Single Chromium Atoms Supported on Titanium Dioxide Nanoparticles for Synergic Catalytic Methane Conversion under Mild Conditions

Article abstract of DOI:10.1002/anie.201913309

Direct conversion of methane to value-added chemicals with high selectivity under mild conditions remains a great challenge in catalysis. Now, single chromium atoms supported on titanium dioxide nanoparticles are reported as an efficient heterogeneous catalyst for direct methane oxidation to C1 oxygenated products with H₂O₂ as oxidant under mild conditions. The highest yield for C1 oxygenated products can be reached as 57.9 mol mol_Cr^?1 with selectivity of around 93 % at 50 °C for 20 h, which is significantly higher than those of most reported catalysts. The superior catalytic performance can be attributed to the synergistic effect between single Cr atoms and TiO₂ support. Combining catalytic kinetics, electron paramagnetic resonance, and control experiment results, the methane conversion mechanism was proposed as a methyl radical pathway to form CH₃OH and CH₃OOH first, and then the generated CH₃OH is further oxidized to HOCH₂OOH and HCOOH.

Full text of DOI:10.1002/anie.201913309

A Journal of the Gesellschaft Deutscher Chemiker
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Accepted Article
Title: Single chromium atoms supported on titanium dioxide
nanoparticles for synergic catalytic methane conversion under
mild condition
Authors: Wei-Guo Song, Qikai Shen, Changyan Cao, Runkun Huang,
Lei Zhu, Xin Zhou, Qinghua Zhang, and Lin Gu
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201913309
Angew. Chem. 10.1002/ange.201913309
Link to VoR: http://dx.doi.org/10.1002/anie.201913309
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10.1002/anie.201913309
Angewandte Chemie International Edition
COMMUNICATION
Single chromium atoms supported on titanium dioxide
nanoparticles for synergic catalytic methane conversion under
mild condition
c
Qikai Shen,[a,b] Changyan Cao,*[a,b] Runkun Huang, [a,b] Lei Zhu, [a,b] Xin Zhou, [a,b] Qinghua Zhang, [ ] Lin
c
Gu[ ] and Weiguo Song*[a,b]
Abstract: Direct conversion of methane to value-added chemicals
with high selectivity under mild conditions remains a great challenge
in catalysis. Herein, we report a new kind of single chromium atoms
supported on titanium dioxide nanoparticles as an efficient
heterogeneous catalyst for direct methane oxidation to C1
oxygenated products with H2O2 as oxidant under mild condition. The
highest yield for C1 oxygenated products can be reached as 57.9
mol/molCr with selectivity of around 93% at 50ºC for 20 h, which is
significantly higher than those of most reported catalysts. The superior
catalytic performance can be attributed to the synergistic effect
between single Cr atoms and TiO2 support. Combining catalytic
kinetics, electron paramagnetic resonance and control experiment
results, methane conversion mechanism was proposed as a methyl
radical pathway to form CH3OH and CH3OOH first, and then the
generated CH3OH is further oxidized to HOCH2OOH and HCOOH.
methane oxidation under mild conditions. Homogeneous
molecular catalysts are usually highly active and selective, but
usually highly corrosive acids were used and it was hard to
recover the catalysts.[8] Many impressive heterogeneous catalysts,
such as Cu and Fe-exchanged zeolites[9], Au-Pd colloids[10] and
single atom catalysts of Rh[11] and Fe[12], were also developed for
direct methane conversion under mild conditions in recent years.
For example, Hutchings et al. reported that aqueous Au-Pd
colloidal catalysts exhibited better catalytic performance than the
same nanoparticles supported on TiO2 for oxidation of CH4 to
methanol with high selectivity (92%) at 50ºC and 3 MPa pressure
with H2O2 and O2 as co-oxidants.[10a] Lee et al. reported a single-
atom Rh catalyst dispersed on zirconia surface for selective
activation of CH4 at 70ºC with H2O2 as an oxidant.[9] Very recently,
Deng et al. reported that graphene-confined single iron atoms
could activate CH4 conversion at room temperature with H2O2 as
an oxidant, and the highest selectivity of C1 oxygenated products
(CH3OH, CH3OOH, HOCH2OOH and HCOOH) was around
94%.[12c]
Although much progress has been achieved with
heterogeneous catalysts for direct methane conversion with
oxidants, two major issues are still needed to be addressed. First,
the newly emerged single-site catalysts have shown impressive
catalytic performance for kinds of reactions.[13] However, only a
few metals were reported for direct methane conversion so far. It
is worth of developing and investigating other single-site metals,
especially non-noble metal catalysts for this reaction. Second, the
oxygenated products are hard to be preserved during the reaction
because it is much easier for them to be further oxidized to CO or
CO2 thermodynamically[14]. Therefore, it is highly important to
control the extent of methane oxidation to selectively obtain the
oxygenated products.[15]
Herein, we report single site chromium atoms supported on
titanium dioxide nanoparticles (denoted as Cr1/TiO2) can be used
as an efficient heterogeneous catalyst for direct methane
oxidation to C1 oxygenated products with H2O2 as an oxidant
under mild conditions. With optimized catalyst, the highest yield
for C1 oxygenated products can be reached as 57.9 mol/molCr
with high selectivity of around 93% at 50ºC for 20 h, which is
significantly higher than those of most reported catalysts. The
superior catalytic performance can be attributed to the synergistic
effect between Cr single atoms and TiO2 support. Combining
catalytic kinetics, electron paramagnetic resonance and control
experiment results, a possible reaction pathway for methane
conversion with Cr1/TiO2 catalyst is proposed as: CH4 is first
oxidized through a radical pathway to form CH3OH and CH3OOH,
and then the generated CH3OH is further oxidized to HOCH2OOH
and HCOOH.
Methane is one of the most promising building blocks for
producing basic chemicals because of its abundant and
inexpensive.[1-2] However, methane is the most inert hydrocarbon
due to the strong C-H bond strength (434 kJ/mol) and its
tetrahedron structure.[3] Activation of methane is a big challenge
and has been regarded as the “holy grail” in catalysis.[4] In the
conventional approaches that has been implemented on a large
scale, methane is converted into liquid hydrocarbons through the
indirect route, such as steam-reforming[5] and Fischer-Tropsch
(FT) synthesis[6]. These processes are usually operated at high
temperature and are highly energy intensive.[7] As an alternative
approach, the direct route for methane conversion into liquid
hydrocarbon products with oxidants (H2O2, O2, etc.) has attracted
great attention in recent years due to the advantages in mild
reaction condition and environmentally-benign.
Several kinds of catalysts, including homogeneous and
heterogeneous catalysts, have been developed for direct
[a]
Q. Shen, Dr. C. Cao, R. Huang, L. Zhu, X. Zhou, Prof. W. Song
Beijing National Laboratory for Molecular Sciences, CAS
Research/Education Center for Excellence in Molecular Sciences
CAS Key Laboratory of Molecular Nanostructure and
Nanotechnology
Institute of Chemistry, Chinese Academy of Sciences
Beijing 100190 (P.R. China)
E-mail: cycao@iccas.ac.cn
wsong@iccas.ac.cn
[b]
[c]
Q. Shen, Dr. C. Cao, R. Huang, L. Zhu, X. Zhou, Prof. W. Song
University of Chinese Academy of Sciences
Beijing 100190 (P.R. China)
Dr. Q. Zhang, Prof. L. Cu
Beijing National Laboratory for Condensed Matter Physic
Institute of Physics, Chinese Academy of Sciences
Beijing 100190 (P.R. China)
Supporting information for this article is given via a link at the end of
the document.
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10.1002/anie.201913309
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COMMUNICATION
Table 1. Comparative catalytic performance of the catalysts for CH4 oxidation.
HCOOH
HOCH2OOH
CH3OOH
CH3OH
CO2
SUM
Metal
[wt%]
Entry
Catalyst
T [°C]
P [MPa]
[μmol]
[μmol]
[μmol]
[μmol]
[μmol]
[μmol]
1a
2
Cr2O3
68.4
1
50
50
50
50
50
50
50
50
50
50
50
50
50
50
25
3
0.2
0.4
1.3
1.4
0.5
2.6
0
0.6
1.7
0.1
0.8
2.1
1.1
0.9
3.1
0
*
*
*
*
*
*
*
*
*
*
*
*
*
*
*
2.4
Cr/SiO2
Cr/Fe2O3
Cr/CeO2
Cr/ZrO2
Cr/TS-1
TiO2
3
3.7
6.3
11.2
28.8
20.2
18.3
36.8
0
3
1
3
12.2
7.7
13.2
10.0
7.5
4
1
3
5
1
3
9.4
6
1
3
15.0
0
16.1
0
7[a]
8
1
3
Cr/TiO2
Cr/TiO2
Cr/TiO2
Cr/TiO2
Cr/TiO2
Cr/TiO2
Cr/TiO2
Cr/TiO2
1
3
2.2
0.7
1.5
2.0
1.0
0.3
0.4
0.4
20.8
12.2
15.8
13.2
7.5
17.5
26.5
24.1
7.9
3.4
2.5
3.8
2.1
1.2
0.4
0.1
0.5
43.9
41.9
45.2
25.2
14.6
4.6
9
0.25
0.5
2
3
10
11
12
13
14
15
3
3
4
3
4.9
8
3
2.1
1.8
1
0.1
3
1.2
1.1
2.8
1
1.3
3.0
5.2
Typical reaction conditions: 0.5 mL H2O2 (30%), 9.5 mL deionized water, 3 MPa CH4, catalysts (Cr: 0.1mg), in a stainless steel auto-clave with a Teflon liner vessel
at 50 °C for 1 h. [a] Catalyst (10 mg).
In a typical reaction, the oxidation of CH4 is performed in water
with H2O2 as an oxidant at 50ºC and 3 MPa pressure in a
stainless-steel autoclave with a Teflon liner vessel. Initially, we
investigated the activity of commercial Cr2O3 for oxidation of CH4,
and the result showed that about 2.5 μmol C1 oxygenated
products was detected in the liquid phase in an hour based on the
1H-NMR analysis. The peaks can be ascribed to CH3OH,
CH3OOH, HOCH2OOH and HCOOH, respectively, which were
similar with those reported in the literatures (Figure S1).
Encouraged by this, 1 wt% Cr supported on commercial SiO2,
ZrO2, TS-1 zeolite, TiO2 nanoparticles and flowerlike α-Fe2O3 and
CeO2 were prepared by incipient wetness followed by calcinations
in air in order to enhancing the dispersion and catalytic activity.
All these supported catalysts were then test for oxidation of CH4
under the same conditions. As shown in Table 1 and Figure S2,
the supported catalysts exhibited much better activities and 1 wt%
Cr/TiO2 showed the best performance with nearly 43.9 μmol C1
oxygenated products, which is about 17 times than that of bare
commercial Cr2O3.
We further evaluated the effect of Cr loading in Cr/TiO2 on the
catalytic performance. Without Cr, bare TiO2 nanoparticles could
not catalyze CH4 oxidation, indicating Cr species were the active
sites. A series of Cr/TiO2 catalysts with different Cr lording from
Figure 1. (a) 13C-NMR and 13C DEPT-135 of oxygenated products in liquid
phase with 1 wt% Cr/TiO2 catalyst by using 13CH4 as the reactant gas. (b)
Catalytic performance of the 1 wt% Cr/TiO2 catalysts for CH4 oxidation with
different reaction time. (c) EPR experiments oxygenated products in liquid
0.25 to 8 wt% were prepared by the same incipient wetness
method. The Cr loadings were close to the theoretical values as
measured by inductively coupled plasma atomic emission
spectrometry (ICP-AES) (Table S1). As shown in Table 1 and
Figure S3, it could be found that the Cr/TiO2 catalysts with loading
of less than 1 wt% showed the highest yields of C1 oxygenated
products. However, the yields significantly decreased as further
increasing the Cr loading. That is probably because of the
agglomeration of active sites with higher Cr content. Therefore,
phase with different catalysts. (d) Catalytic performance with CH4 (3 MPa) and
CH3OH (~15.8 μmol) as reactants over 1 wt% Cr/TiO2.
1 wt% Cr/TiO2 was selected as the optimum catalyst for the
following study.
13C-NMR, 13C DEPT-135 (distortionless enhancement by
polarization transfer) and 2D 1H-13C HMQC (heteronuclear
multiple-quantum correlation) were measured by using 13CH4 as
the reactant gas to further confirm the C1 oxygenated products in
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10.1002/anie.201913309
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liquid phase with 1 wt% Cr/TiO2 catalyst. Combined 1H-NMR and
13C-NMR results, the featured points matched well with CH3OH,
HOCH2OOH, CH3OOH and HCOOH in 2D 1H-13C HMQC
spectrum (Figure 1a and Figure S5). No other liquid products
were detected from NMR analysis and no CO2 was detected from
gas chromatography, indicating the selectivity of C1 oxygenated
products was nearly 100% at 1 h. Compared with the previous
reports, 1 wt% Cr/TiO2 catalyst shows significant higher catalytic
activity and selectivity for direct oxidation of CH4 with H2O2 (Table
S2)[10-11,12b,c]. Especially, it still worked well even at room
temperature or ambient pressure (Table 1 and Figure S4).
The evolution of products with reaction time was then
investigated to elucidate the catalytic kinetics and the reaction
pathway. As shown in Figure 1b, the overall yields of C1
oxygenated products first increased and then started to decline
with the highest value at the reaction time of 20 h. Meanwhile,
CO2 appeared and increased after 8 h, suggesting the liquid C1
oxygenated products were further oxidized to CO2 with prolonged
reaction time. The highest C1 oxygenated products could be
reached as 57.9 mol/molCr with high selectivity of around 92.8%
in 20 h.
Figure 2. (a) HAADF-STEM image of 1 wt% Cr/TiO2. The red circles show
single Cr atoms in the matrix of TiO2. (b) H2-TPR results of catalysts.
field scanning transmission electron microscope (HAADF-STEM)
was further performed to confirm the dispersion state of Cr atoms.
As shown in Figure 2a, isolated bright dots corresponding to
individual Cr atoms dispersed on the TiO2 support were observed.
X-ray photoelectron spectroscopy (XPS) was then used to
characterize the Cr oxidation state. The main peaks located at
586.4 eV and 588.4 eV in Cr 2p spectrum (Figure S7a) can be
ascribed to Cr (III) and Cr (VI), respectively.[17] The presence of
Cr oxidation species was also confirmed by temperature
programmed reduction with H2 (H2-TPR). As shown in Figure 2b,
the peaks at around 245ºC and 305ºC were corresponding to the
reduction of Cr (VI) and Cr (III), respectively. Compared to pure
TiO2, Ti 2p peaks of 1 wt% Cr/TiO2 shift to lower binding energies.
Especially, with the increase of Cr loading, the binding energies
of Ti decreased progressively (Figure S7a), suggesting the
interaction between Cr oxidation species and TiO2 nanoparticles.
Raman spectra also confirmed their interaction, as shown in
Figure S7b. There were progressively blue-shift for the B1g and
MP modes, and red-shift for the Eg and A1g modes of TiO2 with
The presence of CH3OH, CH3OOH, HOCH2OOH, HCOOH and
CO2 suggests a consecutive pathway for CH4 oxidation, which
were observed and proposed previously by many groups.
Basically, for direct CH4 conversion in heterogeneous system, the
activation of oxidants produces oxidative species that can attack
C-H bond of CH4 nucleophilically to form methyl radicals, which
are then reacted with oxygenated species to form oxygenated
products. In order to clarify the reaction mechanism on 1 wt%
Cr/TiO2, electron paramagnetic resonance (EPR) experiments
were carried out after the reaction. As shown in Figure 1c, both
•
•CH3 and OH radicals were detected in the reaction. As TiO2 is
well-known for activating H2O2 to produce surface-stabilized
-
peroxo- or hydroperoxo species (TiO2-O2 or Ti-OOH)[16], the
[18]
the increase of the Cr loading on the TiO2.
generated •CH3 may interact with Ti-OOH to form CH3OOH, which
is also confirmed as the primary product in the initial stage.
At the same time, the generated •CH3 and •OH could also react
to form CH3OH, which was also observed in the product.
Apparently, CH4 is first oxidized to CH3OH and CH3OOH. After 30
min, CH3OOH and HOCH2OOH increased significantly while
CH3OH remained. In addition, HCOOH and CO2 increased
significantly after 8 h. These results suggest that CH3OH formed
in the initial CH4 oxidation stage is further oxidized to HOCH2OOH
and HCOOH. In order to confirm this, we used CH3OH as the
reactant instead of CH4. Indeed, HOCH2OOH and HCOOH were
observed in the products, which agrees well with the hypothesis
(Figure 1d). These observations agreed well with Deng et al. on
FeN4/GN catalyst. CH4 is first oxidized to CH3OH and CH3OOH,
and then the generated CH3OH is further oxidized to HOCH2OOH
and HCOOH.[12c]
The morphology and structure of 1 wt% Cr/TiO2 was then
investigated to elucidate the excellent catalytic performance.
Under bright-field transmission electron microscopy (TEM), only
pure TiO2 nanoparticles were observed and there was no obvious
CrOx particles or clusters found on TiO2 (Figure S6). However,
energy-dispersive spectroscopy EDS mapping images showed
that Cr element was uniformly dispersed on TiO2 nanoparticles
(Figure S6h). Thus, aberration-corrected high-angle annular dark-
Based on the structure characterizations and compared
catalytic results, we can ascribe the high efficiency of 1 wt%
Cr/TiO2 catalyst for direct CH4 oxidation to their synergistic effect.
•
EPR results showed that only very weak OH radicals were
generated with bare TiO2 nanoparticles. Introducing Cr species,
much higher •OH radicals as well as •CH3 radicals were produced,
suggesting single Cr atoms play roles not only in activating C-H
•
bonds of CH4 to form CH3 radicals, but also activating H2O2 to
form •OH radicals. •CH3 radicals could react with surface bound –
OOH of TiO2 to form CH3OOH. Moreover, the synergistic effect
between TiO2 and Cr species lies in TiO2 is a more suitable
support for the formation of highly dispersed single Cr atoms,
which have the maximum atomic efficiency and highest activity.
Besides high activity and selectivity, 1 wt% Cr/TiO2 also
exhibited good recyclability for methane oxidation. After reaction,
the catalyst was separated through centrifugation and directly
used for the next cycle, its catalytic activity decreased greatly. H2-
TPR result showed that the peak at around 220 °C corresponding
to Cr(VI) was nearly disappeared (Figure 2b). However, after
calcinations in air at 400 °C for 2 h, its catalytic activity was similar
with the fresh catalyst and the peak of Cr(VI) in H2-TPR curve was
recovered, suggesting Cr (VI) species in 1 wt% Cr/TiO2 catalyst
played a vital role in direct methane oxidation. Although slightly
activity loss was observed during each cycle, the catalyst
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COMMUNICATION
A new kind of single chromium atoms
supported on titanium dioxide
nanoparticles as an efficient
Qikai Shen, Changyan Cao,* Runkun
Huang, Lei Zhu, Xin Zhou, Qinghua
Zhang, Lin Gu and Weiguo Song*
synergistic heterogeneous catalyst for
direct methane oxidation to C1
oxygenated products with H2O2 as
oxidant under mild condition.
Page No. – Page No.
Single chromium atoms supported on
titanium dioxide nanoparticles for
synergic catalytic methane
conversion under mild condition
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