A viable hydrogen-storage system based on selective formic acid decomposition with a ruthenium catalyst

Article abstract of DOI:10.1002/anie.200800320

(Chemical Equation Presented) The use of formic acid as a hydrogen-storage material has become more feasible thanks to the development of a homogeneous catalytic system of ruthenium water-soluble complexes (Ru/TPPTS; TPPTS = meta-trisulfonated triphenylphosphine) that selectively decomposes HCOOH into H₂ and CO₂. Continuous generation of H₂ of very high purity, over a wide range of pressures, and under mild conditions was achieved.

Full text of DOI:10.1002/anie.200800320

Communications
DOI: 10.1002/anie.200800320
Hydrogen Storage
AViable Hydrogen-Storage System Based On Selective Formic Acid
Decomposition with a Ruthenium Catalyst**
CØline Fellay, Paul J. Dyson, and Gµbor Laurenczy*
Hydrogen represents an important alternative energy feed-
stock for both environmental and economic reasons, and
when combined with fuel-cell technology, very efficient
temperatures a conversion of 90–95% can be achieved
(Figure 1). The total conversion does not reach 100% because
the formate salt added for the activation of the catalyst is not
converted; however, all the formic acid is consumed. In
addition, the catalyst was stable up to 1708C and remained
[
1]
[10]
energy conversion can be achieved. Although the advan-
tages of hydrogen over fossil fuels are numerous, the actual
use of hydrogen as a transportation fuel is limited mainly
because of storage and delivery problems. Conventional
hydrogen-storage methods, such as high-pressure gas contain-
ers and cryogenic liquid/gas containers, have weight and
active after one year in solution.
[
2]
safety issues. Consequently, a great deal of research is being
undertaken to develop new materials, such as metal
[
2,3]
[4]
hydrides
and carbon nanostructures, that store hydrogen
efficiently, although no entirely satisfactory options have been
found so far. Formic acid containing 4.4 wt% of hydrogen, as
well as its conjugate base, formate salt, are well known
[
5–7]
sources of hydrogen
and have previously been reported as
[
8]
potential hydrogen-storage material. Formic acid has the
advantage over other substrates that only gaseous products
are formed (H /CO ), hence preventing the accumulation of
2
2
by-products, which is a limitation for mobile applications.
However, until now potential applications have been limited
by catalyst regeneration requirements, by harsh reaction
conditions, and by poor selectivity. We present herein an
efficient, completely selective, and robust system for hydro-
gen production from formic acid using water-soluble homo-
Figure 1. Effect of temperature on the decomposition of formic acid in
a closed (batch) system: 258C (*) (125 mm Ru), 708C (~), 808C (~),
908C (&
), 1008C (&), 1208C (*) (22 mm Ru); [Ru(H O) ](tos) , 2 equiv
2 6 2
TPPTS, 4m HCOOH/HCOONa (9:1), 2.5 mL H O/D O (1:1); addition
2
2
[
9]
of 0.38 mL HCOOH for recycling.
geneous catalysts.
Decomposition of formic acid was carried out in aqueous
solution using hydrophilic ruthenium-based catalysts, gener-
ated from the highly water-soluble ligand meta-trisulfonated
The thermal decomposition of formic acid into CO and
H O, which depends on the temperature and the formic acid
2
2
+
triphenylphosphine (TPPTS) with either [Ru(H O) ] or,
concentration, becomes nonnegligible at elevated temper-
2
6
[11]
more conveniently, commercially available RuCl3.
atures. No traces of CO, which is known to poison some fuel
[
12]
The catalysts were activated prior to use by reaction with
sodium formate and formic acid and the catalytic decom-
position of formic acid performed under a wide range of
pressures and temperatures. The generated H /CO pressure
cells, could be detected by FTIR spectroscopy (detection
limit of 3 ppm) in a sample of gases generated by using this
catalyst system at 1008C (see the Supporting Information),
because of the rapidity of the reaction and, hence, the short
residence time of formic acid at this temperature.
2
2
was typically between 1 and 220 bar, but no inhibition of
catalytic activity was observed up to a pressure of 750 bar (see
the Supporting Information for details). The rate of formic
acid decomposition increased with temperature, and at all
A continuous system was developed in which formic acid
(98–100%) was added under pressure into a 50 mL reactor
containing 12 mL of catalyst solution. The gases generated
were released at a rate that maintained a constant pressure
inside the reactor. The performance of the optimized catalytic
system (125 mm Ru) obtained at two different temperatures
under continuous conditions are given in Table 1. Although
only the results for [Ru(H O) ](tos) (tos = toluene-4-sulfo-
[
*] C. Fellay, Prof. P. J. Dyson, Dr. G. Laurenczy
Institut des Sciences et IngØnierie Chimiques
Ecole Polytechnique FØdØrale de Lausanne (EPFL)
2
6
2
1015 Lausanne (Switzerland)
nate) are given, RuCl3 also led to successful continuous
hydrogen production.
Fax: (+41)21-693-9780
E-mail: gabor.laurenczy@epfl.ch
The purity of hydrogen obtained with this catalytic system
makes it suitable for all types of fuel cells, and, since a
constant pressure of hydrogen is produced, it can be used
directly in combustion or electric engines, thus avoiding
[
**] We thank the Swiss National Science Foundation for financial
support.
Supporting information for this article is available on the WWW
under http://www.angewandte.org or from the author.
3
966
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Angew. Chem. Int. Ed. 2008, 47, 3966 –3968

Angewandte
Chemie
Table 1: Formic acid consumption rates, hydrogen production rates, and
turnover frequencies (TOF) of the continuous system.
techniques. NMR spectra were recorded on Brucker Avance DRX
[
a]
400 MHz spectrometer and IR spectra on a Perkin–Elmer Spectrum
GX FTIR spectrometer with a gas cell (path length 10 cm).
Kinetic measurements at variable temperature were performed
À1
À1
À1
À1
À1
T [8C] HCOOH input [mL min
] H outflow [mL min ] TOF [h ]
2
[
21]
1
1
00
20
0.21Æ0.01
0.42Æ0.01
140Æ10
290Æ10
230Æ5
460Æ24
in 10 mm medium pressure sapphire NMR tubes. [Ru(H O) ](tos)
2 6 2
(0.056 mmol, 31 mg) was dissolved in a formic acid/sodium formate
(
9:1, 4m) aqueous solution (2.5 mL, 1:1 D O/H O) containing TPPTS
2 2
[
[
a] 50 mL reactor, 12 mL initial solution of HCOOH/HCOONa (9:1, 4m),
Ru(H O) ](tos) (125 mm), TPPTS (250 mm).
(0.112 mmol, 63.9 mg). The active species were formed by heating the
2
6
2
tube at 908C in an electric heating jacket. Recycling of the catalyst
was performed at different temperatures, without protection against
oxygen/air, by cooling the tube to room temperature, depressuriza-
tion, and addition of formic acid (10 mmol, 0.38 mL). The reaction
technical problems due to pressure drops that occur with
high-pressure cylinders. H and CO can easily be separated if
2
2
1
was followed by monitoring the pressure and by H NMR spectros-
desired. Furthermore, prior to the hydrogen consumption
process, the gas pressure can be used advantageously to
generate energy in a similar manner to that of a compressed
copy.
For continuous hydrogen production, a 50 mL Parr autoclave
with a modified output valve allowing control of the gas outflow, was
connected to a high-pressure pump and to a gas flow meter. An initial
aqueous solution of HCOOH/HCOONa (9:1, 4m, 12 mL) was placed
[
13]
air engine.
Formic acid is prepared commercially from carbon mon-
oxide and also as by-product in the synthesis of other
in the autoclave containing [Ru(H
equivalents of TPPTS, and the autoclave was heated externally
activation period). Once the initial amount of formic acid was
2 6 2 3 2
O) ](tos) or RuCl ·xH O, and
2
(
[
14]
chemicals.
Numerous studies have demonstrated formic
[
15]
acid production from carbon dioxide, either electrochemi-
cally or in aqueous
converted, formic acid (98–100%) was added continuously, and the
gases released at a rate that maintained a desired pressure inside the
autoclave.
[
16]
[10,17]
[18]
or supercritical phases, which
require bicarbonate instead of CO or addition of a base.
2
Hence, a “green” CO cycle could be achieved by separation
2
[
19]
Received: January 22, 2008
Published online: April 4, 2008
of the CO generated and its conversion into formic acid
2
using hydrogen from a stationary source (Scheme 1). In the
Keywords: formic acid · homogeneous catalysis ·
.
hydrogen storage · ruthenium
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Angew. Chem. Int. Ed. 2008, 47, 3966 –3968
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
www.angewandte.org 3967

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