Catalytic combustion of methane over Pd/Ce-Zr oxides washcoated monolithic catalysts under oxygen lean conditions

Article abstract of DOI:10.1039/c5ra13223a

Pd-based monolithic catalysts were prepared using cordierites as supports and used as combustion catalysts for methane. The catalytic activity was further promoted by Ce-Zr oxides coatings. Oxides with different Ce/Zr ratios were synthesized via urea co-p

Full text of DOI:10.1039/c5ra13223a

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Catalytic combustion of methane over Pd/Ce–Zr
oxides washcoated monolithic catalysts under
oxygen lean conditions
Cite this: RSC Adv., 2015, 5, 102147
a
a
b
b
a
*
Jianhui Jin, Chuang Li, Chi-Wing Tsang, Bin Xu and Changhai Liang
Pd-based monolithic catalysts were prepared using cordierites as supports and used as combustion
catalysts for methane. The catalytic activity was further promoted by Ce–Zr oxides coatings. Oxides with
different Ce/Zr ratios were synthesized via urea co-precipitation method and fully characterized.
Catalytic performance of the monoliths was evaluated by oxygen-lean methane combustion through
light-off experiments. Steep conversion curves were observed with oxygen completely depleted below
350 ꢀC over all catalysts. Pd supported on ZrO2 ignited the reaction at the lowest temperature while
similar activities for total oxygen conversion were observed for Zr-rich monoliths. Zr-embedded oxides
promoted the activity on oxygen consumption due to the interaction between Zr and Pd at the interface,
dispersing and stabilizing Pd particles on the catalyst surface. The addition of Ce further promoted the
stability of the catalysts. Compared with Pd/Zr, Pd/Ce1Zr2 showed little deactivation after three
successive light-offs and maintained stability after 500 h time-on-stream test. Core/shell Pd particles
model was proposed for the activity. Small Pd metal particles on the surface serve as an activation site
for methane whereas wrapped PdO in the core stabilized by the Ce–Zr oxides acts as oxygen donor.
The extent of reduction of Pd has significant effect on activity in the reaction conditions. Other
impacting factors such as different oxygen concentrations and space velocities were also assessed to
investigate the extent of the influence on methane catalytic combustion.
Received 7th July 2015
Accepted 20th November 2015
DOI: 10.1039/c5ra13223a
www.rsc.org/advances
thermal combustion for total oxidation of methane because heat
and/or energy at lower temperature can be produced without
NOx generation. Many catalysts, such as supported noble metals7
Introduction
Monolithic catalysts are attractive replacements for conven-
tional carriers in heterogeneous gas–solid phase catalytic reac-
tions.1,2 Various gas-phase reactions in industrial applications
such as exhaust gases elimination and VOCs catalytic combus-
tion facilitate the development such as the use of the cordierite
monolithic structure. High mechanical strength, low thermal
expansion coefficient, and low-pressure drop are all character-
istics of monolith catalysts.3 These properties are particularly
suitable for highly exothermic process such as methane
combustion reaction (DH ¼ ꢁ802 kJ molꢁ1). At large mass ow
rate of air and fuel, monolithic honeycomb structure is a better
option for seeking thermal stable catalysts to withstand
decomposition.4 Washcoats of pre-coated oxides are necessary
for cordierite monoliths, which serves as supports for conven-
tional catalysts.
and composite oxides composed of transition metal oxides (Cr,
Mn, Fe, Co, Ni, Cu, etc.),8 perovskites9 and hexaaluminates10 have
been employed for methane combustion. Pd-based catalysts are
known to be the most efficient catalysts for methane combustion
as it is a relatively non-expensive precious metal and could
perform better under certain operating conditions.6 However,
considering their scarcities, much effort has been made to
structurally modify the precious metal containing catalysts,
aiming at reducing their amount of usages while improving both
catalytic and stability performances.11–13 Catalyst supports with
different properties also have signicant impacts on the
activity.14 Owing to the excellent ability to store and release
oxygen molecules (OSC, Oxygen Storage Capacity) and improve
the noble metals dispersion,15 ceria are widely used as promoter
for combustion reactions of various types. Ceria modied with
Zr and many other oxides have been explored to enhance the
redox properties and stabilities of supports, aiming to achieve
better catalytic abilities.16 Among them, Ce1ꢁxZrxO2 was
representative and widely recognized as one of the excellent
catalyst supports.17
Methane emission from coal mine is known as one of the
major source of greenhouse gas.5,6 Catalytic combustion of
methane has been considered as an alternative to conventional
aLaboratory of Advanced Materials and Catalytic Engineering, Dalian University of
Technology, Dalian 116024, China. E-mail: changhai@dlut.edu.cn; Fax: +86 411
84986353; Tel: +86 411 84986353
The redox nature of the reacting gas and pretreatments of
the catalysts both affect the crystallite size, the dispersion and
bECO Environmental Energy Research Institute Limited, Hong Kong, China
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the oxidation state of Pd.18 Conventional catalytic combustion Honeycomb cordierite (D ¼ 0.9 cm, L ¼ 1.7 cm) was pretreated
of methane takes place under oxygen-rich environment,5 the with 10 wt% HNO3 to adjust surface structure attachment, then
presence of oxidized palladium species in the catalysts is it was dipped into the oxides suspension and the coated
considered to be the active component during catalytic oxida- cordierite was then blow-dried. To achieve an average loading of
tion.19,20 The research on oxygen-lean methane combustion 20%, the coating procedure was repeated twice, followed by
could have important implication such as the application in calcining the coated monolith in muffle at 700 ꢀC for 2 h. Using
coal mine methane de-oxygenation processes. However, only conventional impregnation method through dipping the
a few studies were reported under oxygen-lean condition in monolith into the aqueous solution of palladium nitrate, fol-
which methane concentration is much higher than oxygen lowed by drying and then calcining in air at 500 ꢀC for 2 h,
concentration stoichiometrically required for complete a brown-colored catalyst complex was obtained with targeted Pd
methane combustion to water and carbon dioxide. Since many metal loading on the monolith of about 0.3 wt%.
side reactions such as partial oxidation and reforming may
occur, the nature of the active oxygen species, palladium
oxidation state and the combustion mechanism may differ
signicantly from that of the oxygen-rich conditions. Catalysts
developed currently may not be suitable under such a reducing
atmosphere. Traditional supported noble metal catalyst, espe-
cially the Pd/Al2O3, is active catalyst for low temperature
methane combustion under high oxygen/fuel ratio environ-
ment. However, for a controlled process at low oxygen/fuel ratio,
Pd/Al2O3 based catalyst fails to maintain the PdO/Pd ratio due to
a phenomenon called the periodic PdO/Pd ratio change (oscil-
latory behavior) occurs.21 A better catalyst is therefore in need to
slow down catalyst decomposition, inhibit carbon deposition
and metal sintering under this condition. Providing that Pd
supported on Ce–Zr based materials exhibit excellent activities
on methane combustion, especially in the eld of automobile
exhaust elimination serving as three-way converter, similar
catalysts could be explored to investigate the activity in oxygen-
lean condition.
Characterization
The X-ray diffraction (XRD) measurements of oxides were
carried out on a Rigaku D/Max-RB diffractometer with a Cu Ka
radiation a power of 40 kV and 100 mA. Diffraction data was
obtained in the 2q range between 5ꢀ and 90ꢀ with a step of 0.02ꢀ
and scan rate of 10ꢀ minꢁ1
.
Raman spectra of the oxides were recorded using a 325 nm
He/Ne laser source on a homemade device at scanning range of
200–1000 cmꢁ1
.
The surface area, pore volume, and pore size distribution of
the mixed oxides were calculated from nitrogen adsorption–
desorption isotherms at 77 K on Quantachrome Autosorb-iQ.
Prior to the measurements, all samples were degassed at
250 ꢀC for 8 h. The surface areas were calculated from BET plots.
H2-TPR experiments of oxides and Pd impregnated oxides
were performed using Quantachrome ChemBET PULSAR. The
powders were rst purged under He (100 mL minꢁ1) at 300 C
for 30 min and then cooled to room temperature. TPR
ꢀ
The purpose of this work is to obtain a monolith catalyst
formulation that are stable and exhibit high activity for the trace
oxygen removal in methane combustion reaction. First, a series
of Ce–Zr oxides as coatings was synthesized by co-precipitation
method. Pd-based monolithic catalysts with cordierite as
support can then be prepared by impregnation on the surface of
Ce–Zr oxides. The inuence of Ce/Zr ratio, oxygen concentra-
tion, gaseous hourly space velocity (GHSV) on the reaction were
investigated, together with stability tests of light-offs and time-
on-stream experiments to provide a thorough investigation into
the active sites and the promoting effects of Ce–Zr oxides of this
new type of catalyst.
measuremꢁe1nt was carried out in a ow of 10% H2/Ar at a rate of
ꢀ
ꢀ
10 C min up to 800 C.
X-ray photoelectron spectroscopy (XPS, Escalab250, Thermo
Corp.) was carried out to investigate the surface compositions of
the oxides. The XPS measurements were performed using an
X-ray source of Mg Ka (1253.6 eV) with a power of 150 W. Ce 3d,
Zr 3d, and O 1s lines were monitored. All core-level spectra were
corrected by referring the binding energy of the C 1s neutral
carbon peak at 284.6 eV.
TEM images of the catalysts were recorded by FEI Tecnai G2
F30 instrument operated at 200 kV. The catalysts were dispersed
in ethanol and sonicated. A single drop of sample was placed
onto a 200-mesh copper grid coated with an amorphous holey
carbon lm.
Experimental
Materials preparation
Catalyst evaluation
Ce–Zr mixed oxides were synthesized by co-precipitation
method. Ce(NO3)3$6H2O and Zr(NO3)4$5H2O with specic All the experiments were performed in a reactor tube sur-
ratio were dissolved in water sequentially. Urea was then added rounded by a furnace. The monolith catalyst was centered in the
to the mixture (urea : metal ¼ 10 : 1) and the reaction mixture reactor tube with diameter slightly less than the channel's
ꢀ
was slowly heated up to 96 C under stirring and subsequently diameter, and two thermocouples was positioned at top and
the reaction was maintained for 24 h at this temperature. The bottom end of the monolith. A K-type thermocouple was
precipitates were then ltered, washed with cold deionized centered within the reactor tube to detect and control the
water, dried at 120 ꢀC for 12 h and nally calcined at 550 ꢀC for reaction temperature. Temperatures at the bottom and top of
4 h. The nal oxides were denoted as CexZry, where x and y the catalyst bed were also recorded.
correspond to the molar ratios. The oxides were ball-milled in
The catalyst was pretreated by reduction of PdO to Pd under
10 wt% HNO3 solution for 4 h to obtain homogeneous slurry. hydrogen atmosphere, during which any impurities should be
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removed. 10% H2 in Ar with a total ow of 100 mL minꢁ1 was of solid solution and eventually distorted the crystalline
passed through the Pd/CeZr/ceramic monolith bed for 2 h at structure.
400 C with temperature ramp rate of 4 C minꢁ1. The reactor
The BET surface areas for CeO2, Ce2Zr1, Ce1Zr1, Ce1Zr2 and
ZrO2 were 86, 100, 59, 61 and 90 m2 gꢁ1, respectively. The
ꢀ
ꢀ
ꢀ
was then cooled down to 250 C under argon.
In the light-off experiments, a feed gas with 50% of CH4, incorporation of ZrO2 increased the BET surface area at rst,
balanced with 3% of O2 and Ar, was introduced to the catalyst but was dropped signicantly and exhibited poorer crystallinity
bed through mass ow controller with overall ow at 500 mL with further addition. The surface area increased slightly when
minꢁ1 and a GHSV of 30 000 hꢁ1. The reactor temperature was the ratio of Ce/Zr decreased below 1, but were lower than the
ꢀ
ꢀ
raised to the target value starting from 250 C at rate of 1 C respective pure CeO2 and ZrO2, implying the synthesized
minꢁ1. Oxygen concentration was analyzed using an online samples were not simply mechanical mixes of two individual
oxygen detector. Online gas chromatograph was used to identify pure oxides. By using co-precipitation method using Ce(NO3)3
gas components of the product gas at intervals of 10 min. To and Zr(NO3)4 as precursors and urea as precipitating agent, the
evaluate the activity for oxygen consumption, oxygen conversion presence of heterogeneous mixed oxides in the form of CeO2 or
was adopted, which referred to the ratio of the decreased ZrO2-embeded solid solution were thus conrmed. Surface area
concentration of oxygen to oxygen inlet, since minor change in of the catalysts was also calculated, and the results were shown
volume took place during reaction.
in Table 1. Aer coating and thermal treatment, surface area
For steady-state experiment, the reactor temperature was dropped drastically, especially for ceria. Zirconia helped pre-
slowly decreased to 370 ꢀC under argon aer reduction. The venting the incineration of oxides. Compared with reaction
feed gases were introduced for the combustion reaction with results, the surface area was not a dominant factor for catalytic
excess methane for the targeted time period. The oxygen levels activity.
were recorded by the oxygen detector at intervals of 300 min.
Raman spectra were illustrated in Fig. 2, and all samples
showed two characteristic peaks at 440 cmꢁ1 and 650 cmꢁ1
.
Peak at 440 cmꢁ1 corresponds to the F2g Raman active mode of
the uorite-like structure. The peak at 640–660 cmꢁ1 indicates
the intensity of oxygen defects due to the incorporation of Zr
and Eg active mode of tetragonal phase of zirconia, which
belongs to tetragonal displacement of some O atoms from their
ideal position in the uorite structure. With the increasing
doping of zirconia, the peak shis to higher wavenumber,
which may be due to the crystal phase transfer of ZrO2.23 And
the intensity of peaks, so as to the oxygen vacancy increased,
and then decreased at Ce1Zr2 due to the drop of Ce amount in
oxides. Combined with catalytic results, the defects of oxides
caused by doping of zirconia benetted catalytic activity in
comparison with bulk CeO2.
Results and discussion
Catalyst characterization
XRD analysis allows identication of the structures of particles.
As shown in Fig. 1, diffraction peaks correspond to ZrO2 and
uorite cubic phase of CeO2 (Fm3m, JCPDS 34-0394) without
signicant shis. The intensities of CeO2 diffraction peaks
decreased gradually with decreasing Ce/Zr ratio, implying
gradual change from large to small size of crystalline and poor
crystallinity. On the other hand, diffraction peaks of ZrO2 shi
to higher angle, indicating a change of crystalline phases of
ZrO2 (from Fm3m 49-1642 to P42/nmc 50-1089), during which
metastable tetragonal (t00) emerges in the mixed oxides state.22
No monoclinic ZrO2 phase was observed. These results indi-
cated that Zr ions partially entered the ceria lattice in the form
As shown in Fig. 3, H2 consumption is attributed to the
reduction of surface and bulk CeO2, since Zr4+ is an irreducible
cation in this temperature range. Pure CeO2 exhibits two
reduction peaks: the rst peak centered at around 500 ꢀC
corresponds to the reduction of the uppermost layers of Ce4+
;
the second peak centered on 800 ꢀC originates from the
reduction of the bulk CeO2.24 The broad reduction peak of CeO2
and Ce1Zr2 in the temperature range 450–700 ꢀC corresponds to
the reduction of surface Ce4+, while the reduction temperature
for bulk CeO2 is above 800 ꢀC. With further addition of Zr, two
dominant reduction peaks instead of one for surface reduction
Table 1 Surface area, Pd loading of catalysts and catalytic activity of
monoliths promoted by various CeZr coatings for the oxygen-lean
methane combustion
Coating BET (m2 gꢁ1
)
Pd loading T10 (ꢀC) T50 (ꢀC) T90 (ꢀC)
CeO2
3
8
7
5
13
0.28
0.30
0.27
0.29
0.28
317.3
318.9
311.2
311.6
303.0
329.0
330.4
323.0
323.3
322.2
330.2
331.4
324.3
324.2
324.7
Ce2Zr1
Ce1Zr1
Ce1Zr2
ZrO2
Fig. 1 XRD patterns of Ce–Zr oxides with different Ce/Zr ratios.
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Three H2 consumption peaks at about 100, 150, 230 ꢀC were
found for ceria promoted catalysts, while a gentle peak at 150 ꢀC
plus a negative peak for hydrogen release were found for Zr
doped catalysts. For ceria promoted catalyst, the peak at 100 ꢀC
could be assigned to the reduction of surface PdO species, while
ꢀ
peaks at 150 and 230 C could be assigned to the reduction of
Pdn+ species in Ce1ꢁxPdxO2ꢁd solid solution structure.26 For Zr
ꢀ
doped catalysts, negative peaks at 100 C was attributed to the
decomposition product of Pd hydride formed during H2-TPR,27
indicating the existence of metallic Pd in catalysts. Unlike
Pd/CeO2, peaks at 150 ꢀC were much weaker for catalysts doped
with Zr, indicating that Zr weakened the interaction between Pd
and ceria, and PdO was much easier to be reduced. With
increasing doping of Zr, peaks for hydrogen release shied to
lower temperature region and the intensity increased, and
peaks for hydrogen consumption decreased, indicating an
increased in Pd/PdO ratio of the catalysts. The doping of Zr in
ceria lattice weakened the bond of Ce–O–Pd, making Pd expose
to the outer surface of the ceria lattice and keep a moderate
interaction between Pd and the ceria.
Fig. 2 Raman spectra of Ce2Zr1, Ce1Zr1 and Ce1Zr2.
The elemental composition and chemical valence on the
surface of the as-prepared Ce–Zr oxides were detected by XPS to
investigate the relationship of activity and coating oxides. As
exhibited from Fig. 5, the surface ratio of Ce to Zr are 2.7 and
0.33, which are different from the theoretical value for Ce2Zr1
and Ce1Zr2, revealing that Ce or Zr-rich phases appeared on the
surface of each sample, forming a shell/core structure.28
The seemingly complex spectrum of Ce 3d can be decom-
posed into 10 peaks for analyses, the bands v0 and u0 are the
satellites arising from the Ce 3d5/2 and Ce 3d3/2 ionizations of
Ce3+, indicating the generation of oxygen vacancies.29 The
chemical valence of cerium on the surface contains Ce3+ and
Ce4+, with the ratio of Ce3+/Ce as 0.29 and 0.35 for Ce2Zr1 and
Ce1Zr2, respectively. The surface concentration of Ce3+ to Ce on
the as-prepared oxides was enhanced by increasing the addition
of zirconia and thus more oxygen vacancies exist in CeO2 lattice.
O 1s spectra tted with two peaks, one band located at
529.3–529.6 eV and a peak at the higher BE of 531.1–531.4 eV.
were found. The reduction peaks shied to lower temperature
thus the oxides are much easier to be reduced. For Ce1Zr2, peak
belongs to bulk CeO2 reduction shied signicantly to a lower
temperature which is close to the reduction peak of surface
Ce4+, forming two shoulder peaks below 700 ꢀC. A possible
“shell/core structure” with heterogeneous surface elemental
distribution like embedding can be described on the phenom-
enon of bimodal H2-TPR prole.25 Combining the information
from XRD, BET, Raman and TPR, it can be concluded that the
Ce–Zr oxides are a mixture of solid solution and pure Ce or Zr
oxides.
Due to the low loading of oxides on cordierites, Pd impreg-
nated oxides was synthesized to evaluate the interaction
between Pd and oxides. As shown in Fig. 4, the at peaks at
4+
ꢀ
around 500 C were assigned to the reduction of surface Ce
ions, which showed only small difference for the 5 catalysts.
Fig. 3 H2-TPR profiles of the oxides prepared with different Ce/Zr
ratios.
Fig. 4 H2-TPR profiles of PdO/oxides with different Ce/Zr ratios.
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Fig. 6 TEM images of Pd/Ce1Zr (a), (c) and Pd/Zr (b), (d).
Using GC to analyze the formed gases, at the early stage of
reaction before ignition, no signicant hydrogen produced, and
indicating methane was completely oxidized. When ignition
happened, H2 appeared, partial oxidation of methane took
place due to the lack of oxygen. When oxygen was completely
removed, the signal of H2 increased.
Results of light-off experiments were shown in Fig. 7 and
presented in Table 1, where T10, T50 and T90 of the catalysts
correspond to the temperature needed to reach 10, 50 and 90%
oxygen conversion, respectively. Aer catalysts reduction, reac-
tants were introduced at 250 ꢀC at which temperature the
catalysts would not have any activity for oxygen consumption.
With programmed elevation of temperature, the catalytic
activity increased gradually. When the oxygen conversion
increased to some level (at about 20%), a subtle increase in
activity accompanied with large amount of heat release was
observed for all catalysts, and oxygen was completely consumed
Fig. 5 XPS spectra of Ce 3d and O 1s for Ce2Zr1 and Ce1Zr2 oxides.
ꢀ
at below 350 C. Two aspects contribute to this phenomenon.
The former is characteristic of lattice oxygen associated with
cerium or zirconium and the latter belongs most likely to
defective oxide or hydroxyl-like groups. A shi slightly towards
higher BE for the lattice oxygen peak of Ce1Zr2 than Ce2Zr1 is
coherent with the conversion of part of Ce4+ into Ce3+ species.30
Since the Pd/Ce1Zr and Pd/Zr showed similar ignition
performance, TEM images of the two catalysts were shown in
Fig. 6. Pd was hard to nd in Pd/Ce1Zr2 due to the background
of ceria lattice, the low loading of Pd, nely dispersed or
interacted with ceria to form Ce–O–Pd.26 In Pd/Zr catalyst, Pd
particles were clearly dispersed on Zr with particle sizes among
3–8 nm. Compared with catalytic activities and stability tests
below, the main role of ceria is to stabilize Pd.
The rst is heat accumulation: during methane oxidation under
such ow rate, the monolith cannot radiate its surplus of heat to
Oxygen elimination activity
Oxygen conversion was used as the key indicator for assessing
catalytic activity using online oxygen detector with detect limit
of 0.01%. Since methane combustion took place in such scarce
oxygen environment, methane may undergo partial oxidation or
reforming. However, due to the enormous concentration
gradient between methane and products, it is quite difficult to
Fig. 7 Oxygen conversion versus temperature over catalysts with
quantify the outputs precisely through gas chromatography. different Ce/Zr ratios.
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surroundings efficiently, resulting in the rise of reaction between oxygen and methane. The stick coefficient of oxygen is
temperature at catalyst surface. The second corresponds to larger than methane at low temperature, and the surface of
chemical/physical change of the active sites. At the beginning of catalysts are covered by oxygen, inhibiting the adsorption of
the reaction, metallic state of Pd dominates. As temperature methane. When the reaction reached the temperature for
increases, palladium was re-oxidized progressively by oxygen in oxygen desorption, the dissociative adsorption of methane took
the reactants, leading to the formation of Pd/PdO and eventu- place. Once the cracking of methane happened, oxygen was
ally to PdO. The surface of PdO was then reduced again owing to consumed immediately by methane radicals, which resulted in
the reducibility of methane, plus the formation of reaction the steep oxygen conversion. The little difference between the
products such as H2O, CO2, etc. The sudden increase in activity catalysts lied in the ability of methane activation. Due to the low
took place right at the temperature reported where large PdO coverage of oxygen, the ignition temperature was much lower
particle was reduced under CH4/N2.31 There may be a correla- than traditional methane combustion reaction.
tion between activation energies and surface Pd valances: when
Catalysts with different Pd loadings referring to the whole
the surface was dominated by PdO at low temperature, the monolith were prepared using Ce1Zr2 as coating. As exhibited
activation energy was greater, leading to low conversion at low in Fig. 8, by increasing the loading of Pd from 0.17 wt% to
temperature; less energy barrier emerged with sharp increase in 0.48 wt%, temperature for total oxygen conversion decreased
oxygen conversion when metallic Pd was dominant. The nal gradually (from 332.7 ꢀC to 322 ꢀC). And similar activity was
state of palladium strongly depends on the properties of observed for Pd loadings with 0.40 and 0.48 wt%. The light-off
supports. As the addition of Zr increased, the activity of results suggested that there is an optimum Pd content for the
methane combustion decreased rst, followed by a further monolith catalysts in methane combustion.32 Furthermore,
increase at lower Ce/Zr ratio. Pd supported on pure ZrO2 there was no oxygen consumption when reaction temperature
exhibited the best ignition activity while activity for total was below 280 ꢀC, regardless of the amount of Pd loading.
conversion was similar when Ce/Zr ratio equals to or below 1. Oxygen consumption began at temperature close to the
Catalysts supported by Zr-rich oxides were more active than CH4-TPR data on Pd/Al2O3 reported by Baylet et al.31 Since
Ce-rich ones in methane combustion under oxygen-lean catalyst supported by irreducible ZrO2 had the lowest ignition
conditions.
temperature, the results implied that metallic Pd may be the
Comparing catalysts supported by different Ce–Zr oxides, Pd active site for the early stage of the reaction.
maintained at partial +2 states by oxygen contributed from ceria
To understand the role of oxygen in the reaction, different
lattice due to the high OSC of ceria, which can be further concentrations of oxygen were introduced over the catalyst
oxidized by O2 and H2O. On the contrary, Pd supported on supported by Ce1Zr2 while keeping methane concentration at
zirconia-rich oxides is easier to be reduced under such reducing 50% and GHSV at 30 000 hꢁ1. The effect of oxygen concentra-
atmosphere. With the decrease in Ce/Zr ratio, much facile tion was illustrated in Fig. 9. Volcano-like activity was observed
reducibility for zirconia-rich oxides is deduced from H2-TPR. by increasing oxygen concentration from 2% to 5% taking T100
However, oxygen storage capacity decreased with more ZrO2 in as reference. Oxygen elimination activity increased linearly as
the oxides, causing palladium more prone to be reduced to oxygen concentration increased from 2% to 4%. However, T100
metallic Pd. Therefore higher metallic Pd ratio in Pd particles is rose when oxygen concentration was further increased to 5%. By
more benecial for methane combustion in such reaction increasing oxygen partial pressure, surface Pd is more prone to
condition, especially at the ignition period. Unlike methane be oxidized, exposing a thinner layer of active Pd on the surface.
combustion in air, the redox ability of ceria in the catalyst was For conventional methane combustion, optimum ratio of
not prominent for oxygen elimination in excess methane. The
more useful property may be the xation of Pd by oxygen
vacancies which promotes stabilization. However, a relatively
low temperature for total oxygen conversion was found with
catalysts promoted by Ce–Zr mixed oxides. The properties of
supports such as OSC may cause signicant effect at the nal
stage when only scarce amount of oxygen le. Due to the strong
interaction between CeO2 and interfacial Pd, partial PdO was
sustained in the catalyst and further Pd agglomeration was
prevented. It implies that rapid oxygen exchange between the
surface and the lattice has signicant effect on total oxygen
conversion via the maintenance of a desirable PdO/Pd ratio.
Although there is a big difference among the TPR proles, so
as the properties of catalysts. However, the catalytic activity
difference was not apparent. All the 5 catalysts showed steep
oxygen conversion, which was usually found in homogeneous
reactions. However, it was not quite possible since the reaction
temperature was not high enough to support homogeneous
reactions. A possible explanation is the comparative adsorption Fig. 8 Effects of Pd loadings on light-off activity.
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the concentration of oxygen feed. Thus, active metallic Pd sites
were exposed to the gas phase. The PdO core was stabilized by
its interaction with supporting oxides.
One of the advantages of monoliths is low-pressure drop, so
the ow velocities of reactants can be insured to operate at
a wide variety. As shown in Fig. 10, space velocities of 10 000,
30 000 and 50 000 hꢁ1 were chosen to investigate the ow rates
applicability of the monolithic catalysts over the catalyst
promoted by Ce1Zr2. Higher O2 conversion activity was found at
relatively high GHSV though having shorter residence times of
reactant gases. Besides, a gentle oxygen conversion curve was
acquired at lower velocity of 10 000 hꢁ1. The heat generated
during reaction was indicated via outlet temperature. There was
no difference on outlet temperature among the three ow rates
when reaction temperature was below 310 ꢀC with oxygen
conversion below 20%. Outlet temperature increases signi-
cantly when sharp oxygen conversion occurred, and higher
temperature was found at higher velocity. Since methane
combustion is exothermic, higher GHSV would generate more
heat at the surface of the catalyst.35 The elevated surface
temperature accelerated the reaction rates, thus promoting the
consumption of oxygen.
Fig. 9 O2 concentration effects on the conversion of O2 over catalyst
supported by Ce1Zr2.
PdO/Pd was proposed for methane activation.33 A similar
phenomenon exists in oxygen lean conditions. According to
Mars–van Krevelen mechanism,5 methane is adsorbed onto the
active sites and reacts with lattice oxygen on the surface. The
oxygen vacancies are then replenished by gas phase oxygen.
With more oxygen replenished, competitive adsorption takes
place, and the number of vacancies and metallic Pd decreases,
leaving lesser sites for methane adsorptions. Besides, higher
oxygen conversion rate occurred when oxygen concentration
increases due to the generation of more heat during oxidations,
agglomeration of Pd was thus more apt to take place. In all
cases, oxygen was totally consumed below 350 ꢀC, indicating
that the catalyst can keep its activity in a wide range of oxygen
concentration under oxygen-lean conditions.
With reference to the model proposed by Assmann et al.34 the
processes and the core/shell structure of catalyst were illus-
trated in Scheme 1. The pre-reduced particles were totally
oxidized at the early stage of the reaction. Increasing the
temperature under net reducing conditions leaded to a gradual
reduction of the outer PdO layer to metallic Pd. The rate of
metallic Pd layer formation and its thickness were controlled by
Catalyst stability
Stability is one of the key criteria for catalyst assessment, since
the sustainability of structure and surface components are
essential for catalysts to have long time operation. Two kinds of
tests were conducted over the catalyst coated by Ce1Zr2 with
reactants of 50% CH4, 3% O2 and 47% Ar at GHSV of 30 000 hꢁ1
:
three cycles of light-off experiments (Fig. 11a) and time-on-
stream experiment at 370 ꢀC (Fig. 12). Aer 3 times of light-
off, temperature for total oxygen conversion shis to 5 ꢀC
higher. However, the methane activation temperatures
decreased since T50 of the second and the third were lower than
the rst one. The phenomenon reported by Demoulin et al. for
catalytic combustion of methane over Pd/Al2O3 catalyst stated
that under reaction conditions of time-on-stream, activity
Fig. 10 Effect of GHSV on O2 conversion over the catalyst promoted
Scheme 1 The core/shell model of Pd particles and the interaction by Ce1Zr2 (inset shows outlet temperature versus furnace
between Pd and coating oxides.
temperature).
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Similar to Pd/Ce1Zr2, oscillations appeared aer rst ignition.
However, oscillation became severe at the third light-off and
temperature for total oxygen conversion shied to higher
temperature, which indicates zirconia failed to maintain the
stability of catalyst.
Since severe, continuous and ruleless oscillations may
happen when oxygen conversion didn't reach 100%, stability test
of Pd/Ce1Zr2 aer second cycle is attempted to perform at 330
and 350 ꢀC for 12 h. The oxygen conversion maintained 100% for
Pd/Ce1Zr2 at 350 ꢀC. When the reaction temperature lowered to
330 ꢀC, gradual deactivation with gentle oscillation appeared,
and oxygen conversion dropped to 99% aer 12 h test.
To examine the long-time stability for industrial purpose,
a fresh catalyst was used. Time-on-stream stability test was
conducted under a feed gas with 50 vol% CH4,ꢁ31 vol% O2 and
Fig. 11 Stability of the catalyst promoted by (a) Ce1Zr2, (b) Zr through
successive light-off experiments.
ꢀ
balancing Ar at 370 C with GHSV at 30 000 h and kept for
500 h. Fluctuation of inlet and outlet temperatures was
observed during the test, and became greater with prolonged
time. During the 500 h stability test, a gradual deactivation
exhibited with the conversion of oxygen decreasing from 100%
to 98%. Neither sudden deactivation nor severe oscillation was
discovered. The catalyst exhibits excellent stability under excess
methane with scarce oxygen condition, and Pd particles were
well stabilized by Ce–Zr oxides.
increased due to the sintering of Pd particles.36 During the
initial heating, slight aggregation of the palladium phase
occurred owing to the reducing circumstance and hot pots
generated by combustion, which seems to be benecial for the
ignition activity. But sharp decreases in activity were found
during the second and third heating curves when the oxygen
conversion increased to about 80%, and the temperature
interval for activity regaining becomes greater. Only aer the
conversion dropped to a specic low level (about 30%), the
activity increased again. Similar but not such severe oscillation
phenomenon has been reported in the case of conventional
methane combustion.37 Oscillation happened when reaction
Unlike light-off experiments that surface palladium experi-
enced a continuous transition from PdO and Pd, metallic Pd
ꢀ
temperature reached 700–800 C, which was attributed to the
decomposition of PdO. X. Zhang et al. believed oscillation
occurs when the surface of the catalyst undergoes cycles of
oxidation and reduction, between a highly active oxide state and
a less active metal-rich state.38 When the removal of oxygen
from the oxide lattice exceeds the rate of replacement through
adsorption of oxygen, the oxide will be rapidly reduced to the
metal. We speculate the larger margin and lower temperature
for oscillation is due to the scarcity of oxygen, which made PdO
more prone to be reduced. Zhang et al. studied the oscillatory
behavior under various temperatures and methane/oxygen gas
ratios, and ascribed the behavior to the failure of efficient
transformation between PdO and Pd.39 During several times of
temperature elevation and the existence and vanish of oxygen,
the valence of palladium on catalyst surface underwent
continual change throughout the reaction. The surface PdO
particles, especially smaller ones would be easily reduced and
aggregated to larger particles through re-dispersion, leaving less
active site for methane adsorption. Besides, since larger surface
layer of PdO particles were harder to be reduced, they would
benet the initial activity at the early stage of each light-offs.
However, larger PdO particles failed to exchange with gas
phase oxygen efficiently when oxygen conversion reached some
level, resulting in the occurrence of oscillation. Besides, much
higher temperature was needed for the dissociation of methane.
Because Pd/Ce1Zr2 and Pd/Zr had better activity for oxygen
elimination, successive light-off experiments were also per-
formed on Pd/Zr to compare the stability of catalysts (Fig. 11b).
Fig. 12 Stability test of the catalyst Pd/Ce1Zr2 (a) stability after second
cycle of ignition (T ¼ 350 ꢀC and 330 ꢀC), (b) long-time stability for
500 h (T ¼ 370 ꢀC, GHSV ¼ 30 000 hꢁ1).
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may be the main form of surface layer under time-on-stream
circumstance.31 The catalyst is always in the reducing condi-
tion since oxygen is totally consumed. No obvious deactivation
was discovered, which indicated that the ratio of PdO and Pd
was kept constant in the reaction condition and Pd particle size
was maintained due to the stabilization by Ce–Zr oxides.
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ꢀ
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GHSV reached 50 000 hꢁ1
. With similar initial activity,
Pd/Ce1Zr2 kept high stability aer three successive light-offs,
while the activity of Pd/Zr dropped dramatically. The time-on-
stream experiments further conrmed that the Pd/Ce1Zr2
catalyst had good stability aer running for 500 h. The results
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product gas provided insight into the feasibility and optimiza-
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cated that the as-prepared monolithic catalysts may be
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This work was supported by the National Natural Science
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Fundamental Research Funds for the Central Universities
(DUT12YQ03).
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