Oxidation of methane to methanol over Pd@Pt nanoparticles under mild conditions in water

Article abstract of DOI:10.1039/d1cy00273b

Direct methane oxidation into oxygen-containing chemicals under mild conditions has sparked increasing interest. Here, we report Pd@Pt core-shell nanoparticles that efficiently catalyse the direct oxidation of CH₄to CH₃OH in water using H₂O₂as an oxidant under mild conditions. The catalyst presents a methanol productivity of up to 89.3 mol kg_catalyst^?1h^?1with a high selectivity of 92.4% after 30 min at 50 °C, thus outperforming most of the previously reported catalysts. Electron-enriched Pt species in the Pd@Pt nanoparticles were identified by structural and electronic analysis. Pd in the core donates electrons to Pt, leading to higher rates of methane activation. Based on the results of control experiments and kinetic analysis, a consecutive oxidation pathwayviaa radical mechanism is proposed, which includes initial formation of CH₃OOH and CH₃OH followed by further oxidation of CH₃OH to HCHO, HCOOH, and CO₂

Full text of DOI:10.1039/d1cy00273b

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Oxidation of methane to methanol over Pd@Pt
nanoparticles under mild conditions in water†
Cite this: DOI: 10.1039/d1cy00273b
ab
ac
d
d
Jianjun Chen,‡ Sikai Wang,‡ Laurent Peres, Vincent Collière,
d
Pierre Lecante, Yaoqiang Chen* and Ning Yan *a
e
b
Karine Philippot,
Direct methane oxidation into oxygen-containing chemicals under mild conditions has sparked increasing
interest. Here, we report Pd@Pt core–shell nanoparticles that efficiently catalyse the direct oxidation of CH
4
to CH
3 2 2
OH in water using H O as an oxidant under mild conditions. The catalyst presents a methanol
−1
−1
productivity of up to 89.3 mol kgcatalyst
h
with a high selectivity of 92.4% after 30 min at 50 °C, thus
Received 12th February 2021,
Accepted 19th March 2021
outperforming most of the previously reported catalysts. Electron-enriched Pt species in the Pd@Pt
nanoparticles were identified by structural and electronic analysis. Pd in the core donates electrons to Pt,
leading to higher rates of methane activation. Based on the results of control experiments and kinetic
analysis, a consecutive oxidation pathway via a radical mechanism is proposed, which includes initial
DOI: 10.1039/d1cy00273b
rsc.li/catalysis
3 3 3 2
formation of CH OOH and CH OH followed by further oxidation of CH OH to HCHO, HCOOH, and CO .
Among these, oxidation pathways are exothermic and low-
temperature favoured. For instance, direct oxidation of
methane into methanol (CH + 0.5 O2 → 3CH OH) has a
1
. Introduction
Natural gas is receiving great attention as a valuable source of
energy and chemicals. As the main component of natural
gas, methane is extensively applied as a fuel and also
processed into chemicals. To date, large-scale conversion of
methane to liquid hydrocarbons in industry has been carried
out through indirect methane-steam reforming coupled with
4
3
1
,2
0
−1
ΔH
value of −126.4 kJ mol . Over a range of oxidants,
2
98K
2 2 2 2
such as O , H O , and N O, partial oxidation has been proven
3
–5
viable below 200 °C. Compared to the indirect syngas
process, it is simpler and potentially more effective, but there
are two main issues to overcome. First, the C–H bond in
methane is difficult to activate due to the high ionization
6
–10
Fischer–Tropsch synthesis.
These steps normally involve
harsh conditions with high temperature (>400 °C) and
energy, high bond energy, low polarity, and low electron and
1
1–15
24–26
pressure (>10 bar).
Direct conversion of methane into
proton
affinity
of
methane.
Second,
it
is
value-added liquid fuels and chemicals such as methanol,
olefins, hydrogen, and aromatics has thus become an
important research topic attracting interest from both
thermodynamically difficult to keep the formed methanol
because the dissociation energy of the C–H bond in methanol
−
1
−1
(392 kJ mol ) is lower than that in methane (439 kJ mol ).
Under typical reaction conditions, methanol is thus easily
1
6–18
industry and academia.
The direct conversion pathways include methane
6
,27–29
oxidized to CO2.
1
9
20,21
22,23
pyrolysis, oxidative coupling,
and partial oxidation.
Results from decades of research showed that certain
homogeneous and heterogeneous catalysts are active for the
selective oxidation of methane into C1 oxygenates at low
a
Department of Chemical and Biomolecular Engineering, National University of
Singapore, 4 Engineering Drive, Singapore 117585, Singapore.
E-mail: ning.yan@nus.edu.sg
3
0
temperatures. For instance, molecular complexes of Pd, Rh,
Au, Hg, Ru, and Pt directly oxidized methane into
b
26,31,32
Institute of New Energy and Low-carbon Technology, Sichuan University, Chengdu
methanol,
but toxic/corrosive reagents such as
6
10064, China. E-mail: chenyaoqiang@scu.edu.cn
hydrobromic acid, trifluoroacetic acid, oleum, or
hydrochloric acid were heavily used. In heterogeneous
catalysis, zeolite-stabilized Fe and Cu species are known to
c
Joint School of National University of Singapore and Tianjin University,
International Campus of Tianjin University, Binhai New City, Fuzhou 350207,
China
d
CNRS, LCC (Laboratoire de Chimie de Coordination), 205 route de Narbonne,
4
convert CH into methanol via stepwise, separated reaction
3
3–40
41–47
BP44099, F-31077 Toulouse Cedex 4, France
steps.
In a single-step manner,
a few systems have
e
CNRS, CEMES (Centre d'Élaboration des Matériaux et d'Études Structurales), 29
been identified to be effective as well. For instance, iron
atoms confined in graphene showed high selectivity (94%) to
Rue Jeanne-Marvig, BP 4347, F-31055 Toulouse Cedex 4, France
†
Electronic supplementary information (ESI) available. See DOI: 10.1039/
d1cy00273b
These authors contributed equally to this work.
C1 products (CH OOH, CH OH, HOCH OOH and HCOOH) at
3
3
2
4
8
‡
25 °C using H O as an oxidant.
TiO2 nanoparticle
2
2
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4
9
supported Cr catalysts achieved similar selectivity at 50 °C.
NPs, Pd (or Pt) NPs were firstly prepared by NaBH reduction
4
Concerning noble metal-containing catalysts, much progress
has been achieved with Au–Pd colloid-based catalysts.
of the same metal precursors at r.t. Then the Pt (or Pd)
precursor was reduced in a second step at a lower
temperature (∼0 °C). The second step was conducted at a
lower temperature to slow down the reduction of the second
metal precursor, to favor its reduction at the surface of pre-
formed NPs.
5
0–52
5
3
Hutchings et al.
methanol productivity (53.6 mol kgAuPd h ) over Au–Pd
nanoparticles using H as an oxidant. Very recently, Xiao
reported high selectivity (92%) and
−
1
−1
2 2
O
5
4
et al. prepared hydrophobic ZSM-5 as a molecular fence for
in situ H O generation that led to the selective oxidation of
2
2
methane to methanol. This fence permitted the diffusion of
hydrophobic species like methane, oxygen, and hydrogen to
PdAu active sites over the colloidal catalyst, while locally
2
.2 Catalytic performance testing and product analysis
The catalytic performance testing for direct methane
oxidation was performed in a stainless-steel autoclave reactor
concentrating the formed H
interaction with methane.
2 2
O that allowed its rapid
(
2 2
30 mL). The reactor was charged with a H O aqueous
solution (400 μmol, 1 M, 0.4 mL) and 2 mL of each colloidal
solution (1 μmol metal). After sealing, the reactor was purged
with 95%CH
Besides Au, heterogeneous noble metal catalysts for
oxidizing methane to methanol and other oxygenates are
currently not common. The CH3 intermediate is a key
precursor for methanol formation in direct oxidation of
4 2
–5%N (3 MPa) three times to remove residual
air. Then, the autoclave was re-pressurized to 3 MPa and
−
1
heated to the desired temperature at a rate of 2 °C min .
When the reaction temperature reached 50 °C, stirring was
started (800 rpm) and continued for 0.5 h. After reaction, the
reactor was cooled in ice–water (ca. 10 °C) to minimize the
volatility of the liquid products. The gas phase products were
collected using a gas sample bag (100 mL) and analysed by
gas chromatography on an Agilent 7890B gas chromatograph
equipped with two columns of Porapak-Q and 5A and a
2
2,41,55
methane.
with high proportion of highly coordinatively unsaturated
sites are predicted to effectively stabilize the CH
Based on DFT calculation, platinum clusters
3
intermediate
dehydrogenation.
by
hindering
The design of Pt-based colloids by
its
successive
5
6–58
incorporating a second metal to change their geometric and
electronic structures may be a viable approach to identify
suitable catalysts to convert methane to C1 oxygenates.
Motivated by these analyses, in this work, a core–shell
Pd@Pt catalyst dispersed in water was developed. Its
structural characteristics were investigated by TEM, WAXS,
CO-DRIFTS, and XPS. This catalyst was evaluated in the direct
thermal conductivity detector (He carrier gas) using N as an
2
2
internal standard. Only CO was detected in the gas phase.
1
H-NMR (Bruker, 400 MHz) spectroscopy was used to
determine and quantify the liquid phase products. Water
suppression was employed to minimise the signal arising
4 3 2 2
oxidation of CH to CH OH using H O as an oxidant under
2
from water. Typically, 0.2 mL D O, 1.0 mL sample and DMSO
mild reaction conditions. The catalytic performance was
correlated to the structure of the Pd@Pt core–shell
nanoparticles, while the reaction pathway was established on
the basis of control experiments and reaction kinetic
analysis.
1
as an internal standard were placed in a NMR tube. H-NMR
spectra were recorded with a recycle delay of 5 s and a
spinning rate of 64 scans. The methanol selectivity and
productivity
equations:
were
estimated
using
the
following
5
3,54
2
. Experimental methods
Primary oxygenate selectivity = mole of (CH OH + CH OOH)/
3
3
total mole of products.
2
.1 Catalyst synthesis
Monometallic Pt, monometallic Pd, and bimetallic Pt Pd
x
y
nanoparticles (NPs) were synthesized by a wet chemistry
method using water as a solvent to provide the colloidal
catalyst. The studied x/y ratios were 8/1, 4/1, 2/1, 1/1, 1/2, 1/4
and 1/8, respectively. Typically, the procedure for the
Primary oxygenate productivity = mole of (CH
weight of metal (kg) × reaction
time (h))
3 3
OH + CH OOH)/
(
1 1
preparation of the Pt Pd colloid was as follows: an acidic
solution of the PdCl precursor and an aqueous solution of
2
the K PtCl precursor were dissolved in de-ionized water (500
2
4
−
1
2.3 Catalyst characterization
mL) at a total metal concentration of 0.256 mmol L .
Polyvinylpyrrolidone (PVP; MW 40 000) was used as
stabilizer and added to the solution at a metal-to-PVP ratio of
: 1.2. After vigorously stirring for 30 min, a freshly prepared
a
To determine the morphology and size of the Pd@Pt NPs,
transmission electron microscopy (TEM) measurements were
performed. For that purpose, the materials were dispersed on
a 300-mesh copper TEM grid covered with a carbon film. Low
and high resolution transmission electron microscopy (TEM
and HRTEM) observations were carried out with a JEOL JEM-
ARM200F Cold FEG (cold field emission gun) equipped with
a probe corrector and coupled to an EDX spectrometer and
1
NaBH4 (0.1 M) aqueous solution was slowly added to the
metal precursor/PVP mixture at room temperature (r.t.) and
at a metal-to-NaBH ratio of 1 : 5. The colour of the solution
4
immediately turned dark. The stirring was maintained for 1
h. For the core–shell systems, namely Pd@Pt NPs and Pt@Pd
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an electron energy loss spectrometer (EELS). The size
distribution was acquired by measuring a minimum of 300
objects and was given as mean standard deviation according
to a Gaussian fit of the corresponding size distribution.
CO adsorption-in situ diffuse reflectance infrared Fourier
transform spectroscopy (DRIFTS) studies were performed
using a Thermo Nicolet iS50-FT-IR spectrometer equipped
with a gas-dosing system and a ZnSe window. The IR spectra
−
1
were recorded at atmospheric pressure with 4 cm
in
absorbance mode and a resolution of 64 scans. The colloidal
suspensions (containing ca. 5 mg of NPs) were dried in a
freeze dryer for 48 h before their transfer into an in situ FT-IR
1
4
Fig. 1 Representative H-NMR spectrum resulting from CH oxidation
experiments with identification of the formed products.
−
1
chamber. A nitrogen (x) flux with a flow rate of 50 mL min
was applied to clean the catalyst surface before
measurement. Then 5 vol% CO/N gas was introduced into
2
the FT-IR chamber for 30 min at r.t. in order to reach a
saturated coverage. The CO adsorption-IR spectra were
obtained after the signal for physisorbed CO species
disappeared.
The chemical state and the surface composition of the
catalysts were analyzed by X-ray photoelectron spectroscopy
peaks corresponding to CH OH (δ = 3.2), CH OOH (δ = 3.7),
3 3
HCHO (δ = 4.8), HCOOH (δ = 8.1), and dissolved CH (δ = 0.0)
4
were clearly identified. The results of catalytic data are
summarized in Table 1. The two metal chloride precursors,
their mixture (Table 1, entries 10–12) and PVP alone (Table
S1†) displayed no catalytic activity for CH
4
oxidation,
(
XPS) on an XSAM 800 spectrometer (Kratos Co., Ltd.)
suggesting that the metal NPs were the active species. No
carbon-containing products were found in the liquid phase
utilizing Mg Kα radiation operating at a power of 150 W. The
colloidal samples were previously dried in a freeze dryer for
or gas phase when CH
4
was not charged into the reactor
4
8 h. XPS analysis was carried out at 13 kV and 20 mA with a
(Table S1†), ruling out the possibility of PVP conversion into
sweep time of 60 s. Pass energies of 160 eV and 40 eV were
employed to measure the wide scan and high resolution
spectra with a step size of 1.0 eV and 0.1 eV, respectively. The
signal was corrected using the C1s peak level at 284.6 eV. The
XPS spectra were fitted using a Gaussian–Lorentzian function
with a Shirley background using Casa XPS software.
C
1
organic products.
As shown in Table 1, methanol was the only product
detected in the liquid phase using the monometallic Pt NP
catalyst (entry 1). A small amount of CO (6%) was also
generated. Despite the high selectivity (94%), the productivity
2
−
1
−1
was relatively low (3.0 mol molmetal
h ). With the
WAXS analysis. To investigate the average structure of the
NPs, wide-angle X-ray scattering (WAXS) studies were
performed. The powder was sealed in a Lindemann glass
capillary of 1 mm in diameter. Measurements were carried
monometallic Pd NP catalyst (entry 9), formaldehyde was
formed, leading to a reduced selectivity to methanol (80%).
On the other hand, the productivity of methanol increased to
5
9
−
1
−1
7.8 mol molmetal
h , suggesting excellent methane
out using
a
diffractometer dedicated to the study of
activation ability over Pd NPs. Concerning the series of
bimetallic Pt Pd NPs, interestingly, reducing the ratio of Pt
amorphous and nanocrystalline samples equipped with a Mo
radiation source (0.71069 nm) and a solid state detector in
order to achieve low background and effective rejection of
X-ray fluorescence radiation. Typical measurements covered a
range of 129° in 2 theta with a collection time of 64 h. To
access the radial distribution function (RDF), data were
reduced and then Fourier transformed using classic
procedures.
x
y
to Pd (entries 5–8) resulted in a decrease in the methanol
selectivity but an increase in the productivity. For instance,
the bimetallic Pt Pd NP catalyst exhibited a high selectivity
1
1
−
1
−1
(86%) and yield (9.4 mol molmetal h ), exhibiting superior
productivity towards C primary oxygenates to that obtained
1
over the monometallic Pt NPs and Pd NPs. To identify the
most suitable configuration of Pd and Pt in the PdPt
bimetallic NPs for methane oxidation, Pt@Pd and Pd@Pt
NPs were prepared following a seeded growth method and
further tested (entries 13 and 14). Pt@Pd NPs present better
3
. Results and discussion
Metal NPs were synthesized by a classical wet chemistry
1 1
productivity and worse selectivity than Pt Pd does, which is
method using NaBH as a reductant, PVP as a stabilizer and
ascribed to the higher activity but worse methanol selectivity
of exposed Pd. Pd@Pt NPs, on the other hand, provided a
4
water as
a
solvent. The resulting colloidal aqueous
−
1
suspensions were directly evaluated in methane oxidation
higher selectivity (92%) and productivity (13.4 mol molmetal
−
1
−1 −1
using H O as an oxidant at 3 MPa pressure and 50 °C.
h , 89.3 mol kg
h ) toward methanol than alloy PtPd
NPs. Note that Pd@Pt NPs are significantly better in
productivity than Pt enriched Pt Pd NPs which have similar
2
2
catalyst
Initially, the catalytic activity of monometallic Pd,
monometallic Pt, and bimetallic Pd Pt (x/y = 1/8, 1/4, 1/2, 1/
, 2/1, 4/1, 8/1) colloidal catalysts was investigated. A
x
y
x
y
1
surface structures, demonstrating the unique properties of
Pd@Pt prepared by consecutive reduction. The performance
representative NMR spectrum is shown in Fig. 1, where the
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Table 1 Catalytic performance of the NP samples and metal precursors tested for direct methane oxidationa
metal−
1
Concentration of product (mol mol
)
Primary oxygenate
Primary oxygenate
selectivity (%)
productivity (mol
−1
−1
Entry
Catalysts: Pt
x
Pd
y
3
CH OH
3
CH OOH
HCHO
HCOOH
CO
2
molmetal
h )
1
2
3
4
5
6
7
8
9
x/y = 10/0
x/y = 8/1
x/y = 4/1
x/y = 2/1
x/y = 1/1
x/y = 1/2
x/y = 1/4
x/y = 1/8
x/y = 0/10
1.5
2.0
2.7
3.2
3.5
3.7
2.8
3.0
2.4
0
0
0
0
0
1.2
2.6
3.7
3.6
1.5
0
0
0
0
0
0.4
0.9
4.4
1.6
0.6
0
0
0
0
0
0.1
0.1
0.2
0.3
0.4
0.4
0.4
0.5
0.4
0.3
0.5
0.4
0.5
0.3
94
95
93
91
86
76
45
72
80
0
3.0
4.0
5.4
6.4
9.4
12.6
13.0
13.2
7.8
0
0
0.7
3.1
0.4
0
0
0
0
1.0
0
10
11
12
13
14
K
H
2
PtCl
PdCl
PtCl
4
2
4
0
0
2.8
2.3
0
0
3.4
4.4
0
0
0.8
0.3
0
0
73
92
0
0
12.4
13.4
K
2
4
+ H
2
PdCl
4
Pt@Pd
Pd@Pt
a
4 2 2
Catalysis conditions: 1 μmol metal in 2 mL deionized water, 30 bar CH , 0.4 mL of 1 M H O , 50 °C for 0.5 h, 800 rpm.
obtained with the Pd@Pt catalyst is comparable to or even
diffractogram obtained corresponds well to the fcc structure,
just like Pd and Pt, however much better agreement with
pure Pd can be observed. The related crystallographic
parameter was estimated to be 0.3898 nm, intermediate
between values for pure Pd (0.3890 nm) and pure Pt (0.3923
nm). For bimetallic NPs, deviation to Vegard's law was
previously observed and related to core–shell organization
rather than alloying. In the present case, the minimal
deviation to the Pd parameter points to segregation with Pd
at the core and Pt at the shell. Regarding Pt, the structural
order remains ambiguous since as a rule the thin less
better than that of recently published systems like Au–Pd
5
2
54
colloids, AuPd/ZSM-5-R, PdxCu1–xO/C (ref. 60) and Rh/
TiO (ref. 43) (Table S3†). The gain factor of Pd@Pt was
determined by measuring the consumption of H (Table
S4†). The H O utilization efficiency of Pd@Pt is also
2
2 2
O
2
2
5
3
comparable to that of reported Au–Pd systems. The system
is scalable when the reaction volume is increased four times,
although the primary oxygenate productivity is lower
probably because of mass transfer limitation (Table S5†).
TEM analysis revealed the presence of isolated Pd@Pt NPs
together with aggregates of 10–50 nm in size (Fig. S1†). An
average size of 3.4 ± 2.2 nm was determined for the Pd@Pt
NPs. STEM-EDX analysis on the NP aggregates indicated close
mass contents of Pd and Pt (ca. 54 and 46%, respectively; Fig.
S2†). Few isolated pure Pt NPs (Fig. S3†) were also observed.
Given their mean size, the calculated turnover frequency
5
9
(
3
TOF) per surface metal site of Pd@Pt NPs was found to be
4.0 h (Fig. S4 and Table S2†). This result is much higher
−
1
than those of the Pd, Pt or PdPt samples, indicating a
different intrinsic activity.
Fig. 2 presents the STEM-EDX analysis of the Pd@Pt NPs.
The elemental mapping clearly evidenced a predominant
enrichment of Pd in the NP core while the surface layer is
more Pt-enriched, thus indicating NPs with a Pd-core–Pt-shell
structure. The Fourier transform of a STEM-HRTEM dark
field image of Pd@Pt particles showed only one diffraction
pattern attributable to Pt. This result indicates potential
epitaxial growth of Pt over Pd cores (Fig. S5†). The analysis
above suggests that the sample comprises both core–shell
Pd@Pt NPs and pure Pt NPs. Compared with the activity data
of Pt NPs (entry 1, Table 1), the Pd@Pt sample (entry 14)
showed significantly enhanced oxygenate productivity,
supporting the dominant role of the core–shell structure in
providing superior activity.
Fig.
2 Top: STEM-EDX elemental mapping of Pd@Pt NPs where
In order to complete the structural analysis of the Pd@Pt
NPs, WAXS analysis was applied. As shown in Fig. S6,† the
platinum appears in red and palladium in blue. Bottom: Radial
distribution function (RDF) of the Pd@Pt NP sample.
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ordered shell marginally contributes to the scattering and
WAXS alone cannot distinguish between a disordered shell
and a well ordered one in epitaxial growth over a Pd core. In
real space (Fig. 2), the lack of short C–C distances confirms
that PVP contribution to scattering can be neglected. On the
basis of coherence length, the average size for crystallites is
in the range of 3.6–3.9 nm.
XPS analysis was performed on the Pd@Pt nanocatalyst
0
61
(
Fig. S7†). 59% of surface Pt is Pt (Pt4f ∼ 70.8 eV) and 39%
0 62,63
of surface Pd is Pd (Pd3d ∼ 335.0 eV),
likely due to
oxidation of both elements after synthesis. CO adsorption
experiments were carried out on the Pd, Pt and Pd@Pt
catalysts using the DRIFTS technique (Fig. 3). The IR bands
−
1
observed at 1972 and 1913 cm for Pd–PVP NPs can be
assigned to the bridging CO adsorption on the step sites of
6
4–66
Pd NPs or on the Pd(100)/Pd(111) facets.
The
−
1
predominant 2130 cm IR peak corresponds to the linear
2
+
62,67,68
Pd –CO bond,
thus suggesting partial oxidation of
metallic Pd during the sample preparation, which is
consistent with the XPS results. For Pt NPs, only one CO
−
1
absorption band is visible at 2056 cm , which is ascribed to
6
9,70
Fig. 4 (a) Activity and selectivity obtained in methane oxidation at 25,
linearly adsorbed CO on unsaturated Pt sites.
A similar
5
(
0, and 70 °C with 400 μmol of H
b) catalytic performance using CH
reactants over Pd@Pt NPs.
2
O
2
and 30 bar CH
4
over Pd@Pt NPs;
(30 bar) as
phenomenon was observed with Pd@Pt NPs, thus confirming
that Pt is located at the NP surface. However, on Pd@Pt NPs
the CO absorption band appears at a lower wavenumber than
3
OH (3.2 μmol) and CH
4
−
1
−1
that observed on Pt NPs (2051 cm against 2056 cm ). This
shift towards lower frequencies is ascribed to an electron
transfer from Pd to Pt due to their electronegativity
difference.
50 °C evidenced considerably improved catalytic activity
(oxygenate products: 7.0 μmol) and selectivity (92.4%). From
the slope of the Arrhenius plot, the apparent activation
−
1
The Pd@Pt NP catalyst was further investigated. Fig. 4a
shows its catalytic performances obtained at 25, 50, and 70
energy is estimated to be 45 kJ mol , a value that is
comparable to those previously reported with highly active
3
3,52,53
°C, respectively. Pd@Pt NPs provided
a
much lower
catalysts.
To confirm the high selectivity towards C1
productivity (oxygenate product: 2.2 μmol) despite a high
methanol selectivity (89%) at 25 °C. Increasing the
temperature to 70 °C significantly enhanced the yield
primary oxygenate products observed with the Pd@Pt
colloidal catalyst, methanol was used as the reactant instead
of methane (Fig. 4b). Only a low conversion (ca. 10%) of
methanol into formaldehyde was observed under these
conditions, suggesting that Pd@Pt NPs are not active for the
oxidation of methanol, thus minimizing undesired over-
oxidation.
(
oxygenate products: 8.2 μmol) but slightly decreased the
methanol selectivity (87%) due to the favorable formation of
CO as a by-product. Interestingly, the reaction performed at
2
Kinetic analysis was performed by changing the H O
2
2
concentration and CH
reaction is first order with regard to both H
under the tested conditions ([H O ] = 100–400 μmol, P(CH )
4
pressure. As shown in Fig. 5a–c, the
2
O
2
and CH
4
2
2
4
=
10–30 bar). The rate-determining step, therefore, involves
both activation of H and CH . On the basis of H O
2
O
2
4
2 2
consumption, the initial rate was employed to calculate the
pseudo first-order rate constant at the applied temperature.
Fig. 5d presents the variation of catalytic performance of
CH4 oxidation as a function of H O concentration. The
2
2
Pd@Pt NP catalyst showed a volcano-shaped dependence of
the oxygenated products on the amount of H added. A
similar trend was also observed on the selectivity to
methanol. Increasing the H amount from 400 to 4000
2 2
O
2 2
O
μmol led to a remarkable, monotonous decrease in both
oxygenated products (from 7 to 1.2 μmol) and methanol
selectivity (from 92% to 42%). According to the literature,
Fig. 3 In situ DRIFTS spectra of CO chemisorption on Pd, Pt and
Pd@Pt NP samples at r.t.
1
6,49
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increase with the reaction time. In addition, CH OOH and
3
CH OH appeared in the product stream from the beginning,
3
2
while HCHO and CO were only observed after 30 min. The
total yield of products firstly showed an upsurge and then,
decreased from 80 min onwards. This is in agreement with
the consumption rate of H
2 2
O in the reaction medium (Table
S6†). Extra H O was added into the reaction mixture after 80
2
2
min. A similar oxidation rate was attained and thus more
oxygenate products were formed. Besides, no obvious
precipitation for the Pd@Pt NP catalyst was observed at the
end of the reaction.
These results indicate that CH
4
was first converted into
CH OOH, which may be formed via the reaction between
3
3
3,53
˙
CH3 and ˙OOH, or between CH OO˙ and ˙H.
It was
3
previously well established that CH
3
OOH can be easily
1
6,41,53
deoxidized to CH OH.
Further oxidation of CH OH can
3
3
Fig. 5 Dependence of the amount of formed CH
3
OOH on (a) CH
4
48,49
lead to HCHO, HCOOH, and CO2.
Here, when CH OH
3
pressure and (b) concentration of H
for the methane oxidation reaction; (d) effects of H
on the productivity and selectivity over Pd@Pt NPs at 50 °C for 0.5 h.
2
O
2
added; (c) the Arrhenius plot
4
was directly used as the reactant instead of CH (Fig. 4b),
2 2
O
concentration
formation of a small amount of HCHO and CO was
2
1
confirmed by H NMR and GC-TCD analysis, respectively.
3
Combined, CH OOH is a key intermediate in the reaction,
which is formed via CH oxidation and is then converted into
4
3
5,41,48,49,53
H
2
O
2
undergoes splitting supplying ˙OH radicals, and the
activation of CH occurred via a radical mechanism. It was
reported that excess H O leads to termination of the
CH
3
OH, HCHO, HCOOH, and CO
three step reaction mechanism is proposed in
Scheme 1. In step 1, the decomposition of H O catalyzed by
2
.
4
A
2
2
2 2
5
3
reaction and thus reduces the product formation. Note that
the Pd@Pt NP catalyst precipitated to some extent in the
presence of a large amount of H O (4000 μmol) (Fig. S8†).
2 2
Pd@Pt produces H O and O as well as three radicals ˙H,
˙OOH, and ˙OH. Compared with ˙H and ˙OOH, the ˙OH
radical species is known to be highly effective for methane
2
2
Decreasing the H
2
O
2
concentration from 400 to 100 μmol led
activation via subtraction of a H atom to produce ˙CH
3
1
6,22,36
to a decline in both C1 liquid products (from 7 to 2.4 μmol)
and methanol selectivity (from 92% to 84%). This may be
due to the limited amount of available radicals in the
reaction mixture. There is thus an optimum
concentration to achieve the highest methanol selectivity and
yield.
radicals (step (2)).
well-known for its ability to generate ˙OH radicals,
An Fe-based Fenton type catalyst,
7
1,72
was
also investigated (Table S7†). A significantly lower amount of
products was detected over this Fe-based Fenton type catalyst
compared with that over Pd@Pt NPs, demonstrating that the
Pd@Pt nanocatalyst also played an active role in methane
activation in step (2). Considering the first order kinetic
behavior, we propose that both H O and methane are
2 2
H O
The reaction pathway was studied by analyzing the
evolution of the products as a function of time (up to 170
2
2
min, Fig. 6). The amount of CH
3
OOH increased progressively
adsorbed on the surface of NPs with comparable coverage.
dissociates to provide ˙OH radicals at the surface of
Pd@Pt NPs, which then activate adsorbed CH4 by forming
CH radicals. In the solution, ˙OH is able to oxidize H to
form more stable ˙OOH species, so that in the liquid phase
in the first 80 min and decreased afterwards. However, the
quantity of CH OH, HCHO, HCOOH, and CO continued to
2 2
H O
3
2
˙
3
2 2
O
7
3
Fig. 6 Catalytic performance of CH
4
oxidation over Pd@Pt NPs with
Scheme
1
Schematic representation of the proposed radical
various reaction times and reusability investigation.
mechanism.
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OOH would be dominant. As such, selective oxidation of
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