Reversible structural transformation of palladium catalyst supported on La-AhOa probed with X-ray absorption fine structure

Article abstract of DOI:10.1021/jp991857p

Reversible transformation of the palladium catalyst during thermal cycling was investigated with X-ray absorption fine structure. PdO was transformed to Pd at 850°C upon when the air temperature was increased while the reverse transformation occurred at 6

Full text of DOI:10.1021/jp991857p

8124
J. Phys. Chem. B 2000, 104, 8124-8128
Reversible Structural Transformation of Palladium Catalyst Supported on La-Al2O3
Probed with X-ray Absorption Fine Structure
Sung June Cho* and Sung Kyu Kang
Catalytic Combustion Research Team, Korea Institute of Energy Research, 71-2, Jang-dong, Yusung-gu,
Taejon 305-343, Korea
ReceiVed: June 8, 1999; In Final Form: June 1, 2000
Reversible transformation of the palladium catalyst during thermal cycling was investigated with X-ray
absorption fine structure. PdO was transformed to Pd at 850 °C upon when the air temperature was increased
while the reverse transformation occurred at 680 °C. On-line TGA/MS measurements combined with EXAFS/
XANES showed oxygen generation at 680 °C, indicating that the metal-support interaction stabilizes the
catalyst, by transforming to the surface attached PdO and further to Pd metal agglomerates at 850 °C. Thermal
stability of the Pd metal agglomerate was also improved by coating the Pd catalyst with TiO2.
Introduction
from Condea Inc. Its surface area was 90 m2‚g-1. The activation
procedure was the calcination in oxygen at 550 °C for 6 h
followed by the reduction with hydrogen at 550 °C for 4 h.
The Pd loading of the catalyst was 2 wt % on dry basis. The
obtained Pd catalyst was denoted as Pd/La-Al2O3.
The preparation of TiO2 sol following the procedure reported
in the literature is briefly.7 Titanium tetraisopropoxide (Aldrich,
97%) was hydrolyzed in doubly distilled water. Upon the
addition of concentrated HCl, the precipitates were dissolved
to give clear transparent sol. This sol was impregnated onto
Pd/La-Al2O3 and subsequently reduced at 800 °C for 6 h
without calcination. The TiO2-coated Pd catalyst was also
denoted as TiO2/Pd/La-Al2O3.
X-ray absorption measurement was carried out in ambient
air using Beamline 10B at the Photon Factory in Tsukuba. For
high-temperature measurements, a specially designed XAFS cell
with Kapton window (DuPont, 125 µm) was used. The
temperature was raised from room temperature to 900 °C. The
extended X-ray absorption fine structure (EXAFS) data was
analyzed using XFIT and Feff6. The details of EXAFS data
analysis can be found elsewhere.8 Thermogravimetric analysis
(TGA) was carried out with a TA instrument from 30 to 950
°C. Oxygen evolution from the catalyst was monitored with an
on-line mass spectrometer (Genesys, Baltzers) after the removal
of water using acetone/dry ice trap.
Pd catalysts have been used extensively in catalytic combus-
tion. By using Pd catalysts in the combustion process, the
combustion temperature can be lowered to 1200 °C. The
generation of thermal NOx is thereby suppressed dramatically
below 3 ppm since high combustion temperature, ca. 1500 °C,
increased the rate of the formation of NOx from O2 and N2 in
air.1
However, the Pd catalyst is evaporated readily at such a high
temperature, resulting in the loss of catalytic activity.2 Several
combustion systems have been proposed to inhibit deactivation
of the Pd catalyst. Catalytically stabilized combustion with post
combustion zone, allows the catalyst zone to maintain its
temperature, 900°.3
It has been suggested that reversible transformation of the
Pd catalyst between PdO and Pd occurs during the thermal
cycling in the combustion reaction.4 Farrauto et al. proposed
the model for the reversible transformation of Pd catalyst during
thermal cycling based on the weight change from TGA
measurement.5 During thermal cycling, PdO converted to Pd
and the resulting Pd metal agglomerate transformed back to PdO
at 550-650 °C. They suggested a correlation between the
methane combustion activity and the weight change. Electron
microscopic study of Pd catalysts under controlled atmosphere
showed the microscopic features of thermal behavior.6 This
study suggested the formation of a compound like PdAlOx above
800 °C that might cause the deactivation.
Results and Discussion
However, the information on the change of atomic structure
of Pd catalyst during the thermal cycling was very limited. In
the present work, we report the investigation of X-ray absorption
fine structure (XAFS) on the Pd catalyst under thermal treatment
in ambient air in conjunction with the on-line TGA/MS
measurement. Also, the effect of the metal-support interaction
between TiO2 and Pd catalyst on the thermal stability has been
investigated using XAFS.
TGA spectra of the Pd catalyst coupled with on-line MS
spectrum are presented in Figure 1. Pure La-Al2O3 showed
monotonic decrease of weight with the increase of temperature
at a rate of 10 °C‚min-1. There was negligible weight gain of
the sample upon subsequently decreasing the temperature. Thus,
the monotonic decrease of weight seemed to come from the
desorption of adsorbed water.
Pd/La-Al2O3 showed decreases in weight at two distinct
temperatures, namely at 680 and 880 °C. However, subsequent
cooling caused the catalyst to gain weight. The weight changes
at 680 °C can be ascribed to the evolution of oxygen, as shown
by the peak at around 680 °C in the on-line MS spectra of
oxygen (m/e ) 32). There was no oxygen evolution at 880 °C
where the weight change in TGA spectrum occurred. This
Experiment
Pd(NO3)2 (Heesung Engelhard Corp., 19.9% Pd) was im-
pregnated into La-doped Al2O3 (2 mol % La-Al2O3) supplied
* Corresponding author. E-mail: sjcho@kier.re.kr. Fax: +82-42-860-
3133.
10.1021/jp991857p CCC: $19.00 © 2000 American Chemical Society
Published on Web 08/09/2000

Structural Transformation of Pd Catalyst
J. Phys. Chem. B, Vol. 104, No. 34, 2000 8125
Figure 1. TGA spectra of (a) La-Al2O3 and (b) Pd/La-Al2O3, and
the MS spectrum (m/e ) 32) of (c) Pd/La-Al2O3. The ramping rate in
TGA experiment was 10 °C‚min-1. The effluent gas stream in mass
spectroscopic measurement was passed through the dry ice/acetone trap
to remove water.
Figure 3. Near-edge spectra of X-ray absorption at the Pd K edge for
the Pd/La-Al2O3 as a function of heating temperature in ambient air
from the bottom to top.
density of states is strongly enhanced and the absorption can
be increased like the second and third absorption peak in the
XANES spectrum of Pd foil. The XANES spectra of PdO in
Figure 2b shows the strong absorption at the Pd K edge since
the d-p hybridization of Pd2+ increased significantly the
absorption cross section, compared with that of Pd foil.
The XANES feature is known to be very sensitive to the long-
range order up to 10 nm due to a large mean free path of the
photon electron in the energy range of 1-100 eV above the
absorption edge.12 The reference Pd and PdO spectra thus have
a characteristic XANES features which are sensitive to the
change of physicochemical environment of the absorbing atom,
such as the change of oxidation state, hydrogen chemisorption,
the formation of bimetallic particles, etc.13 Therefore, the
reversible transformation of Pd catalyst during the thermal
cycling in ambient air can be monitored using XANES at the
Pd K edge.
Figure 3 shows XANES spectra as a function of the heat
treatment temperature. A rather broad strong single peak
characterized as the PdO was maintained up to 850 °C. Upon
further increase of the temperature above 850 °C, a single peak
became split into two peaks characterized as the Pd, which
suggests the transformation of the PdO structure to Pd. The data
analysis of Pd XANES of the Pd/La-Al2O3 indicated that there
was no change of the PdO entity around 680 °C where the
oxygen was evolved. The results suggest that the spread of Pd
over the surface occurs upon desorption of oxygen from PdO.
Koningsberger and Gates suggested that the metal support
interaction over Al2O3 was very strong.14 They claimed the
EXAFS contribution of a long metal-oxygen distance as a direct
evidence for the metal support interaction. Needle, rod, or planar
shape of the particle on the catalyst support is believed to
maximize the metal-support interaction. Planar shape of the
Pt particle under vacuum and helium was reported inside the
channel of zeolite KL, which might indicate the substantial
metal-support interaction.15 Rodriguez et al. observed that the
needlelike shape was a result of the interaction between Pd and
oxygen at 840 °C.6 They suggested that the reduced Pd particle
over γ-Al2O3 converted to PdO in the range of 350-650 °C
Figure 2. Near-edge spectra of X-ray absorption at the Pd K edge for
(a) Pd foil and (b) PdO. The spectra were measured at room
temperature.
change might be attributed to the evolution of H2O due to
dehydroxylation.
The temperature for the decomposition of PdO to Pd is widely
distributed from 650 to 800 °C in the literature depending on
the support, the presence of dopant like La3+ or metal oxide
like NiO, metal particle size, etc. Widjaja et al. studied the
reduction-oxidation property of Pd catalyst supported on Al2O3
using TPD of oxygen.9 They ascribed the desorption at 700 °C
to the decomposition of PdO and the desorption over 700 °C
from Al2O3. Incorporation of NiO into Pd/Al2O3 led to a change
of the TPD profile: the appearance of the peak at 750 °C.
Groppi et al. report the TPD/TPO measurements over 2.5% Pd/
La-Al2O3 (5 wt % La doping).10 Their result showed that
oxygen is evolved at 700-750 °C.
Figure 2 shows the X-ray absorption near-edge structure
(XANES) of Pd foil and PdO at the Pd K edge. At the Pd K
edge, the absorption transition at 24 348 eV is 1s f 4d dipole
forbidden transition according to the selection rule, ∆l ) 1 and
∆j ) 1, where l and j are the orbital angular momentum and
the total angular moment of the local density of states,
respectively.11 Thus, this transition intensity is suppressed greatly
as shown in Figure 2a. The absorption cross section in the K
edge region of transition metals is strongly influenced by d-s,p
hybridization because the unoccupied bands are d bands. Thus,
in system where the d-s,p hybridization is strong, the p-like

8126 J. Phys. Chem. B, Vol. 104, No. 34, 2000
Cho and Kang
theoretical EXAFS with Feff6 and the subsequent curve fitting
of the experimental EXAFS spectrum using the simulated
amplitude and phase function of EXAFS spectrum.
The best fit of the model structure of PdO is shown in Figure
5. The obtained tetragonal PdO structure consisted of four
oxygen atoms at 0.203 nm, eight palladiums at 0.300 nm, and
four palladiums at 0.340 nm. The difference in the CN in the
second shell was due to the small size of PdO particle. The
reduced Pd particle has the coordination number of Pd-Pd, 8.8
at 0.272 nm, indicating the formation of 1-2 nm Pd particle.
Figure 6 shows the change of the coordination distance (R)
and the coordination number (CN) of the first shell appeared
in r space. The R at 0.20 nm and the CN ca. 4 did not change
up to 850 °C, indicating that the PdO structure was stabilized
on the La-Al2O3 support. At 850 °C, the atomic structure of
the Pd catalyst changed suddenly to Pd metallic state, referred
from the R at 0.27 nm and the CN ca. 6-8. At high
temperatures, the large thermal vibration may hinder an accurate
estimation of the CN. The R may better indicate the change of
the atomic structure. Consistent with the result of XANES, it
was found from the results of EXAFS data analysis that the
reversible transformation between PdO and Pd occurred at 850
and 680 °C with the increase and decrease of the temperature,
respectively.
Based on the results from the TPD/MS experiment, and
XANES and EXAFS of the Pd catalyst, the structural model
for the reversible transformation can be suggested as shown in
Figure 7. The weight change of the Pd catalyst at 680 °C
estimated from TG measurement corresponded to the decom-
position of PdO to Pd, ∼0.25% ( 0.10%, close to the calculated
value, 0.3%. However, the XANES feature did not show the
change of the oxidation state corresponding to the change of
PdO to Pd at the same temperature region. Thus, the fresh PdO
was believed to convert into the surface attached PdO at 680
°C.
The other supporting evidence for the formation of the surface
attached PdO species was found in the change of the Pd-Pd
CN at 0.30 nm. This Pd-Pd CN changed suddenly from 3.0 to
1.6 with the increase of the temperature from 650 to 700 °C.
Below 650 °C, the Pd-Pd CN at 0.30 nm decreased gradually
from 4.8 at room temperature to 3.0 at 650 °C. Above 700 °C,
the Pd-Pd CN also decreased gradually and disappeared at 850
°C. This CN change cannot be the subject of rigorous XAFS
data analysis. However, the sudden change of Pd-Pd CN at
650-700 °C may be qualitatively interpreted as the change of
the PdO size, that is, the reconstruction of the initial PdO into
rather platelike particle by forming the surface attached PdO
upon the desorption of oxygen.
The surface-attached PdO underwent the further structural
change to Pd metal at 880 °C, losing PdO’s character. Both
results from EXAFS and XANES on Pd catalyst supported this
structural change. It has hitherto been suggested that the
conversion of the PdO to Pd is responsible for the decrease of
the catalytic activity in methane combustion. Iglesia reported
that the PdO decomposition to Pd metal above 830 °C led to
very low methane oxidation rates and to very low apparent PdO
dispersions.16 Similarly, Groppi et al. showed that the activity
of methane combustion over the Pd catalyst declined signifi-
cantly upon the complete conversion of PdO to Pd metal.10
Sekizawa et al. also reported that the methane combustion
activity depended on the dispersion and oxidation states of the
palladium through the interaction with the support.17
Figure 4. (a) Typical k3-weighted EXAFS spectrum and (b) its
corresponding Fourier transformation of the Pd/La-Al2O3. The phase
shift was uncorrected.
TABLE 1: Structural Parameters from the Best Fit of
EXAFS Spectra of the Pd Catalyst at the Pd K Edge Using
XFIT and Feff6
sample
pair
N
R (nm)
σ2 (pm2)a
Pd/La-Al2O3
Pd-O
Pd-Pd
Pd-Pd
Pd-O
Pd-Pd
Pd-Pd
Pd-Ti
3.5
4.8
4.2
3.7
4.9
4.2
(4.7)
0.201
0.305
0.343
0.201
0.304
0.342
(0.361)
16.7
57.6
41.6
21.2
68.0
40.0
(79.6)
TiO2/Pd/La-Al2O3
Aged at 1200 °C for 4 h
Pd/La-Al2O3
TiO2/Pd/La-Al2O3
Pd-Pd
Pd-O
Pd-Pd
Pd-Pd
10.6
1.7
5.3
6.4
0.274
0.202
0.275
0.342
56.0
∼ 0
47.8
84.4
a The Debye-Waller factor.
and subsequently PdAlOx above 800 °C. Further, the coexistence
of PdO and Pd over the surface was claimed for heavy metal
loading, ∼10%. In the present work, the Pd metal loading was
controlled to 2 wt %, far from heavy loading.
Upon decreasing the temperature, the two split peaks
combined to give the single feature characterized as PdO at
650-700 °C. This reverse transformation from Pd to PdO was
consistent with the weight gain occurred at 680 °C in TGA
spectra. Thus, the results of XANES suggested that the reversible
transformation between PdO and Pd did not follow the same
pathway upon increasing and decreasing the temperature.
Typical k3-weighted EXAFS spectrum and the corresponding
Fourier transformation are shown in Figure 4. Three distinct
peaks at 0.20, 0.30, and 0.34 nm corresponded to Pd-O, Pd-
Pd, and Pd-Pd bond, respectively, which were identified from
the comparison of amplitude and phase functions with those of
the standards. The results of data analysis of EXAFS are shown
2
in Table 1. The many-body reduction factor, S0 ) 0.90, was
obtained from the curve fitting of Pd foil. The refinement of
the model structure of the Pd catalyst was performed through
an iterative refinement of model structure for the simulation of
The present model for the reversible transformation of Pd
catalyst further suggests the direct transformation from Pd metal

Structural Transformation of Pd Catalyst
J. Phys. Chem. B, Vol. 104, No. 34, 2000 8127
Figure 5. Cluster model of PdO obtained through the iterative refinement of the model structure for the simulation of theoretical EXAFS with
Feff6 and the subsequent curve fitting of the experimental EXAFS spectrum to the obtained theoretical EXAFS spectrum.
Figure 7. Proposed structural model for the reversible transformation
of Pd catalyst in this work. The PdO seems to spread upon the
desorption of oxygen and to agglomerate to form Pd metal particle
upon heating to 850 °C. The reverse transformation of the Pd metal
occurred to PdO at 680 °C.
It was expected that the Pd agglomerate might sinter to form
large bulk Pd metal upon excessive heating above 1000 °C.
We have thereby examined the suppression of Pd metal sintering
using metal oxide coating like TiO2 which is known to have
the strong metal-support interaction.18 Asakura suggested the
strong metal-support interaction between Pd and TiO2 using
EXAFS.19 It was found that the reducibility of Pd on TiO2 was
suppressed, which was ascribed to the metal-support interac-
tion.
The TiO2 sol used as the coating agent in the present work
was initially prepared for the synthesis of titanium incorporated
microporous and mesoporous zeolite.7 Catalytic reaction test
proved that the zeolite contained the incorporated titanium in
the framework. The size of the TiO2 particles measured by
SAXS was ∼10 nm, indicating the presence of polymeric spe-
cies in the sol, that is, nano-sol.20 Investigation on the mech-
anism of the incorporation using EXAFS and XANES suggested
that the dissolved Ti precipitates converted to polymeric Ti
species with a simultaneous partial substitution of isopropyl
ligands with chloride ion and water.21 This polymeric Ti species
Figure 6. Coordination number and coordination distance of the first
shell in r space plotted as a function of heating temperature. The
distances of Pd-O in the palladium oxide reference and Pd-Pd in Pd
metal foil reference are 0.20 and 0.27 nm, respectively.
may further break down during the zeolite synthesis.
Figure 8 compares the Fourier transforms of the EXAFS
spectra of the Pd catalyst with and without TiO2 incorporation.
to PdO at around 680 °C since there was no reoxidation of Pd
metal and adsorption of oxygen before that temperature. In other
words, the Pd metal agglomerate did not spread to form the
surface attached PdO but converted to PdO upon reoxida-
tion.
Incorporation of TiO2 onto Pd catalyst led to an increase of the
third shell intensity corresponding to the Pd-Pd bond at 0.34
nm. Significant change in intensity can be regarded as the Pd-
Ti scattering contribution. Table 1 also summarized the result
of the curve fit of the EXAFS spectra above the Pd K edge.

8128 J. Phys. Chem. B, Vol. 104, No. 34, 2000
Cho and Kang
MS spectroscopy. The atomic structure of the fresh Pd catalyst
was best fitted to the tetragonal palladium oxide. The PdO
appears to spread at 680 °C over the surface of La-Al2O3 to
maximize the metal-support interaction and to transform into
the Pd metal agglomerate upon further heating above 850 °C.
The Pd metal agglomerate was reoxidized to the PdO directly
at 680 °C without the formation of the surface-attached PdO.
The thermal stability of the Pd catalyst has been improved with
the coating of TiO2 probably due to metal-support interaction.
References and Notes
(1) Trimm, D. L. Appl. Catal. 1983, 7, 249.
(2) Hildenbrand, D. L.; Lau, K.-H.; McCarty, J. G. Gas Research
Institute Report GRI-92/0521, 1992.
(3) Pfefferle, L.; Pfefferle, W. C. Catal. ReV. Sci. Eng. 1987, 29, 219.
(4) Dalla Betta, R. A. Catal. Today 1995, 26, 329.
(5) Farrauto, R. J.; Hobson, M. C.; Kennelly, T.; Waterman, E. M.
Appl. Catal. 1992, 81, 227.
Figure 8. Comparison of the Fourier transformation of the Pd/La-
Al2O3 and TiO2/Pd/La-Al2O3 catalyst (a) before and (b) after the
thermal aging at 1200 °C for 4 h. The phase shift was uncorrected
(6) Rodriguez, N. M.; Oh, S. G.; Dalla Betta, R. A.; Baker, R. T. K.
J. Catal. 1995, 157, 676.
(7) Kwak, J. H.; Cho, S. J.; Ryoo, R. Catal. Lett. 1996, 37, 217.
(8) (a) Rehr, J. J.; Albers, R. C.; Zabinsky, S. I. Phys. ReV. Lett. 1992,
69, 3397. (b) Ellis, P. J.; Freeman, H. C. J. Synchrotron Radiat. 1995, 2,
190.
(9) Widjaja, H.; Sekizawa, K.; Eguchi, K.; Arai, H. Catal. Today 1997,
35, 197.
(10) Groppi, G.; Artioli, G.; Cristiani, C.; Lietti, L.; Ramella, C.; Forzatti,
P. Proc. 4th Int. Workshop Catal. Combust., San Diego, CA, April 14-16,
1999 1999, 77.
(11) Koningsber, D. C.; Prins, R. Principles, Application, Techniques
of EXAFS, SEXAFS, and XANES; Wiley: New York, 1988.
(12) Grunes, L. A. Phys. ReV. B 1983, 27, 2711.
(13) Moraweck, B.; Renouprez, A. J.; Hlil, E. K.; Baudoing-Savois, R.
J. Phys. Chem. 1993, 97, 4288.
(14) Koningsberger, D. C.; Gates, B. C. Catal. Lett. 1992, 14, 271.
(15) Cho, S. J.; Ahn, W. S.; Hong, S. B.; Ryoo, R. J. Phys. Chem. 1996,
100, 4996.
(16) Iglesia, E. Proc. 4th Int. Workshop Catal. Combust., San Diego,
CA, April 14-16, 1999 1999, 49.
(17) Sekizawa, K.; Widjaja, H.; Eguchi, K. Proc. 4th Int. Workshop
Catal. Combust., San Diego, CA, April 14-16, 1999 1999, 49.
(18) Tauster, S. J.; Fung, S. C. J. Catal. 1978, 55, 29.
(19) Asakuara, K.; Inukai, J.; Iwasawa, Y. J. Phys. Chem. 1992, 96,
829.
The Ti contribution in the TiO2-coated Pd catalyst appeared at
0.36 nm, of which the estimation of the Pd-Ti CN and the
Debye-Waller factor may be ambiguous due to multiple
scattering effect. However, the distance information appeared
to be reliable to indicate the presence of Ti at the outer shell.
Thermal aging of the Pd catalyst at 1200 °C for 4 h affected
remarkably the Fourier transformation of EXAFS spectrum. The
best-fitted structural parameters were almost the same as that
of bulk Pd foil except for the smaller Pd-Pd CN, 10.6,
suggesting the irreversible transformation of PdO to Pd metal
upon heating at 1200 °C. Meanwhile, the same thermal aging
treatment of the TiO2-coated Pd catalyst at 1200 °C for 4 h did
not affect the Fourier transform of EXAFS spectra as much as
for the Pd catalyst. As shown in Figure 8, the tetragonal PdO
structure was still present after such a severe sintering condition.
The obtained structural parameters were also partly consistent
with those of tetragonal PdO. It was thus concluded that TiO2
coating ameliorated the thermal stability of the Pd catalyst.
Conclusion
(20) Kwak, J. H. Ph.D. Dissertation, Korea Advanced Institute of Science
and Technology, 1996.
(21) Cho, S. J.; Jun, S.; Kwak, J. H.; Kang, S. K. Ryoo, R. Proc. ISES
1997 Solar World Congr., Taejon, Korea, 1997, 591.
The structural model for the reversible transformation has
been proposed using EXAFS and XANES combined with TGA/