Platinum Inhibits Low-Temperature Dry Lean Methane Combustion through Palladium Reduction in Pd?Pt/Al2O3: An In Situ X-ray Absorption Study

Article abstract of DOI:10.1002/cphc.201600993

Palladium–platinum bimetallic catalysts supported on alumina with palladium/platinum molar ratios ranging from 0.25 to 4 are studied in dry lean methane combustion in the temperature range of 200 to 500 °C. Platinum addition decreases the catalyst activity, which cannot be explained by the decrease in dispersion or the structure sensitivity of the reaction. In situ X-ray absorption near-edge structure and extended X-ray absorption fine structure spectroscopy measurements have been conducted for monometallic Pd, Pt, and 2:1 Pd?Pt catalysts. Monometallic palladium is fully oxidized in the full temperature range, whereas platinum addition promotes palladium reduction, even in a reactive oxidizing environment. The Pd/PdO weight ratio in bimetallic Pd?Pt 2:1 catalysts decreases from 98/2 to 10/90 in the 200–500 °C temperature range under the reaction conditions. Thus, platinum promotes the formation of the reduced palladium phase with a significantly lower activity than that of oxidized palladium. The study sheds light on the effect of platinum on the state of the active palladium surface under low-temperature dry lean methane combustion conditions, which is important for methane-emission control devices.

Full text of DOI:10.1002/cphc.201600993

Accepted Article
Title: Platinum inhibits low-temperature dry lean methane combustion via palladium
reduction in Pd-Pt/Al2O3: An in situ X-ray absorption study
Authors: Hanieh Nassiri; Kee Eun Lee; Yongfeng Hu; Robert E. Hayes; Robert W.J.
Scott; Natalia Semagina
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ChemPhysChem
10.1002/cphc.201600993
ARTICLE
Platinum inhibits low-temperature dry lean methane combustion
via palladium reduction in Pd-Pt/Al O :
2
3
An in situ X-ray absorption study
[
a]
[b,c]
[c]
[a]
[b]
Hanieh Nassiri, Kee-Eun Lee,
Yongfeng Hu, Robert E. Hayes, Robert W.J. Scott, Natalia
[
a]
Semagina*
Abstract: Palladium-platinum bimetallic catalysts supported on
alumina with Pd/Pt molar ratios ranging from 0.25 to 4 were studied
in dry lean methane combustion in the temperature range of 200 to
sources, including emissions from natural gas compressor
[
2]
stations and the vent gas from oil wells. The volumetric
efficiency and operating temperature range of current Pd-based
catalysts require improvement for successful device
commercialization, which promotes the search for suitable metal
5
00 degrees Celsius. Pt addition decreases the catalyst activity,
which could not be explained by the decrease in dispersion or the
reaction structure-sensitivity. In situ X-ray absorption near edge
structure (XANES) and extended X-ray absorption fine structure
[
2,4]
promoters and supports.
The active chemical state of Pd in lean methane combustion
has been a matter of debate. The turnover rates increased by an
(
EXAFS) spectroscopy measurements were conducted for
monometallic Pd, Pt and Pd-Pt 2:1 catalysts. Monometallic Pd is fully
oxidized in the full temperature range, whereas Pt addition promotes
Pd reduction even in a reactive oxidizing environment. The Pd/PdO
weight ratio in bimetallic Pd-Pt 2:1 catalysts decreases from 98/2 to
order of magnitude with a 3-fold increase of the Pd/PdO
measured by NH -TPD, and the coexistence of metallic Pd with
PdO was claimed as paramount. However, a further ratio
x
ratio as
3
[
5]
[
5]
increase to 5 was found to be detrimental to catalytic activity.
Farrauto et al. showed the loss of activity when metallic Pd was
1
0/90 in the 200-500 degrees Celsius temperature range under
[
6]
reaction conditions. Thus, Pt promotes the formation of the reduced
Pd phase with a significantly lower activity as compared to the
oxidized Pd. The study sheds light on the effect of Pt on the state of
the active Pd surface under low-temperature dry lean methane
combustion conditions, which are important for methane-emission
control devices.
formed under reducing reaction conditions. The group of
[
7]
Iglesia reported that the methane oxidation rate coefficients at
°
various oxygen pressure at 700 C are a single function of total
2 3
oxygen content in Pd/Al O catalysts with nanoparticle sizes
from 1.8 to 8.8 nm. The conclusion was drawn from oxygen
uptake and evolution measurements, methane combustion
kinetic measurements and thermodynamic calculations. Using in
situ extended X-ray absorption fine structure (EXAFS),
Grunwaldt et al. detected the presence of metallic Pd on a
Introduction
°
Pd/ZrO
2
catalyst at 500-550 C and suggested its beneficial
effect, considering that the activity of pre-reduced catalyst was
Catalyst development for low-temperature natural-gas (NG)
combustion has received significant attention because of the
[8]
higher than that of the oxidized catalyst. An operando Raman
°
study of the Pd/Al
2
O
3
catalyst in the 25-500 C range detected
[
1]
forecasted increase in NG-fuelled devices and the need for
efficient emission control of methane with a greenhouse gas
[9]
only Pd oxide forming under reaction conditions, which is
consistent with thermodynamic expectations and the kinetic
[
2]
potential exceeding that of CO
2
by 23 times. There are many
[10]
2
investigation of Pd/ZrO catalysts by Fujimoto et al. The latter
examples of methane emissions that encompass a wide range
of conditions. For example, the increasing interest in NG-fuelled
vehicles has increased demand for an effective catalyst that can
oxidize methane in a lean wet environment. There are also
study explained the frequently reported requirement of the
coexistence of Pd-PdO by a Mars-van Krevelen mechanism
when CH is activated on site pairs consisting of O atoms and O
vacancies on the PdO surface. These vacancies can also be
x
4
x
4
numerous examples of fugitive CH emission sources that are
easily poisoned by water, which is known for its strong inhibitory
effect on Pd-catalyzed methane combustion. Thus, the
knowledge of the required chemical state of Pd under reaction
conditions and the ability to control it, if necessary, is paramount
to designing an efficient and stable NG combustion catalyst.
lean and dry. A large source of methane emissions is the vent
gas from underground coal mines which are burned to produce
[
3]
heat. The oil and gas industry also has many fugitive emission
In the quest for
a suitable promoter to enhance the
[
a]
H. Nassiri, Prof. R.E. Hayes, Prof. N. Semagina*
Department of Chemical and Materials Engineering
University of Alberta
performance of Pd in methane combustion, platinum has been
[
11-15]
extensively used.
Monometallic Pt is a poor methane
9211 – 116 St., Edmonton T6G 1H9 Canada
[16]
combustion catalyst under lean conditions . In an oxidative
atmosphere, Pt on γ-alumina support is more susceptible to
sintering than Pd; thus, in Pd-Pt formulations, Pd stabilizes Pt as
E-mail: semagina@ualberta.ca
Dr. K.-E. Lee, Prof. R.W.J. Scott
Department of Chemistry
[
b]
c]
University of Saskatchewan
a
result of
a
strong interaction between PdO and the
110 Science Place, Saskatoon S7N 5C9 Canada
[17,18]
support.
In bimetallic catalysts, the chemical states of Pd
[
Dr. K.-E. Lee, Dr. Y. Hu
and Pt can be affected by their ratios, preparation methods, and
catalyst support to enhance or suppress methane
Canadian Light Source Inc.
44 Innovation Boulevard, Saskatoon S7N 2V3 Canada
[
13,15,19-25]
conversion.
The reactivities of Pd and Pt towards
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ChemPhysChem
10.1002/cphc.201600993
ARTICLE
oxygen depend on the temperature and partial pressure of
2 3
Table 1. Characteristics of the synthesized catalysts supported on γ-Al O
[
7,17,26-29]
[27,30]
[7,31]
oxygen,
, metal support
and nanoparticle size.
In
[
b]
Catalyst
Pd/Pt
molar
ratio
Pd
loading,
wt.%
Pt
loading,
wt.%
Dispersion, %
addition to metallic Pd and Pt, oxide species such as PdO, PdO
2
,
[a]
[a]
CH
4
[
32-35]
conversion
PtO, PtO
2
and Pt
3
O
4
may be present.
In bimetallic catalysts,
o
at 350 C,
the formation of the mixed-oxide phases of Pd
x
Pt1-x
O
y
was
[c]
%
[
35]
shown via DFT calculations by Dianat et al. The addition of Pt
to Pd was suggested to stabilize the higher oxidation state of Pd.
Pd
-
4
0.2550
0.1780
0.1520
0.0875
0.0659
0.0328
-
-
43
53
31
23
20
16
13
52
65
28
15
14
4
Oxygen-binding energy in the Pd-Pt catalysts was found to be Pd-Pt 4:1
0.0772
0.1240
0.1700
0.1910
0.2150
0.295
[
36]
higher on Pd-rich surfaces
because of the charge transfer Pd-Pt 2:1
Pd-Pt 1:1
2
1
from Pd to Pt atoms. The extent of Pd segregation to the surface
°
Pd-Pt 1:2
Pd-Pt 1:4
Pt
0.5
0.25
0
after oxidative aging at 750 C was found to be insignificant in
[
37]
[38]
one study but was claimed to be important in another.
An improvement in the dissociation of methane and oxygen
1
[
32]
adsorption was suggested by Persson et al. as the reason for
the higher activity of bimetallic Pd-Pt catalysts as compared to
[a]
[b]
as per NAA;
from CO chemisorption assuming 1:1 CO/metal
[
11]
[c]
monometallic Pd. Kinnunen et al. believed that the proportions
of metal and metal-oxide surface sites determine the catalytic
activity when a small amount of Pt was necessary to enhance
the low-temperature conversion, but a higher Pt content was
stoichiometry; Pd amounts in all tests were kept as 1.2 mg; 1.2 mg of
monometallic Pt was used. Intrinsic kinetic regime in an isothermal operation
o
at 350 C was confirmed.
detrimental due to the suppressed reoxidation of Pd.
A
a)
b)
c)
thermogravimetric analysis of Pd, Pt and Pd-Pt catalysts in air
showed that Pt stabilized Pd in its reduced form and shifted the
onset of deactivation by PdOàPd transformation to lower
°
temperatures in the range of 850 – 950 C, leading to lower
[
14]
intrinsic activity in lean-methane combustion.
At the same
time, Pt improved the sintering resistance of Pd, which resulted
in an overall beneficial effect on methane combustion at low Pt
Figure 1. TEM images of the calcined supported catalysts: (a) Pd, (b) Pt, and
c) Pd-Pt 2:1
[
14]
loadings (Pt mass fraction <10%).
In line with these
(
observations, an ex situ X-ray absorption study of Pd-Pt
°
catalysts oxidized at 750 C revealed that Pt helps Pd to
[39]
As expected,
the addition of Pd to Pt prevented its
maintain in its reduced form when only 20-30% of Pd was
excessive sintering, which also can be seen in the transmission
electron microscopy (TEM) images of the calcined catalysts (Fig.
[
37]
present in its oxide form.
The reported observations of the effect of Pt on Pd
[
14,37]
1
). The richer the catalysts in Pd content are, the lower the
°
covered temperatures above 600 C, which is beyond the range
of interest for low-temperature methane combustion, and did not
investigate the metal chemical states under the reaction
conditions. The current work sheds light on the effect of Pt on
degree of sintering is.
The catalysts were tested in dry lean methane combustion,
and the corresponding methane conversions in the temperature-
programmed reactions are shown in Fig. 2.
Pd/PdO
x
distribution in Pd-Pt/Al
2
O
3
methane combustion
°
catalysts in a low-temperature range of interest (200-550 C)
using an in situ X-ray absorption study and the effect of the
reduced Pd in dry lean methane combustion.
Results and Discussion
Table 1 summarizes the range of synthesized Pd-Pt and
2 3
monometallic catalysts supported on commercial γ-Al O and
their dispersions as determined from CO chemisorption. The
catalyst notation is based on the molar ratio of the two metals
and reflects the metal loadings as determined by neutron
activation analysis (NAA). The samples were pre-calcined in air
at 550 °C for 16 h, which is a recommended commercial
procedure preceding automotive catalytic converter testing for
lean burn applications. For the dispersion determination, 1:1
stoichiometry of the adsorbed CO per surface mole of Pd and/or
Pt was assumed.
Figure 2. Ignition curves recorded in the dry lean methane combustion. Pd
amounts in all tests were kept as 1.2 mg, except for monometallic Pt (also 1.2
mg). The curve for monometallic pre-reduced Pd is not shown as it is identical
to that one of oxidized Pd, within 3% error
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ChemPhysChem
10.1002/cphc.201600993
ARTICLE
Only the first ignition curve is reported for each catalyst, the
duration of which is short enough as compared to the 16-h-long
°
pre-calcination at 550
C so that the effect of in situ
restructurization may be considered negligible. The curve for
monometallic pre-reduced Pd is not shown as it is identical to
that of oxidized Pd within a 3% error. Monometallic Pt shows
extremely low activity as compared to Pd; Pt is indeed known for
[
16]
efficient methane combustion only under fuel-rich conditions.
Because Pt showed very low activity, all tests with monometallic
Pd and bimetallic Pd-Pt catalysts were performed at the same
Pd loading in the reactor (1.2 mg). The methane conversions at
Figure 4. XANES spectra of calcined Pd and Pd-Pt 2:1 samples before
°
3
50 C are also summarized in Table 1.
3
methane combustion reaction: (a) Pd K-edge and (b) Pt L -edge
Obviously (Table 1 and Fig. 2), the higher the Pd-Pt ratio is,
the more efficient the combustion at lower temperatures is. As
metal dispersions also increase with the ratio, a hypothesis on
the negative effect of Pt due to larger particle size was verified
by calculating catalyst activity per mole of surface Pd atoms
because of its significantly higher activity than that of Pt.
Assuming for now the structure-insensitive reaction, surface-
normalized activities (turnover frequency, TOF) should stay
constant per active component irrespective of nanoparticle size.
For the calculation, first order to methane and zero order to
To shed light on the mechanism of the effect of Pt, an in situ
X-ray absorption near edge structure (XANES) and EXAFS
spectroscopy measurements were conducted for the
monometallic Pd, Pt and Pd-Pt 2:1 catalysts, which represent
the central point of the negative effect of Pt (Figs. 2 and 3) with
the dispersion (31%), which is rather similar to Pd (43%). The
measurements were performed in
a
dry lean methane-air
O) over a
mixture (O /CH molar ratio of 134, < 1 ppm H
2
4
2
temperature range of 200 to 500 °C and compared to the
[
10]
hydrogen were assumed
,
and the found rate constants were
standard materials (Pd and Pt foils and PdO). Fig. 4 shows the
o
used to calculate the initial reaction rates at 350 C (Fig. 3).
As seen in Fig. 3, the surface-normalized activities drop by
an order of magnitude with the increase in Pt content in the
bimetallic catalysts, indicating that Pt does not simply dilute the
active Pd surface. Pd-surface-normalized activities were
calculated based on the assumption of the same surface Pd-Pt
ratio as in the bulk. The extent of Pd segregation to the surface
3
Pd K-edge and Pt L -edge XANES spectra of the calcined Pd, Pt
and Pd-Pt samples before the reaction. The Pd K-edge XANES
probes the electronic transition from the 1s to 5p orbitals and is
sensitive to both oxidation state and coordination environment of
[40]
the Pd atoms. The calcined Pd-Pt 2:1 sample compared well
to that of the Pd foil, indicating that the majority of Pd is in the
zero valent state. On the contrary, the spectrum of the calcined
monometallic Pd sample resembled that of PdO, implying that
Pd is mostly oxidized to Pd(II) in this sample. Thus, the
presence of Pt in bimetallic Pd-Pt particles leads to the
stabilization of Pd(0) in the particles towards oxidation, in
°
after oxidative aging at 750 C was found to be insignificant in
[
37]
[38]
one study but was claimed to be important in the another. If
it happened, the decrease in Pd-surface-normalized activity with
Pt addition would be even more dramatic.
Pd-catalyzed methane combustion is known as a structure-
sensitive reaction, with a 65-fold increase in TOF with Pd
agreement with what is already known for the calcined Pd-Pt
[14,37]
systems.
3
The Pt L -edge XANES spectra showed the same
[
27]
dispersion decrease from 50% to 10%. In this work, TOF for
the monometallic Pd catalyst is the largest value, even though
the dispersion in some Pd-Pt catalysts decreases to 16% upon
Pt addition (Table 1). Thus, the effect of Pt addition cannot be
explained by the reaction structure sensitivity either, which
should bring about an increase rather than a decrease in TOF.
features between the Pt and Pd-Pt 2:1 samples, indicating that
there is no difference in the oxidation state of Pt upon Pd
addition.
4
The in situ measurement under a CH /dry air mixture shows
a substantial change in the speciation of Pd upon increases in
temperature. The calcined Pd sample consisted only of PdO;
thus, to compare it with the bimetallic catalyst containing mostly
Pd(0), reduction of the monometallic Pd catalyst was conducted
at 350 °C for 2 h under an H
edge in E space for the supported Pd and Pd-Pt 2:1 under in situ
CH /dry air conditions. The spectra were collected at
2
gas stream. Fig. 5 shows the Pd K-
4
incrementally increasing temperatures in methane/air flows.
Upon increases in temperature, the Pd-Pt 2:1 sample slowly
became more oxidized, whereas rapid oxidation was seen in the
monometallic Pd sample. To understand the change in
speciation of Pd in the Pd-Pt 2:1 and Pd samples, linear
combination fitting using Pd(0) and PdO standards was
performed at each temperature; the results are shown in Table 2
and Fig. 6.
Figure 3. Initial turnover frequencies normalized per surface Pd atoms (or Pt
for monometallic Pt) at 350 C as a function of Pd/Pt ratio assuming the same
o
surface and bulk composition
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ChemPhysChem
10.1002/cphc.201600993
ARTICLE
Figure 7. Pd K-edge EXAFS spectra in R-space for (a) Pd-Pt 2:1 and (b)
4
Figure 5. XANES spectra under CH /dry air conditions for: (a) calcined Pd-Pt
4
reduced Pd in CH /dry air environment
2
:1 sample and (b) reduced Pd sample
Fig. 7 shows the Fourier-transformed Pd K-edge EXAFS plot
in R-space as a function of temperature for the Pd-Pt and
4
Table 2. Linear combination fitting for Pd-Pt 2:1 sample in CH /dry air
environment
4
reduced Pd samples under in situ CH /dry air condition. The
Reaction temperature, °C
Before reaction, calcined
Pd(II) from PdO, wt.%
Pd(0), wt.%
peaks in the 1.9 Å to 2.5 Å range correspond to first-shell Pd-Pd
and Pd-Pt contributions, whereas the peak at 1.5 Å corresponds
to Pd-O first-shell contributions. For the Pd-Pt 2:1 sample, the
amplitude of the Pd-Pd and Pd-Pt contributions decreases while
a new Pd-O contribution emerges beyond 300 °C, which agrees
well with the above linear-combination fitting data. Pt EXAFS
data showed no changes over the entire temperature range
examined (not shown). For the Pd K-edge data shown in Fig.
< 2
4
> 98
96
2
2
3
4
5
00°C
50°C
00°C
00°C
00°C
16
37
62
90
84
63
38
10
The linear combination fitting shows that the Pd-Pt 2:1
sample was slightly oxidized at 200 °C under CH /dry air
conditions, and the level of oxidation increased linearly up to
0% at 500 °C. Even at 500°C, significant amounts of Pd(0) are
7
(a), the intensity of the Pd-M peaks is reduced in amplitude and
4
broadened at higher temperatures due to increased thermal
disorder in the sample. As a consequence of this thermal
broadening, it was not possible to fit EXAFS data accurately at
various temperatures to extract quantitative coordination
numbers for each of the Pd-O, Pd-Pd and Pd-Pt contributions.
The Pd-Pt 2:1 sample at 500 °C shows a significant Pd-O
contribution but still retains weak Pd-M contributions. In addition,
a weak second-shell Pd-Pd contribution from PdO is seen at ca.
9
still present in these particles. The Pd-Pt 2:1 sample still showed
the metallic Pd feature with a slightly declined intensity and an
2 3
edge shift, which was not observed in the Pd/Al O sample. The
EXAFS results below also verify that small amounts of Pd(0) are
still present in the bimetallic sample. In the case of the reduced
monometallic Pd sample, nearly complete Pd oxidation to PdO
is seen at 200 °C, which was completed by 250 °C, and there
were no further spectral changes beyond that temperature. It is
3
Å for the 500 °C bimetallic sample. Fig. 7b shows the Pd K-
edge EXAFS plot in the R-space of the reduced monometallic
Pd sample at various temperatures under CH /dry air conditions;
4
[
41]
known
that surface Pd oxidation is complete at room
beyond 200 °C, the EXAFS mainly shows the Pd-O first-shell
temperature and the bulk oxidation is diffusion limited and takes
place at 200 – 700 °C. The ignition curve measured for the
prereduced Pd catalyst was identical to the oxidized catalyst as
shown in Fig. 2 (within a 3% conversion variation as a typical
measurement error), in agreement with the in situ X-ray
absorption results.
contribution at 1.5 Å and a second-shell Pd-Pd contribution at
3
.0 Å (due to PdO). This agrees with the above XANES data,
which showed that the pure Pd sample is nearly completely
oxidized by 200 °C.
Thus, the in situ X-ray absorption studies revealed that i)
irrespective of Pd’s initial reduced or oxidized state, only PdO
°
exists at 200-500 C under oxidative reaction conditions; ii) the
1
00
state of Pt is not affected by the presence of Pd and does not
change under reaction conditions; and iii) the Pd/PdO weight
ratio in bimetallic Pd-Pt 2:1 catalysts decreases from 98/2 to
Pd-Pt 2:1
Linear fit
8
6
4
2
0
0
0
0
0
1
0/90 in the 200-500 °C range during the combustion.
Coupling these observations with the kinetic data indicating
the negative effect of the addition of Pt to the performance of the
Pd catalyst, which is not connected to surface dilution, size
effect or structure sensitivity (Figs. 2 and 3, Table 1), it follows
that Pt promotes the formation of a reduced Pd phase, which is
significantly less active than in the methane combustion reaction
as opposed to the oxidized Pd. A similar negative effect of
2
00
250
300
350
400
450
500
T(°C)
Figure 6. Linear combination fitting results of the Pd K-edge XANES spectra
showing the percentage of PdO formed as a function of temperature in
[
6]
metallic Pd was reported earlier by Farrauto et al. , and the
active oxidized Pd working state was also claimed in an
4
CH /dry air environment
[
9]
operando Raman study. Metallic Pd is characterized by higher
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ChemPhysChem
10.1002/cphc.201600993
ARTICLE
activation barrier for C-H bond activation in methane as
synthesis of PVP-protected nanoparticles. Bimetallic Pd-Pt nanoparticles
were synthesized with the co-reduction of metal precursors with the
required molar ratios of Pd:Pt. The rest of the process was exactly the
same as that of the monometallic nanoparticles. A dark colloidal was
obtained at the end of synthesis without any precipitates. Nanoparticles
[
42]
compared to PdO clusters. The current study provides in situ
evidence of the importance of oxidized Pd and its dependence
on the co-presence of Pt for dry lean methane combustion in the
2
00-500 °C temperature range.
2 3
were supported on γ-Al O by acetone precipitation to obtain 0.3 wt.%
metal loading. All samples were dried and calcined in air at 550 °C for 16
h. Hereafter, the catalyst after calcination that is ready for the reaction is
referred to as the calcined (or oxidized) catalyst.
Conclusions
Catalyst characterizations. The metal loading on catalysts was
determined by NAA. Samples were irradiated for 110 s in the Cd-
shielded, epithermal site of the reactor core. They were counted for 30
min each on an Aptec CS11-A31C gamma detector approximately 12 h
after irradiation. TEM photographs were taken with a JEOL-2100 TEM
device operating at 200 kV. CO pulse chemisorption analysis was
performed with an AutoChem 2920 instrument (Micromeritics, USA).
Prior to this analysis, approximately 100 mg of the samples was reduced
2 3
Pd-Pt/γ-Al O catalysts with Pd/Pt molar ratios of 0.25 to 4 were
studied in a dry lean methane combustion in the temperature
°
range of 200-500 C, along with monometallic Pt and oxidized
and reduced monometallic Pd catalysts. The samples were pre-
°
calcined at 550 C for 16 h and assessed in the first ignition
temperature-programmed reaction. The higher the Pt proportion,
the lower the catalyst activity per gram of metal(s) and their
surface proportion, as determined by CO chemisorption. The
activity drop was too dramatic and could not be explained by the
dispersion decrease or reaction structure sensitivity. In situ
XANES and EXAFS spectroscopy measurements were
conducted for monometallic Pd, Pt and Pd-Pt 2:1 catalysts.
Monometallic Pd is fully oxidized in the investigated temperature
range, whereas the addition of Pt promotes Pd reduction even
under the reactive oxidizing environment. The Pd/PdO weight
2
in 25 ml/min of 10% H in Ar from room temperature to 550 °C at the rate
of 20 °C/min. Then, pulse chemisorption was performed by dosing 3%
CO in He at room temperature. The volumetric flow rates of CO in He
loop gas and He carrier gas were adjusted to 25 ml/min. CO uptake per
2 3
gram of pure support (γ-Al O ) was also evaluated using 2 g of alumina,
which was subtracted from the values for the supported catalysts.
In situ X-ray absorption (XAS) measurements were conducted at
Canadian Light Source. Full in situ methane oxidation XAS
ratio in bimetallic Pd-Pt 2:1 catalysts decreases from 98/2 to
measurements were performed for the Pd K-edge and Pt L -edge on the
3
°
1
0/90 in the 200-500 C temperature range under reaction
Hard X-ray MicroAnalysis (HXMA) beamline. The energy range used for
conditions. Thus, Pt promotes the formation of a reduced Pd
phase, which is significantly less active in the methane
combustion reaction as compared to the oxidized Pd. The study
sheds light on the effect of Pt on the state of an active Pd
surface under low-temperature dry lean methane combustion
conditions, which is of importance for methane-emission control
devices. Care should be exercised not to extrapolate these
findings on the negative effect of Pt on Pd catalyst performance
in methane-rich combustion, at different temperature ranges, in
the presence of water in the feed (as in automotive converters)
and long-term catalyst stability.
the Pd K-edge was from 24,050 eV to 25,400 eV. Pd K-edge
experiments were conducted using a Si (220) monochromator crystal, Pt
2 3
mirrors and ion chambers filled with an N /Ar gas mixture, while Pt L -
edge spectra were collected using a Si (111) crystal and Rh mirrors. The
samples were loaded into a six-shooter sample cell (Fig. 8) as pressed
pellets of alumina supports containing ca. 2 wt% metal elements and
data was collected in transmission mode. The entire setup for the in situ
XAS measurements is depicted in Fig. 9.
Experimental Section
Materials. Hexachloroplatinic (IV) acid (H
2
PtCl
6
, 8% w/v solution, Sigma-
Figure 8. Six-shooter sample holder for in situ XAS data collection. The total
diameter of the six shooter is 18 mm, and the diameter of each hole for the
sample is 4 mm. The small hole at the top connects to the thermocouple, and
the larger central hole is used for beam alignment
Aldrich), Pd (II) acetate (Pd(AcO) , Sigma-Aldrich), 1,4-dioxane (>99.9%
2
Sigma-Aldrich), poly N-vinylpyrrolidone (PVP, average molecular weight
of 40,000, Sigma-Aldrich), ethylene glycol (>99.9%, Fischer Scientific),
gamma alumina (γ-Al
2
O
3
, average pore size 58 Å, specific surface area
55 m /g, 100 µm particle diameter, Sigma-Aldrich), and Pd oxide (PdO,
9.97% trace metal basis, Sigma-Aldrich) for the in situ EXAFS analysis
2
1
9
were used as received. Milli-Q water (18.2 MΩ.cm) was used.
Catalyst preparation. Pd and Pt monometallic nanoparticles were
synthesized by the one-step alcohol reduction method on the basis of 0.2
2 6
mmol metal. To synthesize Pt nanoparticles, H PtCl and PVP with a
molar ratio of 40 PVP to metal were added to a 500-ml flask and mixed
with 80 vol.% ethylene glycol in a water solution. In the case of Pd
Figure 9. In situ methane oxidation setup at the HXMA beam line (left) and the
sample holder in the quartz tube (right). The blue circled part is a small
furnace for the in situ XAS measurements, and the sample holder within the
quartz tube resides in the furnace
2
nanoparticle synthesis, Pd(AcO) was first dissolved in 10 ml of dioxane
and then mixed with the aforementioned amount of PVP and solvent
solution. The resulting mixture was stirred and heated until it boiled and
then refluxed in air under vigorous stirring for 3 h to complete the
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ChemPhysChem
10.1002/cphc.201600993
ARTICLE
Initially, calcined monometallic and bimetallic samples (Pd and Pd-Pt 2:1)
were measured, and then, in situ methane oxidation measurements were
conducted. For reference compounds, Pd and Pt foils and PdO were
analyzed. Two-dimensional mapping of the sample holder was
conducted before the measurement to find the exact location of each
sample. The range of mapping was ±15 mm from the zero point using the
motorized set-up at the beam line. For methane oxidation under dry
the Government of Saskatchewan, Western Economic
Diversification Canada, the National Research Council Canada,
and the Canadian Institutes of Health Research. We thank Dr.
John Duke (University of Alberta) for NAA, and Dr. Ning Chen
for assistance with XAS measurements on the HXMA beamline.
conditions, 0.1% CH
O) were combined. The samples were incrementally heated from 200
to 500 °C under the flow of methane and air in dry conditions. The flow
rates of CH /N and compressed air used were 154 ml/min and 103
4 2
in N and compressed air (ultra-zero level, <2 ppm
Keywords: bimetallic nanoparticles • reaction mechanism • X-
ray absorption spectroscopy • heterogeneous catalysis •
methane oxidation
H
2
4
2
ml/min, respectively.
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ChemPhysChem
10.1002/cphc.201600993
ARTICLE
Entry for the Table of Contents
ARTICLE
H. Nassiri, K.-E. Lee, Y. Hu, R.E. Hayes,
R.W.J. Scott, N. Semagina
Platinum inhibits low-temperature
dry lean methane combustion via
palladium reduction in Pd-Pt/Al
2 3
O :
Active site “spoiler:” Platinum reduces active PdO to inactive Pd(0) in dry methane
An in situ X-ray absorption study
combustion up to 773 K
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