Low temperature selective oxidation of methane to methanol using titania supported gold palladium copper catalysts

Article abstract of DOI:10.1039/c5cy01586c

The selective oxidation of methane to methanol has been studied using trimetallic AuPdCu/TiO₂ catalysts prepared by incipient wetness impregnation. They are able to catalyse the selective oxidation of methane to methanol under mild aqueous reaction conditions using H₂O₂ as the oxidant. When compared with bimetallic, Au-Pd/TiO₂ analogues, the new trimetallic catalysts present productivities which are up to 5 times greater under the same test conditions, and this is coupled with methanol selectivity of up to 83%. Characterisation shows that whilst Au-Pd is present as Au-core-Pd-shell nanoparticles, copper is present as either Cu or Cu₂O in <5 nm particles.

Full text of DOI:10.1039/c5cy01586c

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Low temperature selective oxidation of methane
to methanol using titania supported gold
palladium copper catalysts†
Cite this: DOI: 10.1039/c5cy01586c
Mohd Hasbi Ab Rahim,‡ Robert D. Armstrong, Ceri Hammond, Nikolaos Dimitratos,
Simon J. Freakley, Michael M. Forde, David J. Morgan, Georgi Lalev,
Robert L. Jenkins, Jose Antonio Lopez-Sanchez, Stuart H. Taylor
*
and Graham J. Hutchings
The selective oxidation of methane to methanol has been studied using trimetallic AuPdCu/TiO2 catalysts
prepared by incipient wetness impregnation. They are able to catalyse the selective oxidation of methane
to methanol under mild aqueous reaction conditions using H2O2 as the oxidant. When compared with bi-
metallic, Au–Pd/TiO2 analogues, the new trimetallic catalysts present productivities which are up to 5 times
greater under the same test conditions, and this is coupled with methanol selectivity of up to 83%. Charac-
terisation shows that whilst Au–Pd is present as Au-core–Pd-shell nanoparticles, copper is present as either
Cu or Cu2O in <5 nm particles.
Received 20th September 2015,
Accepted 10th December 2015
DOI: 10.1039/c5cy01586c
www.rsc.org/catalysis
derivatives has also been reported. High temperature/high
pressure gas phase approaches have been reported,5 though
1. Introduction
The direct oxidation of methane to methanol is a key chal-
lenge within the chemical sciences. Conventional natural gas
reserves are estimated to exceed 200 trillion cubic meters,1
with further exploration ongoing. Owing to its high abun-
dance, natural gas has been suggested as a fuel for society’s
transition away from a petroleum-dependent economy.2 Di-
rect conversion of the major components of natural gas
(methane and ethane) to oxygenated products is a key chal-
lenge in achieving this. However, the direct catalytic
upgrading of these short chain alkanes is yet to be achieved
under mild/green and industrially viable conditions. Pro-
cesses for upgrading methane to value added products have
been commercialised, however due to the relative inertness of
the substrate (ΔHC–H = 439.57 kJ mol−1), these utilise harsh
conditions for its activation.3 Technologies for conversion of
methane to methanol fall into two broad categories; indirect
and direct. Indirect approaches utilise harsh conditions to
convert methane to synthesis gas,4 which is then further
converted to methanol, amongst other bulk chemicals. Al-
though very selective, such processes have high energy and
capital demands. Direct conversion of methane to methane-
these typically show low selectivity towards methanol.
Methane oxidation in the liquid phase has also been studied
extensively. One approach involves oxidation of methane to
yield methyl-esters6–12 and often requires acidic media. The
direct oxidation of methane to methanol is favourable,
however, derivatization incurs additional workup and separa-
tion steps in the liquid phase. Several groups have used
environmentally benign oxidants such as H2O2 in methane
oxidation systems,13–26 though no system has as yet been
deemed viable for commercialisation. Indeed, highly efficient
H2O2 utilisation must be realised if H2O2 is to be used as the
oxidant in the activation of methane. Circumventing the need
to provide a premade H2O2 feed, liquid phase systems have
been reported in which H2O2 is generated in situ from H2 and
O2.12 However these systems used acid promoters and yielded
formic acid or methyl esters as major products.11,12
It has previously been reported that supported AuPd cata-
lysts catalyse the oxidation of methane with H2O2 at 50 °C in
water.20,21,27 The reaction was shown to proceed through for-
mation of methylhydroperoxide which underwent further
conversion to yield methanol and CO2.21 The reaction was
also performed in the absence of added H2O2, with H2O2 in-
stead generated in situ from H2 and O2. In this way AuPd/
TiO2 catalysed both H2O2 synthesis and methane oxidation
reactions.21 We now extend these studies to show the effect
of addition of copper to the previously reported catalyst sys-
tems. Catalysts are studied when H2O2 is both added and
also generated in situ. We have assessed the effect of copper
Cardiff Catalysis Institute, School of Chemistry, Cardiff University Main Building,
Park Place, Cardiff, CF10 3AT, UK. E-mail: Hutch@Cardiff.ac.uk;
Fax: +44 (0)2090874 030; Tel: +44 (0)2920874059
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c5cy01586c
‡ Current Address: Faculty of Industrial Sciences
& Technology, Universiti
Malaysia Pahang, Lebuhraya Tun Razak, 26300, Kuantan, Pahang, Malaysia.
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addition on the rate of H2O2 synthesis, methane activation,
methanol selectivity and the efficiency with which H2O2 is
utilised in products. Through catalyst characterisation stud-
ies, we aim to correlate catalyst structure and function.
solvent (5.6 g MeOH and 2.9 g H2O). The charged autoclave
was then purged three times with 5% H2/CO2 before
pressurising with 5% H2/CO2 (29 bar) and 25% O2/CO2 (11
bar) at 20 °C. The temperature was then allowed to decrease
to 2 °C followed by stirring (at 1200 rpm) of the reaction mix-
ture for 0.5 h. H2O2 productivity was determined by titrating
aliquots of the final solution after reaction with acidified
CeIJSO4)2 (0.01 M) in the presence of two drops of ferroin
indicator.
H2O2 degradation experiments were carried out in a simi-
lar manner as H2O2 synthesis experiments but without
adding the 25% O2/CO2. Furthermore, 0.68 g of H2O from the
8.5 g of solvent was replaced by a 50% H2O2 solution to give
a reaction solvent containing 4 wt% H2O2. The standard reac-
tion conditions for H2O2 hydrogenation included: 0.01 g cata-
lyst, 8.5 g solvent (5.6 g MeOH, 2.22 g H2O and 0.68 g H2O2
(50%)), 29 bar 5% H2/CO2, 2 °C, 1200 rpm, 0.5 h.
2. Materials & methods
2.1 Catalyst preparation
Supported catalysts were prepared by incipient wetness im-
pregnation using chloride precursors. All loadings are given
as wt%. The procedure for synthesis of a 2.5% Au 2.5% Pd
2.5% Cu/TiO2 catalyst is as follows;
Solutions of HAuCl4·3H2O (Johnson Matthey, 4.29 ml, 0.059
M) and CuCl2·2H2O (Sigma Aldrich, 5.3 ml, 0.149 M) were
mixed, and then a solution of PdCl2 (Johnson Matthey, 0.083
g) was added with vigorous stirring. Following dissolution of
the PdCl2, TiO2 (P25, Degussa, 1.85 g) was added with vigor-
ous stirring. Once a homogeneous slurry was formed, drying
was carried out (16 h, 110 °C). The dry catalyst was then
ground to a fine powder and calcined in static air (400 °C, 3
h, 20 °C min−1).
2.3 Catalyst characterisation
Powder X-ray diffraction (XRD) patterns were collected on a
PANalytical MPD diffractometer fitted with a CuKα1 radiation
source (λ = 0.154098 nm) at ambient conditions. Samples
were scanned in the range of 10–70° at 40 kV and 40 mA.
2.2 Catalyst testing
2.2.1 Oxidation of methane with added ex situ H2O2. Cata-
lyst testing was carried out in a 50 ml Teflon-lined stainless
steel Parr autoclave reactor. The reactor was charged with
H2O2 (Sigma Aldrich, 5000 μmol, 0.5 M) and catalyst (1.0 ×
10−5 mol metal equivalent) was added. The reactor was sealed
and purged 3 times with methane. The reactor was then
charged with methane (BOC, 99.999%, 30.5 bar) and the au-
toclave was heated to 50 °C. Once the setpoint temperature
was reached, the system was vigorously stirred (1500 rpm) for
0.5 h. Following this the autoclave was cooled to ca. 12 °C to
minimise loss of volatile products. Post reaction, the
remaining H2O2 was quantified through titration versus
acidified CeIJSO4)2 (8 × 10−3 mol dm−3) of known concentra-
tion, with a Ferroin indicator. Aqueous products were quanti-
fied with solvent suppressed 1H NMR at ambient temperature
on a Bruker Ultrashield 500 MHz spectrometer using a TMS/
CDCl3 internal standard. Gaseous products were quantified
using a Varian 450-GC fitted with a CP-Sil 5CB capillary col-
umn (50 m length, 0.32 mm diameter, carrier gas = He), a
methaniser unit and both FID and TCD detectors.
2.2.2 Oxidation of methane with in situ generated H2O2.
Methane oxidation experiments with in situ generated H2O2
followed the same procedure as outlined in 2.2.1 with the fol-
lowing exceptions. Once sealed, the reactor was purged with
5% H2/N2 (BOC) and then charged with 5% H2/N2 (BOC), 25%
O2/N2 (BOC) and CH4 (BOC, 99.999%) such that the total
pressure equalled 32 bar. The gas phase composition was
0.86% H2/1.72% O2/75.86% CH4/21.55% N2 to ensure the
mixture was outside of the explosive limits.
HR-TEM was performed using
a (LaB6) JEOL 2100
equipped with a high-resolution Gatan digital camera. In
scanning transmission electron microscopy (STEM) mode, a
dark field (HAADF/Z-contrast) detector was used. An EDS sys-
tem (Oxford Instruments) equipped with a 80 mm2 SDD (Sili-
con Drift Detector) X-MaxN 80 T was employed to study the el-
emental composition in point & ID, line scans, layered and
elemental mapping modes. EDS data was analysed using
AZtecTEM software. For HR-TEM analysis, samples were
suspended in DI water and ca.1 μL was added to a TEM grid
and dried. EDS analysis of the samples containing Cu was
carried out using Ni TEM grids and a Beryllium sample
holder.
X-Ray photoelectron spectroscopy (XPS) was performed
using a VG EscaLab 220i spectrometer, using a standard Al-
Kα X-ray source (300 W) and analyzer pass energy of 20 eV.
Samples were mounted using double-sided adhesive tape,
and binding energies were referenced to the C 1s binding en-
ergy of adventitious carbon contamination at 284.7 eV.
Temperature programmed reduction (TPR) profiles were
obtained using a ThermoElectron TPDRO 1100 instrument
fitted with a TCD. Catalysts were pretreated at 90 °C under a
flow of He (20 mL min−1) for 0.5 h and then allowed to cool
to ambient temperature. The flow was then switched to 10%
H2/Ar (15 mL min−1) and the temperature was raised to 700
°C using a linear ramp rate of 5 °C min−1.
3. Results and discussion
3.1 Liquid phase oxidation of methane with preformed H2O2
2.2.3 H2O2 synthesis and decomposition. Hydrogen perox-
ide synthesis and hydrogenation was evaluated using a stain-
less steel Parr autoclave with a nominal volume of 100 ml.
The autoclave was charged with catalyst (0.01 g) and 8.5 g
Our previous studies have shown TiO2 to be an effective sup-
port for methane activation over bimetallic AuPd catalysts.21
Whilst inactive in the reaction, the addition of TiO2 to a
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homogeneous HAuCl4·H2O catalysed methane oxidation reac-
tion resulted in an increase of methanol selectivity and de-
creased selectivity towards formic acid.20 In a similar way,
studies into the oxidation of methane over ZSM-5 catalysts
have reported that deposition of Cu2+ increases methanol
selectivity dramatically by preventing further oxidation to
formic acid. EPR studies showed that Cu2+ was either
scavenging or preventing formation of hydroxyl radicals.24,25
Indeed, theoretical studies have reported Cu to be capable of
oxidising methane to methanol.28 As copper has both been
reported to increase selectivity to primary products and is
able to oxidise methane to methanol we investigated the
possibility of adding Cu as a component to supported AuPd
catalysts.
A series of mono, bi and tri-metallic catalysts comprising
Au/Pd/Cu were prepared and tested for oxidation of methane
with H2O2 added as the oxidant (Table 1). For comparison
with previously published data, Table 1 entry 1 presents pre-
viously published data for the optimal catalyst (2.5% Au 2.5%
Pd/TiO2 prepared by the incipient wetness impregnation
method).21
methylhydroperoxide to methanol at rates which are compa-
rable to AuPd catalysts. Co-deposition of 2.5 wt% Cu and 2.5
wt% Au increased oxygenate productivity with no change in
the product distribution. An added benefit of Au addition
was an increase in efficiency of H2O2 usage to 6.7%. Both
Au–Cu and Pd–Cu catalysts showed lower methanol selectiv-
ity than the Au–Pd catalyst, with methylhydroperoxide the
favoured reaction product (83% and 66% respectively).
Clearly both Au and Pd are required to effectively convert
methylhydroperoxide to methanol. Hence, combining the
high catalytic activity afforded by Cu and high methanol se-
lectivity of AuPd nanoparticles would be desirable in a
trimetallic catalyst. Addition of 2.5 wt% Cu to a 2.5% Au
2.5% Pd catalyst led to a fivefold increase in catalyst produc-
tivity compared with the bimetallic AuPd (Table 1 entry 1).
This constituted a 135% increase in TOF from 0.7 to 1.65
molmethane converted molmetal h−1. Once more, addition of Cu
−1
led to increased efficiency of H2O2 utilisation, reaching
12.2% whilst H2O2 conversion decreased from 93% to just
50%. This suggests that Cu is either altering the AuPd active
sites by (i) changing the redox properties of active sites (ii)
blocking active sites of non-desirable H2O2 decomposition
pathways, or (iii) that Cu is itself altering the rate of H2O2
decomposition. This is explored later in this paper. As with
Au–Cu and Pd–Cu catalysts, addition of Cu shifted the
product distribution to favour the primary product methyl-
hydroperoxide (69% selectivity). Indeed, methanol selectivity
for the AuPdCu catalyst (7.5 wt% metal loading, Table 1 entry
5) was 27.8% which is compared with 49.3% for the Au–Pd
catalyst (Table 1 entry 1). This effect of Cu is consistent with
previous Cu/ZSM-5 studies, whereby Cu2+ was shown to in-
hibit the hydroxyl radical-mediated oxidation of methanol to
formic acid.24,25 The wt% loading of Cu was therefore de-
creased, to determine whether higher methanol selectivity
might be achieved whilst still minimising the unselective de-
composition of H2O2. Comparable activity to 2.5% Au 2.5%
Pd 2.5% Cu/TiO2 was observed upon testing of a 2.5% Au
2.5% Pd 1% Cu/TiO2 catalyst, with TOF (h−1) of 1.65 and 1.40
When deposited onto TiO2 at 2.5 wt% loading, Cu is itself
active (Table 1 entry 2) which is consistent with the ability of
Cu to catalyse Fenton’s type H2O2 conversion and generate
oxygen based radical species.29,30 Indeed, the methane oxida-
tion products methylhydroperoxide, methanol and CO2 were
observed in the reaction mixture. The overall activity of this
catalyst was higher than that of the previously reported bime-
tallic AuPd catalyst (Table 1 entry 1) with a TOF of 1.0 com-
−1
pared to 0.70 molmethane converted molmetal h−1, and H2O2 was
more efficiently utilised with 4.9% of active oxygen retained
in the products. This is compared with 2.1% for AuPd/TiO2.
Moreover, monometallic Cu/TiO2 displayed higher selectivity
towards partially oxygenated products at 96%, which is com-
pared with 90% for the 2.5% Au 2.5% Pd catalyst. The oppo-
site trend was observed in methanol selectivity however, with
49% for the AuPd catalyst and 14% for monometallic Cu.
This suggests that Cu does not catalyse the transformation of
Table 1 Methane oxidation activity of mono, bi or trimetallic Cu/Au/Pd/TiO2 catalysts
Product amount/μmol
Oxygenate
Methanol
Oxygenate
H2O2
H2O2
Entry Catalyst
CH3OH HCOOH CH3OOH CO2 selectivitya/% selectivity/% productivityb TOFc remaind/μmol efficiencye
1
2
3
4
5
2.5% Au 2.5% Pd/TiO2 1.89
2.5% Cu/TiO2 0.76
2.5% Au 2.5% Cu/TiO2 0.91
2.5% Pd 2.5% Cu/TiO2 0.64
2.5% Au 2.5% Pd/2.5% 2.36
Cu/TiO2
0
0
0
0
0
1.57
4.40
6.18
2.30
5.87
0.37 90
0.19 96
0.39 95
0.56 84
0.26 97
49.3
14.2
12.2
18.3
27.8
0.250
0.406
0.739
0.369
1.243
0.70
1.03
1.42
0.59 2434
1.65 2483
383
338
66
2.1
4.9
6.7
5.6
12.2
6
7
2.5% Au 2.5% Pd/1.0% 6.08
Cu/TiO2
0
0
0.94
1.23
0.33 96
0.34 83
82.7
23.8
0.729
0.128
1.40
0.34
842
845
1.5
1.9
2.5% Au 2.5% Pd/TiO2 0.49
f
and 2.5% Cu/TiO2
a
Test conditions: 0.5 h, 50 °C, PIJCH4) = 30 bar, catalyst: 1.0 × 10−5 mol of metal, [H2O2] = 0.5 M (5000 μmol), 1500 rpm. Oxygenate selectivity =
b
−1
c
f
(mol of −o1xygenate/total mol of products) × 100. Oxygenates productivity = moloxygenates kgcat h−1
.
Turn over frequency (TOF) = moloxygenates
d
e
molmetal h−1
.
Assayed by Ce+4 (aq) titration. (mol O2 in products/mol H2O2 converted) × 100. Reaction of physical mixture comprising
2.5% Au 2.5% Pd/TiO2 and 2.5% Cu/TiO2.
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Table 3 H2O2 synthesis and degradation rates for titania-supported
AuPd and AuPdCu catalysts
respectively. Through lowering of the Cu loading, methanol
selectivity increased from 27.8% to 82.7%, with H2O2 conver-
sion also increasing to 83%. Catalyst performance for this
trimetallic catalyst is superior to that of our previously
reported AuPd catalyst (Table 1 entry 1) under liquid phase
conditions.21 Furthermore, addition of Cu led to decreased
CO2 selectivity from 9.7% for Au–Pd/TiO2 to 3.1 and 4.5% fol-
lowing deposition of 2.5 wt% and 1.0 wt% Cu respectively. Fi-
nally, a physical mixture of 2.5% Au 2.5% Pd/TiO2 and 2.5%
Cu/TiO2 was tested and showed a comparatively low TOF of
0.34 and high rate of H2O2 decomposition relative to its
trimetallic analogue. This suggested a synergistic interaction
when Au/Pd and Cu are in close proximity.
Productivity
Degradation
Entry Catalyst
mol kgcat−1 h−1a mol kgcat−1 h−1b
1
2
2.5% Au 2.5% Pd/TiO2
2.5% Au 2.5% Pd 2.5%
Cu/TiO2
83
11
215
143
3
2.5% Au 2.5% Pd 1%
Cu/TiO2
10
184
a
Test conditions: 5% H2/CO2 (29 bar) and 25% O2/CO2 (11 bar), 8.5 g
solvent (2.9 g HPLC water, 5.6 g MeOH) 0.01 g catalyst, 2 °C, 1200
b
rpm, 30 min). Test conditions: 5% H2/CO2 (29 bar), 8.5 g solvent
(5.6 g MeOH, 2.22 g H2O and 0.68 g 50% H2O2), 0.01 g catalyst, 2 °C,
1200 rpm, 30 min.
3.2 Liquid phase oxidation of methane with in situ
generated H2O2
This represents combined rates of H2O2 hydrogenation and
decomposition pathways. Given that addition of Cu also led
to decreased H2O2 synthesis rates, it can therefore be con-
cluded that Cu blocks the sites needed for H2 activation to
H2O2. This indicates that the decomposition pathway would
dominate the degradation rates in Table 3 entries 2 and 3.
The ability of Cu to decompose H2O2 at 50 °C was studied
under an inert atmosphere of N2 (P = 30 bar) with all other
test conditions set as in Table 1 (0.5 h, [H2O2] = 0.5 M, 5000
μmol, 50 °C, 1500 rpm, 1.0 × 10−5 mol of metal − 25.4 mg cat-
Addition of Cu to AuPd catalysts has been shown to enhance
catalytic activity and selectivity for the oxidation of methane
to methanol with added H2O2 as oxidant. A series of
trimetallic AuPdCu/TiO2 catalysts of varying wt% Cu loading
was therefore prepared and assessed for catalytic activity in
the oxidation of methane with H2O2 generated in situ from
H2 and O2. Catalytic data for the optimal bimetallic AuPd cat-
alyst from our previous studies is shown in Table 2 entry 1.
Co-deposition of Cu with AuPd (Table 2 entries 2–5) led to
a significant decrease in catalytic activity for methane oxida-
tion when H2O2 was produced in situ. Deposition of Cu onto
AuPd might alter or block the active sites responsible for
generating H2O2 from dissolved H2 and O2 and thereby limit
oxidant availability for the methane oxidation reaction. To ex-
plore this, catalysts were tested for activity in the synthesis
and degradation of H2O2 under previously reported reaction
conditions.31 It is apparent (Table 3) that addition of Cu to
2.5% Au 2.5% Pd/TiO2 has a detrimental effect upon the net
rate of H2O2 synthesis, with catalyst productivity falling from
83 mol kg−1 h−1 to 10 and 11 for Cu loadings of 1.0 wt% and
2.5 wt% respectively. The rate of H2O2 degradation is also
lower for the trimetallic catalysts (Table 3, entries 2 and 3).
alyst). Upon testing, 2.5% Cu/TiO2 showed a high H2O2 de-
−1
composition rate of 384 molH O
kgcat h−1. This is
converted
2
2
compared with a rate of 0.406 molmethane converted kgcat h−1
when the same catalyst was tested for methane oxidation at
50 °C with added H2O2 (0.5 M, 5000 μmol, Table 1 entry 2).
The low oxygenate productivities shown for trimetallic cata-
lysts in Table 2 can therefore be attributed to competition be-
tween a high rate of Cu-catalysed H2O2 decomposition and
low rates of H2O2 synthesis/methane oxidation reactions.
This was confirmed through addition of 2.5% Cu/TiO2 to a
reaction catalysed by 2.5% Au 2.5% Pd/TiO2. Physical mixing
of these catalysts led oxygenate productivity to fall from
−1
−1
0.11 molmethane converted kgcat h−1 to 0.074 molmethane converted
Table 2 Liquid phase oxidation of methane using heterogeneous Au/Pd/Cu/TiO2 catalysts with in situ generated H2O2
Product amount (μmol)
Oxygenate
Methanol
Oxygenate
H2O2
Entry Catalyst
CH3OH HCOOH MeOOH CO2 selectivitya/% selectivity/% productivityb TOFc remainingd/μmol
1
2
3
4
5
6
7
2.5% Au 2.5% Pd/TiO2
1.31
2.5% Au 2.5% Pd 2.5% Cu/TiO2 0.45
2.5% Au 2.5% Pd 1.0% Cu/TiO2 0.39
2.5% Au 2.5% Pd 0.5% Cu/TiO2 0.31
2.5% Au 2.5% Pd 0.25%Cu/TiO2 0.25
0
0
0
0
0
0
0
0.29
0
0
0
0
0.32 83
0.1 98
0.13 75
0.20 61
0.91 22
68.2
81.8
75.0
60.8
21.6
n/a
0.11
0.320 56
0.090 18
0.068
0.040
0.027
0.020
0.00
0.078
9
0.062 17
0.050 14
2.5% Cu/TiO2
0
0.49
0
0
0
0
0
0
9
2.5% Au 2.5% Pd/TiO2 and
0.1 98
83.1
0.074
0.098
e
2.5%Cu/TiO2
Test conditions: 0.5 h, 50 °C, catalyst: 1.0 × 10−5 mol of metal, 1500 rpm, gases: 0.86% H2/1.72% O2/75.86% CH4/21.55% N2, P(total pressure) =
a
b
−1
c
32 bar. Oxygenate selectivity = (mol of −o1xygenate/total mol of products) × 100. Oxygenates productivity = moloxygenates kgcat h−1
.
Turn over
frequency (TOF) = moloxygenates molmetal h−1
Pd/TiO2 and 2.5% Cu/TiO2.
.
Assayed by Ce+4 (aq) titration. Reaction of physical mixture comprising 2.5 wt% Au 2.5%
d
e
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kgcat h−1 (Table 2, entry 7). This is comparable to the pro-
−1
ductivity of 0.068 (same units) which was observed for 2.5%
Au 2.5% Pd 2.5% Cu/TiO2 under the same reaction condi-
tions (Table 2, entry 2).
3.3 Catalyst characterisation
To correlate catalyst performance and structural/electronic
properties, catalysts were characterised using XRD, TEM, XPS
and H2-TPR.
X-ray diffraction patterns of the mono, bi and tri-metallic
Au/Pd/Cu catalysts from studies in Table 1 were recorded and
are shown in Fig. 1. Comparison with reference patterns for
Cu0 (JCPDS no: 01-085-1326), CuO (JCPDS no: 01-089-5986)
(CuO) and Cu2O (JCPDS no: 01-071-3645) showed no peaks
corresponding to copper species for mono bi or tri-metallic
Cu-containing catalysts. This suggests that the crystallite size
of supported copper species is below the detection limit of
the instrument. Characteristic signals for metallic Au were
observed at 2θ = 38.2°, 44.3° and 64.5° in Au-containing
samples. These are assigned to the (111), (200) and (311)
planes respectively (JCPDS no. 03-065-2870). No Pd phases
were observed in the monometallic Pd catalyst (Fig. 1d). In
all samples the characteristic peaks of the TiO2 (P25) support
were clearly observed (Fig. 1a). Based upon the diffraction
peak at 44.3°, the mean Au crystallite size for 2.5% Au 2.5%
Pd 2.5% Cu/TiO2 was calculated to be 18.5 nm, using the
Scherrer equation. This increased to 23.5 nm for 2.5% Au
2.5% Pd 1.0% Cu/TiO2. Au-catalysts contained Au in a cubic
phase with comparable unit cell parameters to mono-
metallic Au (ICSD no. 53764) as shown in Table 4. No
apparent alloying of Au/Pd and Cu was observed in XRD
diffractograms. Although no reflections indicative of bulk
alloyed nanoparticles were observed, a portion of nano-
particles which are beneath the detection limit of the XRD
method could be present. To determine whether this was
indeed the case, catalysts were studied using HRTEM. Micro-
graphs and corresponding particle size distributions are
shown in Fig. 2. Based upon a sample of 530 nanoparticles
the catalysts; 2.5% Au 2.5% Pd/TiO2, 2.5% Au 2.5% Pd 1.0%
Cu/TiO2 and 2.5% Au 2.5% Pd 2.5% Cu/TiO2 showed average
nanoparticle sizes of 1.32, 1.20 and 1.37 nm respectively. Due
to restrictive particle dimensions, visualisation of core shell
features was not possible. However, previous studies of AuPd/
TiO2 catalysts prepared via the same impregnation method
described in section 2.1 have shown Au-core–Pd-rich shell
nanoparticles to form upon calcination at 400 °C in air.32
Fig. 1 XRD diffractograms of TiO2 deposited with (a) support only, (b)
2.5% Cu, (c) 2.5% Au, (d) 2.5% Pd, (e) 2.5% Au 2.5% Pd, (f) 2.5% Au 2.5%
Pd 1.0% Cu, (g) 2.5% Au 2.5% Pd 2.5% Cu.
We rationalised the disparity between average particle sizes
as determined by XRD (Scherrer, Fig. 1) and HRTEM
(Fig. 2) methods using STEM-EDX. This showed the pres-
ence of ~μm sized gold-rich particles (ESI† Fig. S1). Forma-
tion of large (200 nm–2 μm) Au-rich particles, and Au-core/
Pd-rich shell nanoparticles during impregnation of TiO2
with HAuCl4·6H2O and PdCl2 is consistent with previous
work by Sankar et al.33
XPS spectra were collected to evaluate the oxidation state
and surface composition of Cu, Pd and Au species present in
2.5% Au 2.5% Pd 1% Cu/TiO2 following calcination (Fig. 3).
Copper species of differing oxidation state show characteris-
tic binding energies and satellite structures with Cu2+ (934
eV) species exhibiting a shakeup peak at ca. 10 eV above the
Cu (2p3/2) signal.34,35 These peaks are not observed for Cu+ or
Cu0 and can therefore be used to differentiate between Cu2+
and reduced species.34,36 The Cu (2p) spectra for 2.5% Au
2.5% Pd 2.5% Cu/TiO2, both before and after calcination, are
shown in Fig. 3(a) and (b) respectively. The main peak is ob-
served at a binding energy of 932.8 eV and attributed to re-
duced Cu species (CuI/Cu+) suggesting either Cu2O or metal-
lic Cu. A minor Cu2+ contribution is also observed as a weak
shoulder at 934 eV and a weak satellite peak at 944 eV.
The corresponding Au (4d)/Pd (3d) spectra for the same
catalysts are shown in Fig. 4(a) and (b). It is evident that prior
to calcination the spectrum is composed of Pd (3d) peaks
superimposed on the Au (4d5/2) peak (ca. 335 eV). Pd is
present as Pd2+, as evidenced by the binding energy of
337.6 eV, whilst the presence of any Pd(0) in this uncalcined
sample could not be ascertained due to a strong overlap
with the Au signal.
Table 4 Calculated Au particle properties from XRD
Catalyst
Metal phase Unit cell volume/Å3
2.5% Au/TiO2
2.5% Au 2.5% Pd/TiO2
Au
Au
67.79
67.84
67.73
67.51
67.85
2.5% Au 2.5% Pd 1.0% Cu/TiO2 Au
2.5% Au 2.5% Pd 2.5% Cu/TiO2 Au
Referencea
Au
Following calcination Pd is present as both Pd2+ (337.8 eV)
and Pd(0) (335.3 eV) in an atomic ratio of 6.14 : 1 (Fig. 3b).
a
Reference no. ICSD no. 611624.
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Fig. 2 Representative electron micrographs and associated particle size distributions for: (a, i, ii, iii) 2.5% Au 2.5% Pd/TiO2, (b, iv, v, vi) 2.5% Au 2.5%
Pd 1% Cu/TiO2 and (c, vii, viii, ix) 2.5% Au 2.5% Pd 2.5% Cu/TiO2.
After heat treatment no signal corresponding to Au(4d) is
observed (Fig. 3b) whereas a significant attenuation of the
Au(4f) signal at 83.8 eV is apparent (Fig. 3c and d). The
Au(4f) signal is observable over the Au(4d) signal due to the
difference in the mean free path of the photoelectron at the
different kinetic energies.
A summary of quantified surface-metal compositions of
the uncalcined and calcined catalysts is given in Table 5. Fol-
lowing heat treatment a significant decrease in Au surface
concentration is observed, with the Pd/Au atomic ratio in-
creasing from 0.54 to 15.17. A similar increase in the Cu/Au
ratio also occurred. This indicates that Au core–Pd shell
alloyed nanoparticles are formed upon calcination, and the
increase in Pd/Au ratio is consistent with aforementioned at-
tenuation of the Au signal, which is analogous to the ob-
served attenuation by Pd overlayers as previously reported by
Edwards et al. for AuPd/TiO2 catalysts prepared under compa-
rable conditions.32 Furthermore, quantitative analysis in
Table 5 suggests that following heat treatment the TiO2 sur-
face is enriched with both Pd and Cu. Cu enrichment could
arise from; formation of Au-core–PdCu-shell nanoparticles or
Cu being more highly dispersed across the TiO2 surface than
Au. XRD data shown in Fig. 1 and the low Cu loadings used
support the latter. Little change in the Pd/Cu ratio is appar-
ent in Table 5, indicating that core shell Pd–Cu species are
not forming. XPS surface analysis of 2.5% Pd 2.5% Cu/TiO2
(Table 5 entry 3) shows a Pd : Cu ratio of ca. 1, which further
supports this. Therefore, whereas Pd–Cu alloying has been
Fig. 3 XPS spectra in the Cu (2p) region for 2.5% Au 2.5% Pd 1.0% Cu/
TiO2 before (a) and after (b) calcination at 400 °C.
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Fig. 4 XPS spectra of 2.5% Au 2.5% Pd 1.0% Cu/TiO2 before (a, c) and after (b, d) calcination at 400 °C. Spectra show (a, b) the Au (4d)/Pd (3d)
region and (c, d) the Au (4f) region.
Table 5 Surface elemental composition from XPS data for calcined and uncalcined catalysts
Composition/atom%
Atom
ratio
Atom
ratio
Atom
ratio
Entry
Catalyst
Treatment
Au/Ti
Pd/Ti
Cu/Ti
Pd/Au
Cu/Au
Pd/Cu
1
2
3
2.5% Au 2.5% Pd 2.5% Cu/TiO2
Uncalcined
Calcined
Calcined
0.040
0.003
—
0.022
0.046
0.056
0.022
0.043
0.054
0.54
15.17
—
0.54
14.17
—
1.00
1.07
1.04
2.5% Pd 2.5% Cu/TiO2
reported for supported TiO2 catalysts following heat treat-
ment in hydrogen,37 calcination in static air appears to
favour surface segregation between Pd and Cu. Future EDX
mapping studies at a higher resolution are required to deter-
mine whether Pd and Cu are indeed phase-segregated.
with XPS data from Fig. 3, which suggests that Cu presents
as reduced species following calcination. Fig. 5(e) shows the
reduction profile for 2.5% Au 2.5% Pd/TiO2. This catalyst is
known to contain alloyed AuPd nanoparticles.21 Comparison
with the reduction profile of 2.5% Au 2.5% Pd 1.0% Cu/TiO2
H2-TPR was carried out on Au/Pd/Cu/TiO2 catalysts to
probe the reducibility of metal sites and determine the na-
ture of supported metal species. No reduction was observed
in the TPR of unmodified TiO2 (P25) shown in Fig. 5a. Depo-
sition of 2.5% Cu (Fig. 5b) gave rise to a single major reduc-
tion peak centred at 180 °C. This was assigned to a 2 electron
CuII–Cu0 reduction in highly dispersed CuO species.38 The
monometallic Cu catalyst also presented a second, less
intense reduction centred at 300 °C, which was assigned to
reduction of larger particles of bulk CuO on the surface of
TiO2.39 TPR data suggested that no Au–Cu alloys are present
in bi and trimetallic catalysts (Fig. 5b and c). If Au–Cu
alloys were formed in bimetallic (c) or trimetallic (d) cata-
lysts, an additional reduction peak would be expected at ca.
260–280 °C.38,40 Given that the catalysts in this study were
calcined in air (3 h at 400 °C), alloy formation would not be
expected. Indeed, Bracey et al. have previously reported mini-
mal interaction between Au and Cu under such heat treat-
ment conditions.41 Disappearance of the CuII–Cu0 reduction
event at 180 °C in the trimetallic catalyst (c) is in agreement
Fig. 5 TPR profiles of (a) TiO2, (b) 2.5% Cu/TiO2, (c) 2.5% Au 2.5% Cu/
TiO2, (d) 2.5% Au 2.5 wt% Pd 1.0% Cu/TiO2, (e) 2.5% Au 2.5% Pd/TiO2.
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(Fig. 5d) suggests that the trimetallic catalyst also contains
alloyed AuPd nanoparticles. Meanwhile, the negative peak
observed at ca. 95 °C in Fig. 4(d) and (e) is assigned to
the low temperature decomposition of palladium hydride
species.42,43
and its impact upon reaction rates in partial alkane oxidation
will be explored in a future paper.
Acknowledgements
Catalyst characterisation showed that in trimetallic
AuPdCu/TiO2 catalysts, Au-core–Pd-shell nanoparticles form
upon calcination at 400 °C in static air. This was found to
be consistent with previous studies.21,32 Though no alloying
of Au/Pd and Cu is observed, copper speciation was effected
by the presence of Pd, with CuII favoured in Cu/AuCu cata-
lysts and Cu0/CuI species in trimetallic AuPdCu catalysts.
Trimetallic catalysts showed increased productivity and TOF
for the oxidation of methane with added H2O2 when com-
pared with either bimetallic AuPd catalysts or a physical
mixture of AuPd/TiO2and Cu/TiO2. As CuI can catalyse the
conversion of H2O2 to oxygen based radicals in a CuI/CuII
redox couple, an increase in rate is expected. However the
decrease in the rate of H2O2 conversion and simultaneous
increase in rate of methane oxidation observed following
impregnation of Cu onto 2.5% Au 2.5% Pd/TiO2 (Table 1
entries 5 and 6) suggest an additional role of Cu in blocking
Au/Pd sites, which would otherwise catalyse H2O2 decomposi-
tion in a non-selective way. Future studies should therefore
further characterise these trimetallic catalysts to probe for
Cu–Au/Pd interactions and map the atomic composition/
distribution of constituent nanoparticles.
This work formed part of the Dow Methane Challenge. The
DOW Chemical Company is thanked for their financial
support.
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