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Green Chemistry
“This document is the Accepted Manuscript version of a Published Work that appeared in final form in Green Chemistry,
copyright © The Royal Society of Chemistry 2016 after peer review and technical editing by the publisher. To access the final
edited and published work see http://pubs.rsc.org/en/content/articlelanding/2015/gc/c5gc01589h#!divAbstract

Light Driven Styrene Epoxidation and Hydrogen Generation Using
H2O as an Oxygen Source in a Photoelectrosynthesis Cell
Received 13th July 2015,
Accepted 28th Setember 2015
DOI: 10.1039/ C5GC01589H
www.rsc.org/

P. Farràs,

a,b

a

a

a

*,a,c

C. Di Giovanni, J. N. Clifford, P. Garrido-Barrios, E. Palomares

and A. Llobet

*,a,d

A dye-sensitized photoelectrosynthesis cell (DSPEC) has been prepared for the oxidation of alkenes to epoxides and
evolution of hydrogen using water as an oxygen source and sunlight. A Ru oxidation catalyst, 2,2+, is used in homogeneous
phase in the anodic compartment to oxidize a water-soluble alkene, 4-styrene sulfonic acid (4-HSS), to the corresponding
epoxide that in acidic solution is hydrolyzed to the diol (4-(1,2-dihydroxyethyl)-benzenesulfonic acid). Concomitantly
protons are generated that diffuse through a proton exchange membrane to a Pt cathode where are transformed into
hydrogen. Illumination under 1.5 AMG (100 mW cm-2) together with an external bias of 0.3 V vs. NHE after 24 h, leads to
the generation of 1.28 Coulombs together with the formation of 6.1 µmol of H2 at the cathodic compartment that
corresponds to a faradaic efficiency of 92%. In addition 0.7 mM of the 4-HSS substrate has been oxidized at the anodic
compartment with a conversion yield of 7%. The rate of hydrogen evolution is limited by the oxidation of the organic
substrate, and the TOF for both reactions is measured to be 0.8 ks-1.

INTRODUCTION
Epoxides are highly useful intermediates for the production of
1,2
a variety of important commercial products, and therefore
their synthesis is a subject of substantial academic and
industrial interest. Olefin epoxidation is one of the main paths
that lead to the production of epoxides both at a laboratory
3
and at the industrial scale. While molecular oxygen can be
used for the epoxidation of ethylene, the majority of alkene
epoxidation reactions are carried out using stoichiometric
amounts of alkylhydroperoxides or peracids, with the
formation of the corresponding alcohol or carboxylic acid
respectively as an undesired side product. In addition, from a
safety perspective, peracids are relatively hazardous
chemicals. In this respect, there is a strong need for the
development of new epoxidation methods that make use of
safer oxidants and minimize waste products. The employment
of hydrogen peroxide is an attractive option both on
4-7
environmental and economic grounds giving just water as a
byproduct. Another option is to use water as the source of
oxygen for the epoxidation reaction with the formation of
hydrogen as a valuable side product as indicated in the

a.

Institute of Chemical Research of Catalonia (ICIQ), Avinguda Països Catalans 16,
E-43007 Tarragona, Spain.
Molecular Photonics Laboratory, School of Chemistry, Newcastle University,
Newcastle upon Tyne, NE1 7RU, UK.
c.
ICREA, Avinguda Lluís Companys 28, Barcelona E-08030, Spain.
d.
Departament de Química, Universitat Autònoma de Barcelona, Cerdanyola del
Vallès, E-08193 Barcelona, Spain.
†
Electronic Supplementary Information (ESI) available: Experimental section
containing the preparation of photoanodes and modified membranes, cell
construction and additional spectroscopic, electrochemical, and catalytic data. See
DOI: 10.1039/x0xx00000x
b.

equation below,

-1

which involves the input of 37.35 Kcal mol given its uphill
9,10
thermodynamics that can be provided thermically
or
11
photochemically with sunlight. This strategy also benefits
from the formation of hydrogen as a byproduct that is also a
12,13
clean energy vector.
Given the fact that the worldwide
production of ethylene oxide represents 20 million tons per
14
year, and assuming that the reaction is driven by sunlight and
with a 100 % atom economy, it would represent around 0.9
million tons of hydrogen produced in that process, or 2500
tons of hydrogen per day. In terms of energy, this represents
the production of an impressive 2700 MWh per day just for
this reaction. The same conceptual scheme can be applied for
the oxidation of aldehydes to acids, the oxidation of organic
sulfides to sulfoxides, etc.
To achieve the reaction shown in equation (1), a set of
reactions catalyzed by transition metal complexes is needed.
9
For the thermal case, Ru pincer type of complexes have been
used for the oxidation of aldehydes to carboxylic acids and
formation of hydrogen with a single catalyst in homogeneous
phase.
A different strategy consist on taking advantage of the intrinsic
redox nature of the chemical reactions occurring and separate
the single reaction shown in equation (1) into two half
reactions. This would allow building an electrochemical cell
where the oxidation of the alkene will take place at the anode
and the proton reduction will occur at the cathode, as
depicted for styrene in equations (2) and (3) respectively.

8

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Furthermore, if the reaction is to be driven photochemically, a
light harvesting molecule is needed since sunlight interacts
with H2O and alkenes mainly at a vibrational level. Thus a
series of additional reactions involving light harvesting
molecules capable of interacting with suitable catalysts will
need to be designed in order to break and form the desired
bonds at a reasonable rate.
The oxidation of substrates induced by visible light using water
as an oxygen source has been previously described in the
2+
literature in homogeneous phase using [Ru(bpy)3] (bpy is
2,2’-bypyridine) as a light harvesting molecule coupled to a
sacrificial electron acceptor (SEA), such as sodium persulfate or
2+ 15-19
Co(III) complexes like [Co(NH3)5Cl] .
Other examples
include the use of catalyst-photosensitizer dyad systems that
20-25
can oxidize alcohols, sulfides and alkenes.
On the other
hand the reduction of protons has also been achieved with a
26,27
variety of sacrificial electron donors (SED).
In both cases
the sacrificial agents react fast and in an irreversible manner so
that they favor only the desired reactions. The development of
photoelectrosynthesis cells (DSPEC) to carry out the reaction in
equation (1) implies necessarily the absence of sacrificial
agents and thus introduces further difficulties in its overall
28-37
implementation.
Today there is an urgent need to build
DSPECs and understand and master all the kinetic and
thermodynamic parameters involved in such a cell so that
efficient devices can be obtained.
With all this in mind, we have designed a DSPEC using
FTO/TiO2-P-bpy-Ru (where FTO is fluorine doped tin oxide, PII
2+
bpy-Ru is [Ru (P-bpy)(bpy)2] , P-bpy is 2,2'-bipyridine-4,4'bis(phosphonic acid), bpy is 2,2’-bypyridine) as a photoanode
and a Pt mesh as a cathode. The oxidation catalyst will be
present in homogeneous phase since this configuration gives a
high degree of flexibility with regard to the selection and use
of potential catalysts and reactions. The latter has to comply
with two indispensable requirements: a) the catalyst has to act
efficiently with regard to substrate oxidation and b) the active
site of the catalyst needs to be regenerated very fast by the
photoanode. Finally, a specific proton exchange membrane
based on a combination of Nafion® and poly(3,4ethylenedioxythiophene) (PEDOT) has been designed so that
cationic species in the anodic compartment do not block the
proton channel.
Here on we report a two compartment DSPEC cell for H2
generation and epoxidation of alkenes using visible light and
water and its complete kinetic and thermodynamic
characterization.

RESULTS AND DISCUSSION
+

2.1 Alkene epoxidation catalyzed by 2,2 in homogeneous
phase.
+

We have recently reported a dinuclear Ru-aqua 2,2 complex,
whose structure is shown in Figure 1 and that contains a

dinucleating trianionic ligand backbone (pyrazolate-3,53dicarboxylate, pdc ) that bridges the two Ru metal centers and
a tridentate meridional ligand (2,2’:6’,2”-terpyridine, trpy) that
+
Figure 1. Drawn structure for 2,2 complex and catalytic
scheme proposed for the epoxidation of styrene by the active
+
species derived from 2,2 . In the scheme the 2,2’:6’,2”3terpyridine (trpy) and pyrazolate-3,5-dicarboxylate (pdc )
ligands are not shown.

completes the octahedral geometry around the metal center.
+
Complex 2,2 has been shown to be a powerful epoxidation
catalyst for a variety of alkenes including styrene (TOFi > 2.9 s
1 38
) in organic solvents and using PhIO as sacrificial oxidant.
The catalytic mechanism proposed is shown in Figure 1. In
+
order to introduce the 2,2 catalyst into a DSPEC cell it is
indispensable that it also works in water and is activated by 1e
3+
outer sphere electron transfer oxidants such as [Ru(bpy)3] .
For this purpose we have carried out the photochemically
3+
induced generation of [Ru(bpy)3]
in water using
2+
III
[Co(NH3)5Cl] (Co ) as a sacrificial electron acceptor in the
presence of the water soluble sodium 4-styrene sulfonate as
substrate. Since we have carried out all the reactions at pH =
1.0, the latter is protonated forming 4-styrene sulfonic acid (439
HSS).
+
A deoxygenated solution containing 2,2 0.01 mM/4-HSS 10
2+
III
0.1 mM/Co 10 mM in 4 mL of a
mM/[Ru(bpy)3]
D2O:CF3SO3D mixture (pD 1.0) was irradiated with visible light
for 30 min and generated 1.1 mM 4-(1,2-dihydroxyethyl)benzenesulfonic acid. The latter is a known phenomenon due
to the ring-opening hydrolysis of the epoxide under acidic
conditions, and was already reported by Fukuzumi et al. for
40
the chemical oxidation of epoxides using a Ru catalyst. The
conversion of the initial substrate reached 30% that represents
-1
a TOF of 0.17 s (see Figure S1 for details). This results shows
3+
that [Ru(bpy)3] is capable of activating the catalyst to its
+
active state, 4,4 (see Figure 1 and equation (4)), and that the
latter is capable of oxidizing the styrene substrate to styrene
oxide that is finally hydrolyzed to the corresponding diol
(equations 4-6). The reaction mechanism for this catalyst was
+
38
previously studied in our group. If no 4,4 had been
3+
produced, only the oxidation by [Ru(bpy)3] would have been
observed. In that case, the reaction without catalyst is not
selective and produced only the aldehyde derivative (see
Figure S1).

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Green Chemistry
“This document is the Accepted Manuscript version of a Published Work that appeared in final form in Green Chemistry,
copyright © The Royal Society of Chemistry 2016 after peer review and technical editing by the publisher. To access the final
edited and published work see http://pubs.rsc.org/en/content/articlelanding/2015/gc/c5gc01589h#!divAbstract
Figure 2. Left, schematic drawing of a photoanode. Right, transient absorption spectroscopy (TAS) kinetics of FTO/TiO2-P-bpy-Ru,
TiO2-P-tBu/ photoanodes in a 0.1 M LiClO4/HClO4 aqueous solution (pH 1.0) in the presence of 42.5 mM 4-HSS. Black, in the
+
+
absence of catalyst; red, with the addition of 30 µM 3,3 ; blue, with the addition of 220 µM 3,3 . TAS kinetics were recorded
-2
under 10 mW cm , using laser excitation pulses at 450 nm and recorded at 650 nm.

2.2 The performance of the photoanode
As a photoanode we use FTO coated with TiO2 containing both
II,2+
P-bpy-Ru
and tert-butylphosphonic acid (P-tBu). The latter
minimizes interactions of the catalyst with the TiO2 surface and
thus avoids undesirable back electron transfer (ET) reactions
improving the yield of the productive reactions. The same
strategy has been used in dye sensitized solar cells (DSC) with
41,42
phosphinate amphiphiles.
We label this photoanode as
FTO/TiO2-P-bpy-Ru, TiO2-P-tBu/ and a schematic drawing of
its structure is offered in Figure 2. Transient absorption
spectroscopy (TAS) was carried out in 0.1 M LiClO4/HClO4
aqueous solution at pH = 1.0 with this photoanode in the
presence of the catalyst and substrate in order to check the
capacity of this anode to activate the catalyst with light and to
kinetically characterize the reactions involved.
Figure 2 shows the results obtained upon illuminating under
-2
10 mW cm , using laser excitation pulses at 450 nm and
3+
recording at 650 nm where the anchored [Ru(bpy)3] moiety
has a broad absorption band. In black is shown the kinetics in
the absence of catalyst whereas the red trace contains 30 µM
+
+
3,3 and the blue one 220 µM 3,3 . We add the catalyst in
38
oxidation state 3,3 since, as we have shown previously, the
lower oxidation states are precursor to the real catalytic
+
+
species that are in oxidation states 3,4 and 4,4 as indicated at
the right hand side of Figure 1.
In Figure 3 is depicted a scheme that summarizes the most
significant reactions that can occur at the photoanode
compartment and that will be analyzed in detail here on. In the

absence of catalyst the system decays with a half lifetime of
about 4.1 ms which is attributed to the recombination reaction
shown in eq. 7 (red arrow 6 in Figure 3) once the electron is
injected into the TiO2 conduction band. The latter is known to
43
happen within the picosecond time scale.
+

In the presence of low concentration of catalyst 3,3 the bpyIII,3+
signal decays at a similar rate. However as the catalyst
Ru
concentration is increased from 30 to 220 µM the decay of the
III,3+
bpy-Ru
signal becomes faster by almost an order of

magnitude with a t1/2 ≈ 0.02 ms. A dependence of dye
regeneration on red/ox couple concentration is also not
44,45
uncommon in DSCs.
The notable increase in signal intensity
Figure 3. Scheme showing the most significant reactions that
can occur at the photoanode compartment. Red arrows
indicate unproductive reactions whereas blue arrows are
desired reactions.

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at slower timescales (0.1-1 ms) is attributed to the formation
+
of a new species which we ascribe to the 4,4 state of the
catalyst according to Equations (8) and (9) (blue arrows 3 and 4
+
in Figure 3). The decay of the 4,4 state is due to the oxidation
of the 4-HSS substrate concomitant with the generation of the
+
catalyst, 3,3 species (blue arrow 5 in Figure 3). Due to the low
signal to noise ratio of the kinetic traces we were not able to
record the transient spectra of the long-lived species,
+
however, we note that the 4,4 state of the catalyst (generated
in solution by bulk electrolysis at 1.12 V) also has an
absorption band at 650 nm (see Figure S2).
The fast kinetics observed in the catalyst oxidation all the way
+
to 4,4 together with the generation of oxidized substrate (vide
infra), indicates that the undesired back electron transfer from
the (-)TiO2 and the catalyst (red arrows 7 and 8 in Figure 3)
takes place at longer than ms timescale. On the other hand
this fast forward kinetics enables the use of this set up for the
construction of a DSPEC, as described in the next section.

2.3 The photoelectrosynthesis cell
®

As a proton exchange membrane, Nafion has been widely
11,46
used in fuel cells.
However, in our case, catalytic cationic
species present in solution would adsorb at the surface of the
®
Nafion film blocking the membrane and sequestering the
catalyst. To avoid this, we have deposited a film of poly(3,447
®
ethylenedioxythiophene) (PEDOT) on the surface of Nafion
and generated a new membrane with practically the same
®
conductivity and proton transfer kinetics as Nafion and where
the catalysts is not adsorbed (see SI and Figure S3 for details).
With this membrane we have built a two compartment DSPEC
using FTO/TiO2-P-bpy-Ru, TiO2-P-tBu/ as a photoanode and a
Pt mesh as cathode. The anodic compartment contains a 0.1 M
+
solution of LiClO4 and 2,2 in a 0.1 M perchloric acid solution
Figure 4. Up, linear sweep voltammetry or I-V curve of the
DSPEC upon 1.5 AMG irradiation with a Xe lamp (λ > 400 nm,
-2
100 mW cm ) (red line) or without light (black line) at a scan
-1
rate of 10 mV s . The anodic half-cell contains FTO/TiO2-P-8
-2
bpy-Ru, TiO2-P-tBu/ (Γ = 8.8 x 10 mol cm ) as the working
+
electrode, 0.01 mM 2,2 as catalyst and 4-HSS 20 mM as
substrate. The cathodic half-cell consists of a Pt mesh as
auxiliary electrode. In both cases the supporting electrolyte
solution is made of a 0.1 M LiClO4/HClO4 aqueous solution (pH
1.0) up to a total volume of 9 mL in each compartment. Down,
hydrogen evolution and current intensity measured at 0.3 V vs.
NHE applied bias for the same DSPEC under exactly the same
conditions.

whereas the cathodic compartment contains the same
solution in the absence of the catalyst. LiClO4 is added to
enhance current densities since it decreases recombination
III
events by stabilizing the Ti trap sites (see Figures S4 and S5
48,49
for photocurrent enhancement using LiClO4).
For this type of cells based on TiO2 as semiconductor, a small
50
external electrical bias is generally needed. In the present
case we apply a 0.30 V vs. NHE bias (all redox potentials in this
work are reported vs. NHE), which should be an enough bias as
judged by the LSV voltammetry shown in Figure 4 (up) in the
presence and absence of sun simulated light irradiation. Once
all parameters had been optimized, the DSPEC is run for 24 h
until no more hydrogen is produced under 1.5 AMG (Xe lamp,
-2
λ > 400 nm, 100 mW cm ) with the mentioned 0.30 V applied
external bias. Both photocurrent intensities and hydrogen
evolution are monitored and are depicted in Figure 4 (down).
After 24 h, 1.28 C (13.2 µmols of electrons) have passed
through the external circuit together with the formation of 6.1
µmol of H2 at the cathodic compartment that corresponds to a
faradaic efficiency of 92%. In addition 0.5 mM of the 4-HSS
substrate have been oxidized to 4-(1,2-dihydroxyethyl)benzenesulfonic acid (see Figure S6) at the anodic
compartment. Overall, a 7% conversion yield was found from
the disappearance of the starting material, corresponding to
70% selectivity towards diol formation, which is in accordance
+
with the selectivity of catalyst 2,2 in chemical oxidation
38
-1
reactions. This represents 70 TON with a TOF of 0.8 ks for
the oxidation of the organic substrate and 68 TON with a TOF
-1
of 0.8 ks for the production of hydrogen. In the absence of
the organic substrate or Ru-catalyst or Ru-photosensitizer, the
currents obtained were negligible in all cases (see Figure S5).

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As can be observed in Figure 4 (down), as the photocatalytic
reaction proceeds the current intensity decreases up to
practically zero after 24 h. The main deactivation pathway is
de-attachment of the Ru-phosphonate dye from the surface of
the TiO2 due to the hydration of the bipyridyl ligands (see
Figure S7). This low stability of the dye on the TiO2-surface is a
51
phenomenon that has been reported earlier and that needs
to be dramatically minimized in order to build more stable
devices. Promising solutions involve the use of atomic layer
52
deposition
(ALD),
electropolymerization
of
alkene
53
54
functionalized dyes or dip-coating of polymers, all of which
have proven to improve the stability of photosensitizers in
DSPEC.

3
4
5
6
7
8
9
10
11

Conclusions
The use of water as an oxygen source in combination with
sunlight for the oxidation of organic substrates is one of the
hot topics the green oxidation chemistry is facing today. In the
present paper we present a model DSPEC that transform
alkenes into the corresponding diols via epoxidation catalysis
with simulated sunlight and small potential applied bias. In
addition the byproduct is hydrogen that can be used as a solar
fuel. Here we establish the thermodynamics and kinetics of the
main reactions involved in the cell. We also underline the
unproductive reactions as well as the decomposition pathways
that need to be improved to come up with a technologically
useful device. It is also important to highlight that the present
model DSPEC configuration possesses a high degree of
chemical versatility since the catalyst is used in homogeneous
phase and thus it can be adapted to other substrates provided
the right catalyst is used. For instance, if another Ru catalyst is
found to be more active towards the oxidation of alcohols, the
same DSPEC can be used and only the electrolyte, in this case
the organic substrate and the catalyst in water, will be
different. Even though the electron transfer reactions between
the catalyst in solution and the dye at the electrode is lowered,
new strategies involving the use of supramolecular chemistry
have shown enhanced rates that could be applied in our
55
system.

12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27

Acknowledgements
We thank MINECO (CTQ-2013-49075-R, SEV-2013-0319,
CTQ2014-52974-REDC), the EU COST actions CM1202 and
CM1205, and "LaCaixa Foundation". PG-B thanks "LaCaixa
Foundation" for a PhD grant. CDG is grateful for an FPI grant
from MINECO. EP and JC would like to thank ICIQ and ICREA
for their financial support.

28
29
30
31

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