Effect of palladium on the reducibility of Mn based materials: Correlation with methane oxidation activity

Article abstract of DOI:10.1039/b808289h

Mn-based oxide supports were synthesized using different procedures: (i) carbonate co-precipitation method, leading to the formation of a hexaaluminate crystallized solid (La_0.2Sr_0.3Ba_0.5MnAl ₁₁O₁₉) and (ii) solid-solid diffusion method, leading to the formation of a doped θ-Al₂O₃ crystallized solid (nominal composition: 60 wt% La_0.2Sr_0.3Ba _0.5MnAl₁₁O₁₉ + 40 wt% Al₂O ₃). Impregnation of 1.0 wt%Pd was carried out on both oxides. The solids were tested for the catalytic methane combustion up to 700°C. It was observed that adding palladium resulted in an important increase in the catalytic activity. The combined use of H₂-TPR and XPS techniques reveals that only Mn³⁺/Mn²⁺ redox "couple" is present in the solids, whatever the synthesis procedure used. The fraction Mn³⁺/Mn is proportional to the total Mn content in the solid support, whatever the sample structure (hexaaluminate or doped θ-Al ₂O₃) and its morphology (large crystals or aggregates of small particles, respectively). Pd impregnation and further calcination at 650°C has no significant effect on the Mn³⁺/Mn fraction. However, some changes in Mn³⁺ reduction profile are observed, depending on the solid structure. Indeed, palladium addition strongly affects the manganese reducibility with an important shift of the reduction process to lower temperatures (～100°C). On the basis of redox properties observed for the different catalysts, a Mars-van-Krevelen redox mechanism, with oxygen transfer from support oxides to palladium particles, is proposed to explain the difference in terms of catalytic conversion and stability with respect to a 1.0 wt%Pd/Al₂O₃ reference sample. the Owner Societies.

Full text of DOI:10.1039/b808289h

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www.rsc.org/pccp | Physical Chemistry Chemical Physics
Effect of palladium on the reducibility of Mn based materials: correlation
with methane oxidation activity
a
a
b
a
a
´
A. Baylet,* S. Royer, C. Labrugere, H. Valencia, P. Marecot,
a
a
¨
J. M. Tatibouet and D. Duprez
Received 16th May 2008, Accepted 2nd July 2008
First published as an Advance Article on the web 5th September 2008
DOI: 10.1039/b808289h
Mn-based oxide supports were synthesized using different procedures: (i) carbonate
co-precipitation method, leading to the formation of a hexaaluminate crystallized solid
(La0.2Sr0.3Ba0.5MnAl11O19) and (ii) solid–solid diffusion method, leading to the formation of a
doped y-Al2O3 crystallized solid (nominal composition: 60 wt% La0.2Sr0.3Ba0.5MnAl11O19 + 40
wt% Al2O3). Impregnation of 1.0 wt%Pd was carried out on both oxides. The solids were tested
for the catalytic methane combustion up to 700 1C. It was observed that adding palladium
resulted in an important increase in the catalytic activity. The combined use of H2-TPR and XPS
techniques reveals that only Mn3+/Mn2+ redox ‘‘couple’’ is present in the solids, whatever the
synthesis procedure used. The fraction Mn3+/Mn is proportional to the total Mn content in
the solid support, whatever the sample structure (hexaaluminate or doped y-Al2O3) and its
morphology (large crystals or aggregates of small particles, respectively). Pd impregnation and
further calcination at 650 1C has no significant effect on the Mn3+/Mn fraction. However, some
changes in Mn3+ reduction profile are observed, depending on the solid structure. Indeed,
palladium addition strongly affects the manganese reducibility with an important shift of the
reduction process to lower temperatures (B100 1C). On the basis of redox properties observed for
the different catalysts, a Mars-van-Krevelen redox mechanism, with oxygen transfer from support
oxides to palladium particles, is proposed to explain the difference in terms of catalytic conversion
and stability with respect to a 1.0 wt%Pd/Al2O3 reference sample.
observed that doping hexaaluminate solids by palladium
1. Introduction
allowed to obtain active and stable catalysts. However, their
catalytic activities remained lower than that of the conven-
tional Pd/Al2O3 catalyst, mainly because of the low noble
metal dispersions obtained on such solids (that did not exceed
30% after the calcination step). Impregnation of palladium
over stabilized y-Al2O3 samples (stabilized with cations like
Ba2+, La3+, Sr2+, Mnx+), prepared using a novel coprecipi-
tation procedure,6 were then identified as promising catalysts
since these samples present comparable thermal stabilities
than hexaaluminate catalysts, but with catalytic activities close
to that obtained over the Pd/Al2O3 reference sample. Indeed,
Due to increasing drastic regulations concerning greenhouse
gas emissions, catalytic methane oxidation reaction has been
largely studied over a wide variety of solids.1 In line with this,
catalytic combustion is proposed as an alternative route for
energy production, with low emissions of NOx due to a lower
working temperature than the one used in a conventional
burner.2,3 Among the various catalysts studied for the
methane oxidation reaction, Pd/Al2O3 catalyst1,4 is described
in the literature as a highly active catalyst, although its thermal
stability remains low when reaction temperature exceeds
500 1C. The catalyst deactivation is generally attributed to
the progressive decomposition of active palladium oxide sites
(PdO) into less active palladium metallic sites (Pd1).2 By
contrast, mixed oxides and especially hexaaluminates are
described as thermally stable catalysts5 due to the high crystal-
lization temperature of these compounds. Nevertheless, their
catalytic activities remain low, even when transition metals are
incorporated into the crystal structure. In a recent study,6 we
the higher specific surface areas of the solids (B70 m2 gÀ1
,
compared to 20 m2 gÀ1 for a conventional hexaaluminate)
allow to obtain three times higher palladium dispersion.
Consequently, higher catalytic activity was obtained. More-
over, these samples were found stable in reaction at a tem-
perature at which the reference sample suffered from drastic
deactivation.
The aim of the present study was to elucidate the role of the
reducibility of the oxide support on the catalytic activity and
stability of the Mn-based samples for the methane oxidation
reaction. For this purpose, two model samples (a Mn-doped
hexaaluminate and a Mn-stabilized y-Al2O3) were selected.
Palladium was carried out by conventional impregnation. The
redox properties were studied by H2 temperature programmed
reduction and a thermal decomposition–reoxidation test. XPS
a
´
LACCO, UMR CNRS 6503, Universite de Poitiers, 40 avenue du
Recteur Pineau, 86022 Poitiers Cedex, France
b ICMCB-CNRS, 87 avenue du docteur Schweitzer,
´
´
F-33608 Pessac, France, Centre de Caracterisation des Materiaux
Avances (CeCaMA), Universite Bordeaux I, 351, cours de la
´
´
´
Liberation, F-33405 Talence, France.
E-mail: alexandre.baylet@etu.univ-poitiers.fr;
Fax: +33-(0)5-49-45-34-99; Tel: +33-(0)5-49-45-35-22
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analysis was performed to identify the oxidation states of the
accessible manganese and palladium active phase. The palla-
dium and manganese reducibility results were further corre-
lated to the catalytic conversion and thermal stability of the
solids for the methane oxidation reaction.
equipped with a graphite backmonochromator. Diffraction
patterns were recorded for 2y values comprised between 15
and 801 by steps of 0.041 (dwell time = 2 s). Phase identifi-
cation was made by comparison with JCPDS files.
For transmission electron microscopy, the samples were first
incorporated in a PMMA resin. Layers of 50–70 nm were cut
using ultramicrotomy. Analyses were performed on a Philips
CM120 instrument operating at 120 kV.
2. Experimental
2.1 Oxide sample synthesis and palladium impregnation
A VG 220 i-XL ESCALAB spectrometer equipped with a
non-monochromated Mg Ka source (hn = 1253.6 eV) was
used for the surface analysis at 100 W (10 kV and 10 mA). A
pressure of 10À7 Pa was maintained in the chamber during
analysis. The diameter of the analysed area was about 150 mm.
Surveys (0–1150 eV) were obtained with a constant pass
energy of 150 eV, whereas high resolution spectra were
recorded at a constant pass energy of 40 eV. Powders were
pressed onto small indium foils. Charge neutralization was
required for all insulating samples. The peaks were referenced
to the Al2p maximum at 74.2 Æ 0.12 eV, typical binding
energy for alumina powders. High resolution spectra were
fitted and quantified using the AVANTAGE software pro-
vided by ThermoFisher Scientific.
Three oxide supports were used. The first one, named REF
sample, is a commercial g-Al2O3 obtained from AXENS, as
beads of 2 mm. Before use, beads were hand crushed and
sieved at 100–200 mm. The second one, named HCa sample, is
an hexaaluminate synthesized using the carbonate coprecipi-
tation procedure of nitrate precursors at 40 1C and pH 8.7
Precursor amounts were adjusted to obtain the following
stoichiometry: La0.2Sr0.3Ba0.5MnAl11O19. The third oxide
sample, HP0.60, is a stabilized y-Al2O3 sample prepared by
high temperature solid/solid diffusion of cations precipitated
on the surface of the REF alumina oxide at 40 1C and pH 8.6
After calcination at 1100 1C, the following nominal stoichio-
metry was obtained: 60 wt% (La0.2Sr0.3Ba0.5MnAl11O19)/
40 wt% Al2O3. Calcined samples were further impregnated
with an aqueous solution of palladium nitrate to obtain after
drying and calcination (650 1C for 2 h under air) 1.0 wt% of
Pd on the catalysts.6 The Pd/REF, Pd/HCa, and Pd/HP0.60
catalysts were then obtained.
Sample reducibility was studied by temperature pro-
grammed reduction under H2 on a Micromeritics AutoChemII
instrument. Catalyst samples (195 mg) were pretreated under a
pure O2 flow at 500 1C for 1 h, and cooled down to ambient
temperature. H2-TPR were performed under 1.0 vol%H2 in
Ar, with a temperature ramp of 10 1C minÀ1, starting from
room temperature up to 900 1C. Catalyst was maintained
at this temperature for 30 min to ensure a complete reduction.
H2 consumption was quantified using a TCD previously
calibrated.
2.2 Physical and redox characterizations
Palladium contents in the catalysts were measured by ICP-OES
using a Perkin Elmer Optima 2000 DV apparatus. Specific
surface areas were estimated from N2 adsorption at À196 1C
(BET method), using a Tristar instrument from MICRO-
MERITICS.
For some samples, oxide decomposition and reoxidation
(Pd and Mn) were studied by temperature programmed reac-
tion under oxygen. Experiments were performed on the same
apparatus as for H2-TPR. A catalyst sample of 150 mg was
heated from room temperature to 900 1C at a temperature
ramp of 10 1C minÀ1 and cooled from 900 1C to room
temperature at the same rate. Carrier gas was composed of
1.0 vol% O2 in He. Oxygen release (reduction of Pd2+ and
Mn3+ species) or consumption (reoxidation of Pd1 and Mn2+
species) was monitored using a TCD.
Palladium dispersions and corresponding particle sizes were
obtained by H2 chemisorption in a Micromeritics AutoChemII
instrument. After reduction of the sample under H2 (350 1C for
1 h), and purging under Ar at the same temperature for 3 h, the
sample was cooled down to 70 1C and maintained at this
temperature in order to avoid Pd-H hydride formation during
H2 chemisorption. Then, H2 was pulsed at regular intervals of
time every 10 min. The values of dispersion reported in Table 1
are calculated using eqn (1),8 assuming one hydrogen atom
adsorbed per palladium atom. Details of palladium particle
size calculation can be found in ref. 6.
2.3 Methane oxidation test
The activity of the oxide and Pd-based samples was evaluated for
the oxidation of 1.0 vol% CH4 in excess of oxygen. For each test,
a weight of 27.5 mg of catalyst diluted in 250 mg of carborundum
was introduced between two quartz wool plugs in a U-shaped
X-Ray diffraction patterns of the calcined samples were
recorded on a Siemens D 5005 powder diffractometer using a
Cu Ka radiation (lKa = 0.15186 nm) as an X-ray source and
Table 1 Summary of the sample physical properties and methane conversion properties at 700 1C
Catalyst
SBET/m2 gÀ1
Pd/wt%
Dispersiona/%
Pd crystallite sizeb/nm
X700-initialc/%
X700-12 hd/%
Pd/REF
HCa
Pd/HCa
HP0.60
Pd /HP0.60
a
190.0
18.8
19.5
71.0
51.0
b
1.1
—
1.1
—
67
—
24
—
78
1.4
—
3.8
—
92.1
21.0
38.2
31.0
95.0
64.5
21.0
38.5
31.0
91.5
d
0.9
1.2
c
Determined by eqn (1). Determined by eqn (2). X700-initial: methane conversion measured at 700 1C at this end of the light-off curve. X700-12 h
:
methane conversion measured at 700 1C after 12 h.
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quartz microreactor (id = 5 mm). In order to decrease the
exothermic phenomenon in oxidation, CH4 concentration is fixed
at 1.0 vol%. The reaction flow was then composed of (vol%)
1.0CH4–4.0CO2–4.0H2O–18.2O2–72.8N2. CO2 and H2O were
added to the feed as inhibitors, to simulate exhaust gases. Test
conditions correspond to a gas hourly space velocity (GHSV)
of 415 000 hÀ1 with catalyst weight = 27.5 mg, solid density =
1 g cmÀ3 and total flow rate = 190 mL minÀ1. The test began
with a first temperature ramp (5 1C minÀ1) from 50 to 700 1C.
At the end of the ramp, the reactor was cooled down to
ambient temperature under reaction flow (30 min). Tempera-
ture was then ramped for a second time from ambient
temperature up to 700 1C (ramp = 5 1C minÀ1). At the end
of the second ramp, catalyst temperature was stabilized at
700 1C and maintained at this temperature for 12 h in order to
evaluate the catalyst stability in reaction.
most of the diffraction peaks can be assigned to the hexagonal
alumina-rich barium hexaaluminate (Ba0.75Al11O17.25, JCPDS
file 075-0707) and strontium hexaaluminate (SrAl12O19,
JCPDS file 089-2505) phases (Fig. 1). Weak reflection lines
belonging to a-Al2O3 are observed. Indeed, it is difficult to
obtain a pure hexaaluminate phase, except after calcination at
high temperature (1200 1C).9–12 In order to maintain higher
specific surface areas (SBET), the samples are calcined at only
1100 1C, leading to a contamination of the hexaaluminate
phase by a-Al2O3. Contrarily to what was observed for HCa
and Pd/HCa samples, X-ray diffraction lines attributed to the
hexaaluminate phase are not observed for the HP0.60 based
samples. Indeed, the diffraction peaks are clearly assigned to
the y-Al2O3 crystal structure (JCPDS file no. 023-1009). As for
HCa, small diffraction peaks attributed to a-Al2O3 are de-
tected. For comparison, calcination of the REF support at the
same temperature (not shown) led to a pure a-Al2O3 solid.
Then, the presence of the La3+, Sr2+ and Ba2+ cations allows
to improve the thermal stability of the alumina, delaying the
y-Al2O3 to a-Al2O3 phase transition. The diffractogramms do
not reveal the presence of PdO diffraction peaks at 34 and 551,
whatever the catalysts. This is simply explained by the over-
lapping of PdO diffraction peaks with those of the supports;
added to the low concentration of Pd in the solids.
3. Results and discussion
3.1 Physical and chemical characterizations
Since physical properties were described in a previous work
dealing with synthesis methods,6 only the main properties are
summarized here. X-Ray diffraction patterns obtained for the
different samples are presented in Fig. 1. For samples
prepared by carbonate coprecipitation (HCa and Pd/HCa),
Specific surface areas (SBET) are summarized in Table 1.
Whereas a SBET of 190 m2 gÀ1 is obtained for Pd/REF sample,
Fig. 1 X-Ray diffraction obtained for HCa, Pd/HCa, HP0.60 and Pd/HP0.60 samples (J: SrAl12O4; n: BaAl12O19
; }: SrAl12O19; y: y-Al2O3;
a: a-Al2O3) and TEM micrographs obtained for Pd/HCa and Pd/HP0.60 catalysts after calcination at 1100 1C for 4 h under air.
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Table 2 XPS quantification for HCa, Pd/HCa, HP0.60 and Pd/
HP0.60 samples (at.%, Scofield correction)
from the nominal value. A surface enrichment in La3+ and
a depletion in Sr2+ concentrations are observed, the sur-
face composition being 60 wt% (La0.88SrB0Ba0.33Mn0.72
-
C
O
Al
La Ba Sr
Mn Pd Pd/Mn
Al11O16.87)/40 wt% Al2O3, far from the nominal value 60
wt% (La0.2Sr0.3Ba0.5MnAl11O19)/40 wt% Al2O3. No drastic
changes on the XPS signals are observed between the oxide
support and the catalysts, showing that palladium impregna-
tion and subsequent calcination did not result in important
surface modifications. The atomic percentage of Mn at the
surface of Pd/HCa and Pd/HP0.60 samples is 2.3 and 0.7,
respectively, and those of Pd, 0.4 and 2.5, respectively
(Table 2). The difference for the at.% of Mn is due to the
presence of alumina zones not associated with Mn. Hexa-
aluminate does not cover all the surface. The difference
observed for the at.% of Pd is closely linked to the palladium
particle size previously deduced from TEM analysis (Fig. 1)
and H2-chemisorption (Table 1), large Pd crystallites for Pd/
HCa sample and small Pd particles for Pd/HP0.60 sample.
HCa
19.5 47.6 27.9 0.1 1.4 0.7 2.8
—
—
19.8 48.8 26.3 0.3 1.4 0.7 2.3 0.4 0.17
Pd/HCa
HP0.60
Pd/HP0.60 17.2 53
a
13.0 54.6 20.9 1.6 0.6
1.3
0.7 2.5 3.57
—
—
a
25.2 0.9 0.4
a
Limit of detection.
largely lower SBET are obtained for the HCa (18.8 m2 gÀ1) and
HP0.60 (71 m2 gÀ1) samples. As observed in Table 1, palla-
dium impregnation does not result in drastic change in SBET
,
only a small decrease being observed after impregnation of
palladium over HP0.60.
Different morphologies of materials are clearly observed by
TEM (Fig. 1). Indeed, whereas large crystals (100–200 nm) are
observed for a HCa sample, aggregates of small oxide particles
(20–30 nm), generally observed for the transition alumina
supports, are obtained for HP0.60 sample (and for REF, not
shown). These two different morphologies allow to explain the
3.2 Determination of the manganese oxidation state
3.2.1 Thermodynamical stability of Mnx+ species. The
thermal stability of Mnx+ (with x = 4, 3 and 2) under O2
and H2 atmospheres is determined as a function of tempera-
ture by thermodynamic calculation. Table 3 summarizes the
possible reactions that can occur during oxidation under O2
and reduction under H2. The Gibbs free energies were eva-
luated for these reactions, using the data of Barin et al.13
(Fig. 2). Under oxidative atmosphere (Fig. 2A), thermo-
dynamical calculations show that MnO2 is not stable for
change in SBET
.
Palladium contents vary between 0.9 wt% (Pd/HP0.60) and
1.1 wt% (Pd/REF and Pd/HCa samples). A Pd dispersion of
24% is obtained for Pd/HCa sample, whereas dispersions
higher than 65% are obtained for Pd/REF and Pd/HP0.60
samples (Table 1). This result is explained by the higher SBET
of the REF and HP0.60 samples compared to the HCa one.
Determination of the surface cation concentration was
performed by XPS over HCa, Pd/HCa, HP0.60 and
Pd/HP0.60 (Table 2). For all samples, aluminium oxide is
the main component. Indeed, C, O and Al elements present the
highest atomic percentage. The surface concentration of C,
due to contamination, is the same for all the samples, which
allows a better comparison between materials. Some differ-
ences are observed, depending on the support. Indeed, the
surface composition of HCa oxide is close to the nominal
Table 3 Mn oxidation rate during oxidation by one O2 molecule or
reduction by one H2 molecule
Oxidation
Reduction
2MnO + O2 - 2MnO
Mn(0) - Mn(+II)
MnO + H2 - MnO + H2O
Mn(+II) - Mn(0)
4MnO + O2 - 2Mn2O3
Mn(+II) - Mn(+III)
2MnO + O2 - MnO2
Mn(+II) - Mn(+IV)
6MnO + O2 - 2Mn3O4
Mn(+II) - Mn(+II) +
Mn(+III)
Mn2O3 + H2 - 2MnO + H2O
Mn(+III) - Mn(+II)
MnO2 + H2 - MnO + H2O
Mn(+IV) - Mn(+II)
Mn3O4 + H2 - 3MnO + H2O
Mn(+II) + Mn(+III) -
Mn(+II)
composition,
La0.04Sr0.28Ba0.55Mn1.10Al11O18.78
and
La0.2Sr0.3Ba0.5MnAl11O19, respectively. However, the surface
composition of HP0.60 oxide, obtained by impregnation of
La3+, Sr2+, Ba2+ and Mnx+ cations over g-Al2O3, is different
Fig. 2 Gibbs free energy of Mn oxide formation under O2 atmosphere (A) and under H2 atmosphere (B). Vertical dotted line: calcination
temperature (1100 1C = 1373 K).
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Table 4 Binding energies of Al2p, Mn2p and Pd3d and semi-quantitative Mn3+ fraction in HCa, Pd/HCa, HP0.60 and Pd/HP0.60 samples
Mn2p3/2 Mn3+
area (a.u.) Eb/eV
Mn2p3/2 Mn2+
area (a.u.) Eb/eV
Catalysts
HCa
Al2p maxi eV
74.4
Pd3d5/2 maxi eV
—
Mn3+/Mntot
35
%
1838
(642.0)
3484
(641.8)
772
(642.0)
177
(642.7)
3386
(640.8)
6185
(640.6)
2999
(640.8)
786
(641.5)
Pd/HCa
74.1
74.2
74.2
337.0
—
36
20
18
HP0.60
Pd /HP0.60
336.8
temperatures higher than 1215 K (i.e. DG 4 0), manganese
cations being probably into 3+ valency. Consequently, sam-
ple calcination at 1100 1C (i.e. 1373 K) is supposed to lead to a
maximum oxidation state of manganese equal to 3+. Under
H2 atmosphere (Fig. 2B), reduction of manganese oxide only
leads to the formation of MnO (i.e. DG o 0), regardless of the
reduction temperature (up to 2000 K). The reduction of MnO
into metallic Mn is thermodynamically impossible (DG 4 0)
even at temperature higher than 2000 K. These calculations
suggest that the only reaction to consider during H2-TPR
experiments is the reduction of Mn3+ species into Mn2+
species (eqn (1)), since reduction is performed up to 900 1C:
manganese in our experiments is expected to be limited to
Mn2+ under the H2-TPR (T r 900 1C) and catalytic test (T r
700 1C) conditions.
3.2.2 XPS studies. The oxidation states of surface Mn and
Pd species were determined by XPS (Table 4 and Fig. 3). All
series of spectra for one acquisition were calibrated to the well-
known Al2p position in Al2O3 products. Ba, La and Sr present
similar XPS spectra between catalysts and respective supports.
Concerning Pd in Pd/HCa and Pd/HP0.60, the Pd3d5/2 com-
ponent was found at 337.0 eV, typical of Pd2+ in PdO oxide.
As for Mn, the concentration was found to be at the limit of
detection in Pd/HP0.60, justifying the use of an XPS pass
energy of 40 eV (Fig. 3F). Secondly, the average position of
Mn2p3/2 peak around 641.1 eV corresponds to the 3+ and 2+
oxidation degrees of Mn. This value is in the same range than
those reported for Mn2O3 (641.3–641.7 eV)17–19 and for MnO
(640.5 eV).20 The binding energy of 642.6 eV for Mn4+ in
MnO2 is not observed for Pd/HCa and Pd/HP0.60 cata-
lysts.19–21 The fit proposed for Mn2p has been obtained with
(i) subtraction of the linear background, (ii) use of asymmetric
components, (iii) respect to the energetic difference between
Mn3+ + 1/2 H2 - Mn2+ + H+
(1)
According to our experimental conditions (calcination tem-
perature = 1100 1C), these calculations confirm the interpre-
tation of some academic studies14,15 in which only Mn3+ and
Mn2+ are evidenced by EPR in the Mn-doped hexaaluminate
structure. As properly observed by Strohnmeier and Hercules,16
and according to the calculation results, Mn2+ reduction
into Mn0 only occurs at temperature higher than 1200 1C. It
is then reasonable to suppose that the reduction process of
Fig. 3 Fitted Al2p (A, D), Mn2p (B, E) and Pd3d (C, F) spectra of Pd/HCa and Pd/HP0.60 catalysts.
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Fig. 4 H2-TPR profiles obtained for HCa and HP0.60 samples
(normalized to 1 g of catalyst).
2+ and 3+ components (i.e. around 1.2 eV), and (iv) main-
tain to the height ratio at 0.6 between Mn2p1/2 and Mn2p3/2
for the same chemical bond. The results from the Mn fitting
are presented in Table 4. The main result is the difference of
the Mn3+/(Mn3+ + Mn2+) fraction between HCa and
HP0.60 oxides. Indeed, the Mn3+/(Mn3+ + Mn2+) ratios
are 0.35 and 0.20, respectively. Consequently, the hexa-
aluminate surface is richer in Mn3+ than the one of doped
y-Al2O3. Palladium adding does not influence this Mn3+
fraction. Indeed, the Pd/HCa and Pd/HP0.60 catalysts present
percentages of reducible Mn3+ equal to 0.36 and 0.18, respec-
tively (Table 4), values close to those obtained for the supports
without palladium.
Fig. 5 Hexaaluminate structure (large cations: Ba, Sr, La, medium
cations: O, small cations: Al, Mn).
680 1C is composed of two or more overlapped peaks. The
second peak presents a maximum at 780 1C. The amount of H2
consumed during the first reduction peak (at intermediate
temperature) is attributed to the reduction of the most reactive
manganese species, and can be assigned to the reduction of
Mn3+ ions located into the aluminium interstitial sites near
the mirror plane of the hexaaluminate structure (Fig. 5).16,22
The high temperature reduction process (occurring at 780 1C)
can be attributed to the reduction of Mn3+ cations located
inside the aluminium spinel blocks. Indeed, H2 diffusion
logically occurs at a higher rate between the spinel blocks15
than in the bulk, leading to an easier reduction of the Mn3+
near the mirror planes. The theoretical fraction of trivalent
3.3 Manganese reducibility of HCa and HP0.60 oxides
From the previous results, only Mn3+/Mn2+ and Pd2+/Pd0
redox couples are considered to be responsible for the H2
consumption during TPR experiments. Then, Pd2+ reduction
degree and fraction of Mn3+ initially present in the material
will be calculated from the experimental H2 consumptions.
It was first verified that the H2-TPR profile obtained for the
REF support (alumina alone) did not show any H2 consump-
tion. The H2-TPR profiles obtained for the HCa and HP0.60
oxide catalysts are presented in Fig. 4. The reduction profile
obtained for the HCa support shows two broad reduction
peaks. The first reduction peak that occurs between 230 and
Table 5 Reduction temperatures, H2 consumption and amount of Mn3+ measured for the different samples (normalized to 1 g of catalyst)
Manganese reduction peak
1st peak
Mn content/mmol gÀ1 Mn3+ fractiona/% T/1C
2nd peak
H2 cons./mmol gÀ1 Mn3+b/% T/1C
Catalyst
H2 cons./mmol gÀ1 Mn3+c/%
HCa
1285
1270
775
43
44
27
28
50–700 203
31.5
37.6
27.0
28.0
700–900 74
700–900 40
12.5
6.4
0
Pd/Hca
HP0.60
Pd/HP0.60
100–700 235 + 22Pd
50–700 106
80–700 108
700–900
700–900
0
0
765
0
a
b
c
Total percentage of Mn as Mn3+ in the solid deduced from H2-TPR. Percentage of Mn as Mn3+ reduced during the first peak. Percentage of
Mn as Mn3+ reduced during the second peak.
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Table 6 Reduction temperatures, H2 consumptions and palladium
reduction degrees measured for the different samples (normalized to
1 g of catalyst)
Palladium reduction peak
Catalyst
T/1C
H2 cons./mmol gÀ1
Pd2+ reduced/%
Pd/REF
Pd/HCa
Pd /HP0.60
20–80
20–100
20–80
101
79
102
100
78
100
The reduction behaviour of HP0.60 slightly differs from that
of HCa. Indeed, a similar reduction profile is observed in the
230–680 1C temperature range with one broad reduction peak
consisting in two overlapped peaks. Nevertheless, no H2
consumption is observed at temperature higher than 700 1C.
The calculated fraction of Mn3+ is 27%, not far from the XPS
surface results (i.e. 20%, Table 4). Thus, reduction of the
trivalent manganese in HP0.60 is achieved at intermediate
temperature. This difference can be explained by (i) a lower
content of Mn on the surface (1.3 at.% versus 2.8 at.% for
HCa, Table 2) and (ii) a lower Mn3+/Mn total ratio (27%
versus 43% in HCa, Table 5).
Fig. 6 Correlation between Mn molar content and Mn reducibility
for HCa, Pd/HCa, HP0.60 and Pd/HP0.60 samples.
3.4 Palladium effect on the sample reducibility
manganese in the hexaaluminate, calculated from the total
amount of H2 consumed, is 43% for the HCa sample (Table 5).
However, this value is slightly higher than those reported in
The H2-TPR profiles obtained for the palladium impregnated
samples are presented in Fig. 7A (20–100 1C range) and 7B
(100–900 1C range). The impregnation of palladium results in
a drastic change in the reduction process. First, a sharp peak is
always observed at low temperature (40–80 1C). This peak is
assigned to the reduction of PdO into metallic Pd. The
temperatures of reduction obtained in this work (o70 1C,
Table 5) are in good agreement with those reported in the
literature.23,24 The amounts of H2 consumed allow to calculate
the reduction percentages of PdO into Pd0 (Table 6). It is
evident that palladium reduction is achieved at the end of the
first reduction peak over Pd/REF and Pd/HP0.60, whereas the
Pd/HCa sample presents a slightly different behaviour (with
only 78% of palladium being reduced at the end of the peak).
This result suggests that the palladium reduction occurs in two
steps for Pd/HCa, as already observed by Fuentes et al.25
and Barrera et al.26 over Pd/Al-La and Pd/Al catalysts,
the literature by Artizzu-Duart et al.14 over BaMnxAl12Àx
-
O19Àd samples (x = 1, 2 or 3). The Mn3+ fraction was
reported to increase from 17 to 44% with x varying from 1
to 3 and was not related to the specific surface area of the
materials, 20 and 14 m2 gÀ1, respectively.14 A similar trend is
obtained in this work (Fig. 6) since a linear increase in Mn3+
content is observed with a linear increase in total Mn. Never-
theless, the values reported by Artizzu-Duart et al.14 include
only H2 consumption for temperature lower than 800 1C. We
previously observed by thermodynamic calculation that the
reduction of Mn2+ into metallic manganese cannot occur at
900 1C. Consequently, reduction of Mn2+ into Mn0 cannot
explain the higher amounts of H2 consumed by HCa that
can only be explained by an incomplete reduction of manga-
nese at 800 1C.
Fig. 7 H2-TPR profiles obtained for the different palladium impregnated samples: (A), from the ambient temperature to 100 1C; (B), up to 900 1C
(normalized to 1 g of catalyst).
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respectively. The two reduction processes identified by these
authors are a first reduction of PdO into Pd2O followed by the
reduction of Pd2O into Pd1. Nevertheless, these two reduction
steps were not observed over the Pd/REF sample and only a
more difficult reduction of palladium over Pd/HCa sample was
observed. This difference can only be explained by a stabiliza-
tion of some palladium in its oxidized form by the hexa-
aluminate support through a larger size of PdO particles
on Pd/HCa compared to Pd/HP0.60 and Pd/REF samples
(3.8 versus 1.2 and 1.4 nm, respectively, Table 1).
The higher percentage of reducible Mn3+ at high tempera-
ture observed on the HCa or Pd/HCa samples compared to
the HP0.60 or Pd/HP0.60 samples indicates that on these latter
samples, a larger part of the manganese is located on the
surface and/or subsurface sites. The manganese species located
in the bulk of the hexaaluminate are then more difficult to
reduce. This result suggests that the Mn3+ species is more
accessible on HP0.60 than on HCa.
3.5 How does support reducibility affect the catalytic activity
of palladium for the methane oxidation reaction?
Changes in manganese reducibility are also observed. The first
reduction peak which could be assigned to the manganese reduc-
tion is observed around 210–230 1C. A larger peak is obtained for
the Pd/HCa sample which is due to the contribution of the
remaining PdO reduction (22% of the palladium remains oxi-
dized at the end of the low temperature reduction peak, Table 5).
Percentages of Mn3+, calculated from the amount of H2
consumed, are presented in Table 5. Impregnation of Pd and
subsequent calcination at 650 1C do not affect the total
percentage of Mn3+ in the sample, as previously observed by
XPS (Table 4). Important changes in reducibility are however
observed. Once reduced, the Pd is suggested to increase the Mn
oxide reducibility by promoting H2 spillover.27
The reducibility measurements showed similar behaviour for
HCa and HP0.60 samples in the intermediate temperature
range, between 230–680 1C (Table 5 and Fig. 4). The catalytic
activities of the oxide catalysts (summarized in Table 1 and in
Fig. 8A) correlate with the surface Mn contents since a slightly
higher methane conversion at 700 1C was measured over
HP0.60 sample (31%) than on HCa sample (21%). The
product of the surface area by the Mn content (atMn m2 gÀ1
)
determined by XPS is higher for HP0.60, 92.3 (71 Â 1.3) than
for HCa sample, 52.6 (18.8 Â 2.8). Thus, the higher SBET of
HP0.60 (71 m2 gÀ1) seems to balance the lowest proportion of
Fig. 8 CH4 oxidation activity (A) and stability (B) of Pd/REF, Pd/HCa and Pd/HP0.60 catalysts.
Fig. 9 Proposed scheme of the oxygen transfer between palladium and manganese from the support.
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For Mn-doped materials, a different behaviour is observed on
the long term stability test (Table 1 and Fig. 8). The
Mn-based samples (Pd/HCa and Pd/HP0.60) are more stable
than the Pd/REF sample with time on stream at 700 1C. This
result suggests an interaction between the reducible support and
the active palladium sites. In our previous study,6 we concluded
on the stabilization of the PdOx sites by the Mn-based support.
The H2-TPR results presented in this work clearly suggest that
manganese oxidized sites (Mn3+) and palladium sites: (i) are in
close interaction and (ii) the presence of palladium on the
sample is found to activate the reduction of the manganese
species. Because of the lower activity obtained for Pd supported
on Mn-doped support with respect to Pd supported on an
alumina sample, and the fact that these two samples present a
largely higher thermal stability, an oxygen transfer between
manganese and palladium particles could be proposed. Indeed,
PdO reduction is suggested to be responsible for the deactivation
of palladium catalysts for methane combustion.6 Manganese
reduced sites are reoxidized by oxygen present in the gas phase,
as schematised in Fig. 9 and could transfer its oxygen to avoid
palladium site reduction. This mechanism is supported by the
results obtained by the decomposition and reoxidation test over
the Pd/HCa sample (Fig. 10). It is clearly observed in this figure
that palladium decomposition occurs at a 60 1C higher tem-
perature over Pd/HCa than over Pd/REF (black lines, Fig. 10)
showing a positive effect of the HCa support on the thermal
stability of the palladium oxide clusters. Moreover, reoxidation
also occurs at a 50 1C higher temperature on this sample, the
temperature gap between the two process remaining constant
for the two samples. This result is coherent with the fact that Pd/
HCa is more stable than Pd/REF at 700 1C since PdO decom-
poses and reforms at much higher temperatures on HCa than on
the REF support (B50 1C). Nevertheless, this shift of 50 1C
cannot only explain why Pd/HCa is stable up to 700 1C whereas
Pd/REF deactivates from 500 1C. Then, only an oxygen transfer
from oxidized manganese sites to reduced palladium sites can be
proposed as the reason of the catalyst activity stabilization.
Fig. 10 Thermal decomposition/oxidation under 1.0 vol.%O2/He
obtained for Pd/REF and Pd/HCa (black: heating; grey: cooling).
Mn on the surface. A two steps Mars-van-Krevelen mecha-
nism for CH4 oxidation, involving the Mn3+/Mn2+ redox
couple, is proposed to occur on these solids, as already
proposed for the perovskite-type catalysts28 (Fig. 9). Then,
the amount of manganese in the solid, as well as the accessi-
bility of the reducible manganese on the surface, are key
parameters controlling the catalytic activity. In this process,
oxidized manganese sites react with methane leading to the
reduction of the manganese sites (Fig. 9). Reduced sites are
then reoxidized by gaseous oxygen.
Adding palladium allows to improve the catalytic activity
(i.e. Pd/HCa: 38%, Pd/HP0.60: 95% versus 92% for Pd/REF,
Table 1). Pd/REF catalyst presents the highest activity up to
550 1C (Fig. 8A). A lower activity is observed for the Pd/
HP0.60 sample since the methane conversion starts at around
500 1C to reach a conversion of 90% at 650 1C. Compara-
tively, Pd/HCa presents a very low activity, conversion start-
ing after 550 1C and reaching only 37% at 700 1C. It is
interesting to note that for Pd/REF the methane conversion
increased more slowly above 500 1C, suggesting some palla-
dium deactivation under reaction beyond that temperature.
The thermal stability of palladium under oxygen pressure
(in absence of methane) reported in Fig. 10 allows to explain
this phenomenon. Palladium oxide starts to decompose into
metallic palladium at a temperature around 800 1C. During
temperature decrease ramp, palladium is found to reoxidize
at temperature lower than 550 1C (i.e. beginning of oxygen
consumption). Thus, at temperature above 550 1C, and
because of methane oxidation reaction, palladium oxidized
sites are suggested to slowly reduce under reaction, without
any possible reoxidation by gaseous oxygen. This mechanism
is summarized in Fig. 9, where the progressive reduction of
the palladium particle under reaction is schematized. This
phenomenon results in a progressive deactivation of the Pd/
REF catalyst during the stability test, for which the conversion
decreased from 91.2 to 64.5% after 12 h of reaction.
4. Conclusion
The manganese reducibility in different Pd doped Mn-based
materials is reported in this work. Two Mn-based materials
were prepared using different procedures: carbonate co-precipi-
tation method (La0.2Sr0.3Ba0.5MnAl11O19) and solid–solid
diffusion method (60 wt% La0.2Sr0.3Ba0.5MnAl11O19 + 40
wt% Al2O3). Oxides were impregnated by 1.0 wt%Pd. So
obtained materials were used for the catalytic combustion of
methane. Supports and Pd based catalysts are characterized by
XPS analysis and H2-TPR, and compared to a 1.0 wt%Pd/
Al2O3 reference. The main conclusions are:
1. The concentration in reducible Mn3+ is mainly related to
the total Mn content in the initial support (higher in hexa-
aluminate than in stabilized y-Al2O3).
2. Mn3+ reduction is easier in y-Al2O3 stabilized sample
than in hexa-aluminate, Mn3+ reduction occurring at inter-
mediate temperature.
3. Addition of palladium does not affect the fraction of
Mn3+/Mn observed in the supports alone, but shifts the
manganese reduction to lower temperatures (B100 1C). The
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high catalytic activity of Pd based samples is related to (i) the
Pd dispersion, (ii) the concentration of Pd/Mn surface sites
(i.e. interaction between Pd and Mn).
10 M. Ocal, R. Oukaci, G. Marcelin, B. W. L. Jang and J. J. Spivey,
Catal. Today, 2000, 59, 205.
11 T.-F. Yeh, H.-G. Lee, K.-S. Chu and C.-B. Wang, Mater. Sci.
Eng., A, 2004, 384, 324.
4. On the basis of the TPR and XPS results, a Mars-van-
Krevelen redox mechanism, with transfer of the oxygen from
the support to the palladium particles, is proposed to explain
the difference in terms of catalytic activity and stability with
respect to the Pd/Al2O3 reference sample.
12 T.-F. Yeh, H.-G. Lee, K.-S. Chu and C.-B. Wang, J. Alloys
Compd., 2006, 425, 353.
13 Ihsan Barin, Thermochemical Data of Pure Substances, VCH,
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The French ‘‘Agence De l’Environnement et de la Maıtrise de
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for financially supporting this work. We thank Pr. Groppi and
Pr. Forzatti for helpful discussions.
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