Characterization and activity of supported palladium combustion catalysts

Article abstract of DOI:10.1006/jcat.2002.3515

The catalytic activity of Pd supported on γ-Al₂O₃, Ba-Al₂O₃, and La-Al₂O₃ was studied in complete oxidation of methane when operating in excess of oxygen. The presence of La or Ba improved the thermal stability of alumina support when exposed to ≤ 1000°C. No specific effect was detected when the samples were calcined at 500°C. The difference in combustion activity was due to the difference in surface oxidation states of the Pd particles. The presence of foreign ions in the alumina structure led to surface PdO only. After calcination at 1000°C, La and Ba inhibited the formation of a mixed phase in which Pd in the metallic and oxide phase is present. The large Pd particles showed stronger interaction with Ba and La, respectively, as Ba and La were in the form of small oxide particles well dispersed in the alumina matrix. There was much more interaction between the Pd particles and the foreign ions present in Al₂O₃ than in the case of small and well-dispersed Pd particles.

Full text of DOI:10.1006/jcat.2002.3515

Journal of Catalysis 207, 139–149 (2002)
doi:10.1006/jcat.2002.3515, available online at http://www.idealibrary.com on
Characterization and Activity of Supported Palladium
Combustion Catalysts
∗
∗
∗
P. O. Thevenin,∗,1 E. Pocoroba, L. J. Pettersson, H. Karhu,† I. J. Va¨yrynen,† and S. G. Ja¨ra˚s
∗
KTH—Royal Institute of Technology, Department of Chemical Engineering and Technology, Chemical Technology, Teknikringen 42, SE 10044
Stockholm, Sweden; and †University of Turku, Department of Applied Physics, FIN 20014 Turku, Finland
Received November 21, 2001; revised January 10, 2002; accepted January 16, 2002
increasing number of reviews and general articles on cata-
lytic combustion have been published (4–8). It has been
proven that emission levels below 5 ppm can be reached
for the various pollutants previously mentioned (9). Be-
sides the reduction in the CO, HC and NOx levels in the
emissions, such a technique lowers noise and vibration lev-
els, which may cause premature damage to the gas turbine
components.
In addition, the use of methane would avoid the possi-
ble formation of soot, which can be rather significant when,
for instance, combusting diesel fuel. Moreover, it is a rather
cleanfuelwithnonitrogen-boundcompounds, henceavoid-
ing the possible formation of so-called fuel–NOx . The in-
creasing concern about emission of greenhouse gases such
as methane and carbon dioxide has been accompanied by
more stringent regulations for the emissions of such com-
pounds.
The catalytic activity of Pd supported on γ -Al2O3, Ba-Al2O3,
and La–Al2O3 has been examined in complete oxidation of methane
when operating in excess of oxygen. Two different sizes of Pd par-
ticles have been considered. Foreign ions have a strong influence
with respect to the stabilization of alumina when submitted to a
temperature as high as 1000◦C. In contrast, no specific effect can be
detected when the samples are calcined at 500◦C. Interaction with
the supported palladium particles, observed during the combus-
tion reaction, has been investigated by X-ray photoelectron spec-
troscopy and temperature-programmed oxidation. The difference
in combustion activity is attributed to the difference in surface oxi-
dation states of the Pd particles. The presence of foreign ions in the
alumina structure results in surface PdO only. When supported on
γ -Al2O3, small amounts of metallic Pd can be detected, resulting in
c
a lower ignition temperature.
ꢀ 2002 Elsevier Science (USA)
Key Words: catalytic combustion; methane; palladium; alumina;
metal-support interaction; TPO; XPS.
The combustion of natural gas generates more energy
per mole of carbon dioxide produced when compared to
other fuels (10). While the combustion of methane gener-
ates 890 kJ/mol of CO2 produced, the corresponding value
for n-decane and coal (graphite), for instance, are 678 and
393 kJ/mol, respectively. This property is related to the high
H/C ratio, which means that methane is the most difficult
hydrocarbon to oxidize. It contains only C–H bonds, which
bring the molecule a rather high stability when compared
to compounds containing C–C bonds (11).
Noble metal catalysts are the most active species
for performing the complete oxidation of hydrocarbons.
Palladium is reported to be the most active species in
methane catalytic combustion, when operating under ox-
idizing atmosphere (12). When comparing palladium and
platinum, Pd is more active for the oxidation of short-chain
hydrocarbons whereas Pt exhibits higher activity toward
long-chain hydrocarbons and aromatics (13).
1. INTRODUCTION
Combustion of fuel is one of the key processes in heat and
power generation. A major issue related to this process is
the formation of significant amounts of pollutants that oc-
cursduringthereactionbetweenthehydrocarbonsandoxy-
gen. The main environmental concern focuses on emissions
of carbon monoxide, hydrocarbons, and nitrogen oxides.
Natural gas provides an attractive source of energy since
it is in abundant supply. Methane as a fuel has drawn
increasing attention to various types of utilizations and
has recently found application in various environmentally
friendly processes. It is suitable for firing gas turbine com-
bustion chambers (1) and has been reported as an alterna-
tive fuel for automotive applications (2).
In gas turbine applications, catalytically stabilized com-
bustion, which was first proposed by Pfefferle (3), stabilizes
the reaction of leaner fuel–air mixtures, hence lowering the
adiabatic flame temperature. During the last decades, an
The noble metal is usually dispersed over an inert support
with a large surface area. The high temperature released
during the oxidation of methane requires that the support
and the catalyst maintain their integrity. The stabilization
of alumina support by addition of large ions, and more
specifically La, has been largely reported in the literature
1 To whom correspondence should be addressed. Fax: +46 (0) 8 10 85 79.
E-mail: philippe@ket.kth.se.
139
0021-9517/02 $35.00
c
ꢀ 2002 Elsevier Science (USA)
All rights reserved.

140
THEVENIN ET AL.
(14–16). More recently, the influence of Nd and its inter- The operating parameters were monochromatic Cu-Kα ra-
action with PdO has been considered (17). The influence diation, Ni filter, 30 mA, 40 kV, 2θ scanning from 20 to 80◦,
on the combustion activity of palladium seems to be af- and a scan step size of 0.02. Phase identification was done
fected by a possible interaction with La or Ba present in using the reference database (JCPDS-files) supplied with
the alumina structure. The purpose of the present work is the equipment.
to investigate the metal–support interaction by means of
Transmission and scanning electron microscopy. The
different characterization techniques. Results concerning
morphology, size, and dispersion of the palladium parti-
temperature-programmed oxidation (TPO)and X-raypho-
cles on the alumina washcoat were studied using a trans-
mission electron microscope (TEM—JEOL 2000 FX) op-
erated at 200 kV and a scanning electron microscope
(SEM—Zeiss DSM 940) operated at 20 kV. The TEM
toelectron spectroscopy (XPS) are providing information
concerning these interactions. Specific attention is given to
the role of La and Ba.
microscope was equipped with an EDS (energy dispersive
spectroscopy) detector (LINK AN 10000). The SEM mi-
2. EXPERIMENTAL
croscope was equipped with an EDS detector (LINK QX
2000).
2.1. Catalyst Preparation
Palladium was deposited onto three different supports
Temperature-programmed oxidation. The redox prop-
by incipient-wetness impregnation, as described elsewhere
erties of the samples were measured by means of
(18). Supports with the following chemical composition
temperature-programmed oxidation (TPO) experiments
have been employed: Al2O3 (Condea, Puralox HP 14-
on a Thermoquest TPD/R/O 1100 equipped with a ther-
mal conductivity detector (TCD). Approximately 100 mg
of calcined powder was used for each measurement. Prior
to TPO experiments, the samples were reduced in a 5%
150, 144 m2/g), Ba–Al2O3 (Condea, Puralox SCFa-140 B3,
143 m2/g), and La–Al2O3 (Condea, Puralox SCFa-140 L3,
142 m2/g). The content of Ba and La, respectively, was
3 wt% for both supports. A Pd(NO3)2 solution (Alfa Ae-
H2/Ar atmosphere. Subsequently, TPO experiments have
sar, 8.41 wt%) was used as the metal precursor. The vol-
been performed using a gas mixture of 2.12% O2/He
with a flow rate of 30 cm3 · min−1, increasing the tem-
ume of the impregnation solution was adjusted in order to
achieve a final washcoat with a precious metal loading of
perature from 25 to either 500 or 1000◦C depending on
2.5 wt%.
the calcination temperature of the sample, at the rate of
The catalysts were dried for 12 h at 120◦C prior to calci-
10◦C · min−1
.
nation. Subsequently, the catalyst powder was divided into
two parts. One part was calcined for 4 h in air at 500◦C,
whereas the other portion was calcined at 1000◦C.
The prepared catalysts were then coated onto a mullite
tube (outer diameter, 4 mm) to test their catalytic activity
in complete oxidation of CH4. The dip coating was accom-
plished according to the following method: the powder to
be deposited on the monolith was first dispersed in ethanol
to form a slurry (dry content of 20 wt%) and ball-milled
for 24 h to obtain a homogeneous suspension. The length
of the catalyst layer was 30 mm, corresponding to a weight
of about 20 mg. The coated catalysts are then calcined at
500◦C for 4 h for the first batch and 1000◦C for 4 h for the
second set of catalysts to avoid any thermal deactivation
during reaction.
X-ray photoelectron spectroscopy. The calibration of
the binding energy (BE) was based on the carbon impu-
rity peak at 284.6 eV. Samples were charged approximately
5 eV in the X-ray bombardment, but we did not observe
changes in charging during the typically 7-h measurement
time. The acquisition time of 7 h was employed in order to
improve the signal-to-noise ratio, as due to the relative high
surface area and the small particle size the photoelectron
intensity was very low. Therefore, at least 20 scans were per-
formed at the rate of 1 eV/s, in the desired energy range. The
˚
depth of the analysis was about 1–20 A. In the computer
fitting procedure of spectra with multiple states due to spin–
orbit interaction, intensity ratios and energy separations of
states were kept constant at the theoretical values. Shirley
background due to inelastic photoelectron scattering was
removed prior to Voigt function peak fitting. A pass energy
of 35 eV was used, with typically 1×10−8 Torr pressure dur-
ing the analysis. A Perkin–Elmer PHI 5400 ESCA was used,
with a nonmonochromatized Mg Kα source, using 100 W of
power to prevent sample reduction during the X-ray bom-
bardment. Impurities other than carbon were not observed.
Sensitivity factors used in quantitative analysis for Al 2p,
2.2. Characterization Techniques
BETsurfacearea. TheBETsurfaceareameasurements
were performed by adsorption/desorption of N2 at the liq-
uid nitrogen temperature. The samples were degassed in
vacuum for at least 8 h at 250◦C prior to adsorption mea-
surements.
X-ray powder diffraction. The phase composition of the Ba 3d5/2, La 3d, O 1s, and Pd 3d were 0.234, 7.469, 9.122,
various samples was determined by means of X-ray powder 0.711, and 5.356, respectively (19). The accuracy of the BE
diffraction (XRD), using a Siemens Diffraktometer 5000. values is ca. 0.15 eV.

SUPPORTED PALLADIUM COMBUSTION CATALYSTS
141
TABLE 2
2.3. Activity Tests
Atomic Surface Composition of the Samples after Calcinations
at 500 and 1000◦C, Determined by XPS analysis
Activity tests were performed on catalysts coated on a
mullite tube, located in an annular reactor, as described
elsewhere (20). The use of such annular reactors was first
proposed by McCarty (21) and has since been widely
reported in the literature (22, 23). It is made of quartz and
placed in an 800-mm-long furnace. A K-type thermocou-
ple is inserted in the mullite tube in order to measure the
catalyst temperature during combustion experiments.
During the−t1ransient experiments, a furnace heating-rate
0.94 dm3 · min−1, resulting in a linear gas velocity of
1.0 m · s−1, with a fuel concentration of 1 vol% CH4 in air.
The composition of the product gas was monitored by on-
line gas chromatography (GC Varian 3800) equipped with
a TCD. The conversion calculations are based on the con-
centration of CH4 and corroborated by the yields of CO2
and CO.
Pd/Al2O3
(Pd : Al)
Pd/La–Al2O3
(Pd : La : Al)
Pd/Ba–Al2O3
(Pd : Ba : Al)
Temperature
0.013 : 1a
0.12 : 1
0.0017 : 1
0.013 : 0.012 : 1a
0.084 : 0.27 : 1
0.00089 : 0.026 : 1
0.013 : 0.012 : 1a
0.054 : 0.31 : 1
0.0012 : 0.022 : 1
500◦C
1000◦C
a Calculated bulk atomic ratio.
of 3◦C · min
was used. The total gas flow-rate was
alumina reflect the significant influence of La or Ba. This
effect becomes noteworthy when the calcination tempera-
ture reaches 1000◦C. The surface area of the nondoped sam-
ple undergoes a sharp decrease after calcination at 1000◦C
due to sintering, showing a specific surface area of only
75 m2 · g−1
.
In contrast, the materials containing La or Ba maintain
a notably higher surface area, when compared to neat alu-
mina, and the modified supports exhibit a surface area in
the range 95–105 m2/g. The presence of La or Ba is effec-
tive in reducing the strong sintering of alumina. La is more
effective in thermally stabilizing alumina, probably due to
its larger ionic diameter (14), compared to Ba.
3. RESULTS
3.1. Surface and Bulk Properties of the Prepared Catalysts
The results concerning BET measurements, TEM and
SEM observations, and selected XPS measurements are re-
ported in Tables 1 and 2.
X-ray powder diffraction. The objective of the XRD
BET surface area. After calcination at 500◦C, all sam- measurements was to (a) examine the nature of the Pd par-
ples present similar surface properties with a surface area ticles and (b) investigate the possible modifications of the
of about 144 m2 · g−1, as seen in Table 1. No major differ- alumina-based washcoat. Special attention is given to the
ence can be observed between the pure alumina and the possible formation of La and Ba mixed oxides as well as to
smaples containing Ba or La.
changes in the alumina phases. Concerning the identifica-
When submitted to further calcination, up to 1000◦C, the tion of crystalline structures of Pd, the focus of attention
measurements of the BET surface area of the stabilized was the oxide form, PdO, as it is the most active species
TABLE 1
Surface Properties of the Different Prepared Catalysts
Calcination
temperature
(◦C)
Binding energyc (eV)
Surface areaa
(m2/g)
Pd particle sizeb
(nm)
Sample
Pd 3d5/2
La 3d5/2
Ba 3d5/2
Al 2p
74.5
Pd/Al2O3
500
500
144
143
142
75
5–10
5–10
334.9 (36%)
336.4 (64%)
—
—
780.5
—
Pd/Ba–Al2O3
Pd/La–Al2O3
Pd/Al2O3
334.9 (24%)
336.2 (76%)
—
74.2
74.3
73.8
500
5–10
334.9 (48%)
336.5 (52%)
835.5
839.1
1000
80–100
334.9 (13%)
336.5 (87%)
—
—
Pd/Ba–Al2O3
Pd/La–Al2O3
1000
1000
94
80–100
80–100
336.8
336.7
—
780.7
74.1
74.2
107
835.9
839.2
—
a Measured by BET.
b Determined by TEM.
c All BEs referenced to C 1s = 284.6 eV.

142
THEVENIN ET AL.
FIG. 1. Comparison of the X-ray diffractogram of pure alumina with stabilized alumina after calcination at 500 and 1000◦C.
for the complete oxidation of methane, when operating in
excess of oxygen.
The first observable effect of the crystal structure is cal-
cination temperature (cf. Fig. 1). An increase from 500 to
The tetragonal form of Pd was identified by its XRD re- 1000◦C gives rise to the formation of more crystalline PdO
flection at 2θ = 33.889◦ for the (101) plane and 2θ = 54.759◦ for all samples, with a more intense peak at 2θ = 34◦. After
for the (112) plane. The cubic Pd0 was identified by its XRD calcination at 500◦C PdO is already present in a crystalline
reflection at 2θ = 40.115◦, (111) plane, and 82.096◦, (222) state, at least partially. A similar observation can be made
plane. Owing to the overlapping of alumina and Pd reflec- concerning the crystal structure of alumina. When raising
tions, the other main peaks of PdO (41.948 and 60.207◦) the calcination temperature, the peaks of alumina and Pd
and of Pd0 (46.682◦) could not be used for structure deter- develop into sharper and more intense ones, reflecting an
mination.
important crystal growth, which in the case of alumina may
We observed that the PdO peak for the samples calcined be accompanied by a decrease in the specific surface area
at 500◦C is rather small and broad, reflecting the presence of the support material.
of small crystalline particles of PdO (cf. Fig. 1). On calci-
In contrast, an important difference is observed when
nation at 1000◦C, the shape of the peak becomes sharper comparing the XRD pattern of the pure alumina and Ba- or
and more intense, corresponding to an increase in size of La-stabilized alumina. The pure alumina undergoes severe
the crystalline particle. Despite the calcination at 1000◦C modification of its crystalline structure under calcination at
in air, we can still detect the presence of small amounts 1000◦C, whereas in the presence of additives the transfor-
of cubic Pd(111). Nevertheless, palladium particles exhibit mation of the Al2O3 structure is lower. As reported in the
a rather high PdO/Pd ratio with respect to the crystalline literature (24), the alumina undergoes the following phase
phases, as seen by comparing the intensity of the reflection transformation when increasing the calcination tempera-
at 2θ = 33.889◦ and 2θ = 40.115◦.
ture (see Fig. 2).
Concerning the phase transformation, solid-state diffu-
The La- and Ba-modified aluminas exhibit similar XRD
patterns after calcinations at both 500 and 1000◦C. It seems sion occurs within the defects of the crystal lattice. Atoms,
that the increase in PdO particle size is comparable as well, ions, or vacancies can move via these defects to a lower en-
despite the different composition of the support materials. ergy state, and such movements may be associated with

SUPPORTED PALLADIUM COMBUSTION CATALYSTS
143
FIG. 2. Modification of the crystal phase of alumina under increasing calcination temperatures.
phase transformation. The presence of voluminous ele-
ments within the crystal lattice of alumina strongly lowers
the mobility of the atoms (25).
These transformations are clearly observed on the pure
alumina samples as well. The most evident aspect of these
changes is the emergence of a θ-Al2O3 peak, simultane-
ously with the increase in intensity of the δ-Al2O3 diffrac-
tion peak (when compared to the Pd peaks that have a
similar intensity regardless of the nature of the support).
When considering the samples containing Ba or La ions,
these changes are not that marked and the alumina tends
to remain in the γ -Al2O3 crystal phase.
The addition of Ba or La stabilizes the alumina material
against changes in crystal phase, as reported in earlier litera-
ture (24). This commercial support, with a La or Ba content
of about 3 wt%, exhibits stable behavior, even after calcina-
tionat1000◦Cfor4h. Incomparison, neataluminashowsan
intense modification of the crystal structure. The samples
calcined at 500◦C are mostly amorphous whereas further
calcination up to 1000◦C causes a severe crystal growth
that is accompanied by a decrease in the BET surface
area.
Transmission and scanning electron microscopy. The
results from the microscopy analyses show that the Pd par-
ticles are present in two distinct sizes, resulting from the two
different calcination temperatures. The samples calcined at
500◦Cfor4hcontainPdparticlesthathaveadiameterinthe
range of 2–5 nm whereas the catalysts calcined at 1000◦C
exhibit much larger particles, in the range 80–100 nm. In
both cases, an even Pd dispersion over the support ma-
terial was noticed regardless of the nature of the sup-
port. No substantial concentration of Pd particles could be
detected.
Due to the detection limit of the SEM microscope, the
analysis of the samples concerned only the catalyst cal-
cined at 1000◦C. The catalysts show well-dispersed palla-
dium particles on the washcoat material (cf. Figs. 3A–3C),
independent of the nature of the support material. We
did not observe any large agglomerate of Pd particles or
any strong discrepancy in particle size. This result corrob-
orates the analysis made with the TEM microscope. In this
case, the higher resolution enabled us to observe even par-
ticles as small as about 5 nm. The detection limit of the
TEM (ca. 1 nm) did not allow the observation of smaller
particles.
In addition to the behavior of the metal particles, sinter-
ing of the support material can be observed. We can observe
that the size of the alumina particles increases when raising
thecalcinationtemperaturefrom500to1000◦C. Incontrast,
FIG. 3. SEM photograph of the samples calcined at 1000◦C, sup-
ported on the different materials. (A) Al2O3, (B) La-stabilized Al2O3,
and (C) Ba-stabilized Al2O3.

144
THEVENIN ET AL.
In contrast, the samples calcined at 1000◦C exhibit sin-
gular patterns, depending on the chemical composition of
the washcoat. The samples supported on stabilized alumina
present a first peak with a maximum at about 300◦C, repre-
senting the oxidation of Pd species. The oxidation starts as
soon as the temperature of the catalyst is increased. When
supported on pure alumina, the oxidation of Pd reaches
a maximum at 500◦C only. Complete oxidation is however
reachedatasimilartemperature, i.e., 600◦C. Thepalladium-
free supports do not present any significant activity during
TPO experiments (26). The variations between the differ-
entcatalystswithrespecttotheirTPOprofilescantherefore
be attributed to the modifications in metal–support inter-
action by the stabilizers, La and Ba.
Negative peaks represent an O2 “desorption” when in-
creasing the temperature up to 800◦C. This occurs in two
successive and distinctive steps: a first one at about 720◦C
followed by a second, more intense one at ca. 750◦C. This
decomposition of PdO into metallic Pd is the cause of this
behavior. These two steps observed are directly connected
to the two types of PdO, as previously reported (26). PdO
seems to be present in a bulk form (27) and a surface com-
plex oxide form (21), the later being decomposed at lower
temperatures than the former.
It is important to notice that when supported on La–
Al2O3, the decomposition of PdO takes place in one main
step only, with a major peak at 755◦C. A first peak is still
detectable at 715◦C, but its intensity is negligible when com-
paring the TPO profiles measured over the other catalysts.
FIG. 4. TPO experiments on samples calcined at 500◦C for 4 h.
the samples containing rare earth elements are not subject
to such modification.
Temperature-programmed oxidation. The purpose of
the TPO experiments was to investigate the behavior of
supported Pd catalysts under oxidative conditions. Specific
attention is given to the PdO–Pd decomposition that takes
place at about 750–800◦C. Temperature-programmed oxi-
dation experiments were completed over the samples cal-
cined at 500◦C (Fig. 4) and 1000◦C (Fig. 5). During the TPO
performed on the samples calcined at 500◦C, similar O2 pro-
files are observed, in spite of the nature of the support. We
observed a single peak of O2 consumption at about 300◦C.
The peak is relatively wide, reflecting an O2 consumption
thattakesplaceoveranextendedtemperaturewindow. This
O2 consumption starts at about 250◦C and the oxidation is
complete at 350◦C. A similar observation can be made for
the various catalysts in spite of the different nature of the
support materials.
X-ray photoelectron spectroscopy. It seems that the
presence of La and Ba additives causes a raise in the binding
energy of palladium in the samples calcined at 1000◦C, as
seen in Table 1. After calcination at 500◦C this phenomenon
was not observed, which indicates that a temperature of
500◦C is not high enough to bring about a strong interaction
of Ba or La ions with Pd. In addition, binding energies give
evidencethatsamplescalcinedat1000◦Cwereslightlymore
oxidized than the samples calcined at 500◦C. The samples
containing additives do not present any metallic Pd con-
tribution in the XPS spectra, whereas the neat Al2O3 sup-
port reflect the presence of about 13% Pd0. Furthermore,
the samples calcined at 500◦C and Pd/alumina calcined at
1000◦C show the presence of small amounts of Pd0. Most
likely, the reduction was due to X-ray bombardment in ul-
trahigh vacuum conditions, which only the two most stable
catalysts were able to fully endure.
We observed 780.5–780.7 eV binding energies for barium
3d5/2, which is in good agreement with the results published
by Talo et al. (28). This BE actually corresponds to the lit-
erature value of Ba0, the metallic bulk barium value. How-
ever, metallic Ba in the literature has a strong satellite line
at ca. 787 eV, which is significantly weaker for this catalyst.
Most likely this BE originates from BaO in the dispersed
state. Comparison of the two Pd/Ba–Al2O3 catalysts at the
FIG. 5. TPO experiments on samples calcined at 1000◦C for 4 h.

SUPPORTED PALLADIUM COMBUSTION CATALYSTS
145
FIG. 6. Combustion activity of the catalyst after calcination at 500◦C.
two different calcination temperatures, in Table 2, shows This is in agreement with the results reported by Haack
a 93% drop in surface Ba atomic concentration when the et al. in the case of La-stabilized alumina, with a low La/Al
calcination temperature was raised to 1000◦C.
ratio.
The BEs of lanthanum in both catalysts with different
calcination temperatures were in agreement with previ-
ous studies of dispersed lanthanum, i.e., 835.0–836.1 eV
(28, 29). Our initial lanthanum-loading, La/Al ratio was
3.2. Activity Tests
Results of the activity tests over the various catalysts are
0.27, which can be compared to the range 0.25–0.50 in the represented in Fig. 6 for the samples calcined at 500◦C and
work by Haack et al. (29). In this loading range, accord- Fig. 7 for the ones calcined at 1000◦C.
ing to Bettman et al. in their XRD studies (30), lanthanum
For the samples calcined at 500◦C, no major differences
remains well dispersed in the form of surface lanthanum are observed with respect to the catalytic activity of Pd-
aluminate, LaAlO3. At higher La/Al ratios, ranging from supported catalysts in complete oxidation of methane.
0.75 to 2.0, Haack et al. observed the Al 2p peak at 73.0 eV, Pd/Al2O3, Pd/La–Al2O3, and Pd/Ba–Al2O3 present similar
which corresponds to Al in LaAlO3. In our XPS experi- combustion activity regardless of the nature of the support
ments we observed that the Al 2p BEs were slightly lower material. We observe an ignition of the fuel–air mixture that
for the samples calcined at the higher temperature, which is startsat about 470◦C (T10, temperaturefor10% conversion)
due to the increased formation of LaAlO3, probably in crys- and 50% conversion is reached at about 550◦C. In addition,
talline form. While the bulk concentrations of La are similar the apparent reaction rate for the complete oxidation of
in the catalysts calcined at 500 and 1000◦C, the atomic- methane is similar over the three concerned catalysts.
surface La/Al ratio after calcination at the higher temper-
In contrast, the catalysts calcined at the higher temper-
ature was only ca. 10% of the ratio in the catalyst calcined ature (1000◦C) and presenting larger Pd particles have a
at the lower temperature. This is due to the increased ag- combustion activity that seems to be directly influenced by
glomeration of Al at the higher temperature, which also the chemical composition of the support. In fact, the use
may lead to the formation of LaAlO3 crystals. The BEs for of γ -alumina as a support for Pd particles brings a higher
the La 3d5/2 line are higher than 835 eV, which is attributed activity when compared to the use of stabilized aluminas.
to one form of surface lanthanum (28, 29, 31). When the size There is a difference of at least 100◦C with respect to the
of the particles increases, dispersion should weaken but we T10 and T50 values. The catalysts supported on stabilized
observed only 0.4 eV change in BE toward bulk La2O3, alumina have a similar ignition behavior until the cata-
which is still well dispersed in the La BE range. The O 1s lyst temperature reaches 620◦C. In the temperature win-
line was found at 531.3 eV in the Pd/Al2O3 and Pd/La– dow 620–800◦C, a higher apparent reaction rate over the
Al2O3 samples calcined at 1000◦C without any additional Pd/La–Al2O3 catalyst is observed when compared to the
shoulders, which is characteristic of O in Al2O3, but does Pd/Ba–Al2O3 system. When the temperature is further in-
not offer any additional information regarding La and Al. creased, they retrieve an identical apparent reaction rate.

146
THEVENIN ET AL.
FIG. 7. Combustion activity of the catalyst after calcination at 1000◦C.
The decrease of combustion activity due to the PdO decom- lattice, hinder or at least retard these diffusion processes.
position takes place at about 800◦C for the three catalysts This results in a larger BET surface area when compared to
but is more pronounced over the catalysts supported on Ba- pure alumina. In addition, the alumina particles observed
and La-stabilized alumina.
by TEM are smaller. The influence of this addition of La or
Ba is observable as well when examining the crystal struc-
ture of the support material. In parallel to the decrease in
surface area, the alumina support undergoes modification
of its crystal phase. The presence of new phases (θ-Al2O3,
δ-Al2O3, and traces of α-Al2O3) are noticed when pure alu-
minaiscalcinedat1000◦C, whereasγ -Al2O3 isthedominat-
4. DISCUSSION
4.1. Structure of the Washcoat—Effect of La and Ba
on the Stability of Al2O3
Materials that have different chemical composition were ing phase after thermal treatment at 500◦C. In contrast, the
used to support palladium particles. The calcination of the samples containing the foreign ion exhibit the initial crystal
catalyst powders was performed at two distinctive temper- phase, γ -Al2O3, after calcinations at both 500 and 1000◦C.
atures. In that way, we achieved catalysts with palladium
However, besides this clear positive effect on the thermal
particles of different size, in order to investigate the metal– stability of alumina material, the presence of foreign ion
support interaction with respect to the metal particle size elements has a noteworthy effect on the properties of Pd
and the nature of the support material.
The rate of sintering is an activated process with a dif- evaluation of the catalytic activity in methane combustion.
fusion coefficient that is exponentially dependent on tem- When investigating the XPS line for La, the binding en-
species. This is observed both by XPS analysis and through
perature. The surface diffusion becomes significant near ergy is the same as the one for La in surface LaAlO3. The
the Hu¨ttig temperature (∼0.3 Tm K). When the temper- XPS measurements indicate the presence of La in the form
ature further increases near the Tammann temperature of LaAlO3. This phase is however not detected during XRD
(0.5 Tm K), volume diffusion starts, and migration of inter- analysis. This comparison of the results obtained by XRD
stitials (Frenkel effect) or vacancies (Shottky effect) may be and XPS, with respect to the existence of La, leads to the
involved. For instance, the addition of rare earth elements conclusion that the size of the particles is probably below
such as La has been reported to be suitable for inhibit- the XRD detection limit (ca. 2 nm), at least for the La
˚
ing these diffusion processes (24). Sintering may occur via present in the outer layer (1–20 A). Similar observations are
various diffusion processes: surface, volume, and/or grain made concerning the Ba particles. In addition, we measure a
boundary diffusion (32, 33). The resistance to sintering of decrease in surface atomic concentration in La and Ba when
the catalyst supports is clearly improved by addition of even raising the calcination temperature from 500 to 1000◦C.
small amounts of large ions such as barium or lanthanum.
The observed results are in agreement with previous find-
These large ions (such as +), located in the alumina crystal ings in the literature reporting on the stabilizing effect of

SUPPORTED PALLADIUM COMBUSTION CATALYSTS
147
rare earth additions (14, 15). The BET surface area mea- surface of the catalyst calcined at 1000◦C, when supported
surements are corroborated by the X-ray diffraction pat- on the modified alumina. The reduction of Pd can be due
terns that exhibit the strong influence of foreign ions on the to the photoelectron beam as well. The shift is observed on
thermal stability of the alumina supports.
the samples calcined at 1000◦C only. The XPS line for Pd in
the samples calcined at 500◦C is at the same binding energy,
in spite of the chemical composition of the support. Never-
theless, the TPO experiments (Fig. 5) suggest an influence
of Ba and La as well. Just as for the XPS analysis, the effect
is observable only when the samples are calcined at 1000◦C,
as the samples calcined at 500◦C present almost identical
TPO profiles.
The catalytic activity of the various catalysts in complete
oxidation of methane is illustrated in Figs. 6 and 7. In Fig. 6,
the activity of small particles (calcined at 500◦C) is dis-
played whereas Fig. 7 exhibits the activity of larger par-
ticles (calcined at 1000◦C), both in complete oxidation of
methane.
The activity of the small particles is apparently not af-
fected by the various natures of the support material. The
presence of rare earth elements in the alumina matrix does
not affect the catalytic activity of the supported PdO par-
ticles. All the different catalysts present a similar ignition
path, with a difference in T50 and T10 of fewer than 10◦C.
On the other hand, it seems that the large particles present
singular behavior for each support. The neat alumina pro-
vides a lower ignition temperature whereas the use of La-
and Ba-stabilized Al2O3 materials requires a higher cata-
lyst temperature to reach similar conversion. The catalyst
supported on neat alumina displayed a T50 of about 450◦C.
On the other hand, the modified alumina supports require
an inlet temperature of about 530 and 550◦C for Ba–Al2O3
and La–Al2O3, respectively, to reach a methane conversion
of 50%.
4.2. Structure of Pd—Effect of Addition of La and Ba
Considering the investigation of the palladium particles,
their size is apparently not affected by the presence of La
and Ba in the alumina support. Both microscopy examina-
tions and XRD study show that the size of the palladium
particles is identical and they are well dispersed at the sur-
face of the support. The TEM observation showed that the
Pd particles sinter considerably when the calcination tem-
perature is raised from 500 to 1000◦C. Similar conclusions
can be drawn from the XRD pattern where the peak of
the PdO reflection increases in intensity and getts sharper.
This increase in palladium particle size is not affected by
the nature of the support material on which the metal par-
ticles have been deposited. Furthermore, the intensity and
shape of the PdO peaks are similar in the different samples
considered.
In the three samples calcined at 500◦C reported in
Table 2, the binding energy value of Pd 3d5/2 corresponds to
PdO. It matches results by Shyu et al. (34) and also Otto et al.
(35). Shelef et al. (36) observed similar PdO binding ener-
giesonPd/La-sapphiresamplesafteroxidationat600◦C. No
PdO2 at 338.3 eV, as measured by Otto et al. (35), is present
in our samples. In addition, the surface atomic concentra-
tion measured by XPS reflects the encapsulation of Pd when
increasing the calcination temperature. We notice a strong
decline in the amount of Pd, Ba, and La accessible at the
surface, while the concentration of Al rises considerably.
This probably explains the lower activity of the samples
calcined at 1000◦C when compared to the samples treated
at a lower temperature. In addition, the influence of Ba and
La with respect to the Pd particles is not observed when
considering the bulk properties. The XRD analysis displays
similar patterns regardless of the nature of the support on
which the palladium is dispersed. This is in agreement with
the TEM and SEM observations that imply that the size
of the Pd particles is identical for all the catalysts calcined
atthesametemperature. ThesizeofthePdparticlesisabout
80–100 nm for the samples calcined at 1000◦C. The initial
size, after calcination at 500◦C, is in the 2- to 5-nm range.
The presence of small amounts of metallic Pd is observed
by XRD.
Duringthecombustionofmethaneoverpalladium-based
catalysts, a decrease in activity is observed at about 750◦C.
This phenomenon has been largely reported in the litera-
ture and is due to the decomposition of active palladium
oxide, PdO, into less active metallic palladium (21, 37, 38).
In addition to the increase in ignition temperature ob-
served over the modified support materials, the decrease
in methane combustion activity, observed at about 800◦C,
exhibits different intensity with respect to the nature of the
washcoat. The loss of combustion activity is much more in-
tense when the oxidation takes place over Pd/Ba-stabilized
Al2O3 or Pd/La-stabilized Al2O3, when compared to the
Pd/Al2O3. The decomposition rate of PdO into less active
Pd is increased by the presence of La or Ba in the support.
This is probably due to differences in acidity of the support,
as the basicity of the support may follow the decreasing or-
der Ba–Al2O3 > La–Al2O3 ꢁ Al2O3.
As opposed to the above results, the surface character-
izations (XPS, TPO, and combustion experiments) reflect
a much stronger influence of the additives. By XPS, for in-
stance, we notice a shift in the PdO peak toward the higher
binding energy when the samples contain Ba or La. The
When the particles are larger, the catalysts have dif-
stronger effect is observed for the catalyst supported on ferent ignition behavior depending on the nature of the
La-stabilized alumina. The oxidation state is then affected support. The alumina-supported catalysts, despite a lower
by the presence of La or Ba and only PdO is present at the surface area, are much more active than the samples

148
THEVENIN ET AL.
impregnated on thermally stable materials. This is prob- complete oxidation of methane when operating in excess
ably due to a difference in palladium oxidation state when oxygen.
compared to the samples containing Ba or La. In addition,
The increase in the apparent reaction rate observed be-
the presence of metallic Pd for the samples dispersed on tween 600 and 700◦C for the catalyst supported on La–
alumina may enhance the oxidation of methane, as Pd is re- alumina is confirmed by the TPO experiments. The decom-
sponsible for the dissociation of methane (13). The absence position of PdO into metallic and less active Pd starts at
of such Pd0 when La or Ba are present is responsible for the higher temperature. Only one peak in the higher tempera-
lower combustion activity. Moreover, it is possible to dis- ture region is observed.
tinguish the catalysts with respect to the nature of the ion
The large Pd particles exhibit stronger interaction with
that has been employed to stabilize the alumina support. Ba and La, respectively, as Ba and La are in the form of
The Pd/Ba-stabilized Al2O3 catalyst, calcined at 1000◦C, small oxide particles well dispersed in the alumina matrix.
exhibits a lower combustion activity than Pd/La-stabilized There is much more interaction between the Pd particles
Al2O3, even if the surface areas of the supports are in the and the foreign ions present in Al2O3 than in the case of
same order of magnitude. In that case, the interaction be- small and well-dispersed Pd particles. When increasing the
tween the palladium particles and La or Ba atoms seems calcination temperature, different effects can be observed
responsible for this specific behavior. The surface property with respect to the catalytic properties. The foreign ions
measurements (XPS, TPO) exhibit results that confirm this retard the sintering of the alumina support but result in a
hypothesis.
lower combustion activity due to the absence of the mixed
Pd–PdO phase.
5. CONCLUSIONS
ACKNOWLEDGMENTS
Pd-based catalysts have been prepared by impregnation
of three different support materials. Two of them contain
rare earth ions, Ba and La. The presence of La or Ba im-
proves the thermal stability of alumina support when ex-
posed to temperature as high as 1000◦C. The supports con-
taining 3 wt% of La or Ba maintain a BET surface area of
about 95 m2/g after calcination at 1000◦C, whereas the pure
alumina displays a more intense sintering with a surface
area of only 75 m2/g.
The presence of Ba or La retards the strong sintering of
the alumina support, as confirmed by BET and XRD mea-
surements. In contrast, the presence of these foreign ions
exhibits a negative effect with respect to the combustion
activity.
The increase in calcination temperature from 500 to
1000◦C mainly affects the surface composition and prop-
erties of the catalysts. Calcination at 500◦C leads to cata-
lysts with almost identical properties. In contrast, when in-
creasing the calcination temperature to 1000◦C, significant
changes are observed in both surface composition and be-
havior. One of the most important is the sintering of both
the support and the noble metal particles.
STEM—the Swedish National Energy Administration—is acknowl-
edged for financially supporting this work. Thanks are due to Condea
GmbH for providing the alumina supports. The authors would also like
to thank Dr. M. Ferrandon and Prof. P. G. Menon for valuable discussions
and comments.
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