Oxidative Methane Conversion to Ethane on Highly Oxidized Pd/CeO2 Catalysts Below 400 °C

Article abstract of DOI:10.1002/cssc.201903311

Methane upgrading into more valuable chemicals has received much attention. Herein, we report oxidative methane conversion to ethane using gaseous O₂ at low temperatures (<400 °C) and atmospheric pressure in a continuous reactor. A highly oxidized Pd deposited on ceria could produce ethane with a productivity as high as 0.84 mmol g_cat ^?1 h^?1. The Pd?O?Pd sites, not Pd?O?Ce, were the active sites for the selective ethane production at low temperatures. Density functional theory calculations confirmed that the Pd?O?Pd site is energetically more advantageous for C?C coupling, whereas Pd?O?Ce promotes CH₄ dehydrogenation. The ceria helped Pd maintain a highly oxidic state despite reductive CH₄ flow. This work can provide new insight for methane upgrading into C₂ species.

Full text of DOI:10.1002/cssc.201903311

Accepted Article
Title: Oxidative Methane Conversion to Ethane on Highly Oxidized Pd/
CeO2 Catalysts Below 400 ˚C
Authors: Gihun Kwon, Dongjae Shin, Hojin Jeong, Suman Kalyan
Sahoo, Jaeha Lee, Gunjoo Kim, Juhyuk Choi, Do Heui Kim,
Jeong Woo Han, and Hyunjoo Lee
This manuscript has been accepted after peer review and appears as an
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to this Accepted Article as a result of editing. Readers should obtain
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to ensure accuracy of information. The authors are responsible for the
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To be cited as: ChemSusChem 10.1002/cssc.201903311
Link to VoR: http://dx.doi.org/10.1002/cssc.201903311
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ChemSusChem
10.1002/cssc.201903311
COMMUNICATION
Oxidative Methane Conversion to Ethane on Highly Oxidized
Pd/CeO Catalysts Below 400 ˚C
2
Gihun Kwon†[a], Dongjae Shin†[b], Hojin Jeong , Suman Kalyan Sahoo , Jaeha Lee , Gunjoo Kim ,
[a]
[b]
[c]
[a]
Juhyuk Choi , Do Heui Kim , Jeong Woo Han and Hyunjoo Lee*[a]
[
a]
[c]
*[b]
Abstract: Methane upgrading into more valuable chemicals has
2
using O in a continuous reactor, but the reaction temperature is
received much attention. Herein, we report oxidative methane
typically higher than 700 ˚C, and various side-products were
[
5]
conversion to ethane using gaseous O
2
at low temperatures (< 400 ˚
formed together due to its radical-derived chemistry, causing
higher cost for separation.[ Methanol was produced from a
6]
C) and atmospheric pressure in a continuous reactor. A highly
oxidized Pd deposited on ceria could produce ethane with the
continuous flow reactor using O
low as 0.02 mmol gcat h .
2
, but the productivity was very
The non-oxidative conversion
-
1
-1
-1
-1 [4a, 4c]
productivity as high as 0.84 mmol gcat h . The Pd-O-Pd sites, not Pd-
O-Ce, were the active sites for the selective ethane production at low
temperatures. Density functional theory calculations confirmed that
Pd-O-Pd site is energetically more advantageous for C-C coupling
uses methane only, but it typically require very high temperature
(> 1000 ˚C). Selective production of olefin or benzene have been
reported,[7] but these results were not easily reproduced.
Using a continuous flow reactor is preferred for the direct
whereas Pd-O-Ce prefers CH
4
dehydrogenation. The ceria helped Pd
flow. This work can
maintain highly oxidic state despite reductive CH
4
methane conversion. O
2
as an oxidant, instead of H
2
O
2
or N
Because
2
O, is
preferred for the oxidative methane conversion.[
1b, 2, 8]
provide new insight for methane upgrading into C2 species.
conventional oxidative coupling of methane generates radicals at
high temperatures of >700 ˚C resulting in various side-products,
the catalysts enabling surface reaction at much lower
temperatures are desired.
Direct conversion of methane to value-added chemicals such as
olefins, aromatics, and methanol has recently received much
attention due to the depletion of crude oil and the large reserves
of shale gas. The methane is often emitted from shale gas mines
into air, causing a severe greenhouse effect. The majority of
methane is burned, also producing a significant amount of carbon
dioxide. Converting the methane into more valuable chemicals
would be beneficial for not only producing more benefits but also
for protecting environment. However, direct conversion is very
difficult because the first C-H bond activation requires high energy
We show here that highly oxidized Pd/CeO
catalyze oxidative methane upgrading into ethane at low
temperatures using gaseous O . The studies on methane
coupling at low temperatures below 400 ˚C are rare. Pt/SiO or
Pd/Al have been used for methane coupling below 400 ˚C but
2
catalysts can
[
1]
2
2
2 3
O
they could not complete a catalytic cycle; after flowing methane,
hydrogen gas was required to extract the adsorbed products from
the catalyst surface.[ (≡SiO)
9]
2
Ta-H could have a closed catalytic
(
439 kJ/mol).[2] The inertness of methane hinders the selective
production of valuable chemicals and instead the methane
conversion often results in over-oxidation to CO or severe coke
cycle producing ethane continuously, but methane pressure was
as high as 50 bar, and the ethane productivity was very low as
2
.01 mmol gcat h-1 at 300 ˚C.[10] The detailed comparison for the
-
1
0
[
3]
formation.
catalysts producing ethane from methane at < 400 ˚C is provided
in Table S1.
Despite the difficulties, oxidative or non-oxidative methane
conversion has been reported. The oxidative conversion uses
2
Pd/CeO catalysts, on which Pd nanoparticles are dispersed
oxidants such as H
2
O
2
or O
conditions. Methanol could be directly produced from methane
using H as an oxidant, however, a batch reactor was typically
used and H is more expensive than methanol, making its
practical application improbable. Ethylene could be produced
2
, typically allowing milder reaction
on ceria surface, have been widely used for oxidations, such as
CO oxidation, benzyl alcohol oxidation, and methane
combustion.[ The Pd surface can be easily oxidized and the
formed PdO can behave as an oxidation catalyst.[ The interface
[
4]
2 2
O
11]
2 2
O
12]
between Pd and ceria often behaves as efficient active sites for
oxidation at lower temperatures.[
3b, 13]
Particularly, it was reported
that Pd can be highly oxidized on ceria with a ratio of Pd to O
smaller than 1.[ Methane activation was also studied on Pd; the
energy barrier was lower in PdO than metallic Pd.[15] Various Pd
†
These authors equally contributed
14]
[
[
[
a]
Gihun Kwon, Hojin Jeong, Gunjoo Kim, Juhyuk Choi, Hyunjoo Lee
Department of Chemical and Biomolecular Engineering
Korea Advanced Institute of Science and Technology
Daejeon 34141, South Korea
E-mail: azhyun@kaist.ac.kr
Dongjae Shin, Suman Kalyan Sahoo, Jeong Woo Han
Department of Chemical Engineering
Pohang University of Science and Technology, Pohang
Gyeongbuk 37673, South Korea
E-mail: jwhan@postech.ac.kr
catalysts such as Pd@CeO
reported to be efficient for methane combustion.
In this study, highly oxidized Pd/CeO
2
/Al
2
O
3
or Pd/ZSM-5 have been
[3b, 16]
catalysts can catalyze
b]
c]
2
the oxidative methane conversion producing ethane at low
temperatures. The reaction kinetics were investigated by varying
methane or O partial pressure. Catalysts with different ratios of
2
Pd-O-Pd and Pd-O-Ce sites were prepared and tested for ethane
production. Density functional theory (DFT) calculations were
performed to investigate the reaction mechanism on Pd-O-Pd and
Pd-O-Ce sites.
Jaeha Lee, Do Heui Kim
School of Chemical and Biological Engineering
Institute of Chemical Processes, Seoul National University
Seoul, 08826, South Korea
Supporting information for this article is given via a link at the end of
the document.
1
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ChemSusChem
10.1002/cssc.201903311
COMMUNICATION
1
0
0
0
0
0
.0
.8
.6
.4
.2
.0
70
(a)
(b)
5
0ppm O2
130ppm O2
000ppm O2
1.3% O2
.6% O2
6.5% O2
2.8% O2
50ppm O
2
60
130ppm O2
1
50
40
1000ppm O
2
1.3% O
2.6% O
2
2
2
30
6.5% O2
1
12.8% O2
20
10
0
1
90 210 230 250 270 290 310 330
190 210 230 250 270 290 310 330
o
o
Temperature ( C)
Temperature ( C)
1
.0
.8
.6
.4
.2
.0
0.20
o
(d)
o
o
(c)
3
2
2
2
2
2
10 C
310 C
250 C
o
o
o
90 C
290 C
230 C
0
0
0
0
0
o
o
o
0
0
0
.15
.10
.05
70 C
270 C
210 C
o
50 C
o
30 C
o
10 C
0.00
0
20
40
60
80
100
0
1
2
3
4
CH concentration (%)
O concentration (%)
2
4
Figure 1. Effect of the temperature on the ethane (a) productivity and (b) selectivity with various O
b). In the case of 12.8% O , 78% of CH was used instead. Effect of the (c) CH and (d) O content. The O
with balance N in (d).
2
contents. The CH
4
content was 86% with balance N
2
in (a) and
(
2
4
4
2
2
content was 5% in (c), and the CH
4
content was 8.6%
2
The highly oxidized Pd/CeO
Pd/CeO , which was synthesized with a conventional deposition-
precipitation, at 750 ˚C for 25 h under air. When a high
concentration of CH (86%) was flowed with gaseous O (50 ppm
was produced with a
at 310 ˚C as shown in Figure
2
catalyst was prepared by treating
the C
the highest productivity and selectivity (Figure S5).
The effect of CH and O content was also examined. The
productivity increased linearly as CH content increased as
shown in Figure 1c. This trend was observed in a wide range of
CH content (2.5% ~ 86%) at various temperatures (210 ~ 310
˚C), indicating that CH activation can occur easily on the
Pd/CeO catalyst. On the other hand, the C productivity
became saturated as O content increased as shown in Figure 1d.
The O content at which the ethane productivity reached
saturation increased at higher temperature; 1.4% at 210 ˚C to
2.4% at 310 ˚C. O activation occurs more at higher temperature,
resulting in higher C productivity. These results indicate that
activation is more difficult. When O content was low below 1%,
the Pd/CeO was reduced more easily, terminating the C
production at lower temperatures. Facilitating O activation while
preventing the catalyst reduction is important to maximize the
production.
Whether the active site is Pd-O-Pd or Pd-O-Ce was
investigated. Pd-O-Pd is the main bonding of PdO particles, and
Pd-O-Ce is the site at the interface between Pd and CeO . The
2 6 2
H production was observed in all cases, and the CeO had
2
4
2
4
2
C
2
H
6
4
~
2 6
6.5%) at elevated temperatures, C H
productivity of 0.84 mmol gcat-1
h
-1
4
1a. The C
2
H
6
selectivity was 11% at 310 ˚C with balance CO
2
,
4
and a trace amount of C
of 5 μmol gcat h . When the temperature was 330 ˚C, the C
2
H
4
was also detected with a productivity
2
2 6
H
-
1
-1
2
H
6
2
production suddenly stopped. X-ray photoelectron spectroscopy
XPS) results showed that the Pd surface was reduced and
2
(
extended X-ray absorption fine structure (EXAFS) result also
showed that a strong Pd-Pd peak appeared after the reaction at
2
2 6
H
3
30 ˚C (Figure S1). This indicates that metallic Pd was not the
O
2
2
active phase for methane coupling to produce ethane. The Pd
catalyst after the reaction at 310 ˚C showed the same oxidic
2
2 6
H
2
surface state as the fresh Pd catalyst. The C
produced at 310 ˚C stably for 100 h (Figure S2).
2 6
H could be
2 6
C H
2 2
As the O content decreased, the highly oxidized Pd/CeO
catalyst was reduced at lower temperature, and then the ethane
productivity dropped to nearly zero. Instead, the ethane selectivity
was high; 60% of the ethane selectivity was observed at 190 ˚C
2
Pd-O-Ce is known to be efficient for methane combustion at lower
temperatures.[3b, 13] The ratio of the surface Pd-O-Pd to Pd-O-Ce
was varied by changing calcination temperatures. The 4 wt%
and 50 ppm O
with O , only 4.7 % of the C
Figure S3). Only a small fraction of the produced C
2
as shown in Figure 1b. When C
2 6
H was flowed
2
2
H
6
was converted to CO
2
at 310 ˚C
seems to
(
2
H
6
Pd/CeO
oxidic Pd catalysts with different degrees. XRD patterns and TEM
images were shown in Figure S6 and S7. The Pd/CeO calcined
at 350, 550, 750 ˚C showed the XRD peak for CeO only, while
the Pd/CeO calcined at 900 ˚C also showed the metallic Pd
peaks. EDS mapping images show that Pd was well distributed
on the CeO . The Pd domain size was 2.0, 2.3, 2.9, and 12.7 nm
2
was calcined at various temperatures to prepare the
be converted into CO
supports were tested. The C
2
. Various kinds of metals or metal oxide
production was observed on Pd
and Pt among Pd, Rh, Ru, Ir and Pt (Figure S4). The Pd had a
much higher C productivity at lower temperatures than that of
the Pt. When the Pd was deposited on ZrO , CeO , TiO and SiO
H
2 6
2
2
2
H
6
2
2
2
2
2
,
2
2
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ChemSusChem
10.1002/cssc.201903311
COMMUNICATION
for the Pd/CeO
2
catalysts calcined at 350, 550, 750, and 900 ˚C,
(2) Dissociation of the adsorbed CH
(E1→E2)
4 4 3
(*CH ) into *CH and *H
respectively, when measured by CO chemisorption (Table S2).
These catalysts were compared to commercial PdO with a Pd
domain size of 17.8 nm. The oxidation state of surface Pd was
estimated by XPS (Figure S8). In the Pd/CeO
oxidic Pd 3d5/2 peak appears at two different positions of 336.5 eV
for Pd-O-Pd and 337.6 eV for Pd-O-Ce, while the metallic Pd 3d5/2
peak appears at 335.5 eV.[
deconvoluted and the percentages of Pd-O-Pd or Pd-O-Ce at the
surface were estimated from the peak area ratio in Figure S8. The
(3) Approach of two *CH
*CH into *CH and *H (E2→E3-1)
(4) Coupling of two *CH into *C (E3→E4)
O with an O vacancy left on the surface
3
and two *H (E2→E3) or dissociation of
3
2
2
catalysts, the
3
2 6
H
(5) Formation of *H
(E4→E5)
2
17]
The oxidic XPS peaks were
(6) Desorption of *H
(7) Healing of O vacancy by gas-phase O
*C (E6→FS)
2
O from the surface (E5→E6)
2
and desorption of
2 6
H
2
surface of Pd/CeO calcined at 900 ˚C was reduced after the
calcination. As shown in Figure 2, the fraction of Pd-O-Pd sites
increased from 30% to 41%, 50%, 58%, and 100% for the
Pd/CeO catalysts calcined at 350, 550, 750, 900 ˚C, and bulk
2
PdO, respectively. EXAFS data in Figure S9 also confirmed that
the peak for Pd-O-Pd increased while the peak for Pd-O-Ce
decreased as the calcination temperature increased. Table S3
shows the fitting results of the EXAFS data. The coordination
number for Pd-O-Pd increased from 2.3 to 3.3, 4.3, 5.3, and 8.0
The asterisk mark (*) denotes the chemical species adsorbed
on the surface. Figure 3 shows the relative potential energies of
each elementary step, and Figure S11 shows the optimized
structures of each step and transition states. Table S4 shows the
specific energy difference at each step. In the Pd-O-Ce model,
the largest energy is required when the *H
surface with the energy difference of 1.79 eV (step 6). In the Pd-
O-Pd model, the largest energy is required when the *H O and
2
O is desorbed from the
2
in the Pd/CeO
respectively. When C
Pd atom, the turnover frequency becomes higher as the fraction
of the Pd-O-Pd sites increase. Clearly, the Pd-O-Pd sites, not Pd-
O-Ce, are the active sites for the methane coupling.
2
calcined at 350, 550, 750, 900 ˚C, and bulk PdO,
productivity was estimated per surface
oxygen vacancy are formed with the energy barrier of 1.40 eV
(step 5). Since the barrier for rate-determining step (RDS) is
smaller in the Pd-O-Pd model than that in the Pd-O-Ce model, the
activity for the methane coupling would be higher in the Pd-O-Pd
2 6
H
than that in the Pd-O-Ce, leading to the higher C
After *(CH +H) is formed from the dissociation of *CH
), the *CH might be further dissociated into *(CH +H) (step 3-1).
The reaction barrier of *CH dissociation into *(CH +H) were
compared. The barrier for the *CH dissociation was 1.58 eV on
the Pd-O-Pd model, while it was 1.07 eV on the Pd-O-Ce model.
Especially, the *CH dissociation (step 3-1) was energetically
preferred to the pathway towards the coupling of two *CH into
(step 3) by 2.26 eV on the Pd-O-Ce model. However, the
pathway to *C (step 3) was favored than the pathway to *CH
dissociation (step 3-1) by 0.29 eV on the Pd-O-Pd model. The Pd-
O-Pd would produce C whereas the Pd-O-Ce would rather
promote further dehydrogenation leading to the formation of
CO
.[18] The DFT results indicate that Pd-O-Pd sites can be the
active sites producing C from methane, whereas Pd-O-Ce site
would combust the methane.
2
H
6
productivity.
3
4
(step
2
3
2
3
2
3
3
3
2 6
*C H
2
H
6
3
2 6
H
2
2 6
H
[
3b, 13]
Figure 2. The ratios of the Pd-O-Pd and Pd-O-Ce estimated from XPS Pd 3d
spectra (left axis) and turnover frequency for C
2 6
H production at 250˚C (right
axis) on the Pd/CeO catalysts calcined at 350, 550, 750, 900 ˚C, and bulk PdO.
2
The number of surface Pd atom was estimated from CO uptakes.
DFT calculations were carried out to elucidate how Pd-O-Ce
and Pd-O-Pd sites affect the oxidative methane coupling. Two
models with Pd-O-Ce or Pd-O-Pd sites were constructed to
understand the fundamental effects of these sites on the methane
7 7
conversion. The partially embedded Pd O /Ce33O66(111) surface
(
Figure S10c) was used as the Pd-O-Ce model; all the oxygens
exposed on the PdO particle are Pd-O-Ce sites because all the
oxygens (O1-O8) are neighbored by both Pd and Ce. The
PdO(101) facet was used as the Pd-O-Pd model (Figure S10d);
all the oxygens (O1-O2) are neighbored by Pd with Pd-O-Pd sites
only. The computational details of constructing the models can be
found in supporting information. The mechanistic steps of
oxidative methane conversion considered in the DFT calculations
are as follows:
Figure 3. Reaction profiles of the oxidative methane conversion on Pd-O-Ce
and Pd-O-Pd sites; the rate-determining steps were denoted using dashed
arrows. The blue and green lines indicate the paths (E3-1) leading to CO
formation on Pd-O-Pd and Pd-O-Ce, respectively.
2
Although Pd-O-Pd are the active sites for the methane
coupling, bulk PdO presented lower maximum C productivity
and selectivity than the Pd/CeO catalyst calcined at 750 ˚C
4
(1) Adsorption of gas-phase CH molecule (IS→E1)
2 6
H
2
3
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ChemSusChem
10.1002/cssc.201903311
COMMUNICATION
(
Figure S12). H
data showed that the bulk PdO was reduced at -17 ˚C whereas
smaller Pd nanoparticles supported on CeO was reduced at
higher temperatures above 5 ˚C (Figure S13). The bulk PdO is
reduced too easily while the Pd/CeO can maintain the oxidic Pd
state better in a high concentration of CH flow. The oxidic Pd
2
temperature-programmed reduction (H
2
-TPR)
Ir, and Pt were deposited on the CeO
2
. Metal chloride precursors of
RhCl
3 2
ꞏxH O (99.98%, Sigma-Aldrich), IrCl
3
ꞏxH
2
O (≤100%, Sigma-Aldrich),
(≤100%, Sigma-Aldrich)
2
H PtCl
6 2
ꞏ6H O (≤100%, Sigma-Aldrich), and RuCl
3
2
were used. Other metal oxide supports of TiO
2
(P25, Degussa), ZrO
2
(
Sigma-Aldrich), and SiO (Sigma-Aldrich) were also used instead of CeO
2
2
2
at otherwise the same synthesis conditions. Reaction, characterizations,
and computational details can be found in supporting information.
4
sites are the active sites for the methane coupling, and the ethane
would not be produced in the presence of metallic Pd. The ceria
support helped the Pd domain maintain the highly oxidized state
Acknowledgements
in the reductive condition. The Pd/CeO
2
calcined at 350 ˚C or 550
˚
C showed a larger reduction peak than the other samples,
This research was supported by the National Research
Foundation of Korea (NRF-2016M3D3A1A01913255 and
2
indicating that more H was consumed to reduce the surface with
more Pd-O-Ce bonds.[
19]
O
2
temperature-programmed desorption
was desorbed more in the bulk
PdO, causing more over-oxidation (Figure S14). The surface
oxygen of the Pd/CeO calcined at 750 ˚C started to be desorbed
at the highest temperature, indicating that removing the oxygen
from the catalyst surface is the most difficult. CH -TPR was also
performed and the produced H O was detected using a mass
spectrometer (Figure S15). The H O was detached from the
Pd/CeO calcined at 750 ˚C at the temperature similar to the bulk
PdO, but the peak intensity was lower than that of bulk PdO. The
2
018R1A2A2A05018849). The experiments at PLS were
2 2
(O -TPD) results also show that O
supported in part by MSIP and POSTECH.
2
Keywords: methane, ethane, PdO, heterogeneous catalysis,
4
oxidation
2
2
[
1]
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2
H
2
O was detached at higher temperatures from the Pd/CeO
2
[
2]
3]
calcined at 550 ˚C or 350 ˚C. In DFT calculations, the H
2
O
50, 418-425.
formation or desorption was identified as the most difficult step for
methane coupling reaction on Pd-O-Pd or Pd-O-Ce model. H
could be detached at lower temperatures from the catalysts with
more Pd-O-Pd sites, whereas H O was detached at higher
temperatures from the catalysts with more Pd-O-Ce sites.
In summary, a highly oxidized Pd/CeO catalyst could oxidize
methane directly to ethane in a stable manner using gaseous O
below 400 ˚C. The Pd-O-Pd sites, not Pd-O-Ce, enabled the
oxidative methane coupling. But the ceria helped Pd domain to
maintain a highly oxidized state even under a high concentration
of methane flow with higher C
that the C-C coupling can occur more on Pd-O-Pd site whereas
Pd-O-Ce site would promote dehydrogenation into CO . This work
opens up a new avenue to design heterogeneous catalysts for
oxidative methane conversion into value-added chemicals at
milder conditions.
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2
2 6
H selectivity. DFT study confirmed
2
[
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Entry for the Table of Contents
A highly oxidized Pd deposited on ceria could oxidize methane directly to ethane in a stable manner using gaseous O
The Pd-O-Pd sites, not Pd-O-Ce, enabled the oxidative methane coupling.
2
below 400 ˚C.
6
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