Natural clay-supported palladium catalysts for methane oxidation reaction: Effect of alloying

Article abstract of DOI:10.1039/c9ra06804j

The catalytic combustion of methane (CCM) has been extensively studied owing to the wide use of methane in motor vehicles and power generation turbines. However, the absence of polarizability and the high C-H bond strength are considered to be the main drawbacks that limit its oxidation by traditional catalytic converters. Palladium-based catalysts are recognized as the benchmark catalysts for methane oxidation, especially under oxidizing conditions, and their activity is dependent on different parameters such as size, dispersion, and the nature of the support. Additionally, metal oxides are the most common supports used for CCM; however, they can become saturated with water, especially during steady-state operation at low temperatures, owing to their hydrophilic nature. This causes saturation of the active sites with OH species, which poisons the active centers of the catalyst, prevents activation of methane molecules, and induces catalyst sintering. Herein, we reported the synthesis of a binary palladium nanoalloy on a halloysite nanotube support (PdM?Hal). This one-pot synthesis procedure was performed via ultrasound-enhanced reduction of metal precursors in aqueous solution containing dispersed halloysite nanotubes, using NaBH₄ as reducing agent. Transmission electron microscopy revealed that the synthesized PdM?Hal catalysts preserved the morphology of the pristine support after synthesis and calcination, with good dispersion of the catalyst on the surface of the support. Promoted metal-support interactions revealed enhanced catalytic performance, following the order PdNi > PdCo > Pd > PdCu, with activation energies of 68-94 kJ mol^-1.

Full text of DOI:10.1039/c9ra06804j

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Natural clay-supported palladium catalysts for
methane oxidation reaction: effect of alloying†
Cite this: RSC Adv., 2019, 9, 32928
a
Yahia H. Ahmad,
Assem T. Mohamed,a Khaled A. Mahmoud,b Amina S. Aljabera
a
*
and Siham Y. Al-Qaradawi
The catalytic combustion of methane (CCM) has been extensively studied owing to the wide use of
methane in motor vehicles and power generation turbines. However, the absence of polarizability and
the high C–H bond strength are considered to be the main drawbacks that limit its oxidation by
traditional catalytic converters. Palladium-based catalysts are recognized as the benchmark catalysts for
methane oxidation, especially under oxidizing conditions, and their activity is dependent on different
parameters such as size, dispersion, and the nature of the support. Additionally, metal oxides are the
most common supports used for CCM; however, they can become saturated with water, especially
during steady-state operation at low temperatures, owing to their hydrophilic nature. This causes
saturation of the active sites with OH species, which poisons the active centers of the catalyst, prevents
activation of methane molecules, and induces catalyst sintering. Herein, we reported the synthesis of
a binary palladium nanoalloy on a halloysite nanotube support (PdM@Hal). This one-pot synthesis
procedure was performed via ultrasound-enhanced reduction of metal precursors in aqueous solution
containing dispersed halloysite nanotubes, using NaBH4 as reducing agent. Transmission electron
microscopy revealed that the synthesized PdM@Hal catalysts preserved the morphology of the pristine
support after synthesis and calcination, with good dispersion of the catalyst on the surface of the
support. Promoted metal-support interactions revealed enhanced catalytic performance, following the
Received 28th August 2019
Accepted 3rd October 2019
DOI: 10.1039/c9ra06804j
rsc.li/rsc-advances
order PdNi > PdCo > Pd > PdCu, with activation energies of 68–94 kJ molꢀ1
.
To this end, great efforts have been devoted to developing
efficient catalysts that can oxidize methane completely at low
temperatures. Noble elements, e.g., Pt,6,7 Ir,8,9 Ru,10 Pd,11,12 and
Rh,13,14 have been recognized as highly reactive catalysts for
methane oxidation.15–17 Among them, Pd has been reported to
possess the highest catalytic activity towards methane oxida-
tion, especially under oxidizing conditions.18 However, the
deactivation of catalytic activity at high temperatures induced
by water adsorption, the formation of Pd(OH)2, and sintering as
a result of Ostwald ripening, are reported as the main obstacles
hindering the use of Pd-based catalysts for methane oxidation.
Several factors inuence the oxidation mechanism, catalytic
activity, and the resistance to deactivation of Pd-based catalysts,
such as the type of support, particle size, catalyst-support
interaction, oxidation state of the metal, and the dispersion of
the catalyst.17
One of the main ways to improve the catalytic activity of Pd-
based catalysts for methane oxidation is to alloy Pd with
a second metal, such as Fe, Ni, Co or Cu, as a means of tuning
the activity and stability of a Pd-based catalyst. Alloying with
a second metal can tune the d-band center of Pd, the parameter
that determines the capability of d-electrons to bind chemical
species through electron donation or capture.19–21 This can
ultimately inuence the adsorption/activation of methane, thus
Introduction
As a major constituent in natural gas, methane (CH4) is of great
importance in multifarious applications, especially power
generation. However, the escape of unburnt CH4 from exhaust
gas streams during the combustion process has a deleterious
effect on the environment, because methane as a greenhouse
gas is 20 times as potent as CO2.1 Methane is an appealing fuel
owing to the low concentrations of nitrogen and sulfur
contaminants, along with the low CO2 emission per energy
produced due to the high H/C ratio compared to other
conventional fuels.2,3 However, because of its structural
symmetry and relatively high chemical stability (i.e. it is the
least reactive hydrocarbon4), the rate determining step for the
oxidation reaction is the dissociative adsorption step (the acti-
vation and dissociation of the rst C–H bond), which has a high
energy barrier.5
aDepartment of Chemistry and Earth Sciences, College of Arts and Sciences, Qatar
University, Doha 2713, Qatar. E-mail: siham@qu.edu.qa
bQatar Environment and Energy Research Institute (QEERI), Hamad Bin Khalifa
University (HBKU), Doha 5825, Qatar
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra06804j
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altering the kinetics of the oxidation process.22 For instance, the
poisoning effect of SO2 on the methane oxidation activity of
PtPd/Al2O3 using various gas compositions has been investi-
gated.23 The results revealed that the activity of bimetallic PtPd/
Al2O3 was enhanced relative to the activity of monometallic Pd/
Al2O3 and Pt/Al2O3. The enhanced activity of the bimetallic
sample was attributed to the fact that alloying Pd with Pt
enhanced the resistance to –OH formation and changed the
oxidation mechanism compared to monometallic Pd under the
same experimental conditions. In addition, bimetallic samples
could withstand the deactivation by SO2 for prolonged periods
compared with monometallic species.23 Likewise, methane
oxidation activity has been studied on three-dimensionally
ordered macroporous CeO2 loaded with Pd@Co nanoparticles,
Co@Pd/3DOM CeO2, of different molar ratios in the range of
2.4–13.6 via a modied PVA-protected reduction procedure
using polymethylmethacrylate as a template.24 Co3.5Pd exhibi-
ted the highest methane oxidation activity with T90% ¼ 480 ꢁC
at SV ¼ 40 000 mL gꢀ1 hꢀ1 and the lowest activation energy, Ea
of 58 kJ molꢀ1. The boosted performance was ascribed to the
enhanced oxygen and methane adsorbability and the unique
properties of the core–shell structure.24
Experimental
Materials
Halloysite nanoclay, potassium tetrachloropalladate (K4PdCl4),
copper nitrate trihydrate (Cu(NO3)2$3H2O), cobalt nitrate
hexahydrate (Co(NO3)2$6H2O), nickel nitrate hexahydrate
(Ni(NO3)2$6H2O) and sodium borohydride (NaBH4) were
purchased from Sigma-Aldrich Co. Ltd. All reagents were used
without further purication.
Synthesis of Pd-based catalysts
In a ask containing 500 mL deionized water, 100 milligrams of
halloysite nanotubes were added and the mixture was ultra-
sonicated for 5 h. Aer this time, K4PdCl4 (5 mL, 20 mM) and
MCl2$xH2O (3 mL, 20 mM) (M ¼ Co or Ni or Cu) were added
with continuous stirring for 2 h. Aer that, 5 mL of ice-cooled
0.1 M NaBH4 solution was added dropwise under ultrasound
irradiation, and the nal product was collected by centrifuga-
tion, washed with water and ethanol, and dried in a vacuum
ꢁ
oven at 60 C. The monopalladium counterpart (Pd@Hal) was
synthesized using the same procedure, but with the addition of
K4PdCl4 as the only metal precursor.
The nature of the support is considered to be one of the
main factors affecting the catalytic activity. The role of the
support is to provide a high surface area on which the active
catalyst is dispersed, sufficient interaction with the catalyst
to prevent its sintering, and may give rise to new active sites
for the reaction at the support/catalyst interface.25,26 Various
types of supports have been investigated for the methane
oxidation reaction, such as metal oxides, i.e. Al2O3, ZrO2,
CeO2, TiO2 and SiO2, zeolites,27–30 nitrogen-doped carbon,31
perovskites,32,33 and mesoporous silica supports.34,35 Halloy-
site is a naturally occurring aluminosilicate clay (Al2Si2O5(-
OH)4$nH2O), which has outstanding structural and chemical
merits, such as high surface area, good chemical and thermal
stabilities, and is inexpensive and environmentally friendly.36
Halloysite has a hollow tubular structure with nanometer
dimensions and comprises a two-layered structure. Al–OH
groups are present in the inner surface and Si–O groups
dangle along the outer surface, which are inherent anchoring
sites for various types of materials including polymers,
drugs, metal oxides and metal nanoparticles.37–40 Despite the
promising properties of halloysite as a potential host for
different types of materials, its use as a support for hetero-
geneous catalysis is still rare41–43 and has not been reported
for the methane oxidation reaction.
Characterization
The morphology and composition of the as-synthesized mate-
rials were investigated using a Transmission Electron Micro-
scope (TEM) (FEI Tecnai G2 TF20 UT) equipped with Energy
Dispersive X-ray (EDX) spectroscopy and high-angle annular
dark-eld scanning TEM (HAADF-STEM) operated at 200 kV.
The elemental composition of the as-synthesized catalysts was
determined using Inductively Coupled Plasma-Mass Spec-
trometry (ICP-MS) on a NEXION 300D (PerkinElmer, USA).
Textural properties of the as-synthesized catalysts were deter-
mined from N2 sorption experiments at 77 K. The surface area
was estimated according to the BET (Brunauer–Emmett–Teller)
model. The crystalline phases of the prepared materials were
investigated using Powder X-ray Diffraction (PXRD) on an
X'Pert-Pro MPD diffractometer (PANalytical Co., Netherlands)
˚
with a Cu-Ka source (l ¼ 1.54059 A). The chemical nature and
oxidation states of various elements were investigated via an
XPS spectrophotometer (Kratos Axis Ultra) equipped with
a monochromatic Al-Ka radiation source (1486.6 eV). A Micro-
meritics Autochem 2910 equipped with a thermal conductivity
detector (TCD) was used to investigate H2-TPR. 100 mg of the
test sample was introduced into a testing tube and 10% H2 in Ar
Inspired by this, herein we report a one-pot reduction
method for the rational synthesis of binary metallic PdM@Hal
with a small loading content of a Pd-based catalyst (<1 wt%).
The synthesized PdM@Hal catalysts retained the one-
dimensional nanotubular morphology of the pristine halloy-
site support along with its high surface area. Furthermore, they
was purged over the test sample at a ow rate of 30 mL minꢀ1
.
ꢁ
The sample was then heated up to 850 C at a heating rate of
ꢀ1
ꢁ
5 C min
.
Methane oxidation activity
exhibited pronounced activity and stability towards methane Methane oxidation measurements were performed in a xed-
oxidation, especially PdCo@Hal, which manifested the highest bed quartz reactor (id ¼ 6 mm) at ambient pressure. 50 mg of
activity and stability compared to PdNi@Hal, PdCu@Hal and each catalyst was placed at the center of the reactor and
Pd@Hal.
mounted with quartz wool. Prior to the activity measurements,
the catalyst was oxidized in 20 wt% O2/Ar (50 mL minꢀ1) at
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500 ꢁC for 2 h, where the temperature was increased at images also revealed that the metal nanoparticles were mainly
a constant heating rate of 5 ꢁC minꢀ1. The feed gas, comprising present at the outer surfaces of the halloysite nanotubes. The
1 wt% CH4 and 20 wt% O2 in Ar, was supplied at a rate of 60 particle size distribution of the different Pd-based catalysts (as
mL minꢀ1 (GHSV ¼ 72 000 mL gꢀ1 hꢀ1), which was controlled determined from TEM imaging) is presented in Fig. 1. The
by digital mass ow controllers. Gaseous products were catalysts exhibited a relatively wide range of 4–25 nm, with
analyzed using an infrared gas analyzer (IR200, Yokogawa, average particle sizes of 7–9 nm.
Japan) in order to estimate methane conversion. The reaction
The compositions of the Pd-based catalysts PdNi@Hal were
temperature was recorded via a K-type thermocouple located at investigated using HAADF-STEM analysis. Fig. 2 shows the
the catalyst bed. Stability measurements were investigated at elemental mapping of PdNi@Hal, which implies an even
ꢁ
350 C under the continual ow of the reacting gases.
distribution of the different metals, with an atomic ratio of Pd
to Ni of 3 : 1 as obtained from EDX analysis. The compositions
of the different halloysite nanotube-based catalysts were further
investigated using ICP-MS, and are given in Table 1. The
compositions revealed that the average Pd loading is about
0.9 wt% for all catalysts and the average ratio of the Pd to the
alloying metal is almost 3 : 1.
Fig. 3 shows nitrogen sorption isotherms and the pore size
distributions of the as-synthesized catalysts. According to
IUPAC nomenclature, all isotherms follow type IV isotherms.
All of the catalysts exhibit isotherms similar to halloysite
nanotube support, indicating that the porous tubular structures
were maintained aer metal deposition. The specic surface
areas and pore sizes were calculated from the corresponding
BJH isotherms and are given in Table 2. The metal-loaded hal-
loysite nanotubes exhibited surface areas of 86–90 m2 gꢀ1, pore
radii of 7–9 nm, and pore volumes of 0.43–0.51 cm3 gꢀ1, which
are slightly lower than those of pristine Hal (Fig. S1†) due to the
partial blockage of pores with metallic nanoparticles. The
reasonable surface area exhibited by all catalysts can afford
more active sites for the catalytic oxidation reaction.
Results and discussion
Monometallic and bimetallic palladium alloys were prepared on
halloysite nanotubes via a single-step reduction process using
NaBH4 as reductant followed by annealing of the formed
products in H2 at 450 ꢁC. TEM images of the monometallic and
bimetallic Pd-based catalysts are presented in Fig. 1. The images
show that all catalysts retained the nanotubular morphology of
pristine halloysite nanotubes, along with deformed walls and
edges owing to ultrasonication during the synthesis step. The
Fig. 4 shows the X-ray diffraction patterns of pristine hal-
loysite nanotubes as well as the Pd-supported halloysite nano-
tubes. All diffraction peaks of the halloysite nanotubes are
assigned to monoclinic halloysite (PDF 29-1487).44,45 For
instance, the diffraction peak at 2q ¼ 12.1 is ascribed to the
46
˚
(001) diffraction recognized as halloysite-7 A. Meanwhile, the
diffractions noticed at 20.2 and 24.8 are ascribed to (020) and
(002) planes of the halloysite nanotubes, respectively. In
comparing the XRD pattern of the halloysite support to the
Fig. 1 Low magnification TEM images and particle size distribution of
(a) Pd, (b) PdCo, (c) PdNi, and (d) PdCu@Hal.
Fig. 2 HAADF-EDS elemental mapping of PdNi@Hal.
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Table
1 The metallic composition of various Pd-based catalysts
(of smaller size). This led to the contraction of the Pd lattice
along with the modication of the Pd electronic structure.48–51
H2-TPR proles of PdM@Hal are given in Fig. 4b. The results
exhibit no reduction peaks for the halloysite support (Fig. S2†),
which reveals high chemical stability. All catalyst proles reveal
one reduction peak located at around 45 ꢁC, which can be
assigned to the reduction of PdO. There were no observable
peaks for the other metallic components, due to their low
concentrations. The PdO reduction peak temperature follows
the order PdNi < PdCo < PdCu < Pd@Hal, affirming the easier
PdO reducibility over PdNi@Hal compared to the other cata-
lysts. The interaction of Pd with Ni, Co and Cu is accompanied
by charge transfer from these metals to Pd, which facilitates the
reduction of PdO and shis the reduction peak to lower
temperatures compared to pure Pd.52 Despite the existence of
hydroxyl groups in the halloysite support, which are capable of
interacting with various metal particles, the recorded low
reduction temperature of PdO veries the weak interaction
between the metallic nanoparticles and the support material.
This implies that the metallic nanoparticles are mainly located
at the outer surfaces of the support where only Si–O groups are
available which do not efficiently interact with the catalyst.
XPS analysis was performed to investigate the chemical
composition and valence states of different elements in the
studied catalysts. Deconvolution of Pd(3d) in all catalysts
revealed two peaks at around 335.3 and 340.8 eV, assigned to
Pd(0), while the other two peaks observed at 337.0 and 342.0 eV
are ascribed to Pd(II) oxide (Fig. 5a). A high-resolution spectrum
of Co 2p revealed the existence of two peaks at 780.7 and
794.9 eV assigned to metallic Co, while those observed at 785.5
and 496.9 eV are attributed to Co(II) oxide53 (Fig. 5b). Similarly,
PdNi@Hal displayed two peaks in the Ni 2p region at 851.5 and
856.5 eV, which can be attributed to metallic Ni(0) and Ni(II),
respectively, whereas the peaks located at 861.5 and 880.9 eV are
shake-up peaks originating from multi-electron excitations54
(Fig. 5c). The predominant species of Ni are the oxidized
species, which can be ascribed to the oxophilic nature of Ni
surface atoms, which rapidly react with atmospheric oxygen or
H2O to form oxidized species, i.e. NiO and Ni(OH)2.54,55 Two
peaks were observed in the Cu 2p region at 932.0 and 951.8 eV,
assigned to Cu 2p3/2 and Cu 2p1/2, whereas the other two peaks
observed at the higher binding energy side (942.6 and 963.0 eV)
are satellite peaks encountered by the presence of unlled
3d.9,56 Deconvolution of the Cu 2p peaks revealed two peaks at
expressed in weight (%)
Catalyst
Pd
Co
Ni
Cu
PdNi@Hal
PdCo@Hal
PdCu@Hal
Pd@Hal
0.91
0.90
0.94
0.92
—
0.32
—
0.33
—
—
—
—
0.36
—
—
—
metal-supported samples, it can be noticed that the Hal
conserved its crystalline structure aer the deposition of the Pd
and PdM nanoalloys. Furthermore, the diffractions at 40, 47, 68,
82 and 87ꢁ can be assigned to the (111), (200), (220), (311) and
(222) facets of the face-centered cubic structure of metallic Pd
(JCPDS no. 46-1043), which implies crystallization of Pd over the
halloysite nanotubes.47 The diffraction peaks of single metallic
Ni, Co and Cu species were not detected, implying full inte-
gration of Ni, Co and Cu in the Pd lattice structure with the
formation of bimetallic alloys with no observable undesirable
phases. The diffractions of bimetallic Pd displayed slight shis
to higher 2q values as compared to monometallic Pd, owing to
the decrease in the lattice constants of the PdM nanoalloys
compared to Pd, which can be attributed to the partial
replacement of Pd atoms (of larger size) with Co, Ni or Cu atoms
Fig. 3 Adsorption–desorption isotherms and BJH pore size distribu-
tions (insets) of as-synthesized catalysts.
Table 2 The textural properties of the different Pd-based catalysts
Surface area
Average pore
radius (nm)
Pore volume
Catalyst
(m2 gꢀ1
)
(cm3 gꢀ1
)
PdNi@Hal
PdCo@Hal
PdCu@Hal
Pd@Hal
Hal
85.6
87.2
88.9
89.6
93.4
8.3
7.9
8.6
7.4
9.1
0.51
0.49
0.48
0.43
0.54
Fig. 4 (a) XRD spectra and (b) H2-TPR profiles of the as-synthesized
catalysts.
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Monometallic and bimetallic Pd are the active species for
methane oxidation, hence, all calculations were done on their
basis. The calculated reaction rate on PdNi@Hal was 4.01 ꢂ
10ꢀ2 mol gꢀ1 sꢀ1 at 250 ꢁC, which is 3.8, 1.5 and 1.9 times higher
than those for PdCu, PdCo and Pd@Hal, respectively. Further-
more, the turnover freqꢁ uency of the various investigated cata-
lysts estimated at 250 C followed the order: PdNi (0.32 sꢀ1) >
PdCo (0.24 sꢀ1) > Pd (0.21 sꢀ1) > PdCu@Hal (0.11 sꢀ1).
The apparent activation energies for methane oxidation over
monometallic and bimetallic Pd catalysts were estimated via the
corresponding Arrhenius plots (Fig. 6c). Calculations were
based on data collected at CH4 conversions of less than 10% to
minimize mass transfer limitations. The estimated activation
energies were found to be 68, 80, 94 and 91 kJ molꢀ1 for PdNi,
PdCo, PdCu and Pd, respectively. PdNi and PdCo expressed
lower activation energies compared to PdCu and Pd, which
conrms the enhanced oxidation kinetics.
The stability of different catalysts was examined at 380 ꢁC for
a gas mixture containing 1 wt% methane at a space velocity
GHSV of 72 000 hꢀ1 over a period of 12 h (Fig. 6d). PdCo and
PdNi@Hal exhibited enhanced stability compared to PdCu and
Pd@Hal. Unlike the bimetallic catalysts, monometallic Pd
expressed a higher rate of activity decay with time. This
conrms the role of alloying in the stabilization of Pd.
The mechanism of oxidation of methane on Pd-based cata-
lysts is still a subject of debate. Some studies report that
metallic Pd is the surface active phase,59 whereas other studies
claim that PdO is the active phase for this reaction.60 However,
at high temperatures (specially above 600 ꢁC) PdO is reduced to
Pd even under oxidizing conditions, which leads to catalyst
deactivation.61,62 Hence, it has been argued that the highest
catalytic activity is obtained when a mixed phase of Pd/PdO is
available.63,64 It has been reported that the reaction proceeds
initially via adsorption and dissociation of methane to methyl
and hydrogen radicals (which is considered to be the rate
determining step).5 This step takes place preferentially on
Fig. 5 XPS high resolution spectra of (a) Pd 3d, (b) Co 2p, (c) Ni 2p, and
(d) Cu 2p.
931.9 and 951.7 eV, assigned to metallic Cu, and lower intensity
peaks at 934.2 and 953.7 eV, corresponding to CuO (Fig. 5d).57
The Pd 3d peaks are shied to higher binding energies in the
bimetallic catalysts compared to Pd@Hal (Fig. S3†). This is
attributed to the interaction between Pd and Ni, Co and Cu,
induced by electron transfer between Pd and the other metals,
which alters the electronic structure of Pd, modies its d-band
center, and enhances catalytic performance.58
The catalytic activity of methane oxidation over different Pd-
based catalysts was investigated using a xed-bed reactor at
atmospheric pressure. The reactivity of the samples was
expressed in terms of methane conversion (%) as a function of
temperature (Fig. 6a). For all of the catalysts, the methane
conversion increased with rising the temperature until it
reached 100% conversion. For all investigated Pd-based cata-
lysts, CO was not detected in the outlet stream, which conrms
that methane was completely oxidized to CO2 (i.e. CH4 + 2O2 /
CO2 + 2H2O). All bimetallic catalysts exhibited higher activity
towards methane oxidation than the monometallic Pd
counterpart.
Amongst all of the catalysts, PdCo expressed the highest
methane oxidation activity. For instance, methane conversion
reached 10% at 295 ꢁC over PdNi, while it reached the same
conversion at 299, 308 and 322 ꢁC, in the case of PdCo, Pd and
PdCu, respectively. ꢁSimilarly, the temperature for 50% conver-
sion (T50) was 355 C in the case of PdNi, which is 3, 14 and
37 ꢁC, lower than that of PdCo, Pd and PdCu, respectively.
Furthermore, complete methane conversion was attained over
PdNi at 428 ꢁC; however, it was reached at 420, 444, and 465 ꢁC
in the case of PdCo, Pd and PdCu, respectively (Fig. 6b). The
data obtained revealed that the activity towards methane
oxidation follows the order: PdNi > PdCo > Pd > PdCu.
The rates of methane combustion and the analogous turn-
over frequencies of the as-synthesized Pd-based catalysts are
given in Table 3. Measurements were performed at low rates of
conversion (below 20%) to lessen the diffusion effect.
Fig. 6 (a) Light-off methane oxidation curves. (b) Temperatures of 10,
50, and 90% conversions. (c) Arrhenius plots. (d) Stability tests of
monometallic and bimetallic Pd-catalysts at 380 ꢁC.
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Table 3 Kinetic parameters over Pd-based catalysts at 250 ꢁ
C
r ꢂ 10ꢀ2 (mol gPd sꢀ1
)
Particle size (nm)
D (%)
TOF (sꢀ1
)
Ea (kJ molꢀ1
)
ꢀ1
Catalyst
PdNi@Hal
PdCo@Hal
PdCu@Hal
Pd@Hal
4.01
2.74
1.05
2.07
9.4
10.2
11.8
12.1
13.3
12.3
10.6
10.3
0.32
0.24
0.11
0.21
68
80
94
91
metallic Pd; then, it is followed by subsequent oxidation by the the formation of adsorbed methyl radicals and surface hydroxyl
adsorbed oxide species. Regardless of the nature of the active groups (CH4 + Pd*/PdO ¼ Pd–CH3 + Pd–OH).71,72 This step is
phase in the Pd-based catalyst, the activity and stability of the followed by recombination of surface hydroxyls to form H2O,
catalyst towards CCM is dependent on several factors, such as PdO and surface vacancies (Pd–OH + Pd–OH ¼ Pd–O + H2O +
the crystallite size, specic surface area, nature of support, and Pd*). Pd–O is regenerated by combining of vacancies to O2
Pd content on the surface. Among them, alloying with a second (2Pd* + O2 ¼ 2Pd–O) and the regenerated Pd–O can abstract H
metal can greatly affect the catalytic performance of Pd-based from methyl radicals for further oxidation.71,72 The rate of
catalysts. The impact of Pd alloying on methane combustion methane oxidation is dependent on the rate of O2 activation, in
has rarely been studied and remains a subject of dispute. Willis addition to the rate of formation of hydroxyl species. Ni and Co
et al.22 studied the impact of different bimetallic Pd nano- are recognized for their oxophilic nature as well as high oxygen
crystals on the CCM. Their study revealed that most PdM/Al2O3 storage capacity, so they can stabilize PdO and enhance the
catalysts have similar T100, i.e. 400 ꢁC, and similar activation activation of O2.54,73 This can explain the positive effect of their
energies (78–87 kJ molꢀ1); however, the alloying elements oxides in the methane oxidation reaction. In the case of Cu, the
enhanced the rate of oxidation and turnover frequency (TOF), low oxidation activity can be assigned to the partial coverage of
especially in the case of Ni and Zn. Additionally, the thermal Pd and PdO with CuO, which decreases the number of active
stabilities of catalysts were enhanced in the case of Fe, Co, Ni sites participating in the oxidation reaction.74 Furthermore, Cu
and Zn, as revealed by the improvement of the redox properties has inferior reactivity towards O2 activation compared to Co and
of the PdO active phase. The enhancement of stability was Ni, which may slow down the oxidation kinetics.58,75
attributed to inhibition of PdO sintering in the case of Fe, Co
Compared to metal oxide supports, halloysite nanotubes
and Sn, and an increase of PdO thermal stability in the case of have a great captivating nature compared to traditional metal
Ni and Zn.22 The catalytic activity of different bimetallic Pd oxides and a great ability to conne and stabilize metallic
alloys, i.e. PdPt, PdIr, PdNi, PdCo, PdCu, PdAu, PdAg, and PdRh, nanoparticles and prevent their sintering. Furthermore, the
revealed similar or lower CCM activity compared to mono- halloysite support has a greater hydrothermal stability than
metallic Pd, and only a slight improvement in the catalyst oxides, especially at low temperatures, and hence has a reduced
stability was observed in the case of PdPt and PdAg.65 In poisoning effect on the methane oxidation activity. Such
contrast, it was reported that NiO enhanced the activity of Pd/ a poisoning effect can originate due to water absorption and
Al2O3, owing to the suppression of PdO decomposition at high accumulation of hydroxyl groups on the surface, which retards
temperatures.66 According to the results obtained here, it seems the activation of methane molecules and enhances steam-
that Ni and Co oxides have a positive effect on the activity of induced sintering. Comparing the catalytic activity of
PdO, whereas Cu oxide exhibits a deactivating effect on PdO.
PdM@Hal to that of previously reported catalysts conrms the
Methane oxidation over Pd-based catalysts takes place via promising performance of halloysite nanotubes as a support for
dissociation of CH4 at the vacancy/oxide active sites, yielding the methane oxidation reaction (Table 4). These results will
Table 4 Comparison of PdNi@Hal activity towards methane oxidation with reported catalysts in literature
WHSV (mL
Catalyst
Metal loading (wt%)
0.26
gꢀ1 hꢀ1
)
T10 (ꢁC)
T50 (ꢁC)
T90 (ꢁC)
Ea (kJ molꢀ1
)
Reference
24
Co3.5Pd/three dimensionally ordered
macroporous CeO2
40 000
356
430
480
58
Flame made Pd/TiO2 nanoparticles
NiO@PdO/Al2O3
La(Fe,Pd)O3
La(Mn,Pd)O3
PdO@CeO2/mesoporous silica
5
—
30 000
305
255
350
318
545
524
290
350
ꢃ350
355
>500
370
565
550
342
—
89
—
—
—
—
—
68
67
68
69
0.2
2.28
2.32
1
5
0.81
0.9
>400
>400
ꢃ250
ꢃ300
291
60 000
180 000
70
442
Pd/CeO2
PdNi/halloysite nanotubes
200 000
72 000
ꢃ490
11
295
402
This work
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Conflicts of interest
There are no conicts to declare.
´
23 N. Sadokhina, G. Smedler, U. Nylen, M. Olofsson and
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Acknowledgements
This work was supported by the Qatar National Research Fund
(a member of the Qatar Foundation) [grant numbers NPRP 8- 25 R. M. Heck, R. J. Farrauto and S. T. Gulati, Diesel engine
1912-1-354]. The statements made herein are solely the
responsibility of the authors. The authors acknowledge the
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