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ORIGINAL PAPER
Catalytic Generation of Hydrogen from Formic acid and itsDerivatives: Useful Hydrogen Storage Materials
Bjorn Loges • Albert Boddien • Felix Gartner •
Henrik Junge • Matthias Beller
Published online: 11 May 2010
� Springer Science+Business Media, LLC 2010
Abstract In this account the concept of using formic acid
as a hydrogen storage material is presented. Catalytic
reduction of carbon dioxide and heterogeneously catalyzed
decomposition of formic acid to hydrogen and carbon
dioxide are briefly discussed. In the main part the historic
development and recent examples of homogeneously cat-
alyzed hydrogen generation from formic acid are covered
in detail.
Keywords Formic acid � Hydrogen � Hydrogen storage �Energy � Catalysis � Ruthenium � Carbon dioxide
1 Introduction
A sufficient and benign supply of energy is one of the most
important challenges for the future of human society [1–3].
Among the various known energy carriers, hydrogen could
play an important role. One of the major obstacles to use
hydrogen for energy applications however, is the efficient
storage and handling of hydrogen. Compared to traditional
fuels, hydrogen has a very high gravimetric energy density,
but its volumetric energy density at atmospheric conditions
is too low. To obtain a balanced gravimetric and volu-
metric hydrogen storage density, pure hydrogen can be
stored in compressed gaseous or liquid form [4, 5]. In
addition, it can be adsorbed on porous materials, e.g.
zeolites, metal organic frameworks, or polymers of intrin-
sic microporosity [6–9]. Moreover, inorganic and organic
hydrogen storage materials are known in which chemical
bonds are formed and broken upon storage and release of
hydrogen. Here, mainly hydrides have been explored,
including covalent hydrides such as boranes, salts such as
magnesium hydride, metallic hydrides of the MHx type,
and complex hydrides such as Li(AlH4) [4, 7, 10–17]. In
recent years also few organic materials (‘‘organic
hydrides’’) have emerged as materials for covalent hydro-
gen storage. Due to their easier handling and inherent
energy efficiency especially liquid compounds like meth-
anol and formic acid appear to be practical [18, 19].
In this review the use of formic acid for hydrogen
storage will be discussed. Carbon dioxide hydrogenation to
formic acid, and formic acid decomposition in the presence
of heterogeneous catalysts will be covered briefly, and by
no way comprehensively. Recent advances in homoge-
neous hydrogen generation from (aqueous) formic acid/
metal formate solutions and from formic acid amine mix-
tures will be described in detail.
2 Formic Acid For Hydrogen Storage: The Concept
Based on formic acid and carbon dioxide a sustainable
cycle for energy storage can be conceived (Fig. 1). For
energy storage, carbon dioxide is converted to formic acid
or a formate derivative either electrochemically [20, 21] or
by catalytic hydrogenation [22–24]. The resulting material
is a liquid at ambient conditions, either pure formic acid, an
adduct containing formic acid, or an inorganic formate in
aqueous solution, and can thus be handled, stored, and
transported easily. On the other side of the cycle energy is
released either in a direct formic acid fuel cell, or by
selective on-demand decomposition into carbon dioxide
and hydrogen, which can be used directly in an appropriate
hydrogen oxygen fuel cell [25–30]. If pure hydrogen is
B. Loges � A. Boddien � F. Gartner � H. Junge � M. Beller (&)
Leibniz-Institut fur Katalyse e.V. an der Universitat Rostock,
Albert-Einstein-Str. 29a, 18059 Rostock, Germany
e-mail: matthias.beller@catalysis.de
123
Top Catal (2010) 53:902–914
DOI 10.1007/s11244-010-9522-8
required the gases may also be separated using membrane
techniques [31].
The hydrogen content of pure formic acid is 43 g/kg or
52 g/L, its release from formic acid is thermodynamically
downhill by DG� = –32.8 kJ/mol at room temperature [24]
(Scheme 1).
As early as 1978, the electrochemical reduction of carbon
dioxide to formic acid and its subsequent decomposition on
Pd/C for energy storage have been proposed by Williams
et al. [32]. Later on, a system for solar energy conversion by
reduction of aqueous carbonate was proposed by Halmann
in 1983 [33]. In 1986, a similar concept was described by
Wiener et al. They proposed a Pd/C catalyst to decompose
aqueous formate solutions to obtain hydrogen [34–36].
However, both approaches have not led to any application.
Two decades later, the research on the use of carbon dioxide
for energy storage has been resumed recently [19, 37–39].
Compared to methanol, a competing hydrogen carrier,
the hydrogen density of formic acid is considerably lower
(Table 1). However, to the best of our knowledge, there is
currently no procedure to obtain hydrogen from methanol
at ambient temperature. Additionally, regarding potential
hazards of formic acid, it may generally be considered less
hazardous than methanol. Methanol is highly flammable
and exhibits a metabolic toxicity which affects the central
nervous system and may lead to blindness [40]. Formic
acid is primarily a strong acid with an immediate corrosive
effect causing severe burns (see Table 1) [41]. An addi-
tional hazard of pure formic acid is its decomposition to
gases. On the other hand, dilute formic acid is approved as
a food additive [42].
Obviously, for the actual energy storage cycle, both the
formation and the decomposition of formic acid have to be
improved. However, most research groups focused only on
one of these two steps. In the next two sections, the reduc-
tion of carbon dioxide and heterogeneous decomposition of
formic acid are discussed briefly. Then, the first approaches
and current developments in the homogeneously catalyzed
hydrogen generation from formic acid are shown.
3 The Reduction of Carbon Dioxide to Formic Acid
In the first step of the proposed cycle, formic acid is formed
by reduction from carbon dioxide. One option is the
CatalyticRelease
CatalyticStorage
H2
from Renewable Resources
H2
Usage
CO2
HCO H (derivative)2
Fig. 1 A catalytic cycle for hydrogen storage in formic acid
Scheme 1 The hydrogenation
of carbon dioxide to formic acid
and/or derivatives [24]
Table 1 Comparison of properties: methanol versus formic acid
Methanol Formic acid
Molecular mass 32.042 g/mol 46.026 g/mol
Gravimetric hydrogen density 125 g/kg 43 g/kg
Volumetric hydrogen density 99 g/L 52 g/L
Hazard codes T, F C
Risk statements (R-sentence) 11–23/24/25–39/23/24/25 10–35
Boiling point 65 �C 101 �C
Vapor pressure (20 �C) 130.3 hPa 42.0 hPa
Explosion limits (lower - upper) 6 - 36 vol% 18 - 57 vol%
Flash point 11 �C 48 �C
Workplace exposure limit 200 ppm 5 ppm
LD50 (oral, rat) 5,628 mg/kg 1,100 mg/kg
The data has been derived from material safety data sheets available at commercial suppliers. Material safety data sheets are available from all
suppliers of chemical products upon request or via a website after login, e.g., www.chemdat.info or www.sigmaaldrich.com
Top Catal (2010) 53:902–914 903
123
electrochemical reduction of carbon dioxide in water,
which is covered in Ref. [20, 21]. For a sustainable storage
cycle, this approach depends on the availability of ‘‘car-
bon dioxide-free’’ electricity. Another option is the syn-
thesis of formic acid by hydrogenation of carbon dioxide,
reviewed in [22–24]. However, a major issue of this
reaction are the unfavorable thermodynamics: Formic acid
is formed under reaction conditions, i.e., elevated pres-
sure, but generally decomposes as soon as the pressure is
relieved.
For sustainable hydrogen storage, the hydrogenation of
carbon dioxide depends on the availability of ‘‘carbon
dioxide-free’’ hydrogen and energy. As a guideline, the
additional energy (DG�) required for this reaction is equiv-
alent to approximately 0.14 mol of hydrogen per mole of
formic acid, if the lower heating value of hydrogen in the
fuel cell reaction is considered (242 kJ/mol) [43].
Recently, the use of task specific ionic liquids in order to
facilitate the isolation of formic acid was reported [44, 45].
As a current state of the art catalyst system an iridium PNP-
pincer catalyst has been developed, which reached activi-
ties up to a turnover frequency (TOF) of 150,000 per hour
and an overall turnover number (TON) of 3,500,000,
respectively [46]. It has also been proposed that both
electrochemical reduction and hydrogenation of CO2 could
rely on sunlight as a sustainable source of energy: Elec-
tricity may be obtained via photovoltaics, and hydrogen
may then obtained by electrolysis [21, 32–38]. Alterna-
tively, hydrogen may be obtained by direct photochemical
water splitting in the future [3].
4 Hydrogen Generation from Formic Acid and Its
Derivatives
The second step of the cycle for hydrogen storage is the
selective hydrogen generation from formic acid. This step
also liberates carbon dioxide that may be recycled. It is
important to note that the decomposition of formic acid
may occur via two different pathways (Scheme 2):
dehydrogenation/decarboxylation (A) and dehydration/
decarbonylation (B) [47]. Both reactions are thermody-
namically downhill at standard conditions. As carbon
monoxide is a catalyst poison for fuel cell catalysts, only
the dehydrogenation/decarboxylation (A) pathway is of
interest for hydrogen generation, while the carbon mon-
oxide concentration due to decomposition via the other
pathway must be minimized to the ppm level! Only
reaction systems that selectively catalyze pathway A will
be considered here.
4.1 Decomposition of Formic Acid With
Heterogeneous Catalysts
The heterogeneously catalyzed decomposition of formic
acid to hydrogen and carbon dioxide has been reported
first by Sabatier in 1912 [48]. Since then, this reaction has
been employed in heterogeneous catalysis to study
adsorption and desorption processes during the decom-
position of formic acid vapors as a model substrate on
surfaces [49–53]. An early example was reported by Ri-
enacker et al., who performed this reaction with formic
acid vapors on surfaces of copper–gold and silver–gold
alloys [54]. Later, Rienacker et al. employed the decom-
position of formic acid to measure the activity of many
types of heterogeneous catalysts, mainly metals and alloys
of transition metals, such as iron, nickel, copper, palla-
dium, silver, platinum, and gold [55–57]. Another known
heterogeneously catalyzed decomposition of formic acid is
the photocatalytic conversion on titanium dioxide or other
nanoparticles [58–60], which occurs during photocatalytic
wastewater treatment [61]. As an application of catalytic
formic acid decomposition, Hyde and Poliakoff et al. have
proposed formic acid or formates as hydrogen sources for
hydrogenation reactions ‘‘without gases’’. Here, formic
acid is decomposed in a pressure reactor on a platinum
catalyst at 450 �C, and the resulting supercritical fluid is
fed to a second reactor, where the substrate and further
solvent is added. The product can be collected down-
stream after decompression of the supercritical fluid from
the reactor. This principle has been successfully applied
for hydrogenation of alkenes, alkynes, and carbonyl
compounds [62–64].
With respect to hydrogen generation, Williams et al.
used Pd/C (1 wt% Pd) to obtain around 55 mL of hydrogen
from 4 mol/L aqueous formic acid within 10 min. The
carbon dioxide obtained was trapped in a column filled
with potassium hydroxide pellets [32]. In the approach of
Wiener et al. in the mid-1980s, 900 mL hydrogen was
evolved from a 4 mol/L aqueous sodium formate solution
within 20 min at 70 �C in the presence of a Pd/C catalyst
(10 wt% Pd) [34]. They used a barium hydroxide solution
as carbon dioxide trap. Recently, Xing et al. reinvestigated
the decomposition of formic acid/sodium formate solutions
with noble metal catalysts supported on charcoal. They
obtained around 1,250 mL of gas from a solution of 1 eq.
Scheme 2 Formic acid
decomposition pathways, and
their thermodynamic properties
([114], see also [24])
904 Top Catal (2010) 53:902–914
123
sodium formate in 3 eq. formic acid with a Pd-Au/C cat-
alyst that had been co-deposited with CeO2 at 92 �C, which
corresponds to a TOF of 227 h-1 [65]. Another example is
the report of Iglesia et al., who selectively decomposed
formic acid on gold or platinum on alumina at 80 �C at
rates of more than 1,000 mol H2 per gram gold per hour,
which corresponds to a TOF of about 5 h-1. According to
the authors, it depends on the size of Au domains whether
formic acid dehydrogenation or dehydration, or even water
gas shift reaction (WGS) occurs [66]. The electrochemical
decomposition of formic acid, which is of interest for
wastewater treatment, has also been reported. Carbon
dioxide and hydrogen are generated separately on different
electrodes, and hydrogen can be obtained in high purity
(99.999%) [67].
4.2 Hydrogen Generation from Formic Acid and
Alkaline Formates
In homogeneous catalysis, an early report of a catalytic
decomposition of formic acid to carbon dioxide and
hydrogen dates back to the 1960s, when Coffey showed
formic acid decomposition with several platinum, ruthe-
nium and iridium phosphine complexes [68]. Coffey’s
most active catalyst was [IrH3(PPh3)3] with an initial 8890
turnovers per hour in a refluxing solution of formic acid in
acetic acid (Table 2, entry 1). Though metal carbonyls
were formed, no free carbon monoxide could be detected.
Four years later, Forster and Beck used rhodium and irid-
ium iodocarbonyl compounds in the presence of hydroiodic
acid, achieving a TOF of 4.4 h-1 in 70% aqueous formic
acid at 100 �C (Table 2, entry 2) [69]. Studying homoge-
neously catalyzed WGS, it was found that Ru3(CO)12
catalyzes the decomposition of formate in basic media at
75 �C (Table 2, entry 3) [70]. Later, Otsuka et al. studied
the selective dehydrogenation of formic acid in order to
characterize intermediates for the (homogeneously) cata-
lyzed water gas shift reaction. Their platinum(0) complex
[Pt(2-Pr3P)3] catalyzed the decomposition of formic acid in
acetone/water at 20 �C at a rate of 25 turnovers within the
first 15 min (Table 2, entry 4). A mechanism for the role of
this complex in the water gas shift reaction was proposed,
involving the decomposition of formic acid via the unstable
platinum formate [PtH(HCO2)(2-Pr3P)2] [71]. Strauss,
Whitmire and Shriver showed that the cyclometalated
rhodium triphenylphosphine complex 1 (Scheme 3) is a
precursor for [Rh(HCO2)(PPh3)3], which could be isolated.
The decomposition of formic acid was carried out in a
Table 2 Hydrogen generation from 5 mL TEAF with several homogeneous catalyst precursors and heterogeneous catalysts
Catalyst Substrate T/ �C TON TOF/h-1 Conv. Selectivity Remarks References
1 [IrH3(PPh3)3] HCO2H in acetic
acid (0.75 mol/L)
118 [11,000 8890 n.a. No free CO
detected
[68]
2 RhI/NaI HCO2H in
water 70%
100 n.a. 4.4 n.a. Significant
amount of CO
[69]
3 [Ru3(CO)12] HCO2H in
ethoxyethanol/water
75 50 600 50% n.a. Performed in a
pressure vessel
[70]
4 [Pt(2-Pr3P)2] HCO2H in
acetone/water 70%
20 25 100 Full n.a. [71]
5 1 HCO2H in
toluene 70%
20 [3.5 0.06 n.a. n.a. [72]
6 2 HCO2H/HCO2Na in
water 70%
20 ‘‘Several
hundred’’
3.3 n.a. n.a. [73]
7 RhCl3�3H2O/
NaNO2
HCO2H in water 90 n.a. 126 12.5% n.a. Titration of aq. cat.
soln. with HCO2H
[74]
8 3 HCO2H in
acetone (0.35 mol/L)
R.T. 2,000 (?) *500 Full No free CO,
H2O detected
Performed in NMR
spectrometer
[76]
9 [cp*Mo(PMe3)3H] HCO2H in
pentane (13 mmol/L)
80 n.a. n.a. n.a. n.a. [77]
10 4 HCO2H in THF-d8
(63 mmol/L)
80 10 \2 Full n.a. Performed in NMR
spectrometer
[78]
11 6 HCO2H/HCO2Na
in water
25 80 30 n.a. No free CO
detected
[82]
12 7 HCO2H in water
(2 mol/L)
90 \10,000 14,000 Near
full
No free CO
detected
Also performed in
pressure vessels
[84]
13 [Ru(H2O)6](tos)2/
TPPTS
HCO2H/HCO2Na 9:1
in water (4 mol/L)
120 [40,000 460 n.a. No CO
detected
Continuous setup [87]
The values have been calculated from the data obtained from the respective references
Top Catal (2010) 53:902–914 905
123
solution of formic acid in toluene at 20 �C, although the
turnover frequency of 0.06 h-1 for this catalyst is rather
low (Table 2, entry 5) [72]. In 1982 Trogler and co-
workers reported a binuclear platinum triethylphosphine
catalyst 2 (Scheme 3) that decomposed an aqueous solu-
tion of formic acid in the presence of sodium formate with
a TOF of 3.3 h-1 at 20 �C and at a constant rate of several
100 turnovers (Table 2, entry 6) [73].
Rhodium, iridium, ruthenium, and palladium chloride in
the presence of sodium nitrite, but without any other
ligand, were found to catalyze formic acid decomposition
at 90 �C in aqueous solution by King and Bhattacharyya
(max. TOF 126 h-1) in 1995 (Table 2, entry 7) [74].
Another binuclear complex for formic acid decomposi-
tion was presented by Puddephat et al. in 1998. This binu-
clear ruthenium phosphine complex [Ru2(l-CO)(CO)4
(l-dppm)2] (3) was the most active complex for this reaction
at that time, achieving a TOF of 500 h-1 after quantitative
formic acid decomposition (15 min; Table 2, entry 8). The
reaction was performed in NMR tubes at room temperature
in a solution of formic acid and acetone-d6. The authors
identified various intermediate hydrides and formate com-
plexes by NMR spectroscopy, and isolated [Ru2H(l-H)
(l-CO)(CO)2(l-dppm)2]. In the presence of triethylamine
they successfully performed the reverse reaction, achieving
a maximum HCO2H:NEt3 ratio of 1.2:1 [75, 76].
The formic acid decomposition with an early transition
metal complex, [cp*Mo(PMe3)3H], was reported by Parkin
et al. in 2002 (Table 2, entry 9) [77]. Another approach has
been followed by Lau et al. in 2003. To explore the acid-
ities of Ru–H and Mo-H or W–H hydrides, they
synthesized heterobinuclear bisphosphine complexes 4
containing ruthenium and molybdenum or tungsten for the
interconversion of carbon dioxide/hydrogen and formic
acid (Scheme 4). In NMR studies, they identified a com-
plex bearing a bridging hydride, which was not stable as a
formate, but could be isolated as a tetrafluoroborate.
However, the activity of these complexes does not exceed a
TOF of 30 h-1 for carbon dioxide hydrogenation and 2 h-1
for formic acid decomposition (Table 2, entry 10) [78].
Ogo, Fukuzumi et al. observed stoichiometric hydrogen
evolution treating [Rh(III)(1,4,7-triazacyclononane)
(HCO2)2](OTf) 5 with sodium formate, and they obtained a
dihydride carbonyl complex [79]. Using 1H NMR and mass
spectroscopy together with deuteration experiments and
other spectroscopic studies, they concluded that hydrogen
is generated from the diformate complex 5 by protonation.
They also demonstrated that hydrogen is in fact evolved
from the formate ligands, and that a rhodium carbonyl
complex is formed. Based on their former investigations on
carbon dioxide hydrogenation [80, 81], Fukuzumi et al.
used [Rh(cp*)(bipy)(H2O)](SO4) (6) and similar com-
plexes for the hydrogen generation from aqueous formic
acid with a maximum of 80 turnovers within 5 h at
pH = 3.8 (Table 2, entry 11). Combining spectroscopic
studies and DFT calculations, they demonstrated that for-
mic acid decomposition occurs via a formate complex and
a hydride complex [82]. More recently, he demonstrated
that heteronuclear iridium–ruthenium complexes are highly
active catalysts for hydrogen generation in an aqueous
solution under ambient conditions giving a TOF of about
426 h-1 [83]. Focusing on increased hydrogen output,
Himeda reported the iridium catalyzed decomposition of
formic acid/sodium formate in aqueous solution at 90 �C
[84]. With the iridium complex 7 similar to the system used
earlier in the group of Fukuzumi, he achieved the highest
activity for non-phosphine-containing catalysts below
100 �C with an initial TOF of 14,000 h-1 at 90 �C
(Table 2, entry 12; Scheme 4).
In 2008, Laurenczy et al. and our group independently
revisited the concept of formic acid as a hydrogen storage
material. Based on their expertise in the hydrogenation of
Scheme 3 Selected catalyst precursors for the decomposition of
formic acid 1979–1998
Ru M
R
Ph2P PPh2OC
CO
Lau et al., 2003
M = Mo, WR = H, Me
RhO
HN
NH
NHH
O
O
HO
OTf-
Ogo, Fukuzumi et al., 2005
Ru OH2
N
N
SO42-
Ogo et al., 2002
2+
+
4 5 6
Ir OH2
N
N
Himeda, 2009
2+
7
OH
SO42-
HO
Scheme 4 Selected catalyst precursors for the decomposition of formic acid 2003–2009
906 Top Catal (2010) 53:902–914
123
carbonate, Laurenczy et al. used a water-soluble ruthenium
tppts (tris-m-sulfonated triphenylphosphine trisodium salt,
or 3,30,300-phosphinidynetris(benzenesulfonic acid) triso-
dium salt) catalyst, which released hydrogen from aqueous
solutions of formic acid/sodium formate (9:1) at tempera-
tures of 70–120 �C. At 25 �C, hydrogen generation is also
observed, but conversion is slow. Notably, hydrogen can be
obtained at pressures between 1 and 220 bar, and no loss of
catalytic activity is observed up to 750 bar [85–87]. Under
this rather high pressure, carbon dioxide hydrogenation
takes places with the remaining product gases when the
reaction turns slightly basic after full conversion of formic
acid due to the presence of formate. Remarkably, no carbon
monoxide was detected in the gas phase by high pressure
infrared spectroscopy. In a continuous setup, formic acid is
fed to the autoclave and product gases are released from it.
A TOF of 460 h-1 was obtained, and the catalyst was
stable for more than 90 h, reaching more than 40,000
cycles (Table 2, entry 13). High pressure NMR provided
information for proposing a mechanism with two compet-
ing pathways of formic acid decomposition, one involving
a monohydride, the other a dihydride catalyst species
(Scheme 5).
Starting from the bisphosphine tetraaqua ruthenium
complex 8, formate replaces a water ligand to form com-
plex 8-I. A second water molecule is lost when b-elimi-
nation of a hydrogen atom from formate occurs, forming
the carbon dioxide hydride complex 8-II. Carbon dioxide is
replaced by water to give the monohydride complex 8-III.
Concluding the monohydride mechanism, formic acid
replaces a water ligand and protonates the hydride to form
dihydrogen in 8-IV, which is subsequently displaced by
water to reform formate 8-I. Monohydride 8-III is also the
starting point for the dihydride mechanism. Instead of
formic acid, a formate ion coordinates to give the hydride
formate species 9-I. b-elimination of hydrogen forms the
carbon dioxide complex 9-II, which eliminates carbon
dioxide to form 9-III. From this complex, hydrogen is
eliminated upon addition of a molecule of formic acid. The
authors suggested that the reaction mainly occurs via the
dihydride mechanism. Further investigations on the heter-
ogenization of these ruthenium phosphine catalysts were
successful, but catalyst activity is still very low [88].
4.3 Hydrogen Generation From Formic Acid Amine
Mixtures
Our own approach on formic acid decomposition for
hydrogen generation is based on the formic acid amine
adducts as substrate/solvent, which are also known as tri-
alkylammonium formates. Such compounds are also well
known as hydrogen donors in transfer hydrogenation
reactions [89–93], in particular triethylammonium formate
(TEAF, HCO2H/NEt3 5:2) [94]. In the past there have been
some observations on hydrogen generation during these
transfer hydrogenations, especially with ruthenium and
rhodium catalysts [89, 95, 96]. However, the potential for
hydrogen generation at ambient conditions has never been
explored before our work.
At the start of our work the selective decomposition of
formic acid triethylamine adducts was investigated apply-
ing several homogeneous and heterogeneous catalysts
(Table 3) [97]. With most heterogeneous catalysts
(Table 3, entries 1–7) no or only little hydrogen was
evolved at ambient temperature. In TEAF only Pd/C,
which has been suggested for aqueous formate decompo-
sition for energy storage [32, 34], was active for about
20 min, but then deactivated, too. In order to establish a
first system for hydrogen generation, we also investigated
several soluble transition metal compounds (Table 3,
entries 8–17). Among these, only two ruthenium(II) arene
complexes (Table 3, entries 14–17) were active for more
than 2 h, being [RuCl2(p-cymene)]2 (10) the most active.
Slow formation of hydrogen is also observed with rho-
dium(III) chloride, whereas ruthenium(III) chloride, cop-
per(I) iodide, iron(II) chloride, and iron(III) chloride as
Scheme 5 Mechanism of
formic acid decomposition
catalyzed by the Ru/tppts
system (P = tppts; charges are
omitted for clarity) [87]
Top Catal (2010) 53:902–914 907
123
well as [cpFe(CO)2I] are not active under these conditions.
Interestingly, with rhodium(III) chloride, hydrogen pro-
duction increased four times when the experiment was
continued 6 h [97].
The robustness of the TEAF/10 system for hydrogen
generation was investigated adding water and other sol-
vents including some ionic liquids to the substrate
(Table 4, entries 1–9) [98, 99]. The addition of water,
ethanol or N,N0-dimethylformamide (DMF) has no or only
little effect on the system’s performance. Dimethylsulfox-
ide (DMSO) and tetrahydrofuran (THF) decrease the
hydrogen output. Among the ionic liquids, 1-methyl-3-
octylimidazolium tetrafluoroborate did not have a negative
effect. We also found that the addition of a small amount of
alkaline or earth alkaline bromides or iodides is beneficial
and increased catalyst activities by a factor of up to five
(Table 4, entries 10–12). More recently, Shi and co-
workers have successfully investigated the formic acid/10
system using task specific ionic liquids and sodium formate
as a base [100].
Having established the [RuCl2(p-cymene)]2 (10) as a
reliable and robust system for the catalytic decomposition
Table 3 Hydrogen generation from 5 mL TEAF with several homogeneous catalyst precursors and heterogeneous catalysts
Catalyst T/ �C nmetal/lmol 2 h 3 h
Vgas/mL TON Vgas/mL TON
1 CuO on alumina 13 wt% 80 59.5 1.4 \1 1.7 \1
2 Fe2O3 on silica 5 wt% 40 19.1 0.4 No H2 0.9 No H2
3 Nano Fe2O3 40 59.5 0.6 No H2 – –
4 Nickel on silica 60 wt% 80 595 6.9 \1 7.5 \1
5 Pd/C 10 wt% 40 19.1 19 20 20 22
6 Ru on Fe3O4 5 wt% 40 59.5 12.4 4.2 13.0 4.5
7 RuOx on Fe2O3 5 wt% 40 59.5 2.9 1.0 3.2 1.1
8 CuI 40 59.5 4.3 1.5 6.1 2.1
9 FeCl2 120 120 2.9 \1 4.8 \1
10 FeCl3 120 120 3.9 \1 6.4 \1
11 [cpFe(CO)2I] 40 59.5 3.6 No H2 4.1 No H2
12 RhCl3�xH2O 40 19.1 1.9 2.0 4.3 4.5
13 RuCl3�xH2O 40 19.1 0.6 No H2 – –
14 [RuCl2(benzene)]2 40 59.5 30 10 46 16
15 [RuCl2(p-cymene)]2 40 19.1 12 13 17 19
16 [RuCl2(p-cymene)]2 40 59.5 41 14 61 21
17 [RuCl2(p-cymene)]2 26.5 1,191 215 3.7 345 5.9
Table 4 Hydrogen generation from 5 mL TEAF/59.5 lmol 10 with additives at 40 �C
Catalyst Additive Amount 2 h 3 h References
Vgas/mL TON Vgas/mL TON
1 [RuCl2(p-cymene)]2 – – 41 14 61 21 [97]
2 [RuCl2(p-cymene)]2 Water 5 mL 41 14 62 21 [98]
3 [RuCl2(p-cymene)]2 Ethanol 5 mL 44 15 67 23 [98]
4 [RuCl2(p-cymene)]2 DMF 1 mL 47 16 69 24 [98]
5 RuCl2(p-cymene)]2 DMSO 1 mL 29 10 35 12
6 RuCl2(p-cymene)]2 THF 1 mL 35 12 52 18
7 RuCl2(p-cymene)]2 1-methyl-3-octylimidazolium tetrafluoroborate 1 mL 43 15 66 22
8 RuCl2(p-cymene)]2 1-butyl-3-methylimidazolium tetrafluoroborate 1 mL 31 11 48 16
9 RuCl2(p-cymene)]2 1,3-dimethylimidazolium dimethylphosphate 1 mL 3.5 1.2 4.4 1.5
10 RuCl2(p-cymene)]2 Potassium bromide 595 lmol 101 34 150 51 [99]
11 RuCl2(p-cymene)]2 Magnesium bromide 595 lmol 78 27 120 41 [99]
12 RuCl2(p-cymene)]2 Potassium iodide 595 lmol 279 96 338 116 [99]
908 Top Catal (2010) 53:902–914
123
of TEAF, we investigated the effect of the amine and its
concentration in more detail [97–99].
In a first approach, only tertiary amines and ammonia
were studied, later on other nitrogen-containing com-
pounds such as imines, amides and heterocycles com-
pounds were also added to our study. As shown in Fig. 2, a
relation between the added amine and the activity of the
resulting hydrogen generation system was established. In
general, more basic compounds generate systems with
higher activity. The most active system was obtained
applying a 5:3 mixture of formic acid and DBN (1,5-
diazabicyclo(4.3.0)non-5-ene).
The concentration of the amine also strongly affects the
rate of hydrogen generation. We investigated this effect for
triethylamine and N,N-dimethylbutylamine. In control
experiments, no catalytic hydrogen generation is observed
in the absence of amine. A higher concentration of amine is
beneficial for hydrogen production. For NEt3/HCO2H, an
increase of catalyst activity (TON after 3 h) from 1.2 for a
1:10 mixture to 76 for a 3:4 mixture is observed. For
BuNMe2/HCO2H the TON after 3 h increased from 2.3 for
1:10 mixture to 41 for a 4:5 mixture [98]. For DBN/
HCO2H, 58 turnovers were achieved in 3 h for a 2:5
mixture, and 90 turnovers for a 3:5 mixture [99].
Noteworthy, using the ruthenium phosphine complex
[RuCl2(PPh3)3] we found that hydrogen generation
increased by more than an order of magnitude. A similar
activity is obtained with an in situ catalyst prepared from
ruthenium trichloride hydrate and triphenylphosphine in
DMF. We explored this phenomenon investigating a number
Fig. 2 The influence of different nitrogen containing compounds on
hydrogen evolution from 2:5 mixtures with formic acid using
[RuCl2(p-cymene)]2 (10) at 40 �C [99]
Table 5 Hydrogen generation from 5 mL TEAF with ruthenium/triphenylphosphine catalysts at 40 �C [98]
Ruthenium precursor nRu/lmol Phosphine ligand Ru:P 2 h 3 h References
Vgas/mL TON Vgas/mL TON
1 – – PPh3 – 0.0 – 0.0 – [98]
2 RuCl2(PPh3)3 5.95 – – 260 891 261 893 [97]
3 RuCl3�xH2O 5.95 PPh3 1:3 202 691 204 700 [98]
4 RuBr3�xH2O 5.30 PPh3 1:3.4 230 882 232 891 [98]
5 RuBr3�xH2O 17.1 PPh3 1:3.4 1,154 1,375 1,238 1,475 [98]
6 [RuCl2(p-cymene)]2 6.05 PPh3 1:3 22 73 24 81 [98]
7 [RuCl2(p-cymene)]2 19.0 PPh3 1:3 47 50 52 56 [98]
8 [RuI2(p-cymene)]2 5.97 PPh3 1:3 13 44 17 57 [98]
9 [RuCl2(benzene)]2 5.95 – – 2.5 8.7 3.8 13 [98]
10 [RuCl2(benzene)]2 5.95 PPh3 1:1 1.6 5.5 2.0 6.9 [98]
11 [RuCl2(benzene)]2 5.95 PPh3 1:3 106 361 124 425 [98]
12 [RuCl2(benzene)]2 5.95 PPh3 1:6 116 397 131 450 [98]
13 [RuCl2(benzene)]2 5.95 PPh3 1:20 133 454 147 505 [98]
14 [RuCl2(benzene)]2 19.4 PPh3 1:3 324 340 419 440 [98]
15 [RuCl2(benzene)]2 29.8 PPh3 1:3 450 308 671 459 [98]
16 [RuCl2(C10H16)]2a 5.99 PPh3 1:3 47 159 51 173 [98]
17 Ru(methylallyl)2 COD 5.96 PPh3 1:3 20 68 23 79 [98]
18 Ru(acac)3 5.93 PPh3 1:3 1.1 3.9 1.2 4.0 [98]
19 RuCl2(bipy)2 5.96 PPh3 1:3 0.2 0.85 0.3 1.0 [98]
20 Shvo0s cat.b 5.90 PPh3 1:3 0.0 – 0.0 – [98]
a) [RuCl2(C10H16)]2 = bischloro(l-chloro)bis[(1,2,3,6,7,8,h)-2,7-dimethyl-2,6-octadien-1,8-diyl]diruthenium (IV); b) pretreatment at 120 �C,
reaction at 50 �C
Top Catal (2010) 53:902–914 909
123
of ruthenium precursors and phosphine ligands for the for-
mation of several in situ catalysts. First, ruthenium precur-
sors were investigated with triphenylphosphine in DMF
(Table 5). While ruthenium precursors such as ruthe-
nium(III) chloride and bromide showed a high initial activ-
ity, they are deactivated after 20 min. Significant activity
after the 20 initial minutes was best achieved with ruthenium
(II) g6-arene complex [RuCl2(benzene)]2 (14). With this
precursor, the highest catalyst activities are observed at
lower ruthenium concentrations (Table 5, entries 12, 14, 15).
Varying the ruthenium to phosphine ratio (Table 5, entries
9–13), we observed that three PPh3 ligands are needed to
obtain an improved activity. Adding 6 or 20 equivalents of
PPh3 also increases catalyst activity, but less dramatically.
Next, different phosphine ligands were studied using
precursor 14 and a Ru:P ratio of 1:6 (Table 6). The best
performance among monodentate phosphines is observed
for 14/triphenylphosphine. Among bidentate phosphines,
14/1,2-bis(diphenylphosphino)ethane (dppe) catalyst per-
forms best, but with a long induction period. Only little
hydrogen generation is observed in the presence of alkyl
phosphines.
To prove the concept of using the hydrogen from formic
acid for electricity generation, we coupled one of our
systems to a fuel cell. The gas evolved from a mixture of
5HCO2H/4HexNMe2 in the presence of [RuCl2(benzene)]2
(14)/6 PPh3 at room temperature, contained only hydrogen,
carbon dioxide, and traces of argon. However, to prevent
possible poisoning of the fuel cell by traces of starting
material that might have been below the limit of detection
of our GC, we filtered the gas through a short column of
activated charcoal. After an initial phase of higher activity,
constantly 26 mW (at 370 V) are obtained during 42 h
[98]. After full conversion of formic acid, the system can
be re-activated simply by adding formic acid, which is an
indicator for the robustness of the catalyst.
Using ruthenium in combination with the dppe ligand,
the system was further developed towards practical appli-
cations [101]. Applying 20 mL of 5HCO2H/4HexNMe2 as
substrate a TON of 5,716 within 3 h at 40 �C was reached,
which constituted the highest activity for our approach to
hydrogen generation so far. After gas evolution had ceased,
the catalyst was reactivated 10 times by addition of formic
acid. In the first run, a prolonged induction period was
Table 6 Hydrogen generation from 5 mL TEAF with [RuCl2(benzene)]2 (14)/ligand catalysts at 40 �C [98]
Ruthenium
precursor
nRu/
lmol
Ligandb Ru:P 2 h 3 h References
Vgas/
mL
TON Vgas/
mL
TON
1 [RuCl2(benzene)]2 19.1 PCy3 1:6 1.7 1.8 2.0 2.1 [98]
2 [RuCl2(benzene)]2 19.1 BuPAd2 1:6 1.2 1.3 1.5 1.6 [98]
3 [RuCl2(benzene)]2 19.1 P(o-tolyl)3 1:6 11 12 16 17 [98]
4 [RuCl2(benzene)]2 19.1 P(furyl)3 1:6 164 175 198 211 [98]
5 [RuCl2(benzene)]2 19.1
NPPh
Ph
1:6 4.5 4.8 6.1 6.5 [98]
6 [RuCl2(benzene)]2 19.1 dppm 1:6 2,799 2,624 2,985 2,798
7 [RuCl2(benzene)]2 19.1 dppe 1:6 238 254 1,289 1,376 [98]
8 [RuCl2(benzene)]2 19.1 dppp 1:6 694 740 799 852 [98]
9 [RuCl2(benzene)]2 19.1 dppb 1:6 23 25 27 29 [98]
10 [RuCl2(benzene)]2 19.1 dppf 1:6 73 78 87 93 [98]
11 [RuCl2(benzene)]2 19.1 1,2-bisdiphenylphosphinobenzenea 1:6 5.5 5.9 12 13 [98]
12 [RuCl2(benzene)]2 19.1 Xantphos 1:6 22 24 35 37
13 [RuCl2(benzene)]2 19.1 (S)-1-[(RP)-2-(diphenylphosphino)ferrocenyl]-ethyldi(3,5-
xylyl)phosphine
1:6 41 44 62 66
14 [RuCl2(benzene)]2 19.1
NPCy
Cy
P Cy
Cy
1:6 32 34 49 52
a Pre-treatment 3 min at 40 �C in an ultrasonic bathb dppm bis(diphenylphosphino)methane, dppe 1,2-bis(diphenylphosphino)ethane, dppp 1,3-bis(diphenylphosphino)propane, dppb 1,4-
bis(diphenylphosphino)butane, dppf 1,10-bis(diphenylphosphino)ferrocene
910 Top Catal (2010) 53:902–914
123
observed. The activity did not decrease significantly within
the next 10 runs, and an overall TON [ 60,000 was
achieved (Fig. 4a). Based on these results, a continuous
reactor was set up with 9.55 lmol [RuCl2(benzene)]2 14/6
dppe 17.5 mL HexNMe2 (Fig. 3).
To start the reaction, 4.75 mL formic acid were added.
Then addition of formic acid was continued at a rate of
0.74 mL/h. The gas flow was monitored by a flow meter
and the hydrogen content was measured with a hydrogen
gas sensor, sustained by a GC of the gas collected every
24 h. The system worked for more than 11 days using
commercial 99% formic acid from BASF SE as received.
Gas output and hydrogen concentration were virtually
constant during this time, and no signs of catalyst deacti-
vation were observed (Fig. 4b). Overall, 260,000 turnovers
were achieved, corresponding to TOF [ 900 h-1. This
concept was proven to work in a small prototype model car
driven by a hydrogen/air fuel cell, which has been coupled
to an onboard hydrogen generation system using formic
acid and a similar catalyst.
Recently, the group of Wills studied hydrogen genera-
tion from TEAF with a tethered half-sandwich complexes
of rhodium 11 and ruthenium 12 at room temperature,
obtaining a TOF of around 490 h-1 [102]. In the same
publication, they also presented studies with different
transition metal complexes at 120 �C, among them
[RuCl2(dmso)6]. This is an active catalyst (TOF =
18,000 h-1), which is also stable and recyclable (TON =
25,000, after four cycles). However, addition of one
equivalent of phosphine led to deactivation, and the for-
mate-bridged binuclear complex [Ru2(HCO2)2(CO)4
(PPh3)2] 13 is observed (Scheme 6).
Another interesting aspect of the catalytic decomposi-
tion of formic acid is its photochemical acceleration. In the
early 1990s, there have been two independent reports of
light-accelerated reactions of formic acid and transition
metal complexes in solution. Onishi reported that
Fig. 3 Continuous setup for
hydrogen generation from
formic acid [101]
h
/ L
time / min
Fig. 4 a Recycling of a [RuCl2(benzene)]2/dppe catalyst for hydrogen generation from 5HCO2H/4HexNMe2. b Gas output and hydrogen
concentration from continuous formic acid decomposition with [RuCl2(benzene)]2/dppe in HexNMe2 [101]
Scheme 6 Complexes by Wills et al. for the dehydrogenation of
formic acid
Top Catal (2010) 53:902–914 911
123
irradiation with a 400 W Hg vapor lamp with Pyrex filter
accelerates the reaction of formic acid with [HCo(Ph-
P(Et)2)4], performed in THF at 30 �C, from 0.09 turnovers
to 1.6 turnovers within 3 h, and 3.1 in 6 h [103]. They also
observed a shift of the hydride signal in 1H NMR from a
well defined quintet at -14.76 ppm of the original complex
to a broad singlet -12.47 ppm upon the addition of formic
acid, which did not change neither after ageing the com-
plex for 6 h, nor after subsequent irradiation. At the same
time, King et al. showed that hydrogen is evolved from
aqueous formate solutions with chromium hexacarbonyl
under irradiation with a TON of 18 after 1 h in a setup
similar to Onishi’s [104]. They proposed that one carbon
monoxide ligand dissociates from the chromium center
upon photolysis, which is replaced by a weakly coordinated
solvent molecule (methanol). When formate is added,
hydrogen is evolved. Interestingly this reaction is inhibited
by the addition of pyridine.
While investigating the decomposition of TEAF with
various ruthenium phosphine catalysts, we observed that
this process is also accelerated by illumination with sun-
light [105]. Though the photochemistry of ruthenium
phosphine hydride and carbonyl complexes [106, 107] and
ruthenium g6-arene complexes [108–111] is well-known,
this behavior has rarely been explored for catalysis [112,
113]. When our previous ruthenium phosphine containing
catalyst systems were irradiated with visible light, gas
evolution increased up to 11 times (Fig. 5). We have
studied several phosphine ligands in combination with
[RuCl2(benzene)]2 (14), but the effect is only observed for
aryl phosphines. RuCl3•xH2O and [Ru(cod)(methylallyl)2]
are suitable ruthenium sources, too. The best catalyst pro-
ductivity is observed with a 14/dppe catalyst, where gas
evolution increased from 407 to 2,804 turnovers, which is
an almost 7-fold increase. The highest activity with a
monodentate ligand is observed with PPh3 as ligand.
Additionally, the reaction was performed at different
temperatures. In all cases experiments under irradiation
performed significantly better than dark reactions, so a
temperature effect can be ruled out. Notably, our system is
still active at low temperatures such as 0 �C.
Analyzing the rate of gas evolution, it was found that
irradiation generates faster an active species, and then
accelerates the actual reaction. Based on these observa-
tions, and on further NMR-spectroscopic investigations, we
proposed the mechanism shown in Scheme 7 for this light
accelerated dehydrogenation of formic acid (Scheme 7).
Starting from the ruthenium precursor 14, a ruthenium
hydride phosphine complex 15-I is formed, while the g6-
benzene ligand is cleaved under irradiation. Formic acid
adds to this complex, possibly via intermediates, and a
dihydrogen formate ruthenium complex 15-II is formed.
Dihydrogen loss from 15-II is as well accelerated by light
as it is the subsequent b-elimination of carbon dioxide from
the formate 15-III to reform the ruthenium hydride phos-
phine complex 15-I. Additionally, irradiation prevents the
catalyst from being deactivated. This allows one trigger the
hydrogen generation from formic acid simply by switching
on and off the light source.
5 Conclusion
Although the decomposition of formic acid, especially with
heterogeneous catalyst, has been often studied as a model
reaction for catalyst characterization, only recently this
reaction has received significant attraction for hydrogen
generation. In general, formic acid allows for a simple and
benign storage of hydrogen. On one hand, the industrial
production of formic acid via direct catalytic hydrogenation
of carbon dioxide seems not too distant. On the other hand,
already today several methods for hydrogen generation
Ru
deactivated species
HCO2H
HCO2H
[RuCl2(benzene)]2
PAryl3LnRu + ...
CO2
H2
O
O
H
(PAryl3)mLn-1Ru
O
O
H
Ln
RuLn
H
L
L
L = H-, Cl-, benzenem = 1-3n = 1-3
H H(PAryl3)m
h·
(PAryl3)m
15-I
15-II15-III
14
h·
h·
h·
Scheme 7 Mechanism for the light-accelerated decomposition of
formic acid [105]
Fig. 5 Acceleration of ruthenium-phosphine catalyzed hydrogen
generation from 5 mL TEAF with light (19.1 lmol Ru, 40 �C). aNo hydrogen detected by GC. b No dark experiment, experiment
performed under lab conditions (environmental light)
912 Top Catal (2010) 53:902–914
123
from formic acid are available awaiting application.
Depending on the developed catalysts hydrogen can be
generated either at higher temperatures ([100 �C) and high
pressure as well as at ambient conditions and low temper-
atures. It has been shown by us that systems invented in the
lab can be actually used in ‘‘real world’’ electric applica-
tions, and that scale-up is viable. Yet a less expensive cat-
alyst system is desirable, and investigations are ongoing.
Hydrogen cleaning is easy, but for a higher performance
and prolonged lifetime of the fuel cell, hydrogen and carbon
dioxide should be separated. This would also lead to a better
control of the fuel cell performance, and a higher energy
density of the entire system.
In the future formic acid obtained from sustainable
resources might be also of considerable importance as
hydrogen storage material. Due to its intrinsic properties,
we believe that formic acid will be especially valuable for
niche energy applications, such as small portable devices.
Acknowledgements This work has been supported by the State of
Mecklenburg-Vorpommern, the BMBF, and the DFG (Leibniz-prize
and research training group 1213).
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