Methane Catalytic Combustion over Hierarchical Pd@CeO2/Si-Al2O3: Effect of the Presence of Water

Article abstract of DOI:10.1002/cctc.201402717

The influence of water vapor on methane catalytic combustion was studied over a Pd@CeO₂/Si-Al₂O₃ catalyst, carefully designed to maximize Pd-CeO₂ interaction and prevent metal sintering and compared to a conventional impregnated catalyst with identical chemical composition. Although the nanostructured Pd@CeO₂/Si-Al₂O₃ catalyst is thermally stable, the addition of water to the reaction feed leads to a transient deactivation at low temperatures, consistent with the well documented competitive adsorption. In addition to this, the hierarchically structured catalyst exhibits an additional severe deactivation after methane oxidation in the presence of water vapor at 600°C that can be reversed only by heating the catalyst above 700°C. The presence of water in the reaction feed deactivates the conventional impregnated catalyst less severely and the activity largely returns upon water removal. Catalytic FTIR and CO-chemisorption data indicate that this severe deactivation process in the hierarchical catalyst is due to the formation of stable OH groups on the surface of the ceria nanoparticles. These hydroxyl groups are suggested to significantly inhibit the oxygen spillover from the CeO₂ nanoparticles to Pd, preventing its efficient re-oxidation, as observed by operando X-ray absorption near edge spectroscopy (XANES) experiments. At the same time, their presence can contribute to limit the gas phase accessibility of Pd, as indicated by the decrease of CO chemisorption capability. The presence of hydroxyls plays a minor role on the deactivation of the conventional catalyst at 600°C.

Full text of DOI:10.1002/cctc.201402717

CHEMCATCHEM
FULL PAPERS
DOI: 10.1002/cctc.201402717
Methane Catalytic Combustion over Hierarchical Pd@CeO2/
Si-Al2O3: Effect of the Presence of Water
Matteo Monai,[a] Tiziano Montini,[a] Chen Chen,[b] Emiliano Fonda,[c] Raymond J. Gorte,*[b] and
Paolo Fornasiero*[a, d]
Dedicated to Prof. Gianfranco Pacchioni in the occasion of his 60th birthday.
The influence of water vapor on methane catalytic combustion
was studied over a Pd@CeO2/Si-Al2O3 catalyst, carefully de-
signed to maximize Pd-CeO2 interaction and prevent metal sin-
tering and compared to a conventional impregnated catalyst
with identical chemical composition. Although the nanostruc-
tured Pd@CeO2/Si-Al2O3 catalyst is thermally stable, the addi-
tion of water to the reaction feed leads to a transient deactiva-
tion at low temperatures, consistent with the well documented
competitive adsorption. In addition to this, the hierarchically
structured catalyst exhibits an additional severe deactivation
after methane oxidation in the presence of water vapor at
6008C that can be reversed only by heating the catalyst above
7008C. The presence of water in the reaction feed deactivates
the conventional impregnated catalyst less severely and the
activity largely returns upon water removal. Catalytic FTIR and
CO-chemisorption data indicate that this severe deactivation
process in the hierarchical catalyst is due to the formation of
stable OH groups on the surface of the ceria nanoparticles.
These hydroxyl groups are suggested to significantly inhibit
the oxygen spillover from the CeO2 nanoparticles to Pd, pre-
venting its efficient re-oxidation, as observed by operando X-
ray absorption near edge spectroscopy (XANES) experiments.
At the same time, their presence can contribute to limit the
gas phase accessibility of Pd, as indicated by the decrease of
CO chemisorption capability. The presence of hydroxyls plays
a minor role on the deactivation of the conventional catalyst
at 6008C.
Introduction
Pd-supported catalysts are the most active materials for cata-
lytic combustion of methane at lower temperatures.[1–6] Both
metallic Pd and PdO are active for methane oxidation, but, at
lower temperatures, PdO is reported to be the more active
phase.[7] Although metallic Pd tends to be significantly less
active,[5,8] the PdO phase is not stable at high temperatures
and low O2 partial pressures. The transition temperature at
which the mechanism involving metallic Pd dominates is in-
creased by increasing O2 pressures and by interactions be-
tween Pd and reducible supports.[9] It should be recognized
that the above discussion is a greatly simplified picture of the
reaction on working catalysts because the nature of CꢀH bond
cleavage changes with the Pd oxidation state and because Pd
and PdO can co-exist under non-steady conditions.[8,10]
Ceria (CeO2) is one of the most effective promoters for Pd
oxidation among oxides used as a support and it can, there-
fore, play a pivotal role in enhancing catalytic stability and ac-
tivity for methane combustion.[11,12] CeO2 is known to have two
beneficial functions: it provides oxygen for the catalytic com-
bustion at low temperature by transferring oxygen to Pd[9,13,14]
and it can improve the stability of the active PdO phase at
high temperatures.[15–17] However, pure ceria has limited ther-
mal stability,[18] and direct contact between the ceria and Pd is
required for oxygen transfer.[11,19]
[a] Dr. M. Monai, Dr. T. Montini, Prof.Dr. P. Fornasiero
Department of Chemical and Pharmaceutical Sciences
ICCOM-CNR, Consortium INSTM Trieste Research Unit
University of Trieste
via L. Giorgieri 1, 34127 Trieste (Italy)
E-mail: pfornasiero@units.it
For emissions-control applications in gasoline and lean-burn
engines, the exhaust contains large amounts of water vapor
(5.0–15.0%)[6,20] and water is known to strongly inhibit catalytic
combustion on Pd-based materials.[21] On conventional Pd cat-
alysts, it has been reported that water and methane competi-
tively adsorb on PdO active sites and that water reacts with
PdO to form Pd(OH)2, which is much less reactive with meth-
ane.[22,23] There is some disagreement over whether this deacti-
vation is reversible or irreversible,[24–26] but the inhibition effect
of water is known to be more important at lower tempera-
tures. For example, Burch et al. reported methane-oxidation re-
action rates on Pd/Al2O3 were inhibited by water up to approx-
[b] Dr. C. Chen, Prof. R. J. Gorte
Department of Chemical and Biomolecular Engineering
University of Pennsylvania, 311 A Towne Building
220 South 33rdStreet, Philadelphia, PA 19104 (USA)
E-mail: gorte@seas.upenn.edu
[c] Dr. E. Fonda
SAMBA beamline, Synchrotron SOLEIL
L’Orme des Merisiers, Saint Aubin BP48
91192 Gif sur Yvette CEDEX (France)
[d] Prof.Dr. P. Fornasiero
King Abdullah University of Science and Technology (KAUST)
Thuwal 23955-6900 (Saudi Arabia)
Part of a Special Issue on Palladium Catalysis. A link to the Table of Con-
tents will appear here once the Special Issue is assembled.
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 0000, 00, 1 – 10
&
1
&
ÞÞ
These are not the final page numbers!

CHEMCATCHEM
FULL PAPERS
www.chemcatchem.org
imately 4508C.[22] The importance of water inhibition on Pd
catalysts also depends on the support, with reducible oxides
showing improved resistance to water inhibition.[27] An elegant
explanation for the role of the support comes from the work
of Pfefferle and co-workers,[28] who reported the accumulation
of hydroxyl/water on the oxide support. The support hydrox-
ides are suggested to hinder oxygen mobility from the sup-
port.
light-off measurements on the same catalyst were identical to
that shown here, demonstrating complete reversibility of the
catalyst. As reported in earlier work,[5] there was no dip in the
conversion during either the heating or the cooling cycles,
owing to decomposition of PdO, for space velocities below
1000000 mLgꢀ1 hꢀ1. The addition of 15.0% water to the feed
had a dramatic effect on the light-off curves. First, the methane
conversions were shifted to higher temperatures by approxi-
mately 2008C during both heating and cooling cycles. Second,
transient deactivations were observed during both the heating
and cooling cycles. The decrease in CH4 conversion was small
during the upward temperature ramp, down to 98% of con-
version at about 7508C; but the decreased conversion was
very noticeable with decreasing temperatures, going down to
60% conversion at 6508C.
Transient deactivation for CH4 oxidation is commonly ob-
served with conventional supported-Pd catalysts, even under
dry conditions, and is associated with a PdO!Pd phase transi-
tion.[2] When the temperature is ramped upward, conversions
can decrease with increasing temperature, owing to decompo-
sition of PdO, then return to 100% conversion as temperature
is further increased. The transient deactivation is more evident
during the cooling cycle because the re-oxidation of Pd to the
more active PdO is kinetically controlled,[30] with Pd reoxidation
occurring through a nucleation mechanism that is favored
when some PdO is still present.[8] The transition temperatures
for the decreased conversion during cooling depend on the O2
pressure and can be influenced by the support.[2] The absence
of such transients with Pd@CeO2/Si-Al2O3 under dry conditions
is likely caused by the close contact between Pd and ceria in
the core-shell structures.[5] The presence of these transients
upon the addition of water to the feed suggests water sup-
presses reoxidation of Pd.
To maximize the Pd-ceria interfacial contact and, therefore,
the catalytic activity, some of us have recently developed hier-
archical catalysts in which Pd nanoparticles are first produced
and subsequently surrounded by a very thin, porous ceria
shell, in a tight metal-support contact. These catalysts have
shown exceptional activity for methane oxidation under dry
conditions and increased thermal stability of the catalyst.[29]
Complete methane conversion was achieved on a Pd@CeO2/Si-
Al2O3 catalyst at temperatures below 4008C and at high space
velocities. Significantly, the deactivation associated with the
classical PdO-Pd transition observed in conventional Pd-based
catalysts was not observed in temperature ramping experi-
ments.[5] In the present study, we have extended our investiga-
tion to the effect of water on methane oxidation over
a Pd(1.0%)@CeO2(9.0%)/Si-Al2O3 catalyst. In addition to the
normal inhibition observed on traditional Pd catalysts, we ob-
serve a strong deactivation of the hierarchical catalyst at
6008C, which was attributed to the nanostructured CeO2 shell
being significantly converted to the thermally stable hydroxide.
The catalyst activity could only be restored by heating above
7008C to decompose the cerium hydroxide.
Results and Discussion
The catalytic performance of the Pd@CeO2/Si-Al2O3 catalyst for
CH4 oxidation under dry and wet conditions was characterized
using light-off curves with a heating and cooling rate of
108Cminꢀ1. As shown in Figure 1, under dry conditions CH4
conversion began at approximately 3008C with increasing tem-
perature and was shifted by approximately 50 degrees to
lower temperatures during the cooling cycle. Subsequent
Further evidence that the detrimental effect of H2O is due to
suppressed oxidation of Pd is demonstrated in Figure 2, which
shows light-off curves as a function of the O2 partial pressure.
In these experiments, the CH4 and H2O concentrations were
fixed at 0.5% and 15.0%, respectively, while the O2 concentra-
tion was increased from 2.0% to 14.0%. The magnitude of the
dip in the conversion during the cooling cycles decreased
steadily with increasing O2 concentration, while the tempera-
ture at which the minimum occurred increased, from 6408C
for 2.0% O2 to ꢁ7108C for 14.0% O2. The explanation for both
of these observations is that the higher O2 partial pressures
help to maintain the Pd as PdO.
Steady-state measurements of CH4 oxidation over Pd@CeO2/
Si-Al2O3 in the presence of water at 500 and 6008C were espe-
cially revealing. In these experiments, the catalyst temperature
was ramped to the desired temperature in 0.5% CH4, 2.0% O2
and 15.0% H2O and held at that temperature for a period of
time. Then water was removed from the reaction mixture
while monitoring the CH4 conversion. Results for experiments
performed at 5008C are shown in Figure 3. After the catalyst
temperature reached 5008C, the methane conversion de-
creased slowly from an initial value of 90% to ꢁ60% after 4 h.
When water was removed from the reaction mixture, the activ-
ity was completely restored after less than 10 min. When the
Figure 1. Effect of water on methane combustion light-off curves over
Pd@CeO2/Si-Al2O3. Dry conditions (circles): 0.5% CH4, 2.0% O2, Ar balance,
O2/O2(stoich) =2, GHSV=200000 mLgꢀ1 hꢀ1, heating and cooling rates
108Cminꢀ1. Wet conditions (squares): same conditions as above, with the
addition of 15.0% H2O.
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 0000, 00, 1 – 10
&
2
&
ÞÞ
These are not the final page numbers!

CHEMCATCHEM
FULL PAPERS
www.chemcatchem.org
Figure 3. Steady state aging test under wet reaction conditions at 5008C,
followed by activity recovery under dry reaction conditions. Dry conditions
(circles): 0.5% CH4, 2.0% O2, Ar balance, O2/O2(stoich) =2,
GHSV=200000 mLgꢀ1 hꢀ1, heating and cooling rates 108Cminꢀ1. Wet condi-
tions (squares): same conditions as above, with the addition of 15.0% H2O.
The catalyst deactivation observed in Figure 4 was not
caused by a permanent destruction of the catalyst, as demon-
strated by the data in Figure 5a. In this experiment, the
Pd@CeO2/Si-Al2O3 catalyst was again aged in 0.5% CH4, 2.0%
O2, and 15.0% H2O for 4 h, then the sample was purged under
Ar and cooled to room temperature. Next, the sample temper-
ature was ramped to 8508C at 108Cminꢀ1. The conversions
during this initial temperature ramp were shifted to significant-
ly higher temperatures than that observed even with wet feed
in Figure 1. On the aged catalyst, 100% conversion of CH4 was
not reached until above 6508C. However, the cool-down curve
followed the conversion data observed for dry CH4 oxidation
on the fresh catalyst and subsequent light-off curves for dry
CH4 oxidation were identical to that observed on the fresh cat-
alyst.
Figure 2. Effect of oxygen concentration on methane combustion light-off
curves over Pd@CeO2/Si-Al2O3 under wet reaction conditions: a) 2.0, b) 4.0,
c) 10.0, and d) 14.0%. Conditions: 0.5% CH4, different concentrations of O2,
15.0% H2O, Ar balance, GHSV=200000 mLgꢀ1 hꢀ1, heating and cooling rates
108Cminꢀ1
.
sample was cooled to room temperature at 108Cminꢀ1, the
conversion followed the cool-down curve for dry methane oxi-
dation reported in Figure 1. These results imply that H2O at
5008C does not modify the structural properties of the
Pd@CeO2/Si-Al2O3 catalyst.
It is interesting to note that something similar occurs on the
reference impregnated sample. However, aging at 6008C
under wet conditions for 4 hr leads to a deactivation of the
catalyst that is less marked with respect to that of the hierarch-
ical catalyst under the same experimental conditions (residual
The results from the analo-
gous experiment at 6008C
(Figure 4) were very different.
The CH4 conversion decreased
much more sharply during iso-
thermal aging, reaching only
10% after 4 h. Removing water
from the reaction mixture at
6008C did not completely re-
store the conversion, even after
10 h. Finally, when the sample
was cooled to room temperature
at 108Cminꢀ1, the conversion
did not follow the cool-down
curve for dry methane oxidation
Figure 4. Steady state aging test under wet reaction conditions at 6008C, followed by activity recovery under dry
reaction conditions. Dry conditions (circles): 0.5% CH4, 2.0% O2, Ar balance, O2/O2(stoich) =2,
GHSV=200000 mLgꢀ1 hꢀ1, heating and cooling rates 108Cminꢀ1. Wet conditions (squares): same conditions as
above, with the addition of 15.0% H2O.
reported in Figure 1; but, rather,
the conversions were shifted to
high temperatures.
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 0000, 00, 1 – 10
&
3
&
ÞÞ
These are not the final page numbers!

CHEMCATCHEM
FULL PAPERS
www.chemcatchem.org
Figure 5. Methane combustion light off experiment over a) Pd@CeO2/Si-Al2O3 catalyst and b) reference impregnated Pd-CeO2/Si-Al2O3 catalyst, both aged at
6008C for 4 h under wet reaction conditions and cooled to 1508C under Ar. Squares represent first cycles and circles second cycle. Conditions: 0.5% CH4,
2.0% O2, Ar balance, O2/O2(stoich) =2, GHSV=200000 mLgꢀ1 hꢀ1, heating and cooling rates 108Cminꢀ1
.
conversion ꢁ32% vs ꢁ8%). Upon removing water, a sharp in-
crease in activity is obtained and complete restoration of the
initial activity is achieved after high temperature treatment. In-
terestingly, the light-off curve measured under dry conditions
for this aged impregnated catalyst (Figure 5b) is once again
shifted to higher temperature and the cooling cycles resem-
bles that of the fresh sample as the following activity.
above 7008C. Although an accurate estimate of this amount is
difficult, owing to the steady-state reaction in the background,
we monitored an amount of water from the deactivated cata-
lyst that is slightly higher than the moles of cerium in the
sample. This could suggest that, besides significant formation
of cerium hydroxyls on CeO2, some hydroxyls are also formed
on the alumina.
Simultaneously with the conversion measurements in Fig-
ure 5a, water evolution was also monitored over the
Pd@CeO2/Si-Al2O3 sample, with results shown in Figure 6. For
To determine whether other pretreatment conditions could
cause the deactivation observed in Figure 4, the CH4 conver-
sion was monitored under dry conditions for longer times and
after high-temperature reduction, with results shown in
Figure 7. First, the conversion in a dry mixture of 0.5% CH4 and
2.0% O2 at 6008C showed no decrease from 100%, even after
14 h. Neither the high temperatures nor exposure to CH4 and
O2, in the absence of H2O, are responsible for the catastrophic
loss in conversion. Next, the catalyst was reduced at 6008C in
5.0% H2/Ar. When the catalyst was again exposed to the dry
reaction mixture, 100% conversion was achieved after 6 min.
Figure 6. Water evolution (signal m/z=18) recorded for the experiments
presented in Figure 5a. The horizontal bars correspond to the period during
which complete conversion of methane was observed. Shadow area high-
lights the additional water evolution with respect to that formed by meth-
ane combustion. Squares represent first cycles and circles second cycle.
Solid line represents the catalyst temperature (on the right axis).
the active catalyst, the water production increases with tem-
perature as CH4 conversion increases. When the temperature is
sufficient to completely oxidize the CH4, the water production
is constant. However, with the catalyst deactivated by treat-
ment under wet CH4 oxidation conditions, a significant excess
of water, above that produced by the steady-state oxidation of
CH4, is released from the sample when the temperature is
Figure 7. Catalytic oxidation of methane. Aging test under dry reaction con-
ditions at 6008C, followed by reduction in 5.0% H2/Ar at 6008C and subse-
quent purge in Ar. Conditions: 0.5% CH4, 2.0% O2, Ar balance, O2/
O
2(stoich) =2, GHSV=200000 mLgꢀ1 hꢀ1
.
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 0000, 00, 1 – 10
&
4
&
ÞÞ
These are not the final page numbers!

CHEMCATCHEM
FULL PAPERS
www.chemcatchem.org
Flowing Ar over the catalyst at 6008C for 30 min also had no
effect on the subsequent activity. The effect of catalyst pre-re-
duction was also examined by means of operando XAFS ex-
periments demonstrated that Pd is almost completely reduced
under 5.0% H2/N2 already at 1508C. In addition, the data indi-
cate the presence of 20% CeIII in the fresh catalyst, in accord-
ance with the nanometric dimensions of the ceria crystal-
lites.[31] This contribution was determined to be constant
during all operando X-ray absorption near edge spectroscopy
(XANES) experiments, either under dry or wet conditions. Upon
reduction at 6008C under 5.0% H2/N2, the CeIII content in-
creases up to 45%. Excellent fitting of the Pd EXAFS data was
achieved by considering only the contribution of Pd fcc phase
with a distance of 0.273 nm (0.271 in bulk Pd) in accordance
with a similar study for Pd/PdO on g-Al2O3 (0.274 nm).[32] In the
absence of oxygen (N2 atmosphere), operando XANES experi-
ments demonstrated that PdO almost fully decomposes to Pd
at 6008C, even if in close contact with CeO2 (Figure 8b). The
Figure 9. FTIR spectra of Pd@CeO2/Si-Al2O3 a) fresh sample and after differ-
ent treatments: b) dry reaction condition at 6008C for 4 h, c) wet reaction
condition at 6008C for 4 h, d) wet reaction condition at 6008C for 4 h, fol-
lowed by dry condition at 6008C for 1 h, e) wet reaction condition at 6008C
for 4 h, followed by dry condition at 8008C for 1 h, f) Pd/Si-Al2O3 and con-
ventional impregnated Pd-CeO2/Si-Al2O3 after different treatments g) dry re-
action condition at 6008C for 4 h, h) wet reaction condition at 6008C for 4 h.
lowing treatment of the catalyst in wet CH4-oxidation condi-
tions (Figure 9c), the n(OH) hydroxyl band becomes much
more intense and centered near 3720 cmꢀ1, a frequency that
has been assigned to mono-coordinated and doubly bridged
OH groups bound to ceria. A small shoulder at 3565 cmꢀ1 may
be attributable to triply bridging OH species.[33] Exposing the
catalyst to dry CH4-oxidation conditions for 1 h (Figure 9d)
caused minimal changes; however, heating the sample to
8008C (Figure 9e) greatly reduced the intensity of the n(OH)
stretching band. The fact that this feature is significantly less
evident in the case of Pd/Si-Al2O3 (Figure 9e–f) aged under
wet conditions suggests that the stable hydroxyls are some-
how linked to the defective - nanostructured ceria. Neverthe-
less, since the Al2O3 support presents various n(OH) band in
the same region (Figure 9 f), the intense n(OH) band in Fig-
ure 9c and 9d cannot be definitely assigned to cerium hydrox-
ide. The formation of these stable hydroxyls under conditions
where catalyst deactivation occurs and their removal by condi-
tions that reactivate the catalyst strongly suggest that they are
associated with deactivation. The IR spectra of the reference
impregnated Pd-CeO2/Si-Al2O3 samples are different, indicating
the influence of the preparation method on nature of the sur-
face hydroxyls. Remarkably, the wet aging at 6008C leads to
less marked changes in the population of hydroxyls.
Figure 8. Fraction of PdO obtained from XANES spectra during subsequent
treatments at 6008C under a) dry reaction conditions (0.5% CH4, 2.0% O2, N2
balance), b) pure N2, c) dry reaction conditions, and d) 5.0% O2/N2.
equilibrium is reversed under dry conditions leading to the
almost immediate reoxidation of 25% Pd to PdO followed by
a further slow increase during the following reaction time (Fig-
ure 8c). Only the use of 5.0% O2/N2 leads to a step increase in
the Pd to PdO oxidation (Figure 8d). Notably the catalytic ac-
tivity of the pre-reduced catalyst (Figure 7) is fully recovered
after a comparable time (7–8 min) indicating that partial reoxi-
dation of the Pd is sufficient to guarantee high catalytic activi-
ties.[30]
To understand the changes that occur in the catalyst follow-
ing CH4 oxidation under wet conditions, we examined the
effect of catalyst pretreatment conditions using FTIR and
XANES measurements, with results from ex situ FTIR shown in
Figure 9. The spectrum in Figure 9.a was obtained on the cata-
lyst that had simply been calcined to 8508C. After exposure to
air and mild heating in dry He, a rather broad n(OH) stretching
band is observed in the region of 3700 cmꢀ1. Aging the cata-
lyst under dry CH4-oxidation conditions (Figure 9b), does not
lead to significant modification of the spectrum. However, fol-
Results from the operando XANES experiments on
Pd@CeO2/Si-Al2O3 are reported in Figure 10. Figure 10a shows
XANES spectra of the Pd edge as a function of time as the cat-
alyst is exposed to dry CH4 oxidation conditions at 6008C. The
spectra remain unchanged over a period of 79 min; from
measurements on reference samples (shown in the inset of
Figure 10a), it is apparent that the Pd is present as PdO. Upon
the introduction of 15.0% H2O to the reaction mixture (Fig-
ure 10b), the spectra evolve with time to form a mixture of
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 0000, 00, 1 – 10
&
5
&
ÞÞ
These are not the final page numbers!

CHEMCATCHEM
FULL PAPERS
www.chemcatchem.org
ing occurs with a few hours of aging under the
adopted conditions and there is no significant cover-
age of the catalytic active phase by reduced cerium
oxide or by hydroxyls. However, the initial metal-sup-
port contact area, as indirectly indicated by the metal
dispersion, is decreased, partially accounting for the
lower activity
Pd-CeO2 based catalysts show very good low-tem-
perature activity for methane oxidation,[2] especially
when prepared in a hierarchical structure that maxi-
mizes the contact between the PdO core the sur-
rounding porous ceria.[5] The reactivity of these sys-
tems strongly depends on the oxidation state of Pd,
with PdO showing much higher activity than Pd, so
that the O2 partial pressure in the reaction feed plays
a major role in affecting rates. The intimate contact
between the Pd phase and CeO2 obtained in our
Pd@CeO2/Si-Al2O3 catalysts not only stabilizes Pd par-
Figure 10. Normalized XANES spectra obtained during operando methane combustion
under a) dry and b) wet reaction conditions at 6008C. Conditions: 0.5% CH4, 2.0% O2,
15.0% H2O (if present), N2 balance. The insets report XANES of reference Pd and PdO
and PdO percentage during wet reaction conditions at 6008C, respectively.
both PdO and the less active metallic Pd. As shown in the
inset, approximately 60% of the PdO is converted to Pd after
80 min under these conditions.
ticle size but also facilitates oxygen spillover, reducing the
extent of PdO decomposition at high temperatures and the
subsequent decrease in activity associated with the transition
of PdO to Pd.[5]
CO chemisorption data on fresh, dry, and wet aged samples,
as well as the corresponding high temperature regenerated
ones are reported in Table 1. The data clearly indicate that,
At lower temperatures, the presence of water is known to
significantly alter the activity of all Pd catalysts due to forma-
tion of hydroxyls on the Pd surfaces.[22–26] In conventional im-
pregnated catalysts, such as Pd/Al2O3,[22] the extent of this de-
activation significantly decreases with increasing temperature,
owing to the thermal instability of surface hydroxyls. However,
the presence of water at high temperature can promote Pd
sintering.[34] Notably, on those catalysts, the high temperature
PdO-Pd phase transition leads also to a significant decrease of
catalytic activity even under dry conditions. What we have ob-
served in the present study is that there is an additional deacti-
vation process that occurs on the Pd@CeO2/Si-Al2O3 core-shell
catalysts during oxidation of methane in the presence of water
vapor at high temperatures. After exposure to these condi-
Table 1. Accessible Pd surface area, CO/Pd ratio and apparent particle
size obtained from low temperature CO chemisorption on Pd@CeO2/Si-
Al2O3 and conventional impregnated Pd-CeO2/Si-Al2O3.
Sample
Accessible Pd
surface area
CO/Pd
ratio
[%]
Apparent
particle size[a]
[nm]
[m2 gꢀ1
]
Pd@CeO2/Si-Al2O3
fresh
dry aged
reactivated after dry aged[b]
wet aged
reactivated after wet aged[b]
Pd-CeO2/Si-Al2O3
fresh
dry aged
wet aged
3.0
2.8
3.3
1.1
3.2
67
64
74
27
72
1.7
1.8
1.5
4.2
1.6
[5]
tions, the exceptional activity observed for Pd@CeO2/Si-Al2O3
was lost even under dry conditions, and the catalyst per-
formance resembled that of a conventional supported Pd/CeO2
catalyst.[2] The presence of water induces 1) a shift of the light
off curve to higher temperature and 2) the appearance of the
transient deactivation generally observed in the 600–7508C
range.
2.4
2.2
2.1
53
50
48
2.1
2.2
2.3
[a] Apparent particle size was estimated assuming a spherical geometry.
[b] sample reactivated by thermal treatment at 8508C under 5.0% O2/Ar.
The influence of water on catalytic activity can be under-
stood by considering the reaction mechanism that is generally
accepted for palladium supported on CeO2. In the presence of
PdO, a Mars-van Krevelen mechanism is operative, in which
PdO lattice oxygen atoms are consumed by CH4 to produce
CO2 and water.[7] Re-oxidation of palladium can occur by reac-
tion with O2 from the gas phase or by spillover of O atoms
from CeO2.[27] For each oxygen vacancy formed by oxygen spill-
over, two reduced CeIII ions are generated[35] and these in turn
must be re-oxidized by molecular O2. Therefore, the excellent
catalytic properties of the core-shell Pd@CeO2 catalyst can be
associated with intimate contact between the PdO phase and
the CeO2 nanocrystallites which leads to efficient O spillover,
resulting from the peculiar core-shell structure.[5]
consistently with the stable catalytic activity, the relatively high
accessible Pd surface area of the fresh Pd@CeO2/Si-Al2O3
sample is not altered by the dry aging. Vice versa, the wet
aging at 6008C leads to an appreciable decrease of the capa-
bility to chemisorb CO. Although significant, the observed de-
crease of exposed metal surface area (Table 1) cannot alone ex-
plain the observed dramatic deactivation of the catalytic activi-
ty. Remarkably, the initial Pd accessibility is fully recovered
after the high temperature treatment that eliminates the hy-
droxyls groups and restores the catalytic activity. The results
for the conventional impregnated Pd-CeO2/Si-Al2O3 were differ-
ent and the initial accessible Pd surface area is only marginally
reduced by either dry or wet aging (Table 1). Only minor sinter-
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 0000, 00, 1 – 10
&
6
&
ÞÞ
These are not the final page numbers!

CHEMCATCHEM
FULL PAPERS
www.chemcatchem.org
Consistent with previous results,[6] the presence of water at
medium-low temperature results in a reversible and competi-
tive adsorption with methane and oxygen on the catalyst sur-
face, leading to a moderate catalyst deactivation. The effect of
water in the reaction media at high temperature is more re-
markable and less reversible (Figure 4). IR data shows that, in
the presence of a large amount of water, the oxygen vacancies
present on the nanoceria can react with H2O, forming OH
groups on the surface of the ceria nanoparticles (Figure 9).
Results from density functional theory (DFT) simulations per-
formed on reduced CeO2 (111) indicate that water can dissoci-
ate into an OH group, which fills the oxygen vacancy, and an
H atom which bonds to a surface O atom.[36,37] Therefore, the
reaction of a vacancy, caused by reduction of ceria, with one
water molecule results in the formation of two hydroxyl
groups, without affecting the oxidation state of CeIII ions close
to the original oxygen vacancy site. The hydroxyl groups
formed by the dissociation of water on the oxygen vacancies
can in turn inhibit the oxygen diffusion on the surface of the
CeO2 nanoparticles and stabilize CeIII.[28] Since the oxygen va-
cancies originate from the O back spillover from ceria to the
metal, a process that involves short range interactions,[38] it is
reasonable to expect that the hydroxyls resulting from the re-
oxidation by water are mostly located in the proximity of the
Pd. In this way, an efficient Pd re-oxidation is hindered and
a deactivation of the catalyst occurs (Scheme 1), owing to the
6008C (Figure 4) is a slow process that can be outlined in the
following steps:
1) the recombination of OH groups to form a chemisorbed
water molecule;
2) the desorption of water leaving an oxygen vacancy on the
CeO2 surface;
3) the reaction of O2 from the gas phase with surface O va-
cancies on CeO2ꢀx
;
4) the migration of O species to Pd forming PdO.
Since steps 3 and 4 are also involved in the fast regeneration
of the catalyst after reduction in H2/Ar at 6008C (Figure 7), the
slow recovery observed after aging under wet reaction condi-
tions should be limited by step 1 and/or step 2 (Scheme 2).
Scheme 2. A representation of OH groups recombination to chemisorbed
H2O (step 1) and of water desorption, with an oxygen vacancy formation
(step 2).
Notably, the same steps are involved in the reduction of CeO2-
[39,40]
based materials in H2
and are recognized as the rate deter-
mining steps for the processes operative above 5508C.[41,42]
Investigations of the catalyst that was deactivated under
wet conditions at 6008C (Figure 5) further corroborate the pro-
posed mechanism of high temperature deactivation. Under dry
conditions, the light-off curves for the aged sample are shifted
significantly towards higher temperatures (Figure 5). In addi-
tion to the water generated by methane combustion, the addi-
tional water evolved from the deactivated catalysts in the tem-
perature range from 700 to 8508C (Figure 6) confirms the high
temperature stability of the hydroxyls groups on CeIII and their
role in the deactivation phenomenon. The catalyst deactivated
under wet condition is, therefore, composed of metallic palla-
dium (EXAFS data of Figure 10) surrounded by a shell of hy-
droxylated (IR data of Figure 9) and partially reduced ceria. In
addition to the specific lower activity of Pd with respect to
PdO, the presence of thermally stable OH may also hinder gas-
eous reactants from reaching the surface and limit the active
sites accessibility, as observed by the decrease of CO chemi-
sorption capability (Table 1). Finally, the exceptional high activi-
ty observed under dry condition on the fresh sample is fully re-
covered on the wet aged sample after complete removal of
the hydroxyls on CeO2 (Figure 5).
Scheme 1. A representation of the proposed evolution of the Pd@CeO2/Si-
Al2O3 catalyst accounting for the deactivation observed in the presence of
water.
PdO transformation to Pd as observed by operando XANES ex-
periments (Figure 10).
The proposed reaction Scheme is fully consistent with both
the transient and steady state catalytic data. Under transient
conditions (Figure 1), the rapid increase in temperature does
not allow a significant buildup of hydroxyl groups, the stability
of which decreases with temperature. In steady-state experi-
ments at 5008C (Figure 3), the weak and fully reversible deacti-
vation is still consistent with the competitive adsorption of
water and methane. At this temperature, the occurrence of
ceria back spillover is not a key requirement for having high
activity. The catalytically active PdO is thermally stable at
5008C and Pd can be easily oxidized by gas-phase molecular
oxygen. On the other hand, aging at 6008C under wet reaction
conditions (Figure 4) induces a deeper deactivation and a re-
duction of PdO observed by EXAFS (Figure 10), along the for-
mation of hydroxyl groups (Figure 9). The activity recovery ob-
served when switching from wet to dry reaction conditions at
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 0000, 00, 1 – 10
&
7
&
ÞÞ
These are not the final page numbers!

CHEMCATCHEM
FULL PAPERS
www.chemcatchem.org
catalyst with cerium ammonium nitrate and palladium nitrate, with
intermediate and final calcination steps at 8508C for 5 h.
Conclusions
Methane oxidation on Pd@CeO2/Si-Al2O3 is strongly influenced
by the presence of water during methane oxidation at higher
temperatures. Although the catalyst is thermally stable under
both run-up experiments and steady-state experiments at both
500 and 6008C under dry conditions, the addition of water pro-
gressively deactivates the system. For temperatures below
5008C, deactivation is easily reversed by removing the water and
is, therefore, associated with competitive adsorption on the Pd
surface. At higher temperatures and under reaction conditions,
a second deactivation process is observed that is attributable to
the formation of hydroxides, which are decomposed only by
high-temperature treatment. A similar negative effect due to the
formation of hydroxyls is observed also on reference convention-
al Pd-CeO2 system, as well documented in the literature. Howev-
er, this contribution is less important at the higher temperatures
investigated here. In addition, the formation of hydroxyls in the
Pd@CeO2/Si-Al2O3 has a more important impact on oxygen mo-
bility from CeO2 to the Pd, owing to the higher metal-support in-
terphase. The results show that design of catalysts for methane
oxidation must maximize metal-support interactions, along with
oxygen transfer from a reducible promoter. In fact, some of us
have just demonstrated that a hierarchical Pd@ZrO2/Si-Al2O3 cat-
alyst can both guarantee high activity thanks to the nanoarchi-
tectonic design and also good hydrothermal stability.[43]
Catalytic measurements
Light-off reaction measurements were performed in a U-shaped,
quartz reactor with 25 mg of catalyst that had been sieved for
grain sizes below 150 mm. Reactions were carried out at a total gas
pressure of 1 atm, with the inlet composition controlled by varying
the flow rates of CH4, O2 and Ar, while maintaining the total flow
rate at 83.3 mLminꢀ1, a condition that corresponds to a Gas Hourly
Space Velocity (GHSV) of ꢁ200000 mLgꢀ1 hꢀ1. The heating and
cooling rates were maintained at 108Cminꢀ1 for most measure-
ments. For experiments that included water, the pre-mixed gas-
eous reactant mixture was bubbled through a saturator heated to
the temperature required for the desired water feed concentration.
The composition of the effluent gases was monitored on-line using
a mass spectrometer. Prior to each catalytic test, the catalyst was
treated in 5.0% O2/Ar at 40 mLminꢀ1 for 30 min at 4508C, heating
and cooling at a 108Cminꢀ1 rate.
Fourier transform infrared spectroscopy (FTIR)
Spectra were recorded by using a Mattson Galaxy 2020 FTIR spec-
trometer with a diffuse-reflectance attachment (Collector II) pur-
chased from Spectra-Tech Inc., using a resolution of 16 cmꢀ1. Be-
cause the sample could not be heated to sufficiently high tempera-
tures in the sample stage of the diffuse reflectance unit, the
Pd@CeO2/Si-Al2O3 catalysts were first treated under different condi-
tions in a separate flow reactor, cooled to room temperature in He,
and then transferred to a sample holder for the spectroscopic
measurements. In the diffused reflectance cell, the catalyst was de-
gassed in dry flowing He at 2008C before data collection. All spec-
tra were recorded at room temperature in dry He.
Experimental Section
Catalyst synthesis
Potassium tetrachloropalladate(II) (99.95%) and palladium nitrate
(99.95%) were purchased from ChemPUR. Cerium ammonium ni-
trate (CAN, 99.99%), sodium methoxide (25% in methanol), phos-
phoric acid (85%), 11-mercaptoundecanoic acid (MUA, 95%), do-
decanoic acid (99%), triethoxy(octyl)silane (TEOOS, 97.5%), and all
the solvents (analytical grade) was purchased from Sigma–Aldrich.
Sodium borohydride (98%+) was purchased from Acros Organics.
Al2O3 support was kindly provided by SASOL. Prior to use, Al2O3
was stabilized by calcination at 9508C for 24 h. After this treat-
ment, the material is composed by g-Al2O3 and possesses a surface
area of 100 m2 gꢀ1 by BET (Brunauer–Emmett–Teller) measure-
ments, with a suitable pore size for core-shell deposition.[5]
Operando XAFS measurements
X-Ray Absorption Fine Structure experiments were performed at
the SAMBA beamline[46] of Synchrotron SOLEIL (France) with a Si
220 double crystal monochromator. The monochromator was kept
fully tuned and harmonics were rejected by a pair of Pd-coated, Si
mirrors. Spectra were measured in transmission mode using ioniza-
tion chambers as detectors. One chamber was used as the baseline
monitor and two other chambers were used to continuously check
the stability of the energy scale by placing one after the sample
and the other after a reference foil. Because we used Pd mirrors to
reject harmonics, the baseline was checked to ensure the absence
of a residual Pd signal, owing to nonlinearity in the detectors. The
experiments were conducted using the transmission and fluores-
cence cell fully described in [47]. 8 mg of catalyst, diluted 1:10 by
weight with BN, and a total gas flow rate of 12 mLminꢀ1 (0.5%
CH4, 2.0% O2, 15.0% H2O if needed, and N2 balance) were used for
Ce LIII edge experiments. 95 mg of catalyst and a total gas flow
rate of 130 mLminꢀ1 (0.5% CH4, 2.0% O2, 15.0% H2O if needed,
and N2 balance) were used for Pd K edge experiments. Products
analysis systems was a Cirrus-MKS mass spectrometer, and activity
comparable to that observed during catalytic test were observed.
The supramolecular Pd@CeO2 core-shell structures were prepared
according to published procedures.[44] Briefly, the method is based
on the self-assembly between CeIV tetradecyl alkoxide and func-
tionalized Pd nanoparticles protected by 11-mercaptoundecanoic
acid (MUA). A controlled hydrolysis in the presence of dodecanoic
acid leads to the formation of the Pd@CeO2 structures, in which
the CeO2 shell is composed of small crystallites (ꢁ3 nm) organized
around the preformed metal particles. The core-shell units were
deposited from solution onto modified, hydrophobic g-Al2O3,
which was prepared by treating the calcined g-Al2O3 with TEOOS,
as discussed elsewhere.[5] Before use, the Pd(1.0%)@CeO2(9.0%)/Si-
Al2O3 catalysts were first calcined to 8508C in air for 5 h to remove
the organic components and to activate the catalyst.[45]
The fractions of CeIII and Pd0 in the samples were determined by
fitting the X-ray absorption near edge spectroscopy (XANES) part
of the spectrum using a linear combination of spectra for CeAlO3
and CeO2 to fit data for Ce and a combination of spectra for PdO
and a Pd foil to fit data for Pd. All measurements were performed
Synthesis of conventional impregnated Pd(1.0%)-CeO2(9.0%)/Si-
Al2O3 was achieved by consecutive impregnation of the same Si-
Al2O3 used for the preparation of the above described hierarchical
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 0000, 00, 1 – 10
&
8
&
ÞÞ
These are not the final page numbers!

CHEMCATCHEM
FULL PAPERS
www.chemcatchem.org
during the same beam session. EXAFS data were fit with Feff8.4[48]
and Horae[49] software packages. Fits were performed in r-space
and a Hanning window was applied in k-space from 3 to 13 Aꢀ1
(the window was zero outside these values).
[15] T. X. T. Sayle, M. Cantoni, U. M. Bhatta, S. C. Parker, S. R. Hall, G. Mçbus,
M. Molinari, D. Reid, S. Seal, D. C. Sayle, Chem. Mater. 2012, 24, 1811–
1821.
[16] E. Rocchini, M. Vicario, J. Llorca, C. de Leitenburg, G. Dolcetti, A. Trovar-
elli, J. Catal. 2002, 211, 407–421.
[17] A. Trovarelli, Catal. Rev. 1996, 38, 439–520.
[18] G. Zhou, P. R. Shah, T. Montini, P. Fornasiero, R. J. Gorte, Surf. Sci. 2007,
601, 2512–2519.
Chemisorption measurements
[19] M. Cargnello, V. V. T. Doan-Nguyen, T. R. Gordon, R. E. Diaz, E. A. Stach,
R. J. Gorte, P. Fornasiero, C. B. Murray, Science 2013, 341, 771–773.
[20] P. Papaefthimiou, T. Ioannides, X. E. Verykios, Appl. Therm. Eng. 1998, 18,
1005–1012.
Metal accessibility was estimated by means of low temperature CO
chemisorption measurements by using
a Micromeritics ASAP
2020C automatic analyzer. Typically, 150 mg of sample were pre-re-
duced at 808C under 5.0% H2/Ar for 30 min, degassed at the same
temperature for 30 min and at 3508C for 4 h. The samples were al-
lowed to cool to room temperature and then cooled further to
about ꢀ808C using an acetone ice cold-trap. A range of CO pres-
sure of 2–20 Torr was used. The apparent equilibrium was consid-
ered to be reached when the pressure change was less than
0.01% for 11 consecutive readings taken at 30 s intervals. Ad-
sorbed volumes were determined by extrapolation of the linear
part of the adsorption isotherm to zero pressure. A chemisorption
stoichiometry ratio, CO/Pd, of 1:1 was assumed.
[21] R. J. Farrauto, Science 2012, 337, 659–660.
[22] R. Burch, F. J. Urbano, P. K. Loader, Appl. Catal. A 1995, 123, 173–184.
[23] K. Persson, L. D. Pfefferle, W. Schwartz, A. Ersson, S. G. Jꢅrꢆs, Appl. Catal.
B 2007, 74, 242–250.
[24] P. Hurtado, S. Ordonez, H. Sastre, F. V. Diez, Appl. Catal. B 2004, 47, 85–
93.
[25] R. Kikuchi, S. Maeda, K. Sasaki, S. Wennerstrçm, Y. Ozawa, K. Eguchi,
Appl. Catal. A 2003, 239, 169–179.
[26] D. L. Mowery, R. L. McCormick, Appl. Catal. B 2001, 34, 287–297.
[27] D. Ciuparu, L. Pfefferle, Appl. Catal. A 2001, 209, 415–428.
[28] W. R. Schwartz, D. Ciuparu, L. D. Pfefferle, J. Phys. Chem. C 2012, 116,
8587–8593.
[29] L. De Rogatis, M. Cargnello, V. Gombac, B. Lorenzut, T. Montini, P. Forna-
siero, ChemSusChem 2010, 3, 24–42.
Acknowledgements
[30] A. K. Datye, J. Bravo, T. R. Nelson, P. Atanasova, M. Lyubovsky, L. Pfef-
ferle, Appl. Catal. A 2000, 198, 179–196.
[31] F. Zhang, P. Wang, J. Koberstein, S. Khalid, S.-W. S. Chan, Surf. Sci. 2004,
563, 74–82.
[32] J. Keating, G. Sankar, T. I. Hyde, S. Kohara, K. Ohara, Phys. Chem. Chem.
Phys. 2013, 15, 8555–8565.
[33] A. Badri, C. Binet, J. Lavalley, J. Chem. Soc. Faraday Trans. 1996, 92,
4669–4673.
[34] A. Setiawan, J. Friggieri, E. M. Kennedy, B. Z. Dlugogorski, M. Stocken-
huber, Catal. Sci. Technol. 2014, 4, 1793–1802.
[35] F. Esch, S. Fabris, L. Zhou, T. Montini, C. Africh, P. Fornasiero, G. Comelli,
R. Rosei, Science 2005, 309, 752–755.
[36] M. Watkins, A. Foster, A. Shluger, J. Phys. Chem. C 2007, 111, 15337–
15341.
[37] M. Fronzi, S. Piccinin, B. Delley, E. Traversa, C. Stampfl, Phys. Chem.
Chem. Phys. 2009, 11, 9188–9199.
Dr. Matteo Cargnello (UPenn, Philadelphia) is acknowledged for
discussions. M.M., T.M. and P.F. acknowledge financial support
from University of Trieste trough the grant FRA2013 “Advanced
core-shell based catalysts for methane catalytic combustion”, the
EU for the grant FP7-NMP-2012-SMALL-6 (project ID 310651) and
MIUR (Rome) for the grant HI-PHUTURE (protocol 2010N3T9M4).
C.C., and R.J.G. were supported by the Department of Energy,
Office of Basic Energy Sciences, Chemical Sciences, Geosciences
and Biosciences Division, Grant No. DE-FG02-13ER16380. We ac-
knowledge SOLEIL for provision of synchrotron radiation facilities
and we would like to thank V. Briois and G. Alizon for assistance
in using beamline SAMBA.
[38] G. N. Vayssilov, Y. Lykhach, A. Migani, T. Staudt, G. P. Petrova, N. Tsud, T.
Skꢄla, A. Bruix, F. Illas, K. C. Prince, V. Matolꢂn, K. M. Neyman, J. Libuda,
Nat. Mater. 2011, 10, 310–315.
Keywords: ceria · methane · oxidation · palladium · water
[39] J. El Fallah, S. Boujana, H. Dexpert, A. Kiennemann, J. Majerus, O. Touret,
F. Villain, F. Le Normand, J. Phys. Chem. 1994, 98, 5522–5533.
[40] S. Bernal, J. J. Calvino, G. A. Cifredo, J. M. Rodriguez-Izquierdo, J. Phys.
Chem. 1995, 99, 11794–11796.
chemistry
[1] P. Hurtado, S. OrdꢁÇez, A. Vega, F. V. Dꢂez, Chemosphere 2004, 55, 681–
689.
[2] S. Colussi, C. de Leitenburg, G. Dolcetti, A. Trovarelli, J. Alloys Compd.
2004, 374, 387–392.
[3] G. Corro, C. Cano, J. L. G. Fierro, J. Mol. Catal. A 2010, 315, 35–42.
[4] A. M. Venezia, G. Di Carlo, G. Pantaleo, L. F. Liotta, G. Melaet, N. Kruse,
Appl. Catal. B 2009, 88, 430–437.
[5] M. Cargnello, J. J. Delgado Jaꢃn, J. C. Hernꢄndez Garrido, K. Bakhmutsky,
T. Montini, J. J. Calvino Gꢄmez, R. J. Gorte, P. Fornasiero, Science 2012,
337, 713–717.
[6] P. Gꢃlin, M. Primet, Appl. Catal. B 2002, 39, 1–37.
[7] S. Yang, A. Maroto-Valiente, M. Benito-Gonzalez, I. Rodriguez-Ramos, A.
Guerrero-Ruiz, Appl. Catal. B 2000, 28, 223–233.
[8] D. Ciuparu, M. R. Lyubovsky, E. Altman, L. D. Pfefferle, A. Datye, Catal.
Rev. 2002, 44, 593–649.
[9] W. R. Schwartz, L. D. Pfefferle, J. Phys. Chem. C 2012, 116, 8571–8578.
[10] Y.-H. C. Chin, C. Buda, M. Neurock, E. Iglesia, J. Am. Chem. Soc. 2013,
135, 15425–15442.
[11] S. Colussi, A. Trovarelli, G. Groppi, J. Llorca, Catal. Commun. 2007, 8,
1263–1266.
ˇ
[41] P. Fornasiero, T. Montini, M. Graziani, J. Kaspar, A. B. Hungrꢂa, A. Martꢂ-
nez-Arias, J. C. Conesa, Phys. Chem. Chem. Phys. 2002, 4, 149–159.
ˇ
[42] P. Fornasiero, J. Kaspar, M. Graziani, Appl. Catal. B 1999, 22, L11–L14.
[43] C. Chen, Y.-H. Yeh, M. Cargnello, C. B. Murray, P. Fornasiero, R. J. Gorte,
ACS Catal. 2014, 4, 3902–3909.
[44] M. Cargnello, N. L. Wieder, T. Montini, R. J. Gorte, P. Fornasiero J. Am.
Chem. Soc. 2010, 132, 1402–1409.
[45] C. Chen, J. Cao, M. Cargnello, P. Fornasiero, R. J. Gorte, J. Catal. 2013,
306, 109–115.
[46] V. Briois, E. Fonda, S. Belin, L. Barthe, C. La Fontaine, F. Langlois, M. Rib-
bens, F. Villain, in UVX 2010-10e Colloque sur les Sources Cohꢀrentes et In-
cohꢀrentes UV, VUV et X; Applications et Dꢀveloppements Rꢀcents, EDP
Sciences, Les Ulis, France, 2011, pp. 41–47.
[47] C. La Fontaine, L. Barthe, A. Rochet, V. Briois, Catal. Today 2013, 205,
148–158.
[48] A. Ankudinov, B. Ravel, J. Rehr, S. Conradson, Phys. Rev. B 1998, 58,
7565–7576.
[49] B. Ravel, M. Newville, J. Synchrotron Radiat. 2005, 12, 537–541.
[12] A. D. Mayernick, M. J. Janik, J. Catal. 2011, 278, 16–25.
[13] C. Doornkamp, V. Ponec, J. Mol. Catal. A 2000, 162, 19–32.
[14] P. Mars, D. W. van Krevelen, Chem. Eng. Sci. 1954, 3, 41–59.
Received: September 8, 2014
Published online on && &&, 0000
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 0000, 00, 1 – 10
&
9
&
ÞÞ
These are not the final page numbers!

FULL PAPERS
M. Monai, T. Montini, C. Chen, E. Fonda,
R. J. Gorte,* P. Fornasiero*
&& – &&
Methane Catalytic Combustion over
Hierarchical Pd@CeO2/Si-Al2O3: Effect
of the Presence of Water
A bridge over troubled water: Excess
water has a detrimental effect on the
performance of hierarchical Pd@CeO2/
Si-Al2O3 under methane combustion.
The origin of this effect has been stud-
ied by a combination of reactivity ex-
periments, IR spectroscopy and operan-
do EXAFS spectroscopy, observing that
the formation of OH groups on the sur-
face of partially reduced CeO2 hinders O
spillover and favors progressive forma-
tion of reduced Pd.
ꢀ 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
ChemCatChem 0000, 00, 1 – 10
&10
&
ÞÞ
These are not the final page numbers!