Hydrogen generation: Catalytic acceleration and control by light

Article abstract of DOI:10.1039/b908121f

The ruthenium-catalyzed generation of hydrogen from formic acid is significantly accelerated by light.

Full text of DOI:10.1039/b908121f

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Hydrogen generation: catalytic acceleration and control by lightw
Bjorn Loges, Albert Boddien, Henrik Junge, James R. Noyes, Wolfgang Baumann
¨
and Matthias Beller*
Received (in Cambridge, UK) 23rd April 2009, Accepted 28th May 2009
First published as an Advance Article on the web 15th June 2009
DOI: 10.1039/b908121f
The ruthenium-catalyzed generation of hydrogen from formic
acid is significantly accelerated by light.
The sufficient and sustainable supply of energy remains one, if
not the most, important challenge for our future. An abundant
2
4
source of energy is sunlight, which provides about 2.9 Â 10
J
Scheme 1 Light-accelerated hydrogen generation from formate.
TON) of 0.09 to 1.6 within 3 hours in the HCo(PhP(Et) ) -
per year, of which about 50% (UV/Vis) can be used for
1
photochemistry. In the coming years it will be vital to develop
(
2
4
novel efficient processes to convert the energy of light into
chemical energy. Among the various energy carriers hydrogen
is a promising candidate, which can be efficiently used in
fuel cells generating electricity and water. Nevertheless,
advancements in hydrogen technology such as the generation
of hydrogen from suitable starting materials, its storage and
conversion to electrical energy are needed.
2
catalyzed H evolution upon applying a 400 W Hg vapor lamp
12
with a pyrex filter. At the same time, King et al. showed that
hydrogen is evolved from aqueous formate solutions with
chromium hexacarbonyl under irradiation (TON = 18 after
13
1
other organic compounds with heterogeneous catalysts is used in
h). In addition, (UV-)photooxidation of formic acid and
14
environmental chemistry, e.g. for wastewater treatment.
2
3
Recently, we and the group of Laurenczy have demonstrated
for the first time that hydrogen can be generated from formic
acid at ambient conditions and directly applied in fuel cells.
While the reverse reaction (hydrogenation of carbon dioxide)
Clearly, in all these examples only small amounts of hydrogen
are generated, which makes a combination with fuel cells or the
direct use in energy applications not applicable.
In order to investigate the influence of light on our hydrogen
generation reaction in detail, we exposed a solution of
5HCO HÁ2NEt (5 mL) and various in situ-generated ruthenium
4
has been studied extensively since then, the whole idea of
using formic acid as an energy carrier has received considerable
5
attention only lately. Based on the hydrogen evolution from
6
alcohols, we showed that the commercially available
2
3
catalysts to irradiation. The reactions were performed at 40 1C in
the presence of 320 ppm Ru (Ru : P = 1 : 6). As light source a
PerkinElmer Cermax 300 W xenon-arc lamp with a filter
designed to approximate the spectrum of sunlight in the
UV/Vis region has been applied. The filter cuts off UV below
[
RuCl
from formic acid in the presence of amines at ambient conditions.
3 3
Improved catalytic activity can be obtained with [RuCl (PPh ) ]
2 2
(p-cymene)] catalyst is suitable for hydrogen generation
2
2
15
as catalyst, while Laurenczy and co-workers reported a
3
system based on a water soluble ruthenium catalyst. More
380 nm and IR wavelengths between 780 and 1200 nm.
Additionally, we performed experiments in the dark. Selected
results are summarized in Fig. 1. In control experiments
in the presence of triphenylphosphine without any
ruthenium precursor or using [RuCl (benzene)] alone no
recently, we demonstrated an increase of the catalyst activity
with in situ-generated ruthenium phosphine catalyst systems at
7
8
room temperature. Notably, both Fukuzumi et al. and
2
2
9
Laurenczy and co-workers reported details of the mechanism
positive influence of light is observed (Fig. 1, entries 1 and 2).
Remarkably, in the presence of both [RuCl (benzene)] and
of formic acid/formate decomposition in aqueous solution. In
2
2
addition, Zhou et al. presented heterogeneous catalysts for
1
hydrogen supply from formic acid/sodium formate.
0
Here, we demonstrate for the first time that visible light
significantly accelerates and controls the ruthenium-catalyzed
hydrogen generation from HCO
(Scheme 1). Although the photochemistry of ruthenium phos-
phine complexes has been investigated extensively in the past,
2
H in the presence of amines
11
6d,e
the catalytic exploitation of these properties has been scarce.
Also, only a few light-accelerated hydrogen generation reactions
in the presence of other metal catalysts are known. For example,
Onishi reported in 1993 an increase of the turnover number
Leibniz-Institut fu¨r Katalyse e.V. an der Universita¨t Rostock,
Albert-Einstein-Str. 29a, 18059 Rostock, Germany.
E-mail: matthias.beller@catalysis.de; Fax: +49 381 1281 5000
w Electronic supplementary information (ESI) available: Experimental
details and technical data. See DOI: 10.1039/b908121f
Fig. 1 The influence of light on the decomposition of 5 mL 5HCO H/
2
a
NEt with ruthenium catalyst systems (320 ppm Ru). No hydrogen
2
3
b
detected by GC; no dark experiment, performed under lab condi-
tions, environmental light.
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Chem. Commun., 2009, 4185–4187 | 4185

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aryl phosphines
a considerable accelerating effect of
irradiation is seen (Fig. 1, entries 3–6). For example in the
presence of PPh the turnover number increased from 322 to
3
1
649 during irradiation (Fig. 1, entry 3). Likewise with sub-
stituted triphenylphosphines, both with electron-donating or
electron-withdrawing substituents (Fig. 1, entries 4 and 5), the
positive effect of light is observed. Compared to triphenyl-
phosphine, catalyst activities in the dark are only about 24%
and 63% for these ligands, respectively. Notably, the combi-
nation of bidentate ligand 1,2-bis-(diphenyl-phosphino)ethane
Fig. 2 Typical curves for gas evolution from 5HCO
RuCl (benzene)] + 12PPh (320 ppm Ru).
2
HÁ2 NEt
3
with
(
dppe) and [RuCl
804 vs. 407 in the dark) reaching 90% conversion after 2 h
Fig. 1, entry 6). Tricyclohexylphosphine as a common
2 2
(benzene)] showed the best activity (TON:
[
2
2
3
2
the second effect, which prevents catalyst deactivation, is more
evident: in the dark, there is almost no activity after 1 h, while gas
evolution is stable under irradiation.
(
electron-donating alkyl-phosphine ligand retains its inhibiting
effect also upon irradiation (Fig. 1, entry 7). Catalysts containing
triphenylphosphite showed only low activity and no significant
effect of irradiation is detected (Fig. 1, entry 8).
In order to elucidate the mechanism of the photoactivation
1
H
NMR studies simulating the pretreatment with
(benzene)] and equivalents of PPh (3 equiv.
per Ru) were performed in DMF-d . The signal of
[
RuCl
2
2
6
3
Clearly, the observed effects do not simply rely on the metal
precursor. With other ruthenium complexes (Fig. 1, entries 10
and 11), a photoactivation of the catalyst is observed, too. The
PPh
6
3
7
Z -coordinated benzene, a singlet at 5.79 ppm, is shifted upfield
to become a doublet at 5.55 ppm (JPH = 0.78 Hz; Fig. 3, A and B)
active species derived from [Ru(cod)(methylallyl) ] even showed
2
directly after addition of PPh
3
. These values are within the
the most pronounced effect upon irradiation (1160% increase of
gas evolution), but is not very active in the dark (TON = 115). A
positive effect (39% increase of gas evolution) of light is observed
same range as those reported by Zelonka and Baird and by
6
1
Bennett and Smith for [RuX (benzene)(PR )] complexes.
2
3
Apparently, a similar ruthenium phosphine complex is
with the catalyst derived from RuCl
3
ÁxH
2
O. Also when the
formed. The reaction is completed by heating to 80 1C for
molecular-defined ruthenium hydrido complex [RuH (PPh ) ] is
4 3 3
2
h, which is underlined by the new multiplet at 7.8 ppm that
used, irradiation accelerates the reaction by 27% (Fig. 1, entry
is attributed to protons of triphenylphosphine. Traces of
non-coordinated benzene at 7.38 ppm which appear after
heating are due to the beginning of the displacement of the
1
4
3 3
2). However, [RuH (PPh ) ] is overall less active than a
comparable in situ system (Fig. 1, entries 3, 10 and 11).
In the presence of light significant amounts of hydrogen are
generated even at 0 1C (Table 1). This is an interesting observa-
tion because the system can be combined with a fuel cell for low
temperature applications, e.g. appliances in colder areas. To the
best of our knowledge this is the first example of hydrogen
generation from organic compounds at such low temperatures
6
Z -coordinated benzene and/or to equilibrium in solution.
6
After 30 min irradiation, however, no Z -coordinated benzene
is left, and the singlet at 7.38 ppm increases dramatically. Such
6
a photochemical displacement of Z -arene ligands in the
presence of other ligands is in agreement with previous
1
6b,17
organometallic studies.
(Table 1, entries 1–4). At all temperatures studied the influence of
6
The proposed photo-assisted dissociation of the Z -benzene
from the metal center is also supported by the fact that gas
light is observed. Obviously, by increasing the temperature the
hydrogen generation can be increased. Thus, the observed
photochemical activation can be used to save thermal energy.
A careful analysis of the gas evolution in the presence of
evolution with pre-irradiated [RuCl
catalysts (curve A and B in Fig. 2) is similar to gas evolution
with catalysts formed from RuCl O (see ESIw) both in the
2 2 3
(benzene)] + 12PPh
ÁxH
2
[RuCl (benzene)] reveals that irradiation has at least two effects
2 2
3
dark and under irradiation. In addition to the photo-induced
activation of the ruthenium arene complexes, there is also a
clear photo-assisted regeneration of the active catalyst
in different phases of the reaction: first, within the induction
period of 10 to 20 minutes, a photoinductive effect occurs
generating the active catalyst species; and second, when the
catalyst is almost deactivated in the dark a reactivation takes
place under irradiation (Fig. 2, curve C and D); thus the reaction
is photoassisted. Irradiating the pre-catalyst solution for 30 min
prior to addition of the substrate, the initial photoinductive effect
can be mimicked. Indeed, a catalyst is formed which is highly
active under both conditions (Fig. 2, curve A and B). However,
Table 1 Influence of irradiation and temperature on the performance
of the [RuCl
2
(benzene)]
Dark
gas/mL
25
2
/12PPh
3
(320 ppm Ru) catalyst system
Irradiation
Entry
T/1C
V
TON
V
gas/mL
TON
1
1
2
3
4
0
25
40
45
26
217
322
426
182
1238
1545
1863
195
1321
1649
1987
Fig.
3
H
NMR of the irradiated pre-catalyst solution.
203
302
399
(A) [RuCl (benzene)] in DMF-d ; (B) 9.55 mmol [RuCl (benzene)] ,
2
2
7
2
2
6
3 7
equiv. PPh , dissolved in 1.0 mL DMF-d ; (C) after 2 h at 80 1C;
D) after 30 min irradiation with Xe-arc lamp with filter.
(
4
186 | Chem. Commun., 2009, 4185–4187
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Fig. 4 (A) Light-triggered gas evolution from 5 mL 5HCO
2
HÁ2NEt
3
with [RuCl
2
(benzene)] /12PPh
2
3
(320 ppm Ru); (B) differential plot of gas
evolution in a light-triggered experiment.
reaction. The best catalyst productivity is observed with a
RuCl (benzene)] /dppe catalyst. Here, the catalyst activity is
[
2
2
almost double the activity of the best non-light activated system.
Notably, we demonstrated for the first time that hydrogen
evolution can be triggered by switching on and off the light source.
Notes and references
1
Calculated from the ASTM G137 air mass 1.5 spectra. Data are
available from the Renewable Energy Resource Center of National
Renewable Energy Laboratory of the U.S. Department of Energy:
http://rredc.nrel.gov/solar/spectra/am1.5/
¨
D. Wohrle, in Photochemie, ed. D. Wo
W.-D. Stohrer, Wiley-VCH, Weinheim, 1998, pp. 113–177.
Please
hrle, M. W. Tausch,
also
see:
¨
2
3
4
5
6
B. Loges, A. Boddien, H. Junge and M. Beller, Angew. Chem., 2008,
1
20, 4026–4029 (Angew. Chem., Int. Ed., 2008, 47, 3962–3965).
C. Fellay, P. J. Dyson and G. Laurenczy, Angew. Chem., 2008, 120,
030–4032 (Angew. Chem., Int. Ed., 2008, 47, 3966–3968).
4
The Handbook of Homogeneous Hydrogenation, ed. J. G. de Vries,
C. J. Elsevier, Wiley-VCH, Weinheim, 2008.
Scheme 2 Proposed mechanism for the Ru-catalyzed decomposition
of formic acid with a [RuCl (benzene)] /PAryl catalyst.
2
2
3
(a) S. Enthaler, ChemSusChem, 2008, 1, 801–804; (b) F. Joo
ChemSusChem, 2008, 1, 805–808.
(a) H. Junge and M. Beller, Tetrahedron Lett., 2005, 46, 1031–1034;
b) H. Junge, B. Loges and M. Beller, Chem. Commun., 2007,
22–524; (c) B. Loges, H. Junge, B. Spilker, C. Fischer and
´
,
occurring (Fig. 4). While the origin of this effect is not fully clear,
it offers the opportunity to trigger hydrogen generation. In fact, it
is possible to control the formation of hydrogen with light!
As depicted in Fig. 4A the experiments were run for 45 min
with light irradiation and then without irradiation (continuous
line). In addition, the dashed lines represent typical experiments
under permanent irradiation and in the dark, respectively. After
an initial formation of a stable photoactive catalyst, gas evolution
can be almost stopped switching off the light source, and restarted
after turning it on again. In a complementary experiment irradia-
tion with light after 45 min further activates the catalyst (Fig. 4B)
in the first cycle of irradiation. In the following irradiation cycles,
hydrogen generation is started and stopped by turning the light
on or off. This possibility of triggering the reaction with light
provides further evidence for a photo-assisted reaction.
(
5
M. Beller, Chem.-Ing.-Tech., 2007, 79, 741–753, for earlier work
see; (d) D. Morton and D. J. Cole-Hamilton, J. Chem. Soc., Chem.
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7 (a) A. Boddien, B. Loges, H. Junge and M. Beller, ChemSusChem,
008, 1, 751–758; (b) H. Junge, A. Boddien, F. Capitta, B. Loges,
J. R. Noyes, S. Gladiali and M. Beller, Tetrahedron Lett., 2009, 50,
603–1606.
8 S. Fukuzumi, T. Kobayashi and T. Suenobu, ChemSusChem, 2008,
, 827–834.
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009, 15, 3752–3760.
2
1
1
9
2
1
1
0 X. Zhou, Y. Huang, W. Xing, C. Liu, J. Liao and T. Lu, Chem.
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M. G. Bradley, Inorg. Chem., 1977, 16, 744–748.
According to all these observations, we propose the mechan-
ism shown in Scheme 2. The initial pre-catalyst is activated by
irradiation with visible light to give an aryl phosphine-ligated
ruthenium hydride species. Deactivation of this active catalyst is
prevented by light. Furthermore, the release of hydrogen and/or
carbon dioxide from the catalyst is also influenced by light.
In summary, we present the first light-accelerated hydrogen
generation reaction from formic acid with a catalyst system
based on a ruthenium precursor and aryl phosphines. The
overall activity of our light-accelerated ruthenium complex is
more than 100 fold higher compared to systems previously
12 M. Onishi, J. Mol. Catal. A: Chem., 1993, 80, 145–149.
1
3 D. E. Linn, R. B. King and A. D. King, J. Mol. Catal. A: Chem.,
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1
1
(
D. W. Bahnemann, Chem. Rev., 1995, 95, 69–96.
5 For further details, please see ESIw or the product specifications
and the Cermax Engineering Guide at http://optoelectronics.
perkinelmer.com/.
1
16 (a) R. A. Zelonka and M. C. Baird, Can. J. Chem., 1972, 50,
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1
2,13
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In comparison to the
1
7 (a) T. Hayashida and H. Nagashima, Organometallics, 2002, 21,
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non photo-assisted system, a more than one order of magnitude
increase of gas evolution is achieved in the light-accelerated
3
This journal is ꢀc The Royal Society of Chemistry 2009
Chem. Commun., 2009, 4185–4187 | 4187