WDS'07 Proceedings of Contributed Papers, Part III, 128–133, 2007.
ISBN 978-80-7378-025-8 © MATFYZPRESS
Ceria Reduction via Ce-Sn Bimetallic Bonding
M. Škoda, M. Cabala, L. Sedláček, F. Šutara, V. Matolín
Charles University, Faculty of Mathematics and Physics, Department of Surface and Plasma Physics,
V Holešovičkách 2, 18000 Prague 8, Czech Republic.
K. C. Prince and T. Skála
Sincrotrone Trieste, Strada Statale 14, km 163.5, 34012 Basovizza-Trieste, Italy.
V. Cháb
Institute of Physics, Academy of Sciences of the Czech Republic, Cukrovarnická 10, 16253 Prague 6, Czech Republic.
Abstract. Tin was deposited in the amount of 0.4 and 0.8 nm onto CeO2(111) surface
by molecular beam deposition at the temperature of 520 K. The interaction of tin with
cerium oxide was studied with use of XPS, UPS and RPES (Resonant Photoelectron
Spectroscopy). The strong tin – ceria interaction led to the Ce(4-nx)+Snxn+O22compound formation. It was accompanied with partial reduction of CeO2 observed as a
giant 4f resonance enhancement of Ce3+ species referring to defects in ceria. CeO2 and
SnO2 oxides were formed after oxygen treatment at 520 K. The strong Ce - Sn interaction and Sn – Ce charge transfer that lead to a weakening of cerium – oxygen bond
and consequently to the formation of oxygen deficient active sites on the ceria surface
can be a key of understanding the high catalytic activity of the SnOx/CeOx catalysts.
Introduction
Cerium oxide (ceria) has been a subject of increasing interest in recent years due to its technological
importance in a wide field of various electronic and optical applications. Cerium oxide also constitutes important
components of the catalysts used in commercially produced catalytic converters for CO oxidation, and have also
shown significant activity in syngas reactions and hydrocarbon conversion. The CO oxidation activity has
attracted wide interest due to the increasing number of applications, above all in the catalytic treatment of
automobile exhaust emission.
Ceria can be found in two stable stoichiometries: CeO2 and Ce2O3. The electronic structure of the CeO2
dioxide is characterized by unoccupied 4f states of Ce4+ (4f0) whilst the Ce2O3 trioxide has a Ce3+ (4f1)
configuration [Fabris et al., 2005]. Different 4f configurations for Ce4+ and Ce3+ result in different core level and
valence band (VB) structure. Photoelectron spectroscopy represents a powerful tool of Ce 4f state investigation.
There are many spectroscopic data showing different 4f configurations using Ce 3d and Ce 4d core level and Ce
VB spectra [Napetsching et al., 2004; Berner et al., 2002; Xiao et al., 2003, Fujimori, 1983; Mullins et al., 1998;
Overbury et al., 1999] including resonant photoemission technique in the Ce 4d – 4f photoabsorption region
[Matsumoto et al., 1994; Shimada et al., 2002; Iwasaki et al., 2002; Witkowski et al. 1997; Kucherenko et al.,
2002].
One of the important properties of ceria in terms of use in catalysis is named oxygen storage capacity
(OSC), as it acts as an oxygen reservoir regulating the partial pressure of oxygen near the catalyst surface
[Taylor, 1993; Trovarelli, 1996], which can provide oxygen to the gas mixture. The key factor for this property
is the reversible transformation from Ce4+ to Ce3+ oxidation state resulting in a formation of oxygen vacancies on
the cerium oxide surface. This transition corresponds in general to the crystal structure transition from the cubic
fluorite lattice (Fm3m space group) of CeO2 to the hexagonal lattice of Ce2O3 [Berner et al., 2002; Xiao et al.,
2003; Taylor, 1993]. The bulk Ce2O3 has a hexagonal crystal structure (P-3m1) characterized by stacking of
complete Ce and O layers with – Ce3+ – O2- – Ce3+ – O2- – O2- – repeat in [0001] direction [Trovarelli, 1996].
However, Berner and Schierbaum [2002] proposed for Pt supported CeO2 films the transformation of an – O2- –
Ce4- – O2- – layer sequence (CeO2) into – O2-3/4 – Ce3+ – O2-3/4 – (Ce2O3) where O2-3/4 indicates oxygen hexagonal
layer if a quarter of O2- ions is removed.
The addition of a suitable metal atom to the cubic structure of ceria may promote reducible behavior,
increase the OSC and consequently improve the catalytic activity for CO oxidation of the system, perhaps via the
creation of active sites at the oxide–metal boundary [Trovarelli et al., 1996; Boffa et al., 1994]. A number of
studies of ceria-zirconia systems in order to increase the oxygen storage/relace capacity were done, for ex.
[Gonzáles-Velasko et al., 1999; Fornasiero et al., 1996]. Ceria-zirconia solid solutions are reported to have three
to five times the OSC than ceria-only systems [Hori et al., 1998].
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ŠKODA ET AL.: CERIA REDUCTION VIA Ce-Sn BIMETALLIC BONDING
Mihaiu et al. [2006] dealt with the study of the thermal decomposition of tin and cerium precursors, in
nonisothermal and isothermal conditions, in order to obtain Sn-Ce-O materials with different nominal Sn:Ce
atomic ratio with use of simultaneous thermogravimetry and thermal analysis, powder X-ray diffraction, IR
spectroscopy and BET surface area measurements. Preferential formation of the cubic fluorite-type structure for
samples with Sn:Ce atomic ratio ≤ 1 could be evidence up to 600 °C. Selective oxidation of CO in excess of
hydrogen over CuO/CexSn1-xO2 (1 – x = 0.1-0.5) mixed oxide catalysts was studied by Chen [2006]. This study
showed a good activity and selectivity of the 7% CuO/Ce0.9Sn0.1O2 catalyst. Shapovalov et al., [2007] presented
density functional theory calculations for CeO2(111) surface doped with Au, Ag and Cu showing that the bond
between the oxygen atoms and the oxide was weakened by presence of the dopant. This property can increase
OSC of the catalyst.
In order to understand these interactions at a fundamental level we have studied interaction with ceria
overlayer prepared on single crystal surface Cu(111), which provides stable ceria overlayers [Šutara et al., to be
published].
Experimental
The experiments were carried out at the Materials Science Beamline (MSB) at the Elettra synchrotron light
source in Trieste. The MSB is a bending magnet beamline using a plane grating monochromator with a tuning
range from 40 to 900 eV. The ultra-high vacuum (UHV) experimental chamber was equipped with a
multichannel 150 mm mean radius electron energy analyzer (Phoibos 150), rear view LEED optics, a dual
Mg/Al X-ray source, Ce evaporation source and an ion gun.
The photoelectron spectra were acquired at different photon energies: Al K-α (hν = 1486.6 eV) for Ce 3d
and Cu 2p core levels and variable excitation energy in the interval 115 - 130 eV for the resonant photoelectron
spectroscopy of the Ce 4f states. The photoelectron spectra were taken at normal-emission with photons incident
at 60° with respect to the surface normal.
The sample was a copper crystal disc made be MaTeck GmbH of 8 mm diameter and 2 mm thickness,
oriented to within 0.2° of the (111) plane. The cleaning procedure consisted of cycles of sputtering and flashing
to 800 K in order to obtain sharp LEED pattern indicating a good crystalline surface quality. The surface
cleanliness was checked by XPS.
The Ce metal was deposited at constant deposition rate from an electron beam evaporator by using the Mo
crucible onto the clean Cu(111) surface at 520 K in 5 x 10-7 mbar of O2. The quantitative XPS analysis of Cu 2p
intensity permitted to calibrate the deposition rate. Thickness of the CeO2 film can be estimated from a relative
decrease of the Cu 2p signal. Assuming the electron mean free path is 1.12 nm from the TPP formula [Tanuma et
al., 1993] this gives the deposition rate of 0.03 nm per minute. For calculating the ceria film thickness in
fractions of monolayer we considered 0.31 nm thickness of 1 ML of CeO2 (taken for bulk cerium dioxide
crystallographic data).
Tin was deposited onto CeO2(111) surface by molecular beam evaporation at 520 K. The deposition rate
was kept constant of 0.1 nm per minute. The deposition rate was calculated from a relative decrease of Cu 2p
signal during deposition using formula [Tanuma et al., 1993] with assumption of the electron mean free path in
CeO2 of 1.5 nm.
Results
The CeO2(111) epitaxial films of thickness of 1.5 nm (5 ML of CeO2) were grown on the Cu(111) substrate
kept at 520 K in the oxygen pressure of 5 x 10-7 mbar. The preparation and characterization of the ceria layer are
described in detail in [Šutara et al., to be published]. Disappearance of Cu(111) spots on LEED diffraction
pattern indicated a total coverage of the copper substrate by the ceria layer. The LEED pattern corresponding to
the (1.5 x 1.5) superstructure was interpreted as a formation of the continuous CeO2(111)/Cu(111) epitaxial
overlayer [Šutara et al., to be published], analogous to Siokou and Nix [1999], who obtained continuous ceria
films on Cu(111) substrate for 20 ML thick overlayers grown at room temperature.
The electronic structure of CeO2 is characterized by unoccupied 4f states of Ce4+ (4f 0) whilst Ce2O3 has a
3+
Ce (4f 1) configuration [Fabris et al., 2005]. A set of resonant photoelectron spectra in the Ce 4d – 4f
photoabsorption region at photon energies between 115 and 130 eV is presented in Fig. 1. At photon energy 115
eV there is no resonance. We recognize two resonant features located at binding energy (BE) 1.5 eV and 4.5 eV
that reach their maxima at hν = 122 eV and 124.5 eV, respectively. Valence band spectra obtained at these onresonance photon energies together with hν = 115 eV off-resonance excitation, are presented in Fig. 1.
Tin in the amount of 0.8 nm was deposited onto CeO2(111) surface by molecular beam deposition. The
substrate was kept at the temperature of 520 K. The interaction of tin with cerium oxide was studied with use of
XPS, UPS and RPES. The RPES spectra obtained after tin deposition indicated a giant 4f resonance
enhancement of the Ce3+ species (maximum resonance at hν = 122 eV), see Fig. 2.
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ŠKODA ET AL.: CERIA REDUCTION VIA Ce-Sn BIMETALLIC BONDING
Intensity [arb. units]
hν =
115 eV
122 eV
124.5 eV
’122’ - ’115’
’124.5’ - ’115’
8
6
4
2
0
BE [eV]
Figure 1. Valence band spectra of the CeO2(111) thin film measured at different photon energies: 115 eV (offresonance); 122 eV (Ce,3+ resonance); 124.5 (Ce4+ resonance).
Intensity [arb. units]
hν =
8
115 eV
122 eV
124.5 eV
6
4
2
0
BE [eV]
Figure 2. Valence band spectra of tin deposited onto 5 ML CeO2(111) thin film measured at different photon
energies: 115 eV (off-resonance); 122 eV (Ce0, 3+ resonance); 124.5 (Ce4+ resonance).
Ce 3d core level XPS spectra presented in Fig. 3, show considerable changes after tin deposition, first of all
typical for partial reduction of CeO2 [Overbury et al., 1999; Henderson et al., 2003]. The Ce 3d XPS spectra
taken at different photoelectron escaping angles (with various surface sensitivity) showed that the Ce4+ → Ce3+
transition occurred not only on the surface but also in the ceria film depth.
The hypothesis of mixed Ce-Sn-O oxide formation is supported by high resolution Sn 4d spectrum taken at
hν = 115 eV in Fig. 4, which consists of two doublets corresponding to tin metal and to a new chemical state
giving a chemical shift to higher binding energy. This chemical state is not tin oxide as can be seen from Fig. 5,
where the Sn 4d reference spectrum of SnO2 measured at the same condition is presented. This reference SnO2
sample was obtained by a radio-frequency (RF) oxygen plasma oxidation of the film presented in Fig. 4. This
observation can be interpreted as mixed oxide formation caused by strong tin – cerium oxide interaction.
Because O 1s peak intensity does not change upon tin deposition, we can tentatively describe the mixed oxide as
Ce(4-nx)+Snxn+O22- compound. Thus, the compound can be described as a product of the transition:
Ce 4 + O22 − → Ce ( 4 − nx ) + Snxn + O22 −
(1)
The quantitative XPS analysis shows that in this case x = 0.3.
The comparison of high resolution Sn 4d spectrum of SnO2 reference (Fig. 5) with spectrum for the Ce(4nx)+
Snxn+O22- compound, see Fig. 4, reveals that the Snn+ intensity exhibits a narrower shape with higher binding
energy. Evidently, the Sn electronic structure is different from SnO2, probably due to strong hybridization of Ce
4f and Sn s,p orbitals which is characteristic for CexSny compounds [Holgado et al., 2005]. The compound
formation should be accompanied by a charge transfer from Sn to unoccupied 4f0 orbitals of Ce-O complex. This
behavior is confirmed also by RPES results presented above.
In order to study dependence of the Sn 4d spectrum on the quantity of deposited tin, half amount of 0.4 nm
of tin was evaporated on clean CeO2(111) substrate. It can be clearly seen in Fig. 6b that the contribution of
metallic tin is smaller than in Fig. 4 whilst the main part of the Sn 4d spectra corresponds to the state of the
130
ŠKODA ET AL.: CERIA REDUCTION VIA Ce-Sn BIMETALLIC BONDING
Intensity [arb. units]
formed compound. In order to investigate the mixed oxide reactivity towards oxygen adsorption, tin was heated
at 520 K in oxygen atmosphere of 5 x 10-7 mbar for 5 min. Fig. 6c shows an increase of tin oxide character states
and on the other hand an decrease of mixed oxide tin states after O2 treatment. Metallic tin at 24 eV was completely oxidized. A great deal of the Sn-Ce bimetallic bonds was suppressed; CeO2 and SnO2 oxide were formed.
Ce 3d5/2
Ce 3d3/2
920
910
900
890
880
BE [eV]
Figure 3. Ce 3d XPS spectra; 5 ML CeO2(111) film (dotted line), tin deposited onto CeO2(111) thin film (solid
line).
Sn
hν = 115 eV
Sn 4d3/2 (Sn metal)
Sn 4d5/2 (Sn-Ce-O)
Sn 4d5/2 (Sn metal)
Intensity [arb. units]
Sn 4d3/2 (Sn-Ce-O)
29
28
27
26
25
24
23
22
BE [eV]
Figure 4. Sn 4d spectra of tin deposited onto CeO2(111) thin film.
Intensity [arb. units]
hν = 115 eV
29
28
27
26
25
24
BE [eV]
Figure 5. Reference Sn 4d spectra of SnO2(111) thin film.
O 1s spectra are presented in Figs. 6a,b,c. According to [Henderson et al., 2003] showing a trend of O 1s
core level shift to higher binding energy after CeO2 reduction, the peak component at 529.6 eV in Fig. 7a,b,c can
be associated to CeO2, whilst the feature at 530 eV in Figs. 7b,c to surface reduced ceria Ce2O3. The small
intensity component at binding energy of 531.5 eV on the main O 1s feature is typically assigned to water and
CO2 adsorption or to O2- anions located near to oxygen vacancy sites [Henderson et al., 2003; Holgado et al.,
2000]., The Ce2O3 feature appeared after the deposition of tin (even in higher intensity than CeO2 component compare Figs. 7a and 7b), and the Ce2O3 feature intensity remarkably fell after oxygen treatment. These facts
confirm the hypothesis of reduction of ceria after tin deposition and oxidation of ceria after oxygen treatment.
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ŠKODA ET AL.: CERIA REDUCTION VIA Ce-Sn BIMETALLIC BONDING
Figure 6. Sn 4d spectra: tin deposited onto CeO2(111) thin film (dotted line) and after oxidation (solid line) (a);
Sn 4d spectra deconvolution of tin deposited onto CeO2(111) thin film (b); Sn 4d spectra deconvolution of tin
deposited onto CeO2(111) thin film after oxidation (c)
Figure 7. O 1s XPS spectra of 5 ML CeO2(111) film (a); O 1s spectra of tin deposited onto CeO2(111) (b); O 1s
spectra of tin deposited onto CeO2(111) after oxidation (c)
Conclusion
The model studies performed on a well defined Sn/CeO2(111)/Cu(111) system show that high catalytic
activity of the SnOx/CeOx catalysts can be explained by strong Ce - Sn interaction accompanied by Sn – Ce
charge transfer that leads to weakening of cerium – oxygen bond and consequently to the formation of oxygen
deficient sites in the ceria film.
Acknowledgements. This work is a part of the research programs No. MSM 0021620834 and LC06058 that are
financed by the Ministry of Education of the Czech Republic.
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