainly metallic copper with an averaged Cu Cu
˚
bond distance of 2.56 A. The CN of copper in the reduced cat-
alyst was 5.6 approximately. However, hydrogenation of CO2
on the reduced Cu/ZrO2 catalyst at 673 K might cause the
oxidation of the metallic copper (Cu(0)) to Cu2O (Cu(I)) or
CuO (Cu (II)). The EXAFS data of the used catalyst showed
that about 1.9 nearest oxygen atoms surrounding the copper
˚
atoms with the Cu O bond distance of 1.91 A. Note that the
interactions between copper and ZrO2 was not observed by
in situ EXAFS spectroscopy during the hydrogenation pro-
cess. Evidently, little high-temperature reduction feature was
found in the TPR experiments (see Fig. 1).
Fig. 3. In situ XANES spectra of (a) calcined, (b) reduced Cu/ZrO2 catalysts,
and (c) the catalyst that was measured during CO2 hydrogenation at 673 K.
The component fits of the spectrum (c) are also shown in (d).
The possible reaction path for Cu(0) that involved in the
hydrogenation of CO2 with H2 may be described as follows.
The first and second shells Cu Cu bond distances in the re-
determined. The in situ XANES spectra of the Cu/ZrO2 cat-
alyst in the calcined and reduced states and during the hydro-
genation of CO2 at 673 K are shown in Fig. 3. The pre-edge
XANES spectra of the calcined Cu/ZrO2 catalyst exhibited
a very weak absorbance feature for the 1s-to-3d transition
(8975–8980 eV) which is forbidden by the selection rule in
the case of perfect octahedral symmetry [8,9]. A shoulder at
8984–8988 eV and an intense feature at about 8995–9002 eV
were attributed to the 1s-to-4p transition that indicated the ex-
istence of Cu(II) species. The XANES spectra of the reduced
Cu/ZrO2 catalyst was very similar to that of the Cu foil. The
metallic copper is, in general, very active for catalytic hydro-
genation of CO2 [10]. The pre-edge band at 8981–8984 eV
may be due to the dipole-allowed 1s-to-4p transition of Cu(I)
that might be formed via oxidation of copper by CO2.
˚
duced Cu/ZrO2 catalyst were 2.56 and 3.57 A, respectively.
Stoichiometrically, two moles of CO2 interacted with the
metallic copper and formed Cu(II) and Cu(I) species with
yields of CO(g) and CH3OH(g). When oxygen was inserted
into the metallic copper in the CO2 hydrogenation process,
˚
the Cu Cu bond distance was decreased by 0.05 A and a
˚
formation of Cu O bonding (1.91 A) was observed. In the
presence of excess H2, the Cu(II) and Cu(I) species was re-
duced to metallic copper promptly.
4. Conclusions
The XANES spectra were also expressed mathematically
in a LC XANES fit vectors, using the absorption data within
the energy range of 8960–9040 eV. XANES spectra of stan-
dard samples such as CuO, Cu2O and Cu foil were measured
on the Wiggler beamline. The fit of the XANES spectra dur-
ing the hydrogenation of CO2 on ZrO2 at 673 K is shown in
Fig. 3(d). It was found that about 76% of the Cu(0) species
was oxidized to Cu(I) (27%) and Cu(II) (49%) on ZrO2 in
the catalytic hydrogenation of CO2 to yield CO and CH3OH.
The in situ EXAFS spectra were also recorded and ana-
The EXAFS data indicated that the calcined Cu/ZrO2 has a
Cu O bond distance of 1.95 A with about 3.1 nearest oxygen
˚
atoms surrounding. Reduction of the catalyst in hydrogen at
˚
573 K led to the formation of Cu Cu bonds (2.56 A) with a
CN of about 5.6. Mainly CO and CH3OH were yielded in the
hydrogenation of CO2 catalyzed by Cu/ZrO2 at 673 K. The
XANES spectra shows that Cu(I) was formed via oxidation of
Cu(0) by CO2. About 76% of the Cu(0) species was oxidized
to Cu(I) (27%) and Cu(II) (49%) on ZrO2 in the catalytic
hydrogenation of CO2 to yield CO and CH3OH.
lyzed in the k range of 3.5–12.5 A−1. An over 99% reliability
˚

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[5] J. Weigel, A. Koeppel, A. Bailker, A. Wokaun, Langmuir 12 (1996)
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