Low temperature selective oxidation of methane using gold-palladium colloids

Article abstract of DOI:10.1016/j.cattod.2018.12.017

Methane upgrading into energy-dense liquid derivatives (such as methanol or mid-range hydrocarbons) is a highly desirable process to increase its utilisation. The selective oxidation of methane using hydrogen peroxide has been investigated using unsupported gold-palladium nanoparticles prepared using colloidal methods. The effect of the reaction conditions and the catalyst parameters have been systematically investigated. Poly(vinyl)pyrrolidone (PVP) stabilised Au-Pd colloids produce methyl hydroperoxide as the primary reaction product, which is subsequently converted to methanol with high oxygenate selectivity. The stability and re-use characteristics of the colloidal catalyst have also been assessed for methane oxidation with hydrogen peroxide.

Full text of DOI:10.1016/j.cattod.2018.12.017

CatalysisTodayxxx(xxxx)xxx–xxx
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Low temperature selective oxidation of methane using gold-palladium
colloids
Rebecca McVickera, Nishtha Agarwala, Simon J. Freakleya,b, Qian Hea, Sultan Althahbanc,
Stuart H. Taylora, Christopher. J. Kielya,c, Graham J. Hutchingsa,⁎
a Cardiff Catalysis Institute, School of Chemistry, Cardiff University, Main Building, Park Place, Cardiff, CF10 3AT, UK
b Department of Chemistry, University of Bath, 1 South, Claverton Down, Bath, BA2 7AY, UK
c Department of Materials Science and Engineering, Lehigh University, 5 East Packer Avenue, Bethlehem, PA, 18015, USA
A R T I C L E I N F O
A B S T R A C T
Keywords:
Methane
Oxidation
Gold palladium
Unsupported nanoparticles
Methane upgrading into energy-dense liquid derivatives (such as methanol or mid-range hydrocarbons) is a
highly desirable process to increase its utilisation. The selective oxidation of methane using hydrogen peroxide
has been investigated using unsupported gold-palladium nanoparticles prepared using colloidal methods. The
effect of the reaction conditions and the catalyst parameters have been systematically investigated. Poly(vinyl)
pyrrolidone (PVP) stabilised Au-Pd colloids produce methyl hydroperoxide as the primary reaction product,
which is subsequently converted to methanol with high oxygenate selectivity. The stability and re-use char-
acteristics of the colloidal catalyst have also been assessed for methane oxidation with hydrogen peroxide.
1. Introduction
oleum and tri-fluoroacetic acid [10]. These oxidants also helped in in-
creasing methanol selectivity at high methane conversion by protecting
Natural gas can be considered as a versatile fuel. It currently sup-
plies 22% of the worldwide energy demand and its use is expected to
grow faster than both oil and coal [1]. Methane, our most abundant
hydrocarbon, is the primary component of natural gas and is also
generated as a by-product of oil refining and chemical processing. Since
methane has the potential to be a carbon source for the synthesis of
commodity chemicals, its transformation to other products is of utmost
importance and direct activation of methane has been identified as one
of the grand challenges for chemists [2]. Current industrial utilisation
of methane into value-added products is performed indirectly by pro-
ducing synthesis gas that can be transformed to methanol or higher
hydrocarbons via Fischer-Tropsch synthesis [3].
A single-step direct activation of methane, preferably under low
reaction temperature conditions, could significantly reduce processing
costs thus making liquid phase approaches viable. Several high tem-
perature gas phase routes have also been investigated over the past but
have been limited due to low methanol selectivity and total oxidation to
CO2 [4–7]. In the liquid phase, homogeneous catalysts based on Pt, Pd,
Os have been extensively studied and provide valuable information
about the mechanism of methane oxidation and active site generation
[8–10]. However, these catalysts require harsh reaction conditions in-
volving temperatures of at least 180 °C with strong oxidants such as
the product as methyl bisulphate and methyl trifluoroacetate respec-
tively but in turn required hydrolysis of the product to obtain methanol
meaning that the catalytic cycle was not closed [10,11]. Monometallic
Au and Pd catalysts have also been investigated for low temperature
methane oxidation [10,12]. However, without the presence of strong
oxidants the catalysts were not effective in activating methane to pro-
duce oxygenated products. Mild oxidation in aqueous conditions with
homogeneous chloroauric acid led to precipitation of Au° and deacti-
vation of the catalyst [9]. Methane oxidation to methanol has also been
attempted using Fe-based complexes utilising more benign oxidants
such as hydrogen peroxide, which generates water as the by-product.
Hammond et al. have previously reported use of Fe-ZSM-5 and Cu
modified Fe-ZSM-5 for methane activation to methylhydroperoxide,
methanol and formic acid in aqueous media at 50 °C [13,14].
Au-Pd catalysts prepared by sol immobilisation have been shown to
be effective for primary C–H bond activation in toluene using O2 at
elevated temperatures (∼160 °C) [15]. Such Au-Pd nanoparticles have
also been shown to be catalytically active for the oxidation of alcohols
and the direct synthesis of H2O2 [16,17].The aforementioned reactions
are considered to be linked by the presence of a hydroperoxy inter-
mediate which can be formed by oxidants like H2O2 and tert-butyl
hydroperoxide (TBHP) [8,13]. Since the formation of the hydroperoxy
⁎ Corresponding author.
E-mail address: hutch@cardiff.ac.uk (G.J. Hutchings).
https://doi.org/10.1016/j.cattod.2018.12.017
Received 15 August 2018; Received in revised form 30 November 2018; Accepted 7 December 2018
0920-5861/©2018ElsevierB.V.Allrightsreserved.
Pleasecitethisarticleas:McVicker,R.,CatalysisToday,https://doi.org/10.1016/j.cattod.2018.12.017

R. McVicker et al.
CatalysisTodayxxx(xxxx)xxx–xxx
intermediates was also found in methane oxidation, Au-Pd alloy par-
ticles have also been tested for the methane oxidation reaction. TiO2
supported bimetallic Au-Pd nanoparticles were shown to be active for
methane oxidation at 50 °C using H2O2 as the oxidant [18,19]. The
synergistic effect of gold and palladium was found to not only increase
the catalytic activity but also to improve selectivity and the efficiency of
the reaction. Since supported bimetallic Au-Pd catalysts have been ex-
tensively studied for the direct synthesis of H2O2 [17,20], they were
also employed for methane oxidation using H2O2 produced in an in-situ
fashion by using a mixture of H2, O2, CH4 and N2. A similar productivity
but higher methanol selectivity was observed when using the in-situ
generated H2O2.
Recently, Agarwal et al. demonstrated the use of unsupported Au-Pd
nanoparticles for methane oxidation to methanol [21]. Higher oxyge-
nate productivity levels of 29.4 mol kgcat h−1 were obtained when
using unsupported poly(vinyl)pyrrolidone (PVP) polymer stabilised Au-
Pd colloids in the presence of H2O2, as compared to 0.03 mol kgcat h-1
for a conventional sol-immobilised Au-Pd/TiO2 solid catalyst. Mole-
cular O2 was also used as an oxidant, which resulted in an increase in
oxygenate productivity to 53.6 mol kgcat h-1 and incorporation of O2
was demonstrated in the liquid oxygenate products [21]. In the study
reported here, systematic investigations are carried out to evaluate
methane oxidation reactions using the unsupported Au-Pd colloid in the
presence of H2O2 but in the absence of O2. Au-Pd nanoparticles were
synthesised via a standard colloid preparation method using PVP as the
stabiliser and NaBH4 as the reductant [22]. The effect of reaction
parameters (such as temperature, pressure, and stirring speed) on the
catalytic activity has been evaluated. Investigations on the effect of
varying the catalyst preparation parameters (such as metal concentra-
tion, chloride concentration and PVP molecular weights) have also been
performed. Extended reactions were also carried out to study the evo-
lution of the total product distribution and product selectivity in order
to determine the reaction pathway.
2.3. Product analysis
The amount of H2O2 remaining at the end of each reaction was
quantified by titrating a portion of the reaction mixture against acid-
ified Ce(SO4) solution of known concentration using ferroin as in-
dicator. The only gas phase product of the reactions was CO2, which
was quantified by GC using a Varian 450-GC equipped with FID & TCD
detectors, a methaniser and CP-SiL5CB column (50 m, 0.33 mm dia-
meter, He carrier gas). 1H-NMR studies were carried out to quantify the
amounts of liquid phase products (see SI Table S1) using a Bruker
500 MHz NMR equipped with a solvent suppression system to minimise
the signal arising from the aqueous solvent (see SI Fig. S1). An internal
standard containing 1% TMS in CDCl3 (99.9% D) was placed in a sealed
tube and used to quantify the amount of product after calibration
against known standards.
2.4. Catalyst characterisation
2.4.1. UV–vis spectroscopy
UV–vis spectra were recorded on a Jasco V-570 UV/VIS/NIR spec-
trophotometer over a wavelength range of 200–700 nm using a data
interval of 1 nm. Samples were placed in quartz cuvettes for analysis
which was carried out at room temperature.
2.4.2. X-ray photoelectron spectroscopy
A Kratos Axis Ultra DLS photoelectron spectrometer was used to
collect XPS spectra using a monochromatic Al Kα X-ray radiation source
operating at a power of 120 W. Colloidal samples were pipetted onto
clean glass slides and the residual solvent removed by the pumping
system of the fast entry airlock of the spectrometer. Data was collected
with pass energies of 160 eV for survey spectra, and 40 eV for the high-
resolution scans with step sizes of 1 and 0.1 eV respectively. All samples
were analysed using a slot aperture and in hybrid spectroscopy mode,
which utilizes both magnetic and electrostatic lenses; in this mode, the
analysis area is a 700 μm × 300 μm rectangle. For all samples, the
Kratos immersion lens system was used for charge neutralization and
the spectra subsequently referenced to the C(1s) line taken to be
285 eV. The sample also exhibited a peak at 99.4 eV which corre-
sponded to the elemental Si(2p) peak arising from the SiO2 substrate.
2. Experimental
2.1. Catalyst preparation
An aqueous solution of HAuCl4 precursor (Sigma Aldrich) and an
acidic solution of PdCl2 (Sigma Aldrich) precursor (in 0.5 M HCl) were
prepared of the desired concentration in de-ionised water. Poly(vinyl)
pyrrolidone (PVP, average molecular weight 10,000 Da, Sigma Aldrich)
was added as a stabilizer to give the required metal-to-PVP ratio (ty-
pically 1:1.2 by weight). After 2–3 min of stirring, a freshly prepared
0.1 M NaBH4 (Sigma Aldrich) solution was added such that the molar
ratio of NaBH4-to-metal was 5:1. This produced a dark brown colloid
that was the left stirring for 30 min to ensure all the metal precursor
salts were reduced to metallic nanoparticles. At this stage, colloid
preparation was complete, and the material was stored in glass media
bottles prior to use.
2.4.3. Electron microscopy
Materials for TEM analysis were prepared by dispersing the colloid
onto a continuous carbon film supported on a 300-mesh copper TEM
grid and allowing the solvent to evaporate Specimens were examined
using the bright-field imaging mode in a JEOL 2000FX transmission
electron microscope operating at 200 kV equipped with an Oxford
Instruments X-ray energy dispersive (XEDS) spectrometer system.
3. Results and discussion
3.1. Catalyst characterisation
The colloidal catalyst was characterised using a variety of techni-
ques. UV–vis spectroscopy (Fig. S2) was used to confirm the colloid
formed was a Au-Pd alloy and not a physical mixture of Au and Pd
nanoparticles. The broad plasmon band between 500–550 nm that is
normally associated with Au nanoparticles was found to be absent in
the bimetallic colloids indicating the formation of an alloy rather than
gold and palladium nanoparticles [15,22]. The colloidal catalyst was
also characterised by transmission electron microscopy to determine
the particle size distribution of the nanoparticles (Fig. 1A). Un-
supported nanoparticles were found to have a narrow particle size
distribution with a mean diameter of 2.8 nm. The colloidal Au-Pd pre-
pared with PVP has been observed to primarily consist of multiply
twinned icosahedral structures [21,23].
2.2. Catalyst testing
Methane oxidation was carried out in a 50 mL Teflon-lined stainless
steel Parr autoclave reactor [13,21]. The reactor was charged with
10 mL of colloid and H2O2 (Sigma Aldrich, 50% wt in water). The
charged autoclave was then sealed and purged three times with me-
thane (99.999%, Air Products). It was then pressurized with methane
(to 30 bar) and heated to 50 °C. Once the reactor had reached the set
temperature, stirring at 1500 rpm was commenced. After 30 min
heating and stirring the reaction was stopped and the reactor vessel was
cooled to below 10 °C using an ice bath. The gas containing the reaction
products was removed for analysis in a gas-sampling bag.
XPS was used to analyse the colloidal samples and is shown in
2

R. McVicker et al.
CatalysisTodayxxx(xxxx)xxx–xxx
Fig. 1. a) Representative BF-TEM image and particle size distribution and b) XPS analysis of the Au-Pd-PVP colloid. Plot (a) corresponds to Au(4f) and (b) corre-
sponds to Pd(3d). Plot (b) also shows presence of Au(4d) signals.
Fig. 1B. XPS analysis showed both Au and Pd to be metallic in nature
with the presence of minor PdCl2 components. The bimetallic Au-Pd
colloid showed interactions between the Au and Pd since a negative
shift in binding energies was observed when compared to the mono-
metallic colloids (Fig. S3). This shift is attributed to a charge transfer
from Pd to Au and is known to increase the Au s-state occupancy in-
dicating alloy formation [24,25].
3.2. Catalytic activity
Previously, methane oxidation was carried out with aqueous Au-Pd
colloids in the presence of molecular O2 and a small amount of H2O2 at
50 °C [21]. It was shown that molecular oxygen was acting as the oxi-
dant, but reactions performed in the absence of H2O2 showed no gen-
eration of oxygenated products, confirming that H2O2 was necessary to
initiate the reaction. Hence, reactions have been carried out with excess
H2O2 (5 mmol) as the oxidant for different times to investigate temporal
selectivity towards reaction products between 5–240 min. The evolu-
tion of the overall product distribution as a function of time, along with
oxygenate selectivity towards different products, is shown in Fig. 2. As
the reaction proceeded, the total amount of reaction products generated
also increased over time. A high selectivity towards methyl hydroper-
oxide was observed for short reaction times, but this gradually
Fig. 2. Time-on-line analysis of product distribution for methane oxidation
using Au-Pd-PVP colloids with H2O2. Test Conditions - 5000 μmol H2O2, 50 °C,
total volume 10 ml, pCH4 30 bar, 1500 rpm, 7.57 μmol metal per reaction.
Colloid; PVP : metal = 1.2:1, Au : Pd = 1:1 M, [metal] = 7.57 × 10−4 M.
decreases with time-on-line. The reaction was performed for 10 min
with 1 mmol and this also showed a high selectivity to methyl
3

R. McVicker et al.
CatalysisTodayxxx(xxxx)xxx–xxx
Scheme 1. Proposed reaction scheme based on temporal selectivity.
hydroperoxide (Table S2 Entry 1). Methanol was also observed in this
case which indicates that the primary product of the reaction is methyl
hydroperoxide, which subsequently is converted to methanol over time.
Methylhydroperoxide and methanol are subsequently oxidised to form
formic acid, demonstrating that the reaction proceeds through the same
series of intermediates which were observed in previous studies with
Au-Pd colloids when using both H2O2 and O2 [21]. Scheme 1 shows the
proposed reaction pathway and suggests the final over-oxidation pro-
duct of all three liquid oxygenates to be CO2, the amount of which was
observed to increase over time. Along with this, a decrease in oxygenate
selectivity from 94% after 30 min to 61% after 240 min was observed
due to the overoxidation. After 4 h, most of H2O2 was consumed and
methyl hydroperoxide was found to have been transformed into me-
thanol and formic acid. This is in line with the observations reported
previously for supported AuPd catalysts [26,27]. It is also interesting to
note that the same intermediates have also been reported for CH4
oxidation reactions with H2O2 using Fe and Cu-based zeolite catalyst
systems which could suggest similar reaction pathways are in operation
[13,18,28,29]. In a previous study by Chadwick and co-workers, for-
maldehyde was also observed as a product [29]. As shown in Fig. S1,
formaldehyde was not observed in our reactions; however since the
signal for formaldehyde (methane diol, s, δ = 4.8 ppm) could be ob-
scured by the broad water signal, its presence or absence of for-
maldehyde could not be confirmed in our study.
Fig. 3. Methane oxidation reactions carried out at various temperatures using
Au-Pd-PVP colloids and H2O2. Test Conditions - 1000 μmol H2O2, total volume
10 ml, pCH4 30 bar, 0.5 h, 1500 rpm, 7.57 μmol metal per reaction. Colloid; PVP
: metal = 1.2:1, Au : Pd = 1:1 M, [metal] = 7.57 × 10−4 M.
than that for reactions carried out with unsupported Au-Pd-PVP col-
loids in presence of molecular O2 [21]. No residual H2O2 was detected
after 30 min at temperatures above 70 °C which might be due to an
increase in thermal degradation [30].
Further investigations were carried out by varying other reaction
parameters. The effect of varying methane pressure and thus the
amount of the dissolved reactant was studied at 50 °C with Au-Pd-PVP
colloids in the presence of 1000 μmol H2O2. The solubility of methane
was found to be 0.74 g(CH4)/kg(H2O) at 50 bar, which is higher than
0.3 g(CH4)/kg(H2O) at 30 bar [31,32]. Thus, a series of CH4 oxidation
reactions were carried out where different pressures of CH4 between 10
and 50 bar were employed. The results of these reactions are shown in
Fig. 4. It was found that increasing CH4 pressure resulted in a very
Two consecutive reactions were carried out with the Au-Pd colloid
over 10 min. intervals to evaluate the stability of the colloids over short
reaction times (SI, Table S2). After an initial 10 min of reaction, a
sample was taken for analysis and the H2O2 was replenished to run a
second reaction immediately afterwards. The results shown in SI, Table
S2 demonstrate that double the total amount of products are produced
after two sequential 10 min reactions, then after the first 10 min reac-
tion alone indicating that the TOF and productivity of the colloids is
maintained over multiple reaction cycles.
Time-on-line analysis also included a reaction which was halted
after 0 min (i.e. when the reactor had reached 50 °C) to determine if
there was any product generation at lower temperatures during the
reactor heat-up stage. As shown in Fig. 2, 15 μmol of products were
formed before a temperature 50 °C was even reached. This lack of in-
duction period was also previously observed with PVP stabilized Au-Pd
colloids where products were obtained at room temperature [21].
Reactions were also carried out with H2O2 as the oxidant at various
temperatures ranging from 5 °C to 90 °C (Fig. 3). A total of 4.5 μmol of
products were obtained at 5 °C with high oxygenate selectivity of 87%
and only 0.6 μmol of CO2 produced. As the temperature was increased,
the conversion of methane increased. However, the increase in the re-
action temperature also caused a substantial decrease in the selectivity
to the desired liquid oxygenated products from 87% to 40% at 90 °C.
This is caused by the increased rate of over-oxidation to CO2, which has
been previously observed for CH4 oxidation reactions at higher tem-
perature [27]. The activation energy for methane oxidation with H2O2
was calculated as 18 kJ/mol. This activation energy value was lower
Fig. 4. Methane oxidation reactions carried out using colloidal Au-Pd nano-
particles and H2O2 under various pressures of CH4. Test Conditions - 1000 μmol
H2O2, 50 °C, total volume 10 ml, 0.5 h, 1500 rpm, 7.57 μmol metal per reaction.
Colloid; PVP : metal = 1.2:1, Au : Pd = 1:1 M, [metal] = 7.57 × 10−4 M.
4

R. McVicker et al.
CatalysisTodayxxx(xxxx)xxx–xxx
was employed. No significant change in the amount of products gen-
erated and no clear trend in oxygenate selectivity and activity was
observed on varying the stirring speed. These results imply that the
reaction is not mass transport limited. However, a significant increase
in the H2O2 decomposition rate was observed upon increasing the
stirring speed. Nearly 40% of the initial H2O2 remained after 30 min of
reaction time when the stirring speed was set to 250 rpm, whereas only
10% remained at the end of the reaction when using a stirring speed of
1500 rpm. At stirring speeds of 1000 rpm, the highest total amount of
products were observed. These optimised parameters were combined to
achieve even higher total product amounts of 42 μmol and a higher TOF
of 11.2 h−1 (as compared to 4.2 h−1 when the reaction was performed
under standard conditions) (SI, Table S3). A reaction was performed at
60 °C with 40 bar CH4 pressure and a high selectivity ( = 81%) to
oxygenated products was maintained. However, the oxidant usage was
found to be less efficient under the optimised conditions in relation to
the ratio of H2O2 consumed-to-products generated, increasing from 36
to 110 when compared to a reaction carried out under standard con-
ditions. The optimised reaction was carried out with 5000 μmol of H2O2
which resulted in lower efficiency of H2O2 usage. These observations
are in line with our previous study on the effect of H2O2 concentration
on methane oxidation activity [21].
An investigation into effect of metal concentration was carried out
by conducting reactions by diluting the colloid sample with water.
Samples of a typical Au-Pd colloid ([metal] = 7.57 × 10−4 M) were
diluted to 3.79 × 10−4 M and 1.89 × 10−4 M and were used to carry
out CH4 oxidation reactions. It was considered preferable to dilute a
standard colloid rather than prepare new colloids with different metal
concentrations in order to avoid any possible effects that altering the
metal concentration during nanoparticle preparation might have on
particle size or composition. The results are presented in Fig. 7 and
show that changing the metal concentration in the reactor has minimal
effect on both H2O2 consumption and on the selectivity towards oxy-
genated products. The total amount of products produced, however,
appears to increase linearly with metal concentration, which might be
due to removal of mass transfer limitations due to removal of support,
which was also observed in CH4oxidation in the presence of O2 [21].
The reaction with a metal concentration of 1.89 × 10−4 M produced
18 μmol of products, which increased to 23 μmol when the metal con-
centration was raised to 7.57 × 10-4 M. Since the colloidal catalysts
were prepared using metal chloride precursors, any change in metal
concentration could also lead to a change in the Cl concentration, it was
necessary to investigate the effect of Cl concentration on the activity of
the colloids without modifying the metal concentration. With this aim
in mind, reactions were carried out with a standard Au-Pd colloid
having a metal concentration of 7.57 × 10−4 M where various amounts
Fig. 5. Methane oxidation reactions carried out using colloidal Au-Pd nano-
particles and various initial concentrations of H2O2. Test Conditions - 50 °C, total
volume 10 ml, 30 bar, 0.5 h, 1500 rpm, 7.57 μmol metal per reaction. Colloid;
PVP : metal = 1.2:1, Au : Pd = 1:1 M, [metal] = 7.57 × 10−4 M.
significant increase from 9.8 to 35 μmol in the total amount of products
formed. Oxygenate selectivity was also improved by roughly 11% on
increasing the CH4 pressure from 10 to 40 bar. At 5 bar total pressure, a
high CO2 selectivity was observed which corresponded to an oxygenate
selectivity of only 30%. Conversely, at higher CH4 pressures, a high
selectivity to primary oxygenates was observed with oxygenate se-
lectivity reaching > 90% at a pressure of 30 bar. The initial H2O2
concentration employed was also varied systematically in order to
probe its influence on methane oxidation activity. Compared to pre-
vious studies, a similar activity trend was observed on varying the H2O2
concentration (Fig. 5) [21]. As the initial H2O2 concentration is in-
creased, oxygenate selectivity remains high (> 85%) while the per-
centage of H2O2 remaining decreases from 35 to 18% across the con-
centration range investigated. In agreement with previous studies, it
was also found that higher initial concentrations of H2O2 resulted in an
increase in the total amount of products formed [13,21]. Increasing the
initial H2O2 concentration from 500 to 5000 μmol yielded an increase
of roughly 10 μmol in the total amount of products generated. Further
increasing the H2O2 concentration beyond 5000 μmol did not result in
any more products being generated. It was also found that increasing
the initial H2O2 concentration reduced the efficiency of H2O2 usage.
The ratio of H2O2 consumed-to-products generated increased from 24
to 383 when the initial H2O2 concentration was increased from
500 μmol to 10,000 μmol.
Fig. 6 shows the results for CH4 oxidation reactions carried out with
colloidal Au-Pd nanoparticles, where a range of different stirring speeds
Fig. 6. Methane oxidation reactions carried out using colloidal Au-Pd nano-
particles and H2O2 where the system was subjected to various stirring speeds.
Test Conditions - 1000 μmol H2O2, 50 °C, total volume 10 ml, 30 bar, 0.5 h,
7.57 μmol metal per reaction. Colloid; PVP : metal = 1.2:1, Au : Pd = 1:1 M,
[metal] = 7.57 × 10−4 M.
Fig. 7. Methane oxidation reactions carried out with H2O2 and colloidal Au-Pd
nanoparticles with various metal concentrations. Test Conditions - 1000 μmol
H2O2, 50 °C, total volume 10 ml, pCH4 30 bar, 0.5 h, 7.57 μmol metal per re-
action. Colloid; PVP : metal = 1.2:1, Au : Pd = 1:1 M.
5

R. McVicker et al.
CatalysisTodayxxx(xxxx)xxx–xxx
was unaffected by changing in PVP molecular weight from 10 to
360 kDa, although the 3.5 kDa PVP colloid consumed 20% less oxidant
than the other colloid catalysts tested. Overall, these results indicate
that varying the PVP molecular weight in the stabilising ligand only had
a modest effect on the activity of the Au-Pd colloid catalyst.
4. Conclusions
Methane oxidation reactions have been carried out in the presence
of H2O2 and colloidal Au-Pd nanoparticles prepared using PVP as a
stabilising ligand. These reactions were found to proceed via the same
reaction pathway as for CH4 oxidation reactions catalysed by supported
Au-Pd nanoparticles. In both systems, CH3OOH is produced as the
primary product which undergoes subsequent transformation to form
CH3OH and HCOOH before complete oxidation to CO2. Reactions were
also carried out at different temperatures, demonstrating the efficacy of
unsupported Au-Pd colloids at room temperature. An activation energy
of 18 kJ/mol was calculated from analysis of the reaction kinetics.
Further investigations were performed in order to optimise individual
reaction parameters such as CH4 pressure, oxidant concentration and
stirring speed. The CH4 oxidation reaction when carried out by com-
bining the results of our optimisation study, resulted in a high pro-
ductivity value of 74.4 mol kg−cat1 h−1. This demonstrates the high in-
trinsic activity of unsupported Au-Pd nanoparticles for this particular
reaction. This productivity value also compares very favourably to
methane monooxygenase (MMO) and Fe-Cu/ZSM-5 catalysts which
have productivities of 5.1 mol kg−cat1 h-1 and 16.5 mol kgc−at1 h-1 respec-
tively [13,14,33]. However, it should be noted that MMO utilises
oxygen as the terminal oxidant and this remains the target for catalyst
design as the current study uses H2O2 as an initiator to enable O2 to be
utilised [21,34,35].
Fig. 8. CH4 oxidation reactions carried out with H2O2 and colloidal Au-Pd
nanoparticles with various amounts of NaCl added to the reactor. Test
Conditions - 1000 μmol H2O2, 50 °C, total volume 10 ml, pCH4 30 bar, 0.5 h,
7.57 μmol metal per reaction. Colloid; PVP : metal = 1.2:1, Au : Pd = 1:1 M,
[metal] = 7.57 × 10−4 M.
Studies carried out to monitor the effects of parameters associated
with the preparation procedure found that increasing the metal con-
centration of the Au-Pd colloids employed resulted in an increase in the
amount of products generated during a reaction. Changes in Cl con-
centration was found to have no significant effect on the catalytic ac-
tivity. However, some increase in activity was observed on increasing
the molecular weight of polymer PVP used in the stabilising ligand, but
it was only a modest effect considering that the change in PVP mole-
cular weight investigated was over two orders of magnitude.
Overall, the colloidal Au-Pd nanoparticles, in the presence of H2O2
have been found to be effective catalysts for CH4 oxidation, and a better
mechanistic understanding has been attained through systematic in-
vestigation of various parameters associated of the system. It has been
shown that even though the Au-Pd nanoparticles display a high level of
intrinsic activity for this reaction, further exploitation of unsupported
nanoparticles as catalysts for industrial use will necessitate an in-
vestigation into the factors effecting colloid stability.
Fig. 9. Effect of PVP molecular weight on catalytic activity of colloidal Au-Pd
nanoparticles for CH4 oxidation with H2O2. Test Conditions - 1000 μmol H2O2,
50 °C, total volume 10 ml, pCH4 30 bar, 0.5 h, 7.57 μmol metal per reaction.
Colloid; PVP : metal = 1.2:1, Au : Pd = 1:1 M, [metal] = 7.57 × 10−4 M.
of NaCl were added to the reactor (Fig. 8). The H2O2 consumption and
selectivity to oxygenated products remained stable throughout the in-
vestigation. No clear trend was observed for the total amount of pro-
ducts generated as the sodium chloride concentration increased.
Overall it is clear that increasing the Cl concentration in this system
does not have a beneficial effect on the activity of the Au-Pd nano-
particles. From these results, it can be concluded that the increase in the
amount of products generated upon increasing the metal concentration
for this system was not the caused by the concurrent increase in Cl
concentration.
Acknowledgements
Another parameter that was studied was the effect of varying the
amount of stabilising polymer employed. The Au-Pd colloid used in
these reactions was prepared with PVP having a molecular weight
(number average) of 10 kDa. Three colloids were prepared using the sol
method employing the same metal concentration of 7.57 × 10−4 M,
but using various molecular weights of PVP. Comparative methane
oxidation experiments were then carried out using these colloids and
H2O2 as the oxidant as shown in Fig. 9. A slight increase in the amount
of products generated was observed as molecular weight of the PVP
used was increased. Increasing the PVP molecular weight from 3.5 to
40 kDa results in an increase of 10 μmol in the total products generated,
but further increasing the PVP molecular weight from 40 to 360 kDa
only created an extra 2 μmol of products. Again, only a moderate im-
provement from 85 to 98% was observed in oxygenate selectivity
considering the PVP molecular weight was increased by two orders of
magnitude from 3.5 kDa to 360 kDa. The measured H2O2 consumption
We acknowledge Cardiff University for financial support as part of
the MAXNET Energy Consortium. C.J.K. acknowledges funding from
the NSF Major Research Instrumentation program (grant MRI/DMR-
1040229). S.M.A. thanks the Saudi Arabian government for his Ph.D.
scholarship. All results are reported in the main text and supplementary
materials.
References
[1] International Energy Agency, Resources to Reserves 2013 - Oil, Gas and Coal
Technologies for the Energy Markets of the Future, (2013), https://doi.org/10.
1787/9789264090705-en.
[2] R.G. Bergman, Organometallic chemistry: C-H activation, Nature 446 (2007)
391–394, https://doi.org/10.1038/446391a.
[3] D. Wilhelm, D. Simbeck, A. Karp, R. Dickenson, Syngas production for gas-to-liquids
applications: technologies, issues and outlook, Fuel Process. Technol. 71 (2001)
6

R. McVicker et al.
CatalysisTodayxxx(xxxx)xxx–xxx
designing supported gold-palladium bimetallic catalysts for the direct synthesis of
hydrogen peroxide, Acc. Chem. Res. 47 (2013) 845–854, https://doi.org/10.1021/
ar400177c.
139–148, https://doi.org/10.1016/S0378-3820(01)00140-0.
[4] J.-P. Lange, K.P. de Jong, J. Ansorge, P.J.A. Tijm, Keys to methane conversion
technologies, Stud. Surf. Sci. Catal. 107 (1997) 81–86, https://doi.org/10.1016/
S0167-2991(97)80320-5.
[5] M. Ravi, M. Ranocchiari, J.A. van Bokhoven, The direct catalytic oxidation of
methane to methanol-a critical assessment, Angew. Chem. Int. Ed. 56 (2017)
16464–16483, https://doi.org/10.1002/anie.201702550.
[6] A.R. Kulkarni, Z.-J. Zhao, S. Siahrostami, J.K. Nørskov, F. Studt, Cation-exchanged
zeolites for the selective oxidation of methane to methanol, Catal. Sci. Technol. 8
(2018) 114–123, https://doi.org/10.1039/C7CY01229B.
[21] N. Agarwal, S.J. Freakley, R.U. McVicker, S.M. Althahban, N. Dimitratos, Q. He,
D.J. Morgan, R.L. Jenkins, D.J. Willock, S.H. Taylor, C.J. Kiely, G.J. Hutchings,
Aqueous Au-Pd colloids catalyze selective CH 4 oxidation to CH 3 OH with O 2 under
mild conditions, Science (80-.) 358 (2017) 223–227, https://doi.org/10.1126/
science.aan6515.
[22] J.A. Lopez-Sanchez, N. Dimitratos, P. Miedziak, E. Ntainjua, J.K. Edwards,
D. Morgan, A.F. Carley, R. Tiruvalam, C.J. Kiely, G.J. Hutchings, Au–Pd supported
nanocrystals prepared by a sol immobilisation technique as catalysts for selective
chemical synthesis, Phys. Chem. Chem. Phys. 10 (2008) 1921, https://doi.org/10.
1039/b719345a.
[23] J. Pritchard, M. Piccinini, R. Tiruvalam, Q. He, N. Dimitratos, J.A. Lopez-Sanchez,
D.J. Morgan, A.F. Carley, J.K. Edwards, C.J. Kiely, G.J. Hutchings, Effect of heat
treatment on Au–Pd catalysts synthesized by sol immobilisation for the direct
synthesis of hydrogen peroxide and benzyl alcoholoxidation, Catal. Sci. Technol. 3
(2013) 308–317, https://doi.org/10.1039/C2CY20234D.
[24] A. Venezia, V. La Parola, G. Deganello, B. Pawelec, J.L. Fierro, Synergetic effect of
gold in Au/Pd catalysts during hydrodesulfurization reactions of model compounds,
J. Catal. 215 (2003) 317–325, https://doi.org/10.1016/S0021-9517(03)00005-8.
[25] D. Wang, X. Cui, Q. Xiao, Y. Hu, Z. Wang, Y.M. Yiu, T.K. Sham, Electronic behaviour
of Au-Pt alloys and the 4f binding energy shift anomaly in Au bimetallics- X-ray
spectroscopy studies, AIP Adv. 8 (2018) 065210, , https://doi.org/10.1063/1.
5027251.
[26] M.H. Ab Rahim, M.M. Forde, R.L. Jenkins, C. Hammond, Q. He, N. Dimitratos,
J.A. Lopez-Sanchez, A.F. Carley, S.H. Taylor, D.J. Willock, D.M. Murphy, C.J. Kiely,
G.J. Hutchings, Oxidation of methane to methanol with hydrogen peroxide using
supported gold-palladium alloy nanoparticles, Angew. Chem. 125 (2013)
1318–1322, https://doi.org/10.1002/ange.201207717.
[27] M.H. Ab Rahim, M.M. Forde, C. Hammond, R.L. Jenkins, N. Dimitratos, J.A. Lopez-
Sanchez, A.F. Carley, S.H. Taylor, D.J. Willock, G.J. Hutchings, Systematic study of
the oxidation of methane using supported gold palladium nanoparticles under mild
aqueous conditions, Top. Catal. 56 (2013) 1843–1857, https://doi.org/10.1007/
s11244-013-0121-3.
[7] C. Hammond, S. Conrad, I. Hermans, Oxidative methane upgrading, ChemSusChem.
5 (2012) 1668–1686, https://doi.org/10.1002/cssc.201200299.
[8] R.A. Periana, D.J. Taube, S. Gamble, H. Taube, T. Satoh, H. Fujii, methanol deri-
vative platinum catalysts for the high-yield oxidation of methane to a methanol
derivative, Science (80-.) 280 (1998) 560–564, https://doi.org/10.1126/science.
280.5363.560.
[9] Q. Yuan, W. Deng, Q. Zhang, Y. Wang, Osmium-catalyzed selective oxidations of
methane and ethane with hydrogen peroxide in aqueous medium, Adv. Synth.
Catal. 349 (2007) 1199–1209, https://doi.org/10.1002/adsc.200600438.
[10] L.C. Kao, A.C. Hutson, A. Sen, Low-temperature, plladium(I.I.)-catalyzed, solution-
phase oxidation of methane to a methanol derivative, J. Am. Chem. Soc. 113 (1991)
700–701, https://doi.org/10.1021/ja00002a063.
[11] M. Ahlquist, R.J. Nielsen, R.A. Periana, W.A. Goddard III, Product protection, the
key to developing high performance methane selective oxidation catalysts, J. Am.
Chem. Soc. 131 (2009) 17110–17115, https://doi.org/10.1021/ja903930e.
[12] C.J. Jones, D. Taube, V.R. Ziatdinov, R.A. Periana, R.J. Nielsen, J. Oxgaard,
W.A. Goddard, Selective oxidation of methane to methanol catalyzed, with C-H
activation, by homogeneous, cationic gold, Angew. Chem. Int. Ed. 43 (2004)
4626–4629, https://doi.org/10.1002/anie.200461055.
[13] C. Hammond, M.M. Forde, M.H. Ab Rahim, A. Thetford, Q. He, R.L. Jenkins,
N. Dimitratos, J.A. Lopez-Sanchez, N.F. Dummer, D.M. Murphy, A.F. Carley,
S.H. Taylor, D.J. Willock, E.E. Stangland, J. Kang, H. Hagen, C.J. Kiely,
G.J. Hutchings, Direct catalytic conversion of methane to methanol in an aqueous
medium by using copper-promoted Fe-ZSM-5, Angew. Chem. Int. Ed. 51 (2012)
5129–5133, https://doi.org/10.1002/anie.201108706.
[14] C. Hammond, R.L. Jenkins, N. Dimitratos, J.A. Lopez-Sanchez, M.H. ab Rahim,
M.M. Forde, A. Thetford, D.M. Murphy, H. Hagen, E.E. Stangland, J.M. Moulijn,
S.H. Taylor, D.J. Willock, G.J. Hutchings, Catalytic and mechanistic insights of the
Low-temperature selective oxidation of methane over Cu-promoted Fe-ZSM-5,
Chem. A Eur. J. 18 (2012) 15735–15745, https://doi.org/10.1002/chem.
201202802.
[15] L. Kesavan, R. Tiruvalam, M. Hasbi, A. Rahim, M. Izham, D.I. Enache, R.L. Jenkins,
N. Dimitratos, J.A. Lopez-sanchez, S.H. Taylor, D.W. Knight, C.J. Kiely,
G.J. Hutchings, Solvent-Free oxidation of primary carbon-hydrogen bonds in to-
luene using Au-Pd alloy nanoparticles, Science (80-.) 331 (2011) 195–199.
[16] A. Villa, N. Dimitratos, C.E. Chan-Thaw, C. Hammond, L. Prati, G.J. Hutchings,
Glycerol oxidation using gold-containing catalysts, Acc. Chem. Res. 48 (2015)
1403–1412, https://doi.org/10.1021/ar500426g.
[17] J. Pritchard, L. Kesavan, M. Piccinini, Q. He, R. Tiruvalam, N. Dimitratos,
J.A. Lopez-Sanchez, A.F. Carley, J.K. Edwards, C.J. Kiely, G.J. Hutchings, Direct
synthesis of hydrogen peroxide and benzyl alcohol oxidation using Au-Pd catalysts
prepared by sol immobilization, Langmuir 26 (2010) 16568–16577, https://doi.
org/10.1021/la101597q.
[18] M.H. Ab Rahim, M.M. Forde, R.L. Jenkins, C. Hammond, Q. He, N. Dimitratos,
J.A. Lopez-Sanchez, A.F. Carley, S.H. Taylor, D.J. Willock, D.M. Murphy, C.J. Kiely,
G.J. Hutchings, Oxidation of methane to methanol with hydrogen peroxide using
supported gold-palladium alloy nanoparticles, Angew. Chem. Int. Ed. 52 (2013)
1280–1284, https://doi.org/10.1002/anie.201207717.
[28] M.H. Ab Rahim, R.D. Armstrong, C. Hammond, N. Dimitratos, S.J. Freakley,
M.M. Forde, D.J. Morgan, G. Lalev, R.L. Jenkins, J.A. Lopez-Sanchez, S.H. Taylor,
G.J. Hutchings, Low temperature selective oxidation of methane to methanol using
titania supported gold palladium copper catalysts, Catal. Sci. Technol. 6 (2016)
3410–3418, https://doi.org/10.1039/C5CY01586C.
[29] S. Al-Shihri, C.J. Richard, D. Chadwick, Selective oxidation of methane to methanol
over ZSM-5 catalysts in aqueous hydrogen peroxide: role of formaldehyde,
ChemCatChem 9 (2017) 1276–1283, https://doi.org/10.1002/cctc.201601563.
[30] G.B. Shul’pin, G.V. Nizova, Y.N. Kozlov, L. Gonzalez Cuervo, G. Süss-Fink,
Hydrogen peroxide oxygenation of alkanes including methane and ethane catalyzed
by iron complexes in acetonitrile, Adv. Synth. Catal. 346 (2004) 317–332, https://
doi.org/10.1002/adsc.200303147.
[31] Z. Duan, N. Møller, J. Greenberg, J.H. Weare, The prediction of methane solubility
in natural waters to high ionic strength from 0 to 250°C and from 0 to 1600 bar,
Geochim. Cosmochim. Acta. 56 (1992) 1451–1460, https://doi.org/10.1016/0016-
7037(92)90215-5.
[32] J. Kiepe, S. Horstmann, A. Kai Fischer, J. Gmehling, Experimental determination
and prediction of gas solubility data for methane + water solutions containing
different monovalent electrolytes, Ind. Eng. Chem. Res. 42 (2003) 5392–5398,
https://doi.org/10.1021/IE030386X.
[33] J. Colby, D.I. Stirling, H. Dalton, The soluble methane mono-oxygenase of
Methylococcus capsulatus (Bath), Biochem. J. 165 (1977) 395–402.
[34] J. Shan, M. Li, L.F. Allard, S. Lee, M. Flytzani-Stephanopoulos, Mild oxidation of
methane to methanol or acetic acid on supported isolated rhodium catalysts,
Nature= 551 (2017) 605–608, https://doi.org/10.1038/nature24640.
[35] V.L. Sushkevich, D. Palagin, M. Ranocchiari, J.A. van Bokhoven, Selective anae-
robic oxidation of methane enables direct synthesis of methanol, Science (80-.) 356
(2017) 523–527, https://doi.org/10.1016/S0166-9834(00)80103-7.
[19] C. Williams, J.H. Carter, N.F. Dummer, Y.K. Chow, D.J. Morgan, S. Yacob, P. Serna,
D.J. Willock, R.J. Meyer, S.H. Taylor, G.J. Hutchings, Selective oxidation of me-
thane to methanol using supported AuPd catalysts prepared by stabilizer-free sol-
immobilization, ACS Catal. 8 (2018) 2567–2576, https://doi.org/10.1021/acscatal.
7b04417.
[20] J.K. Edwards, S.J. Freakley, A.F. Carley, C.J. Kiely, G.J. Hutchings, Strategies for
7