Experimental and theoretical investigation of oxidative methane activation on Pd-Pt catalysts

Article abstract of DOI:10.1039/C9RA00735K

Density functional theory (DFT) and measurements of rate are used to provide evidence for the rate determining step (RDS) and requirements of the active site for CH₄ combustion on Pd-Pt bimetallic catalysts in five different distinct kinetic regimes. These five regimes exhibit different rate equations for methane combustion due to the reaction rate constants and diverse dominant adsorbed species for these different kinetically relevant steps. Oxygen chemical potential at the Pd-Pt surface was replaced by oxygen pressure, reflecting the kinetic coupling between C-H and O═O bond cleavage steps. C-H bond cleavage occurs on different active sites in five of these kinetic regimes, evolving from vacancy-vacancy (*-*) to oxygen-vacancy (O*-*), oxygen-oxygen (O*-O*) site pairs, monolayer Pd-O, and ultimately to oxide bulk with Pd-O site pairs as the oxygen chemical potential increases. It is easier to form a metallic surface at low oxygen pressure, implying minimal O* coverage. The sole kinetically relevant step on uncovered Pd-Pt surfaces for methane combustion is O═O bond cleavage. The supply of oxygen is obviously more important than the supply of methane in regime (I). As vacancies become less available on metallic surfaces, C-H bond cleavage occurs via O*-* paired sites, the energy barrier of which is much higher than that on uncovered Pd-Pt surfaces. In this regime (II), O═O bond cleavage is still an irreversible process because O* will be consumed by the rapidly formed products of methane dissociation. For the oxygen saturated surfaces in regime (III), C-H bond cleavage occurs on two adjacent adsorbed oxygens that form OH and weak CH₃-O bond interactions, resulting in a low activity for methane combustion. On the oxidation surfaces (IV and V), exposed metal atoms and their adjacent exposed lattice oxygen were the active sites, leading to a large decrease in C-H bond cleavage energy barrier, deduced from both experiment and theory. The increase of the metallic oxide thickness (increase of oxygen potential) increases the methane combustion turnover rates on Pd-Pt catalysts.

Full text of DOI:10.1039/C9RA00735K

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Experimental and theoretical investigation of
oxidative methane activation on Pd–Pt catalysts†
Cite this: RSC Adv., 2019, 9, 11385
abc
a
a
a
Zehao Huang, Zheming Chen, Lijuan Fu and Zhigang Zhanga
*
*
Wenjie Qi,
Density functional theory (DFT) and measurements of rate are used to provide evidence for the rate determining
step (RDS) and requirements of the active site for CH4 combustion on Pd–Pt bimetallic catalysts in five different
distinct kinetic regimes. These five regimes exhibit different rate equations for methane combustion due to the
reaction rate constants and diverse dominant adsorbed species for these different kinetically relevant steps.
Oxygen chemical potential at the Pd–Pt surface was replaced by oxygen pressure, reflecting the kinetic
coupling between C–H and O]O bond cleavage steps. C–H bond cleavage occurs on different active sites
in five of these kinetic regimes, evolving from vacancy–vacancy (*–*) to oxygen–vacancy (O*–*), oxygen–
oxygen (O*–O*) site pairs, monolayer Pd–O, and ultimately to oxide bulk with Pd–O site pairs as the oxygen
chemical potential increases. It is easier to form a metallic surface at low oxygen pressure, implying minimal
O* coverage. The sole kinetically relevant step on uncovered Pd–Pt surfaces for methane combustion is
O]O bond cleavage. The supply of oxygen is obviously more important than the supply of methane in
regime (I). As vacancies become less available on metallic surfaces, C–H bond cleavage occurs via O*–*
paired sites, the energy barrier of which is much higher than that on uncovered Pd–Pt surfaces. In this regime
(II), O]O bond cleavage is still an irreversible process because O* will be consumed by the rapidly formed
products of methane dissociation. For the oxygen saturated surfaces in regime (III), C–H bond cleavage
occurs on two adjacent adsorbed oxygens that form OH and weak CH3–O bond interactions, resulting in
a low activity for methane combustion. On the oxidation surfaces (IV and V), exposed metal atoms and their
adjacent exposed lattice oxygen were the active sites, leading to a large decrease in C–H bond cleavage
energy barrier, deduced from both experiment and theory. The increase of the metallic oxide thickness
(increase of oxygen potential) increases the methane combustion turnover rates on Pd–Pt catalysts.
Received 28th January 2019
Accepted 23rd March 2019
DOI: 10.1039/c9ra00735k
rsc.li/rsc-advances
relative to the initial CH4 adsorbed states, while the energy
barrier for initial C–H bond cleavage on Pd(111) is a much larger
value of 92 kJ molꢀ1.2 Actually, some other metallic surfaces also
1. Introduction
Pd catalysts are widely used in emission reduction and energy
transformation due to their considerable high activity for
methane combustion. Although the reaction of methane
oxidation has been studied for decades, the oxidation state of
the active surfaces and its detailed mechanism are still being
discussed. The transformation of PdO to metallic Pd leads to
lower activity at high temperature, implying that PdO has a very
essential role in the complete combustion of methane. For
instance, Antony et al.1 reported that the initial C–H bond
cleavage barrier for CH4 is very low with a value of 55.2 kJ molꢀ1
exhibit a higher activation energy for initial C–H bond cleavage
of CH4 than that of PdO(101), such as Pt(111) with a value of
90 kJ molꢀ1 (ref. 2) and Ru(0001) with a value of 83 kJ molꢀ1
.
3
Besides, PdO(101) even showed high selectivity for initial C–H
bond cleavage of propane at temperatures between ꢁ150 and
200 K.4
Although the initial activity of Pd is admirable, poor stability
is a restricting factor for its application. Persson et al.5 found
that the stability of the Pd catalyst is very poor, and its initially
high activity is difficult to maintain during operation. Some
experimental results indicated that water (added or generated)
may cause a signicant irreversible loss in the activity of Pd
catalysts.6,7 However, some other studies showed that H2O
removal could lead to the best recovery of initial activity for
methane combustion.8,9 Pt is also a good catalyst for methane
combustion, although it has a low activity under the conditions
of lean combustion. Therefore, it has quite different chemical
characteristics to Pd catalysts.10,11 Meanwhile, Pt has a better
water and sulfur poisoning resistance capability, as compared
aKey Laboratory of Advanced Manufacturing Technology for Automobile Parts,
Ministry of Education, Chongqing University of Technology, Chongqing 400050,
China. E-mail: wenjieqi@cqut.edu.cn; fulijuan@cqut.edu.cn
bDepartment of Chemistry, Fujian Province University Key Laboratory of Green Energy
and Environment Catalysis, Ningde Normal University, Ningde, 352100, People's
Republic of China
cKey Laboratory of Low-grade Energy Utilization Technologies and Systems, Ministry of
Education of PRC, Chongqing University, Chongqing 400044, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra00735k
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with Pd. The Pd oxidation state is a very important parameter states were determined by using the method of linear synchronous
for methane combustion on Pd/Al2O3.12 Meanwhile, the super- transit/quadratic synchronous transit (LST/QST).19
cial oxides of Pt also have a high activity for methane
combustion, which was also predicted.13
According to previous results of XRD patterns, PdxPt1ꢀxO is the
only bulk oxide, namely some Pd atoms were replaced by the same
Some previous studies14 have shown that bimetallic Pt–Pd number of Pt atoms.9 To investigate the thermodynamic stability
catalysts have better catalytic activities in the presence of water. of PdxPt1ꢀxO, its Gibbs free energy of formation was calculated
Actually, the activity of bimetallic Pd–Pt catalysts for methane with DFT. Even though the transformation processes of the metal
combustion has been observed to increase with time when the to metallic oxide are difficult to calculate, transformation
Pt content reaches up to 35%.5 Maione et al.15 initially consid- temperature is easy to acquired via the calculation of the Gibbs
ered that Pd–Pt clusters undergo less distinct sintering, leading free energy of formation. The approximation form for the Gibbs
to a higher thermal stability compared to pure Pd clusters. free energy of formation of PdxPt1ꢀxO can be given by:20,21
Besides, the existence of Pt4+, Pd4+ and a metallic PdPt alloy in
DGðT; p; xÞ ¼ Eoxide ꢀ ½xEPd þ ð1 ꢀ xÞEPtꢃ
Pd–Pt/Al2O3 has also been identied.15 Mixed Pd–Pt oxide
ꢀ
ꢁ
phases cannot be excluded, although they have never been
observed, which could also explain the reason why the surface
reactions on Pd and Pd–Pt catalysts are different. Besides, the
bimetallic PdPt alloys possess high stability under reducing
conditions.16 Furthermore, gas composition is another impor-
tant aspect that has inuences on the durability and activity of
Pd catalysts. Bugosh et al.17 concluded that the methane
combustion rate on Pd–Pt bimetallic catalysts strongly depends
on O2 concentration, especially near the stoichiometric point.
Therefore, the inuence of O2 concentration on the activity of
Pt–Pd catalysts is investigated in this study.
The kinetic mechanisms of methane combustion and the
selectivity of chemisorbed oxygen in methane activation on Pd
and Pt catalysts have been widely studied. However, the kinetic
mechanisms of methane combustion on Pd–Pt bimetallic
catalysts have rarely been reported, especially for the case under
variable CH4–O2 conditions. In this paper, the effects of varying
oxygen pressure from 0 to 30 kPa at methane pressures of 2, 3,
and 4 kPa are studied. Furthermore, the kinetic consequences
of C–H bond cleavage for CH4–O2 catalysis on Pd–Pt catalysts
are investigated. Also, the cases of increasing oxygen pressures
are studied. Five kinds of catalysts with different amounts of Pt
and Pd via both theory and experiment were analyzed, obtain-
ing distinct reaction pathways for methane C–H bond cleavage.
À
Á
p
p0
ꢀ0:5 EO þ m~O p0; T þ kB ln
2
2
ꢂ
ꢂ
ꢃ
ꢃ
x
þTkB x ln
þ lnð1 ꢀ xÞ
(1)
1 ꢀ x
where Eoxide is the total energy of the mixed oxide per metal
atom. EPt and EPd are the energies of a bulk Pt and Pd atom,
respectively. The last two terms are the oxygen chemical
potential and the mixing entropy, respectively. Besides,
m~O (p0,T) is the chemical potential, which is measured at stan-
2
dard atmospheric pressure. The difference of the standard
oxygen chemical potential is estimated by other research as:21
Dm~O (p0,T) ¼ ꢀ0.1159T + 10.0775
(2)
2
2.2 Catalyst synthesis and analysis
Bimetallic catalysts were prepared by incipient wetness
impregnation of g-Al2O3 with mixed solutions of PtCl4 and
Pd(NO3)2. Using the incipient wetness impregnation method,
Pd–Pt/g-Al2O3 bimetallic catalysts with varied compositions
were prepared: Pd1.0Pt0/g-Al2O3, Pd0.75Pt0.25/g-Al2O3, Pd0.5Pt0.5
/
g-Al2O3, Pd0.25Pt0.75/g-Al2O3 and Pd0Pt1.0/g-Al2O3. The catalysts
achieved a loading of around 188 mmol metallic atoms per gram
of catalyst power. Then, all samples were calcined at 923 K aer
drying for over 10 h in air.
2. Methods
2.1 Details of computation
A thermogravimetric analyzer (TG 209F3) was used to
measure the decomposition and reformation of these metallic
oxides. Inductively coupled plasma atomic emission spectros-
copy (ICP-AES) was used to determine the amounts of metals on
these catalysts. The dispersion of Pd–Pt bimetallic clusters in
these samples was determined by CO adsorption measure-
ments. The measured and following calculated values of these
catalysts are summarized in Table 1. The Pd–Pt dispersion of
these catalysts was used to calculate the pre-exponential factor
and apparent activation energy for methane combustion.
In this study, the generalized gradient approximation (GGA) and
the Perdew–Burke–Ernzerhof (PBE) functional were adopted to
perform the DFT calculations. Cambridge Sequential Total Energy
Package 8.0 (CASTEP 8.0) program package18 was used to deter-
mine the most stable structure and obtain the activation energy.
The optimal geometry, accurate simulation and electronic struc-
tures were acquired with a 380 eV plane wave cut-off energy. 6 ꢂ 6
ꢂ 1 and 6 ꢂ 6 ꢂ 6 Monkhorst–Pack k-points were used for
geometry optimization of the surfaces and the primitive cells,
respectively. Besides, the ultraso pseudopotential (US-PP) was
adopted to describe the interactions between core and valence
electrons. Fermi smearing with a value of 0.2 eV was utilized in this
study. The energy convergence criteria for energy, self-consistent
eld, maximum displacement and maximum force, respectively,
3. Results and discussion
3.1 Metallic oxide-to-metal transformation of Pd–Pt catalysts
with values of 2.0 ꢂ 10ꢀ5 eV per atom, 2.0 ꢂ 10ꢀ6 eV per atom, 2.0 20% O2/N2 was used in the phase transformation experiments for
ꢂ 10ꢀ3 A and 0.05 eV A were utilized. Finally, the transition all catalysts. A continuous ow of the gas mixture at 200
ꢀ1
˚
˚
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Table 1 Actual metal loading, CO uptake and metal cluster dispersion of the different catalysts
mmol CO uptake
Pd–Pt dispersion (%)
from CO uptake
Average cluster
diameter (nm)
Sample
Actual wt% Pd
Actual wt% Pt
per g catalyst
Pd1.0Pt0
2.0
1.7
1.0
0.6
0
0
9.7
11.3
9.8
10.1
15.1
9.3
9.4
7.2
6.0
8.9
12.0
11.6
15.8
18.7
12.5
Pd0.75Pt0.25
Pd0.50Pt0.50
Pd0.25Pt0.75
Pd0Pt1.0
0.9
1.8
2.5
3.5
ml minꢀ1 was passed over the samples and the variance rate of upon rising temperature. Signicantly, the oxygen uptake and
temperature was 5 K minꢀ1. The sample was heated from room release for the Pt-rich catalysts are negligible compared to the
temperature to 1223 K, cooled to 373 K, then heated again to 1223 other catalysts, reecting that the Pt atoms in Pt enrichment
K, and nally cooled to 373 K. The rst and second cycles were catalysts will hinder the oxidation of Pd.
mainly used to clean up the surface hydroxyls as well as adsorbed
The optimized geometries of the oxide phases PdxPt1ꢀxO
water vapor (see ESI, Fig. S1†). Therefore, the third cycle was used with x ¼ 0, 0.25, 0.5, 0.75 and 1 are shown in Fig. 2. For each
to analysis the phase transformation. The third cycle of these composition and each type, the total energies of all super-cells
catalysts is shown in Fig. 1. For the Pd catalyst, a typical hysteresis were calculated to acquire the minimum energy congura-
curve for Pd catalysts can be observed in the PdO / Pd / PdO tions. The minimum formation energies of the PdxPt1ꢀx
O
transformation when sequentially increasing the temperature conguration at 0 K were calculated to evaluate the thermo-
and then cooling in air, as shown in Fig. 1a. Some scientists22,23 dynamic stability. Although some previous reports showed
also reported that the hysteresis was also observed in the reaction that Pt3O4 and PtO2 phases are thermodynamically stable, only
of methane combustion on Pd. The weight of the air exposed PdxPt1ꢀxO phases are detected by XRD, probably due to kinetic
catalyst starts to decrease when the temperature reaches around reasons.9 At the oxygen pressure of 20 kPa, PdO decomposes
1080 K, reecting the onset of the PdO / Pd transformation.
directly into metallic Pd at temperatures higher than ꢁ1056 K
In the case of Pd0.75Pt0.25, the hysteresis still exists. However, (Fig. 3), which is consistent with the experimental value of
the onset transformation temperature decreased to 1020 K, 1080 K. Transformation temperature decreased with the
which is lower than that for the Pd catalyst. The onset trans- increment of Pt content, and the trend is consistent with the
formation temperature decreased with the increment of Pt experimental results. Also, a high O2 pressure (oxygen chem-
content. Besides, the Pd–Pt bimetallic catalyst with a higher Pt ical potential) aids the formation of metallic oxide
content exhibited a higher oxygen release onset temperature (PdxPt1ꢀxO).
Fig. 1 Third cycle: a TG curing of the catalysts in a flowing 20% O2 and 80% N2 stream at a constant rate of 5 K minꢀ1. (a) Pd1.0Pt0, (b) Pd0.75Pt0.25
,
(c) Pd0.5Pt0.5, (d) Pd0.25Pt0.75, (e) Pd0Pt1.0, and (f) a pellet of g-Al2O3 without Pd or Pt content. The plots of the first and second cycles are shown in
Fig. S1.†
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Fig. 2 Atomic super-cells of PdxPtxꢀ1O. Blue, green, and red spheres represent Pt, Pd, and O atoms, respectively.
kPa) and methane (2, 3 and 4 kPa) pressures. Fig. 4 shows the
rst order rate constants for methane combustion as a function
of O2 pressure on these catalysts. A rough judgment shows that
turnover rates of methane are proportional to methane pres-
sure. For the Pd-containing catalysts, the linear correlation
between the rst order rate constants and O2 pressure was
established under the condition of a high O2 pressure (over 2–6
kPa). However, this conclusion is no longer suitable for the Pt-
containing catalysts at low pressure (below 2–6 kPa), and the
experimental data at low O2 pressure are irregular.
The turning point of the rst order rate constant for these
catalysts reects the different kinetic consequences and the
catalytic surfaces at different oxygen chemical potentials. For
the Pt-containing catalysts, the rst order rate constant as
a single valued function of O2/CH4 ratio (for an oxygen pressure
of Pt-containing catalyst in the range of 0 to ꢁ4 kPa) can be
written as follows:
3.2 Kinetic dependence of CH4–O2 reactions
For all the experimental conditions, to achieve the same space
velocity of 48 000 hꢀ1, a continuous ow of gas mixture at 200
ml minꢀ1 was passed over 0.17 g of catalyst and g-Al2O3 powder.
The catalyst powders were mixed uniformly with 30 times their
amount of g-Al2O3 to rule out inter-particle heat and mass
transfer restrictions. The catalysts were held in place between two
layers of quartz wool and a K-type thermocouple was inserted in
the packed catalyst bed. All catalyst and g-Al2O3 powder mixtures
were heated at 723 K with a 10% H2/N2 feed for 1 h, and then held
in owing 100% N2 for 1 h before pumping the CH4/O2/N2 gas
mixture. Every working condition was maintained for 30 min
before the measurement, then another gas component was
added and this procedure was repeated. Concentrations of the
exhaust gas were measured with gas chromatography.
The reaction rates for methane combustion were acquired
via kinetic measurements over a broad range of oxygen (0–30
ꢄ
ꢅ
A
rCH
O2
4
¼ kI;app
(3)
(4)
CH4
CH4
ꢄ
ꢅ
ꢄ
ꢅ
rCH
O2
4
ln
¼ ln kI;app þ A ln
CH4
CH4
where A represents the slope of the straight line shown in Fig. 5.
Goodness of Fit (R2) in these two regimes, I and II, for the Pt-
containing catalysts was over 0.82. The O2 (a) and CH4 (b)
dependence of the rates is listed in Table 2.
The region in which the rst order rate constant increases
with O2/CH4 ratio indicates that the Pt-containing clusters still
maintain a metallic surface. Moreover, the phase and structure
of these metallic clusters did not change over the range of O2/
CH4 ratios, which is dened here as regime I. When the O2/CH4
ratio is greater than a specic value (0.08–0.12 for the Pt-
containing catalysts with different contents of Pt and Pd), the
rst-order rate constant decreases with O2/CH4 ratio. The Pt-
containing clusters remain metallic meanwhile the O*
coverage of these clusters increases with the increase of O2/CH4
Fig. 3 Variation of phase transition temperature for PdxPt1ꢀxO at
different oxygen pressures.
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Fig. 4 First order rate constants for methane combustion as a function of O2 pressure on the catalysts at 2 kPa (B), 3 kPa (-) and 4 kPa (:). (a)
Pd1.0Pt0-723 K, (b) Pd0.75Pt0.25–723 K, (c) Pd0.5Pt0.5-723 K, (d) Pd0.25Pt0.75-723 K, (e) Pd0Pt1.0-723 K, and (f) Pd0Pt1.0-873 K.
ratio that denes regime II. For the Pt catalyst, CO2 was not
detected at the temperature of 723 K even for a high O2 pressure
(30 kPa). However, CO2 could be detected for the temperature of
873 K, suggesting that the rst order rate constant for methane
combustion is independent of CH4/O2. Hence, the reaction
rates for methane combustion are independent of O2 pressure
but proportional to CH4 pressure in regime III (O2/CH4 > 3–4).
For Pd0.75Pt0.25 and Pd0.5Pt0.5, the rst order rate constant
increases with the increment of O2 pressure mainly due to the
growing number of active sites (Pd–O). These kinetic interpre-
tations are consistent with all measured rate data and with
theoretical estimated overall reaction activation barriers that
will be discussed in the next section.
3.3 Rate equations and their micro-kinetic analysis for CH4
combustion on these catalytic surfaces
For Pd and Pt catalysts, the kinetic models for methane
combustion include three types: Eley–Rideal (ER), Langmuir–
Hinselwood (LH) and Mars–van Krevelen (MvK). In the case of
ER mechanisms, an alternative reactant has already been
adsorbed on the metallic surface, and the reaction takes place
between the adsorbed reactant and another reactant in the gas
phase. LH models consider that reactions take place between
carbon compounds and oxygen adsorbed on the metallic
surface. They describe reactant oxidation over metal oxides via
MvK mechanisms. Surface lattice oxygen formed via the disso-
ciation of oxygen from the gas phase. The reactions take place
through alternating oxidation and reduction of the catalytic
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Fig. 5 First-order constant (rCH (CH4)ꢀ1) for methane oxidation as a single valued function of O2/CH4 ratio on (a) Pd0.75Pt0.25, (b) Pd0.5Pt0.5, (c)
4
Pd0.25Pt0.75, and (d) Pd0Pt1.0. “A” represents the slope of fitting a straight line. The left region and right region of the dash line are regime I and
regime II, respectively.
Table 2 The O2 (a) and CH4 (b) dependence of the rates; these values
In our previous study, the activity and stability of methane
combustion on these catalysts under methane lean conditions
were calculated by using eqn (4) and Fig. 5a
were studied, suggesting that Pd0.75Pt0.25 was the most prom-
Regime I
Regime II
ising catalyst.9 Generally, a transition in methane conversion
reects the variation of the different kinetic regimes. Here, the
measured and calculated activation enthalpies and entropies
are compared. These parameters are related to the methane
and/or oxygen dissociation rate, according to:
Catalyst
a
b
a
b
Pd0.75Pt0.25
Pd0.5Pt0.5
Pd0.25Pt0.75
Pd0Pt1.0
1.1
1.0
1.0
1.1
ꢀ0.1
0
ꢀ0.82
ꢀ0.72
ꢀ0.72
ꢀ1.38
1.82
1.72
1.72
2.38
0
ꢄ
ꢅ
ꢄ
ꢅ
ꢀ0.1
kBT
h
DS
DEapparent
koverall
¼
exp
exp
ꢀ
a
The rates of methane oxidation in regimes I and II can be written as
R
RT
rCH ¼ kapp(O2)a(CH4)b, a ¼ A and b ¼ ꢀA + 1.
ꢄ
ꢅ
4
DEapparent
¼ A0 exp
ꢀ
;
(5)
RT
surfaces. Hence, LH and ER models are suitable for Pt while
MvK is likely to occur for Pd since the surface oxidation states
were observed under lean conditions. The mechanisms of O2
dissociation and C–H bond cleavage on these catalytic surfaces
are very different from each other. Elementary steps for CH4
catalytic combustion in different regimes are shown in Table 3.
Six kinds of catalytic surfaces with different oxygen chemical
potentials are shown in Fig. 6. In this paper, the types of active
sites are respectively expressed as *–* (Fig. 6a), *–O* (Fig. 6b
and c), O*–O* (Fig. 6d) and Pd–O (Fig. 6e and f) for C–H bond
cleavage with the increase of oxygen chemical potential. The
optimized geometry of IS, TS and FS for methane and oxygen
dissociation on these surfaces (Fig. S2–S4†) and their parame-
ters (Table S1†) are presented in the ESI.†
where A0 denotes the pre-exponential factor, T is the tempera-
ture, h is the Planck constant and kB is the Boltzmann constant.
Experimental results of the regression tting of the temperature
dependent rate constant for these catalysts are shown in Fig. 7.
Reaction rate equations (eqn (6), (8)–(10)) for these different
kinetic regimes were obtained by the assumption of different
active sites (derived in eqn (S29)†). The measured apparent
activation energies are consistent with the calculated energy
barriers of the overall reactions, which are listed in Table 4.
These apparent activation energies reect the combined effects
of the methane and oxygen activation on the different catalytic
surfaces. Apparent activation energies were calculated from the
reaction rate constants, which were acquired from the reaction
rate expressions in every kinetic regime. In this paper, these
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Table 3 Elementary steps for CH4 catalytic combustion in different regimesa
Rate and equilibrium
constant
Classication
Step
Elementary reaction step
O2 þ *#O*2
O2 dissociation
1.1
1.2
1.3
2.1
2.2
2.3
2.4
k1,1f, k1,1r
k1,2f, k1,2r
k1,3f, k1,3r
k2,1f
O2* + * # 2O*
O*2 þ 2V#2VO þ *
CH4 þ * þ */CH*3 þ H*
CH4 þ * þ O*/CH*3 þ OH*
CH4 + 2O* / CH3O* + OH*
CH4 þ VO þ */CH*3 þ VOH
CH4 dissociation
k2,2f
k2,3f
k2,4f
a
*, V, VO, and VOH represent metallic vacancy, oxygen vacancy, lattice oxygen, and hydroxyl adsorbed on oxygen vacancy, respectively.
simulations and results of these kinetic regimes are reported in since O* will be immediately depleted by other species. O2
the order of increasing O* coverage or oxygen chemical poten- dissociation is irreversible due to the high adsorption
tial. We discuss every kinetic regime and give the kinetic energy of O in this kinetic regime, especially for the clean
interpretation in the below sections.
metallic surfaces (Fig. S3†). Assumptions of irreversible
3.3.1 Methane C–H bond cleavage on metallic surfaces O]O and C–H bond cleavage lead to rates being propor-
without O coverage (*–*, in regime I). For low O2 pressure, tional to oxygen chemical potential (derived in the ESI, eqn
a massive metallic vacancy forms, nevertheless O2 is lacking (S16)–(S19)†).
Fig. 6 Optimized structures of different catalytic surfaces. (a) Pt(111), (b) 0.25 ML O, (c) 0.75 ML O and (d) O* saturated on Pt(111) surfaces, (e)
monolayer PdO(101) on Pt(100) and (f) PdO(101).
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active metallic vacancies were formed for methane and oxygen
dissociation. The calculated activation energy of methane rst
C–H bond dissociation on Pd(111) and Pt(111) is 83 and
78 kJ molꢀ1, respectively. Further, the activation energy
decreases in the case of the Pd–Pt bimetallic substrate atomic
layer (79 and 75 kJ molꢀ1 for Pd/Pd–Pt(111) and Pt/Pd–Pt(111),
respectively). The O]O activation barrier is much lower due to
the high binding energies on clean metallic surfaces, resulting
in a strong endothermal reaction for oxygen dissociation.24
According to eqn (6), the overall reaction activation energy in
regime I can be written as:
DEI ¼ E1,2f + (E1.1f ꢀ E1,1r) ¼ E1.2f ꢀ QO
.
(7)
2
Hence, the calculated overall reaction activation energy on Pt
in regime I is 31 kJ molꢀ1, being close to the measured apparent
activation energy with a value of 5 kJ molꢀ1 (Table 4). Obviously,
this value is much smaller than the value of C–H activation
energy (73 kJ molꢀ1). Therefore, turnover rates for methane
combustion are determined by O2 pressure and independent of
methane concentration. Besides, the modeled Pd/PtPd(111) and
Pt/PtPd(111) surfaces were built as Pd-rich and Pt-rich bime-
tallic catalytic surfaces, respectively. The differences in methane
activation energies at different Pd–Pt (111) facets were minor,
with a value difference of only 4 kJ molꢀ1. Besides, the differ-
ences between methane activation on 1 ML O/Pt/PdPt(111) and
1 ML O/Pt(111) were also compared, with a difference value less
than 4 kJ molꢀ1. Hence, the Pd : Pt molar ratio has little effect
on the activation energy of methane C–H bond cleavage at
similar facets.
3.3.2 Methane C–H bond cleavage on metallic surfaces at
intermediate O* coverage (*–O*, in regime II). The transition in
methane conversion reects an increase in oxygen coverage and
oxygen and chemical potential, which causes a lack of the
paired vacancy *–* and rates to ultimately become limited by
methane activation on O*–*. In this case, rates are given by
(details can be found in the ESI, eqn (S20)–(S23)†):
Fig. 7 Arrhenius plots of the methane first order rate coefficient versus
1000/T for methane combustion on different catalysts. (a) Oxygen
pressure at 2 kPa for Pd0.75Pt0.25,Pd0.5Pt0.5, Pd0.25Pt0.75 and Pt1.0, and
0.5 kPa for Pd1.0; “low metal loading” means that metal loading is
0.35 wt%; (b) oxygen pressure at 20 kPa for all catalysts. R-square
values for these experimental data are larger than 0.93.
2
2k2:2f Pm
2
rO*ꢀ*
¼
(8)
k1:2f K1:1 PO
k1:2f K1:1PO
r*ꢀ*
¼
(6)
2
Apparently, the turnover rates for methane combustion in
this regime decrease with an increment of O2 pressure
(decreasing number of active sites), which has been veried by
experiment. O* coverage increases with increasing oxygen
A 2 ꢂ 2 unit cell consisting of four metallic atom layers was
used to model the M(111) surface (Fig. 6a). In this case, enough
Table 4 Comparison between apparent activation energy and activation energy of the overall reaction: A, B, and C represent pre-exponential
factor, measured apparent activation energy and DFT-calculated activation energy for overall reaction, respectively
Kinetic regime
Reaction rate
I
k
II
2k2:2f Pm
1:2f K1:1 PO
III
k2.3fPm
IV and V
k2.4fPm
2
2
1:2f K1:1PO
2
k
Apparent activation energy
Kinetic parameters A, B(C)
E1.2f ꢀ QO
2E2.2f ꢀ (E1.2f ꢀ QO
)
2
E2.3f
E2.4f
2
Pd1.0
1.5 ꢂ 1010, 145(163)
5.8 ꢂ 105, 65(61)
8.2 ꢂ 105, 62
4.4 ꢂ 105, 62(67)
1.9 ꢂ 107, 101(110)
Pd0.75Pt0.25
Pd0.5Pt0.5
Pd0.25Pt0.75
Pt1.0
92, 8
8.4 ꢂ 106, 87(81)
148, 12
248, 15
52, 5(31)
9.2 ꢂ 106, 89
5.6 ꢂ 106, 79(79)
5.0 ꢂ 106, 81(77)
1.6 ꢂ 1010, 160(175)
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pressure. For low O* coverage (0.25 ML O), methane dissocia- The measured apparent activation energy (81 kJ molꢀ1) for
tion occurs on O*–* with an activation barrier of 118 and methane combustion over Pt in regime III is consistent with the
105 kJ molꢀ1 for Pt(111) and Pd(111), respectively. The atomic DFT calculated value (77 kJ molꢀ1). This kinetic interpretation is
model of 0.75 ML O monolayer was chosen to express the higher also suitable for the low pressure region of methane combus-
oxygen coverage. The adsorption energy of O decreases with the tion on the Pd catalyst.
increment of O* coverage, leading to more active adsorption
3.3.4 Methane C–H bond cleavage on metallic oxide
oxygen atoms (higher adsorption energy of H). Meanwhile, high surfaces (Pd–O, in regimes IV and V). Pd atoms more easily
atomic oxygen coverage increases the coordination number of segregate on the Pd–Pt bimetallic surface compared to Pt and
the metal atoms, which makes the C–H bond activation barriers then a Pd monolayer oxide is formed under the condition of
higher.
high O2 pressure in kinetic regime IV. Our DFT results indicate
In this kinetic regime, the apparent activation energies that monolayer Pd oxide has poor activity, while the activity of
reect the collective effects of the O*2 dissociation barrier, the the two monolayer Pd oxide was greatly improved (110 /
C–H activation energies on O*–* and the heat of O2 adsorption. 67 kJ molꢀ1). Although the monometallic Pt catalyst is hard to
Although their contributions cannot be measured by experi- oxidize at high temperature, the monometallic Pd catalyst can
ments, DFT calculations of methane activation on O*–* could maintain the oxide states even at high temperature.27,28 Exten-
provide signicant evidence in this matter. According to eqn (8), sive research has shown that methane activation takes place on
the overall reaction activation energy in regime II can be given a paired Pd–O site. Bossch et al.29 showed that O2 would be
by:
better adsorbed by Pd atoms, and thereaer O*2 would dissociate
into the oxygen vacancy. Gremminger et al.30 showed that for
Pd–Pt/Al2O3 catalysts under lean combustion conditions, Pd
segregates to the surface of the bimetallic clusters. Barcaro
et al.31 reported that Pt segregation takes place in the (111)
surface while Pd segregation takes place in (100) facets in the
case of the optimal Pd135Pt48 and Pd110Pt91 clusters. The
segregation of Pd in the (100) facets could promote the forma-
tion of PdO(101).32,33 The kinetic parameters in this regime are
difficult to measure due to the interference of Pd oxide for the
Pd-rich catalysts. However, the pre-exponential factor and
measured apparent activation energy of Pd0.25Pt0.75 at high
DEII ¼ 2E2.2f ꢀ (E1.2f ꢀ QO
)
(9)
2
3.3.3 Methane C–H bond cleavage on metallic surfaces
completely covered by O*(O*–O*, in regime III). In the case of
the kinetic regime III, the number of active sites (O*–O*) did not
change; the methane combustion rate was no longer affected by
O2 pressure. Apparent energies in this regime were only deter-
mined by C–H bond activation barriers on O*–O* paired sites.
For the Pt catalyst, the turnover rate of methane ultimately
dropped to a constant value, which did not vary with O2 pres-
sure. Experimental results showed that the number of active
sites no longer changed.25,26 These data indicate that turnover
rates can be described by a simple rate equation in regime III
(see ESI, eqn (S24)–(S26)†):
oxygen pressure are very different to those of Pd0.5Pd0.5
,
Pd0.75Pt0.75 and Pd. Furthermore, the measured apparent acti-
vation energy of Pd0.25Pt0.75 is 101 kJ molꢀ1, which is close to the
value of the calculated value (110 kJ molꢀ1). Hence, the
formation of monolayer PdO for the Pt-rich catalysts leads to the
low activity of methane combustion.
In the case of kinetic regime V, active sites were stable, and
they did not vary with O2 pressure. Hence, the reaction rate of
methane remained at a constant value in this kinetic regime.
Both the O2 dissociation and C–H bond dissociation of methane
on Pd–O pairs are irreversible in regimes IV and V. Therefore,
the reaction rate only related to the partial pressure of methane,
given by
rO*–O* ¼ k2.3fPm,
(10)
where k2.3f is the rate constant for the rst C–H bond cleavage of
methane on O*–O*. According to eqn (10), the overall reaction
activation energy in regime III can be given by:
DEIII ¼ E2.3f
(11)
For the O* saturated surface, paired O*–O* sites are formed
for methane dissociation instead of the paired O*–* and *–*
sites. Chin et al.25 reported that O* coverage depends only on O2
pressure, whether CH4 exists or not. Although the activation
energy barrier for O]O bond cleavage is higher than that for
the C–H bond (228 vs. 175 kJ molꢀ1), the advantage of high
oxygen coverage makes the elementary reaction C–H bond
cleavage, which is the RDS. Clearly, the C–H activation barrier
on O*–O* is higher than that on O*–* due to the weaker C–O
bond strength compared to the C–Pd and/or C–Pt bond,
resulting in the poor activity for methane lean combustion on
the Pt catalyst and rich combustion on the Pd catalyst. Besides,
the apparent energy for the Pd–Pt bimetallic catalysts in kinetic
regime III is difficult to measure even under the condition of
higher temperature due to the interference of regimes IV and V.
rPd–O ¼ k2.4fPm
(12)
Hence, the overall reaction activation energy in regimes IV
and V for the oxide surfaces can be given by:
DEIV,V ¼ E2.4f
(13)
Pd atoms in Pd-containing catalysts more easily segregate to
the surfaces of clusters at high O2 pressure.34 For this reason,
the oxide layers exist at high oxygen chemical potential for the
Pd-containing catalysts. The C–H activation barrier on mono-
layer oxide PdO(101)/Pt(100) is smaller than that on O*–O*
paired sites (110 kJ molꢀ1 vs. 163 kJ molꢀ1 for O* saturated
Pd(111)). However, this value is much higher than that of
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PdO(101) (61 kJ molꢀ1). Besides, the activation energy for two
mono-layers of PdO(101) on Pt(100) is retained with a value of
67 kJ molꢀ1, indicating that activation of Pd-containing cata-
lysts is mainly determined by the two outmost oxide layers. For
the Pd-rich catalysts, multiple Pd oxide layers are easily formed,
resulting in a high activity for methane combustion in the case
of high oxygen pressure.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
Financial assistance from the National Natural Science Foun-
dation of China (Nos. 51276207 and 50876118) is gratefully
acknowledged. We also would like to thank Open Fund of
Fujian Province University Key Laboratory of Green Energy and
Environment Catalysis (No. FJ-GEEC201803).
4. Conclusion
The kinetic consequences of C–H cleavage for CH4–O2 on
bimetallic platinum–palladium catalysts were investigated in
this study. Methane combustion at different O2 pressures was
divided into ve kinetic regimes. These kinetic regimes exhibit
different kinetic dependences and unique rate equations.
Methane activation occurs on two adjacent metal sites (*–*),
metal site and oxygen ion site pairs (O*–*), two adjacent oxygen
sites (O*–O*), and Me–O site pairs located on a metallic cluster
and/or oxide cluster. C–H bond cleavage on these different
active sites has markedly different entropies and activation
energies, as conrmed here from measurement and calcula-
tion. In the narrow regime I, O2 species were almost completely
depleted, C–H cleavage becomes kinetically irrelevant and O]O
bond cleavage/O2 supply on Pd–Pt clusters limits the turnover
rate of methane. The measured apparent activation energies are
in the range of 8–15 kJ molꢀ1, which are much smaller than the
energy barrier of methane dissociation on Pd–Pt(111) clusters.
The barriers for C–H bond cleavage on *–* range from 75 to
79 kJ molꢀ1, depending on the Pd : Pt mole ratio. Clearly, the
supply of oxygen is insufficient due to the low oxygen pressure
in this regime. C–H bond cleavage is kinetically relevant in the
other four regimes, yet it proceeds via different paths on these
different site pairs, depending on oxygen pressure and Pd : Pt
mole ratio.
C–H bond cleavage on O*–* proceeds via a hydrogen
abstraction route that produces OH* and CH*3. The binding
strength of CH*3 is correlated with the coverage of oxygen, which
inuences the energy barriers for C–H bond cleavage on O*–*.
Hence, the energy barrier for C–H bond cleavage on O*–*
increases with increasing O* coverage. The kinetic parameters
for C–H bond cleavage on O*–O* in kinetic regime III are
difficult to measure due to the interference of C–H bond
cleavage on Pd–O in kinetic regimes IV and V. However, these
parameters could be measured via methane activation on O*–
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