Effect of surface oxygen on the activation of methane on palladium and platinum surfaces

Article abstract of DOI:10.1021/cs500510x

Active surfaces for the catalytic oxidation of CH₄ on Pd and Pt were identified using infrared reflection absorption spectroscopy (IRAS) with a wide spectrum range (4000-450 cm^-1) that is capable of measuring both the surface species and changes specific to the surface. Both the metallic Pd and Pt surfaces were found to be significantly less active for the oxidation of CH₄ than the PdO surface under near-stoichiometric conditions. However, the metallic surfaces become very active under oxygen-poor conditions. This work represents the first evidence of such high activity for CH₄ oxidation on metallic Pt-group metal surfaces. A comparison of the catalytic behaviors of CH₄ and CO oxidation demonstrates that the chemisorbed oxygen with near-saturated coverage suppresses the activation of CH₄; additionally, the involvement of an unoccupied surface site, surface cation, or metallic atom can significantly enhance the activity for the activation of CH₄. The current results also demonstrate that oxygen is depleted sharply when switching from the combustion to the reforming region in the partial oxidation of methane to syngas.

Full text of DOI:10.1021/cs500510x

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Article
The Effect of Surface Oxygen on the Activation
of Methane on Palladium and Platinum Surfaces
Xuefei Weng, Hongjia Ren, Ming-shu Chen, and Huilin Wan
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/cs500510x • Publication Date (Web): 27 Jun 2014
Downloaded from http://pubs.acs.org on July 4, 2014
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The Effect of Surface Oxygen on the Activation of Methane on Palladium and Platinum
Surfaces
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Xuefei Weng, Hongjia Ren, Mingshu Chen*, Huilin Wan
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State Key Laboratory of Physical Chemistry of Solid Surfaces, National Engineering Laboratory
for Green Chemical Productions of Alcohols–Ethers–Esters, Department of Chemistry, College
of Chemistry and Chemical Engineering, Xiamen University, Xiamen 361005, Fujian, CHINA.
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Abstract: Active surfaces for the catalytic oxidation of CH4 on Pd and Pt were identified using
infrared reflection absorption spectroscopy (IRAS) with a wide spectrum range (4000ꢀ450 cmꢀ1)
that is capable of measuring both the surface species and changes specific to the surface. Both the
metallic Pd and Pt surfaces were found to be significantly less active for the oxidation of CH4
than the PdO surface under nearꢀstoichiometric conditions. However, the metallic surfaces
become very active under oxygenꢀpoor conditions. This work represents the first evidence of
such high activity for CH4 oxidation on metallic Ptꢀgroup metal surfaces. A comparison of the
catalytic behaviors of CH4 and CO oxidation demonstrates that the chemisorbed oxygen with
nearꢀsaturated coverage suppresses the activation of CH4; additionally, the involvement of an
unoccupied surface site, surface cation or metallic atom can significantly enhance the activity for
the activation of CH4. The current results also demonstrate that oxygen is depleted sharply when
switching from the combustion to the reforming region in the partial oxidation of methane to
syngas.
KEYWORDS: Methane oxidation; Palladium; Platinum; Active surface; IRAS
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1. Introduction
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Methane is a clean, lowꢀcarbon, and highꢀefficiency energy source and is abundantly found
in natural gas, coal bed gas, shale gas, and CH4 clathrates.1ꢀ7 Methane is also a potential
greenhouse gas, where catalytic oxidation can be used to reduce emissions of CH4.8ꢀ12 The
relative abundance of CH4 makes it an attractive alternative to oil. The direct conversion of
methane to more valuable chemicals, such as its selective oxidation to methanol and
formaldehyde, oxidative coupling to ethylene, and aromatic dehydrogenation to benzene, has
been thwarted by the low selectivity, activity, and stability of existing catalysts.1ꢀ3,13ꢀ22 The partial
oxidation of CH4 (POM) to synthetic gas (syngas, CO+H2), followed by FischerꢀTropsch
chemistry,1ꢀ3,13,14 has become a potential process for the development of liquefied alcohols,
synthetic fuels, and light alkenes (key chemicals in industry). Although the partial oxidation of
CH4,1ꢀ3,13,14,20ꢀ24 as well as the activation of CH4,25ꢀ41 has been extensively studied in recent years,
the active surfaces and the mechanism leading to the formation of CO and H2, by either a direct
or combustionꢀreforming pathway, have not been fully clarified. The oxidation of CO and H2
progresses rapidly on Pdꢀ, Rhꢀ and Ptꢀbased catalysts,42ꢀ48 and the direct POM products may
follow from subsequent oxidation to CO2 and H2O in the front part of the catalyst bed under O2ꢀ
rich conditions, resulting in the appearance of complete combustion. In such a case, the rear part
of the catalyst bed suffers from O2 deficiency; as a result, the CH4 reforms CO2 and H2O to
reproduce CO and H2. The relative contribution of each process is difficult to identify because
the products are similar. Moreover, because the oxidation of CH4 is a highly exothermic reaction,
temperature gradients and reactant gradients are typically encountered during the catalytic partial
oxidation of CH4.49ꢀ51 In most cases, a hot spot of several hundred Kelvin arises, and the reaction
conditions change from oxidizing to reducing, which can also affect the nature of the active
sites.49ꢀ51 The complexity of this process, along with the support and additive effects, results in
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controversial knowledge regarding the nature of the catalytically active sites and the catalytic
reaction mechanism for the oxidation of CH4.1ꢀ3,13,14,20ꢀ22,49ꢀ55
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Due to the significantly faster progression of the oxidation of CH4 on Ptꢀgroup metals
compared to those for reforming, only a few percent of the packed catalyst is sufficient to fulfill
the combustion reaction.28,49,51ꢀ53 Moreover, a very sharp reactant gradient occurs in the front part
of the catalyst bed for POM.51ꢀ53,56,57 Because such high inhomogeneity is encountered in POM
on the highꢀsurfaceꢀarea supported catalysts, it is difficult to elucidate a reliable structureꢀactivity
relationship. Recently, Chin et al. performed detailed experiments combined with a theoretical
approach, offering significant insight into the reaction mechanism for POM.29,34,42,43 Our
previous in situ spectroscopic characterizations and kinetic measurements confirmed that
combustionꢀreforming is the main process by which the formation of syngas occurs, while direct
partial oxidation is also possible for very short contact times (e.g., an extremely thin catalyst
bed).20ꢀ22 It was also found that the catalysts of supported Rh, Pd, and Pt were in the fully
oxidized state as observed by in situ Raman spectroscopy before the ignition of POM, but turned
to be a metallic state upon ignition.20ꢀ22 However, it is still not known whether an oxidized region
exists at the very front of the catalyst bed with metalꢀoxide amounts that are below the detection
limit of in situ Raman spectroscopy.
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In this study, resistively heated polycrystalline Pd and Pt thin foils served as model catalysts
for the oxidation of CH4, similar to the experimental setup in several previous reports.56,58ꢀ62 We
emphasize that the surface temperature was measured directly by a thermocouple attached to the
metal foil; additionally, the studied surface was under the same reaction atmosphere at each
moment, allowing our setup to overcome the hot spot and reactant gradient problems
encountered in the combustion and partial oxidation of CH4. The reaction products were mainly
found to be CO2 and H2O using an online gas chromatograph, and the reaction rate was measured
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continuously by monitoring the total pressure change in a batch reactor, which is similar to the
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results in our previous study of CO oxidation on Pd . Such experiments, in which the oxygen
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pressure decreases from its initial value to depletion, imitate the processes that occur in the front
part of the catalyst bed, where the O2 partial pressure decreases from the initial value to complete
depletion under realistic POM reaction conditions. Moreover, in situ infrared reflection
absorption spectroscopy (IRAS) with a wide spectrum range (4000ꢀ450 cmꢀ1), which covers the
stretching vibrations for most oxides, was combined to collect surface information under these
realistic catalytic reaction conditions.48 Furthermore, reactions under conditions of excess CH4
were examined. The most active surfaces for the catalytic oxidation of CH4 over Pd and Pt were
successfully identified. PdO was found to be significantly more active than both the metallic Pd
and Pt at nearꢀstoichiometric reaction conditions. However, the most active regions were found
to be the metallic Pd and Pt surfaces under oxygenꢀpoor conditions, where the surfaces were not
saturated by chemisorbed oxygen. The oxidation of CO on the Pd surface, which is a wellꢀ
documented catalytic reaction, was compared to obtain a better understanding of the observed
phenomenon for the oxidation of CH4.
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2. Experimental Section
Experiments were carried out in a batch reactor (0.5 L) connected to a vacuum system and
equipped with basic surface science techniques, including an ion gun for surface cleaning using
Ar+ sputtering, lowꢀenergy electron diffraction (LEED) and Auger electron spectroscopy (AES).
The reactor was also furnished with in situ infrared reflection absorption spectrometry (IRAS)
and onꢀline Agilent gas chromatography (GC) techniques as well as a MKS precise pressure
sensor. Details concerning the system used for our model catalysis study are provided in our
previous publications.48,63 The in situ reactor can accommodate a pressure range of UHV (ultraꢀ
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high vacuum) to 1 atm and sample temperatures of 80–1,200 K. The wide spectrum range of in
situ IRAS allows for the measurement of surface oxide formation and reduction and can be used
to establish an authentic relationship between the surface structure and its catalytic activity.
The Pd and Pt thin foils measured 0.1 mm in thickness (Alfa Aesar) and were cleaned by
chemical treatment before introduction into the vacuum chamber. The samples were heated
resistively, while their temperatures were measured using a Cꢀtype thermocouple (5%Re/95%Wꢀ
26%Re/74%W). Prior to the reaction, the samples were further cleaned and oxidized with O2 at
600 K for 5 min and then were annealed under UHV at 900–1,000 K to remove the residual
oxygen. Average surface atom densities of 1.53×1015 and 1.47×1015 atoms/cm2 for Pd and Pt,
respectively, were used to estimate the turnover rate.
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The possible products of the catalytic oxidation of CH4 on Pd and Pt, respectively, are CO2
and H2O (complete oxidation) and CO and H2 (partial oxidation). When crosschecked with an
onꢀline Agilent gas chromatograph,63 the main products were confirmed as CO2 and H2O (>
99.9%) under the reaction conditions examined in this study. This is consistent with the general
kinetics results of POM, namely, that feeding sufficient oxygen (CH4:O2=1:2) yields complete
combustion products.1ꢀ3,13,14 Additionally, the oxidation of either CO or H2 on both Pd and Pt
occurs at significantly faster rates than that of CH4.28,29,42,45ꢀ48 The saturation pressures of CO2
and H2O are both below 10ꢀ6 Torr at the temperature of liquid nitrogen (78 K), while those of
CH4, O2 and CO are approximately 10.2, 160 and 360 Torr, respectively; in contrast, H2 is not
condensable at this temperature. Therefore, when using a liquid nitrogen trap with initial
pressures of CH4 and O2 that are less than 10 and 160 Torr, respectively, the CO2 and H2O
formed during the reaction will predominantly be trapped, while CO and H2 will not be trapped.
Therefore, the CH4 oxidation rate (CH4 + 2O2 = CO2↓ + 2H2O↓) can be continually monitored
and computed from the decrease in total pressure. Partial oxidation, CH4 + 1/2 O2 = CO + 2H2,
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will result in an increase in the total pressure. Hence, a decrease of the total pressure is correlated
with the consumption of CH4 and O2.
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Highꢀpurity CH4 and O2 (Hong Kong Specialty Gases Co., Ltd.) were further cleaned in a
liquid nitrogen trap before introduction into the reaction cell. The cell was filled with a CH4:O2
gas mixture at the desired ratio and specified pressure at room temperature. Then, approximately
oneꢀseventh of the reactor was immersed into the liquid nitrogen bath to further clean the reactant
gases and to remove the majority of the combustion products of CO2 and H2O. The sample
temperatures were then increased to the reaction temperature to initiate the kinetic measurements.
In the present study, pressure changes were measured using a MKS precision pressure sensor.
The surface changes occurring under the reaction conditions, in particular, the formation and
reduction of surface oxides, were characterized by our homeꢀbuilt in situ IRAS. The
polycrystalline Pd and Pt foils were cleaned by preꢀoxidizing in oxygen followed by annealing at
1,000 K in UHV for 10 min to completely remove any residual oxygen.46ꢀ48,64 The obtained clean
surfaces were used to collect the IRAS background spectrum.
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3. Results and Discussion
Figure 1 shows in situ IRAS spectra for the oxidation of CH4 on a Pd foil at 643 K. Upon
the exposure of the clean Pd foil to the reactant mixture (CH4 and O2 of 9 and 1 Torr,
respectively), while heating to 643 K, the Pd surface was oxidized to form PdO, as indicated by
the two IR peaks at approximately 645 and 600 cmꢀ1. The assignments of these IR peaks were
addressed previously in detail.48 The formation of PdO was also confirmed by in situ Raman
spectroscopy and was found to be consistent with the results presented in several previous
approaches.56,58ꢀ62,65ꢀ69 The ignition of the POM on Rh/Al2O3 was found to be potentially related
to the amount of Rh present in the higher oxidation state;51,67 this finding also sustains the
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interpretation that the oxide is an active surface. It should be noted that owing to the liquid
nitrogen trap, the formation products of H2O and CO2 were held at very low levels in the gas
phase, as can be observed in the in situ IR spectra (Figure 1).
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Figure 1. In situ IRAS spectra depict the formation and reduction of the Pd surfaceꢀoxide during
the oxidation of CH4 at 643 K on a polycrystalline Pd thin foil. The bands of PdO as well as the
gaseous phases of CH4, CO2, and H2O are indicated in the figure. The initial CH4 and O2
pressures were 9 and 1 Torr in the batch reactor. The reaction times in seconds are labeled on the
figure by corresponding spectra. The insets show the IRAS technique and cartoonꢀlike surface
structures of the PdO and chemisorbed oxygen covered surfaces (O/Pd).
The reaction kinetics were monitored based on changes in pressure, as shown in Figure 2A. The
computed reaction rate in TOF (the turn over frequency, i.e., CH4 molecules converted per
surface Pd atom per second) is displayed in Figure 2B. The CH4 conversion rate of the TOF was
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calculated based on the geometric Pd metal surface area, assuming an average Pd surface atom
density of 1.53×1015 atoms/cmꢀ2. The total pressure decreases rapidly at the beginning,
concurrently accompanied by the appearance of PdO (Figures 1 and 2A). As the reaction
proceeds, the O2 partial pressure decreases quickly, while the CH4 partial pressure remains at a
nearly constant level because of the significant excess quantity (Figure 2A). The amount of PdO
formed increases at the beginning, then is reduced at very low O2 pressures (<0.2 Torr), in
addition to an observed decrease in the reaction rate (Figure 2B). It should be noted that the asꢀ
formed PdO includes the bulk oxide, while the catalytic reaction takes place only on the surface;
hence, the reaction rate need not follow the intensity of the PdO IR band. The observed decrease
of the reaction rate as the O2 pressure decreased and the PdO reduced leads to a complicated
explanation, which may be related to the reaction order with respect to O2 pressure as well as to
the decrease of the effective active area of PdO. After approximately 890 seconds, the PdO peaks
vanish, and the reaction rate reaches its lowest point (the TOF has decreased from 20 to 5). The
CH4 oxidation rate then increases on this reduced surface as the oxygen partial pressure further
decreases. This sequence can be reproduced at various reaction temperatures. The critical
pressure ratio of O2/CH4 increases as the reaction temperature increases, as can be observed from
the extent of the pressure drop from the critical point to the end of the reaction, as shown in
Figure 3A.
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Figure 2. (A) The changes of the total pressure (black line) and partial pressure (blue line) are
displayed as functions of the reaction time for a typical experiment of CH4 oxidation on a
polycrystalline Pd thin foil at 643 K. The initial CH4 and O2 pressures are 9 and 1 Torr,
respectively. (B) The computed reaction rate (pink line) and IR intensity of PdO (black line) are
displayed as functions of the reaction time. The insets show cartoonꢀlike surface structure
models, according to the in situ IRAS results.
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Figure 3. The changes in total pressure (black lines) and the computed reaction rates (pink
lines) as functions of the reaction time for CH4 oxidation over polycrystalline Pd and Pt thin foils
at the indicated reaction temperatures. (A) Over a Pd surface with an initial CH4/O2 ratio of 9/1
and a total pressure of 10 Torr and (B) over a Pt surface with an initial CH4/O2 ratio of 10/3 and a
total pressure of 13 Torr.
For comparison, CH4 oxidation was conducted on a polycrystalline Pt foil. With an initial
CH4/O2 pressure ratio of 10/3, the CH4 oxidation rate on Pt at 750 K was significantly lower than
that on Pd at 643 K, although the rate increases continuously as the reaction proceeds, i.e., the O2
partial pressure decreases (Figure 4A). The CH4 conversion rate of TOF was calculated based on
the geometric Pt metal surface area. The finding of a significantly lower rate for the oxidation of
CH4 on Pt compared to Pd was also reported previously.56,59,70 Additionally, the ignition
temperature for the oxidation of CH4 on Pt/Al2O3 increased with an increase in oxygen
concentration in the reactant mixture,67 which also corroborates the suppression effect of oxygen.
At a critical point of a CH4/O2 pressure ratio of approximately 20/1, the CH4 reaction rate
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increases quickly and achieves a maximum that is approximately 2 orders of magnitude higher
than the rate achieved at nearꢀstoichiometric conditions. Such effect of O2 pressure on the
reaction rate for the oxidation of CH4 was also observed on supported Pt catalysts, where a
maximum TOF of approximately 1,300 was achieved with an CH4/O2 pressure ratio of
approximately 10/1 at 873 K.42 We emphasize that the critical O2/CH4 pressure ratio increases
with the reaction temperature, as shown in Figure 3B. Because platinum oxides were not detected
(a band with a frequency of 500ꢀ700 cmꢀ1 would be expected 71,72) by in situ IRAS and Raman
spectroscopies during the oxidation of CH4 on the Pt thin foil, it is reasonable to conclude that
metallic Pt is the active surface under the present conditions, consistent with a recent report.42
Here, it should be noted that platinum oxide does form for oxideꢀsupported Pt nanoparticles
before the point of ignition under POM conditions, but reduces to the metallic state upon
ignitation.21,22 These results demonstrate that PdO is significantly more active for CH4 oxidation
than either metallic Pd or Pt under comparable reaction conditions. Previous kinetic
measurements on Pd also yielded observations of a significant drop of the reaction rate upon the
transformation of PdO to the metallic Pd phase as the reaction temperature increased to values
above 950 K (1.5 Torr O2 + 0.3 Torr CH4).56 A recent in situ surface Xꢀray diffraction study also
confirmed PdO as one of the active surfaces for CH4 oxidation.66 The temperatureꢀprogrammed
desorption of light alkanes revealed that CH4 molecules bond more strongly on PdO surfaces
than on Pd surfaces due to the formation of a sigmaꢀcomplex,68 which may be one reason for the
corresponding higher reaction rate.
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Figure 4. The changes of the total pressure (black line) and the computed reaction rates (pink
line) as functions of the reaction time for (A) CH4 oxidation over a polycrystalline Pt thin foil at
750 K with an initial CH4/O2 ratio of 10/3 and a total pressure of 13 Torr; (B) CO oxidation over
the polycrystalline Pd thin foil at 523 K with an initial CO/O2 ratio of 1/1.2 and a total pressure
of 5 Torr. The insets show cartoonꢀlike surface structure models based on IRAS measurements.
The above results demonstrate that significantly higher reaction rates (Figures 2, 3 and 4)，
specifically, 2ꢀ3 orders of magnitude higher than those observed at nearꢀstoichiometric
conditions, are achieved on Pd and Pt surfaces under oxygenꢀpoor conditions. Such high rates are
even more remarkable when compared to those observed on PdO surfaces, especially at higher
reaction temperatures (Figure 3). To better understand this phenomenon, CO oxidation was
performed in the same reactor using the same Pd sample. It should be noted that such a sharp
enhancement of the reaction rate as a function of the CO partial pressure has previously been
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observed for the oxidation of CO on Pd, Rh, and Pt metals.46ꢀ48 As observed in Figure 4B, CO
oxidation was conducted on the Pd thin foil at 523 K in the same reactor as that used for CH4
oxidation. Beginning with an oxygenꢀrich condition, the extent of the total pressure decrease
increases with reaction progress, i.e., the reaction rate increases as the CO partial pressure
decreases (a negative order of 1.2 with respect to the CO pressure), see also Figure 5A. A very
sharp pressure drop, corresponding to a TOF of 2,000ꢀ3,000 (2ꢀ3 orders of magnitude higher than
that observed at the nearꢀstoichiometric condition), is observed at the critical CO/O2 pressure
ratio. It has been well established that Pd and Pt surfaces are dominated by CO, even under
oxygenꢀrich conditions during CO oxidation; this fact results in COꢀdesorption limiting of the
reaction rate.42ꢀ48,73 As the CO partial pressure decreases, the surface coverage of the
chemisorbed CO decreases, vacating more surface sites for O2 activation and leading to a very
sharp growth of the reaction rate at low levels of surface oxygen coverage. Then, the reaction rate
either achieves a maximum value at suitable surface coverages of CO and O or meets the mass
transfer limit of CO, resulting in the shift from a COꢀdominant surface to a chemisorbed oxygenꢀ
dominant surface.48 It should be noted that in situ IRAS with a wide spectrum range has
demonstrated that PdO does not appear for CO oxidation at the above conditions.48
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In contrast, the CH4 conversion rates (TOF) on metallic Pd and Pt surfaces are low under the
aboveꢀexamined conditions, as shown in Figures 2, 4 and 5. However, the dissociation of O2 on
Pd and Pt surfaces is well known to occur quickly,74ꢀ76 which can also be substantiated from the
oxidation of CO with a TOF of several hundreds to several thousands at lower reaction
temperatures of 450ꢀ550 K (Figures 4 and 5).46ꢀ48 The lower CH4 reaction rate should then be
limited by the activation of CH4 on the Pt and Pd surfaces. Moreover, surface CHx (x=0ꢀ3)
species were not observed by subtracting the in situ IRAS spectra obtained under the reaction
conditions, when using the comparable gas phase CH4 as a background. Based on these facts, the
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surfaces of Pd and Pt under the CH4 oxidation conditions should be dominated by chemisorbed
oxygen, if the specific oxides do not present.
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Accordingly, the initial lower CH4 conversion rate observed on the Pt surface and the
increase of the reaction rate with decreasing O2 partial pressure (a negative order of
approximately 1.4 with respect to the O2 pressure) demonstrate that the surfaceꢀchemisorbed
oxygen at nearꢀsaturation coverage suppresses the activation of CH4, i.e., CH4 activation requires
unoccupied Pt sites. This is also evidenced by the similarity of the trend lines of the changes in
the total pressure and rate for the oxidation of CH4 on Pt as functions of the reaction time, in
addition to the trends recorded for the oxidation of CO on Pd and Pt (See Figure 5) 48. It has also
been proposed that a surface that is unsaturated with chemisorbed oxygen (oxygenꢀvacancy pair:
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*
O*ꢀ ) is more active than a surface that is saturated with chemisorbed oxygen, based on kinetic
measurements for the oxidation of CH4 on supported Pt nanoparticles and DFT calculations.28,34
Therefore, with decreasing in O2 pressure, the coverage of the surface chemisorbed oxygen (θO)
decreases, which then releases additional active sites on the surface (uncovered surface Pt atoms)
for the activation of CH4, leading to an increase in the reaction rate (a negative order of 1.4
respect with O2, Figure 5B). As the critical point is approached, the number of available surface
Pt sites increases significantly, resulting in a sharp increase in the reaction rate. This continues
until a maximum reaction rate has been achieved. After the maximum point, the rate decreases
with a further decrease of the O2 pressure due to the presence of an insufficient amount of surface
oxygen species.
Even at a CH4/O2 pressure ratio of 10/1, PdO initially forms on the Pd surface; hence, this
system exhibits a higher reaction rate and changes by +0.9 orders with respect to the O2 pressure
(Figure 5). PdO is reduced at low O2 partial pressures (Figures 1 and 2); after this occurs, the
behavior of CH4 oxidation on the metallic Pd at low O2 pressure is similar to that on the Pt
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surface. There are common features between the two systems, including the increase in the
reaction rates (TOFs) with decreasing CO partial pressure with respect to CO oxidation and with
decreasing O2 with respect to CH4 oxidation, and the occurrence of a sharp increase at the critical
point. It should be noted that deꢀN2O efficiency on Pd/Al2O3 catalysts in the presence of CH4 and
O2 is also significantly suppressed by O2, due to the scavenging of strongly adsorbed oxygen
species by the hydrocarbon.78
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Figure 5. The reaction order dependence for (A) CO oxidation over a Pd foil at 523 K on
CO (approximately 0ꢀ3 Torr CO, 1.5ꢀ3 Torr O2) and (B) CH4 oxidation over a Pd foil at 643 K
and over a Pt foil at 750 K on O2 (approximately 10 Torr CH4, and 0ꢀ10 Torr O2).
Furthermore, it should be emphasized that O2 can dissociate on metallic Pd surfaces to form
a chemisorbed oxygen species below the room temperature,74ꢀ76,79ꢀ82 while the formation of a
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bulk oxide requires a higher temperature and sufficient O2 partial pressure.48,83,84 The reduction
of PdO as the O2 partial pressure decreases under the CH4 oxidation conditions demonstrates that
the O2 partial pressure is essential to PdO stability in the presence of a certain amount of the
reducing gas.84 Such results also suggest that the activation/dissociation of O2 on the PdO surface
is slower than that on the metallic Pd surface as well as the fact that the lattice oxygen can
directly engage in CH4 oxidation.
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The above comparisons demonstrate that strongly chemisorbed oxygen species are involved
in the oxidation of both CH4 and CO on Pd and Pt, when no oxide has formed. Under nearꢀ
stoichiometric conditions, the surface is dominated by chemisorbed oxygen for the oxidation of
CH4, but chemisorbed CO dominates the surface for the oxidation of CO. The state of higher
activity occurs under oxygenꢀpoor conditions on a chemisorbed oxygenꢀdeficient surface during
CH4 oxidation, but under oxygenꢀrich conditions on a chemisorbed oxygenꢀrich surface during
CO oxidation. These results reveal that the Pd and Pt surfaces are rich in surface chemisorbed
oxygen for CH4 oxidation, while such oxygenꢀrich surfaces are highly active for the oxidation of
CO. This is the reason why CO2 and H2O are the main products during the oxidation of CH4 on
Pd and Pt under the present experimental conditions, except when O2 is nearly depleted. This
supports a mechanism of combustion followed by subsequent reforming for the partial oxidation
of methane.1ꢀ3,13,14 The very sharp pressure drop observed at the critical point also confirms the
complete oxidation of CH4 because partial oxidation (CH4 + 0.5O2 = CO + 2H2), including both
the direct and combustionꢀreforming paths, would result in a pressure increase.
The significantly different activities observed for the oxidation of CH4 on Pd and Pt surfaces
as a function of O2 pressure (Figure 5) demonstrate that the combination of Pd and Pt on a
supported catalyst should enhance the overall activity of the catalyst in a wide range of O2
pressures in which PdO may play the key role under oxygenꢀrich conditions, while Pt may be the
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key catalytically active surfaces under oxygenꢀpoor conditions. The better performance observed
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on a bimetallic PdꢀPt catalyst for the oxidation of CH4 can be attributed partially to the effect
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of alloying, which modifies the redox properties of Pd nanoparticles, in addition to the existence
of different active surfaces of PdO and metallic Pt.
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4. Conclusions
In summary, both PdO and metallic Pd were identified by the homeꢀbuilt wide spectrum
range in situ IRAS as catalytically active surfaces for the oxidation of CH4, depending on the
reaction temperature and the partial pressure of O2. On a bulkꢀlike Pt foil, only the metallic
surface presents as the active surface. Near stoichiometric reaction conditions, the PdO surface is
significantly more active for CH4 oxidation than either the metallic Pd or Pt surfaces. The
dissociated adsorption of O2 occurs significantly faster than that of CH4 on the Pd and Pt
surfaces, leading to a surface dominated by chemisorbed oxygen on both the metallic Pd and Pt
surfaces, which then suppresses the activation of CH4. The most active region is achieved under
oxygenꢀpoor conditions. The study reveals that the involvement of a surface metal or cation site
significantly enhances the activation rate of CH4.
ASSOCIATED CONTENT
AUTHOR INFORMATION
Corresponding Author
*Email: chenms@xmu.edu.cn. Tel.& Fax: 86ꢀ592ꢀ2183723
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Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
We gratefully acknowledge the support received by this work from the National Basic
Research Program of China (973 program: 2013CB933102), the Major Project of the Chinese
Ministry of Education (No: 309019), the National Natural Science Foundation of China
(21033006, 21073149, 21273178), the Program for Changjiang Scholars and Innovative
Research Team in University (No. IRT1036), and the Ph.D. Programs Foundation of the Chinese
Ministry of Education (No. 20110121110010). We thank Ying Lin and Ying Zeng for their
excellent assistance during the in situ measurements.
REFERENCES
1. Pena, M. A.; Gomez, J. P.; Fierro, J. L. G. Appl. Catal. A 1996, 144, 7ꢀ57.
2. Lunsford, J. H. Catal. Today 2000, 63,165ꢀ174.
3. Hickman, D. A.; Schmidt, L. D. Science 1993, 259, 343ꢀ346.
4. Murray, E. P.; Tsai, T.; Barnett, S. A. Nature 1999, 400, 649ꢀ651.
5. Trimm, D. L.; Onsan, Z. I. Catal. Rev. –Sci. Eng. 2001, 43, 31ꢀ84.
6. Park, S. D.; Vohs,J. M.; Gorte, R. J. Nature 2000, 404, 265ꢀ267.
7. Boswell, R. Science 2009, 325, 957ꢀ958.
8. Oh, S. E.; Mitchell, P. J.; Siewert, R. M. J. Catal. 1991, 132, 287ꢀ301.
9. Ciuparu, D.; Lyubovsky, M. R.; Altman, E.; Pfefferle, L. D.; Datye, A. Catal. Rev. –Sci. Eng.
2002, 44, 593ꢀ649.
10. Gelin, P.; Primet, M. Appl. Catal. B 2002, 39, 1ꢀ37.
11. Cargnello, M.; Jaen, J. J. D.; Garrido, J. C. H.; Bakhmutsky, K.; Montini, T.; Gamez,J. J. C.;
Gorte, R. J.; Fornasiero, P. Science 2012, 337, 713ꢀ717.
12. Lu, Y.; Michalow, K. A.; Matam, S. K.; Winkler, A.; Maegli, A. E.; Yoon, S.; Heel, A.;
Weidenkaff, A.; Ferri, D. Appl. Catal. B 2014, 144, 631ꢀ643.
18
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Page 19 of 23
ACS Catalysis
1
2
3
4
5
6
7
8
13. Enger, B. C.; Lødeng, R.; Holmen, A. Appl. Catal. A 2008, 346, 1ꢀ27.
14. Branco, J. B.; Ferreira, A. C.; do Rego, A. M. B.; Ferraria, A. M.; AlmeidaꢀGaschet, T. ACS
Catal. 2012, 2, 2482ꢀ2489.
15. Lee, J. S.; Oyama, S. T. Catal. Rev.-Sci. Eng., 1988, 30, 249ꢀ280.
16. Wang, L. S.; Tao, L. X.; Xie, M. S.; Xu, G. F.; Huang, J. S.; Xu, Y. D. Catal. Lett. 1993, 21,
35ꢀ41.
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
17. Periana, R. A.; Taube, D. J.; Gamble, S.; Taube, H.; Satoh, T.; Fujii, H. Science 1998, 280,
560ꢀ564.
18. Otsuka, K.; Wang, Y. Appl. Catal. A 2001, 222, 145ꢀ161.
19. Tomita, A.; Nakajima, J.; Hibino, T. Angew. Chem. Int. Ed. 2008, 47, 1462ꢀ1464.
20. Wei, Z. W. (Wei Zheng Weng); Ming, S. C. (Ming Shu Chen); Hui, L. W. (Hui Lin Wan)
Chem. Record 2002, 2, 102ꢀ112.
21. Wang, M. L.; Zheng, H. Z.; Li, J. M.; Weng, W. Z.; Xia, W. S.; Huang, C. J.; Wan, H. L.
Chem. -An Asian J. 2011, 6, 580ꢀ589.
22. Liu, Y.; Huang, F. Y.; Li, J. M.; Weng, W. Z.; Luo, C. R.; Wang, M. L.; Xia, W. S.; Huang,
C. J.; Wan, H. L. J. Catal. 2008, 256, 192ꢀ203.
23. Chalupka, K. A.; Jozwiak, W. K.; Rynkowski, J.; Maniukiewicz, W.; Casale, S.; Dzwigaj, S.
Appl. Catal. B 2014, 146, 227ꢀ236.
24. Cimino, S.; Mancino, G.; Lisi, L. Appl. Catal. B 2013, 138, 342ꢀ352.
25. Jones, W. D. Acc. Chem. Res. 2003, 36, 140ꢀ146.
26. Juurlink, L. B. F.; Killelea, D. R.; Utz, A. L. Prog. Surf. Sci. 2009, 84, 69ꢀ134.
27. Schwarz, H. Angew. Chem. Int. Ed. 2011, 50, 10096ꢀ10115.
28. Chin, Y. H.; Buda, C.; Neurock, M.; Iglesia, E. J. Am. Chem. Soc. 2011, 133, 15958ꢀ15978.
29. Choudhary, T. V.; Aksoylu, E.; Goodman, D. W. Catal. Rev.-Sci. Eng. 2003, 45, 151ꢀ203.
30. Luntz, A. C.; Bethune, D. S. J. Chem. Phys. 1989, 90, 1274ꢀ1280.
31. Duarte, R. B.; Safonova, O. V.; Krumeich, F.; Makosch, M.; van Bokhoven, J. A. ACS Catal.
2013, 3, 1956ꢀ1964.
32. Kondo, T.; Sasaki, T.; Yamamoto, S. J. Chem. Phys. 2003, 118, 760ꢀ767.
33. Fuhrmann, T.; Kinne, M.; Whelan, C. M.; Zhu, J. F.; Denecke, R.; Steinruck, H. P. Chem.
Phys. Lett. 2004, 390, 208ꢀ213.
34. Wei, J. M.; Iglesia, E. J. Phys. Chem. B 2004, 108, 4094ꢀ4103.
19
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ACS Catalysis
Page 20 of 23
1
2
3
4
5
6
7
8
9
35. Tait, S. L.; Dohnalek, Z.; Campbell, C. T.; Kay, B. D. Surf. Sci. 2005, 591, 90ꢀ107.
36. Marsh, A. L.; Becraft, K. A.; Somorjai, G. A. J. Phys. Chem. B 2005, 109, 13619ꢀ13622.
37. Psofogiannakis, G.; StꢀAmant, A.; Ternan,M. J. Phys. Chem. B 2006, 110, 24593ꢀ24605.
38. Cushing, G. W.; Navin, J. K.; Donald, S. B.; Valadez, L.; Johanek, V.; Harrison, I. J. Phys.
Chem. C 2010, 114, 17222, & 22790ꢀ22790.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
39. Enterkin, J. A.; Setthapun, W.; Elam, J. W.; Christensen, S. T.; Rabuffetti, F. A.; Marks, L.
D.; Stair, P. C.; Poeppelmeier, K. R.; Marchall, C. L. ACS Catal. 2011, 1, 629ꢀ635.
40. Zhang, R. G.; Song, L. Z.; Wang, Y. H. Appl. Surf. Sci. 2012, 258, 7154ꢀ7160.
41. Navin, J. K.; Donald, S. B.; Tinney, D. G.; Cushing, G. W.; Harrison, I. J. Chem. Phys. 2012,
136, 061101(ꢀ4).
42. Chin, Y. H.; Buda, C.; Neurock, M.; Iglesia, E. J. Catal. 2011, 283, 10ꢀ24.
43. Chin, Y. H.; Buda, C.; Neurock, M.; Iglesia, E. J. Am. Chem. Soc. 2011, 133, 15958ꢀ15978.
44. Su, X. C.; Cremer, P. S.; Shen, Y. R.; Somorjai, G. A. J. Am. Chem. Soc. 1997, 119, 3994ꢀ
4000.
45. Ackermann, M. D.; Pedersen, T. M.; Hendriksen, B. L. M.; Robach, O.; Bobaru, S. C.; Popa,
I.; Quiros, C.; Kim, H.; Hammer, B.; Ferrer, S.; Frenken, J. W. M. Phys. Rev. Lett. 2005, 95,
255505.
46. Chen, M. S.; Cai, Y.; Yan, Z.; Gath, K. K.; Axnanda,S.; Goodman, G. W. Surf. Sci. 2007,
601, 5326ꢀ5331.
47. Gao, F.; Goodman, D. W. Langmuir 2012, 26, 16540ꢀ16551.
48. Chen, M. S.; Wang, X. V.; Zhang, L. H.; Tang, Z. Y.; Wan, H. L. Langmuir 2010, 26,
18113ꢀ18118.
49. Boucouvalas, Y. ; Zhang, Z. L.; Verykios, X. E. Catal. Lett. 1994, 27, 131ꢀ142.
50. Dissanayake, D.; Rosynek, M. P.; Kharas, K. C. C.; Lunsford, J. H. J. Phys. Chem. 1993, 97,
36644ꢀ36646.
51. Grunwaldt, J. ꢀD.; Hannemann, S.; Schroer, C. G.; Baiker, A. J. Phys. Chem. B 2006, 110,
8674ꢀ8680.
52. Donazzi, A.; Beretta, A.; Groppi, G.; Forzatti, P. J. Catal. 2008, 255, 241ꢀ258 & 259ꢀ268.
53. Korup, O.; Goldsmith, C. F.; Weinberg, G.; Geske, M.; Kandemir, T.; Schlögl, R.; Horn, R.
J. Catal. 2013, 297, 1ꢀ16.
20
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Page 21 of 23
ACS Catalysis
1
2
3
4
5
6
54. Campa, M. C.; Ferraris, G.; Gazzoli, D.; Pettiti, I.; Pietrogiacomi, D. Appl. Catal. B 2013,
142, 423ꢀ431.
7
8
9
55. Meng, L. ; Lin, J. J.; Pu, Z.Y.; Luo, L. F.; Jia, A. P.; Huang, W. X. ; Luo, M. F. ; Lu, J. Q.
Appl. Catal. B 2013, 119, 117ꢀ122.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
56. Zhu, G. H.; Han, J. Y.; Zernlyanov, D. Y.; Ribeiro, F. H. J. Phys. Chem. B 2005, 109, 2331ꢀ
2337.
57. Rodrigues, M. T. S.; Silva, R. B.; Rocha, M. G. C.; Bargiela, P.; Noronha, F. B.; Brandao, S.
T. Catal. Today 2012, 197, 137ꢀ143.
58. Haack, L. P.; Otto, K. Catal. Lett. 1995, 34, 31ꢀ40.
59. Aryafar, M.; Zaera, F. Catal. Lett. 1997, 48, 173ꢀ183.
60. Monteiro, R. S.; Zemlyanov, D.; Storey, J. M.; Ribeiro, F. H. J. Catal. 2001, 199, 291ꢀ301;
& 2001, 201, 37ꢀ45..
61. Xu, J.; Ouyang, LK.; Mao, W.; Yang, XJ.; Xu, XC.; Su, JJ.; Zhuang, TZ.; Li, H.; Han, YF.,
ACS Catal. 2012, 2, 261ꢀ269.
62. Han, J. Y.; Zemlyanov, D. Y.; Ribeiro, F. H. Catal. Today 2006, 117, 506ꢀ513.
63. Zhen, Y. P.; Zhang, L. H.; Wang, S. L.; Ding, D.; Chen, M. S.; Wan, H. L. Langmuir 2013,
29, 9090ꢀ9097.
64. Penner, S.; Bera, P.; Pedersen, S.; Ngo, L. T.; Harris, J. J. W.; Campbell, C. T. J. Phys.
Chem. B 2006, 110, 24577ꢀ24584.
65. Descorme, C.; Jacobs, P. W.; Somorjai, G. A. J. Catal. 1998, 178, 668ꢀ678.
66. Hellman, A.; Resta, A.; Martin, N. M.; Gustafson, J.; Trinchero, A.; Carlsson, P. A.; Balmes,
O.; Felici, R.; van Rijn, R.; Frenken, J. W. M.; Andersen, J. N.; Lundgren, E.; Gronbeck, H.
J. Phys. Chem. Lett. 2012, 3, 678ꢀ682.
67. Bourane, A.; Cao, C.; Hohn, K.L. Appl. Catal. A 2006, 302, 224ꢀ231; & 2008, 344, 78ꢀ87.
68. Weaver, J. F.; Hakanoglu, C.; Hawkins, J. M.; Asthagiri, A. J. Chem. Phys. 2010, 132,
024709 (ꢀ10).
69. Munoz, F. F.; Baker, R. T.; Leyva, A. G.; Fuentes, R. O. Appl. Catal. B 2013, 136, 122ꢀ132.
70. Fang, Y.; Yao, Y. Ind. Eng. Chem. Prod. Res. Dev. 1980, 19, 293ꢀ298.
71. Graham, G. W.; Weber, W. H.; McBride, J. R.; Peters, C. R. J. Raman Spectrosc. 1991, 22,
1ꢀ9.
72. Chan, H. Y. H.; Zou, S.; Weaver, M. J. J. Phys. Chem. B 1999, 103, 11141ꢀ11151.
21
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Page 22 of 23
1
2
3
4
5
6
73. Satsuma, A.; Osaki, K.; Yanagihara, M.; Ohyama, J.; Shimizu, K. Appl. Catal. B 2013, 132,
511ꢀ518.
7
8
9
74. Engel, T.; Ertl, G. A. J. Phys. Chem. 1978, 69, 1267ꢀ1281.
75. Berlowitz, P. J.; Peden, C. H. F.; Goodman, D. W. J. Phys. Chem. 1988, 92, 5213ꢀ5221.
76. Liu, J. ; Xu, M.; Zaera, F. Catal. Lett. 1996, 37, 9ꢀ13.
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
77. Getman, R. B.; Schneider, W. F.; Smeltz, A. D.; Delgass, W. N.; Ribeiro, F. H., Phys. Rev.
Lett., 2009, 102, 076101.
78. Konsolakis, M. ; Yentekakis, I. V.; Pekridis,G.; Kaklidis, N.; Psarras, A. C.; Marnellos, G.
E. Appl. Catal. B 2013, 138, 191ꢀ198.
79. Jones, I. Z.; Bennett, R. A.; Bowker, M. Surf. Sci. 1999, 439, 235ꢀ248.
80. Gabasch, H.; KnopꢀGericke, A.; Schloegl, R.; Borasio, M.; Weilach, C.; Rupprechter, G.;
Penner, S.; Jenewein, B.; Hayek, K.; Kloetzer, B. Phys. Chem. Chem. Phys. 2007, 9, 533ꢀ
540.
81. Campbell, C. T.; Ertl, G.; Kuipers, H.; Segner, J. A. Surf. Sci. 1981, 107, 220ꢀ236.
82. Guo, X.; Hoffman, A.; Yates Jr., J. T. J. Chem. Phys. 1989, 90, 5787ꢀ5792.
83. Leisenberger, F. P.; Koller, G.; Sock, M.; Surnev, S.; Ramsey, M. G.; Netzer, F. P.; Klötzer,
B.; Hayek, K. Surf. Sci. 2000, 445, 380ꢀ393.
84. Campbell, C. T. Phys. Rev. Lett. 2006, 96, 066106.
85. Castellazzi, P.; Groppi, G.; Forzatti, P. Appl. Catal. B 2010, 95, 303ꢀ311.
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