Investigation of Methane Oxidation Reactions Over a Dual-Bed Catalyst System using 18O Labelled DRIFTS coupling

Article abstract of DOI:10.1002/cssc.201601165

Low loading Pd-supported (0.2 wt % Pd) Y-stabilized zirconia (YSZ) and LaMnO₃(LM) perovskite were associated to study the partial oxidation of methane using labelled¹⁸O₂in the gas phase. Synthesis gas production was demonstrated to occur through an indirect reaction in which oxygen is first consumed in the total methane combustion. A Mars–van Krevelen mechanism was observed over Pd/YSZ at 425 °C to yield C¹⁶O₂and C¹⁶O. A significant enhancement of the Pd/YSZ catalyst activity was achieved by the association of LM–Pd/YSZ in a dual catalyst bed, resulting in a significant increase of the oxidation rate. Vibration bands of adsorbed formate species, assumed to be intermediates to the gas production, were observed by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) coupling experiments. It was proposed that LM enables the generation of highly active singlet O₂, which is activated on the YSZ oxygen vacancies to assist a rapid recovery of surface PdO and increase formate decomposition into CO and H₂in Pd-supported catalyst.

Full text of DOI:10.1002/cssc.201601165

Accepted Article
Title: Investigation of methane oxidation reactions over Lanthanum
Manganites-Pd/YSZ dual bed by 18O labeling-DRIFTS coupling
Authors: Melissandre Richard, Daniel Duprez, Nicolas Bion, and
Fabien Can
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Investigation of methane oxidation reactions over Lanthanum
Manganites-Pd/YSZ dual bed by 18O labeling-DRIFTS coupling
Melissandre Richard,[a] Daniel Duprez,[a] Nicolas Bion[a] and Fabien Can,*,[a]
Abstract: Low loading palladium supported yttrium-stabilized zirconia
(0.2%wtPd/YSZ) and LaMnO3 perovskite were associated to study the
partial oxidation of methane using labeled 18O2 in the gas phase.
Synthesis gas production was demonstrated to occur via an indirect
reaction where oxygen is first consumed in the total methane
combustion. A Mars-Van-Krevelen mechanism was observed over
Pd/YSZ at 425°C to yield C16O2 and C16O. A significant enhancement
of the Pd/YSZ catalyst activity was achieved by the association of
LaMnO3+Pd/YSZ in a dual catalyst bed. It results a significant
increase of the oxidation rate. Vibration bands of adsorbed formate
species, assumed as intermediate to synthesis gas production, were
observed by DRIFTS coupling experiments. It was proposed that
LaMnO3 enables the generation of highly active singlet O2 which is
activated on the YSZ oxygen vacancies to assist a rapid recovery of
surface PdO and increase formate decomposition into CO and H2 in
Pd-supported catalyst.
of total oxidation with a suitable H2/CO ratio of 2 for downstream
processes such as methanol synthesis and Fischer-Tropsch
synthesis for instance.[7,8]
CH4 + ½O2  CO + 2H2
∆Hퟎퟐퟗퟖ = −ퟑퟓ. ퟕ 퐤퐉 퐦퐨퐥−ퟏ
(2)
As for total oxidation, noble metals [9] or perovskites [4] can be used
for producing synthesis gas with excellent yield. Depending on the
CH4:O2 ratio, methane conversion can be selectively oriented, on
the same catalyst, towards Eq. 1 or 2, but not necessarily at the
same temperature. It is reported that partial oxidation of methane
(POM) may occur via two different mechanisms, i.e., direct POM,
assuming that methane is directly converted to CO and H2 without
initial deep oxidation, or indirect POM. In this latter case, oxygen
is claimed to be first consumed in the combustion reaction (Eq. 1).
The remaining methane is converted to synthesis gas by dry or
steam reforming.
The relevance of using Lanthanum Manganites-Pd/YSZ dual
catalytic bed for methane oxidation at moderate temperature was
explained in our recent works.[10,11] Remarkable performances
were obtained on yttrium-stabilized zirconia-based palladium (0.2
wt-% Pd / 8%molY2O3-ZrO2 noted Pd/YSZ) in both 18O2/16O2
exchange activity and methane oxidation when LaMnO3
perovskite was placed in a first bed upstream. These results were
suggested to be related to the formation of highly active singlet
oxygen species through LaMnO3 bed facilitating the activation of
O2 molecule on the YSZ surface from 425°C. The concentration
of oxygen vacancies in the YSZ oxide played also an important
role in oxygen activation. The latter allowed a drastic improvement
of methane oxidation rate in dual-bed system inducing the
participation of oxygen atoms of YSZ solid through a Mars-van
Krevelen (MvK) mechanism.
The purpose of the present study was to investigate the partial
oxidation of methane over the same dual-bed system by coupling
in situ infrared spectroscopy and mass spectrometry (MS)
analysis. A DRIFTS (diffuse reflectance infrared Fourier transform
spectroscopy) cell was added to isotopic exchange set-up
enabling both gas phase and surface study to highlight adsorbed
intermediate species in dual-bed configuration.
Introduction
Methane is the main component of natural gas. It is also present
in great proportion in biogas and shale gas. The proven reserves
of natural gas would be close to 2x1014 m3 in 2013 while the
production of biogas amounts to 1012 m3/year in Europe.[1]
Methane combustion (Eq. 1) is widely used in industry to produce
heat and electricity.
CH4 + 2O2  CO2 + 2H2O
∆Hퟎퟐퟗퟖ = −ퟖퟎퟐ 퐤퐉 퐦퐨퐥−ퟏ
(1)
The reaction can be catalyzed by noble metals [2,3] or oxides such
as perovskites.[4] The main problem in combustion processes is
to control the temperature of reaction to avoid NOx formation
when air, instead of oxygen, is used as combustion agent. But
methane is also a feared greenhouse gas whose global warming
potential (GWP) is 25 times that of carbon dioxide at a time
[5]
horizon of 100 years
even though a GWP of 10 has been
reported in the past.[6] As CH4 is mixed in flue gases with more
reactive compounds, it is essential to improve the catalyst activity
to get CH4 oxidation at the lowest possible temperature.
Partial oxidation (Eq. 2) is an alternative process to transform
methane into synthesis gas, a more valuable product than those
Results and Discussion
Catalytic systems for methane oxidation
[a]
Dr. M. Richard, Dr. D. Duprez, Dr. N. Bion, Dr. F. Can
University of Poitiers, CNRS, UMR 7285
Institut de Chimie des Milieux et Matériaux de Poitiers (IC2MP)
4 rue Michel Brunet – TSA 51106 – 86073 Cedex 9, France
E-mail: fabien.can@univ-poitiers.fr
18O labeling-DRIFTS coupling was used to study the four following
catalytic systems for partial methane oxidation (POM) reaction,
namely LaMnO3 single solid (denoted hereafter as LM), Pd/YSZ,
LM-YSZ and LM-Pd/YSZ dual-beds. Note that the ratio 40%w LM
and 60%w YSZ or Pd/YSZ was chosen for dual-bed composition
according to previous results.[11] The reaction was performed at
Supporting information for this article is given via a link at the end of
the document.
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10.1002/cssc.201601165
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425°C with CH4:18O2 (2:1 molar ratio) reactive mixture in the
closed recycling device (see experimental section).
conclude that activity of a supported-palladium catalyst is due to
the ability of palladium oxide (PdOx) to chemisorb oxygen.
Additionally, Takahashi et al.[14] agree that PdO enhances oxygen
adsorption, lowering the activation energy for methane oxidation.
The methane combustion step is then assumed to result in a
PdO/Pd redox combustion mechanism serving as porthole for gas
phase oxygen to dissociate, migrate to the oxide support, and
exchange with oxygen from the oxide support. Additionally,
mechanisms for the combustion of methane over PdO-based
catalysts need to include steps for oxygen and hydroxyl exchange
between the Pd phase and the support.[15] The best catalytic
performances were obtained on dual LM-Pd/YSZ bed for which
the production of H2 occurs after only 20 min compared to 35 min
for Pd/YSZ. This result highlights the beneficial impact of the
LaMnO3 placed upstream on both methane combustion and POM
reaction.
Furthermore, products containing 16O atoms from bulk YSZ
catalyst are first observed in gas phase during both steps of
reaction (C16O2, then C16O). It is thus confirmed that the reaction
proceeds through a Mars-van Krevelen (MvK) mechanism, in
which methane is oxidized by lattice oxygen ions on the surfaces
of YSZ. To the opposite, for LM perovskite tested alone, labelled
C18O2 is first observed during methane combustion, illustrating
suprafacial mechanism where lattice oxygen of this solid are not
involved in the reaction in our conditions.
Figure 1 compares the four catalytic formulations on the basis of
18O2 (m/z 36) conversion and H2 (m/z 2) production versus time
on stream. Note that single YSZ solid does not exhibit any activity
at 425°C, in agreement with previous work that showed a high
temperature activity of yttrium-stabilized zirconia for methane
oxidation.[12]
As previously reported [10], two steps are identified according to
the evolution of the reaction products: (a) in the first minutes of
reaction, a rapid consumption of CH4 and 18O2 is observed
simultaneously with an increasing of CO2 (Figure 2B) and H2O
signals (see supporting information, Figure S1). Clearly, only
methane combustion reaction occurs in this step; (b) then, after
the almost total consumption of O2, the partial methane oxidation
could explain the concomitant appearance of syngas H2 and CO.
The main advantage of this reaction procedure is to have an
overview of the performance of the catalytic system both in
methane combustion and in partial oxidation (POM).
18O labeling-DRIFTS coupling on Pd/YSZ catalyst
IR bands corresponding to the formation of gaseous CH4 and CO2
species were observed during POM reaction over LM-Pd/YSZ
system (see supporting information, Figure S2). But in this case
the adsorbed species were not detected on perovskite surface
due to a low IR reflection of the black material. Therefore 18O
labelling-DRIFTS coupling was focused on Pd/YSZ single bed,
since essential steps of the CH4 reaction are located on this
catalyst. Figure 2A shows the DRIFTS spectra recorded during
methane oxidation reaction on Pd/YSZ catalyst at 425°C in which
typical IR bands corresponding of gaseous CH4 (at 3015 and
1304 cm-1) and CO2 (broad bands at 2260-2400 cm-1) were also
observed. After 35 min of reaction, new IR bands appeared in the
range of 1675-2095 cm-1 which can be attributed to the formation
of carbon monoxide adsorbed on Pd. A detailed assignment of
these IR bands is given in the next section. Moreover, the discrete
appearance of 2143 cm-1 centered IR band after 40 min of
reaction seems to confirm the C16O formation in gas phase. Note
also the occurrence of two negative bands at 3684 and 3640 cm-
1 in the OH region. This hydroxyl groups can probably participate
in the formation of intermediate adsorbed species such as
Figure 1. Time evolution of m/z=2 and 36 (H2 and 18O2) signals during POM
reaction at 425°C over a) LM, b) Pd/YSZ single beds and c) LM-YSZ, d) LM-
Pd/YSZ dual-beds.
Differences in catalytic activity occur for both steps over the four
catalytic systems. First, combustion of methane is enhanced
when LM is placed upstream compared to corresponding single
YSZ or Pd/YSZ beds. In a very recent work, it was demonstrated
by oxygen isotopic exchange that LaMnO3 activates the O2
molecule on the YSZ oxygen vacancy sites resulting in significant
improvement in total methane oxidation activity.[10,11] Then, H2
production was observed only for supported-Pd catalysts (curves
b) and d)). As expected, the presence of Pd (or PdO) particles is
crucial at this temperature to produce H2 in the second reaction
step. Even if the nature of active sites of Pd-based catalysts for
methane oxidation has been a matter of debate, Farrauto et al.[13]
‒
hydrogen-carbonate (HCO3 ), formate (HCOO‒) or acetate
(CH3COO‒) species.
The variations with time of IR bands areas are plotted in Figure
2B for CH4(g), CO2(g) and CO(ads) and compared to MS profiles. A
correlation can clearly be observed between DRIFTS and MS
data. At the beginning of the experiment (<35 min), the rapid rise
of the CO2 concomitant with the decrease of CH4 bands
integration observed by infrared spectroscopy confirms that
methane combustion is the predominant pathway. During this first
step, sufficient oxygen concentration allows the total oxidation of
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surface species to CO2 and H2O. The IR bands of adsorbed
carbonyl species quickly emerged after 35 min of reaction to
reach a maximum at 40 min before their slowly decay. At the
same time, MS profile showed the formation of H2 (m/z 2) and
C16O (m/z 28) in gaseous phase. Water-gas shift (WGS) reaction
between CO and H2O and methane decomposition cannot be
excluded to explain, in part, the observed H2 production. The
possible occurrence of WGS could also justify the small increase
of CO2(g) during the second step of reaction. In contrast, the fact
that CO2 was not consumed when H2 and CO are emitted
indicates that partial oxidation of methane to synthesis gas did not
involve dry reforming reaction at 425°C. We previously propose
two reactions to explain the hydrogen formation, namely steam
reforming and direct partial oxidation of CH4 with lattice oxygen
atoms. The evolution of the curve corresponding to 18 m/z (see
supporting information, Figure S1) does not allow validating the
former reaction at 425°C, whereas the latter reaction has been
observed when CH4 was introduced alone at the same
temperature on the dual-bed reactor, in agreement with the
results of Lefferts et al.[12]
In addition, the rise from 425°C to 700°C (rate 10°C/min) after 90
min of POM reaction leads to the desorption of adsorbed carbonyl
species from catalyst surface together with a decrease of CH4(g)
and CO2(g) IR bands (see supporting information, Figure S3). To
the contrary, centered bands at 2143 and 2093 cm-1 are clearly
distinguished and assigned to C16O and C18O gaseous molecules
respectively.
To confirm the formation of carbon monoxide in the second step
of reaction and the assignment of IR bands observed in the range
of 1675-2093 cm-1, the same POM experiment was performed at
425°C on a pre-exchanged Pd-YSZ catalyst, i.e. after introduction
18
of a significant amount of
O
(s)
in YSZ lattice (see experimental
section). The observed MS profiles of C16O (m/z 28) and C18O
(m/z 30) are represented in Figure 3A. As expected with this pre-
exchanged material, the formation of labeled molecule was
mainly detected in the gas phase. Figure 3B shows a comparison
of DRIFTS spectra for (a) raw and (b) pre-exchanged Pd-YSZ bed
in the range of 1700-2400 cm-1. An isotopic shift of 47 and 37 cm-
1 to lower wavenumber was clearly observed for IR bands at 2056
and 1915 cm-1 respectively. Note that the shift was also significant
concerning CO2(g) bands. An uniform isotopic shift, corresponding
to
a frequency ratio (i =
C16O/C18O) of 1.02 between
corresponding wavenumbers of spectra (a) and (b), demonstrates
their assignments to the same carbonyl species adsorbed on Pd.
Figure 3. A. MS signals of C16O and C18O (m/z 28 and m/z 30) recorded during
POM reaction at 425°C over 18O2 pre-exchanged Pd/YSZ catalyst. B.
Comparison of DRIFTS spectra between (a) raw and (b) pre-exchanged Pd/YSZ
materials with 18O2.
CO adsorption experiments
The assignment of adsorbed carbonyl bands observed on
Pd/YSZ surface by 18O labeling-DRIFTS coupling was studied in
more details by performing CO adsorption experiments.
Stretching vibration frequencies of the adsorbed molecules were
measured in different steady-state conditions, described below,
and compared to the literature and POM experimental results
(Figure 4). The first experiment was carried out in a closed FTIR
cell containing a self-supporting wafer of Pd/YSZ material (c.a. 15
Figure 2. POM reaction over Pd/YSZ at 425°C. A. DRIFTS spectra vs. time. B.
Comparison between MS profiles and IR band integrations.
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mg.cm-2). Successive doses of carbon monoxide (Pressure = 0.1
to 100 mbar in a volume of 0.9 cm3) were introduced at ambient
temperature on the wafer pre-reduced in H2 at 450 °C and then
evacuated at 10-6 mbar. Spectrum (a) was recorded after
saturation of the surface by CO pulses. Two other experiments
were carried out in the DRIFTS cell containing 100 mg of catalyst.
After the same oxidative pre-treatment at 700°C than the one
used prior to the POM reaction, 30 mbar of CO was admitted in
the cell at 425°C. Spectra (b) and (c) were recorded at 425°C and
after cooling down to room temperature respectively.
Figure 4 presents these normalised IR spectra in the range of
1700-2200 cm-1, compared with that recorded during POM
reaction at 425°C (spectrum d)). Note that the presence of
gaseous molecule was observed with centered IR band at 2143
cm-1. Metallic palladium forms carbonyls that are generally
characterized by bands at wavenumbers below 2100 cm-1, due to
the back-donation of electrons from the metal d bands to CO 2*
antibonding orbital. Additionally, the vibrational frequencies of the
C−O bond stretching depend on the different adsorption sites of
CO on Pd.[16] Generally, the (CO) bound around 2100 cm-1 is
assigned to linearly bound CO molecules.[17] CO adsorbed on
bridge sites presents distinct features in IR spectra below 2000
cm-1. To the opposite, the spectral ranges in which IR bands have
been attributed to surface carbonyls of Pdn+ are ranked between
2215-1930 cm-1. Linear Pd2+−CO complexes were observed at
2215-2145 cm-1; Pd+−CO species are characterized by bands
between 2140 and 2110 cm-1 and bridging carbonyls
Pd+−CO−Pd+ were suggested to absorb in the range 1995-1975
cm-1.[16]
On admission of successive CO doses on Pd/YSZ wafer at room
temperature, IR bands recorded at 2094, 1981, 1942 and 1879
cm-1 increase until surface saturation (spectrum (a)). Note that
attributions can be difficult in the light of the outcome results of
the literature, but the band at 2094 cm-1 is tentatively assigned to
monocarbonyls species bound to Pd metal particles (linear Pd0‒
CO), while bands at 1981 and 1942 cm-1 are attributed to
compressed and isolated bridged carbonyls respectively where a
single CO molecule is adsorbed on two Pd0 sites (Pd2‒CO).
Compressed term means that two bridged species are quite close
to interact together; in other words each Pd adsorption center is
bound to two CO bridging molecules. Such a configuration would
be preferentially found on (111) faces of larger Pd metal particles
due to their pillbox morphology.[19] The shoulder at 1879 cm-1 is
associated with triply bridging carbonyls species (Pd3‒CO).[18]
Table 1. Assignment of IR bands (νCO) corresponding to CO adsorbed
species on Pd0 sites observed in Figure 4 [18,19]
Pd0-CO
(cm-1)
(Pd0)2-CO[a]
(cm-1)
(Pd0)2-CO[b]
(cm-1)
(Pd0)3-CO
(cm-1)
Conditions
FTIR-Tamb.
2094
2094
1981
1973
1942
1915
1879
n.d.
DRIFTS-Tamb.
DRIFTS-425°C
POM-425°C
2070
2056
1940
1915
n.d.
n.d.
n.d.
n.d.
[a] Compressed carbonyls. [b] Isolated bridged carbonyls.
It has been observed that DRIFTS spectra, realized at ambient
temperature (spectrum (b)), were quite similar to FTIR spectra
whereas an increase at 425°C (spectrum (c)) resulted in a red
shift of vibrations modes towards lower wavenumbers (2070 and
1940 cm-1 for Pd0‒CO and Pd2‒CO respectively). Comparing this
latter with spectra recorded during POM reaction at 425°C
(spectra (d)), a light shift of the bands of CO adsorbed species
has also been denoted, resulting in IR bands at 2056 and 1915
cm-1 for linear and bridged adsorbed carbonyls. This shifting could
be linked to a difference of surface coverage during the catalytic
oxidation reaction, together with a temperature effect. Additionally
the reducing environment of the POM reaction can also contribute
to enhance the red shift of the 2* back-donation by strengthening
the metallic palladium state. Note that the amount of linear Pd0‒
CO adsorbed species, less stable than bridged one, is smaller at
425°C.
Figure 4. CO adsorption spectra over Pd/YSZ. Experimental conditions: (a)
Transmission FTIR cell at ambient temperature; DRIFTS cell (b) at ambient
temperature, (c) at 425°C and (d) during POM reaction at 425°C.
Spectrum (a) shows four stretching bands of carbonyl species
(CO) adsorbed on Pd/YSZ surface at 2094, 1981, 1942
(shoulder) and 1879 (shoulder) cm-1. These bands were assigned
to different Pd adsorption sites, listed in Table 1, in accordance
with previous studies on oxide supported-Pd systems [18,19]. IR
vibration bands observed on spectra recorded in the DRIFTS cell
were attributed by analogy with the bands listed in Table 1.
Additionally, adsorbed species (carbonates, formates) were also
detected during IR adsorption experiments together with
perturbations in OH vibrations region.
Observation of intermediate species
In order to highlight the formation of intermediate species during
POM reaction, further analysis of DRIFTS spectra was performed
in which the spectrum obtained at 35.0 min of reaction over
Pd/YSZ from Figure 2A, is subtracted to the following spectra to
highlight the adsorbed species. Results reported in Figure 5 show
that distinct IR bands appear simultaneously from 39.6 min at
1355, 1382, 1576 and 2879 cm-1. According to previous
numerous studies [20-23], these wavenumbers are characteristic of
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stretching and deforming modes of the formate species (HCOO‒)
adsorbed on the YSZ oxide, as reported in Table 2. Note that
maximum of IR formate bands intensity was obtained as of 47.6
min.
The formation of formate species adsorbed on YSZ surface can
be explained by two hypotheses. They can be (i) formed from
carbon monoxide or (ii) an intermediate species driving the CO
yield. Characteristic HCOO‒ bands were not observed previous to
40 min of reaction which means that they could react very quickly
to produce the products mentioned upstream.
monoxide is not responsible for the formate species formation on
YSZ surface which invalidates the first hypothesis mentioned
before. Nevertheless, when the temperature was raised at 700°C,
results show the exclusive formation of CO2 in gas phase, without
syngas production (Figure 7). IR spectra recorded at the same
time and represented in Figure 6B also confirm the CO2
production (2260-2400 cm-1) with a simultaneous consumption of
formate adsorbed species (negative IR bands). This result is in
[24]
accordance with previous study
suggesting a decomposition
of formate species to produce CO2 molecule. Finally, in the region
of hydroxyl groups, negative IR bands around 3671-3675 cm-1
can be attributed to water desorption.
Table 2. Assignment of the adsorbed formate vibration bands.

asOCO + CH
Catalysts
CH

asOCO[a]
sOCO[b]
CH
overtone
(cm-1)
(cm-1)
(cm-1)
1576
1574
(cm-1)
(cm-1)
Pd/YSZ
YSZ
2879
2880
1355
1357
1382
1383
n.d.
2968
[a] Suffix “as” for asymmetric. [b] Suffix “s” for symmetric
Figure 6. Identification of formate adsorbed species on YSZ surface during
POM reaction A. at 425°C and B. between 425-700 °C.
Figure 5. Identified formate species (HCOO‒) adsorbed on Pd/YSZ surface
during POM reaction at 425°C.
Even if the first option cannot be totally excluded, the second one
was preferentially admitted for a number of reasons (see
discussion). On the one hand, when the temperature of reaction
increases, IR bands of formate species disappear while CO
production increases in gas phase. It indicates that carbon
monoxide could be formed after HCOO‒ decomposition.
Supplementary POM experiment was performed over YSZ oxide,
without Pd deposition, at 425°C using isotopic exchange-DRIFTS
coupling. Figure 6A shows IR spectra recorded during this
reaction and MS signals are reported in Figure 7. Surprisingly, IR
bands assigned to formate adsorbed species were clearly
observed at 1357, 1383, 1575 and 2880 cm-1 with a relatively high
intensity and increasing with time on stream. A new vibration
frequency was also detected at 2968 cm-1 which is attributed to a
combination between asOCO and CH modes (Table 2).
Figure 7. MS profiles of gas phase from POM reaction over YSZ support.
A last important point to stress is the formation of intermediate
HCOO‒ species from lattice oxygen atoms of YSZ material
involving MvK mechanism during POM reaction. Indeed, no
isotopic shifting of stretching OCO modes was observed while
labeled isotope 18O2 was introduced in gas phase at the beginning
of the reaction. Thus, the decomposition of HC16O16O‒
intermediate species can explain the formation of unlabeled
Concurrently, MS profiles (Figure 7) shows a poor methane
conversion with relatively no 18O2 consumption. It means that
gaseous oxygen is not consumed in YSZ system at 425°C even
in presence of CH4. This observation proves that carbon
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products C16O2 and C16O during POM reaction on Pd/YSZ and
dual LM-Pd/YSZ bed catalysts.
states. However, at low to moderate temperatures (T<450°C) the
recombination of surface hydroxyl and water desorption from the
catalyst can likely control reactivity of methane oxidation over
PdO, making the water desorption from the surface the rate-
limiting step for methane oxidation.[31] The poisoning effect of
water was proposed to be due to the formation of palladium (II)
hydroxide where the breaking of the OH bond in the hydroxide is
the determining-step.[32] The influence of water on the catalyst
activity is also affected by the type of host materials where high
oxygen mobility supports show potential to moderate inhibition by
water, accelerating hydroxyl desorption.[33] In addition, the oxygen
mobility of the support can strongly affect the oxidation/reduction
behavior of palladium particles together with the reversibility of
water poisoning.
The initial step in methane activation on noble metal involves
assisted hydrogen abstraction.[34] The subsequent interaction of
the methyl or methylene radicals with the adsorbed oxygen has
been proposed to lead to either direct combustion to carbon
dioxide and water or the formation of chemisorbed formaldehyde
via methoxide, methyl peroxide, or methylene oxide
intermediates.[30,35] Consequently, the first step of the POM
experiments presented herein (Figure 1, Figure 2B) is in
accordance with the literature data above presented. Methane
conversion is enhanced over palladium containing catalyst by
assisting the complete CH4 combustion together with CO2 yield,
leading to oxygen consumption. Thereafter, the remaining
methane is converted to synthesis gas by direct or indirect POM.
Over non-noble catalysts, the first step for methane oxidation is
also the dissociative adsorption of methane leading to Metal-CH3
bond together with a hydroxyl group formation. This first step can
Discussion of reaction mechanism
As mentioned previously, the role of formate species is an
essential step in the mechanism of synthesis gas production
during the partial oxidation of methane. Several mechanisms are
developed to highlight the formation mechanism of formate
species. It is reported that both acidic hydroxyls and O2− ions
could play an important role in the generation of these species.
Carbon monoxide can hypothetically reacts with hydroxyls to form
formate species, leading to the consumption of an OH group,
revealing that OH groups surface could also behave as active
sites for CO transformations. For instance, by means of in-situ
transmission FTIR experiments, Lefferts et al. reports that
hydroxyl groups on the ZrO2 support are involved in the formate
yield by reaction with CO. In the presence of metallic supported
catalyst (Pt/ZrO2) formate decomposition can thereafter lead to H2
and CO2 production.[25] On ceria-based catalysts, Shido and
Iwasawa[26] used infrared spectroscopy to conclude that bidentate
formate species are produced by the reaction of CO with terminal
hydroxyl groups of the ceria surface and that these species can
be decomposed into H2 and carbonate species. More recently, Li
et al.[27] have invoked a redox process to explain the mechanism
of formation of formate species. These authors conclude that CO
adsorbs on metal sites and reacts with the neighbouring
framework oxygens. The reduced oxide can furthermore be
oxidized by CO2. Even if this process cannot be totally excluded,
since CO adsorption has revealed the presence of metallic Pd
sites (Figure 4), formate compounds are rather assumed to be
responsible of the syngas production as intermediate species of
CH4 activation. Whatever the active sites considered (i.e. PdO or
O—(s)), surface formate formation involves dissociation to
adsorbed methyl or methylene radicals as a result of the removal
of hydrogen atoms from the methane. CO, CO2, H2 and H2O being
the major decomposition products of formate intermediate
species, in agreement with the results presented here.
The nature of active sites of Pd catalysts for methane oxidation
has been a matter of debate for a long time. It has been thought
that the active form of palladium is an oxidized state, but the
precise nature of the active sites is still controversy since it
remains conflicting evidence on the activity of different oxidation
states of Pd for methane oxidation. Nevertheless, it has been
reported that methane is activated on a site pair consisting of an
oxygen atom and an oxygen vacancy site on the PdO surface,
suggesting a redox mechanism[28,29] where surface oxygen on the
palladium crystallites is much higher active for methane oxidation
than bulk oxide of palladium due to the higher ability of the oxygen
sorption processes.[30] Methane interacts with oxygen on the PdO
surface which is rapidly restored by diffusion of bulk oxygen to the
surface, which perfectly matches with the dramatic enhancement
of methane oxidation observed on the LM-Pd/YSZ dual bed
system (Figure 1). To the opposite, methane oxidation over the
metallic Pd surface is controlled by competitive adsorption of
methane and oxygen.
[36]
be catalysed by Lewis acid sites and adjacent lattice oxygen
but Steghuis et al. [37] agrees that methane is rather homolytically
dissociated over coordinatively unsaturated oxygen sites (O—
)
(s)
of yttria-zirconia materials to form methoxy and subsequently
formate adsorbed species. Zhu et al.[24] also suggest that formate
species are formed by methane reaction with lattice oxygen atoms
of YSZ catalyst, as illustrated in Figure 6. The rate-determining
step is suggested to be likely the splitting of a Metal-CH3 bond.
Thereafter, C−H bond in the methoxy is dissociated to form a
second OH group and the remaining OCH2 forms
a
dioxymethylene which is considered as formaldehyde
precursor.[38,39] Formaldehyde is never observed due to rapid
conversion to formate. In a recent DFT study, Cooper et al.
reported an energetically favourable methane C–H bond
cleavage over YSZ in the presence of oxygen.[40] In this
mechanism, oxygen is activated on the YSZ surface as a
superoxide like species. On the neighbouring surface oxygen ions,
H abstraction occurs and the C–H bond of the methane
undergoes homolytic breaking, forming hydroxyl group with an
overall -1 charge, the additional electron density being transferred
to the superoxide species.
Finally, the methane C–H bond activation therefore implies the
activation of O2 and the formation of the methyl radical would be
the first step of the formation of formaldehyde or formate species.
This mechanism occurs at 425°C with appearance of the vibration
bands of the formate species on the spectra collected over single
YSZ (Figure 6). No formation of gaseous products was evidenced
Activation of the methane molecule is often proposed to be the
rate-limiting step of the reaction over both the Pd and the PdO
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at this temperature because of the stability of the formate species
on the YSZ surface in agreement with the work of Zhu et al. who
demonstrated in the same domain of temperature that the rate-
determining step of POM over YSZ (with 12mol%Y2O3) was not
activation of methane but decomposition of the formate.[24] In
single YSZ, dissociation of superoxide species is limited and
production of combustion product is not observed. An increase of
the temperature is needed to decompose the formate species
generating CO2 and H2O (Figure 7). In the dual bed system we
demonstrated in a previous work[10,11] that the first LaMnO3 bed
acted as a generator of highly reactive intermediate singlet (1Δg)
molecular oxygen making its dissociative activation possible at
lower temperature over the YSZ surface. We observed here that
it increases the rate of decomposition of the formate species since
the generation combustion CO2 and H2O products was observed
for LM-YSZ in larger amount than for single LM catalyst.
Accordingly, the dramatic enhancement of synthesis gas
production in the LaMnO3-Pd/YSZ dual bed system is tentatively
attributed to higher activation of O2 molecule on YSZ surface
vacancies, that enables a rapid recovery of surface PdO species
required for the decomposition of adsorbed formate species to CO
and CO2 molecules in gas phase. In absence of oxygen, oxidation
of formate species into CO2 and H2O is inhibited and selectivity to
synthesis gas generation is increased due to the formates
dehydration into CO and H2O and formates dehydrogenation into
CO2 and H2.[41,42] However if the formation of synthesis gas was
observed on pure YSZ at very high temperature by Zhu et al.,[43]
the necessity of Pd particles is essential at moderate
temperatures.
results obtained with bare YSZ material, we supposed that this
former plays a role of intermediate species in POM reaction. In
this case, formate species results from methane activation and
lattice oxygen atoms of YSZ.
Experimental Section
Materials
The preparation and characterization of the materials used in this study
have already been described elsewhere [44]. The citrate route was selected
to synthesize LaMnO3 perovskite in which equimolar amounts of two metal
nitrates precursors, La(NO3)3·6H2O (Sigma Aldrich, >99.0%) and
Mn(NO3)2·4H2O (Alfa Aesar, 98%), were mixed in 25 cm3 distilled water.
After addition of anhydrous citric acid (Alfa Aesar, >99.5%), the solution
was heated at 90°C for 2h to evaporate excess H2O under magnetic
stirring (600 rpm), then dried at 110°C overnight. After milling, the sample
was submitted to a static thermal treatment in an oven up to 200°C (ramp
rate 1°C min-1) to eliminate the citric acid. The resulting LaMnO3 catalyst
was finally calcined at 750°C for 2h with a ramp rate of 5°C min-1 by using
a quartz tubular reactor under air flow (50 cm3 min-1) and exhibits a specific
surface area of 20 m2 g-1. In Figure 8, powder X-ray diffraction (XRD)
pattern shows well-crystallized samples without any presence of
amorphous regions corresponding to the expected LaMnO3 phase. The
mean crystallite size of bulk perovskite determined from line-broadening
of the most intense reflections at 32.6° using the Scherrer equation is
≈14.8 nm.
Conclusions
The objective of this work was to investigate the mechanism of
low-temperature methane oxidation over a highly active catalyst.
An innovative dual-bed system, LaMnO3-Pd/YSZ was studied
showing a remarkable improvement of the catalytic oxidation
activity in both methane combustion and synthesis gas
production. Arrangement of a DRIFTS cell in a closed recycling
system of isotopic exchange set-up allows the observation of
adsorbed intermediates species on catalytic surface during the
partial oxidation of methane (POM) reaction at 425°C and enables
to highlight the synthesis gas production mechanism. While the
use of labeled oxygen (18O2) reactant, unlabeled oxidation
products (C16O2, H216O, C16O) were mainly detected by mass
spectrometry in gas phase involving the participation of lattice
oxygen atoms from YSZ material in the reaction via a so-called
Mars van Krevelen mechanism. Similar behavior was found for
Pd/YSZ catalysts which was used to perform 18O labeling-DRIFTS
coupling experiments. At this temperature, CO molecule is
adsorbed on Pd sites revealing infrared bands in the 1675-2095
cm-1 spectral range assigned to linear and bridged carbonyl
adsorbed species. The attribution of these vibration frequencies
was confirmed with isotopic shifting observed in the case of the
reaction over a pre-exchanged catalyst. IR analysis also reveals
the formation of formate species (HCOO‒) adsorbed on YSZ
surface during POM reaction. According to previous study and
Figure 8. XRD pattern of LaMnO3 material. Reference corresponds to the
expected LaMnO3 phase (ISCD file 98-008-2226).
YSZ (8mol% Y2O3-stabilized ZrO2) oxide was provided by TOSOH. This
commercial material was calcined at 750°C (5°C min-1) under air flow (50
cm3 min-1) for 2h before use. YSZ exhibits a specific surface area of 13 m2
g-1 and a cubic fluorite structure [1]. The mean crystallite size of the YSZ
oxide is ≈18.2 nm. Finally YSZ-supported Pd catalysts (Pd/YSZ) were
prepared by wet impregnation of the appropriate amount of nitrate
precursor Pd(NO2)2(NH3)2 (Alfa Aesar, 8.91mass% Pd) to obtain 0.2 wt% Pd.
The excess water was evaporated by using sand bath at 60°C. The
resulting solid was dried at 120°C overnight and calcined at 700°C during
4h under air flow (60 cm3 min-1). Amount of 0.297 wt% Pd was determined
by inductively coupled plasma optical emission spectroscopy (ICP-OES).
18O labeling-DRIFTS coupling set up
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The 18O labeling-DRIFTS coupling set up is depicted in Figure 9 and
consisted of three main parts: (i) a DRIFTS cell connected to (ii) a closed
recycling system and (iii) the analysis section.
Before the reaction, sample were additionally subjected to an oxidation
activation step at 700 °C for 1h under a 16O2 flow before cooling to 425°C
at which the system was purged for 30 min using primary vacuum. Then,
105 mbar of a mixture of CH4:18O2 in a molar ratio 2:1 was introduced in
the reactor. The m/z values and corresponding gaseous molecules
reported in Table 3 were monitored continuously during the POM reaction
as a function of time.
The DRIFTS cell (Praying MantisTM from Harrick Scientific Products)
consists of a high temperature reaction chamber placed in a highly efficient
diffuse collection system (6 mirrors) that minimizes the detection of the
specular component. The reaction chamber was engineered to perform
studies under controlled temperatures (up to 910°C) and pressures (from
133 μPa to 133 kPa). Catalyst is deposited under a wire mesh in a ceramic
crucible covered with a dome having two KBr windows transparent to IR
radiation. The objective of that DRIFTS cell was to observe adsorbates on
the catalyst surface. Note that the dead volume (≈3 cm3), allowing
observation of gaseous reactants and products present within the reactor,
can also reduce temporal resolution of the IR technique. In our case, the
utilization of recycling pump allows to curtail this problem.
A POM experiments was performed on a pre-exchanged Pd-YSZ catalyst.
After same pre-treatment (oxidation and purge), several consecutive
18O2/16O2 exchange reactions were undertaken at 700°C for 20-30 min by
introducing each time 50 mbar of 18O2 labeled molecule in the reactor. The
formation of 16O18O and 16O2 isotopes in the gas phase indicates that
exchange reaction takes place in the catalyst involving YSZ lattice 16O
atoms. We estimate after consecutive experiments that around 98% of
oxygen atoms are exchanged in the material (i.e. 98% of lattice oxygen
atoms of YSZ oxide are labeled (18O). After a purge step at 425°C, POM
experiments were performed by using protocol described above.
Both inlet/oulet ports provided for introducing/evacuating reaction chamber
are connected to
a closed recycling system already described
elsewhere[45]. The whole system (DRIFTS cell + closed system) represents
a volume of about 90 cm3 for a recycling flow of 170 mL.s-1.
The analytical instrumentation includes a mass spectrometer (Pfeiffer
Vacuum, QMS 200 quadrupole, Faraday detector) to monitor the gas-
phase composition (mass-to-charge m/z signals) and connected to the
closed recycle system. A controlled valve was used to limit the operating
pressure of the mass spectrometer ionization chamber at 10-6 mbar. The
DRIFTS cell was placed in a Bruker (Equinox 55) IR spectrometer with a
MCT (Mercure-Cadmium-Tellure) detector. Spectra were collected in a
range of 400 to 4000 cm-1, accumulating 64 scans with a spectral
resolution of 4 cm-1. Reference spectra was recorded over a KBr powder
sample at ambient temperature under primary vacuum. Spectra were
plotted as a function of wavenumber using OPUS® Bruker software after
Fourier transform mathematical treatment of single beam interferogram.
Table 3. Gaseous molecules recorded during POM experiments and
corresponding m/z signals.
m/z
2
Molecule
H2
m/z
28
Molecule
m/z
36
Molecule
18O2
N2,
C16O,C16O2
C18O,
C16O18O,
C18O2
CH4
C16O2
15
30
44
H216O, 18O2,
16O2
C16O18
O
18
20
32
34
46
48
16O18
O
16O18
O
C18O2
H218O
FTIR CO-adsorption
A self-supporting wafer (ca. 15 mg cm-2) was prepared by pressing Pd/YSZ
powder at 100 bar. Then, the sample was introduced in the IR cell
connected to a vacuum and adsorption system. Prior to the adsorption
measurement, the catalyst was pre-treated at 450°C (ramp rate 10°C min-
1) under H2 flow (30 cm3 min-1) for 1h. Then, the system was purged at the
same temperature for 45 min by using secondary vacuum (10-3 Pa). The
wafer was cooled at room temperature under vacuum in the T-shaped IR
cell provided with two KBr windows transparent to IR beam.
Successive doses of carbon monoxide (Pressure = 0.1 to 100 mbar in a
volume of 0.9 cm3) were introduced at ambient temperature on the wafer
until surface saturation. A spectrum was collected after each addition of
CO in the IR cell, in a range of 400 to 4000 cm-1, accumulating 64 scans
with a spectral resolution of 4 cm-1. Spectra (a) in Figure 4 was recorded
after introduction of around 1.8×10-5 mol of CO. Nicolet 6700 apparatus
with a MCT detector was used to realize FTIR CO-adsorption experiments.
Absorbance spectra were plotted as a function of wavenumber using
OMNIC® software.
Figure 9. Schematic representation of experimental set up.
Partial oxidation of methane
Keywords: Partial oxidation • Methane • Isotopic exchange •
DRIFTS • Yttrium-stabilized zirconia
POM experiments were performed by using the set up described above.
Isotherm experiments at 425°C were undertaken on 30 mg LaMnO3 (+SiC),
110 mg Pd/YSZ and 75 mg dual bed material or mechanical mixture placed
in the DRIFTS cell reactor.
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The dual LaMnO3-0.2%wtPd/YSZ
catalytic system is highly active for
CH4 partial oxidation at moderate
temperatures. The role of LaMnO3 is
to generate active singlet oxygen
species in order to improve the
oxygen activation step on the YSZ
surface. The rate of oxidation of
methane which followed a MvK
mechanism is then drastically
increased and intermediate formate
species can subsequently be
Melissandre Richard, Daniel Duprez,
Nicolas Bion, Fabien can*
Page No. – Page No.
Investigation of methane oxidation
reactions over Lanthanum
Manganites-Pd/YSZ dual bed by 18
labeling-DRIFTS coupling
O
decomposed into CO and H2.
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