Gold-palladium colloids as catalysts for hydrogen peroxide synthesis, degradation and methane oxidation: Effect of the PVP stabiliser

Article abstract of DOI:10.1039/d0cy00915f

The reactivity of AuPd nanoparticle catalysts prepared by sol immobilisation is often explained by a structure activity relationship based solely on particle size or composition. In this contribution, we compare colloidal AuPd nanoparticles stabilised with polyvinylpyrrolidone (PVP) with the same AuPd nanoparticles supported on TiO2 for the direct synthesis of hydrogen peroxide and methane oxidation to methanol. We show that while the particles have similar rates of H2O2 synthesis, supporting the particles can affect the rates of H2O2 decomposition and hence the effectiveness of the catalyst for reactions which rely on H2O2 as an initiator or oxidant. We demonstrate that the absence of PVP results in high rates of H2O2 decomposition in methane oxidation experiments but this can be minimised by the addition of PVP to the reactor. These results also show that for AuPd alloys, both polymer stabiliser and support effects need to be taken into account when describing the activity of the nanoparticles and the active sites should in fact be thought of as a metal-support-polymer interface with many degrees of freedom. This journal is

Full text of DOI:10.1039/d0cy00915f

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Gold–palladium colloids as catalysts for hydrogen
peroxide synthesis, degradation and methane
oxidation: effect of the PVP stabiliser†
Cite this: DOI: 10.1039/d0cy00915f
*
Simon J. Freakley,
a Nishtha Agarwal,b Rebecca U. McVicker,b
b
Sultan Althahban,cd Richard J. Lewis, b David J. Morgan,
Nikolaos Dimitratos,e
b
Christopher J. Kielybc and Graham J. Hutchings
*
The reactivity of AuPd nanoparticle catalysts prepared by sol immobilisation is often explained by a
structure activity relationship based solely on particle size or composition. In this contribution, we
compare colloidal AuPd nanoparticles stabilised with polyvinylpyrrolidone (PVP) with the same AuPd
nanoparticles supported on TiO2 for the direct synthesis of hydrogen peroxide and methane oxidation
to methanol. We show that while the particles have similar rates of H2O2 synthesis, supporting the
particles can affect the rates of H2O2 decomposition and hence the effectiveness of the catalyst for
reactions which rely on H2O2 as an initiator or oxidant. We demonstrate that the absence of PVP results
in high rates of H2O2 decomposition in methane oxidation experiments but this can be minimised by
the addition of PVP to the reactor. These results also show that for AuPd alloys, both polymer stabiliser
and support effects need to be taken into account when describing the activity of the nanoparticles
and the active sites should in fact be thought of as a metal–support–polymer interface with many
degrees of freedom.
Received 5th May 2020,
Accepted 6th August 2020
DOI: 10.1039/d0cy00915f
rsc.li/catalysis
remove the strongly bound polymer.5–7 Therefore, in reality
the reported performance of the catalyst material not only
Introduction
The preparation of supported precious metal catalysts by
colloidal nanoparticle synthesis methods can result in highly
depends on the nature of the metal particle (size, shape,
oxidation state) but also the nature of the composite particle–
polymer surface.
active materials for
a
range of important chemical
processes.1,2 The advantage of solution phase synthesis of
metal nanoparticles is the high degree of control over the
particle size, shape and composition that can be achieved
before immobilisation onto high surface area supports.3
Typically, colloidal methods involve fast chemical reduction
of metal precursors and steric stabilisation of the growing
nanoparticles by polymer additives.4 In many cases these
catalysts are used without high temperature oxidative heat
treatments or washing protocols specifically designed to
Reaction rates, product selectivity and enantioselectivity can
all be altered by the interaction of the polymer ligand or
organic modifier with the metal surface.8 For instance, the
introduction of chiral modifiers such as cinchonidine
derivatives or napthyl-ethylamine to Pt catalysts have been
shown to impart increased enantioselectivity in activated
ketone hydrogenation.9–11 Polymer additives can also be used
to tune the oxidation states of metal surfaces through binding
of electron donating or withdrawing groups which in turn
effects reactant binding strengths.12 Steric effects can also be
observed in many cases with changes in reaction selectivity
observed in the presence of polymer additives.13 Despite these
potential advantageous degrees of control available by polymer
addition, detrimental effects such as site blocking by strong
adsorption can reduce activity – especially in gas phase
reactions.14–16 This requires careful removal of polymer
additives prior to reaction without significantly changing the
nanostructure of the catalyst material which has been carefully
controlled in the colloidal synthesis of the nanoparticles.
Polyvinylpyrrolidone (PVP) is a common polymer additive
used in the preparation of precious metal catalysts for
a Department of Chemistry, University of Bath, Claverton Down, Bath, BA2 7AY,
UK. E-mail: s.freakley@bath.ac.uk
b Cardiff Catalysis Institute and School of Chemistry, Main Building, Park Place,
Cardiff, CF10 3AT, UK. E-mail: hutch@cardiff.ac.uk
c Department of Materials Science and Engineering, Lehigh University, 5 East
Packer Avenue, Bethlehem, PA 18015, USA
d Department of Mechanical Engineering, Jazan University, Jazan 82822, Saudi
Arabia
e Department of Industrial Chemistry, Alma Mater Studiorum-University of Bologna,
Viale Risorgimento, 40136, Bologna, Italy
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
d0cy00915f
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reactions such as glycerol oxidation using Au and Pd based Results and discussion
catalysts supported on TiO2.17–20 We recently demonstrated
that while effective catalysts could be prepared in the absence
of additives, the presence of PVP and polyvinylalcohol (PVA)
has significant effects on product selectivity which is often
not considered in structure activity relationships.21,22 Similar
catalyst structures based on Au–Pd particles have been shown
to be highly active for reactions such as the direct synthesis
of hydrogen peroxide (H2O2) from molecular hydrogen and
oxygen which would represent an attractive alternative to the
current indirect anthraquinone process for on-site H2O2
production.23–27 Catalysts with varying alloy compositions
and nanostructure have been extensively studied for this
reaction and have been shown to be highly active for H2O2
synthesis.27 Titania supported Au–Pd catalysts prepared by
colloidal methods with small particle sizes tend to have
significant activity for H2O2 degradation by over
hydrogenation and decomposition.28 To date, the challenge
of minimising the subsequent degradation of H2O2 (via
hydrogenation and decomposition pathways) has only been
achieved with a few specific catalyst systems in the absence
of acid and halide additives.29–31
Recent studies by Giorgianni et al. reported that the addition of
PVA to Pd catalysts results in enhanced H2O2 yields due to
hindered H2O2 back diffusion to the metal surface via
hydrophobic interactions in addition to greater observed catalyst
stability when PVA is present.38 In this current study on H2O2
synthesis and methane oxidation, we focus on PVP as an
additive because in methane oxidation reactions traces of MeOH
have been previously observed as a result of PVA contamination
or oxidative degradation under our reaction conditions. A series
Au–Pd colloidal nanoparticle solutions were synthesised
according to previously described procedures.37 The colloidal
solutions of Au–Pd nanoparticles (Au : Pd = 1 : 1 by moles) were
synthesised via NaBH4 reduction of metal salts and stabilised by
PVP. A portion of the AuPd–PVP colloidal nanoparticles were
immobilised onto TiO2 (1 wt% metal loading) and dried at 110
°C to provide comparative supported samples. Both the
supported and unsupported AuPd–PVP materials were then
tested for the direct synthesis of H2O2 and methane oxidation,
using experimental conditions which have both been previously
described in the literature.28,37
Unsupported colloidal nanoparticles can be active for a
number of reactions including glucose oxidation,32 aromatic
alcohol oxidation33 and CO2 reduction.34 Nomura et al. have
previously reported that unsupported AuPd colloidal particles
can be active for the direct synthesis of H2O2 from H2 and
O2.35 Recent studies by Deguchi et al. have shown that a
range of bimetallic colloidal particles stabilised with PVP,
including Pd–Ir and Pd–Pt, can be active catalysts – however
the role of the polymer additive in all these cases has not
been explicitly addressed.36 Recently we demonstrated that
using colloidal AuPd nanoparticles stabilised with PVP, it was
possible to achieve selective oxidation of methane to
methanol at 25–50 °C with O2 incorporation using H2O2 as
an initiator.37 A significant result in this study showed that
the background H2O2 decomposition rate was minimal for
unsupported colloidal AuPd–PVP particles at 50 °C, allowing
the radical reaction process to propagate rather than
terminate; however the H2O2 decomposition rate increased
significantly when then the same particles were supported on
TiO2 making this an ineffective catalyst.37
This result clearly demonstrated that colloidal particles
stabilised by polymers can have significantly different
behaviour in reactions involving H2O2 than analogous ‘bare’
particles supported on TiO2 and while colloidal catalyst
systems could have limitations in terms of long term stability
it is possible to use them to propose structure–activity
relations. In this study, we aim to elucidate if the PVP
stabilisers present on AuPd particles effect the H2O2
synthesis and methane oxidation reaction networks and
investigate if the reactivity in these systems should be
considered as not only a result of interactions with the
surface atoms on the metal nanoparticle, but rather with an
entity more akin to a metal core–polymer shell composite.
The colloidal metal nanoparticles and the supported
colloidal nanoparticles have been previously characterised by
X-ray photoelectron spectroscopy.37 XPS analysis of the
colloidal AuPd–PVP nanoparticle solution in the Au(4f) and
Pd(3d)/Au(4d) regions, after drop casting onto a solvent
cleaned silicon wafer, showed that both elements were
present in predominantly the metallic state (Au(4f) ∼ 83–84
eV, Pd(3d) ∼ 334.5–335.5 eV, the range in energies varying
depending on particle size and support interactions) as could
be expected from the presence of the strong reducing agent
at the high molar ratio used in the preparation. For the
unsupported colloid, a Au signal at 85.5 eV and a Pd signal
(ca. 388 eV) were also detected and attributed to residual or
leached metal chlorides. XPS analysis of the immobilised
nanoparticles exhibit both Au and Pd binding energies ca. 1
eV lower compared to the unsupported colloid. Such low Pd
binding energies may be attributed to a particle–support
interaction making the zero-valent supported Pd more
electron rich. The corresponding Au(4f7/2) signal exhibits a
binding energy of 83.1 eV and lower than the characteristic
84 eV for bulk Au, although such binding energies have been
widely reported, and are typically attributed to small, low
coordination atoms, charge transfer from PVP to Au, and
charge transfer between Pd and Au, increasing the s-state
occupancy of Au indicating alloy formation.39,40
TEM analysis (Fig. 1a and b) showed that the mean size of
the bimetallic particles in the colloidal solution was 3.0 2.0
nm, which increased slightly to 4.1
1.3 nm when the
particles were immobilised onto the support. This is in good
agreement with our previous studies on these materials
where the apparent increase in size occurs as a result of the
AuPd nanoparticle flattening slightly and faceting to form an
interface with the TiO2 support (Fig. 1c and d).28
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Fig. 1 BF-TEM images and corresponding particle size distributions of (a) colloidal and (b) TiO2 supported Au–Pd nanoparticles prepared by sol-
immobilisation using PVP as the stabiliser. (c) HAADF image of Au–Pd–PVP colloidal particles drop cast onto a C TEM gird. (d) HR-TEM phase
contrast lattice image of a typical AuPd–PVP particle supported on TiO2.
Previous studies have demonstrated that colloidal Pd and
AuPd–PVP nanoparticles, in the presence of strong acid and
halide additives, can produce H2O2 via the direct reaction of
H2 and O2.41 These un-supported particles were stabilised by
PVP and were shown to exhibit appreciable rates for direct
H2O2 synthesis suggesting that the presence of a support is
not essential for high activity in this reaction. We carried out
comparative tests between our colloidal AuPd–PVP materials
in the unsupported and TiO2 supported state ensuring that
each reaction had the equivalent moles of metal (0.66 μmol).
The results, shown in Table 1, entries 1–3, demonstrate that
over a 30 min reaction period the unsupported colloidal Au–
Pd–PVP nanoparticles were capable of producing 0.10 wt%
H2O2 compared to 0.07 wt% H2O2 when using TiO2
supported counterparts; their corresponding apparent
turnover frequencies (TOFs) based on total moles of metal
−1
were 8.0
×
102 and 5.2
×
102 molH O molmetal
h1
2
2
respectively. The TiO2 support alone showed no background
activity towards H2O2 synthesis or H2O2 degradation (via
over-hydrogenation or decomposition). Next, the amount of
AuPd–PVP colloidal nanoparticle solution used was varied to
identify the kinetic regime in which the reaction was
operating. Fig. 2 shows that increasing the amount of Au–Pd
colloidal nanoparticle solution added to the reaction results
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Table 1 Summary of catalytic testing results for the various AuPd–PVP catalyst systems tested for H2O2 synthesis, degradation and decomposition
H2O2
Apparent reaction rate at 30 min
(molH O molmetal−1 h−1 (molH O molsurface−1 h−1 d
H2O2 degradationb
(%)
H2O2 decomp.c
(%)
a
PVP:
M
Entry
Catalyst
(wt%)
)
)
2
2
2
2
1
2
3
4
5
6
7
8
AuPd colloid
1% AuPd/TiO2
TiO2
1% AuPd/C
AuPd colloid
AuPd colloid
AuPd colloid
AuPd colloid
1.2
1.2
0
1.2
0.005
0.1
20
0.10
0.07
0.00
0.13
0.08
0.11
0.12
0.07
8.0 × 102
5.2 × 102
—
2.7 × 104
3.2 × 104
—
13
11
0
9
9
11
13
11
1
7
0
1
0
1
1
7
10 × 103
6.2 × 102
8.5 × 102
9.1 × 102
5.2 × 102
—
2.8 × 104
2.6 × 104
2.7 × 104
—
0
a
H2O2 synthesis conditions: 5% H2/CO2 (29 bar) and 25% O2/CO2 (11 bar), 8.5 g solvent (2.9 g HPLC 5.6 g MeOH), 0.66 μmol metal, reaction
b
temperature = 2 °C, stirring rate = 1200 rpm, reaction time = 30 min. H2O2 degradation conditions: 5% H2/CO2 (29 bar), 8.5 g solvent (5.6 g
MeOH, 2.22 g H2O and 0.68 g 50% H2O2), 0.66 μmol metal, reaction temperature = 2 °C, stirring rate = 1200 rpm, reaction time = 30 min.
c
H2O2 decomposition conditions: 25% O2/CO2 (29 bar), 8.5 g solvent (5.6 g MeOH, 2.22 g H2O and 0.68 g 50% H2O2), 0.66 μmol metal,
reaction temperature = 2 °C, stirring rate = 1200 rpm, reaction time = 30 min. d As determined from measured TEM particle size distributions.
in a linear increase in the amount of H2O2 produced,
reaching 0.29 wt% after 30 min reaction, which was the point
where the 2.9 mL H2O used as solvent was completely
replaced by the aqueous AuPd–PVP colloidal solution. This
result indicated that no external gas–liquid diffusion
limitations were occurring when increasing amounts of the
colloidal catalyst were used in the reaction. This finding is in
contrast to our previous studies using TiO2 supported AuPd
catalysts where a plateau is observed in H2O2 yield with
increasing catalyst mass due to mass transfer effects.42
Over a 30 min reaction time, significant H2O2 degradation
is likely when using Pd containing nanoparticles in the 2–5
nm size range, with halide and acid stabilisers typically used
to minimize degradation pathways, suggesting that the
reported TOFs after 30 min of reaction are under-estimates of
the actual TOFs, because some fraction of the H2O2 produced
will have been destroyed by subsequent over-hydrogenation
or decomposition reactions.43 Fig. 3 shows a comparison in
terms of wt% H2O2 produced of the colloidal metal
nanoparticle solution and supported AuPd catalyst as a
function of time for this reaction. It can be seen that at very
short reaction times, (∼2 min) where the contribution from
H2O2 degradation pathways can be assumed to be negligible,
that the colloidal AuPd–PVP nanoparticle solution has a
higher initial rate of H2O2 synthesis than the corresponding
supported AuPd nanoparticles. A turnover frequency of 4630
molH O molmetal h1 was determined for the unsupported
−1
2
2
colloid as compared to 2140 molH O molmetal h1 for the
−1
2 2
TiO2 supported AuPd nanoparticles based on the total moles
of metal present in this regime where subsequent reactions
are minimised.
Fig.
3 H2O2 produced (wt%) as a function of reaction time by
Fig. 2 H2O2 produced with increasing amounts of unsupported Au–Pd
colloid added to the reaction solution up to complete replacement of
the H2O in the solvent system (2.9 ml). H2O2 synthesis conditions: 5%
H2/CO2 (29 bar) and 25% O2/CO2 (11 bar), 8.5 g solvent (2.9 g HPLC
5.6 g MeOH), colloidal solution concentration [0.66 mMmetal], reaction
temperature = 2 °C, stirring rate = 1200 rpm, reaction time = 30 min.
unsupported and TiO2 supported AuPd–PVP colloid nanoparticles. wt%
H2O2 produced – filled symbols; colloidal particles (■), supported
particles (●). H2O2 synthesis conditions: 5% H2/CO2 (29 bar) and 25%
O2/CO2 (11 bar), 8.5 g solvent (2.9 g HPLC 5.6 g MeOH), 0.66 μmol
metal, reaction temperature = 2 °C, stirring rate = 1200 rpm;, reaction
time = 30 min.
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Due to the measured difference in particle size
distribution of the two catalytic systems, the total number of
surface atoms available in each reaction was estimated using
the particle size distributions obtained by TEM and applying
the same model for atom packing to each system. This
approximation assumes activity is related to the total number
of exposed surface atoms and is not correlated with specific
surface sites such as edges or corner atoms. In addition it
assumes that the polymer coverage of the metal surface is
consistent between both samples. This analysis gave 1.2 ×
1017 surface atoms for the colloidal system assuming
spherical particles versus 5.7 × 1016 for the TiO2 supported
catalyst assuming hemispherical supported particles meaning
that the colloidal samples have ∼2.1 times the exposed metal
surface. Based on the amount of H2O2 produced after 2 min
of reaction, where the contribution from subsequent
reactions (hydrogenation and decomposition) is minimal, we
find that the moles of H2O2 produced at each exposed
surface site is comparable between the colloidal metal
nanoparticle solution (1.7 × 10−21 molH O metal site−1 h−1)
colloidal particles. Assuming that the H2O2 hydrogenation
and decomposition pathways are independent of each other,
this suggests that either it is the act of supporting the
particles which generates
a metal–metal oxide support
interface or the removal of PVP on washing the solid which
changes the predominant H2O2 degradation pathway. Our
previous studies have shown that supporting Au colloids on
crystalline metal oxides such as TiO2 results in a more highly
faceted particle structure than when they are supported on
amorphous carbon structures.14,15,44 We prepared an
analogous sample by supporting the same colloidal AuPd
nanoparticle solution on an activated carbon. This sample
showed higher H2O2 production, but also no H2O2
decomposition activity, supporting the notion that
immobilisation of the AuPd colloid onto TiO2 P25 surface
results in increased H2O2 decomposition rates possibly
through the formation of highly faceted surfaces as the
particle restructures to minimise the energy needed to
interact with the underlying support lattice and form an
extended intimate support/metal interface. This highlights
the possibility that the underlying support material can
possibly indirectly effect the decomposition pathways of
H2O2 on the surfaces of the alloy nanoparticle attached to it.
Numerous observations that supported catalysts prepared
from PVP-stabilised colloids show low activity in gas phase
reactions, such as CO oxidation, due to the presence of
residual polymer suggest that the polymer is still present to
some extent after the preparation of the supported catalyst
material.15,16 Further studies by Han et al. report the stability
of the PVP polymer under H2O2 synthesis conditions similar
to those used within this work supporting the hypothesis that
PVP is stable to the reaction conditions used in this study.6
To further investigate if the presence of PVP contributes to
these effects colloidal Au–Pd nanoparticles were prepared
using a variety of PVP : metal ratios between 20 and 0.005 in
order to study the influence of PVP concentration and
particle size on the reaction, (Table 1 entries 5–7, Fig. 4). The
mean particle size of the colloidal particles ranged from 6
1.9 nm for a PVP/metal ratio of 0.05 to 1.9 1.3 nm for a
PVP/metal ratio of 20. This decrease in mean size correlates
well with the increasing amount of polymer allowing the
formation of a higher number of smaller nuclei during the
preparation. Fig. 4 shows that as the average particle size of
the AuPd nanoparticles decreases, the net amount of H2O2
produced after 30 min of reaction increases from 0.08 wt%
for 6 nm particles to 0.12 wt% for 1.9 nm particles. In all
cases, the colloidal AuPd–PVP particles showed minimal
H2O2 decomposition (Table 1, entry 5–7), even when the
solution contained smaller particles (i.e., a greater number of
exposed metal surface atoms) producing higher amounts of
H2O2 over the 30 min reaction. Fig. 4b reports the normalised
rate per surface atom as approximated by the TEM derived
particle size distributions for a range of catalysts prepared
with various PVP to metal ratios for H2O2 synthesis,
degradation and decomposition. No significant change in
apparent TOF per surface atom was observed over the mean
2
2
and the supported AuPd nanoparticles (1.5 × 10−21 molH O
2
2
per metal site per h). Considering the assumptions made
during this comparison this suggests that H2O2 formation
occurs on the surface of the Au–Pd nanoparticles at similar
rates in both catalytic systems when minimal H2O2
degradation pathways are operating. Due to the large
difference in catalyst mass between colloidal and supported
nanoparticle catalysts rates normalised to both total moles of
metal and moles of metal surface for 30 min reactions are
reported in Table 1 assuming no significant change in the
catalyst structure takes place.
Experiments were also carried out to investigate the effect
that supporting the colloidal AuPd–PVP nanoparticles on
TiO2 had on the degradation of a 4 wt% H2O2 solution, which
included contributions from both H2O2 hydrogenation and
decomposition pathways. Comparing the unsupported
colloidal AuPd–PVP solution to the corresponding TiO2
supported catalyst, the total degradation after 30 min
(Table 1, entries 1–2) was similar for both situations.
However, when normalising to the difference in surface sites
between the colloidal and supported nanoparticles, it is clear
to see that the supported particles have significant higher
rates per metal surface site by a factor of ∼2.5. By comparing
the rates of H2O2 decomposition under a 25% O2/CO2
atmosphere to remove over-hydrogenation from the possible
reaction pathways, it was observed that the colloidal AuPd–
PVP solution decomposed only 1% of the H2O2 present
compared to the supported particles which decomposed 7%
of the initial H2O2 suggesting a significant difference in how
the two samples interact with H2O2.
The decomposition of H2O2 occurs much faster when the
Au–Pd particles are supported on TiO2, despite the supported
particles having larger mean particle size and therefore less
exposed surface available (by roughly a factor of 2) meaning
an increase in observed decomposition rate of about 14 times
for the supported particles as compared to the unsupported
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Fig. 4 a) Catalytic activity results for unsupported AuPd–PVP colloidal nanoparticle solutions prepared with various amounts of PVP giving rise
to different mean nanoparticle sizes. b) Rate normalised to available metal surface as determined by TEM particle size distributions. Key to
symbols; wt% H2O2 produced (■), H2O2 degradation (△) and H2O2 decomposition (○). H2O2 synthesis conditions: 5% H2/CO2 (29 bar) and 25%
O2/CO2 (11 bar), 8.5 g solvent (2.9 g HPLC 5.6 g MeOH), 0.66 μmol metal, reaction temperature = 2 °C, stirring rate = 1200 rpm, reaction time
= 30 min. H2O2 degradation conditions: 5% H2/CO2 (29 bar), 8.5 g solvent (5.6 g MeOH, 2.22 g H2O and 0.68 g 50% H2O2), 0.66 μmol metal,
reaction temperature = 2 °C, stirring rate = 1200 rpm, reaction time = 30 min. H2O2 decomposition conditions: 25% O2/CO2 (29 bar), 8.5 g
solvent (5.6 g MeOH, 2.22 g H2O and 0.68 g 50% H2O2), 0.66 μmol metal, reaction temperature = 2 °C, stirring rate = 1200 rpm, reaction time
= 30 min.
particle size range 1.9 to 6.0 nm, however it should be noted
that these reaction rates were determined after 30 min so do
not represent initial rates which are independent of
subsequent reaction processes. This suggests, due to the
minimised H2O2 decomposition in all colloidal samples
compared to the supported sample, that it is the effect of
supporting the particles and their interaction with the TiO2
lattice that initiates H2O2 decomposition in this system and
that the main loss of H2O2 selectivity using colloidal particles
results from over hydrogenation.
supported AuPd catalysts in a similar manner with a slightly
broader particle size distribution compared to conventional
immobilisation of polymer stabilised colloidal solutions.22
This colloidal nanoparticle solution prepared without
polymer addition (denoted SF-stabiliser free) was also tested
for direct H2O2 synthesis (Table 1, entry 8). Interestingly they
showed activity towards H2O2 production and also increased
rates of H2O2 decomposition suggesting that the greater
accessibility to the surface in the absence of PVP results in
To support our conclusions, we investigated the
decomposition of H2O2 by simply stirring both supported
and unsupported catalytic systems in glass vials containing 4
wt% H2O2 under air at ambient conditions, rather than at 2
°C and under a 29 bar pressure of 5% H2/CO2 as in the
regular H2O2 synthesis experiments. Fig. 5 shows that when
TiO2 and the AuPd–PVP sol were stirred separately with the
starting H2O2 solution, approximately 4–5% of the H2O2 was
decomposed over the 30 min reaction period. In contrast,
when the supported 1wt% AuPd/TiO2 material was tested,
containing the same amount of metal, bubbles of gas
generated from the decomposition of H2O2 were clearly
visible originating from the catalyst. Over the same 30 min
time period, approximately 80% of the H2O2 was
decomposed. Furthermore, addition of varying extra amounts
of PVP to the solution did not suppress the rapid gas
generation.
Fig. 5 H2O2 decomposition results as a function of time at ambient
conditions. Key to symbols: blank (□), TiO2 only (○), unsupported
colloidal AuPd–PVP particles (◊) and TiO2 supported AuPd particles (△).
H2O2 decomposition conditions: 4 wt% H2O2 in H2O (10 mL, HPLC
grade), in each case 0.66 μmol metal, 9 mg of TiO2 added under
magnetic stirring at ambient temperature and pressure.
Astruc and co-workers have demonstrated that it is
possible to prepare colloidal Au catalysts in the absence of
polymer stabilisers relying solely on electrostatic stabilisation
of the metal particles by residual salts in the colloidal
solution.45 We recently showed that it is possible to prepare
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higher H2O2 decomposition rates in these colloidal systems.
Similar tests under ambient conditions using this stabiliser
free AuPd nanoparticle solution clearly showed the evolution
of bubbles as a result of H2O2 decomposition, which could
be supressed by the addition of PVP to the reaction mixture.
This is in direct contrast to the situation found when using
the TiO2 supported AuPd catalyst, where there seemed to be
an interplay between the presence of PVP and the effect of
efficiency in producing oxygenates. On adding extra PVP to
the supported catalyst an improvement was noted with
respect to H2O2 efficiency and oxygenate production from a
H2O2: product ratio from 570 to 376, however, the activity of
unsupported colloidal system still remained over an order of
magnitude better. This demonstrated that the support
induced H2O2 decomposition could not be fully negated by
the addition of further PVP stabiliser.
supporting
the
nanoparticles
on
inducing
H2O2
Further experiments were carried out to investigate if the
presence of polymer stabiliser was crucial in achieving the
high efficiency of methane oxidation with H2O2 and O2
using colloidal AuPd–PVP nanoparticles. Fig. 6 shows a
stark comparison of the catalytic performance of the
colloidal metal nanoparticle system synthesised with and
without PVP polymer. Without the PVP polymer being
present, H2O2 consumption during the 30 min reaction
period at 50 °C was over 95%, producing minimal
oxygenated products. This compares to only 32% H2O2
consumption when the colloid incorporated the PVP ligand.
The lower H2O2 consumption in this latter case resulted in
significantly higher oxygenated product formation showing
that the polymer is crucial in achieving high reactivity,
presumably– by controlling the rate of H2O2 decomposition.
We then took the stabiliser free colloidal solution prepared
in the absence of PVP and added PVP to the methane
oxidation reaction. The catalytic performance was
significantly improved and indeed, the activity approached
that of the colloidal solution prepared with PVP. The H2O2
consumption was 48% with significant oxygenate production
demonstrating that it is in fact the polymer additive
significantly effects the rate of H2O2 decomposition on the
metal surface and therefore the overall efficiency of
methane oxidation. The PVP ligand shell presumably
controls the rate of H2O2 diffusion to the metal surface and
therefore the production of radicals that can interact with
the solubilised methane, whose concentration may also be
enhanced by the hydrophobic alkyl chains in the organic
layer around the metal surface.
decomposition. The addition of N-methyl pyrrolidone as an
analogue of the PVP monomer unit was not able to suppress
the decomposition in the same way as adding PVP to the
stabiliser free colloid, implying that the presence of the
hydrophobic alkyl chain backbone could be crucial in
controlling the rates of H2O2 decomposition in colloidal
nanoparticle solutions.
This difference in the H2O2 decomposition rates in the
absence of pressurised CO2 could have significant
implications in reactions involving aqueous solutions of
H2O2 as an oxidant or initiator in the absence of acid
stabilisers due to the reactivity of support nanoparticles
being significantly different to colloidal nanoparticles in
terms of H2O2 decomposition. Our previous studies on
methane oxidation using unsupported and TiO2 supported
colloidal AuPd–PVP nanoparticles have shown that
unsupported colloidal catalysts are capable of oxidising
methane in the presence of H2O2 to produce methanol under
mild conditions in which gas phase O2 incorporation
occurred with high efficiency with respect to H2O2.37,40 It was
also demonstrated that the TiO2 supported AuPd colloidal
nanoparticles showed low activity in this particular reaction
due to high levels of H2O2 decomposition.
To extend our previous study and elucidate the role of PVP
in this reaction we conducted further methane oxidation
experiments with H2O2 only as an oxidant using the 1 wt%
AuPd/TiO2 catalyst with extra PVP added to the reaction to
identify if this unwanted H2O2 decomposition activity,
initiated by supporting the AuPd–PVP particles, could be
suppressed. Table 2 shows a summary of the catalytic testing
results for a colloidal AuPd catalyst prepared with PVP, the
TiO2 supported AuPd–PVP nanoparticles, and the same
supported nanoparticles with the addition of extra PVP
(giving a metal to PVP ratio 1.2 by weight in the reaction).
The results clearly show the superior efficiency of the
unsupported AuPd–PVP colloidal system with respect to H2O2
In conclusion, we have investigated the role of the PVP
ligand shell and TiO2 support in controlling the reactions of
H2O2 on AuPd–PVP colloidal particles. These particles display
significant activity towards H2O2 direct synthesis,
hydrogenation and decomposition as well as methane
oxidation using H2O2 and O2. We have shown that supporting
the PVP stabilised AuPd nanoparticles on TiO2 increases
Table 2 Summary of methane oxidation reaction results using unsupported AuPd–PVP colloidal nanoparticles, TiO2 supported AuPd nanoparticles and
supported AuPd nanoparticles with additional PVP present (equivalent to metal/PVP ratio of 1.2) using H2O2 as the oxidant
Product amount (μmol)
Oxygenate
Catalyst
CH3OH
CH3OOH
HCOOH
CO2
selectivity/(%)
H2O2/products
AuPd–PVP colloid
AuPd/TiO2
AuPd/TiO2 + PVP
3.19
0.43
0.86
9.76
0.00
0.43
7.04
0.00
0.00
3.09
1.23
1.51
86
26
46
36
575
326
Reaction conditions; 1000 μmol H2O2, reaction temperature = 50 °C, total volume = 10 mL, 30 bar CH4, reaction time = 30 min, stirring rate =
1500 rpm, 6.6 μmol metal used per reaction.
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Fig. 6 Comparative methane oxidation tests for AuPd–PVP colloidal nanoparticle sample prepared with PVP (Au–Pd–PVP), stabiliser free (Au–Pd–SF)
and stabiliser free with the addition of PVP (Au–Pd–SF + PVP) equivalent to metal/PVP ratio of 1.2. Reaction conditions: pressure (CH4) = 30 bar,
pressure (O2) = 5 bar, amount of catalyst: 10 mL colloid – 6.6 μmol of metal, 50 μmol H2O2, reaction temperature – 50 °C (with a ramp rate of
2.25 °C min−1), stirring rate – 1500 rpm, reaction time – 30 min.
activity towards H2O2 decomposition which cannot effectively
be suppressed by the addition of extra PVP under our H2O2
synthesis or methane oxidation reaction conditions. This
could be a result of the formation of more highly faceted
alloy particles and an extended metal–support interface –
which is absent in the unsupported colloidal AuPd–PVP
nanoparticle catalyst. The unsupported particles, when
stabilised with PVP, show limited H2O2 decomposition
activity while being able to produce H2O2 at similar rates to
supported nanoparticles. However, the use of colloidal
particles in extended reactions under flow conditions
ratio (typically 1 : 1.2 (wt/wt)). After 2–3 min of stirring, freshly
prepared 0.1 M sodium borohydride (NaBH4, Sigma Aldrich)
solution was added such that the molar ratio of NaBH4-to-
metal was 5 : 1 (mol/mol). This produced a dark brown
colloid which was then left stirring for 30 minutes to ensure
that all the metal precursors were reduced to the metallic
form. The colloid was concentrated using a roto-evaporator
to give a nominal metal loading of 0.66 mmol L−1. The
colloid was stored in glass media bottles prior to use. For
supported catalysts, the sol prepared as described above was
immobilized onto a TiO2 (P25, Degussa, 1.98 g) or activated
carbon (Darco G60) (1.98 g) in the following manner. A
sufficient amount of support material was added to ensure a
1 wt% metal loading and the solution was acidified to pH 1
using sulphuric acid to enhance and achieve more
homogeneous deposition of nanoparticles. The supernatant
solution became clear over a 1 h period after support
addition, indicating the deposition process was complete.
The sol-immobilized catalyst was then filtered, washed
thoroughly with distilled water and then left to dry in an oven
at 110 °C for 16 h.
remains
a challenge to translation of this activity to
industrial application. Using methane oxidation as a test
reaction we demonstrated that this high H2O2 decomposition
rate is detrimental to the overall efficacy in this case. We also
have shown that the colloidal systems require the presence of
polymer stabilisers to control the rate of H2O2
decomposition. These results clearly demonstrate that for
AuPd alloys prepared by colloidal methods, both polymer
stabiliser and support effects need to be taken into account
when describing the activity of the nanoparticles for a range
of reactions involving H2O2 such as selective oxidations or
reactions that use H2O2 or other peroxides as an initiator for
oxidation.
Direct synthesis of H2O2
Hydrogen peroxide synthesis was evaluated using a Parr
Instruments stainless steel autoclave with a nominal volume
of 100 mL and a maximum working pressure of 14 MPa
according to our previous optimisation studies.46 To test each
catalyst for H2O2 synthesis, the autoclave was charged with
catalyst (in each case 0.66 μmol), and solvent (5.6 g MeOH
and 2.9 g H2O). The charged autoclave was then purged three
times with 5% H2/CO2 (7 bar) before filling with 5% H2/CO2
(29 bar), followed by the addition of 25% O2/CO2 (11 bar).
The temperature was then decreased to 2 °C followed by
stirring (1200 rpm) of the reaction mixture for 0.5 h. H2O2
productivity was determined by titrating aliquots of the final
Experimental details
Catalyst preparation
Bimetallic Au–Pd nanoparticles were prepared by standard
colloidal methods. An aqueous solution of HAuCl4 precursor
(Strem Chemicals) and acidic solution of PdCl2 (Sigma
Aldrich) precursor (in 0.58 M HCl) were dissolved in 800 mL
of de-ionized water (Au : Pd = 1 : 1 by moles) to give a total
metal concentration of 0.16 mmol L−1. Polyvinylpyrrolidone
(PVP, average molecular weight 1 300 000, Sigma Aldrich) was
added as a stabilizer to give the required metal-to-PVP weight
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solution after reaction with acidified Ce(SO4)2 (0.01 M) in the
presence of ferroin indicator. Duplicate reactions gave H2O2
productivities that were typically consistent to 2% based on
multiple titration results.
adding potassium titanium oxalate solution (0.5 wt% in
water, Sigma Aldrich) to form the yellow pertitanic acid
complex with a characteristic absorption peak at 390 nm.
Gaseous products were quantified using a Varian 450-GC
fitted with a CP-Sil 5CB capillary column (50 m length, 0.32
mm diameter, carrier gas = He), a methaniser unit and both
FID and TCD detectors.
Degradation of H2O2
The autoclave was charged with MeOH (5.6 g), H2O2 (50 wt%,
0.69 g), HPLC grade H2O (2.21 g) and catalyst (0.66 μmol), with
the solvent composition equivalent to a 4 wt% H2O2 solution.
From the solution, 2 aliquots of 0.05 g were removed and
titrated with acidified Ce(SO4)2 solution using ferroin as an
indicator to determine an accurate concentration of H2O2 at
the start of the reaction. The autoclave was pressurised with
5% H2/CO2 (29 bar) and cooled to 2 °C. Upon reaching 2 °C the
reaction mixture was stirred at 1200 rpm for 0.5 h. After the
reaction was complete the catalyst was removed from the
reaction solvents and as previously described, i.e., two aliquots
(approximately 0.05 g) were titrated against an acidified
Ce(SO4)2 solution using ferroin as an indicator. The
degradation activity is reported as the percentage of the initial
4 wt% degraded. Duplicate reactions gave degradation values
that were typically consistent to 1% H2O2 decomposed based
on multiple titration results.
Catalyst characterization
TEM. Materials for TEM analysis were prepared by
dispersing the colloidal or supported catalyst 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.
Explanation of determination of number of surface atoms
is reported in the ESI† material. The same approach was used
for each catalyst material for comparative purposes assuming
spherical particles for colloidal nanoparticles and
hemispherical particles for supported nanoparticles.
H2O2 degradation conducted in the presence of 5% H2/
CO2 (29 bar) represents the sum of both hydrogenation and
decomposition pathways, while the use of 25% O2/CO2 (29
bar) allows for catalytic activity towards H2O2 decomposition
alone to be determined. All reaction conditions were as
described above, apart from the gas atmosphere for the H2O2
decomposition studies.
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
We thank Cardiff University for funding as part of the MaxNet
for Chemical Energy Conversion.
Methane oxidation reaction conditions
References
Methane oxidation was carried out in a 50 mL glass-lined
stainless steel Parr autoclave reactor. The reactor was charged
with either colloidal or supported catalyst (0.66 μmol metal
per reaction) and H2O2 (amount defined in figure captions,
Sigma Aldrich, 50 wt% in water). The charged autoclave was
sealed and purged three times with methane (10 bar
99.999%, Air Products). It was then pressurized with methane
(30 bar) and in some cases oxygen (5 bar, BOC). The mixture
was stirred at 1500 rpm and heated to 50 °C at a ramp rate of
2.25 °C min−1 and maintained at the reaction temperature
for 30 min. At the end of the reaction, the autoclave was
cooled in ice to a temperature below 10 °C in order to
minimize the loss of volatile products. The reaction gas was
removed for analysis in a gas sampling bag.
1 C. J. Jia and F. Schüth, Phys. Chem. Chem. Phys., 2011, 13,
2457–2487.
2 J. Quinson, S. Neumann, T. Wannmacher, L. Kacenauskaite,
M. Inaba, J. Bucher, F. Bizzotto, S. B. Simonsen, L. Theil
Kuhn, D. Bujak, A. Zana, M. Arenz and S. Kunz, Angew.
Chem., Int. Ed., 2018, 57, 12338–12341.
3 L. Prati and A. Villa, Acc. Chem. Res., 2014, 47, 855–863.
4 P. Sonström and M. Bäumer, Phys. Chem. Chem. Phys.,
2011, 13, 19270–19284.
5 S. Maity and M. Eswaramoorthy, J. Mater. Chem. A, 2016, 4,
3233–3237.
6 G. H. Han, S. H. Lee, M. G. Seo and K. Y. Lee, RSC Adv.,
2020, 10, 19952–19960.
7 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 and G. J. Hutchings, Catal. Sci.
Technol., 2013, 3, 308–317.
8 L. F. de L. e Freitas, B. Puértolas, J. Zhang, B. Wang, A. S.
Hoffman, S. R. Bare, J. Pérez-Ramírez, J. W. Medlin and E.
Nikolla, ACS Catal., 2020, 5202–5207.
Product analysis for the methane oxidation reaction
1
Liquid phase product analysis was carried out using H-NMR
on a Bruker 500 MHz instrument equipped with a solvent
suppression system. Tetramethylsilane (TMS) in CDCl3 was
used as an internal standard. The H2O2 concentration was
determined using a titanium oxalate spectrophotometric
method (Agilent, Cary 60). In this procedure, 0.05 to 1.0 mL
of reaction sample was acidified using dilute H2SO4 before
9 M. Studer, S. Burkhardt and H. U. Blaser, Chem. Commun.,
1999, 1727–1728.
This journal is © The Royal Society of Chemistry 2020
Catal. Sci. Technol.

View Article Online
Paper
Catalysis Science & Technology
10 H. U. Blaser and M. Studer, Acc. Chem. Res., 2007, 40,
1348–1356.
11 B. Minder, M. Schürch, T. Mallat, A. Baiker, T. Heinz and A.
Pfaltz, J. Catal., 1996, 160, 261–268.
12 S. Haesuwannakij, T. Kimura, Y. Furutani, K. Okumura, K.
Kokubo, T. Sakata, H. Yasuda, Y. Yakiyama and H. Sakurai,
Sci. Rep., 2017, 7, 1–8.
13 L. M. Rossi, J. L. Fiorio, M. A. S. Garcia and C. P. Ferraz,
Dalton Trans., 2018, 47, 5889–5915.
29 G. M. Lari, B. Puértolas, M. Shahrokhi, N. López and J.
Pérez-Ramírez, Angew. Chem., 2017, 129, 1801–1805.
30 S. J. Freakley, Q. He, J. H. Harrhy, L. Lu, D. A. Crole, D. J.
Morgan, E. N. Ntainjua, J. K. Edwards, A. F. Carley, A. Y.
Borisevich, C. J. Kiely and G. J. Hutchings, Science,
2016, 351, 965–968.
31 J. K. Edwards, B. Solsona, E. Ntainjua, N. A. F. Carley, A. A.
Herzing, C. J. Kiely and G. J. Hutchings, Science, 2009, 323,
1037–1041.
14 J. A. Lopez-Sanchez, N. Dimitratos, C. Hammond, G. L.
Brett, L. Kesavan, S. White, P. Miedziak, R. Tiruvalam, R. L.
Jenkins, A. F. Carley, D. Knight, C. J. Kiely and G. J.
Hutchings, Nat. Chem., 2011, 3, 551–556.
15 B. Donoeva and P. E. de Jongh, ChemCatChem, 2018, 10,
989–997.
16 G. M. Lari, E. Nowicka, D. J. Morgan, S. A. Kondrat and G. J.
Hutchings, Phys. Chem. Chem. Phys., 2015, 17, 23236–23244.
17 A. Villa, N. Dimitratos, C. E. Chan-Thaw, C. Hammond, L.
Prati and G. J. Hutchings, Acc. Chem. Res., 2015, 48,
1403–1412.
18 J. Xu, H. Zhang, Y. Zhao, B. Yu, S. Chen, Y. Li, L. Hao and Z.
Liu, Green Chem., 2013, 15, 1520–1525.
19 A. Villa, C. Campione and L. Prati, Catal. Lett., 2007, 115,
133–136.
20 W. C. Ketchie, M. Murayama and R. J. Davis, J. Catal.,
2007, 250, 264–273.
21 L. Abis, N. Dimitritatos, M. Sankar, S. J. Freakley and G. J.
Hutchings, Top. Catal., 2019, 1–9.
32 P. Beltrame, M. Comotti, C. Della Pina and M. Rossi, Appl.
Catal., A, 2006, 297, 1–7.
33 J. Zhao, W. Y. Hernández, W. Zhou, Y. Yang, E. I. Vovk, M.
Capron and V. Ordomsky, ChemCatChem, 2020, 12, 238–247.
34 A. García-Trenco, E. R. White, A. Regoutz, D. J. Payne, M. S. P.
Shaffer and C. K. Williams, ACS Catal., 2017, 7, 1186–1196.
35 Y. Nomura, T. Ishihara, Y. Hata, K. Kitawaki, K. Kaneko and
H. Matsumoto, ChemSusChem, 2008, 1, 619–621.
36 T. Deguchi, H. Yamano, S. Takenouchi and M. Iwamoto,
Catal. Sci. Technol., 2018, 8, 1002–1015.
37 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 and G. J. Hutchings, Science,
2017, 358, 223–227.
38 G. Giorgianni, S. Abate, G. Centi and S. Perathoner,
ChemCatChem, 2019, 11, 550–559.
39 J. Radnik, C. Mohr and P. Claus, Phys. Chem. Chem. Phys.,
2003, 5, 172–177.
40 R. McVicker, N. Agarwal, S. J. Freakley, Q. He, S. Althahban,
S. H. Taylor, C. J. Kiely and G. J. Hutchings, Catal. Today,
2020, 342, 32–38.
41 T. Ishihara, R. Nakashima, Y. Ooishi, H. Hagiwara, M.
Matsuka and S. Ida, Catal. Today, 2015, 248, 35–39.
42 J. K. Edwards, B. E. Solsona, P. Landon, A. F. Carley, A.
Herzing, C. J. Kiely and G. J. Hutchings, J. Catal., 2005, 236,
69–79.
22 L. Abis, S. J. Freakley, G. Dodekatos, D. J. Morgan, M.
Sankar, N. Dimitratos, Q. He, C. J. Kiely and G. J. Hutchings,
ChemCatChem, 2017, 9, 2914–2918.
23 S. Kanungo, L. van Haandel, E. J. M. Hensen, J. C. Schouten
and M. F. Neira d'Angelo, J. Catal., 2019, 370, 200–209.
24 S. Kanungo, V. Paunovic, J. C. Schouten and M. F. Neira
D'Angelo, Nano Lett., 2017, 17, 6481–6486.
25 F. Menegazzo, M. Manzoli, M. Signoretto, F. Pinna and G.
Strukul, Catal. Today, 2015, 248, 18–27.
26 E. Ghedini, F. Menegazzo, M. Signoretto, M. Manzoli, F.
Pinna and G. Strukul, J. Catal., 2010, 273, 266–273.
27 N. M. Wilson, P. Priyadarshini, S. Kunz and D. W. Flaherty,
J. Catal., 2018, 357, 163–175.
28 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 and G. J. Hutchings, Langmuir, 2010, 26,
16568–16577.
43 Y. F. Han and J. H. Lunsford, J. Catal., 2005, 230, 313–316.
44 R. C. Tiruvalam, J. C. Pritchard, N. Dimitratos, J. A. Lopez-
Sanchez, J. K. Edwards, A. F. Carley, G. J. Hutchings and
C. J. Kiely, Faraday Discuss., 2011, 152, 63–86.
45 C. Deraedt, L. Salmon, S. Gatard, R. Ciganda, R. Hernandez,
J. Ruiz and D. Astruc, Chem. Commun., 2014, 50,
14194–14196.
46 M. Piccinini, E. Ntainjua, N. J. K. Edwards, A. F. Carley, J. A.
Moulijn and G. J. Hutchings, Phys. Chem. Chem. Phys.,
2010, 12, 2488–2492.
Catal. Sci. Technol.
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