The Direct Synthesis of H2O2 Using TS-1 Supported Catalysts

Article abstract of DOI:10.1002/cctc.201900100

In this study we show that using AuPd nanoparticles supported on a commercial titanium silicate (TS-1) prepared using a wet co-impregnation method it is possible to produce hydrogen peroxide from molecular H₂ and O₂ via the direct synthesis reaction. The effect of Au: Pd ratio and calcination temperature is evaluated as well as the role of Pt addition to the AuPd supported catalysts. The effect of Pt addition to AuPt nanoparticles is observed to result in a significant improvement in catalytic activity and selectivity to hydrogen peroxide with detailed characterisation indicating this is a result of selectively tuning the ratio of Pd oxidation states.

Full text of DOI:10.1002/cctc.201900100

Accepted Article
Title: The Direct Synthesis of H2O2 Using TS-1 Supported Catalysts
Authors: Richard Lewis, Kenji Ueura, Yulimasa Fukuta, Simon
Freakley, Liqun Kang, Ryan Wang, Qian He, Jennifer
Edwards, David Morgan, Yamamoto Yasushi, and Graham
John Hutchings
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of the final Version of Record (VoR). This work is currently citable by
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To be cited as: ChemCatChem 10.1002/cctc.201900100
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ChemCatChem
10.1002/cctc.201900100
FULL PAPER
The Direct Synthesis of H
2
O Using TS-1 Supported Catalysts.
2
[a]
[b]
[b]
[a,c]
[d]
[d]
Richard J. Lewis, KenjiUeura, Yukimasa Fukuta, Simon J. Freakley, Liqun Kang, Ryan Wang,
Qian He,[a] Jennifer. K. Edwards,[a] David J. Morgan,[a,e] Yasushi Yamamoto[b] and Graham J.
Hutchings*[a]
oxide[3-9] (via the integrated HPPO process) and cyclohexanone
Abstract: In this study we show that using gold palladium
oxime[10-12] (via cyclohexanone ammoximation) which are key
nanoparticles supported on a commercial titanium silicate (TS-1)
intermediates for the production of polyurethane and Nylon-6
prepared using a wet co-impregnation method it is possible to produce
respectively. In recent years Solvay, a technology leader with an
2 2
hydrogen peroxide from molecular H and O via the direct synthesis
extended product portfolio in H
production capacity to meet growing demand for propylene
oxide.[13] Other significant applications of H
are found in, but
2 2 2 2
O , have doubled their H O
reaction. The effect of Au: Pd ratio and calcination temperature is
evaluated as well as the role of platinum addition to the AuPd
supported catalysts. The effect of platinum addition to gold-palladium
nanoparticles is observed to result in a significant improvement in
catalytic activity and selectivity to hydrogen peroxide with detailed
characterisation indicating this is a result of selectively tuning the ratio
of palladium oxidation states.
2
O
2
[
14-16]
are by no means limited to, alkene epoxidation,
organo-
sulphur oxidation,[
17-19]
aromatic side chain oxidation[20,21] and
ketone oxidation.[22-25]
2 2
Currently the vast majority (95%) of global demand for H O is
met via the anthraquinone oxidation (AO) process, first developed
by Riedl and Pfleiderer of BASF in 1939.[26] The AO process offers
Introduction.
2 2
extremely high efficiency, producing H O concentrations in
excess of 70 wt.% through numerous distillation steps. However,
there are some concerns around its carbon efficiency, in particular
Hydrogen peroxide (H
industrial oxidant, with a particularly high active oxygen content
47.1 %), second only to molecular O . In comparison to other
2 2
O ) is a powerful, environmentally friendly
with the need to continually replenish the anthraquinone H
2
carrier
(
2
molecule,
which
is
deactivated
through
unselective
hydrogenation.[ In addition, the high infrastructure costs and the
27]
commonly utilised oxidising agents such as tBuOOH, NaClO and
permanganate, which all produce hazardous by-products that
overall complexity of the AO process (in particular the choice of
require downstream separation from product stream, H
2
O
2
solvent system) prohibits production of H
so H is often transported at concentrations greatly exceeding
that required by the end user, with costs associated with the
dilution of H to appropriate concentrations, often between 5
and 10 wt.%. Furthermore, the instability of H , at relatively mild
2 2
O at point of use and
produces only H O as a by-product. Although finding increasing
2
2 2
O
use in the treatment of industrial waste streams, due to increasing
legislation preventing the use of chloride containing oxidants,
2
O
2
H
2
O
2
is primarily utilised in the paper / pulp bleaching and textile
2 2
O
industries as well as in the chemical synthesis sector. Currently
temperatures, requires the use of acidic stabilising agents to
[
1]
[28-30]
global demand for H
and is expected to exceed 5.5 million tons by 2020. This
demand is driven primarily from the chemical synthesis sector
2
O
2
is growing at a rate of 4% per annum
prevent its decomposition to H
2
O.
The use of such stabilising
[
2]
agents can result in a decrease in reactor lifetime, through
corrosion, and pass on significant costs to the end user who must
ensure they are removed from product streams in order to prevent
the build-up of impurities.
2 2
with H O finding application in the production of propylene
The shortcomings of the anthraquinone process can be overcome
via the direct synthesis of H
2 2 2 2
O from H and O , outlined in Figure
[
a]
Dr. R. Lewis, Dr. S. Freakley, Dr. Q. He, Dr. J. Edwards, Dr. D.
Morgan and Prof. G. Hutchings.
1, which provides a greener, more atom efficient process for H O
2 2
Cardiff Catalysis Institute, School of Chemistry, Cardiff University,
Main Building, Park Place, Cardiff, CF10 3AT, United Kingdom
E-mail: Hutch@cardiff.ac.uk
generation and has the potential to be adopted at point of use. In
particular, Pd-based catalysts have been demonstrated to offer
2 2
O
.[31-34] However,
high activity towards the direct synthesis of H
catalytic selectivity is often a concern and requires the use of
halide and strong acid additives to inhibit H degradation and
the production of H O via decomposition and hydrogenation
pathways,[35-37] both of which are more thermodynamically
favourable than the formation of H
[
[
b]
c]
Mr. K. Ueura, Dr. Y. Fukuta and Mr. Y. Yamamoto
UBE Industries,1978-5, Kogushi, Ube, Yamaguchi 755-8633, Japan
Dr. S. Freakley, Department of Chemistry, University of Bath,
Claverton Down, Bath BA2 7AY, United Kingdom.
2
O
2
2
[
d]
e]
Mr. L Kang, Dr. R. Wang
Department of Chemical Engineering, University College London,
Roberts Building, Torrington Place, London WC1E 7JE, United
Kingdom.
2 2
O .
[
Dr. D. Morgan
HarwellXPS, Research Complex at Harwell (RCaH), Didcot, OX11
0FA, United Kingdom.
Supporting information for this article is given via a link at the end of the
document.
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ChemCatChem
10.1002/cctc.201900100
FULL PAPER
oxide produced annually, and the vast majority utilised in the
production of polyurethane via the HPPO process. Furthermore,
a growing interest has been placed on Au[
68,69]
and Au-Pd
nanoparticles supported on TS-1 for the in situ generation of H
[70,71]
2
O
2
for a variety of selective oxidation reactions. We have previously
investigated the role of zeolites such as TS-1, HZSM-5 and
zeolite-Y as supports for precious metals in the direct synthesis of
O [
72,73]
as well as CO oxidation,[74] with H
H
2
2
2 2
O productivities for
2 2
Figure 1. Reaction pathways associated with the direct synthesis of H O from
the zeolite supported catalysts much greater than the analogous
catalysts supported on common oxide supports including TiO
,[75]
2 2
H and O .
2
O [
76]
and SiO
[77]
likley due to their strong acid characters.
Al
Building on these findings we now investigate the use of a
commercial TS-1 as a support for AuPdPt nanoparticles for H
synthesis from molecular H and O
2
3
2
Building on the work of Landon et al.[38] and Haruta and co-
workers,[39] Edwards et al.[40] were the first to demonstrate that
2
O
2
high selectivity towards
H
2
O
2
can be achieved through
2
2
.
combination of Pd with Au, with the development of a Au core-
PdO shell often reported as a key factor in the enhancement of
catalytic selectivity.[41,42] Indeed, these bimetallic Au-Pd catalysts
have received significant attention by both experimentalists[43-46]
and theoreticians[47,48] with a combination of electronic, structural
and isolation effects likely to be responsible for the synergy
observed in Au-Pd systems.
Results and Discussion.
Our initial screening of TS-1 supported monometallic (Au, Pd, Pt)
and bimetallic (Au-Pd, Pd-Pt, Pt-Au) catalysts for the direct
synthesis of H O and its subsequent degradation (comprising
2 2
Numerous studies have investigated the modification of
over-hydrogenation and decomposition) are shown in Table 1. It
was observed that the monometallic 5% Au / TS-1 catalyst has
supported Pd nanoparticles through the addition of other precious
metals such as Ag,[49,50] Rh,[51] Ru,[52] Ir[53] and Pt.[54-57] With
-1 -1
Deguchi et al.[53] recently reporting that dramatic enhancements
2 2
limited activity towards H O synthesis (2 molH2O2kgcat h ), in
comparison the 5% Pd / TS-1 catalyst was shown to have much
in catalytic activity can be achieved through the introduction of
very low concentrations of Pt into a Pd-polyvinylpyrrolidone
-
greater activity towards both H
)
O
2 2
synthesis (131 molH2O2kgcat 1h-
1
-1 -1
2 2
and the subsequent degradation of H O (459 molH2O2kgcat h ).
colloid, with the high H
cause for a doubling in H
with the work of Bernardotto et al.
introduction of a small amount of Pt into a supported Pd catalyst
Pd: Pt ratio = 13) leads to a significant enhancement in activity
towards H synthesis in comparison to the analogous mono-
metallic Pd catalyst. This has been attributed to a change in Pd
2
activating abilities of Pt suggested as the
This is consistent with many previous studies investigating Au-
Pd nanoparticles supported on various oxide support materials,
2
O
2
synthesis rates. This is in keeping
[58]
who report that the
-
1 -1 76
with the Pd supported Al
2
O
3
(200 molH2O2kgcat h ) and SiO
2
-
1 -1 77
(
488 molH2O2kgcat h )
catalysts demonstrating high H O
2
2
(
degradation values. In keeping with many previous studies into
co-impregnation of Au and Pd to produce a bimetallic catalyst we
observe enhanced catalytic activity when both Au and Pd are
immobilised onto the same support with the activity of the bi-
2
O
2
[
58]
nanoparticle morphology
Pd surface.[
an experimental and theoretical approach to investigate the role
of Pt incorporation into a Pd catalyst. They report that the addition
of low concentrations of Pt promote the hydrogenation of
and the stabilisation of the oxidised
More recently Quon et al.[60] have combined both
55,59]
-1 -1
metallic 2.5% Au-2.5% Pd / TS-1 catalyst (100 molH2O2kgcat h ),
greater than that observed over a physical mixture of the mono-
metallic catalysts with an analogous metal loading (87
-1 -1
molH2O2kgcat h ). However, catalytic activity of the bi-metallic
AuPd / TS-1 catalyst does not supersede that of the mono-
metallic Pd supported catalyst, which had previously been
molecular O
dissociation of H
similar increase in
2
, leading to the production of H
, with a doubling of H production rate and
selectivity. We have previously
2 2
O and inhibit the
2
O
2
2 2
O
a
H
2
reported on a range of oxide supports, including TiO2,[ MgO
75]
[78]
investigated the addition of Pt to AuPd supported nanoparticles
and demonstrated an enhancement in catalytic selectivity towards
the direct synthesis of H O via the inhibition of the subsequent
2 2
and Al
2
O3,[76] and this is attributed to a lack of complete alloying
and only partial formation of the Au-core-PdO shell nanoparticle
morphology often adopted on oxide supports. This is in keeping
with our previous work investigating the catalytic activity of Au-Pd
hydrogenation/decomposition pathways, when supported on
CeO [
61]
.[62]
2
and TiO
2
catalysts supported on SiO [
77]
and highlights how the choice of
formation.
Indeed it has previously been reported that supports such as SiO
that have low isoelectric points yield catalysts that are highly
[
63]
2
Since its first development by EniChem
titanium silicate-1 (TS-1) has widely been reported to offer high
selectivity and efficiency in oxidation reactions when used
alongside pre-formed H O2. In particular TS-1 has been reported
2
to show excellent activity for aromatic hydroxylation,
oxidation[66] and alkene epoxidation.[67] With the combination of
the hydrophobicity of the silicate lattice and the high coordination
ability of the framework TiIV sites key to the unique catalytic
properties observed for TS-1. In particular the catalytic conversion
of propylene to propylene oxide is a pertinent example of a major
industrial process that utilises TS-1, with over 1.2 Mt of propylene
the MFI-type zeolite
2 2
support can enhance catalytic activity toward H O
2
[
78]
[
64,65]
selective towards H
2
O ,
2
with Drago et al. investigating the
.[79] It should
alkane
acidity of TS-1, finding it to be similar to that of SiO
2
also be noted that the activity of the 2.5% Au- 2.5% Pd / TS-1
catalyst is significantly greater than that of the well-studied 2.5%
Au- 2.5% Pd / TiO
Pd / SiO
.[77]
2
catalyst[75] and the analogous 2.5% Au- 2.5%
2
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In addition, it is observed that the incorporation of Au into a TS-1
supported Pt catalyst can result in a remarkable synergistic
enhancement in catalytic activity towards the synthesis of H O
2 2
Table 2. Summary of porosity and surface area of TS-1 supported catalysts.
Catalyst
Surface
V
Micropore
/
[a]
3
-1
area
/
cm g
2
-1
m g
with the activity of the bimetallic 2.5% Au- 2.5% Pt / TS-1 catalyst
TS-1
421
410
0.212
0.147
-
(
98 molH2O2kgcat 1h-1) much greater than that of the 5% Au / TS-1
*
2
.5%Au-2.5%Pd/TS-1 (Dried)
or 5% Pt / TS-1 catalysts. Indeed as reported previously for the
.5% Au- 2.5% Pd / TS-1 catalyst the co-impregnation of Au and
2
.5%Au-2.5%Pd/TS-1 (400 °C, 3 h air)
°
384
0.139
2
*
Catalyst dried only static air, 110 C, 16 h.
Pt onto the same support surface is observed to offer enhanced
activity compared to that of a physical mixture of the two mono-
metallic analogues (13 molH2O2kgcat 1h-1) and demonstrates the
beneficial role of Au addition to precious metals which are active
[
a]
Surface area determined from nitrogen adsorption measurements using the
BET equation.
-
Analysis by XRD (Figure S.3) reveals that, as with FTIR analysis,
there is no apparent change in the structure of the TS-1 support,
upon the impregnation of precious metals and subsequent
calcination at 400 °C, according to the main reflection peaks
associated with TS-1 (θ = 23.1°, 23.9°, 24.4° and 24.5°), with
these reflections often cited in literature to demonstrate the typical
MFI structure.[81] In particular the single reflection at 24.2°
indicates the orthorhombic symmetry of TS-1.[63] However, it
should be noted that our analysis by XRD does not account for
the reflections below θ = 10°.[82] Upon impregnation of the metals
and calcination at 400 - 800 °C reflections associated with Au (θ
2 2
towards H O synthesis.
Table. 1. Catalytic activity of TS-1 supported monometallic and bimetallic
catalysts towards H synthesis and its subsequent degradation.
Catalyst
2 2
O
Productivity /
2
H O
2
Degradation /
-
1
-1 (a)
-1 -1 (b)
molH2O2kgcat
h
molH2O2kgcat h
5
5
% Au / TS-1
% Pd / TS-1
2
131
29
100
87
98
13
114
0
6
459
119
316
210
255
59
5
% Pt / TS-1
.5% Au – 2.5% Pd / TS-1
.5 % Au / TS-1 + 2.5% Pd / TS-1
.5% Au – 2.5% Pt / TS-1
.5 % Au / TS-1 + 2.5% Pt / TS-1
2
2
2
=
38°, 44° and 70°) are observed, suggesting Au is present as
2
large, poorly dispersed, nanoparticles, regardless of calcination
temperature. This is corroborated via EDX analysis (Figure S.4.),
which reveals the presence of large Au nanoparticles. Conversely,
no reflections associated with Pd can be observed upon
calcination at 400 °C. However, upon calcination at 600 ⁰C
2
.5% Pd – 2.5% Pt / TS-1
286
0
TS-1
2
.5% Au – 2.5% Pd / TiO
TiO
.5% Au – 2.5% Pd / SiO
SiO
2
64
0
235
0
2
2
2
74
0
488
158
reflections associated with PdO (θ
=
34⁰) are observed,
2
a)
suggesting the agglomeration of metal nanoparticles at elevated
temperatures. Further analysis by transmission electron
microscopy (TEM) corroborates this finding with a significant
increase in mean particle size observed with increasing
calcination temperature, from 6.4 nm to 30.5 nm as calcination
temperature increases from 400 to 800 ⁰C (Figure S.5). However
it should be noted that the 2.5% Au- 2.5% Pd / TS-1 catalyst
calcined at 400 °C shows a relatively tight mean particle size, with
no nanoparticles observed in excess of 50 nm, which is atypical
of catalysts prepared by a wet-impregnation methodology and is
ascribed in part to the large surface area of the TS-1 support.
Investigation of the dried only catalyst by X-ray photoelectron
spectroscopy (XPS) reveals a surface Pd:Au ratio of 0.5,
suggesting, on average, a significant surface enrichment by Au in
the uncalcined sample. In contrast upon calcination at 400 °C a
significant increase in Pd : Au ratio is observed (Pd:Au = 19)
consistent with the development of a Au-core PdO shell
morphology with increasing calcination temperature and identical
H
2
O
2
direct synthesis reaction conditions: Catalyst (0.01g), H
2
O (2.9g), MeOH (5.6g),
(160 psi), 0.5 h, 2 C 1200 rpm.
degradation reaction conditions: Catalyst (0.01g), H (50 wt.% 0.68 g) H
O
5
% H
2
/ CO
2
(420 psi), 25% O
2
/ CO
2
b)
H
2
O
2
2
O
2
2
O
O
(
2 2
2.22g), MeOH (5.6g), 5% H / CO (420 psi), 0.5 h, 2 C 1200 rpm.
Investigation of the calcined 2.5% Au - 2.5% Pd /TS-1 catalyst by
Fourier-transform infrared spectroscopy (FTIR) (Figure S.1)
reveals no discernible change in the observed positions of the
absorption bands compared to a standard unmodified material
suggesting that the structure of TS-1 remains unchanged during
catalysts preparation. Indeed, no change in the structure of the
titanosilicate can be observed upon impregnation of Au and Pd
and subsequent calcination at temperatures up to 800 °C. This is
unsurprising given the high thermal stability of TS-1.[79]It is
possible to observe four distinct infrared bands in the FTIR
spectra of AuPd / TS-1,with bands at 800 cm-1 and 1100 cm-1
assigned to lattice modes associated with internal linkages in
tetrahedral SiO
vibrations of SiO
4
, the band at 990 cm-1 assigned to stretching
tetrahedra bound to Ti atoms as Si-O-Ti
4
linkages and the band at 1240 cm-1 assigned to tetrahedral Ti
present in the TS-1 framework.[80] The details of the textural
properties of TS-1 and the supported AuPd catalyst are
summarised in Table 2 (Figure S.2). It can be seen that upon
immobilisation of metal nanoparticles total surface area and total
to that reported previously for the analogous 2.5% Au-2.5% Pd /
75
2
TiO catalyst. As calcination temperature increases beyond
400 °C the surface atomic ratio of Au: Pd decreases significantly
from 19 to 2.6, (Table S.1) possibly as a result of Au surface
migration or particle agglomeration as indicated by XRD and TEM
analysis.
2
-1
3
-1
pore volume decreases slightly from 421 m g and 0.216 cm g
for the as received TS-1 sample to 384 m2g-1 and 0.139 cm g
3
-1
Investigation into the effect of calcination temperature on catalytic
upon co-impregnation of the precious metals, followed by
calcination. We ascribe this decrease to result from the deposition
of metal nanoparticles inside the zeolitic pore structure.
2 2
activity towards H O formation and its subsequent degradation
can be seen in Table 3. A direct correlation is drawn between
calcination temperature and catalytic activity; with increasing
2 2
calcination temperature catalyst activity towards both H O
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ChemCatChem
10.1002/cctc.201900100
FULL PAPER
synthesis and degradation decreases. Evaluation of catalyst
activity upon re-use revealed that the dried only catalyst retained
Productivity
Degradation
500
approximately 58 % of its initial H
2
O
2
synthesis activity. This loss
140
in catalytic activity is attributed to leaching of active metals from
the support with significant loss of Au and Pd observed via MP-
AES analysis for the dried only catalyst (Table S.2). This leaching
is observed to decrease with increasing calcination temperature
with similar findings previously reported for 2.5% Au- 2.5% Pd /
1
20
00
400
1
300
8
0
, prepared via a similar methodology.[75] When calcined at
60
200
100
0
TiO
4
2
00 C catalytic activity decreased from 152 molH2O2kgcat-1h-1 for
⁰
4
0
0
0
the dried only sample to 100 molH2O2kgcat-1h-1 but crucially
remained stable upon second use, with minimal leaching of Au
and Pd observed. As discussed above a decrease in catalytic
activity is observed to correlate with an increase in mean particle
size as calcination temperature increases and this is in keeping
with previous findings in the literature.[83]
2
0
20
40
60
80
100
Pd content / %
Figure 2. The effect of Au: Pd (wt/wt) ratio on catalytic activity of AuPd / TS-1
towards the direct synthesis and degradation of H
direct synthesis reaction conditions: Catalyst (0.01g), H
2
2
O .
Table 3. The effect of calcination temperature on the catalytic activity of 2.5%
H
O
2 2
2
O (2.9g),
O
Au – 2.5% Pd / TS-1 towards H
2
O
2
synthesis and its subsequent degradation.
MeOH (5.6g), 5% H
rpm. H degradation reaction conditions: Catalyst (0.01g), H
0.68 g) H O (2.22g), MeOH (5.6g), 5% H / CO
(420 psi), 0.5 h, 2 OC 1200 rpm.
2
/ CO
2
(420 psi), 25% O
2
/ CO
2
(160 psi), 0.5 h, 2 C 1200
Calcination
Initial Productivity /
Re-use
2
H O
2
Degradation /
2
O
2
2 2
O
(50 wt.%
O
-1 -1 (a)
-1 -1 (b)
2
2
2
temperature /
C
molH2O2kgcat
h
Productivity /
molH2O2kgcat
h
-
1 -1
molH2O2kgcat
a)
h
Our previous studies have reported the activity of Pt containing
Au-Pd supported catalysts towards the direct synthesis of
(
Dried Only*
152
128
100
35
89
98
413
392
316
181
117
[61,62]
H
2
O
2
,
Building on our initial findings and in an attempt to
3
4
6
8
00
00
00
00
improve catalytic selectivity, we investigated the effect of Pt
addition to 2.5% Au- 2.5% Pd / TS-1 on catalytic activity towards
H O synthesis and its subsequent degradation, with the results
2 2
100
35
30
30
a)H
shown in Figure 3. As can be observed, the addition of a small
amount of Pt (0.2 %) to a supported AuPd/TS-1 catalyst reduces
2
O
2
direct synthesis reaction conditions: Catalyst (0.01g), H
/ CO (420 psi), 25% O / CO
degradation reaction conditions: Catalyst (0.01g), H
2
O (2.9g),
MeOH (5.6g), 5% H
rpm. H
2
2
2
2
(160 psi), 0.5 h, 2 OC 1200
b)
O
2 2
2
O
O
2
(50 wt.%
2 2 2
O (2.22g), MeOH (5.6g), 5% H / CO (420 psi), 0.5 h, 2 C 1200 rpm.
°
-
the extent of H
2
O
2
degradation significantly from 316 molH2O2kgcat
0
*
.68 g) H
Catalyst dried only static air, 110 C, 16 h.
1h for the 2.5% Au- 2.5% Pd / TS-1 catalyst, to 93 molH2O2kgcat
-1
-
1h for the 2.4% Au – 2.4% Pd -0.2% Pt / TS-1 catalyst. This
-1
2 2
Further investigation into the effect of Au : Pd ratio on H O
formation and degradation can be seen in Figure 2. As previously
2 2 2
reported for an analogous SiO O
catalyst[ activity towards H
synthesis correlates well with total Pd content, possibly due to
incomplete formation of the Au-core-PdO shell morphology often
observed on oxide supports. As with activity towards H
synthesis, catalytic degradation activity is also directly related to
Pd content, with the 5% Au / TS-1 catalyst reported to have a
minimal degradation activity (6 molH2O2kgcat h ), while the
analogous Pd catalyst is observed to offer much greater
2 2
corresponds with an increase in the rate of H O synthesis, from
-1 -1
1
00 to 167 molH2O2kgcat h . However, further addition of Pt results
77]
in an increase in degradation activity, with this metric rising to a
-1 -1
maxima of 247 molH2O2kgcat
h
for the 2% Au-2%Pd-f 1% Pt/ TS-
1
catalyst. A corresponding decrease in H O synthesis rate is
2 2
2
O
2
-1 -1
observed (69 molH2O2kgcat h ) and this is in keeping with our
previous investigations into the effect of Pt introduction into Au-
[61]
.[62]
Pd supported on CeO
2
and TiO
2
-1 -1
-1 -1
degradation activity (459 molH2O2kgcat h ), similar to that
previously reported for the analogous SiO
supported catalyst.[77]
2
Analysis by both XRD (Figure S.6) and EDX (Figure S.4) indicates
the presence of poorly dispersed Au nanoparticles regardless of
Au: Pd ratio, with XRD reflections for Au (θ= 38, 44 and 70°)
observed in all samples.
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Productivity
Degradation
supported catalysts, analysis by EDS (Figure 3) does not provide
any evidence for a Pd-rich outer later. Further introduction of Pt
results in a minor decrease in the Pd: Au ratio, again this is in
keeping with our previous studies.[61,62] Perhaps more
interestingly the addition of Pt is observed to significantly enhance
1
1
1
1
1
80
60
40
20
00
350
300
250
200
150
100
50
0
0
2+
0
Pd content, with the Pd :Pd ratio increasing significantly from
2+
2
1 (all Pd present as Pd )for the 2.5% Au-2.5% Pd / TiO catalyst
to 0.55 with the incorporation of a small amount of Pt, with further
8
6
4
2
0
0
0
0
0
0
0
Pt addition enhancing Pd content, with the low selectivity of Pd
towards H
well reported in the literature.[51] However Ouyang
et al.[84] have reported the enhanced selectivity and activity of
2
O
2
0
2+
supported Pd catalysts containing Pd -Pd
comparison to those catalysts with predominance of either Pd
oxidation state. With this ascribed to the propensity of H to
dissociate on Pd0 and the enhanced stability of O
on PdO
surfaces, with the maintenance of the O-O bond required for the
formation of H over H O. It is therefore possible to relate the
ensembles in
-
2
0
2
4
6
8
10 12 14 16 18 20 22
2
Total Pt content / % (Au : Pd = 1)
2
Figure 3. The effect of Pt incorporation into AuPd / TS-1 on catalytic activity
towards the direct synthesis and degradation of H
direct synthesis reaction conditions: Catalyst (0.01g), H
MeOH (5.6g), 5% H / CO (420 psi), 25% O / CO
(160 psi), 0.5 h, 2 OC 1200
rpm. H degradation reaction conditions: Catalyst (0.01g), H (50 wt.%
.68 g) H (420 psi), 0.5 h, 2 C 1200 rpm.
2
O
2
.
2
O
2
2
H
O
2 2
2
O (2.9g),
enhanced catalytic performance of the 2.4% Au – 2.4% Pd -0.2%
Pt / TS-1 catalyst due to the development of these Pd domains of
mixed oxidation state. While the increased degradation rates
observed at higher Pt loadings results from a significant increase
2
2
2
2
2
O
2
2 2
O
O
0
2 2 2
O (2.22g), MeOH (5.6g), 5% H / CO
0
Investigation of the AuPdPt / TS-1 catalyst series by XRD is
shown in Figure S.7, as with the bimetallic AuPd / TS-1 catalysts
no reflections associated with PdO are observed, in comparison
reflections associated with Au can be observed in all Au
in Pd content.
Table 4. Effect of Pt incorporation on Au: Pd atomic ratio of supported Au-
Pd / TS-1 catalysts as determined by XPS analysis.
Pd2+ :Pd
0
Catalyst
Pd:
Au*
19
22
18
⁰
⁰
containing samples (θ = 38 , 44 ) and potentially indicates the
poor dispersion of Au on the support. This is in keeping with our
analysis of the 2.4% Au – 2.4% Pd -0.2% Pt / TS-1 catalyst by
Scanning Transmission Electron Microscopy (STEM) coupled
with X-ray Energy Dispersive Spectroscopy (X-EDS) as seen in
Figure 4 (STEM-ADF and corresponding X-EDS seen in Figure
S.8) which reveals that metal nanoparticles immobilised on the
titanosilicate surface consist of a combination of large (> 20 nm)
Au-rich nanoparticles, which also contain Pd and Pt. In addition
smaller (< 10 nm) nanoparticles are observed that contain both
Au and Pd in similar quantities, while there also exists Pd-rich
clusters in the range of 2 nm.
2.5% Au – 2.5% Pd / TS-1
1.00
0.55
0.36
0.35
n.d.
2
.4 % Au – 2.4% Pd -0.2% Pt / TS-1
.35% Au – 2.35% Pd -0.3% Pt / TS-1
2.28% Au – 2.28% Pd -0.44% Pt / TS-1
% Au – 2% Pd – 1% Pt / TS-1
2
17
n.d.
2
⁰
⁰
-1
All catalysts calcined 400 C, 3 h, 20 Cmin , static air.
n.d: not determined. * Expected value = 1.9
Particle size derived from transmission electron microscopy, seen
in Table 5 and Figure S.9, reveals that mean particle size is similar
for the 2.5% Au – 2.5% Pd / TS-1 (6.4 nm) and 2.4% Au – 2.4%
Pd -0.2% Pt / TS-1 (7.2 nm) catalysts despite the distinct contrast
in catalytic performance. The observation that mean particle size
increases only slightly upon addition of small quantities of Pt
suggests that the enhancement in catalytic synthesis activity
cannot be associated with metal dispersion. With further addition
of Pt mean particle size decreases significantly to 2.6 nm for the
2
% Au– 2% Pd -1% Pt / TS-1 catalyst, with a corresponding
decrease in catalytic selectivity towards H . We have previously
reported a correlation between mean nanoparticle size and
catalytic selectivity towards H for AuPd catalysts supported on
TiO with larger AuPd nanoparticles displaying a higher selectivity
towards H
2
O
2
2
O
2
Figure 4. HAADF images of 2.4% Au – 2.4% Pd – 0.2% Pt / TS-1 calcined
sample and corresponding EDS spectra. Note the presence of large Au-rich,
2
[85]
Pd-rich and Au-Pd alloy particles.
2
O
2.
It is therefore possible to correlate this significant
decrease in mean particle size with a loss of catalytic selectivity.
Further analysis by XPS can be seen in Table 4, as with the 2.5%
Au- 2.5% Pd / TS-1 catalyst the observed Pd :Au surface atomic
ratios for the AuPdPt / TS-1 catalysts are much higher than those
calculated from the nominal compositions. It is observed that upon
the introduction of a small amount of Pt the Pd: Au surface atomic
ratio increases marginally from 19 for the 2.5% Au – 2.5% Pd /
TS-1 catalyst to 22 for the 2.4% Au – 2.4% Pd -0.2% Pt / TS-1
catalyst. Although this may be a result of a minor enhancement of
the Au-core PdO-shell morphology, often observed on oxide
Table 5. Particle size of AuPdPt / TS-1 catalysts as determined by TEM.
Catalyst
Particle size / nm
Standard deviation)
6.4 (3.6)
(
2.5% Au – 2.5% Pd / TS-1
2.4% Au – 2.4% Pd -0.2% Pt / TS-1
7.2 (4.27)
2
% Au – 2% Pd – 1% Pt / TS-1
2.6 (0.99)
All catalysts calcined 400 ⁰ C, 3 h, 20 ⁰ Cmin , static air.
-1
For any heterogeneous catalyst operating in a three-phase
system the possibility of the leaching of the active phase and
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ChemCatChem
10.1002/cctc.201900100
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resulting homogeneous contribution towards activity or
deactivation of the catalyst is of great concern, with the activity of
catalysts represent a promising system to explore the direct
synthesis of H O .
2 2
2 2
homogeneous Pd towards the formation of H O previously
reported by Dissanayke and Lunsford.[86] We have already
established the need for the use of an appropriate heat treatment
procedure to impart catalyst stability, with a minimum calcination
temperature of 400 °C required to produce a stable AuPd / TS-1
catalyst (Table 2). Further investigation into the catalytic stability
of the optimal 2.4% Au- 2.4% Pd-0.2% Pt / TS-1 catalyst exposed
to an identical heat treatment regime can be seen in Figure 5.
Experimental Section.
Catalyst Preparation.
Au-Pd and Au-Pd-Pt / TS-1 catalysts have been prepared (on a
weight metal basis) by wet co-impregnation of metal salts, based
on methodology previously reported in the literature.
[75]
The
procedure to produce 2 g of 2.4% Au – 2.4% Pd- 0.2% Pt / TS-1
is outlined below, with a similar methodology utilised for mono-
and bi-metallic catalysts.
180
160
140
120
100
-
1
PdCl
(
(
2
(8.00 ml, 6 mgml , Sigma Aldrich), HAuCl
4
.3H
3.9184 ml, 12.25 mgml , Strem Chemicals) and H PtCl
0.4211 ml, 9.5 mg ml , Sigma Aldrich) were placed in a 50 ml
2
O solution
-1
2
6
.6H O
2
-1
2
round bottom flask, with total volume fixed to 16 ml using H O
80
60
40
20
0
(HPLC grade). The resulting mixture was heated to 65 °C in a
thermostatically controlled oil bath with stirring (1000 rpm). Upon
reaching 65 °C TS-1 (1.90 g, ACS Materials) was added over the
course of 5 minutes. The resulting slurry was then heated to 85 °C
for 16 h to allow for complete evaporation of water. The resulting
solid material was ground prior to calcination in static air (400 °C,
Use 1
Use 2
Use 3
-1
3h, 20 °C min ).
Figure 5. Catalyst stability of 2.4% Au- 2.4% Pd- 0.2% Pt / TS-1 catalyst towards
the direct synthesis and degradation of H
direct synthesis reaction conditions: Catalyst (0.01g), H
Direct Synthesis of H
Hydrogen peroxide synthesis was evaluated using a Parr
Instruments stainless steel autoclave with a nominal volume of
2
2
O .
O .
2 2
H
O
2 2
2
O (2.9g),
MeOH (5.6g), 5% H
rpm.
/ CO (420 psi), 25% O / CO
2 2 2 2
(160 psi), 0.5 h, 2 OC 1200
1
00 ml and a maximum working pressure of 14 MPa. To test each
catalyst for H synthesis, the autoclave was charged with
catalyst (0.01 g), solvent (5.6 g MeOH and 2.9 g H O). The
charged autoclave was then purged three times with 5% H / CO
0.7 MPa) before filling with 5% H / CO to a pressure of 2.9 MPa,
followed by the addition of 25 % O / CO (1.1 MPa). The
2
O
2
As can be seen catalytic activity is observed to decrease
marginally with re-use, from 167 to 156 molH2O2kgcat 1h-1, with this
loss ascribed to the leaching of Pd and Pt over the first two uses,
as determined by MP-AES analysis of the digested post-reaction
catalyst (Table S.3). Total leaching of Pd and Pt over the first two
uses is observed to be 2.6 and 2.2 % of the total individual
elemental loading respectively. It should be noted that after two
uses no further loss of either Pd or Pt is detected. Interestingly no
loss of Au is observed, which is keeping with our previous work
2
-
2
2
(
2
2
2
2
temperature was then decreased to 2 °C followed by stirring (1200
rpm) of the reaction mixture for 0.5 h. The above reaction
parameters represent the optimum conditions we have previously
used for the synthesis of H
has been previously been shown to be optimal for H
with the higher solubility of H in MeOH compared to H
.[87]
factor in achieving enhanced yields of H productivity
was determined by titrating aliquots (approximately 0.5 g) of the
final solution after reaction with acidified Ce(SO (0.01 M) in the
.[75] Our choice of reaction solvent
synthesis,
O a key
2 2
O
2
O
2
[75]
2
investigating the stability of 2.5% Au- 2.5% Pd/ TiO .
2
2
2
O
2
2 2
H O
Conclusions.
4 2
)
We have investigated the activity of TS-1 supported AuPd and
AuPdPt catalysts prepared by a conventional co-impregnation
methodology for the direct synthesis of H O . Furthermore, we
2 2
presence of ferroin indicator. Catalyst productivities are reported
−1 −1
as molH2O2kgcat h .
establish a means of imparting catalyst stability through choice of
appropriate heat treatment while maintaining the MFI structure of
the zeolite. Through the introduction of small concentrations of Pt
into a supported AuPd/TS-1 catalyst it is possible to significantly
Catalyst Reusability in the Direct Synthesis of H
In order to determine catalyst reusability a similar procedure to
that outlined above for the direct synthesis of H is followed
utilising 50 mg of catalyst. Following the initial test the catalyst is
recovered by filtration and dried (110 °C, 16 h, air), from the
2 2
O .
2
O
2
enhance catalytic selectivity towards H
AuPd / TS-1 in a Au:Pd:Pt ratio of 1 : 1 : 0.2 is shown to suppress
catalytic activity towards H degradation, thus improving overall
yields of H . We ascribe this improvement in catalytic selectivity
to the development of Pd domains of mixed oxidation state, well
known for its high selectivity towards H . We believe that these
2 2
O . The addition of Pt to
recovered catalyst sample 10 mg is used to conduct a H
synthesis test as outlined above.
2 2
O
2
O
2
2
O
2
2
O
2
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ChemCatChem
10.1002/cctc.201900100
FULL PAPER
Degradation of H
Catalytic activity towards H
manner similar to the direct synthesis activity of a catalyst. The
autoclave was charged with MeOH (5.6 g), H (50 wt. % 0.69
g) HPLC standard H O (2.21 g) and catalyst (0.01 g), with the
solvent composition equivalent to a 4 wt. % H solution. From
the solution 2 aliquots of 0.05 g were removed and titrated with
acidified Ce(SO solution using ferroin as an indicator to
determine an accurate concentration of H at the start of the
reaction. The autoclave was pressurised with 2.9 MPa 5 % H
CO and cooled to 2 °C. Upon reaching 2 °C the reaction mixture
2
O
2
.
JEOL JEM-ARM200CF microscope in Diamond Laboratory. The
TEM specimen investigated was 2.4% Au – 2.4% Pd – 0.2% Pt /
TS-1 prepared by co-impregnation of precious metals onto the
titanosilicate support, calcined at 400 ⁰C, 3 h, 20 ⁰Cmin , static
air.
2
2
O degradation was determined in a
-1
2
O
2
2
2
O
2
Metal leaching was quantified using microwave plasma - atomic
emission spectroscopy (MP-AES). Post-reaction solid catalysts
were digested (10 mg catalyst, 10 ml aqua-regia, 16 h) prior to
analysis using an Agilent 4100 MP-AES.
4 2
)
2
O
2
2
/
2
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 two aliquots (approximately 0.05 g) were titrated
Acknowledgements.
4 2
against an acidified Ce(SO ) solution using ferroin as an indicator.
The degradation activity is reported as molH2O2kgcat h .
The authors wish to thank UBE Industries, Ltd. for financial
support and the Cardiff University electron microscope facility for
the transmission electron microscopy. We also thank Diamond
Light Source (DLS) and electron Physical Science Imaging
Centre(ePSIC) for access (Instrument E01 session number
EM19246) and support that contributed to the results presented.
XPS data collection was performed at the EPSRC National
Facility for XPS (‘HarwellXPS’), operated by Cardiff University
and UCL, under contract No. PR16195.
-1 -1
Catalyst Characterisation.
Fourier-transform infrared spectroscopy (FTIR) was carried out
with a Bruker Tensor 27 spectrometer fitted with a HgCdTe (MCT)
detector and operated with OPUS software.
N
(
2
isotherms were collected on a Micromeritics 3Flex. Samples
ca. 0.020 g) were degassed (150 °C, 6 h) prior to analysis.
Analyses were carried out at 77 K with P measured continuously.
Free space was measured post- analysis with He. Pore size
Keywords: Hydrogen Peroxide, Au-Pd-Pt, TS-1, Green
0
Chemistry
analysis was carried out using Micromeritics 3Flex software, N
Cylindrical Pores- Oxide Surface DFT Model.
2
-
[
1
[
2
[
3
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1
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Entry for the Table of Contents
Layout 1:
FULL PAPER
When three metals are better than
two: The introduction of small
quantities of Pt into AuPd
Richard J. Lewis, Kenji Ueura, Yukimasa
Fukuta, Simon J. Freakley, Liqun Kang,
Ryan Wang, Qian He, Jennifer K.
180
nanoparticles supported on the
titanosilicate TS-1 significantly
enhances catalytic activity and
160
Edwards, David J. Morgan, Yasushi
Yamamoto and Graham J. Hutchings*
140
120
100
selectivity towards H
from molecular H and O
modification of Pd electronic structure.
2
O
2
synthesis
Page No. – Page No.
The direct synthesis of H
supported catalysts.
8
0
2 2
O using TS-1
through
60
2
2
4
0
0
0
2
Au
AuPd
Pd
AuPdPt
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