Yttrium stabilization and Pt addition to Pd/ZrO2catalyst for the oxidation of methane in the presence of ethylene and water

Article abstract of DOI:10.1039/d0ra10773e

Catalytic oxidation is the most efficient method of minimizing the emissions of harmful pollutants and greenhouse gases. In this study, ZrO₂-supported Pd catalysts are investigated for the catalytic oxidation of methane and ethylene. Pd/Y₂O₃-stabilized ZrO₂(Pd/YSZ) catalysts show attractive catalytic activity for methane and ethylene oxidation. The ZrO₂support containing up to 8 mol% Y₂O₃improves the water resistance and hydrothermal stability of the catalyst. All catalysts are characterized by X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), O₂-temperature-programmed desorption (O₂-TPD), and CO-chemisorption techniques. It shows that high Pd dispersion and Pd-PdO reciprocation on the Pd/YSZ catalyst results in relatively high stability.In situdiffuse reflectance infrared Fourier-transform (DRIFT) experiments are performed to study the reaction over the surface of the catalyst. Compared with bimetallic catalysts (Pd?:?Pt), the same amounts of Pd and Pt supported on ZrO₂and Y₂O₃-stabilized ZrO₂catalysts show enhanced activity for methane and ethylene oxidation, respectively. A mixed hydrocarbon feed, containing methane and ethylene, lowers the CH₄light-off temperature by approximately 80 °C. This shows that ethylene addition has a promotional effect on the light-off temperature of methane.

Full text of DOI:10.1039/d0ra10773e

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Yttrium stabilization and Pt addition to Pd/ZrO2
catalyst for the oxidation of methane in the
presence of ethylene and water†
Cite this: RSC Adv., 2021, 11, 11910
a
a
Junyu Hao,a Omar El Tallb and Aamir Farooq
*
*
Hassnain Abbas Khan,
Catalytic oxidation is the most efficient method of minimizing the emissions of harmful pollutants and
greenhouse gases. In this study, ZrO2-supported Pd catalysts are investigated for the catalytic oxidation
of methane and ethylene. Pd/Y2O3-stabilized ZrO2 (Pd/YSZ) catalysts show attractive catalytic activity for
methane and ethylene oxidation. The ZrO2 support containing up to 8 mol% Y2O3 improves the water
resistance and hydrothermal stability of the catalyst. All catalysts are characterized by X-ray diffraction
(XRD), Brunauer–Emmett–Teller (BET), O2-temperature-programmed desorption (O2-TPD), and CO-
chemisorption techniques. It shows that high Pd dispersion and Pd–PdO reciprocation on the Pd/YSZ
catalyst results in relatively high stability. In situ diffuse reflectance infrared Fourier-transform (DRIFT)
experiments are performed to study the reaction over the surface of the catalyst. Compared with
bimetallic catalysts (Pd : Pt), the same amounts of Pd and Pt supported on ZrO2 and Y2O3-stabilized
ZrO2 catalysts show enhanced activity for methane and ethylene oxidation, respectively. A mixed
hydrocarbon feed, containing methane and ethylene, lowers the CH4 light-off temperature by
approximately 80 ꢀC. This shows that ethylene addition has a promotional effect on the light-off
temperature of methane.
Received 23rd December 2020
Accepted 6th March 2021
DOI: 10.1039/d0ra10773e
rsc.li/rsc-advances
CH4 + 2O2 / CO2 + 2H2O; DHr ¼ ꢁ803 kJ molꢁ1
.
Introduction
Hydrocarbons (HCs), such as methane (CH4) and ethylene
(C2H4), are an environmental concern. Methane is the second
major contributor to climate change.1 Furthermore, it has
a radiative forcing efficiency that is approximately 120 times
more than that of CO2.2–4 Environmental energy security direc-
tives are promoting the use of natural-gas vehicles because of
the low value of C/H compared to that of gasoline implies lower
CO2 emissions.5 However it contributes signicantly to the
release of unburned methane besides fugitive emissions.6
Accordingly, converting methane to a less potent greenhouse
gas, along with overall emission reduction, is necessary.
Compared to collecting and storing methane, catalytically
converting it to carbon dioxide is a thermodynamically favor-
able reaction. The largest activation barrier is associated with
the dissociation of strong C–H bonds (435 kJ molꢁ1), and
catalysts can play a crucial role in overcoming this barrier.7
Ethylene is one of the most important intermediates of the
hydrocarbon combustion process. It is a harmful pollutant that
is emitted into the environment because of the incomplete
combustion in power plants and vehicles. Similar to methane,
ethylene can be converted to carbon dioxide over a catalyst.8
C2H4 + 3O2 / 2CO2 + 2H2O; DHr ¼ ꢁ1322 kJ molꢁ1
.
Pd and Pt are regarded as highly stable catalysts for the total
combustion of CH4 and C2H4, respectively.9,10 However, the
deactivation due to sintering at high reaction temperature (400–
600 C) and wet (H2O) feeds remains a challenge.9,10 Attempts
ꢀ
have been made to resolve catalyst deactivation by adding Pt to
Pd-based monometallic catalysts.11–14 Nevertheless, the inu-
ence of Pt on the improvement of bimetallic Pd–Pt catalysts is
inconclusive. Though some literature state that alloying Pt to Pd
increases catalytic activities,8,12,15,16 others argue that the impact
of adding Pt to Pd is not obvious.14,17,18 Persson et al. compared
the catalytic activity of Pd and bimetallic Pt–Pd catalysts and
found that the addition of Pt to Pd has detrimental effects and it
even lowers the performance of the monometallic Pd catalyst.14
In another study by Strobel et al., the effects of different molar
ratios of Pt to Pd were compared. It was observed that higher Pt
aClean Combustion Research Center, Physical Science and Engineering Division, King
Abdullah University of Science and Technology (KAUST), Thuwal 23955-6900, Saudi
Arabia. E-mail: hassnain.khan@kaust.edu.sa; aamir.farooq@kaust.edu.sa
bKAUST Core Labs, King Abdullah University of Science and Technology (KAUST),
Thuwal, 23955-6900, Saudi Arabia
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra10773e
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molar ratios have adverse effects on the activity of Pd catalysts.17 prepared catalysts are investigated. The effect of dopants on the
However, notably, the addition of Pt has promotional effects on structural and redox properties is elucidated by characterizing
the dispersion of supported Pd catalysts.8
the Mx/ZrO2 and Mx/YSZ (8 mol% Y2O3–ZrO2) catalysts. Our
Among other factors, support materials have an important study will help to understand the inuence of promoters on the
inuence on the catalytic properties. The inuence of the performance of Pd/ZrO2 catalysts. This will help design for
support originates from its interaction with the depositing commercial/practical applications.
metal species. The dispersion, oxidation states, thermal stabil-
ities, and oxygen mobility are governed by the nature of the
Experimental
Materials
support materials.19–23 These characteristics directly inuence
the catalytic performance of the active novel metal. Several
Zirconium(IV) oxide powder (5 mm) and chloroplatinic acid
hexahydrate were acquired from Sigma-Aldrich. Tetraamine-
palladium(II) chloride monohydrate was supplied by Alfa Aesar.
YSZ (TZ-8Y) was purchased from Tosoh Corporation, Japan.
Millipore deionized (DI) water was used as the solvent during
catalyst synthesis.
studies have been conducted on Pd supported on Al2O3, TiO2,
CeO2, SnO2, MgO, zeolites,24,25 SiO2,26 and modied Al2O3–TiO2
(ref. 27) catalysts. CH4 combustion follows the Mars–Van Kre-
velen mechanism over supported Pd catalysts.28 Therefore, the
rate of release of active oxygen species by the support materials
and their replenishment could affect the catalytic activity for
CH4 combustion.
Catalyst synthesis
Yoshida et al. reported that the Pd on moderately acidic
oxides, such as ZrO2 and Al2O3, is more acidic than the Pd
supported on strong acidic or basic oxides.29 ZrO2 has more
surface oxygen vacancies and relatively high oxygen ion
conductivity, which help maintain the PdO states during the
reaction.30,31 Therefore, several reports claim that Pd/ZrO2
shows better catalytic performances in wet and dry conditions,
compared to Pd supported on Al2O3 support.32,33 CeO2 has
a relatively high ability to store and release oxygen, and the
addition of CeO2 to ZrO2 leads to an increase in activity, though
the catalysts are severally deactivated at low temperatures in
real exhaust conditions where moisture is signicantly high.26
Owing to the low temperature of vehicle exhausts during cold
starts, it is challenging to effectively oxidize hydrocarbon
emissions. Therefore, it is desirable to have good catalytic
materials, which show activity at low temperatures. Pd/ZrO2 has
the potential to meet this challenge.22,32,34–41 It is generally
accepted that PdO–Pd is active for the combustion of CH4.14
ZrO2 offers oxygen exchange for Pd–PdO during the CH4
oxidation, and the lattice oxygen transport by ZrO2 greatly
contributes to the oxygen pool of the supported PdO.23,42,43 ZrO2
has different phases, which also inuence the catalytic perfor-
mance.34,36,43 There is no clear information available whether
the Pd supported on the monoclinic phase of ZrO2 is an active
or tetragonal phase. However, it has been communicated in
The wet impregnation method was used to synthesize all the
catalysts. Briey, 1.0 g of the support materials (YSZ and ZrO2)
was vacuum dried for 12 h to remove moisture and clean the
pores. The dried support was dispersed in 180 mL of DI water
and sonicated for 60 min at 25 ꢀC. To the sonicated dispersion,
20 mL of an aqueous solution of the metal salt (1 wt% metal
loading) was added dropwise. The resulting suspension was
mounted onto a rotary evaporator and rotated for 3 h to gently
mix the support and salt solution. The solvent was desiccated
under reduced pressure at temperature close to the boiling
point of water (80 ꢀC). The powder sample was collected and
ꢀ
dried in a vacuum oven at 120 C for the complete removal of
the solvent in the poreꢀs. The dried sample was calcined in
a muffle furnace at 600 C in air for 3 h. The temperature was
ramped up at 2 C minꢁ1. The Pd–Pt bimetallic catalysts were
ꢀ
synthesized by the same method, keeping the overall metal
loading at 1.0 wt% (0.5 Pd : 05 Pt).
Catalyst characterization
Catalyst characterization details and procedures are given in the
ESI† le.
Activity measurement
previous reports that the tetragonal phase of ZrO2 is more active A U-shaped quartz reactor with an inner diameter of 4 mm was
for CH4 combustion with relatively high hydrothermal used to test the performance of the catalysts in CH4 and C2H4
stability.41 Additionally, the phase transition in ZrO2 during the oxidation. The powdered catalyst was pressed to uniform-sized
reaction hinders the performance and leads to the sintering and pellets and sieved to 45–60 mm. The reactor was heated in an
burial of Pd metal.44 The metastable tetragonal ZrO2 phase with electric furnace using an advanced temperature controller. A
high surface area shows promising combustion properties, and thermocouple (K-type) was placed inside the reactor, touching
the catalyst remains stable during the test duration.33,45
the catalyst bed, to record the real-time temperature. Gas-phase
Y2O3 (excellent Oꢁ2 conductor)-stabilized ZrO2(YSZ) is an reactions without the catalyst bed inside the reactor were per-
increasingly signicant catalyst carrier. Adding a specic formed in the temperature range of 200–700 ꢀC, and no
amount of an oxide, such as Y2O3, to ZrO2 stabilizes the signicant conversions were observed. The composition of the
tetragonal phase of ZrO2 and improves its properties.46
feed gases was maintained at 1.0 vol% CH4/20% O2 diluted with
In this work, Pd, Pt, and their alloys are supported on nitrogen to maintain the total ow of 33.33 mL minꢁ1. Subse-
commercial ZrO2 and YSZ support materials to study their quently, 25 mg of the dilute catalyst was placed inside a reactor
performance for HC (CH4 and C2H4) oxidation. Furthermore, using quartz wool, which corresponds to a GHSV of 80 000 mL
the water resistance and hydrothermal stabilities of the gcatꢁ1 hꢁ1. The residence time was varied by increasing the ow
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Table 1 Conversion temperatures (ꢀC) for HC oxidation over the
prepared catalysts
Conversion
Methane
Ethylene
Mix-feeda
Catalyst
T10% T50% T90% T10% T50% T90% T10% T50% T90%
Pd/ZrO2
Pd/YSZ
276 330 422 224 270 325 267 326 443
275 315 354 210 260 324 190 311 340
PdPt/ZrO2 310 392 600 103 160 275 300 380 600
PdPt/YSZ 330 415 527 76 118 227 307 403 450
Pt/ZrO2
Pt/YSZ
405 568 675
405 550 645
53
50
88 173 400 545 700
76 110 350 540 700
a
Mixed feed gas composed of 1.0% of CH4 and 1000 ppm of C2H4, 20%
of O2, balance N2.
Fig.
1 Light-off temperature profiles of CH4 oxidation over the
synthesized catalysts (mass of catalyst: 0.025 g; total flow rate: 33.33
presented in Fig. S1.† The steady state conversion rate can be
achieved at a certain temperature with increasing ow rate. A
further increase in ow rate will have little effect on the
conversion of methane. At higher ow rates the mass transfer
limitations thus become negligible.47
mL ꢁm1inꢁ1; 1 volume% CH4 in 20% O2, balance N2; GHSV: 80 000 mL
gcat
h
ꢁ1).
rates of the feed gases and decreasing the weight of the catalyst.
All the reactions were performed at 1 atm pressure. All the
reactant and product gases were analyzed using an online gas
chromatograph equipped with a thermal conductivity detector
(TCD) and ame ionization detector (FID). The conversion
activities of the catalyst were calculated as follows:
Catalytic performance for ethylene combustion. An initial
gas concentration of 1000 ppm was used to study the combus-
tion of C2H4 over Pd, Pt, and PdPt catalysts on YSZ and ZrO2
supports. Table 1 presents the T10, T50, and T90 temperatures. As
expected, the activity trends shied inversely to CH4 oxidation
on the prepared catalysts, as shown in Fig. 2. Monometallic Pt
on YSZ was highly active for C2H4 oxidation. Complete oxida-
tion was achieved at a low temperature of 125 ꢀC. Consistent
with previous reports, ZrO2- and YSZ-supported Pd were the
least active. However, the Y2O3 promotional effect was prom-
inent, and Pd supported on YSZ showed better catalytic
performance as compared to Pd/ZrO2. The weakly supported Pd
and Pt catalytic performance for C2H4 and CH4, respectively,
CHi4n ꢁ CHo4ut
CH4 conversion ð%Þ ¼
ꢂ 100;
CHi4n
C2Hi4n ꢁ C2Ho4ut
C2H4 conversion ð%Þ ¼
ꢂ 100:
C2Hi4n
Results and discussion
Catalytic performance
Catalytic performance for methane oxidation. Fig. 1 shows
the light-off temperature proles of all the prepared catalysts for
the standard dry methane feed conditions. The light-off
temperatures (T10%) of Pd/ZrO2, Pd/YSZ, Pt/ZrO2, Pt/YSZ, PdPt/
ZrO2, and PdPt/YSZ were 276 ꢀC, 275 ꢀC, 405 ꢀC, 405 ꢀC, 310 ꢀC,
and 330 ꢀC, respectively. The light-off temperatures recorded for
all the catalysts are reported in Table 1. Bimetallic catalysts
prepared with equal ratios (0.5 Pd : 0.5 Pt) were found to be less
active than the reference monometallic Pd catalyst. Pt sup-
ported on ZrO2 was the least active and could not achieve 100%
conversion even at 700 ꢀC. YSZ-supported catalysts did not show
any promotional effect in lowering the light-off temperature.
However, relatively high activity was recorded when the
temperature was increased. Signicantly, 100% conversion was
achieved at 360 ꢀC. The methane conversion is directly relevant
to the intrinsic reaction kinetics and mass transfer. Experi-
mentally, it was validated that methane conversion presented
Fig. 2 Light-off temperature profile of C2H4 oxidation over the synthe-
sized catalysts (mass of catalyst: 0.025 g; total flow rate: 33.33 mL minꢁ1
;
an inverse proportion to the space velocity, and the results are 1000 ppm C2H4/22% O2 in balance Ar; GHSV: 80 000 mL gcatꢁ1 hꢁ1).
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can be explained by the exothermicity of ethylene oxidation. The
C2H4 oxidation to CO2 was completed when CH4 oxidation
began below 180 ꢀC. C2H4 locally increased the catalyst bed
temperature; thus, the CH4 was activated at a low temperature.
At relatively high temperatures, the reaction rates were
controlled by methane combustion, and, therefore, no prom-
inent effect was observed in the presence of C2H4.
Active surface area
Metal dispersion is typically determined using H2, O2, or CO
chemisorption because these methods are commonly avail-
able and can be applied to highly diluted catalysts.48 Table 2
shows the dispersion and metal surface area of the prepared
catalysts. X-ray uorescence spectroscopy (XRF) was used to
quantify the impregnated metal components on all the
prepared catalysts. The metal dispersion, specic metal
surface area and average particle size of Pd, Pt and PtPd were
measured by CO chemisorption method. The metal disper-
sions were: Pd/YSZ(6.73), Pd/ZrO2(6.37), Pt/YSZ(9.71), Pt/
ZrO2(5.4), PtPd/YSZ(12.2), and PtPd/ZrO2(4.27). The results
showed that metals were well dispersed on the support surface
during the impregnation step. Y2O3 addition to ZrO2 increased
the dispersion. It was noted that all the metals deposited on
the YSZ supports were smaller than those on the ZrO2
supports. Particle size inuenced the rate of reaction, and, for
dispersed catalysts, the rate of reaction was proportional to the
exposed active sites. The catalytic activities of the prepared
catalysts were in good agreement with the dispersion of Pd, Pt,
and Pd–Pt supported on YSZ and ZrO2.
Fig. 3 Conversion profiles for CH4 oxidation over the synthesꢁiz1ed
catalysts (mass of catalyst: 0.025 g; total flow rate: 33.33 mL min ;ꢁ11
volume% CH4 in N2, 1000 ppm C2H4, 20% O2; GHSV: 80 000 mL gcat
h
ꢁ1).
have been extensively reported for lean conditions. To improve
the performance, bimetallic catalytic systems are preferred for
the oxidation of mixed hydrocarbon feeds.
Catalytic performance for mixed hydrocarbon feed
combustion. Exhaust gas is composed of a wide range of
hydrocarbons; however, two of the major constituents with high
heat sink are methane and ethylene. Therefore, the co-oxidation
of CH4 and C2H4 was studied on all the prepared catalysts by
adding 1000 ppm of ethylene/Argon as diluter gas, which is
quantitatively 0.03% of the total fuel (1.0 vol% CH4) in feed gas.
The low concentration of C2H4 was fully converted <99% below
160 ꢀC. Fig. 3 shows the light-off temperature curves for CH4
with low concentration of C2H4. Notably, the light-off temper-
ature of pure CH4 was signicantly shied to relatively low
temperatures for the Pd/YSZ catalyst. The Pd/ZrO2 catalyst
showed no prominent effect. However, a slight increase in T90%
temperature was evident. This may be due to the deactivation of
Pd/ZrO2. This deactivation may be ascribed to the reaction of
water and sintering due to high temperature. In the presence of
C2H4, the CH4 light-off temperature shied to relatively low
temperatures over YSZ-supported Pd, Pt, and Pd–Pt. It was
recorded that the T10% of CH4 reduced to 30 ꢀC on a scale in the
presence of C2H4. This superior conversion at low temperatures
Phase identication by XRD
Fig. 4 shows the XRD patterns of the support and pristine
catalysts. YSZ exhibited the tetragonal phase of pure zirconia.
No evidence of Y2O3 was observed by XRD. Fig. 4(a–d) shows the
diffraction patterns for the YSZ support and Pd, Pt, and Pd–Pt
supported on the YSZ catalysts. The peaks at 30.2 [101], 50.2
[112], and 60.2 [211] correspond to the blueprints of the
tetragonal phase of ZrO2 in the joint committee on powder
diffraction standards (JCPDS). The pristine and spent catalyst
(see Fig. S2†) XRD patterns did not show any peaks for Pd and Pt
metals. Fig. 4(e and f) presents the diffraction patterns for the
ZrO2 support and Pd, Pt, and Pd–Pt supported on the ZrO2
catalysts. The ZrO2 support and catalysts only presented the
monoclinic phase. The peaks at 24.6, 28.6 (111), and 31.9 (111)
Table 2 CO chemisorption and XRF quantification for the pristine catalysts
CO adsorption
Dispersion
(%)
Particle size
(nm)
Catalyst (pristine)
(cm3 gꢁ1
)
Metal Sa (m2 gꢁ1
)
Metal percentage (%)
Pd/ZrO2
Pd/YSZ
Pt/ZrO2
Pt/YSZ
PdPt/ZrO2
PdPt/YSZ
0.13
0.14
0.06
0.12
0.09
0.20
6.37
6.73
5.4
9.71
4.27
12.2
0.28
0.30
0.13
0.26
0.19
0.42
14.7
12.9
17.47
9.72
26.2
1.01
0.98
0.98
1.10
0.4 : 0.52
0.50 : 0.50
7.6861
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Fig. 4 X-ray diffraction patterns of supports and pristine catalysts
calcined at 600 ꢀC for 3 h in air. (a) YSZ (b) Pd/YSZ (c) Pt/YSZ (d) PdPt/
YSZ (e) ZrO2 (f) Pd/ZrO2 (g) PdPt/ZrO2 and (h) Pt/ZrO2.
Fig. 5 STEM and EDX elemental mapping (a and b) PdPt/YSZ, (c and d)
PdPt/ZrO2.
O2-TPD analysis
correspond to JCPDS card no. [JCPDS 37-1484], which was
assigned to the pure monoclinic phase of ZrO2. There was no
evidence of Pd, Pt, or alloy detected in the XRD patterns. This
indicated that the impregnated metals were uniformly
dispersed in the support macro and microporous matrix.
Another possible reason was the low weight percentage of Pd
and Pt in the catalyst. However, the presence of Pt and Pd was
conrmed by XRF and TEM EDX color mapping (shown in
Fig. 5). It was conrmed that Pt and Pd were present in the
alloyed form. No evidence of individual metal was found in the
TEM scan.
CH4 oxidation is strongly reliant on the oxidation state of the
active metal in the catalyst. There are many studies on the active
phase of the metal for C–H bond activation. There is
a consensus that PdO is highly active. In this study, PdO was
closely investigated, where Pd–PdO and Pt–PtO decomposition
and CH4 oxidation were compared. To completely understand
the CH4 oxidation, O2-temperature-programmed desorption
(O2-TPD) analysis was conducted. The products were analyzed
using a TCD and an in-line quadrupole mass spectrometer. The
TCD signals were matched with the MS signals to conrm the
species for quantitative analysis. The O2-TPD signals were
deconvoluted to determine the different temperatures at which
the PdO species decomposed to metallic Pd, as shown in Fig. 6.
Notably, multiple PdO decomposition temperatures could be
resolved. As shown in Fig. 6(a), the Pd/YSZ catalyst showed the
highest quantities of the PdO species. The initial peak was
The surface areas of the pristine and spent catalyst were
measured by the Brunauer–Emmett–Teller (BET) method using
krypton and nitrogen as adsorbate gases; the results are given in
Table S1.† As shown in Fig. S3,† the N2-isotherms for ZrO2 and
Pd, Pt, and Pd–Pt supported catalysts showed that the samples
exhibited type-IV isotherms with H4 hysteresis loops in the
relative pressure range of 0.5–1.0 P/Po, indicating mesoporous
(intraparticle) structures. The YSZ support showed type-II
adsorption–desorption isotherms, indicating that YSZ had
a mesoporous structure. It also showed hysteresis (H3);
however, the disordered desorption isotherm showed that the
pore structure was not well dened or uniform. The pore size
distribution calculated by the Barrett–Joyner–Halenda (BJH)
method was 12.1 nm. The BET surface area of the YSZ support
was 9.0 m2 gꢁ1. Aer the impregnation and calcination at
ꢀ
600 C, the surface area did not change signicantly. Notably,
the pores were mesopores and the intraparticle spaces were
void, reducing the diffusion barriers when the reaction was
performed. As the combustion reaction generates moisture,
CO2, during the reaction, catalysts with fewer diffusion limita-
tions provide rapid transport of reactants and products. Similar
behaviors were reported for the Pd/ZrO2 catalysts, where the
catalysts with relatively high pore diameters of the support
materials displayed better performance.33,49
Fig. 6 PdO decomposition analysis results using O2-TPD. (a) Pd/YSZ,
(b) Pd/ZrO2, (c) PdPt/YSZ, and (d) PdPt/ZrO2.
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for comparison purposes. Fig. 7(a) shows the spectrum for Pd/
YSZ, and Fig. 7(b) shows the spectrum for the Pd/ZrO2 cata-
lyst. The broad peaks with doubleheaders at 2225–2360 cmꢁ1
were assigned to CO2. The sharp peak at 3016 cmꢁ1 corre-
sponded to CH4. The area highlighted with dotted rectangle was
assigned to the hydroxyl region, which ranged from 3200 to
3800 cmꢁ1. It was difficult to resolve the ne IR signals for OH
groups because of the inferior signal-to-noise ratio on YSZ and
ZrO2 supported on active metals (Pd, Pt). The small low-
intensity signal at 3734 cmꢁ1 may represent the Pd–OH
masked by the OH signals of the YSZ carrier of Pd. Pd/ZrO2
presented broad signals in the 3200–3800 cmꢁ1 region. A
similar study was conducted to investigate the OH peaks on Pd
supported on Al2O3. Signals at six wavenumbers were recorded.
The signals at 3770, 3723, and 3680 cmꢁ1 were allocated to
hydroxyl on the support materials (Al2O3) and the three signals
at wavenumbers 3556, 3697, and 3733 cmꢁ1 were attributed to
OH on PdO.50 Ciuparu et al. also conrmed the same wave-
numbers assigned to OH signals associated with metal oxides.51
The signals at 3634 and 3628 cmꢁ1 were assigned to terminal
OH species that are attached to a single atom.14
Fig. 8 shows the results of the DRIFT analysis for the Pd–Pt
catalysts supported on YSZ and ZrO2. OH accumulation was
minimized in the alloy catalysts on both the supports. Never-
theless, the alloy catalyst did not show superior catalytic
performance to Pd supported on YSZ or ZrO2. The –OH groups
that can poison the catalyst via the formation of Pd(OH)2 were
mainly derived from the reaction. Pd supported on YSZ was
resistant to OH accumulation. However, the ZrO2-supported
catalyst showed gradual increases in –OH intensity, which was
considered to be the reason for their deactivation in relatively
long-duration reactions.
Fig. 7 DRIFT spectra for (a) Pd/YSZ and (b) Pd/ZrO2. The analyses were
performed at a flow of 2% CH4/N2 and O2 (20 vol%) at temperatures of
250 ꢀC (i–iii), 350 ꢀC (iv–vi), and 550 ꢀC (vii–ix) in a continuous flow
(33.33 mL minꢁ1) heated cell.
observed at 492 ꢀC, and there was a high-intensity peak at
594 ꢀC with small shoulders at 517 ꢀC and 551 ꢀC. Fig. 6(b)
shows low-intensity peaks at 471 ꢀC, 555 ꢀC, 621 ꢀC, and 625 ꢀC
for Pd/ZrO2. As shown in Fig. 6(c), Pd–Pt/YSZ showed peaks at
413 ꢀC and 703 ꢀC. Pt–Pd/ZrO2 showed a considerably low-
ꢀ
ꢀ
ꢀ
intensity peak at 361 C and two peaks at 417 C and 453 C.
No evidence of oxygen from PtO decomposition was observed in
the TCD and MS signals in the temperature range of 25–850 ꢀC.
Therefore, it may be concluded that CH4 oxidation is more
favored for PdO states and Pt metal is the active state for C2H4
oxidation. As the temperature increased, Pd started to provide
active sites for the C2H4 activation, and similarly, Pt metal
started to partially oxidize and behave as an active component
for the CH4 oxidation. Generally, Pt and Pd alloy resist the
formation of oxides, but with increasing temperature the metals
start to segregate and some PdO–Pd or PdO–Pt sites emerge,
which can activate the C–H bond and result in the oxidation of
CH4 to CO2. Most of the catalytic processes are redox or electron
transfer processes, where the metal–substrate has to act as
a donor–acceptor couple, and the metals can accept or donate
electrons to form intermediate radicals that quickly form
products. The redox properties of the ZrO2 support increased
signicantly when it was doped with Y2O3, resulting in more
PdO species with varying bond strengths. Overall, this doping
resulted in the release of more oxygen from the Pd catalyst.
Cyclic stability test
To validate the promotional effect of Y on ZrO2 and Pt on Pd
toward water resistance and deactivation prevention, experi-
ments in cyclic (dry 4 wet) patterns were performed. The
reaction time was prolonged up to 30 h at a GHSV of 80 000 mL
gcatꢁ1 hꢁ1 in a continuous run. Fig. 9 shows the time on stream
activity data for 1% CH4/1000 ppm C2H4 combustion. To
generate wet conditions, 12 vol% water in the form of steam was
In situ DRIFT analysis
Diffuse reectance infrared Fourier-transform (DRIFT) analysis
was performed using a Thermo Nicolet 6700 Fourier-transform
infrared (FTIR) machine in diffusive reectance mode. The
FTIR machine was mounted with a high-temperature stainless
steel reaction cell. The sample cell was covered with a dome-like
cap, which had KBr windows and a liquid nitrogen-cooled
detector. The DRIFT analysis was conducted using wave-
numbers in the range of 800–4000 cmꢁ1 at the spectral resolu-
tion of 4 cmꢁ1. To measure the accurate temperature, a K-type
thermocouple was placed inside the sample holder. MKS
mass ow controllers were connected for the controlled ow of
all the gases. All the catalyst samples were degassed and pre-
heated before the analysis. The same feed gas compositions
Fig. 8 DRIFT spectra for (a) Pd–Pt/YSZ and (b) PdPt/ZrO2. The anal-
yses were carried out at a flow (33.33 mL minꢁ1) of 2% CH4/N2 and O2
(20 vol%) at temperatures of 250 ꢀC (i, ii, iii), 350 ꢀC (iv, v, vi), and 550 ꢀC
were used during the DRIFT analysis and in the quartz reactor (vii, viii, ix) in a heated cell.
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ꢃ Bimetallic catalysts supported on YSZ are more resistant to
deactivation in wet feed gas condition than ZrO2-supported
bimetallic catalysts. The resistance to deactivation is associated
with the YSZ promotional effect rather than the Pt addition to
the Pd catalyst.
ꢃ In situ DRIFT analysis shows OH species on the surface of
both ZrO2- and YSZ-supported catalysts. During CH4 oxidation
at 350 ꢀC, however, the desorption of OH is slow on the Pd/ZrO2
catalysts, and accumulation is noticed on extended durations,
which could be the reason for the deactivation. The steady-state
accumulation of hydroxyl is recorded over the bimetallic cata-
lysts. It might be because the water effect is small for alloy
catalysts or due to the relatively low water of reaction since T50
ꢀ
and T100 are relatively high (400–450 C).
Fig. 9 Cyclic stability test results for the prepared catalysts under dry
and wet (12 vol% H2O) conditions. Feed composition: 1.0 vol% CH4,
100ꢁ01ppm C2H4, 22% O2 diluted with nitrogen at a GHSV of 80 000 mL
Further investigation is required to optimize the Pd/Pt ratio.
Many reports claim that relatively high Pt loading causes
adverse effects and an increase in light-off temperature, but
relatively low Pt loading has a promotional effect and causes
resistance to deactivation in wet feed conditions.
gcat hꢁ1
.
added along with the HCs for these experiments. As seen in
Fig. 9, the catalytic activities of the catalysts supported on ZrO2
were progressively degraded when the reaction environment
was switched from dry to wet feed conditions. The Pd catalyst
supported on YSZ showed promising activity throughout the
cycling, and no prominent degradation in activity was observed
for both reaction environments. The Pd–Pt catalyst supported
on both YSZ and ZrO2 showed a steep decrease in activity. This
showed that the addition of Pt at an equal ratio to Pd did not
inhibit the water effect since Pt is very sensitive to water at
relatively low temperatures. In this study, the deactivation of
Pd–Pt/YSZ and Pd–Pt/ZrO2 was likely due to the Pt component
and not because of Pd, since Pd in the monometallic catalyst
remained stable for the entire duration. The Y2O3 promotional
effect on the stability of the catalysts was prominent on the
tested catalysts. However, the addition of Pt to Pd/ZrO2 and Pd/
YSZ catalyst did not inhibit the deactivation.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
This work was funded by Competitive Research Grant (CRG-
3402) awarded by the Office of Sponsored Research (OSR) at
King Abdullah University of Science and Technology (KAUST).
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