Selective Oxidation of Methane to Methanol Using Supported AuPd Catalysts Prepared by Stabilizer-Free Sol-Immobilization

Article abstract of DOI:10.1021/acscatal.7b04417

The selective oxidation of methane to methanol, using H₂O₂, under mild reaction conditions was studied using bimetallic 1 wt % AuPd/TiO₂ prepared by stabilizer-free sol-immobilization. The as-prepared catalysts exhibited low, unselective oxidation activity and deleterious H₂O₂ decomposition, which was ascribed to the small mean particle size of the supported AuPd nanoparticles. Heat treatments were employed to facilitate particle size growth, yielding an improvement in the catalyst turnover frequency and decreasing the H₂O₂ decomposition rate. The effect of support phase was studied by preparing a range of AuPd catalysts supported on rutile TiO₂. The low surface area rutile TiO₂ yielded catalysts with effective oxygenate production but poor H₂O₂ utilization. The influence of the rutile-TiO₂ support was investigated further by producing catalysts with a lower metal loading to maintain a consistent metal loading per square meter compared to the 1 wt % AuPd/P25 TiO₂ catalyst. When calcined at 800 °C, the 0.13 wt % AuPd catalyst demonstrated significantly improved turnover frequency of 103 h^-1. In contrast, the turnover frequency was found to be ca. 2 h^-1 for the rutile-supported 1 wt % AuPd catalyst calcined at 800 °C. The catalysts were probed by electron microscopy and X-ray photoelectron spectroscopy to understand the influence of particle size and oxidation state on the utilization of H₂O₂ and oxygenate productivity. This work shows that the key to highly active catalysts involves the prevention of deleterious H₂O₂ decomposition, and this can be achieved through carefully controlling the nanoparticle size, metal loading, and metal oxidation state.

Full text of DOI:10.1021/acscatal.7b04417

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Article
Selective oxidation of methane to methanol using supported
AuPd catalysts prepared by stabiliser-free sol-immobilisation
Christopher Williams, James H. Carter, Nicholas F. Dummer, Ying Kit Chow, David John Morgan, Sara
Yacob, Pedro Serna, David J. Willock, Randall J. Meyer, Stuart H Taylor, and Graham J. Hutchings
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.7b04417 • Publication Date (Web): 15 Feb 2018
Downloaded from http://pubs.acs.org on February 16, 2018
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Selective oxidation of methane to methanol using supported AuPd
catalysts prepared by stabiliser-free sol-immobilisation
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Christopher Williams[a], James H. Carter[a], Nicholas F. Dummer[a], Y. Kit Chow[a], David J.
Morgan[a],Sara Yacob[b], Pedro Serna[b], David J. Willock[a], Randall J. Meyer[b], Stuart H.
Taylor[a] and Graham J. Hutchings*[a]
[a]
[b]
Cardiff Catalysis Institute, School of Chemistry, Cardiff University, CF10 3AT
ExxonMobil Research and Engineering Company, Corporate Strategic Research,
Annandale, NJ 08801, USA
*Corresponding email: hutch@cardiff.ac.uk
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Abstract
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The selective oxidation of methane to methanol, using H2O2, under mild reaction conditions
was studied using bimetallic 1 wt. % AuPd/TiO2 prepared by stabiliser-free sol-immobilisation.
The as-prepared catalysts exhibited low, unselective oxidation activity and deleterious H2O2
decomposition, which was ascribed to the small mean particle size of the supported AuPd
nanoparticles. Heat treatments were employed to facilitate particle size growth, yielding an
improvement in the catalyst turn-over-frequency and decreasing the H2O2 decomposition
rate. The effect of support phase was studied by preparing a range of AuPd catalysts
supported on rutile TiO2. The low surface area rutile TiO2 yielded catalysts with effective
oxygenate production, but poor H2O2 utilisation. The influence of the rutile-TiO2 support was
investigated further by producing catalysts with a lower metal loading to maintain a
consistent metal loading per m2 to the 1 wt.% AuPd/ P25 TiO2 catalyst. When calcined at 800
°C the 0.13 wt.% AuPd catalyst demonstrated significantly improved turn-over frequency of
103 h-1. In contrast, the turn-over frequency was found to be ca. 2 h-1 for the rutile-supported
1 wt. % AuPd catalyst calcined at 800 °C. The catalysts were probed by electron microscopy
and XPS to understand the influence of particle size and oxidation state on the utilisation of
H2O2 and oxygenate productivity. This work shows that the key to highly active catalysts
involves the prevention of deleterious H2O2 decomposition and this can be achieved through
carefully controlling the nanoparticle size, metal loading and metal oxidation state.
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Keywords: methane, C-H, methanol, AuPd, particle size, partial methane oxidation
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Introduction
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The direct oxidation of methane to methanol is a key challenge within the scientific
community.1,2 However, the direct transformation to methanol is complicated by the intrinsic
inertness of methane, which lacks a dipole moment and usually requires harsh conditions for
the activation of the kinetically stable C-H bond (ΔHC-H = 440 kJ mol-1).3 Current industrial
methanol production is an energy-intensive and an indirect process that converts methane to
synthesis gas before further conversion to methanol and other bulk chemicals.4,5 This process
demonstrates high selectivity towards the desired products producing 85 million metric
tonnes of methanol per annum as well as other valuable commodities.6
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A single step approach, applicable under low reaction temperature would therefore
drastically reduce processing costs for the production of methanol. Additionally, both
affordable and readily available, natural gas has the potential to be an alternative feedstock
for chemicals and high value fuels, reducing the global dependence and prolonging current
petroleum reserves. Natural gas is a viable prospect for an abundant feedstock with proven
reserves of ca.190 trillion cubic metres.7 Furthermore, with many natural gas reserves found
in remote locations, direct conversion to methanol at the point of extraction would provide a
feedstock with reduced hazards and transportation costs.
Homogeneous catalytic applications have provided important information for low
temperature approaches, active site regeneration and high selectivities. However, typically
these processes require harsh reaction conditions or sacrificial reagents.8–11 Contrastingly,
Hutchings and co-workers12 have reported the use of an environmentally benign H2O:H2O2
system, efficiently oxidising methane to methanol under low reaction temperatures (50 °C).
The use of Fe-ZSM5 was inspired by methane mono-oxygenase, which carries out a one-step
oxidation of methane to methanol under mild, aqueous conditions. The Fe- and Cu- promoted
Fe-ZSM5 catalysts demonstrated considerable activity for the oxidation of methane to
methanol, achieving turnover frequencies (TOFs) of > 2200 h-1 and methanol selectivities
above 80 %.13
Alternatively, C-H bond activation has been extensively studied using noble metal catalysts
such as AuPd.14,15 Kesavan et al. 16 reported the selective oxidation of the methyl group C-H
bonds in toluene over TiO2- and carbon- supported AuPd catalysts using O2. Demonstrating
high selectivity towards benzyl benzoate the differences in catalytic activity were attributed
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to different AuPd morphologies due to support-metal interactions. However, direct oxidation
of methane to methanol with oxygen has proved a significant challenge. Ab Rahim et al.17,18
reported on the systematic study of AuPd/TiO2 catalysts for efficient formation of methanol
from methane with H2O2. These catalysts were synthesized by wet impregnation from
chloride precursors onto the support, typically producing a broad range of particle sizes. The
catalysts were comprised of alloyed AuPd nanoparticles, with particle sizes ranging from 5-20
nm and Au-rich nanoparticles 100-200 nm in size.19 The presence of sub-nm AuPd particles
and clusters of Pd were also identified, reflecting the poor control of metal dispersion and
speciation provided by wet impregnation. Even so, these AuPd/TiO2 catalysts showed
moderate activity, demonstrating the highest methanol (49%) and oxygenate selectivity
(90%) of supports investigated, including carbon.20
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Despite exhibiting lower oxygenate formation, TOF and methanol selectivity than their Fe-
ZSM-5 counterparts, AuPd catalysts are an attractive class of catalyst because they could be
developed to generate H2O2 in situ from H2 and O2. Ab Rahim et al., applied this approach
-1
with AuPd/TiO2, reporting a productivity of 0.50 mol kgcat-1 h-1 compared to 0.116 mol kgcat
h-1 when similar amounts of preformed H2O2 (250 µmol).18 However, an improvement in
methanol selectivity (68%) and three-fold improvement in H2O2 reactivity were observed
during in situ peroxide generation.
Recently Agarwal et al.21 showed significant improvements in activity when unsupported
AuPd nanoparticles were used. Colloidal AuPd prepared using poly(vinyl pyrrolidine) showed
-1
high oxygenate productivity with preformed H2O2 (29.4 mol kgcat h-1). Notably, the
introduction of oxygen with preformed H2O2 resulted in a substantial increase in oxygenate
productivity to 53.6 mol kgcat-1 h-1. The use of 18O2 demonstrated significant incorporation of
the gas phase O2 into the methanol produced (70%). Crucially however, the rate of H2O2
decomposition was drastically reduced by elimination of a metal-support interaction and the
resulting high productivity was attributed to the controlled H2O2 breakdown.
Industrially, in situ generation of H2O2 is more desirable than using pre-formed H2O2 as the
oxidant, a necessary requirement for the use of MFI-zeolite based catalysts. In order to realise
this application, a solid understanding of the chemical reactivity of H2O2 with supported AuPd
catalysts is required. In contrast to impregnation techniques, catalyst synthesis by sol-
immobilisation (SI) facilitates the control of particle sizes, composition and morphology.22,23
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This methodology has been used to tune catalyst activity and enhance product selectivity for
several reactions, including benzyl alcohol oxidation and hydrogenation of furfural.24,25 In this
investigation, the role of AuPd particle size, support and oxidation state is considered for the
selective oxidation of methane to methanol using added H2O2. The particle size distribution
of AuPd is controlled using SI and carefully manipulated to evaluate the influence of
nanoparticle size on the effective utilisation of H2O2 and the production of methanol.
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Experimental
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Preparation of AuPd/TiO2 by sol-immobilisation
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For 0.5 wt.% Au-0.5 wt.% Pd/TiO2, aqueous solutions of HAuCl4.xH2O (Aldrich 99.9% 1.667 mL,
12.25mg mL-1) and PdCl2 (Aldrich 0.816 mL, 6 mg mL-1) were added to deionised water (800
mL) under vigorous stirring at room temperature. The resulting solution was stirred for 2 min
before the addition of freshly prepared NaBH4 (7.23 mL, 0.1 M) such that the molar ratio of
NaBH4 to metal was 5:1.16,26 Upon the addition of NaBH4, the mixture turned dark
brown/black and was then vigorously stirred for 30 min before addition of the TiO2 (P25
Degussa, 1.98 g) support. The solution was acidified to pH 1 after 1 min by drop-wise addition
of sulfuric acid (>95%) and stirred for 1 h. The suspension was then filtered under vacuum
and washed thoroughly with distilled water and then dried in a conventional oven for 16 h at
110 °C. Calcinations were performed under static air at various temperatures for 3 h (ramp
rate of 20 °C min-1), while reduction of the catalysts was carried out under flowing 5% H2/Ar
for 4 h (ramp rate = 10 °C min-1).
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A similar method was followed for the preparation of 0.065 wt.% Au- 0.065 wt.% Pd/rutile
TiO2 using aqueous solutions of HAuCl4.xH2O (Aldrich 99.9% 0.1061 mL, 12.25mg mL-1) and
PdCl2 (Aldrich 99.9% 0.2167 mL, 6 mg mL-1), with pre-calcined TiO2 (P25 Degussa, 1.9974 g)
support.
In order to help differentiate between the prepared catalysts, the following nomenclature
was adopted: “metal loading% AuPd/phase of TiO2, temperature of preparation, calcination
temperature”. For example, a 1 wt. % AuPd catalyst prepared at room temperature on P25
TiO2 and calcined at 800 °C would be abbreviated as 1%AuPd/P25-RT-800. A table of the
catalysts prepared and their abbreviations is presented in the Supporting Information (Table
S1).
Oxidation of methane using H2O2
The oxidation of methane was carried out using a 50 mL Parr stainless steel autoclave reactor.
The autoclave was fitted with a removable Teflon liner (35 mL volume).
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Reactions were carried out using a 10 mL reaction mixture comprising an aqueous solution of
H2O2 (10 mL, 0.5 M, 5000 μmol) and the catalyst (10 mg). Prior to use the reactor was purged
with methane to remove residual air before being pressurised with methane to 30.5 bar. The
autoclave was then heated to the desired reaction temperature (50 °C) using the pre-set
programme. At reaction temperature (t= 0 min), the solution was vigorously stirred at 1500
RPM and both heating and stirring were maintained for the required length (typically 30 min).
Following this, the stirring was stopped and the temperature was reduced to 10 °C using an
ice bath to minimise the loss of volatile products. Gaseous samples were removed by
extraction into a gas bag for analysis via gas chromatography. The Varian-GC was equipped
with a CP-SIL5CB column (50 m, 0.33 mm internal diameter) fitted with a methaniser and
analysed by a flame ionisation detector (FID). The reaction mixture was filtered to remove
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1
catalyst particulates and analysed by H NMR, using a Bruker 500 MHz Ultra-shield NMR
spectrometer. A sample NMR spectrum is presented in Figure S1. All 1H NMR samples were
analysed against a calibrated insert containing tetramethylsilane (TMS) in deuterated
chloroform (99.9 % D). The remaining H2O2 was determined by titration with acidified
Ce(SO4)2.
Reactions to determine the decomposition rate of H2O2 over catalyst samples
The decomposition of H2O2 was carried out in a 35 mL glass vial. A typical reaction was carried
out using 10 mL reaction mixture comprising an aqueous solution of H2O2 (10 mL, 0.5 M, 5000
μmol) and a measured amount of solid catalyst (10 mg). At intervals, H2O2 concentration was
determined by titration of 0.1 mL sample of reaction solution against an acidified Ce(SO4)2
solution of known concentration using one drop of ferroin indicator (0.025 M). The reactions
were carried out at room temperature, with stirring maintained at 1000 RPM.
Catalyst Characterisation
Powder X-ray Diffraction (XRD) patterns were collected using a PANalytical X’PertPRO X-ray
diffractometer, with Cu Kα radiation source (λ = 0.154 nm) and a Ni filter at ambient
conditions. Samples were recorded between 15-80 ° at 40 kV and 40 mA with step sizes of
0.0167°.Transmission electron microscopy (TEM) was performed on a JEOL JEM-2100
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operating at 200 kV. Samples were prepared by dispersion in ethanol by sonication and
deposited onto 300 mesh copper grids coated with holey carbon film. X-ray photoelectron
spectroscopy (XPS) was performed using a Kratos Axis Ultra DLD spectrometer, using
monochromatic Al Kα X-ray source operating at 120 W. Data was collected with pass energies
of 160 eV for survey spectra, and 40 eV for higher resolution scans. The system was operated
in the Hybrid mode, using a combination of magnetic immersion and electrostatic lenses and
acquired over an area approximately 300 × 700 µm2. A magnetically confined charge
compensation system was used to minimize charging of the sample surface, and all spectra
were taken with a 90 ° take of angle. A base pressure of ca. 1×10-9 Torr was maintained during
collection of the spectra. Each spectrum was calibrated against adventitious carbon, using the
C 1s region, to 284.8 eV and CasaXPS was used for peak fitting, with integrated areas
corrected for elemental sensitivity using modified Wagner factors as supplied by Kratos.
Experimental binding energies are reported with confidence of ±0.2 eV. To assess any
reduction of Pd(II) species during analysis, Pd(3d)/Au(4d) spectra were acquired both at the
start and again at the end of analysis, with negligible reduction of Pd(II) noted during the total
analysis period.
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Surface area measurements were performed at -196 °C on a Quantachrome Quadrasorb SI
instrument after each sample was evacuated for 1 h at 120 °C. Surface areas were calculated
using Brunauer–Emmet–Teller (BET) theory over the range P/P0 = 0.05-0.2.
Energy dispersive X-ray analysis (EDX) was performed on a Tescan Maia3 field emission gun
scanning electron microscope (FEG-SEM) fitted with an Oxford Instruments XMAXN 80 Images
were acquired using the secondary electron and backscattered electron detectors. Samples
were dispersed as a powder onto adhesive carbon Leit discs mounted onto aluminium stubs.
Results and Discussion
The effect of the nanoparticle size
To evaluate the effect of supported metal particle size, a series of 1 wt. % AuPd/TiO2 (P25)
catalysts were prepared using stabiliser-free sol-immobilisation. This technique can produce
supported catalysts with a narrow nanoparticle size distribution which can then be
manipulated with post-synthesis heat treatments.27 Furthermore, the deposition
temperature during catalyst synthesis was previously reported to affect the size of the
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nanoparticles. Rogers et al.28 previously reported an increase in particle size for poly(vinyl
alcohol) stabilised Au nanoparticles after increasing the temperature during colloidal
deposition from 25 to 70 °C. The mean particle size of resulting Au nanoparticles increased
from 2.3 nm to 3.3 nm respectively.28 Therefore, both room temperature (RT) and 70 °C were
investigated so that the effect of small changes to the nanoparticle size on the resultant
methane oxidation activity could be established. The mean particle size of 1%AuPd/P25-RT-
dried and 1%AuPd/P25-70-dried catalysts are reported in Table 1 (Entries 3 and 7). These
were calculated from representative TEM images (see supporting information Fig. S2a and
Fig. S3a) from approximately 250 particles. Increasing the catalyst preparation temperature
from RT to 70 °C increased the mean particle size from 4.0 nm to 5.2 nm, consistent with the
previously reported size increase by Rogers et al.28 The dried-only catalysts possess a particle
size distribution of 2-10 nm, as shown in Figure S2a and S3a. This profile is also consistent
with previously published reports where catalysts were prepared using stabiliser-free
methods for sol-immobilisation.29
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Initially, control reactions with no catalyst or just the support, TiO2, were carried out (Table 1,
Entries 1 and 2), where no oxygenates were observed. The small quantity of CO2 detected
was considered to be adventitious. The dried-only catalysts were then screened for methane
oxidation and the catalytic activity data is presented in Table 1 (Entries 3 and 7). Despite the
almost complete decomposition of H2O2 over both catalysts, no selective oxidation products
were detected. We consider that the small particle size of the AuPd alloy could facilitate a
rapid decomposition of the H2O2 to H2O, which would prevent the possibility of methane
reacting with active hydroperoxyl- or hydroxyl- radicals.30,31 Previously, it was shown that such
materials exhibit high rates of H2O2 productivity in the direct synthesis of H2O2, as well as high
H2O2 decomposition rates (> 100 µmol min-1).31
Larger modifications to the mean particle size were achieved through post-synthesis heat
treatments at 400, 600 or 800 °C. The catalysts calcined at 400 °C (Table 1, Entries 4 and 8)
did not produce oxygenates, despite an increase in the metal nanoparticle size to 6.5 nm and
7.5 nm respectively. As with the dried-only catalysts, there was no H2O2 remaining after the
reaction. The complete decomposition of H2O2 by the dried-only catalysts and catalysts
calcined at 400 °C is typical of materials with small particle sizes31 and explains the low
productivities observed for methane oxidation (0.054 mol kgcat-1 h-1).
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Table 1: The effect of post-synthesis heat treatment on the catalytic activity of 1wt.% AuPd/TiO2 (P25) for the oxidation of methane.[a] (Entries 3-6; room
temperature, 7-10; 70°C)
3
4
5
6
7
8
9
Total
Products [μmol]
TOF[f]
Mean
Particle
Size[h]
[nm]
H2O2
Productivity[e]
[mol kg(cat)-1 h-1] [h-1]
Oxygenate
selectivity [d]
[%]
H2O2
Remaining[g]
[%]
Heat
Treatment
Decompositi
on Rate[g][i]
[μmol min-1]
Entry
Methyl
Formic
Acid[b]
[c]
Methanol[b]
hydroperoxide CO2
[b]
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Blank
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0.24
0.20
0
0
0
0
97.6
97.4
0.3
n/a
n/a
n/a
557
199
105
99
TiO2
0.039
0.054
0.054
0.063
0.172
0.032
0.018
0.057
0.312
0
n/a
3
RT, Dried
0
0
0.27
0.27
0.16
0.17
0.16
0.14
0.16
0.16
0
0.78
0.77
0.91
2.47
0.50
0.28
0.89
4.87
4.0 ± 1.4
6.5 ± 2.1
8.3 ± 2.0
19.0 ± 5.1
5.2 ± 1.4
7.5 ± 2.0
11.1 ± 2.4
19.7 ± 6.2
4
RT, 400 °C
RT, 600 °C
RT, 800 °C
70 °C, Dried
70 °C, 400 °C
70 °C, 600 °C
70 °C, 800 °C
0
0
0
0.3
5
0.03
0.23
0
0.13
0.50
0
46.6
80.8
0
1.7
6
42.1
0.3
7
278
251
145
31
8
0
0
0
0.5
9
0
0.12
42.5
13.3
10
0.29
1.12
90.0
56.1
[a]Standard reaction conditions: time: 30 min, temperature: 50 °C, PCH4: 30.5 bar, stirring rate: 1500 rpm, all catalysts (1 wt. % total): 7.24x10-7 mol of metals equal to 10 mg for solid catalysts, volume: 10 mL of
H2O.[H2O2]: 0.5 M. Catalysts were prepared by SI at room temperature (entries 3-6) or at 70 °C (entries 7-10). Catalyst is dried at 110 °C, 10 °C min-1, 16 h, static air. Calcined at various temperatures (20 °C min-1,
3 h, static air). [b] Analysed by 1H NMR spectroscopy with 1 % TMS in CDCl3 internal standard. [c] Analysed by gas chromatography using an FID methaniser . Values obtained using CO2 calibration curve. [d]
Oxygenate selectivity calculated as (moles oxygenates/total moles of products) x 100. [e] Total productivity calculated as (moles(products) /weight(catalyst))/time). [f] TOF: Turn-over frequency, calculated as
(moles(products) / total moles(metal) ) / time (h). [g]Remaining H2O2 assayed by Ce4+ (aq.) titration. Calculated as (moles(initial)/moles(final) x100. [h] Determined by transmission electron microscopy. [i] H2O2
decomposition reaction conditions: time: 30mins, temperature: 24 °C, atmospheric pressure, stirring rate: 1000rpm, all catalysts (1 wt. % total): 7.24x10-7 mol of metals equal to 10 mg for solid catalysts, volume:
10 mL of H2O.[H2O2]: 0.5 M. Maximum experimental error; Methanol: 10%, Methyl Hydroperoxide: 16%, CO2: 22%, H2O2: 9%.
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In order to better establish the relative rate of H2O2 decomposition, a series of control
experiments at room temperature were performed. The catalysts were stirred in a dilute H2O2
solution and the decomposition rate was determined through analysis of the solution as a
function of time. The decomposition rates expressed as μmol min-1 are reported in Table 1
and the time-on-line data is shown in supporting information Figure S4 and S5. The rate of
H2O2 decomposition over 1%AuPd/P25-RT-dried was found to be 557 μmol min-1 and 278
μmol min-1 for 1%AuPd/P25-70-dried. Less than 10 % of the H2O2 was remaining after 30 min
and indicates that the small AuPd particles are capable of rapidly decomposing H2O2.
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Following heat treatment at 600 °C (Table 1, Entries 5 and 9) the particle sizes of the catalysts
increased to ca. 10 nm, where the onset of catalytic activity was observed, albeit with low
productivity and relatively high H2O2 decomposition rates. The rate of H2O2 decomposition
decreased to ca. 100 µmol min-1 over these catalysts, although just 1.7 % of H2O2 remained
after the oxidation reaction. The rate of H2O2 decomposition over catalysts with metal
nanoparticles less than 8 nm in diameter is unfavourably high and does not facilitate the
production of methanol.
Heat treatment at 800 °C (Table 1, Entries 6 and 10) resulted in a significant growth of the
mean nanoparticle size to ca. 19 nm in 1%AuPd/P25-RT-800 and 1%AuPd/P25-70-800
catalysts, which was matched by a large improvement in catalyst productivity (0.172 mol and
0.312 kgcat-1 h-1 respectively), as well as oxygenate selectivity and remaining H2O2. The H2O2
decomposition rates of the 1%AuPd/P25-RT-800 and 1%AuPd/P25-70-800 were found to be
ca. 100 and 30 µmol min-1 respectively, which emphasises that larger nanoparticles are
required to ensure a sufficient concentration of reactive oxidising radicals, which facilitates
the activation of methane.
Particle size distributions for both 1%AuPd/P25-RT-800 and 1%AuPd/P25-70-800 (Fig. S1d
and S2d) demonstrate quite similar profiles and average particle sizes: 19.0 ± 5.1 and 19.7 ±
6.2 nm, respectively. Upon closer inspection, however, the presence of nanoparticles below
9 nm in the 1%AuPd/P25-RT-800 catalyst were observed, in contrast to 1%AuPd/P25-70-800.
The presence of these sub-10 nm particles could be a contributing factor in the lower activity
observed with 1%AuPd/P25-RT-800, as these particles demonstrably catalyse the deleterious
decomposition of H2O2, however the number of sub-10 nm metal nanoparticles in
1%AuPd/P25-RT-800 represents a small proportion of the total population of supported
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nanoparticles. Another possible explanation concerns the standard deviation of the average
particle size measurements, which suggests that the actual differences in the supported
nanoparticle sizes are greater than suggested by the average value. There are, however, clear
trends associated with the average particle sizes, the rates of H2O2 decomposition and the
quantity of reaction products. These relationships are illustrated in Figure 1 a), b) and c).
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Fig. 1. The structure-activity relationship of 1 wt. % AuPd/ TiO2 (P25) for methane oxidation: a) Lower H2O2
decomposition rates are consistent with the larger quantities of products after the 0.5 h reaction; b) Lower H2O2
decomposition rates lead to a higher productivity after the 30 min reaction; c) Larger mean particle sizes result
in higher productivities. ● = RT and ◊ = 70 °C.
The oxidation state of Pd has previously been demonstrated to affect the rate of H2O2
decomposition32,33. XPS was therefore used to investigate a potential link between H2O2
decomposition rates (and therefore oxygenate production) and the oxidation state of Pd in
the samples. The corresponding Au 4d and Pd 3d spectra are illustrated in Figure S7 and S8,
with elemental analysis provided in Table S2. It is evident that the preparation method,
involving NaBH4 as the reducing agent, leads to the formation of the reduced metal species
only. Expectedly, calcination of the catalysts resulted in the formation of oxidised species,
with only Pd2+ present after calcination at 400 °C (Fig. S7 and S8) (Table S2, Entries 2-4 and 6-
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8). Overall, the XPS analysis of Pd 3d region of the RT and 70 °C prepared samples was similar
and the oxidation state of the Pd was not affected by the temperature of the preparation
method. In order to confirm the metal loadings and Au:Pd ratio, elemental analysis was
carried out on the dried samples using MP-AES after digestion of the catalysts using aqua
regia. Due to inefficient digestion of TiO2 support when using aqua regia, complementary
SEM-EDX analysis was also carried out. The measured metal loadings are presented in Table
S2 and are very close to that of the nominal values, although the 1%AuPd-P25-70-800 had a
lower loading (0.89 wt.%) than 1%AuPd-P25-RT-800 (0.97 wt.%). Additionally, these SEM-EDX
measurements were also consistent with the MP-AES data. The lower loading of metal in
1%AuPd-P25-70-800 could actually be beneficial as there are fewer sites to decompose H2O2
in the system. It was previously reported that increasing the mass of 5 wt. % AuPd/TiO2
catalysts could lower the production of methanol.18 The importance of the concentration of
metal is discussed below. TOFs were calculated according to the total metal loading
determined using MP-AES (Table 1). The most active catalyst, 1%AuPd-P25-70-800, exhibited
a TOF of 4.87 h-1, comparable to those previously reported.20
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The effect of the support phase
The increase in oxygenate productivity noted when the 1 wt. % AuPd/TiO2 (P25) catalysts
were calcined at 800 °C was ascribed to the increase in mean AuPd particle size, which
prevented the deleterious decomposition of H2O2. However, the P25 TiO2 support also
underwent a phase transition to rutile, as evidenced by the characteristic XRD reflections (Fig.
S6). Therefore, an analogous series of catalysts were prepared on rutile TiO2. Nitrogen
adsorption measurements indicated that the BET surface area decreased from 45 m2 g-1 for
the P25 material to 6 m2 g-1 for the rutile material. Catalysts were prepared with 1 wt.% AuPd
at both room temperature and at 70 °C for consistency with the previous samples and were
tested for the selective oxidation of methane with H2O2 (Table 2) after the same heat
treatments. The XRD patterns of the supported rutile catalysts are shown in Fig. S9, and
indicate that no major phase changes occur to the catalyst support. Consistent with the
results in Table 1, selective methane oxidation over 1 wt.% AuPd/rutile was not observed for
the dried-only catalysts prepared at RT or 70 °C (Table 2, Entries 2 and 6). These catalysts
exhibited high H2O2 decomposition activity (Fig. S12 and S13), which we consider is due to the
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particle size and presence ofmetallic Pd in the AuPd nanoparticles. Analysis of the rutile-
supported catalysts by TEM revealed an interesting phenomenon that occurs during
preparation (Fig. S10 and S11). Deposition of stabiliser-free AuPd sols onto the rutile TiO2
support resulted in particle agglomerates on the dried-only catalyst, rather than discrete
nanoparticles. Heat treatment of these parent catalysts subsequently resulted in the
preferential formation of distinct, hemi-spherical nanoparticles, which increase in size with
increasing heat treatment temperatures (Fig. S10 and S11). Interestingly, the AuPd/rutile
catalysts calcined at 400 °C exhibited selective catalytic activity towards oxygenates (Table 2,
Entries 3-5 and 7-9), in contrast to the analogous catalysts supported on P25. After calcination
at 800 °C, the mean particle size increased to 19 - 23 nm, consistent with the catalysts
supported on the untreated TiO2 support.
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Table 2: The effect of support pre-calcination for 1wt.% AuPd/rutile TiO2 prepared at room temperature (Entries 2-5) and at 70 °C (Entries 6-9)[a]
1
2
3
4
5
6
7
8
9
Total
H2O2
Decomposition
Rate
Products [μmol]
Productivity TOF[f]
Oxygenate
selectivity
[d] [%]
H2O2
Remaining[g]
[%]
Mean Particle
Size[h]
Heat
[e]
Entry
Treatment
-1
[nm]
Formic
Methyl
[mol kg(cat)
[μmol min-1]
[c]
Methanol[b]
CO2
[h-1]
Acid[b] hydroperoxide[b]
h-1]
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1
2
3
4
5
6
7
8
9
rutile
0
0
0
0
0
0
0
0
0
0
0
0.33
0.26
0.29
0.31
0.18
0.27
0.15
0.15
0.16
0
0.040
0.052
0.109
0.176
0.163
0.053
0.214
0.161
0.243
0
97.5
0.2
2.2
4.5
2.1
0.2
1.4
23.8
7.8
n/a
n/a
n/a
830
251
147
317
530
236
243
158
RT, Dried
0
0
0
0.73
1.51
2.45
2.27
0.83
3.35
2.53
3.80
RT, 400 °C
RT, 600 °C
RT, 800 °C
70 °C, Dried
70 °C, 400 °C
70 °C, 600 °C
70 °C, 800 °C
0.26
0.22
0.45
0
0
47.9
65.4
83.4
0
8.7 ± 3.9
10.4 ± 2.3
19.6 ± 5.9
n/a
0.36
0.21
0
0.36
0.30
0.38
0.56
0.73
0.73
86.2
87.1
86.5
8.0 ± 2.4
10.8 ± 2.9
22.6 ± 10.7
[a]Standard reaction conditions: time: 30 min, temperature: 50 °C, PCH4: 30.5 bar, stirring rate: 1500 rpm, all catalysts (1 wt. % total): 7.24x10-7 mol of metals equal to 10 mg for solid catalysts, volume: 10 mL of
H2O.[H2O2]: 0.5 M. TiO2 support pretreated by calcination at 800 °C prior to use in catalyst preparation (denoted rutile). Catalysts were prepared by SI at room temperature (entries 2-5) or at 70 °C (entries 6-9). Catalyst
is dried at 110 °C, 10 °C min-1, 16 h, static air. Calcined at various temperatures (20 °C min-1, 3 h, static air). [b] Analysed by 1H NMR spectroscopy with 1 % TMS in CDCl3 internal standard. [c] Analysed by gas
chromatography using an FID methaniser. Values obtained using CO2 calibration curve. [d] Oxygenate selectivity calculated as (moles oxygenates/total moles of products) x 100. [e] Total productivity calculated as
(moles(products) /weight(catalyst))/time). [f] TOF: Turn-over frequency, calculated as (moles(products) / total moles(metal) ) / time (h). [g]Remaining H2O2 assayed by Ce4+(aq.) titration. Calculated as
(moles(initial)/moles(final) x100. [h] Determined by transmission electron microscopy.[i] H2O2 decomposition reaction conditions: time: 30mins, temperature: 24 °C, atmospheric pressure, stirring rate: 1000 rpm, all
catalysts (1 wt. % total): 7.24x10-7 mol of metals equal to 10 mg for solid catalysts, volume: 10 mL of H2O.[H2O2]: 0.5 M. Maximum experimental error; Methanol: 11%, Methyl Hydroperoxide: 16%, CO2: 22%, H2O2: 10%.
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However, the concentration of H2O2 remaining is inconsistent with our earlier observations
where the presence of larger AuPd particles resulted in high percentage of H2O2 remaining
(Table 1). For example, the 1%AuPd-rutile-RT-800 catalyst had a mean particle size of ca. 20
nm and exhibited high oxygenate selectivity, but with < 10 % H2O2 remaining (Table 2). The
rate of H2O2 decomposition of the calcined catalysts was investigated and found to be
between 150 and 317 µmol min-1 for the catalysts prepared at room temperature and 70 °C,
respectively (Table 2, entries 3-5 and 7-9, and Fig. S12 and S13). The surface composition, as
determined by XPS, indicates that between 40-60% of the Pd is present as Pd0 (Fig. S14 and
S15, and Table S3), whereas the P25 supported catalysts were composed entirely of Pd2+.
Previous studies on the direct synthesis of hydrogen peroxide have demonstrated Pd0 leads
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to higher rates of H2O2 decomposition compared to Pd2+ 34 The expected effect on catalytic
.
activity would be detrimental and result in inefficient utilisation of H2O2. To observe the effect
of Pd oxidation state on the catalytic activity, two samples of 1%AuPd/P25 were heated under
a reducing environment (5% H2/Ar) at 400 or 800 °C and screened for methane oxidation. The
results of these tests are presented in Table S4 and show almost complete consumption of
H2O2, and almost no catalytic activity. These data confirm that Pd0 promotes H2O2
decomposition at the expense of methane oxidation activity.
The large mean particle sizes and the presence of Pd0 observed with the heat-treated catalysts
supported on rutile TiO2 suggest that this combination can facilitate both the selective
oxidation of methane whilst exhibiting an increased rate of H2O2 decomposition. This
combination however does not facilitate efficient utilisation of H2O2. This is evident in Fig. 2,
which depicts comparable relationships to those plotted in Fig. 1. The mean particle size of
the rutile-supported catalysts does not correlate well with H2O2 decomposition or total
products, as the Pd oxidation state is evidently an important parameter in determining these
rates. Analogous elemental analysis using MP-AES and EDX was carried out on the rutile
supported catalysts to confirm the expected metal loadings (Table S3). In each case, the metal
loadings are close to the nominal loadings, although the catalysts prepared at 70 °C had a
marginally lower total metal loading (0.91 wt. %), consistent with the P25-supported
catalysts, suggesting that the interaction of the nanoparticles and the support is less
favourable at elevated temperatures. Additionally, the increased performance of the
1%AuPd-rutile-70-800 compared with 1%AuPd-rutile-RT-800 could, in part, be due to the
lower concentration of metal. Not only does this increase the TOF, but the lower metal
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loading is also consistent with higher productivity, which as suggested above, could
contribute to suppressing the deleterious decomposition of H2O2.
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Fig. 2. The structure-activity relationship of 1 wt. % AuPd/ TiO2 (rutile) for methane oxidation: a) In contrast to
the P25-supported catalyst, there is not a clear correlation between the H2O2 decomposition rate and the mean
particle size; b) The H2O2 decomposition rate correlates well with the total products after the 0.5 h reaction: c)
The correlation between mean particle size and total products for catalysts prepared on rutile is poor, indicating
that there are other factors affecting the activity. ● = RT and ◊ = 70 °C.
The effect of surface area: metal loading ratio
The reduction in the surface area caused by the high temperature phase-change at 800 °C
resulted in an increase in the supported metal to surface area ratio. Therefore, the metal
loading on rutile TiO2 was reduced from 0.167 %AuPd m-2 to the equivalent loading of 0.022
wt. % AuPd m-2, which is consistent with the metal:surface area of the 1 wt. % AuPd/P25 TiO2.
In this case, a nominal 0.13 wt. % loading of AuPd was used to prepare catalysts at room
temperature on rutile TiO2. As expected, the mean AuPd particle sizes of the 0.13 wt. % rutile
TiO2 catalysts were comparable to the 1 wt. % AuPd/P25 TiO2 catalysts (Table 1) although
interestingly, the dried catalyst featured nanoparticles that had formed agglomerates as was
observed in the 1 wt.% AuPd/rutile TiO2 catalysts. Fig. S16 illustrates the observed
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agglomeration, and similarly shows the formation of hemispherical nanoparticles after heat
treatment at 400 °C. The 0.13 wt. % AuPd catalysts were active after calcination at 400 °C
(Table 3), where the mean particle size was 6.5 nm. Furthermore, the rate of H2O2
decomposition for all heat-treated catalysts was low and in general, > 90 % remained after
the 30 min reaction. The most likely explanation for this difference in catalytic activity is that
the 0.13 wt. % AuPd catalyst had significantly less metal present per moles of reactant and,
therefore, the rate of H2O2 decomposition was lower. The total productivity was calculated
to be 0.68 mol kgcat-1 h-1, which corresponded to a turnover frequency of 103 h-1, significantly
higher than the 1 wt. % AuPd catalysts (2-4 h-1) and any previously reported supported-AuPd
bimetallic catalyst for this reaction.18,20 As above, the TOFs were calculated according to the
MP-AES measurements, which indicated the total metal loading to be 0.11 wt.%, although at
these very low loadings the limit of reliable quantification is approached.
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Table 3: The effect of reduced metal loading on pre-calcined support for the oxidation of methane (0.13 wt.% AuPd/rutile TiO2) [a]
Total
Mean
Particle
Size[h]
[nm]
H2O2
Products [μmol]
TOF [f]
[h-1]
7.00
23.0
35.5
103
Oxygenate
selectivity[d]
[%]
H2O2
Remaining[g]
[%]
Productivity[e]
Heat
Decomposition
Entry
Treatment
Rate[g][i]
Formic
Methyl
[c]
Methanol[b]
CO2
[mol kg(cat)-1 h-1]
[μmol min-1]
Acid[b]
hydroperoxide[b]
1
Dried
400 °C
600 °C
800 °C
0
0
0
0.23
0.25
0.33
0.27
0
0.046
8.9
n/a
215
10.7
25.9
21.4
15
16
17
18
19
20
21
22
2
3
4
0.06
0.13
0.32
0.00
0.00
0.19
0.41
0.86
2.65
64.8
75.0
90.7
0.152
55.1
75.5
64.3
6.5 ± 1.7
9.3 ± 3.2
19.1 ± 4.5
0.234
0.677
[a]Standard reaction conditions: time: 30 min, temperature: 50 °C, PCH4: 30.5 bar, stirring rate: 1500 rpm, all catalysts (1 wt. % total): 9.41x10-8 mol of metals equal to 10 mg for solid catalysts, volume: 10 mL of H2O.[H2O2]:
23
0.5 M. TiO2 support pretreated by calcination at 800 °C prior to use for catalyst preparation (denoted rutile TiO2). Catalyst prepared by SI at 70 °C. Catalyst is dried at 110 °C, 10 °C min-1, 16 h, static air. Calcined at various
temperatures (20 °C min-1, 3 h, static air). [b] Analysed by 1H NMR NMR spectroscopy with 1% TMS in CDCl2 internal standard. [c] Analysed by gas chromatography using a FID methaniser. Values obtained using CO2
calibration curve. [d] Oxygenate selectivity calculated as (moles oxygenates/total moles of products) x 100. [e] Total productivity calculated as (moles(oxygenates) /weight(catalyst))/time). [f] TOF: Turn-over frequency,
calculated as (moles(products) / total moles(metal) ) / time (h). [g]Remaining H2O2 assayed by Ce4+(aq.) titration. Calculated as (moles(initial)/moles(final) x100. [g] Determined by transmission electron microscopy. [h] [i]
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H2O2 decomposition reaction conditions: time: 30mins, temperature: 24 °C, atmospheric pressure, stirring rate: 1000rpm, all catalysts (1wt. % total): 7.24x10-7 mol of metals equal to 10 mg for solid catalysts, volume: 10 mL
of H2O.[H2O2]: 0.5 M. Maximum experimental error; Methanol: 13%, Formic Acid 2%: , Methyl hydroperoxide: 15%, CO2: 20%, H2O2: 9%.
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The rate of H2O2 decomposition at room temperature was much lower after calcination (Table
3, Fig. S17), as expected based on the methane oxidation reactions and the lower
concentration of metal in the reaction. Over each catalyst, the rates were measured at < 26
μmol min-1 despite the presence of ca. 50 % Pd0, as shown from XPS analysis (Table S5 and
Fig. S18). This is consistent with the 1 wt.% AuPd/rutile TiO2 catalysts that also featured high
concentrations of Pd0 after calcination, suggesting that rutile stabilises Pd0 (or inhibits Pd
oxidation). In addition, the XRD patterns of the 0.13 wt.% AuPd catalysts exhibited the same
reflections as those in the 1 wt% AuPd/rutle catalysts (Fig. S19). The reduction in metal
loading also decreased the concentration of H2O2 decomposing metal, which facilitates a
slower turnover of H2O2 and a higher methane oxidation activity. This observation supports
the work of Han35 et al. and Landon36 et al. who demonstrated the proportional increase in
H2O2 hydrogenation with increasing catalyst mass. The results from the present work
indicates that the concentration of metal is an important parameter in determining methane
oxidation activity in AuPd catalysts. This relationship is illustrated in Fig. 3, demonstrating the
dependence of TOF as a function of the actual metal loading, as determined from MP-AES.
Catalysts calcined at 800 °C were selected due to a comparable metal nanoparticle size of ca.
20 nm. Further inspection (Fig. 3, Inset) shows metal loadings closer to 1 wt. % where it is
evident that a lower metal loading is conducive to an improved TOF of methanol and is likely
caused by a decrease in the rate of deleterious H2O2 decomposition. These findings further
underline the sensitivity of this reaction to the properties of the catalyst.
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Fig. 3. The relationship between the actual metal loading determined using MP-AES and the TOF for a series of
AuPd/TiO2 catalysts calcined at 800 °C and with similar average particle sizes (ca. 20 nm). Inset: Close-up of the
graph where the metal loading is between 0.88 - 1 wt.%.
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Conclusions
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The influence of nanoparticle size and Pd oxidation state on the selective methane oxidation
using AuPd/TiO2 has been systematically investigated. Using stabiliser-free sol-
immobilisation, the initial particle size distribution was controlled and modification of the
deposition temperature allowed small differences in the initial particle size to be induced.
These dried-only catalysts were not active for methane oxidation due to their ability to rapidly
decompose H2O2. Calcining the catalysts at 400, 600 and 800 °C resulted in significant
increases in the mean particle sizes, which yielded improvements in the H2O2 stability. In
catalysts with larger than ca. 10 nm metal nanoparticles, the production of oxygenates was
observed. After calcination at 800 °C the P25 TiO2 transformed from approximately 80%
anatase and 20% rutile to 100% rutile. To understand the influence of the support phase,
catalysts were prepared using calcined P25 TiO2, which produced a pure rutile phase. The
stabilisation of Pd0 in these catalysts, even after high temperature calcination, resulted in
rapid H2O2 decomposition. The metal loading was proportionally lowered to account for the
lower surface area of the rutile catalyst compared to the P25 TiO2. These 0.13 wt.%
AuPd/rutile TiO2 catalysts exhibited extremely high activity, particularly when calcined at 800
°C: The TOF of 0.13%AuPd/rutile-RT-800 was 103 h-1. This is significantly higher than
previously reported AuPd catalysts. We consider that the low concentration of metal in the
low-loaded catalysts limited the H2O2 decomposition to a favourable rate. Therefore, while
the supported nanoparticle size is an important parameter, it is not the only consideration
that must be made in developing highly active, efficient heterogeneous catalysts for the
selective oxidation of methane to methanol using supported AuPd nanoparticles. The size,
oxidation state and concentration of supported nanoparticles must be controlled to maximise
the efficiency and performance of the resulting catalyst.
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Acknowledgements
The authors wish to thank ExxonMobil for financial support and the Cardiff University electron
microscopy facility for the transmission (TEM) and scanning electron microscopy (SEM).
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Associated Content
Summary of prepared catalysts, typical 1H NMR spectrum of post-reaction reaction solution,
TEM, H2O2 decomposition reaction data, XRD, XPS, MP-AES and SEM-EDX analysis of catalysts.
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