Photocatalytic Formic Acid Conversion on CdS Nanocrystals with Controllable Selectivity for H2 or CO

Article abstract of DOI:10.1002/anie.201502773

Formic acid is considered a promising energy carrier and hydrogen storage material for a carbon-neutral economy. We present an inexpensive system for the selective room-temperature photocatalytic conversion of formic acid into either hydrogen or carbon monoxide. Under visible-light irradiation (λ>420 nm, 1 sun), suspensions of ligand-capped cadmium sulfide nanocrystals in formic acid/sodium formate release up to 116±14 mmolH₂g_cat^-1h^-1 with >99% selectivity when combined with a cobalt co-catalyst; the quantum yield at λ=460 nm was 21.2±2.7%. In the absence of capping ligands, suspensions of the same photocatalyst in aqueous sodium formate generate up to 102±13 mmolCOg_cat^-1h^-1 with >95% selectivity and 19.7±2.7% quantum yield. H₂ and CO production was sustained for more than one week with turnover numbers greater than 6×10⁵ and 3×10⁶, respectively.

Full text of DOI:10.1002/anie.201502773

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201502773
German Edition: DOI: 10.1002/ange.201502773
Photocatalysis
Photocatalytic Formic Acid Conversion on CdS Nanocrystals with
Controllable Selectivity for H2 or CO**
Moritz F. Kuehnel, David W. Wakerley, Katherine L. Orchard, and Erwin Reisner*
Abstract: Formic acid is considered a promising energy carrier
and hydrogen storage material for a carbon-neutral economy.
We present an inexpensive system for the selective room-
temperature photocatalytic conversion of formic acid into
either hydrogen or carbon monoxide. Under visible-light
irradiation (l > 420 nm, 1 sun), suspensions of ligand-capped
cadmium sulfide nanocrystals in formic acid/sodium formate
release up to 116 Æ 14 mmolH2 gcatÀ1 hÀ1 with > 99% selectivity
when combined with a cobalt co-catalyst; the quantum yield at
l = 460 nm was 21.2 Æ 2.7%. In the absence of capping
ligands, suspensions of the same photocatalyst in aqueous
sodium formate generate up to 102 Æ 13 mmolCOgcatÀ1 hÀ1
with > 95% selectivity and 19.7 Æ 2.7% quantum yield. H2
and CO production was sustained for more than one week with
turnover numbers greater than 6 ” 105 and 3 ” 106, respectively.
a potential renewable fuel of high energy density. Its low
toxicity and high gravimetric hydrogen content of 4.4%
render FA a promising hydrogen storage material, with CO2
as the only by-product of H2 release.[2] CO2 recycling by mild
homogeneous hydrogenation of CO2 to FA has become
a feasible process to store H2 derived from renewable
sources.[3] In addition, a growing number of synthetic
catalysts[4] and enzymes[5] promote the storage of electrical
energy by electrochemical reduction of CO2 to FA. FA is also
a major product of biomass processing.[6]
Although FA dehydrogenation (FA-to-H2) is an exergonic
process (Scheme 1), efficient liberation of H2 requires addi-
tional energy input (i.e. high temperatures or light) unless
precious-metal-based catalysts are employed.[7,8] The high
T
he replacement of conventional fossil fuels with a CO2-
neutral energy cycle is a key global challenge for developing
a sustainable economy. Hydrogen holds promise as a secon-
dary energy vector for use in fuel cells, but its safe storage and
transport remain the subject of intensive research.[1] Formic
acid (HCO2H, FA) has received considerable attention as
Scheme 1. Thermodynamics of formic acid decomposition pathways.[16]
cost and low abundance of these catalysts, however, precludes
scalability and thus a widespread application. Precious-metal-
free alternatives typically require elevated temperatures and
organic solvents, limiting their potential use in portable
applications and decreasing their overall energy density.[9]
Only recently, the photochemical decomposition of FA has
attracted increasing interest as an alternative approach to
generate H2 from FA at room temperature. Photocatalysts
based on Pd,[10] AuPd,[11] Pt,[12] Rh,[13] and Ru[14] have
demonstrated activities up to 154 mmolH2 gcatÀ1 hÀ1. Exam-
ples of precious-metal-free catalysts are scarce and show
substantially lower activity.[15]
Despite the pivotal role of nanocrystalline semiconduc-
tors (commonly referred to as quantum dots, QDs)[17] in
photovoltaics[18] and artificial photosynthesis,[19] little is
known about their activity towards photochemical FA-to-H2
conversion. Cadmium sulfide is among the most studied QD
materials owing to its ease of preparation, low cost, and high
absorption of visible light. Whereas bulk CdS powder shows
limited photocatalytic FA-to-H2 activity,[20] enhanced activity
has been achieved by confinement on a titanate nanotube
support,[21] by construction of CdS-ZnS heterojunctions,[22]
and by introduction of precious-metal co-catalysts such as
Pt[21a,23] and Ru.[22,24] A precious-metal-free hybrid system
comprised of CdS and a H2-producing enzyme (hydrogenase)
exhibited low selectivity and suffered from enzyme inhib-
ition.[25] Moreover, none of these systems show flexibility with
respect to the reaction products.
[*] Dr. M. F. Kuehnel,[+] D. W. Wakerley,[+] Dr. K. L. Orchard,
Dr. E. Reisner
Christian Doppler Laboratory for Sustainable SynGas Chemistry
Department of Chemistry, University of Cambridge
Lensfield Road, CB2 1EW, Cambridge (UK)
E-mail: reisner@ch.cam.ac.uk
Homepage: http://www-reisner.ch.cam.ac.uk
[+] These authors contributed equally to this work.
[**] This work was supported by the Christian Doppler Research
Association (Austrian Federal Ministry of Science, Research, and
Economy and the National Foundation for Research, Technology
and Development), the OMV Group, the EPSRC (EP/H00338X/2 to
E.R.), the Isaac Newton Trust, the German Research Foundation
(M.F.K.), and the Advanced Institute for Materials Research-
Cambridge Joint Research Centre (K.L.O.). XPS spectra were
obtained at the National EPSRC XPS User’s Service (NEXUS) at
Newcastle University, an EPSRC Mid-Range Facility. We thank Dr.
Christine A. Caputo and Georgina A. M. Hutton for helpful dis-
cussions and Benjamin C. M. Martindale for help with XPS
spectroscopy. Additional data related to this publication is available
at the University of Cambridge data repository (http://www.
repository.cam.ac.uk/handle/1810/248441).
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201502773.
ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co.
KGaA. This is an open access article under the terms of the Creative
Commons Attribution License, which permits use, distribution and
reproduction in any medium, provided the original work is properly
cited.
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FA is known to undergo two pathways of decomposition,
to give either H2 or carbon monoxide (Scheme 1). CO is
a valuable synthon in the chemical industry and synthesis gas
(a mixture of CO and H2) can be used to generate liquid fuels
such as methanol and hydrocarbons by the Fischer–Tropsch
process.[26] CO is currently produced from fossil sources, but
despite its critical importance, the sustainable generation of
CO from FA has received little attention, and no chemical
storage process for CO is currently available.[27] FA contains
more than 60 wt% CO, which can be released upon treatment
with excess dehydrating agents, such as conc. H2SO4, but
catalytic FA-to-CO conversion typically requires high tem-
peratures.[28] Consequently, selective photocatalytic FA-to-
CO has not been reported. This work presents an inexpensive
and highly active CdS-based photocatalyst that efficiently
uses FA as a clean storage material for the controlled
generation of either H2 or CO under ambient conditions.
Monodisperse CdS nanocrystals with 3-mercaptopro-
pionic acid (MPA) capping ligands (QD-MPA) were prepared
according to literature procedures from oleic acid capped CdS
nanoparticles by basic ligand exchange (lmax = 443 nm, D =
4.4 Æ 0.4 nm, Figure S1).[19c,29] Under simulated solar light
irradiation (AM1.5G, 100 mWcmÀ2, l > 420 nm, 258C, see
Supporting Information for details), dispersions of QD-MPA
in 4.0m sodium formate in FA (Laboratory Reagent
Grade, > 90%) generated H2 at initial rates of 52.1 Æ
6.6 mmolH2 gcatÀ1 hÀ1 (Figure 1A, Table 1). The addition of
up to 0.5 mm CoCl2·6H2O resulted in enhanced rates of up to
116 Æ 14 mmolH2 gcatÀ1 hÀ1 (QD/Co ratio ca. 500:1, see Fig-
ure S2 and Tables S1 and S2 for optimization details). Only
traces of CO were detected in the headspace gas (670 Æ
146 ppm absolute, 0.614 Æ 0.065% with respect to H2).
When the full solar spectrum was used for irradiation, the
rate increased to 218 Æ 22 mmolH2 gcatÀ1 hÀ1 (Figure S3),
showing that both the visible and UV portion of the solar
spectrum are used for catalysis; a dependence of the catalyst
on light was confirmed by varying the light intensity (Fig-
ure S4). At l = 460 nm, the external quantum yield (EQY)
was 21.2 Æ 2.7% (Table S3). The CO2/H2 ratio increased over
time (Figure S5), but remained below the theoretical 1:1
stoichiometry (Scheme 1), presumably because of a higher
Figure 1. Photocatalytic dehydrogenation of FA: A) Initial activity of
QD-MPA with and without added co-catalyst (1 h of irradiation);
B) long-term activity of QD-MPA [AM1.5G, 100 mWcmÀ2, l>420 nm;
0.91 mm QD-MPA (176 mgmLÀ1), 4.0m NaHCO2 in FA; CoCl2 =0.5 mm
CoCl2·6H2O, MPA=140 mm 3-mercaptopropionic acid].
solubility of CO2 over H2 in FA. H2 generation even proceeds
in FA without added sodium formate, albeit at a lower rate of
1.4 Æ 0.3 mmolH2 gcatÀ1 hÀ1. No H2 was formed in the dark or
without photocatalyst (Table S2).
Long-term experiments were performed to demonstrate
the stability of this system (Figure 1B, Table S4). H2 evolution
was sustained for more than 7 days, even in the absence of
a co-catalyst. The rate of H2 evolution gradually decreased
during the first 48 h of irradiation, stabilizing at 4.4 Æ
1.3 mmolH2 gcatÀ1 hÀ1 and 0.52 Æ 0.17 mmolH2 gcatÀ1 hÀ1 with
and without co-catalyst, respectively. When excess MPA
(140 mm) was added to the QD-MPA/CoCl2 system prior to
irradiation, a lower initial rate of 18.7 Æ 2.9 mmolH2 gcatÀ1 hÀ1
was observed, but this activity remained constant over the
course of one week. More than 3 molH2 gcatÀ1 were generated
after one week, which corresponds to over 600000 turnovers
per QD. Transmission electron microscopy (TEM) after
photocatalysis indicated the formation of aggregates that
retained nanocrystalline features (Figure S6). Particle aggre-
gation over time was monitored in situ by UV/Vis spectros-
copy. When QD-MPA is added to FA, a red-shift of the
absorption maximum indicates aggregation, which increases
further during irradiation (Figure S7A).[30] In the presence of
Table 1: Comparison of selected photocatalysts for visible-light-driven FA-to-H2 conversion under ambient conditions (see Table S5 for more
examples).
Catalyst
Activity[a] [mmolH2 gcatÀ1 hÀ1
]
Selectivity[b] [%]
EQY [%]
Lifetime [h]
Ref.
Pd-C3N4
AuPd-TiO2
Pt-CdS
Ru-CdS/ZnS
53.4
100
99.7
n/a
n/a
n/a
20
n/a
“trace CO”
98.8Æ0.1
99.4Æ0.1
98.9Æ0.1
n/a
>6
>9
[10b]
[11b]
[23a]
[22]
[22]
[25]
[14]
[15c]
17.7[c]
15.6
21.4
20
n/a
3.1
n/a
n/a
n/a
1.22
>30
>40
>40
>3.5
>5
>24
>168
>168
>24
5.85Æ0.09
1.24Æ0.02
0.356
CdS/ZnS
H2ase-CdS[d]
[{RuCl2(PhH)}2] + 12PPh3
[Fe3(CO)12] + PPh3, tpy[f]
QD-MPA
QD-MPA/CoCl2
QD-MPA/CoCl2
154[e]
2.7[e]
52.1Æ6.6
116Æ14
218Æ22[c]
this work
this work
this work
21.2Æ2.7
n/a
[a] For an accurate comparison, published data were converted into gravimetric activity using the mass of the entire photocatalyst used in the reaction.
[b] Selectivity=100%”nH2/(nH2 +nCO). [c] Full solar spectrum irradiation. [d] H2ase=hydrogenase. [e] l>380 nm. [f] tpy=2,2’:6’,2’’-terpyridine.
n/a=not available.
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MPA, a red-shift was observed upon addition to FA, but no
further change occurred during irradiation (Figure S7B),
suggesting that MPA enhances the lifetime of QD-MPA
during photocatalysis by preventing aggregation.
To the best of our knowledge, the catalytic activity and
lifetime of QD-MPA/CoCl2 surpasses all previously reported
heterogeneous photocatalysts for FA-to-H2 conversion under
ambient conditions, including those based on precious metals
(Table 1, see Table S5 for a more comprehensive compar-
ison). The most active heterogeneous catalyst to date, a Pd-
C3N4 nanocomposite, evolves 53.4 mmolH2 gcatÀ1 hÀ1 with
quantitative H2 selectivity for up to 6 h. Without added co-
catalyst, QD-MPA shows a comparable activity, whereas QD-
MPA/CoCl2 is more than twice as active and shows an
improved long-term stability. A Ru-based homogeneous
photocatalyst was reported to achieve 154 mmolH2 gcatÀ1 hÀ1
in DMF solution, but no selectivity was reported.[14] The best
precious-metal-free photocatalyst, CdS-ZnS particles, can
evolve up to 1.24 Æ 0.02 mmolH2 gcatÀ1 hÀ1 under irradiation
with visible light (selectivity not reported),[22] more than two
orders of magnitude less than QD-MPA/CoCl2.
Insight into the nature of the active catalyst was sought by
separating QD-MPA/CoCl2 from the reaction mixture by
centrifugation after 1 h photocatalysis. When the solid residue
was re-dispersed in fresh reaction medium without added co-
catalyst, the observed H2 evolution activity was similar to
QD-MPA in the absence of co-catalyst, suggesting that the
active catalyst is not attached to the QDs. The QD-free
supernatant did not show any activity (Figure S8, Table S6).
Inductively coupled plasma optical emission spectrometry
(ICP-OES) measurements of the solid confirm the absence of
Co (Co/Cd atomic ratio in the solid less than (4.8 Æ 1.9) ”
10À4:1; in contrast to 0.44:1 in the entire sample before
catalysis). These findings and the absence of an induction
period for H2 evolution with QD-MPA/CoCl2 lend further
support to the homogeneous nature of the active co-catalyst.
Cobalt species are known to function as homogeneous
hydrogen-evolving catalysts in the presence of nanocrystal-
line semiconductors.[31] Incorporation of cobalt is also known
to enhance the activity of formate oxidation electrocata-
lysts[32] and there is precedent for homogeneous oxidation of
FA by Co3+ ions.[33] The mechanism of FA-to-H2 conversion
could, therefore, consist of formate oxidation by photogen-
erated holes and subsequent proton reduction by photo-
excited electrons.[20,34] In this case, the reductive half reaction,
the oxidative half reaction, or both are enhanced by Co
catalysis.
Figure 2. Photocatalytic dehydration of FA (AM1.5G, 100 mWcmÀ2
,
l>420 nm): A) Effect of solvent on the product selectivity of CdS QDs
in the absence of CoCl2 co-catalyst (see Table S3 for conditions);
B) long term activity of QD-BF4 [0.0611 mm QD-BF4 (14.4 mgmLÀ1),
4.0m NaHCO2 in 2.5m aqueous KOH/CO2 pH 9.7].
of N,N-dimethylformamide (DMF) to remove the oleic acid
capping groups.[35] Ligand stripping led to a blue-shift of the
absorption maximum indicative of a decrease in QD size
À
(Figure S9B). This is presumably a result of etching by BF4
ions, as shown for CdTe QDs.[36] Larger QD precursors (lmax
=
466 nm, D = 6.0 Æ 0.9 nm) were, therefore, used to obtain
QD-BF4 with a diameter similar to QD-MPA (lmax = 445 nm,
D = 4.9 Æ 0.7 nm, Figure S9). QD-BF4 showed a drastically
enhanced photocatalytic FA-to-CO activity in water com-
pared to QD-MPA (Figure 2A). Under optimized conditions,
up to 102 Æ 13 mmolCOgcatÀ1 hÀ1 are formed with 96.3 Æ
0.1% selectivity (4.0m NaHCO2 in 2.5m KOH/CO2 pH 9.7;
see Figures S10–S12 and Tables S7 and S8 for optimization
details). In FA solution, QD-BF4 showed a decomposition
selectivity of FA towards H2 that was comparable to QD-
MPA, but with lower activity (Figure 2A, Table S2). The
selectivity switchover to CO is, therefore, not a result of
ligand removal, but promoted by the basic aqueous environ-
ment. This is further corroborated by a strong pH-depend-
ence of the product selectivity, with FA-to-H2 conversion
becoming more pronounced in neutral and acidic solution
(Figure S10, Table S7); the addition of MPA to QD-BF4 in
aqueous formate solution did not affect the product selectiv-
ity (see below). The effects of water on the gas-phase
photocatalytic FA decomposition have been previously
documented.[37]
Aqueous QD-BF4 is remarkably robust and sustained CO
production over the course of one week with no detectable
À1
With the reactivity of nanocrystalline CdS towards
formate established, we sought possibilities to tune the
reaction pathway towards FA-to-CO conversion [Scheme 1,
Eq. (2)]. In aqueous formate solution, the activity of QD-
MPA was much lower than in FA but showed a reversed
selectivity, with CO as the main decomposition product (60 Æ
7% CO, Figure 2A, Table S2). To enhance the photocatalytic
activity in aqueous solution, we studied the effect of modify-
ing the QD surface. Ligand-free, charge-stabilized CdS
nanocrystals (QD-BF4) were prepared from oleic acid
capped CdS nanocrystals by following a modified reactive
ligand stripping procedure using [Me3O]BF4 in the presence
decrease in activity; more than 14 molCOgcat were gener-
ated, which corresponds to 3000000 turnovers per QD
(Figure 2B, Table S9). No activity was seen in the absence
of light (Table S2, entry 16; Figure S13), confirming that
irradiation provides the necessary activation energy for
formate dehydration. Since irradiation of CdS generates
electron/hole pairs, we propose that the reaction mechanism
is centered around the charge separation by the QD.
Assuming that one photon is required to generate one CO
molecule, a minimum EQY of 19.7 Æ 2.7% was recorded at
l = 460 nm (Table S10, see the Supporting Information for
further details). Formate was established as the sole source of
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CO through IR spectroscopy by using 13C-labeled sodium
formate (Figure S14). TEM analysis after catalysis indicates
the formation of nanostructured aggregates (Figure S15). This
was corroborated through an observed red-shift in the UV/
Vis spectrum (Figure S16, see above).
Addition of CoCl2 to QD-BF4 or QD-MPA did not
increase the photocatalytic activity in aqueous solution
(Table S2), indicating that the CdS particle surface itself
plays an essential role in the catalytic activity. Mechanistic
studies were performed to further support this hypothesis
(Tables S11 and S12). Addition of excess Na2S to an active
sample of QD-BF4 in aqueous sodium formate solution
resulted in a sudden drop in catalytic activity (Figure 3A),
In summary, we have developed an inexpensive and highly
active photocatalyst system for sunlight-driven conversion of
formic acid with controlled selectivity and long-term stability.
Ligand-capped CdS quantum dots with a cobalt co-catalyst
generate H2 with unprecedented activity and > 99% selec-
tivity when dispersed in formic acid at room temperature. In
contrast, CO formation is strongly favored in aqueous
solution after ligand stripping and proceeds with high activity
and efficiency. With this first example of selective photo-
catalytic formic acid to CO conversion, we introduce formic
acid as a renewable CO storage material with more than
60 wt% capacity. This work demonstrates that careful match-
ing of engineered particle surfaces with optimized reaction
media enables a novel flexibility in the sustainable use of
formic acid to generate valuable chemical feedstocks.
Keywords: CdS · formic acid · hydrogen · photocatalysis ·
quantum dots
How to cite: Angew. Chem. Int. Ed. 2015, 54, 9627–9631
Angew. Chem. 2015, 127, 9763–9767
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suggesting that sulfide ions block the catalytically active sites;
addition of MPA resulted in a similar effect (Figure 3B). At
the same time, no increase in H2 evolution was observed in
either case, proving that S2À ions are not simply acting as
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´
Adrovic, D. Volbers, R. Wyrwich, M. Dçblinger, A. S. Susha,
Published online: July 16, 2015
Angew. Chem. Int. Ed. 2015, 54, 9627 –9631
ꢀ 2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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