Self-oscillations during methane oxidation over Pd/Al2O3: Variations of Pd oxidation state and their effect on Pd catalytic activity

Article abstract of DOI:10.1016/j.apcata.2016.04.024

Oscillations of the rate of methane oxidation under methane-rich conditions over Pd/Al₂O₃ were studied by in situ thermogravimetry and mass-spectrometry. Structural transformations of Pd in the oscillation cycle went through fast reduction of Pd oxide, metal carbonization, removal of deposited carbon as CO₂ and gradual oxidation of metallic Pd. Simultaneous measurement of the weight changes and CO₂ production during oscillations helped to elucidate a relationship between catalytic activity of Pd and its oxidation state. Metallic Pd showed the highest activity which dropped sharply as metal surface became oxidized and then increased gradually as Pd oxidized further into PdO_0.3.

Full text of DOI:10.1016/j.apcata.2016.04.024

Applied Catalysis A: General 522 (2016) 40–44
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Self-oscillations during methane oxidation over Pd/Al O : Variations
2
3
of Pd oxidation state and their effect on Pd catalytic activity
V.Yu. Bychkov , Yu.P. Tulenin , M.M. Slinko , A.K. Khudorozhkovb,c, V.I. Bukhtiyarovb,c
a,∗
a
a
,
d
a
S. Sokolov , V.N. Korchak
Semenov Institute of Chemical Physics, Russian Academy of Sciences, Kosygina str. 4, 119991, GSP-1, Moscow, Russia
Boreskov Institute of Catalysis, Siberian Branch of the Russian Academy of Sciences, Acad. Lavrentiev Pr. 5, 630090, Novosibirsk, Russia
Novosibirsk State University, Pirogova st. 5, 630090, Novosibirsk, Russia
a
b
c
d
Leibniz Institute for Catalysis, Albert-Einstein-Strasse 29a, 18059, Rostock, Germany
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 2 March 2016
Received in revised form 18 April 2016
Accepted 22 April 2016
Available online 27 April 2016
2 3
Oscillations of the rate of methane oxidation under methane-rich conditions over Pd/Al O were studied
by in situ thermogravimetry and mass-spectrometry. Structural transformations of Pd in the oscillation
cycle went through fast reduction of Pd oxide, metal carbonization, removal of deposited carbon as
CO2 and gradual oxidation of metallic Pd. Simultaneous measurement of the weight changes and CO2
production during oscillations helped to elucidate a relationship between catalytic activity of Pd and its
oxidation state. Metallic Pd showed the highest activity which dropped sharply as metal surface became
oxidized and then increased gradually as Pd oxidized further into PdO0.3.
Keywords:
Methane oxidation
Palladium catalysts
Self-oscillations
©
2016 Elsevier B.V. All rights reserved.
1
. Introduction
lates carbon and releases it in form of CO2 [4]. Coupling TG and MS
analyses revealed that the states of Pd when it accumulates car-
bon and when it becomes oxidized are clearly separated on time
scale. With these two processes resolved, TG analysis can be used
for experimental measurement of both Pd carbonization and oxida-
tion rates during methane oxidation reaction. For instance, with the
combined TG-MS analyses we demonstrated that in the oscillatory
regime of methane oxidation, carbon dissolution/removal proceeds
∼30 times faster than Pd oxidation [6]. A simultaneous rise of Pd
Oxidation of methane over Pd foil or powder was often in
focus of catalytic research in view of Pd application for catalytic
combustion. The rate of methane oxidation in methane-rich mix-
tures was shown to oscillate at certain reaction conditions [1–6].
It was demonstrated in Refs. [1–3] that the oxidation state of Pd
in foil changes from metallic Pd to PdO in a given oscillatory cycle.
Catalytic activity of oxidized and reduced Pd in small alkane oxi-
dation was approached in numerous publications over the past
two decades [7–24]. The authors of Refs. [7–10] concluded that
oxidized Pd is more active, while others reported higher activ-
ity of reduced Pd [11] or partially reduced PdOx species [12]. The
advent of advanced experimental techniques and computational
modelling methods at the turn of the century did not result in a
conclusive answer to the above question [13–24].
weigh and CO concentration in the products observed in our other
2
work on methane oxidation indicated higher catalytic activity of Pd
at higher oxidation state [4]. However, because of rather large size
of the studied Pd particles (∼1 ␮m [4] or ∼75 ␮m [6]), the gravimet-
ric data did not allow quantitative determination of Pd oxidation
state at each point. At this size range, oxygen diffusion into Pd bulk
is not fast enough to achieve a uniform oxidation of entire particle.
The size of Pd particles on supported catalysts is on nanoscale,
hence such particles are more likely to change their oxidation state
uniformly. Oscillatory oxidation of methane over supported Pd
catalysts was studied earlier in Refs. [25–29]. The authors relate
periodic changes of weight to oxidation-reduction cycles of Pd.
In situ XAS [28] and QEXAFS [29] methane oxidation experiments
In our previous contributions [4–6], we reported on oscillatory
oxidation of methane and other low alkanes over Pd foil and powder
studied by in situ thermogravimetry (TG) combined with mass-
spectrometry (MS). It was established that in a single oscillatory
cycle, Pd not only changes its oxidation state, but also accumu-
over 5% Pd/Al O3 were performed in a flow-through reactor with a
2
sufficiently long catalytic bed to detect different states of Pd along
it. The methods allowed detection of altering oxidized and metal-
lic Pd phases, carbon deposits on metallic Pd and variations in the
∗ Corresponding author.
E-mail addresses: bychkov@chph.ras.ru, bychkov@polymer.chph.ras.ru
V.Yu. Bychkov).
(
http://dx.doi.org/10.1016/j.apcata.2016.04.024
926-860X/© 2016 Elsevier B.V. All rights reserved.
0

V.Yu. Bychkov et al. / Applied Catalysis A: General 522 (2016) 40–44
41
state of Pd. During oscillations, the bed contained up to 3 sequen-
360 ºС
a
7
6
5
4
3
2
6.E-07
4.E-07
tial zones with Pd in different states. Yet, catalytic activity of Pd
in a particular state could not be determined since the out stream
contained products from all zones.
tg
In the current work, we aimed to gain a mechanistic insight
into periodic changes in the state of supported Pd particles during
oscillatory methane oxidation and to find a quantitative corre-
lation between Pd oxidation state and its catalytic activity. To
this end, Pd/Al O catalysts with different Pd particle size pre-
O
2
H O
2
2
.E-07
2
3
CO2
pared in earlier study [30] were tested in methane oxidation under
methane-rich conditions in a thermogravimetric instrument cou-
pled with a mass-spectrometer.
0.E+00
2
0
40
60
80
100
120
140
160
Time, min
2
. Experimental
a
b
a
b
The catalysts were prepared by an incipient wetness impregna-
2
tion method. ␥-Al O supplied by Sasol (TKA-432, SBET = 215 m /g)
2
3
7
6
6
5
5
.1
.6
.1
.6
.1
O
2
3
.E-07
and ␣-Al O obtained by prolonged calcination of ␥-Al O at
2
3
2
3
◦
2
◦
1
150 C (SBET = 6 m /g) were dried at 120 C for 2 h and then impreg-
2
H O
nated with Pd(NO3) solution. Pd loading was set to 1 wt.% for
2
2.E-07
1.E-07
0.E+00
␥
-Al O and to 5 wt.% for ␣-Al O . The catalysts were denoted
CO2
tg
2
3
2
3
as 1Pd/Al and 5Pd/Al respectively. All impregnated samples were
◦
◦
C
dried at 120 C for 3 h and then calcined at 400 C for 4 h. The prepa-
ration method is described in detail elsewhere [31].
D
E
Pd particle morphology and size distribution were determined
earlier [30] by transmission electron microscopy (TEM, JEM-2010,
JEOL Co., Japan), the samples were prepared by dispensing catalysts
powder on holey carbon supported on copper grid. Particle size
distribution was calculated in “AnalySIS iTEM” v.5 software (Soft
Imaging System GmbH), size distribution histograms were drawn
from 200 to 700 measurements.
Catalytic experiments were carried out in a tubular quartz
flow-through reactor (i.d. 5 mm), operated at atmospheric pressure
under shallow bed conditions. Catalyst weight was 20 mg. A mix-
1
20
125
130
135
140
145
150
Time, min
Fig. 1. (a) Oscillations of catalyst weight and gas components during methane oxi-
dation over 5Pd/Al at 360 C in CH4 O2 Ar He flow in TGA-MS instrument. (b)
Last 3 cycles of the oscillations shown in a. Intervals a-b and b-a are the high- and
low-activity phases respectively. Separate experiments with identical catalyst load-
ings were stopped at points C, D, and E and the samples weight loss was measured
during reduction in CO/He mixture.
◦
ture containing 41.5 vol.% CH , 7.5 vol.% O , 2 vol.% Ar and 49 vol.%
He was fed at 20–30 mL/min.
4
2
ing Information in Figs. S2a,b and S3 respectively. An average Pd
particles size in 1Pd/Al was 16 ± 7 nm, while Pd/␣-Al O contained
2
3
Thermogravimetric analysis (TGA) was performed on Setaram
SETSYS EVOLUTION 16/18 instrument coupled with Pfeiffer
OmniStar GSD 301 quadrupole mass-spectrometer. The operation
unit of the thermogravimetric setup consisted of a vertical flow
tubular furnace with an inner alumina tube. The inner diameter
of the alumina tube was 20 mm and the length of the controlled
temperature zone was ∼30 mm. The instrument is described in
detail elsewhere [32]. The catalyst samples were loaded in a
quartz cup suspended in the center of a heated zone. The gas
mixture of 41.5 vol.% CH , 7.5 vol.% O , 2 vol.% Ar and 49 vol.% He
large particles agglomerates of various sizes. They can be seen as
dark structures in Fig. S2b among which smaller ones marked by
white arrows measured ∼30 nm across whereas larger aggregates
were well over 100 nm, hence meaningful particle size evaluation
was not possible.
Self-oscillations of the rate of methane oxidation over 1Pd/Al
and 5Pd/Al in the in situ TGA experiments were observed at 400
and 360 C respectively. Fig. 1a shows oscillations of the 5Pd/Al
weight and of O , CO and H O MS signals during methane oxi-
2
2
2
4
2
◦
dation at 360 C. The waveform of all oscillations changed with
time on stream starting from a simple “mono-phase” (first 3–4
oscillation cycles in Fig. 1a) and gradually progressing to a more
complex “double-phase” form (last 3–4 oscillation cycles in Fig. 1a).
The mono-phase oscillations demonstrated periodic increase and
decrease of the catalyst weight. The double-phase oscillations have
characteristic points a and b where abrupt changes of the catalyst
weight and the reaction rate took place (Fig. 1b). The oscillation
periods then can be divided into the high-activity phase a-b and
the low-activity phase b-a. The weight of 5Pd/Al remained nearly
constant during a-b and increases noticeably during b-a.
was fed through the top of the tube, the products were sampled
below the catalyst via a stainless capillary connected to the mass-
spectrometer. The flow rate of the gas mixture was maintained at
2
0 mL/min by the flow controller built into the SETSYS instrument.
During the experiments, 40 mg of a catalyst were heated at 10/min
to a selected temperature and held at this temperature for the rest
of the experiment. The weight change was normalized to the initial
sample weight and further on will be expressed as mg/gcat. Atomic
mass units (AMU) of 2 (H ), 15 (CH ), 18 (H O), 28 (CO, CO ), 32 (O ),
2
4
2
2
2
4
0 (Ar), and 44 (CO ) were detected. Ar MS signal was applied as
2
internal standard for MS measurement of the gaseous products con-
centrations. CO concentration in the outflow was very low under
all experimental conditions.
Fig. 2a and b shows oscillations observed on 1Pd/Al during in
◦
situ TG experiment at 400 C. As in case of 5Pd/Al, the oscillation
cycles can be divided into the a-b and phases where the catalyst has
high and low activity respectively. Unlike the oscillations on 5Pd/Al,
waveforms of the oscillations on 1Pd/Al did not evolve with time
on stream.
3
. Results
Size and morphology of Pd particles were characterized by TEM
in our previous work [30]. The micrographs of the catalysts and the
Pd particles size distribution on 1Pd/Al can be found in the Support-
In order to verify whether the weight changes during the b-a
phase were related to the oxygen content in the catalyst, the exper-
iments with reproducible oscillations over 5Pd/Al were stopped

4
2
V.Yu. Bychkov et al. / Applied Catalysis A: General 522 (2016) 40–44
Table 1
a
3
2
1
0
3.E-07
Weight loss due to PdOx reduction measured on 5Pd/Al in different states. The in-situ
O
2
TGA experiment shown in Fig. 1b was stopped at points C, D and E, the sample was
cooled to room temperature and then heated in 10 vol.% CO/He flow. Pre-reduced
and pre-oxidized 5Pd/Al were tested in separate experiments.
2
1
0
.E-07
.E-07
.E+00
Sample pretreatment
Weight loss (mg/g cat)
CO
2
◦
pre-reduced in H2 at 500
C
D
E
C
0.07
0.71
0.76
1.46
6.42
tg
◦
pre-oxidized in O2 at 500
C
-1.E-07
1
15
135
155
175
195
Time, min
its oxidation state was somewhat higher. Correspondingly, these
observations indicate that the abrupt weight loss at point b (Fig. 1b)
is not due to Pd reduction, but rather to oxidative removal of carbon
accumulated during the a-b phase, while the weight gain in the b-a
phase is a result of Pd oxidation.
a
b
a
b
tg
8
.5
7
4
3
.E-07
.E-07
7
6
4. Discussion
О
2
In the following section we will compare Pd powder and sup-
ported Pd catalysts in oscillatory oxidation of methane in terms
of TG and MS waveforms, search for a Pd oxidation state – cat-
alytic activity correlation and compare our results to the current
knowledge. As demonstrated in our earlier report on methane and
ethane oxidation over 75 ␮m Pd powder, oscillations parameters
continuously evolve with time on stream [6]. The evolution was
caused by a significant development of Pd surface and gradual dis-
solution of carbon in Pd bulk. 5Pd/Al sample also demonstrated
some evolution of oscillations waveform, but much weaker than
that observed on the Pd powder. A decrease in the maximal cata-
lyst weight with rising number of oscillations that can be seen in
Fig. 1a may be related to a gradual oxygen removal from progres-
sively deeper layers of large Pd particles at each following cycle.
Here it should be recalled that Pd in 5Pd/Al prior to the TG experi-
2.E-07
1.E-07
0.E+00
Н
2
О
.5
СО
2
6
2
10
215
220
Time, min
225
Fig. 2. (a) Oscillations of catalyst weight and gas components during methane oxi-
dation over 1Pd/Al at 400 C in CH4 O2 Ar He flow in TGA-MS instrument. (b)
Selected 4 cycles of the oscillations shown in a. Intervals a-b and b-a are the high-
◦
and low-activity phases respectively.
3
0
1.0E-07
5.0E-08
0.0E+00
b
c
a
2
2
2
2
2
2
9.9
9.8
9.7
9.6
9.5
9.4
◦
ments was in form of PdO as the precursor was calcined at 400 C in
air. PdO in deeper layers of large particles could not be completely
reduced in the first cycle, but the degree of reduction increased with
each following cycle until metallic Pd was formed through entire
particles volume. 1Pd/Al sample did not show any significant evo-
lutionary effects (Fig. 2ɑ), most likely because 16 nm Pd particles
in 1Pd/Al are too small to undergo morphological transformation.
Also, such particles have no “deep” layers to dissolve irreversibly
oxygen or carbon in Pd bulk during the oscillations.
d
0
100
200
300
400
In our studies on methane and ethane oxidation over Pd foil
or powder [4–6], we assigned periodic patterns in the TG and MS
waves to changes in the state of Pd (Fig. S1). The following sequence
of transformations was set forth: fast PdOx reduction to metallic
state at point a; carbonization of metallic Pd; removal of carbon as
Temperature, C
Fig. 3. Weight variations (a, c) and CO2 evolution (b, d) during heating of 5Pd/Al
sample in 10% CO/He flow: (a, b) the sample pre-oxidized in 5% O2/He mixture at
00 C; (c, d) the sample after the oscillation stopped at point E.
◦
5
CO up to point b; gradual oxidation of the upper Pd layer followed
2
again by fast reduction. In the a-b phase, Pd remains in a reduced
state and contains dissolved carbon. The latter reacts with adsorbed
oxygen thus stabilizing Pd in the reduced state. It is possible only
when the O2 concentration in the gas phase is low, i.e. when the
catalyst is active and the O2 conversion is high. The b-a phase is
characterized by low activity resulting in low O2 conversion, its
high concentration in the reaction mixture and subsequently Pd
oxidation.
at points C, D and E as shown in Fig. 1b. Then the sample was
cooled in vacuum to 30 C and heated in 10 vol.% CO/He mixture
to observe the weight loss occurring due to PdOx reduction. The
◦
maximum CO evolution and weight loss were observed between
2
◦
1
40 and 190 C (Fig. 3). The values obtained in this experiment are
found in Table 1 along with the weight loss measured on 5Pd/Al
◦
pretreated in H2 or in 5 vol.% O /He flow at 500 C. As expected,
2
the pre-reduced catalyst showed very little weight loss, while pre-
oxidized one lost 6.4 mg/g thus showing the highest lost among
all differently pretreated 5Pd/Al samples. However, the weight loss
of 7.5 mg/g corresponding to a complete Pd2+ → Pd reduction on
The same stages are observed on the supported Pd catalysts in
Figs. 1b and 2b suggesting that the oscillations on bulk and sup-
ported Pd are of the same nature. However, a noticeable weight
gain due to carbon dissolution in metal starting after point a on Pd
powder is not observed on 5Pd/Al and 1Pd/Al as evident from Figs.
1b and 2b. Here it should be recalled that the surface/volume ratio
of bulk Pd is lower than that of supported Pd because the former
has larger particles. For both bulk and supported Pd catalysts, PdOx
0
4
0 mg of 5Pd/Al is not reached here. The samples taken off stream
at points C and D lost almost equal and relatively small amount
of oxygen suggesting that Pd in particles and aggregates at these
points of the cycle was substantially reduced, whereas at point E

V.Yu. Bychkov et al. / Applied Catalysis A: General 522 (2016) 40–44
1.2
43
a
b
a
3
2
1
.E-07
.E-07
.E-07
1
0
0
0
0
.0
.8
.6
.4
.2
CO
2
O
2
0
.E+00
0.0
-0.05
3
54
355
356
357
358
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
Time, min
x in PdOx
◦
Fig. 5. Effect of the Pd oxidation state in 1Pd/Al on the CO2 evolution rate. The
oxidation state was calculated from the weight changes from the minimum to the
maximum in the b-a phase. Squares show CO2 evolution rate at point b on partially
carbonized reduced Pd.
Fig. 4. Oscillations of CO2 and O2 during methane oxidation over 5Pd/Al at 360
in CH4 O2 Ar He flow in a flow-through quartz tubular reactor.
C
reduction and carbon dissolution occur concurrently at the begin-
ning of the a-b phase. Because of the low surface/volume ratio in
case of 75 ␮m Pd powder, more carbon is dissolved in particles
bulk than oxygen removed from particles surface which results in
a well-resolved weight maximum in the a-b phase seen in Fig. S1.
In case of the supported catalysts, higher fraction of Pd participates
in redox cycles thanks to higher availability of the metal surface.
Therefore, per mol of Pd, more oxygen is removed from 5Pd/Al and
which may differ from the oxidation state reached in CH /O2 mix-
4
ture. The trend in the CO2 formation rate in Fig. 5 is ascending, so
higher activity is expected at higher oxidation state, most probably
with a maximum encountered at certain x. The latter is supported
by findings made in the study of methane oxidation on the (111)
face of Pd monocrystal [17]. The measurements were performed on
the crystal surface oxidized to a different degree and the oxide with
the empirical formula PdO0.8 was found to be the most active. The
maximum in catalytic activity of oxidized Pd was not encountered
in our case because in the oscillatory regime, PdOx is reduced before
the most active oxidation state is reached. The maximum rate of
CO2 formation was detected before point b (shown as squares)
where x = 0, i.e. on partially carbonized reduced Pd. These values
by far exceed the rates achieved at x = 0.3. Thus, metallic Pd can
exhibit the lowest and the highest catalytic activity depending on
the gas phase composition. In particular, maximum and minimum
in the activity are registered in oxygen-depleted and oxygen-rich
atmosphere respectively. The reason behind this difference is the
rate of O2 chemisorption on metallic Pd surface which is signifi-
1
Pd/Al than from bulk Pd. The limit of carbon dissolution in Pd is
PdC0.13 [33,34], whereas Pd/O of 1 can be reached if all Pd is oxi-
dized to PdO. Hence, it is logical to assume that the weight gain from
carbon dissolution in Pd on the supported catalysts is absorbed by
the weight loss from oxygen removal during PdOx reduction.
An incidence of C accumulation on 5Pd/Al was confirmed in the
methane oxidation experiment performed in a flow-through quartz
tubular reactor. MS signal of CO shown in Fig. 4 exhibits a negative
2
peak immediately after point a. At this point Pd becomes reduced
and highly active affording higher conversion of O2 and CH . A
4
portion of carbon from converted CH4 is not oxidized to CO , but
2
dissolved in Pd until saturation (PdC0.13) is reached. Hence, a drop in
CO2 concentration that appears at higher methane oxidation rate.
The weight of 5Pd/Al saturated by carbon remains nearly constant
during the high activity a-b phase followed by the weight loss at
point b due to the oxidative carbon removal (Fig. 1b). It is possi-
ble to observe this loss since Pd oxidation that comes after carbon
removal is relatively slow. Thus, the weight minimum immediately
after point b corresponds to metallic Pd and the weight increase that
follows is due to Pd oxidation.
Methane oxidation in the oscillatory mode allows monitoring
parallel changes of Pd catalytic activity and its oxidation state over a
number of reproducible oscillation cycles. We have selected 1Pd/Al
for this study because Pd particles on this catalyst are relatively
small and all atoms in such particles are likely to respond to changes
in gas phase composition. Hence, changes in their oxidation state
calculated from the weight changes should be accurate. Fig. S4
shows evolution of the stoichiometric coefficient x in PdOx and
cantly higher compared to that of CH . As a result, the surface of
4
metallic Pd in oxygen-rich CH /O2 mixture is covered by adsorbed
4
oxygen that blocks it for methane molecules.
The findings made in our work are in agreement with literature
reports on the subject. Studies on stationary oxidation of methane
in oxygen-rich mixtures over Pd catalysts indicate higher catalytic
activity at higher oxidation state of Pd [7–10,13,15,16,18]. How-
ever, complete reduction of PdOx and carbon accumulation do not
occur at constant excess of oxygen in gas phase, thus the catalytic
activity of reduced Pd is never reached. Also, oxygen depletion was
shown to stimulate catalytic activity of Pd. An isotope method used
in [14] revealed that a probability of reactive CH collision increases
4
with decreasing coverage of Pd surface with oxygen. The authors of
Ref. [19] reported that the activity in methane oxidation was higher
under oxygen-lean conditions than in oxygen-rich feed where Pd
is more oxidized. In Ref. [20] metallic Pd surface was found to
be significantly less active in methane oxidation than PdO surface
under near-stoichiometric conditions. However, the metallic sur-
face became very active under oxygen-lean conditions. According
CO MS signal over the b-a phase during which Pd loses carbon and
2
becomes oxidized. The value of x was assumed to be 0 at point b and
at the weight minimum right after it because carbon removal does
not change Pd oxidation state. At the weight maximum occurring at
to Ref. [21], methane oxidation over Pd/Al O3 under oxygen-lean
2
point a, the value of x reached 0.3. The rate of CO evolution shown
conditions gave TOF rise from 0.05 to 0.25 with Pd/PdOx ratio
increase from 1 to 2.5, i.e. the activity dropped at higher oxida-
tion state of Pd. The results of DFT calculations presented in Refs.
[22–24] were contradictory. Authors of Ref. [22] report that on PdO
clusters (predominantly PdO(101) surfaces), both Pd and O ions
activate C H bonds more efficiently than Pd atoms in Pd0 clusters,
either free or with adsorbed O*. Calculations performed by authors
2
as a function of x in Fig. 5 almost doubled in the 0 ≤ x ≤ 0.3 inter-
val indicating that the activity increases with Pd oxidation state.
Increase of Pd catalytic activity with its oxidation state was also
reported by R. Burch et al. who studied methane oxidation over
Pd/Al O [9]. Yet, the oxidation state of Pd in this work was mea-
2
3
sured ex situ in oxygen monolayers consumed by pre-reduced Pd,

4
4
V.Yu. Bychkov et al. / Applied Catalysis A: General 522 (2016) 40–44
of Ref. [23] show that both thick films of PdO (101) and metallic Pd
afford high methane conversion.
References
[
[
[
[
[
[
1] D. König, W.H. Weber, B.D. Poindexter, J.R. Mcbride, G.W. Graham, K. Otto,
Catal. Lett. 29 (1994) 329–338.
2] G.W. Graham, D. König, B.D. Poindexter, J.T. Remillard, W.H. Weber, Topics
Catal. 8 (1999) 35–43.
Knowing the activity of different states of metallic Pd is impor-
tant for kinetic modelling of methane oxidation over Pd. Composed
of elementary reactions, such models may be quite complex as, for
instance, the one presented in Ref. [35]. It was developed to model
the hysteresis effect occurring during methane oxidation under
methane-lean conditions and comprised 43 elementary reactions.
Basing on the results of Burch et al. [9] the authors of Ref. [35]
assume that the rate of methane reaction on Pd surface is slow,
and the activation energy of the CH + 2Pd(s) → CH (s) + H(s) reac-
3] X.L. Zhang, C.S.-M. Lee, D.M.P. Mingos, D.O. Hayward, Appl. Catal. A Gen. 240
(
2003) 183–197.
4] V.Yu. Bychkov, Yu.P. Tyulenin, M.M. Slinko, D.P. Shashkin, V.N. Korchak, J.
Catal. 267 (2009) 181–187.
5] V.Yu. Bychkov, Yu.P. Tyulenin, M.M. Slinko, V.N. Korchak, Catal. Lett. 141
(
2011) 602–607.
6] V.Yu. Bychkov, Yu.P. Tyulenin, A.Ya. Gorenberg, S. Sokolov, V.N. Korchak, Appl.
Catal. A Gen. 485 (2014) 1–9.
4
3
tion is high – 160 kJ/mol. However, our results indicate that the
rate of such reaction on truly metallic Pd surface is high, whereas
the processes occurring on the oxygen-blocked Pd surface under
methane-lean conditions cannot be accurately described by the
above elementary equation.
[7] J.G. McCarty, Catal. Today 26 (1995) 283.
[
[
8] R. Burch, F.J. Urbano, Appl. Catal. A 124 (1995) 121.
9] R. Burch, P.K. Loader, F.J. Urbano, Catal. Today 27 (1996) 243–248.
[
10] J.N. Carstens, S.C. Su, A.T. Bell, J. Catal. 176 (1998) 136–142.
[11] M. Lyubovsky, L. Pfefferle, Catal. Today 47 (1999) 29.
[
[
12] D. Ciuparu, L. Pfefferle, Appl. Cata. A: Gen. 218 (2001) 197–209.
13] J. Xu, L. Ouyang, W. Mao, X.-J. Yang, X.-C. Xu, J.-J. Su, T.-Z. Zhuang, H. Li, Y.-F.
Han, ACS Catal. 2 (2012) 261–269.
5
. Conclusions
[14] Y.-H. Chin, E. Iglesia, J. Phys. Chem. C 115 (2011) 17845–17855.
[
15] S. Peng-ont, P. Praserthdam, F. Matei, D. Ciuparu, S. Brosda, C.G. Vayenas,
Catal. Lett. 142 (2012) 1336–1343.
Methane oxidation in oscillatory regime under methane-rich
[
16] O. Demoulin, M. Navez, P. Ruiz, Appl. Catal. A Gen. 295 (2005) 59–70.
conditions was studied over supported Pd catalysts by in situ
thermogravimetric analysis coupled with mass-spectroscopy.
[17] H. Gabasch, K. Hayek, B. Klötzer, W. Unterberger, E. Kleimenov, D. Teschner, S.
Zafeiratos, M. Hävecker, A. Knop-Gericke, R. Schlögl, B. Aszalos-Kiss, D.
Zemlyanov, J. Phys. Chem. C 111 (2007) 7957–7962.
[18] G. Zhu, J. Han, D.Yu. Zemlyanov, F.H. Ribeiro, J. Phys. Chem. B 109 (2005)
2331–2337.
Simultaneous changes in CO and O concentrations in the outflow
2
2
stream and in the catalyst weight suggest that during oscillations,
Pd atoms in supported particles progress through periodic redox
transformations very similar to those observed in Pd powder under
the methane-rich reaction conditions. The states of Pd are: rapid
reduction of PdOx to metallic Pd; carbonization of metallic Pd;
[
19] M. Lyubovsky, L.L. Smith, M. Castaldi, H. Karim, B. Nentwick, S. Etemad, R.
LaPierre, W.C. Pfefferle, Catal. Today 83 (2003) 71–84.
[
20] X. Weng, H. Ren, M. Chen, H. Wan, ACS Catal. (2014) 2598.
[21] N.M. Kinnunen, J.T. Hirvi, T. Venalainen, M. Suvanto, T.A. Pakkanen, Appl. Cata.
A: Gen. 397 (2011) 54–61.
[
22] Y.-H. Chin, C. Buda, M. Neurock, E. Iglesia, J. Am. Chem. Soc. 135 (2013)
5425–15442.
removal of dissolved carbon in form of CO ; gradual oxidation of
2
1
Pd to PdOx. The in situ thermogravimetric experiments on the cat-
alyst containing 16 nm Pd particles allowed monitoring changes
in Pd oxidation state during activity oscillations. Catalytic activity
[23] A. Hellman, A. Resta, N.M. Martin, J. Gustafson, A. Trinchero, P.-A. Carlsson, O.
Balmes, R. Felici, R. van Rijn, J.W.M. Frenken, J.N. Andersen, E. Lundgren, H.
Grönbeck, J. Phys. Chem. Lett. 3 (2012) 678–682.
[
24] N.M. Kinnunen, J.T. Hirvi, M. Suvanto, T.A. Pakkanen, J. Phys. Chem. C 115
(2011) 19197–19202.
0
increased as Pd became oxidized to PdO0.3 which is in agreement
with earlier literature reports, but the highest catalytic activity was
observed after a rapid reduction step as Pd went over to metallic
state. High activity of the metallic state is specific for methane-rich
conditions where the metal surface remains substantially free of
chemisorbed oxygen and thus available for C H bond activation in
methane molecules.
[25] Y. Deng, T.G. Nevell, Faraday Discuss. 105 (1996) 33–46.
[
[
[
26] Y. Deng, T.G. Nevell, J. Mol. Catal. A 142 (1999) 51–60.
27] U.S. Ozkan, M.W. Kumthekar, G. Karakas, J. Catal. 171 (1997) 67.
28] B. Kimmerle, A. Baiker, J.-D. Grunwaldtz, Phys. Chem. Chem. Phys. 12 (2010)
2288–2291.
[
[
[
[
[
[
29] J. Stötzel, B. Kimmerle, M. Nachtegaal, J.-D. Grunwaldtz, J. Phys. Chem. C 116
(
2012) 599–609.
30] V.Yu. Bychkov, Yu.P. Tyulenin, M.M. Slinko, A.K. Khudorozhkov, V.I.
Bukhtiyarov, S. Sokolov, V.N. Korchak, Cat. Commun. 77 (2016) 103–107.
31] A.Yu. Stakheev, A.M. Batkin, N.S. Telegina, G.O. Bragina, V.I. Zaikovsky, I.P.
Prosvirin, A.K. Khudorozhkov, V.I. Bukhtiyarov, Topics Catal. 56 (2013) 306.
32] V.Yu. Bychkov, Yu.P. Tyulenin, V.N. Korchak, E.L. Aptekar, Appl. Catal. A Gen.
304 (2006) 21–29.
33] S.B. Ziemecki, G.A. Jones, D.G. Swartzfger, R.L. Harlow, J. Faber Jr., J. Am. Chem.
Soc. 107 (1985) 4547–4548.
34] J.A. McCaulley, J. Phys. Chem. 97 (1993) 10372–10379.
Acknowledgements
This work was supported by the Russian Science Foundation
(
grant N 15-13-30034). Catalysts preparation and microscopic
examination performed in Boreskov Institute of Catalysis were
partly supported by the Skolkovo Foundation (Grant Agreement
for Russian educational organizations no. 5 of 30.12.2015).
[35] X. Zheng, Y. Shi, X. Wang, N. Cai, Combust. Sci. Technol. 187 (2015)
044–1064.
1
Appendix A. Supplementary data
Supplementary data associated with this article can be found,
in the online version, at http://dx.doi.org/10.1016/j.apcata.2016.04.
024.