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This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 4607–4613 4607
Cite this: Phys. Chem. Chem. Phys., 2011, 13, 4607–4613
In situ Raman and in situ XRD analysis of PdO reduction and Pd1
oxidation supported on c-Al2O3 catalyst under different atmospheres
Alexandre Baylet,*aPatrice Marecot,
aDaniel Duprez,
aPaola Castellazzi,
b
Gianpiero Groppiband Pio Forzatti
b
Received 27th July 2010, Accepted 14th December 2010
DOI: 10.1039/c0cp01331e
Reduction of Pd1 and decomposition of palladium oxide supported on g-alumina were studied at
atmospheric pressure under different atmospheres (H2, CH4, He) over a 4 wt% Pd/Al2O3 catalyst
(mean palladium particle size: 5 nm with 50% of small particles of size below 5 nm). During
temperature programmed tests (reduction, decomposition and oxidation) the crystal domain
behaviour of the PdO/Pd1 phase was evaluated by in situ Raman spectroscopy and in situ
XRD analysis. Under H2/N2, the reduction of small PdO particles (o5 nm) occurs at room
temperature, whereas reduction of larger particles (45 nm) starts at 100 1C and is achieved at
150 1C. Subsequent oxidation in O2/N2 leads to reoxidation of small crystal domain at ambient
temperature while oxidation of large particles starts at 300 1C. Under CH4/N2, the small particle
reduction occurs between 240 and 250 1C while large particle reduction is fast and occurs between
280 and 290 1C. Subsequent reoxidation of the catalyst reduced in CH4/N2 shows that small and
large particle oxidation of Pd1 starts also at 300 1C. Under He, no small particle decomposition is
observed probably due to strong interactions between particles and support whereas large particle
reduction occurs between 700 and 750 1C. After thermal decomposition under He, the oxidation
starts at 300 1C. Thus, the reduction phenomenon (small and large crystal domain) depends on
the nature of the reducing agent (H2, CH4, He). However, whatever the reduction or
decomposition treatment or the crystal domain, Pd1 oxidation starts at 300 1C and is completed
only at temperatures higher than 550 1C. Under lean conditions, with or without water, the
palladium consists of reduced sites of palladium (Pd1, Pdd+ with d o 2 or PdOx with x o 1)
randomly distributed on palladium particles.
1. Introduction
Palladium is widely used as automotive catalyst (in substitution
of or in addition to Pt and Rh) for the abatement of HC, CO
and NOx emissions.1 It is the most active precious metal for
methane combustion in excess of oxygen (lean condition). For
that reason, palladium catalysts are strongly recommended
for depollution of natural gas-powered vehicles and in catalytic
processes for energy production from natural gas. Aging of
these catalysts is often caused by decomposition of the active
phase into metallic palladium. Pd1 is less active than oxidized
palladium in CH4 oxidation. Moreover, metallic palladium
particles sinter more easily than PdO particles, resulting in
an irreversible loss of activity.2 These properties have been
extensively detailed in past reviews on catalytic combustion of
methane over Pd-based catalysts.3–5 Many studies have been
devoted to better describe the optimum state of PdOx in
reaction: either chemisorbed oxygen on Pd1 or a PdO skin
on a Pd metal core or bulk PdO;4 most of these studies
concluded that the two latter forms would be the most active
state of palladium supported catalysts.6 For this purpose
many investigations (experimental or theoretical) on the
stability of the palladium oxide particles during reduction
and palladium metal reconstruction during oxidation have
been done over various catalysts: (i) single Pd crystal; (ii)
polycrystalline Pd foil;7–12 (iii) and Pd supported catalysts13–17
during in situ or ex situ experiments. However, the experi-
mental conditions may change the oxidation state (ultra high
vacuum, 10�10 atm, is required for atomic emission spectrometry
(AES), X-ray photoelectron spectroscopy (XPS), secondary
ion mass spectrometry (SIMS) or low-energy electron diffrac-
tion (LEED) analysis) or modify palladium particle morpho-
logy upon cooling for ex situ analysis (X-ray diffraction (XRD)
or Raman spectroscopy). From these studies, reduction models
show a core-shell development of the catalyst surface, whereas
a LACCO, UMR CNRS 6503, Universite de Poitiers, 40,Avenue du Recteur Pineau, 86022 Poitiers, Cedex, France.E-mail: alexandre.baylet@univ-poitiers.fr;Fax: +33-(0)5-49-45-34-99; Tel: +33-(0)5-49-45-39-98
b Laboratory of Catalysis and Catalytic Processes,Dipartimento di Energia, Politecnico di Milano,Piazza Leonardo da Vinci 32, 20133 Milano, Italy
PCCP Dynamic Article Links
www.rsc.org/pccp PAPER
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4608 Phys. Chem. Chem. Phys., 2011, 13, 4607–4613 This journal is c the Owner Societies 2011
the experiments give two different ways depending on the
reducing agent: (i) reduction with H2 occurs via a shrinking
core mechanism; whereas (ii) reduction with CH4 occurs in
a ‘‘cauliflower-like structure’’ implying roughening of the
surface and so an increase of palladium oxide surface area.18
The oxidation process generally occurs in two steps: (i) oxygen
diffusion into Pd metal followed by (ii) oxygen diffusion
into PdO once the bulk oxide layer was formed. The oxygen
uptake was linearly proportional to the square root of the
time of treatment, No = K(P,T)t1/2. Thus the dissolution of
oxygen atoms into Pd metal followed the Mott-Cabrera
model19 with diffusion coefficient (DO–Pd) of 10�16 cm2 s�1
at 327 1C and activation energy of 60–85 kJ mol�1.20 The
diffusion of oxygen through the bulk oxide layer again follows
the Mott–Cabrera parabolic diffusion law with diffusion
coefficient (DO–PdO) of 10�18 cm2 s�1 at 327 1C and activation
energy of 111–116 kJ mol�1.20
In this work, the reduction or decomposition of PdO under
H2, CH4, lean mixtures (CH4/O2 or CH4/O2/H2O) or He and
the oxidation of Pd1 under O2 have been studied during
temperature programmed experiments by in situ Raman spectro-
scopy and in situ XRD analysis. The XRD technique, which is
more sensitive to disorder in the cationic sublattice compared
to anionic sublattice and allows characterising only the largest
particles, has been used in conjunction with Raman spectro-
scopy, which is primarily sensitive to oxygen-cation vibrations
(coherence length of 1 nm) and provides both short as well as
long range ordering to study the phase evolution of palladium
under different atmospheres. The catalyst was a 4 wt% Pd
supported on g-Al2O3. The 4 wt% Pd loading provided high
signal sensitivity and intensity during XRD and Raman
detection of Pd1 and PdO crystallites.
2. Experimental
2.1 Catalyst preparation
Alumina powder supplied by Sasols was used as precious
metal support. The support was calcined at 950 1C for 4 h
before palladium impregnation. The final alumina powder had
a specific surface area of 100 m2 g�1. The alumina powder
was sieved to retain particles of sizes between 140–200 nm. The
g-Al2O3 support was impregnated with palladium nitrate
dissolved in water. The weight of the salt precursor was
calculated to obtain a theoretical metal loading of 4 wt%.
For this purpose, the Pd salt solution (Pd(NO3)2, Alfa Aesar,
density of 1.427, 14.67 wt% Pd) was adjusted to fill only the
g-Al2O3 porosity, (0.49 cm3 g�1 equivalent to a mean alumina
pore diameter of 12.3 nm with the assumption of cylindrical
pores: d = 2V S�1). The solution containing the palladium
precursor was then added dropwise. The solid was dried under
air at 110 1C for 2 h. The resulting powder was calcined under
air flow at 600 1C for 10 h (ramp = 5 1C min�1).
2.2 Chemical and physical characterization
Specific surface areas were estimated from N2 adsorption
at �196 1C (BET method), using a Micromeritics Tristar
apparatus. The experimental Pd loading was determined by
atomic absorption method on a Varian AA110 apparatus.
Palladium dispersion was obtained by H2 chemisorption.
Analysis has been carried out on a Micromeritics AutoChemII
instrument. After reduction of the sample under 2% H2 in He
at 500 1C for 1 h and purge under Ar (at the same temperature
for 2 h), H2 pulses were injected after cooling at 70 1C at
regular intervals. Dispersion was calculated using eqn (1),
supposing the chemisorption of one hydrogen atom per
palladium atom:
Dð%Þ ¼ 2� P� VH2�mcata
106 � R� T � nPdð1Þ
with, P, atmospheric pressure (in Pa); V, volume of adsorbed
H2 (mL); R, 8.314 J K�1 mol�1; T, 295 K; mcata, sample weight
(g); nPd, mole of palladium as obtained by ICP.
Palladium particle size (A) was determined using eqn (2),
supposing hemispherical particles:
daverage ¼6CaPM � 109
rDNavð2Þ
with Ca, the concentration of surface metal atoms, equal to
1.27 � 1019 atoms m�2, PM Pd atomic mass, r, Pd volumetric
mass, equal to 12.02 � 106 g m�3,22 D metal dispersion and
Nav Avogadro number.
2.3 In situ temperature programmed characterization
2.3.1 H2 adsorption at low temperature. The H2 adsorption
properties of the supported Pd catalyst at low temperature
were evaluated on a Micromeritics AutoChemII instrument.
Prior to H2 adsorption experiments, 150 mg of the catalysts
were pre-treated under reduction treatment. The gas composi-
tion was 5% H2 in Ar. The reaction temperature range was
between 0 and 80 1C with a flow-rate of 50 mL min�1. The
amount of H2 consumption during the H2 adsorption is
measured by a thermal conductivity detector (TCD).
2.3.2 CH4-TPR. Temperature programmed reduction
under a methane containing atmosphere was studied using a
fixed bed tubular quartz microreactor (I.D. = 7 mm) at
atmospheric pressure, placed within an electrically heated
furnace. The catalytic bed consisted of 60 mg of catalytic
powder (74–105 mm), diluted by 180 mg of quartz. CH4-TPR
was carried out in the following conditions: 0.5% CH4 in He,
150 mL min�1 at STP (standard temperature and pressure),
GHSV (gas hourly space velocity) = 150 000 mL g�1 h�1; the
temperature was ramped from room temperature up to 500 1C
at 15 1C min�1. Reactants and product compositions at the
outlet of the reactor were monitored by a mass spectrometer
with quadrupole detector (Balzers QMS 422).
2.3.3 In situ Raman spectroscopy. In situ Raman measure-
ments were carried out on a Horiba Jobin Yvon HR800UV
LabRam spectrometer using green light laser (514 nm).
For measurements, the laser was focused on sample aggregate
(o2 mm). The in situ Raman analysis was carried out at room
temperature on the fresh sample in order to check the PdO
peak and to find the best compromise between acquisition time
and peak intensity. The acquisition time was 20 s. The two
main PdO peaks are located at 445 and 640 cm�1. Mamede
et al.21 have carried out in situ Raman analysis experiments on
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1 wt% Pd/g-Al2O3 using a spectrometer equipped with a
Nd :YAG laser (excitation line at 532 nm). They found a slight
shift at lower values, 427 and 626 cm�1. They attributed the
Raman lines to the Raman active Eg and B1g vibration modes of
PdO. The other weak and broader bands were attributed to a
resonance effect induced by the use of a laser excitation near the
excitation state of PdO. Bell et al.15 found the main PdO peak at
651 cm�1 on 10 wt% Pd/ZrO2 with the 514.5 nm wavelength of
the Ar ion laser and Arai et al.22 observed evolution of the main
PdO peak between 626 cm�1 (40 1C) and 633 cm�1 (600 1C) on
sputtered PdOx thin film with the 632.8 nm wavelength of the
He–Ne laser. A relevant study, carried out by Otto et al.,23 has
demonstrated that Raman spectroscopy of palladium oxide on
g-alumina was useful for a quantitative and non-destructive
analysis. They studied a wide range of palladium content, between
0.05 and 20 wt%. They showed that for palladium loading lower
than 0.2 wt%, some palladium may interact with the support and
thus may not contribute to the signal related to crystalline PdO.
Moreover, McBride et al. have shown that the analysis depth for
Raman measurements at 514.5 nm was about 10 nm24 and that
for palladium loading lower than 2.5 wt%, the Raman signal
should be proportional to the concentration. These results were
confirmed by Otto et al.23 Indeed for palladium content higher
than 2 wt%, there were no linear relations between Raman signal
and Pd loading. They assumed the effect of the presence of large
particles and wide variations in the particle size. To conclude, the
Raman technique is able to give useful information about the
smallest particles present in the catalyst.
The experimental procedure is described as following: (i) the
sample was pre-treated under 1%O2/N2 at 600 1C for 1 h with a
total flow rate of 50 mL min�1. Raman spectra were recorded
after cooling to 30 1C under the same atmosphere; (ii) after
purge under N2, the catalyst sample was treated under different
gaseous atmospheres with a total flow rate of 50 mL min�1,
either under reducing conditions (2% H2/N2, 2% CH4/N2), or
in lean conditions (2% CH4/5% O2/N2 or 2% CH4/5%
O2/H2O/N2) or in pure He; (iii) heating step at 10 1C min�1
was stopped at different temperature values for 10 min in order
to record Raman spectra under N2. For each spectrum acquisi-
tion, CH4 was removed from the feed because its presence
modified the signal intensity.25 In certain experiments, the
heating was stopped after 30 min to evaluate the kinetics of
the PdO/Pd1 transformation. Virtually no further transforma-
tion was observed between 10 and 30 min. The state of the
catalyst is essentially a function of the temperature: after 10 min
and beyond, it depends little on the time at which the sample is
maintained at a given temperature.
2.3.4 In situ XRD analysis. Contrary to Raman spectro-
scopy, XRD analysis allows characterizing only the largest
particles. It is commonly admitted that diffractograms of
nanoparticles deposited on alumina supports can give useful
information only when particle sizes are greater than 4–5 nm.
Diffractograms obtained for in situ experiments were obtained
on a Bruker AXS D8 advance powder diffractometer using
Cu Ka radiation (lKa = 0.15186 nm). Patterns were recorded
for 2y values between 251 and 851 in 0.0301 steps, with step
duration of 0.2 s (i.e. 6 min 40 s for the whole diffractogram).
The 2y values used in order to identify PdO and Pd1 peaks are
35, 40, 54 and 591 and 39, 46 and 821, respectively. The cell
support for XRD measurements was composed of Kanthal
with the main diffraction peaks at 44.480, 64.779 and 82.2841
(relative intensity: 100/20/50) with the following dimensions:
depth o 1 mm, width = 10 mm, length = 20 mm. The
experimental procedure is described as follows: (i) the sample
was pre-treated under 1% O2/N2 at 600 1C for 1 h with a total
flow rate of 50 mL min�1. After cooling down to 30 1C under
the same atmosphere, the XRD diffractogram was recorded;
(ii) after purge under N2, the sample was treated in the same
conditions as for Raman studies: reducing atmosphere
(2% H2/N2, 2% CH4/N2), lean conditions (2% CH4/5% O2/N2
or 2% CH4/5% O2/H2O/N2) or pure He; (iii) the heating step
at 10 1C min�1 was stopped at different temperature values for
10 min in order to record the XRD pattern. In this experiment,
CH4 was kept in the feed because there was no modification of
the signal intensity. As for the Raman spectra, some XRD
spectra were recorded after 30 min at a given temperature: no
detectable transformation was recorded for times longer than
10 min at the same temperature.
3. Results and discussion
3.1 Characterization
The Pd loading is close to the theoretical value, 3.94 wt%, and the
specific surface area is around 105 m2 g�1. The 4 wt% Pd sample
shows a dispersion of 22% (i.e. mean particle size of 5 nm). The
crystallite size estimated on the basis of the width at half-height of
PdO peaks was about 6 nm, in good agreement with results of H2
chemisorption experiments. TEM pictures reveal that particle size
was smaller than 10 nm and the palladium particle size evaluated
from the TEM picture is given in Fig. 1. More than 50% of the
particles have a particle size diameter lower than 5 nm and 30%
lower than 4 nm. More information on the catalyst characteriza-
tion, including XRD patterns and TEM pictures, can be found in
our previous work.26 It can be expected that Raman spectroscopy
should only give useful information on particles smaller than
4 nm while XRD will be more sensitive to bigger particles of
size 44–5 nm.
Fig. 1 Palladium particle size distribution from TEM picture.26
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3.2 Reduction under H2/N2 followed by oxidation under
O2/N2
PdO reduction by H2/N2 and Pd1 oxidation were followed by
in situ Raman spectroscopy. The overall Raman spectrum is
given in Fig. 2A and the zoom of the main PdO band at
640 cm�1 is presented in Fig. 2B. For the sake of clarity, in the
next parts, only the zoom of the PdO band will be presented.
The PdO band located at 640 cm�1 is in the same frequency
range (650 cm�1) as the band observed by Bell et al. obtained
with the 514.5 nm wavelength of the Ar.18 At room tempera-
ture, both phenomenon, PdO reduction by H2 and Pd1 oxida-
tion by O2, occur easily. Raman spectroscopy shows that small
particles are reduced at ambient temperature but this technique
cannot give clear information on the biggest particles. In order
to clarify the behaviour of PdO particles bigger than 5 nm,
in situ XRD analysis was carried out. The overall diffractogram
obtained during reduction under H2/N2 is presented in Fig. 3.
As mentioned for Raman spectra, in the next parts, only the 2yrange of the PdO peak will be presented. The results show
that it is not possible to reduce large PdO particles under H2 at
room temperature. Higher temperatures are required to clearly
observe this reduction, which starts between 50 and 100 1C and
it is completed between 100 and 150 1C. Indeed the peak at
33.91 disappears completely. In the same time, Pd1 crystalli-
sation seems to be achieved at 150 1C. However, H2-TPR
measurements carried out on Pd catalysts show that PdO
reduction occurs at low temperature (o100 1C).27,29,30 The
shift in the reduction temperature between XRD and H2-TPR
measurements could be due to the difference between the
phenomenon detected. In the case of H2, the H2 consump-
tion reveals the reduction of PdO oxide whereas XRD
analysis reveals PdO phase transformation into Pd1 metallic
phase.
Reduction under H2 at ambient temperature was further
complicated by the potential formation of palladium hydride.
Some works have been carried out in order to study the
kinetics and mechanism of the dissolution of H atoms in
palladium supported catalysts28–30 or palladium single crystal.31–33
We carried out H2 adsorption experiment on pre-reduced
sample (H2 + PdO - Pd1 + H2O) in order to evaluate the
effect of the low ambient temperature on the reduction process.
Fig. 4 shows the evolution of H2 signal during two heating
[0–80 1C] and cooling [80–0 1C] cycles under H2. It can be
observed that H release occurs during heating at 68 1C and H
uptake occurs during cooling at 18 1C. Palladium crystallites
absorb H to form PdH (a or b) phase (yH2 + 2Pdb 2 2PdbHy).
The a phase contains less H atoms than the b phase. Both
phases of Pd metal have the fcc structure but with different
lattice parameters (0.39 and 0.40 nm, respectively) whereas in
the PdO structure, Pd atoms are in planar coordination with
four O atoms and the O atoms are in tetrahedral coordination
with four Pd atoms. The dominant phase of PdH depends on
H pressure and temperature.
After reduction treatment, Pd1 oxidation was carried out
under O2/N2 and followed by in situ XRD. PdO and Pd1 peak
evolutions located in the 30–421 range are shown in Fig. 5.
The intensity of the Pd1 reflection rapidly decreases above
300 1C, completely disappearing at 400 1C. On the other
hand, only a very broad PdO peak appears at 350 1C which
only gradually sharpens up to 550 1C, suggesting that reoxida-
tion occurs in the 300–400 1C temperature range, forming
highly disordered PdO, which only crystallizes at higher
temperatures.
3.3 Reduction under CH4/N2 followed by oxidation under
O2/N2
Small PdO particle reduction by CH4/N2 was followed by
in situ Raman spectroscopy. The zoom of the PdO band is
presented in Fig. 6A. Under CH4/N2, the reduction of small
PdO particle into Pd1 starts at 240 1C and is completed at
250 1C (i.e. relative Raman signal being equal to 0). This
reduction was followed by CH4-TPR as described in section
2.3.2. Methane consumption is given in Fig. 6B. The tempera-
ture of the reduction process is similar to those obtained by
Raman. Indeed, the reduction starts at 250 1C and is complete
at 310 1C. This temperature range difference could be attributed
to the size effect distribution. In the case of Raman, only small
palladium particles are analyzed, whereas in CH4-TPR,
methane consumption is due the overall palladium content
(i.e. every particle). In order to follow the large PdO particle
reduction, in situ XRD analysis was carried out. The results
presented in Fig. 7 shows that reduction is a fast process
occurring in a narrow temperature range between 280 and
290 1C with simultaneous Pd1 reconstruction. After the reduc-
tion treatment, Pd1 oxidation was carried out under O2/N2
and followed by in situ Raman and XRD analysis. Small
particles are not reoxidized even after oxidation treatment at
300 1C (Fig. 8A). It can be assumed that after reduction under
CH4 conditions, the surface is covered by C or unburned CH4
located in part on the metal. Pd1 particles could only be
reoxidized when these species are removed from the metal
surface. Moreover, Liu et al. showed, thanks to a density
functional theory study, that the C-forming reaction from
CH4 is structure sensitive and is easier on kink and step sites,
which are more abundant on small particles.34 However, large
Pd1 particle reoxidation follows the same process as after
reduction under H2/N2, i.e. slow gradual phenomenon starting
only at 300 1C and finishing at a temperature higher than
550 1C. XRD PdO and Pd1 peak evolution located in the
30–421 range is shown in Fig. 8B. The oxidation process after
CH4/N2 reduction may include three steps: (i) oxidation ofFig. 2 Overall Raman spectra (A) and zoom of the PdO band
(B) after reduction under 2% H2/He.
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residual C at the surface; (ii) O� and O2� migration in the Pd1
bulk; (iii) reconstruction of PdO crystals.
3.4 Thermal decomposition under He followed by oxidation
under O2/N2
PdO decomposition under He and Pd1 oxidation by O2/N2 was
only followed by in situ XRD analysis. Indeed, Raman results
are not shown here due to the experimental limitation at
700 1C. However, at this temperature, surface PdO was still
present. The zoom on the PdO peak during He decomposition
and oxidation treatment are presented in Fig. 9A and Fig. 9B,
Fig. 3 Overall XRD diffractogram after reduction under 2% H2/He.
Fig. 4 H2-TPR at low temperature [0–80 1C].
Fig. 5 XRD diffractograms focused on PdO and Pd1 peaks after
oxidation under 1% O2/N2.
Fig. 6 Raman spectra focused on PdO band (A) after reduction
under 2% CH4/N2 and CH4-TPR; (B) under 0.5% CH4/He.
Fig. 7 XRD diffractogram focused on PdO and Pd1 peaks after
reduction under 2% CH4/N2.
Fig. 8 Raman spectra focused on PdO band after reduction under
2% CH4/N2 followed by oxidation under 1% O2/N2 (A) and XRD
diffractogram focused on PdO and Pd1 peaks after oxidation under
1% O2/N2 (B).
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respectively. Decomposition of large PdO particles starts at
750 1C and is a gradual phenomenon. Total decomposition of
PdO is completed at 800 1C. This decomposition process can
be due to (i) release of O from the surface toward the gas
atmosphere and (ii) O� or O2� anion migration from bulk to
the surface. This second step allows maintaining constant
oxidation of the surface and would explain the Raman results.
Large Pd1 particle reoxidation under O2/N2 follows the same
slow process as after reduction under H2/N2 or CH4/N2. The
results of Pd1 oxidation under O2/N2 followed by in situ
Raman corroborate our previous results on 1% Pd/y-Al2O3
catalyst during temperature programmed desorption of O2.27
It was found that PdO decomposition into metallic palladium
started at 790 1C and was completed at 920 1C, whereas Pd1
was found to reoxidize during cooling at a temperature lower
than 550 1C (i.e. beginning of oxygen consumption) and is
completed at 400 1C.
3.5 Lean conditions
As already mentioned, palladium is used in many oxidation
processes (gas turbine, NGV catalytic converter). In order to
obtain information on the surface and bulk behaviour of the
4 wt% Pd/g-Al2O3 catalyst during CH4 oxidation, with or
without water, in situ Raman spectroscopy and in situ XRD
analysis were used.
3.5.1 Lean condition without water: CH4/O2/N2. The
evolution of the PdO Raman band and PdO diffraction peak
during temperature combustion test under lean conditions
without water (2% CH4/5% O2/N2) are shown in Fig. 10A
and Fig. 10B, respectively. In dry reaction medium, between
200 and 300 1C, the Raman PdO peak area decreases but its
surface seems to be stable beyond 350 1C. It can be assumed
that the PdO particles are modified during CH4 combustion.
However, it is difficult to conclude if the PdO surface is
composed of a mixture of PdO and Pd1 phases or of an under-
stoichiometric PdOx phase (x o 1). Moreover, until 600 1C,
there is no reduction of large PdO particles (Fig. 10B) and thus
CH4 does not modify bulk PdO.
3.5.2 Lean condition in presence of water: CH4/O2/H2O/N2.
In our previous study,26 the methane conversion on 1%, 2%
and 4 wt% Pd/g-Al2O3 was followed during temperature
programmed combustion in the presence of water (2% CH4/5%
O2/2% H2O/N2) and transient experiment (successive Pd
oxidation/reduction cycles). For the 2% and 4% catalysts
with similar Pd particle sizes, 5 nm, determined by H2
chemisorption, Pd catalyst reactivation and PdO reformation
both occurred gradually with comparable time scales. These
results suggested a role of a mixed PdO/Pd1 phase in temporarily
enhancing CH4 combustion activity, but no definite conclu-
sions were given on the issue of whether complete or partial
re-oxidation of Pd was required to achieve maximum CH4
combustion activity. In this complementary work (Fig. 11A),
in situ Raman spectroscopy reveals that small PdO particles
are partially reduced during lean conditions in the presence of
water (2% CH4/5% O2/2% H2O/N2) whereas large PdO
particles are not modified (Fig. 11B). The state of palladium
in reaction is intermediary between PdO and Pd1 but, like for
dry condition, it is not possible to conclude on the nature of
the active site: either a PdO thin film covering a metallic
palladium core PdO/Pd1 phase or a palladium suboxide
(PdOx, x o 1). Specchia et al. have studied the surface
chemistry and surface reactivity of palladium particles over
ceria-zirconia support.35 The authors assumed that the improved
catalytic activity of the fresh catalyst for CH4 combustion at
low temperature is due to its constitution, partly oxidized very
small Pd metal particles and dispersed Pd oxide species. The
progressive oxidation of highly dispersed small Pd particles to
PdOx as well as the coalescence of dispersed Pd oxide species
result in fully oxidized PdOx particles, at least at the surface,
which are less active (in the low temperature range) with
respect to the active species of the fresh catalyst. Moreover,
it is also possible to take into consideration reduction
assistance between particles of different sizes. For instance,
Martin and Duprez36 showed that big particles of rhodium on
alumina reduced before smaller ones. However, it should be
necessary to know the degree of intimacy of the biggest and
the smallest particles and the mean distance between them. At
this stage, it is difficult to conclude definitively.
4. Conclusion
The reactivity study of the PdO and Pd1 particles of the 4 wt%
Pd/Al2O3 catalysts under different atmospheres, characterized
by in situ XRD and Raman analysis, highlighted the following
behaviours:
(1) The reduction behaviour depends on the reducing agent:
H2 can reduce PdO particles at much lower temperatures
(100–150 1C) than methane (280–290 1C) while PdO decomposi-
tion into Pd1 in neutral gas (He) occurs only above 700 1C.
(2) Whatever the reduction or decomposition treatment,
re-oxidation of large Pd1 particles is a gradual phenomenon,
starting at 300 1C and being completed at temperatures higher
than 550 1C. The oxidation process may occur in three steps:
(i) oxidation of the C species of the surface (if CH4 is the
reducer) and of the surface Pd1; (ii) O migration in the Pd1
bulk (DO–Pd1) and (iii) PdO reconstruction (DO–PdO).
(3) Finally, during experiments in lean conditions, with or
without water, the PdO surface is partially reduced whereas
bulk PdO is not modified.
Fig. 9 XRD diffractogram focused on PdO and Pd1 peaks during
thermal decomposition under He (A) and during oxidation under
1% O2/N2 (B).
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This journal is c the Owner Societies 2011 Phys. Chem. Chem. Phys., 2011, 13, 4607–4613 4613
Acknowledgements
The French ‘‘Agence De l’Environnement et de la Maıtrise de
l’Energie’’ (ADEME) is acknowledged for financially supporting
this work.
References
1 R. J. Farrauto, M. C. Hobson, T. Kennelly and E. M. Waterman,Appl. Catal., A, 1992, 81, 227–237.
2 J. Chen and E. Ruckenstein, J. Catal., 1981, 69, 254–273.3 D. Ciuparu, M. R. Lyubovsky, E. Altamn, L. Pfefferle andA. Datye, Catal. Rev. Sci. Eng., 2002, 44, 593–647.
4 P. Gelin and M. Primet, Appl. Catal., B, 2002, 39, 1–37.5 T. V. Choudhary, S. Banerjee and V. R. Choudhary, Appl. Catal.,A, 2002, 234, 1–23.
6 J. G. McCarty, Catal. Today, 1995, 26, 283–283.7 G. Ketteler, D. F. Ogletree, H. Bluhm, H. Liu, E. L. D. Hebenstreitand M. Salmeron, J. Am. Chem. Soc., 2005, 127, 18269–18273.
8 E. H. Voogt, A. J. M. Mens, O. L. J. Gijzeman and J. W. Geus,Surf. Sci., 1997, 373, 210–220.
9 D. Zemlyanov, B. Aszalos-Kiss, E. Kleimenov, D. Teschner,S. Zafeiratos, M. Haevecker, A. Knop-Gericke, R. Schlogl,H. Gabasch, W. Unterberger, K. Hayek and B. Klotzer, Surf.Sci., 2006, 600, 983–994.
10 H. Gabasch, W. Unterberger, K. Hayek, B. Klotzer,E. Kleimenov, D. Teschner, S. Zafeiratos, M. Haevecker,A. Knop-Gericke, R. Schlogl, J. Han, F. H. Ribeiro, B. Aszalos-Kiss,T. Curtin and D. Zemlyanov, Surf. Sci., 2006, 600, 2980–2989.
11 J. Han, D. Y. Zemlyanov and F. H. Ribeiro, Surf. Sci., 2006, 600,2730–2744.
12 G. Zhu, J. Han, D. Y. Zemlyanov and F. H. Ribeiro, J. Am. Chem.Soc., 2004, 126, 9896–9897.
13 R. J. Farrauto, J. K. Lampert, M. C. Hobson andE. M. Waterman, Appl. Catal., B, 1995, 6, 263–270.
14 Y. S. Ho, C. B. Wang and C. T. Yeh, J. Mol. Catal. A: Chem.,1996, 112, 287–294.
15 S. Su, J. N. Cartens and A. Bell, J. Catal., 1998, 176, 125–135.16 D. Roth, P. Gelin, A. Kaddouri, E. Garbowski, M. Primet and
E. Tena, Catal. Today, 2006, 122, 134–138.17 K. Fujimoto, F. H. Ribeiro, M. Avalos-Borja and E. Iglesia,
J. Catal., 1998, 179, 431–442.18 A. K. Datye, J. Bravo, T. R. Nelson, P. Atanasova, M. Lyubovsky
and L. Pfefferle, Appl. Catal., A, 2000, 198, 179–196.19 A. S. Khanna, Introduction to High Temperature Oxidation and
Corrosion, ASM International, 2002.20 J. Han, D. Y. Zemlyanov and F. H. Ribeiro, Surf. Sci., 2006, 600,
2752–2761.21 A. S. Mamede, G. Leclerq, E. Payen, P. Granger and J. Grimblot,
J. Mol. Struct., 2003, 651–653, 353–364.22 T. Arai, T. Shima, T. Nakano and J. Tominaga, Thin Solid Films,
2007, 515, 4774–4777.23 K. Otto, C. P. Hubbard, W. H. Weber and G. W. Graham, Appl.
Catal., B, 1992, 1, 317–327.24 J. R. McBride, K. C. Hass and W. H. Weber, Phys. Rev.
B: Condens. Matter, 1991, 44, 5016–5028.25 M. Boulova, A. Gaskov and G. Lucazeau, Sens. Actuators, B,
2001, 81, 99–106.26 P. Castellazzi, G. Groppi, P. Forzatti, A. Baylet, P. Marecot and
D. Duprez, Catal. Today, 2010, 155, 18–26.27 A. Baylet, S. Royer, C. Labrugere, H. Valencia, P. Marecot,
J. M. Tatibouet and D. Duprez, Phys. Chem. Chem. Phys., 2008,10, 5983–5992.
28 D. Wang, J. D. Clewley, T. B. Flanagan, R. Balasubramaniam andK. L. Shanahan, Acta Mater., 2002, 50, 259–275.
29 C. Neyertz, M. A. Volpe and C. Gigola, Catal. Today, 2000, 57,255–260.
30 C. Amorim and M. A. Keane, J. Colloid Interface Sci., 2008, 322,196–208.
31 F. Leardini, J. F. Fernadez, J. Bodega and C. Sanchez, J. Phys.Chem. Solids, 2008, 69, 116–127.
32 C. W. Chou, T. P. Perng and C. T. Yeh, J. Phys. Chem. B, 2001,105, 9113–9117.
33 T. Kuji, Y. Matsumura, H. Uchida and T. Aizawa, J. AlloysCompd., 2002, 330–332, 718–722.
34 Z. P. Liu and P. Hu, J. Am. Chem. Soc., 2003, 125, 1958–1967.35 S. Specchia, E. Finocchio, G. Busca, P. Palmisano and
V. Specchia, J. Catal., 2009, 263, 134–145.36 D. Martin and D. Duprez, Appl. Catal., A, 1995, 131,
297–307.
Fig. 10 Raman spectra (A) and XRD diffractogram (B) focused on
PdO and Pd1 peaks during lean condition reaction under 2% CH4/5%
O2/N2.
Fig. 11 Raman spectra (A) and XRD diffractogram (B) focused on
PdO and Pd1 peaks during lean condition reaction under 2% CH4/5%
O2/2% H2O/N2.
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