Earth-abundant photocatalytic systems for the visible-light-driven reduction of CO2 to CO

Article abstract of DOI:10.1039/c6gc03527b

Herein, we report a highly selective photocatalytic system, based on an in situ copper photosensitizer and an iron catalyst, for the reduction of CO₂ to CO. Turnover numbers (TON) up to 487 (5 h) with selectivities up to 99% and Φ_CO = 13.3% were observed. Stern-Volmer analysis allowed us to establish a reductive quenching mechanism between the Cu PS and electron donor.

Full text of DOI:10.1039/c6gc03527b

Green Chemistry
View Article Online
View Journal
COMMUNICATION
Earth-abundant photocatalytic systems for the
visible-light-driven reduction of CO2 to CO†
Cite this: DOI: 10.1039/c6gc03527b
Received 21st December 2016,
Accepted 17th February 2017
Alonso Rosas-Hernández,‡ Christoph Steinlechner,‡ Henrik Junge and
Matthias Beller*
DOI: 10.1039/c6gc03527b
rsc.li/greenchem
Herein, we report a highly selective photocatalytic system, based tion of CO2.8 For instance, Bonin, Robert et al. reported the
on an in situ copper photosensitizer and an iron catalyst, for the use of an iron tetraphenylporphyrin complex together with an
reduction of CO2 to CO. Turnover numbers (TON) up to 487 (5 h) iridium-based PS to reduce CO2 selectively to CO with a turn-
with selectivities up to 99% and ΦCO = 13.3% were observed. over number (TON) of 140.8g The substitution of the Ir PS by
Stern–Volmer analysis allowed us to establish a reductive quench- an organic dye decreased the activity of the catalytic system by
ing mechanism between the Cu PS and electron donor.
half. A CoII complex supported by a cyclic pentadentate ligand
has been also used as Cat in the presence of an Ir PS, driving a
selective CO2-to-CO conversion with a TON of 270.8e A photo-
catalytic system comprising a quaterpyridine CoII or FeII
complex as Cat and a ruthenium-based PS led to CO2
reduction into CO with high selectivity (98%) and TONs
between 497 and 2660.8a When an organic dye is used instead
of the Ru PS, the system converted CO2 to CO with a TON of
Over the last 50 years, the use of fossil fuels has increased the
concentration of greenhouse gas carbon dioxide in the atmo-
sphere.1 Technologies to control CO2 production and reduce
its accumulation in the atmosphere are highly desired. In this
context, photochemical reduction of CO2 provides a pathway to
convert a waste product into more valuable molecules whereby
the energy from sunlight is stored.2 Thus, not only carbon
dioxide can be used as C1-feedstock but also this approach
allows to harvest the energy from sunlight, helping the tran-
sition towards a more sustainable energy source.
1365 (based on Cat, c = 0.005 mM) and a quantum yield ΦCO
=
1.1%. In addition, Ishitani and co-workers recently reported
the use of a dimeric CuI complex, supported by a tetradentate
ligand, as PS for the CO2 reduction to CO with a selectivity of
78% over H2 production.9 The catalytic process was performed
in the presence of an FeII complex as Cat and 1,3-dimethyl-2-
phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) as sacrificial
electron donor (SD). The efficiency and durability for CO for-
mation were high, with ΦCO = 6.7% and TONCO = 270. Apart
from all these interesting developments, the use of abundant-
metal complexes for efficient and durable photocatalytic
systems for selective CO2 reduction with visible light still
remains rare.
Inspired by the seminal work of Lehn and co-workers,3
several molecularly defined systems have been developed to
mediate the photochemical reduction of CO2.2a In these works
usually a multicomponent system is used employing a photo-
sensitizer (PS) and a catalyst (Cat) which is responsible for
CO2-coordination and its subsequent reduction. Ru(II) poly-
pyridyl and cyclometalated Ir(III) complexes are often used as
PS due to their strong oxidation and reduction power and
extended lifetimes of their excited states.4 In addition, tran-
sition metal complexes based on Ru,5 Re,6 and Ir7 have been
successfully applied as Cat. Due to the obvious advantages, the
use of more earth-abundant metals for both PS and Cat is
desired in these materials.
In our research group, several heteroleptic CuI complexes
were developed in the last years and used as PS in photo-
catalytic water reduction.10 To render an operationally simpler
process, we also established a photocatalytic system where the
synthesis and isolation of the corresponding Cu PS is
bypassed.11 Accordingly, the heteroleptic emissive Cu complex
is formed in situ, allowing a facile screening of a broad range
of ligands in order to easily correlate the structural features of
the PS with the observed catalytic activity.
Here, we describe for the first time the application of this
process to the photochemical reduction of CO2. More specifi-
cally, we employed cyclopentadienone iron complexes as Cat,12
generating a fully earth-abundant catalytic system.
Recently, several complexes based on non-precious tran-
sition metals have been described as Cat in the photoreduc-
Leibniz Institute for Catalysis at the University of Rostock, Albert-Einstein-Straße 29a,
18059 Rostock, Germany. E-mail: matthias.beller@catalysis.de
†Electronic supplementary information (ESI) available: Experimental details,
quantum yield calculations and emission-quenching experiments procedure.
See DOI: 10.1039/c6gc03527b
‡These authors contributed equally to this work.
This journal is © The Royal Society of Chemistry 2017
Green Chem.

View Article Online
Communication
Green Chemistry
We started our investigations using iron cyclopentadienone and 6). Under these experimental conditions, the presence of
complex 1 as CO2-reduction catalyst. In situ generation of the the heteroleptic Cu PS is favoured over the inactive homoleptic
Cu PS was performed with equimolar quantities of a simple species.
copper salt ([Cu(MeCN)4]PF6), a bidentate phosphine ligand
Considering the limitations of BNAH as SD (i.e. inner-filter
(xantphos), and a phenantroline derivate (bathocuproine, 5); effect and rapid back-electron transfer from oxidized pro-
see Scheme 1. We observed that upon dissolution of all com- ducts),15 we turned our attention to BIH, a suitable electron
ponents in a 5 : 1 (v/v) mixture of N-methyl-2-pyrrolidone donor recently used in photocatalytic reactions.8a,9 In fact,
(NMP) and triethanolamine (TEOA), with the addition of a SD applying BIH, the photocatalytic activity of the system raised
(1-benzyl-1,4-dihydrinicotinamide, BNAH) and subsequent up to a TONCO = 487 with a selectivity of 99% (Table 1, entry 1).
visible light irradiation for 5 h, CO2 was reduced to CO with a Thus, BIH was used as SD in the following photocatalytic
TON of 245 and a selectivity of 86% over H2 production investigations.
(Table S3,† entry 1). This result suggests that the copper-based
Notably, using an isolated sample of Cu complex 5 instead
PS is easily formed in situ and it is able to mediate a photo- of the in situ generated PS, the catalytic activity was slightly
induced electron transfer from the SD to the CO2-reduction lower (Table S4,† entry 2), which proves that the heteroleptic
catalyst 1 (vide infra).
Cu PS is effectively assembled in situ.
It is worthy to point out that an amine (BNAH) has been
Excited state lifetimes of heteroleptic CuI PS are heavily
employed as sacrificial electron donor. In a large-scale use of influenced by exciplex quenching of the resulted flattened CuII
this technology, photochemical reduction of CO2 needs to be complex upon light irradiation.14 Commonly, steric effects of
coupled with water oxidation reaction. So water can be used the diimine ligand influence the degree of excited state distor-
as an electron source, just as in the case of photosynthesis. tion. Thus, aiming to increase the excited state lifetime of the
Since the oxidation of water is also a challenging reaction by Cu PS and the catalytic activity of the system as well, we tested
itself,13 it is a common practice to optimize both reactions several sterically demanding 1,10-phenantroline derivates as
separately. Thus, sacrificial donors such as amines are used ligands (Table 1, entries 1–6). However, when diimine 7 – con-
i
to explore the activity of potential catalysts in CO2 taining Bu groups in the 2 and 9 positions of the phenantro-
photoreduction.
line moiety – was used instead of bathocuproine 5, the activity
Notably, when dissolving a heteroleptic CuI complex with of CO2 photoreduction was diminished (entry 1 vs. entry 4,
both diimine and diphosphine ligands an equilibrium with Table 1). Interestingly, when applying ligand 9 instead of
the homoleptic CuI(N–N)2 complex (N–N = diimine ligand) is bathocuproine, the catalytic activity is almost similar (Table 1,
established.10d Evidently, this equilibrium can be affected by entry 6). We assume that not only the steric effects of the sub-
the ratio between the diimine and diphosphine ligand in the stituents in position 2 and 9 of the phenantroline play a major
in situ formation of the Cu PS, thus influencing the catalytic role, but also the electronic properties of the various substitu-
activity of the system as well. In fact, when the reaction was ents in the remaining positions.
performed in the absence of a source of diphosphine ligand,
the generation of CO dropped dramatically (Table S2,†
entry 2). In this case, only a homoleptic CuI(N–N)2 complex
can be formed, suggesting that the photophysical properties of
Table 1 Photochemical reduction of CO2 with visible light, iron-based
Cat and in situ Cu PSa
such copper complex are not adequate to drive the photoche-
mical reduction of CO2.14 Accordingly, when an excess of
diphosphine ligand was used (i.e. 2 or 3 equiv.) the catalytic
activity of the system was improved (TONCO = 271 and 270 and
selectivity = 95% and 97%, respectively; Table S2,† entries 5
N–N
ligand
[Fe]
complex
TONb
(CO)
TONb
(H2)
Selec.
(CO)
Entry
1
2c
3
4
5
6
7
8
9
5
5
6
7
8
9
5
5
5
1
1
1
1
1
1
2
3
4
487
64
7
15
17
8
26
31
9
99%
81%
95%
97%
94%
94%
97%
94%
88%
360
310
404
467
254
260
80
16
11
a Reaction conditions: NMP/TEOA (5 : 1, v/v) 7.5 mL;
5
μmol
[Cu(MeCN)4]PF6; 15 μmol xantphos P–P ligand; 5 μmol N–N ligand;
1 μmol Fe catalyst; BIH 0.1 M; Hg-lamp (light output 1.5 W) equipped
with a 400–700 nm filter. b Determined using calibrated GC; TON =
n(CO)/n(catalyst). c Reaction performed using MeCN/TEOA (5 : 1, v/v) as
solvent mixture.
Scheme 1 Molecular structure of the cyclopentadienone iron com-
plexes used as Cat and the in situ formed Cu PS.
Green Chem.
This journal is © The Royal Society of Chemistry 2017

View Article Online
Green Chemistry
Communication
Steric effects caused by bidentate phosphine ligands also
influence the degree of excited state distortion as well as the
equilibrium between homoleptic and heteroleptic spe-
cies.10d,11 For instance, if the flexible diphosphine ligand
DPEPhos 11 was used instead of xantphos (Table S1,† entry 1),
the catalytic activity of the system decreased (TONCO = 340). On
the other hand, applying the sterically demanding thixantphos
10 led to catalytic activities comparable to xantphos
(Table S1,† entry 2). Since it has been previously reported that
the ratio between homoleptic and heteroleptic species is com-
parable for all three diphosphines,11 the observed differences
in activity are mainly due to the restrictions imposed by such
ligands during the flattening distortion of the copper com-
plexes. Obviously, the lower catalytic activity displayed by 11 is
due to the less steric hindrance of the substituents on the
phosphorous atom and the smaller bite angle (102°), which
both are responsible for preventing flattening distortion and
exciplex quenching by nucleophilic attack of
molecule.
a solvent
Notably, we observed that the activity of the photocatalytic
system is strongly influenced by the structure of the
cyclopentadienone iron catalyst. Comparing the activities of
complexes 1 and 2 (Table 1, entries 1 and 7), it becomes
evident that increasing the steric hindrance of the substituents
at the silicon atom has a negative influence on the activity of
the system. In addition, when a heteroatom (oxygen) is present
in the saturated cycle of the ligand (Table 1, entry 9), the cata-
lytic performance decreased dramatically. Specifically, the
TON is reduced from 487 to 80. Attempts to increase the cata-
lyst productivities (TON) by elongation of irradiation time
failed, suggesting that the photocatalytic system is no longer
active after 5 h.
Fig. 1 Linear plots of ratio of fluorescence intensities of in situ Cu PS 5
in the absence and presence of (a) BIH (y = 0.672x + 1.013, R2 = 0.94)
and (b) 1, according to the Stern–Volmer equation.
In order to get insights into the mechanism of the photo-
chemical CO2 reduction catalyzed by iron complex 1 in the
presence of in situ Cu PS 5, the quantum yield of the overall
catalytic photoredox cycle reduction was determined by using 107–1010 M−1
s
−1, suggesting that reductive quenching of Cu
the following equation:
PS with the electron donor BIH can reasonably operate in this
photocatalytic system. On the other hand, the fluorescence of
Cu PS 5 was not attenuated by iron catalyst 1. That excludes
the possibility of an oxidative quenching mechanism (Fig. 1b)
produced CO molecules
ΦCOð%Þ ¼
ꢀ 100%:
incident photons
The number of incident photons was determined using an by the catalyst itself. These observations are in agreement
iron actinometer (see ESI† for details). After 2 h of irradiation with the reported redox potentials of the involved species. For
using monochromatic light (415 nm), 2.5 × 10−4 mol of CO instance, whereas an electron transfer from BIH (Eox = 0.33 V
were quantified. Hence, the calculated quantum yield for this vs. SCE)15 to the excited state of the PS (Cu PS−/Cu PS*, E0 =
molecular system is 13.3%, which is twice as high as the pre- 1.03 V)10b is thermodynamically feasible, an electron transfer
viously reported catalytic system for CO2 photoreduction based from the excited state of the PS (Cu PS*/Cu PS+, E0 = −1.35 V
on a dimeric Cu PS and an Fe phenanthroline catalyst.8
vs. SCE)10b to the iron complex 1 (Ered = −1.65 V vs. SCE) is
To probe the mechanism of photoinduced electron trans- not possible.
fer in this catalytic system, we further carried out quenching
Clearly, a reductive quenching mechanism operates in the
experiments of the in situ Cu PS 5 excited state by the iron photocatalytic CO2 reduction mediate by complex 1 and Cu PS
catalyst 1 and by the electron donor (BIH) using Stern– 5. Accordingly, a photoinduced electron transfer between the
Volmer analysis (see ESI† for details). At an excitation wave- Cu PS and BIH takes place first, resulting in the generation of
length of 450 nm, the luminescence at 550 nm of the Cu PS the reduced state of PS (Cu PS−). Subsequent reduction of the
was effectively quenched by BIH with an apparent quenching iron catalyst by Cu PS−, generates the active catalyst species
rate constant of 1.1 × 108 M−1 s−1 (Fig. 1a). For comparison, that mediates the photoreduction of CO2 to CO, as previously
typical quenching constants for Ru PS are in the order of observed when an Ir PS was used.12
This journal is © The Royal Society of Chemistry 2017
Green Chem.

View Article Online
Communication
Green Chemistry
6 (a) G. Sahara and O. Ishitani, Inorg. Chem., 2015, 54, 5096;
(b) A. Nakada, K. Koike, T. Nakashima, T. Morimoto and
O. Ishitani, Inorg. Chem., 2015, 54, 1800; (c) A. Nakada,
K. Koike, K. Maeda and O. Ishitani, Green Chem., 2016, 18,
139; (d) E. Kato, H. Takeda, K. Koike, K. Ohkubo and
O. Ishitani, Chem. Sci., 2015, 6, 3003; (e) C. Liu,
K. D. Dubois, M. E. Louis, A. S. Vorushilov and G. Li, ACS
Catal., 2013, 3, 655; (f) J. Schneider, K. Q. Vuong,
J. A. Calladine, X.-Z. Sun, A. C. Whitwood, M. W. George
and R. N. Perutz, Inorg. Chem., 2011, 50, 11877;
(g) J. Ettedgui, Y. Diskin-Posner, L. Weiner and
R. Neumann, J. Am. Chem. Soc., 2011, 133, 188;
(h) H. Takeda, K. Koike, H. Inoue and O. Ishitani, J. Am.
Chem. Soc., 2008, 130, 2023.
7 (a) R. O. Reithmeier, S. Meister, A. Siebel and B. Rieger,
Dalton Trans., 2015, 44, 6466; (b) R. O. Reithmeier,
S. Meister, B. Rieger, A. Siebel, M. Tschurl, U. Heiz and
E. Herdtweck, Dalton Trans., 2014, 43, 13259; (c) S. Sato,
T. Morikawa, T. Kajino and O. Ishitani, Angew. Chem., Int.
Ed., 2013, 52, 988.
Conclusions
In summary, we have described a novel selective photocatalytic
system for CO2 reduction based on earth-abundant elements,
combining a copper-based photosensitizer and a molecularly-
defined iron catalyst. The Cu PS was easily generated in situ
using a copper salt, bidentate phosphines, and easily available
diimine ligands. When such a luminescent Cu complex was
combined with an iron cyclopentadienone catalyst in the pres-
ence of light and a suitable electron donor, carbon dioxide is
selectively reduced to CO with a TON up to 487 and a ΦCO
=
13.3%. According to mechanistic investigations, the photo-
catalytic reduction of CO2 proceeded via reductive quenching
of the excited Cu PS with BIH and further electron transfer
from the resulting reduced species of Cu PS to the iron
catalyst.
Acknowledgements
This work has been supported by the state of Mecklenburg-
Vorpommern, the BMBF, and the EU fund H2020-
MSCA-ITN-2015 in Horizon 2020 as part of the project
NoNoMeCat (675020). A. R.-H. acknowledges financial support
from the DAAD.
8 (a) Z. Guo, S. Cheng, C. Cometto, E. Anxolabéhère-Mallart,
S.-M. Ng, C.-C. Ko, G. Liu, L. Chen, M. Robert and
T.-C. Lau, J. Am. Chem. Soc., 2016, 138, 9413–9416;
(b) P. L. Cheung, C. W. Machan, A. Y. S. Malkhasian,
J. Agarwal and C. P. Kubiak, Inorg. Chem., 2016, 55, 3192–
3198; (c) P. G. Alsabeh, A. Rosas-Hernández, E. Barsch,
H. Junge, R. Ludwig and M. Beller, Catal. Sci. Technol.,
2016, 6, 3623–3630; (d) F. Wang, B. Cao, W.-P. To,
C.-W. Tse, K. Li, X.-Y. Chang, C. Zang, S. L.-F. Chan and
C.-M. Che, Catal. Sci. Technol., 2016, 6, 7408–7420;
(e) L. Chen, Z. Guo, X.-G. Wei, C. Gallenkamp, J. Bonin,
E. Anxolabéhère-Mallart, K.-C. Lau, T.-C. Lau and
M. Robert, J. Am. Chem. Soc., 2015, 137, 10918–10921; (f) S.
L.-F. Chan, T. L. Lam, C. Yang, S.-C. Yan and N. M. Cheng,
Chem. Commun., 2015, 51, 7799–7801; (g) J. Bonin,
M. Robert and M. Routier, J. Am. Chem. Soc., 2014, 136,
16768–16771; (h) H. Takeda, H. Koizumi, K. Okamoto and
O. Ishitani, Chem. Commun., 2014, 50, 1491–1493;
(i) V. S. Thoi, N. Kornienko, C. G. Margarit, P. Yang and
C. J. Chang, J. Am. Chem. Soc., 2013, 135, 14413–14424.
9 H. Takeda, K. Ohashi, A. Sekine and O. Ishitani, J. Am.
Chem. Soc., 2016, 138, 4354–4357.
Notes and references
1 M. Aresta, in Carbon Dioxide as Chemical Feedstock, ed.
M. Aresta, Wiley-VCH, Weinheim, 2010, pp. 1–14.
2 (a) Y. Yamazaki, H. Takeda and O. Ishitani, J. Photochem.
Photobiol., C, 2015, 25, 106–137; (b) T. Yui, Y. Tamaki,
K. Sekizawa and O. Ishitani, Top. Curr. Chem., 2011, 303,
151–184; (c) A. J. Morris, G. J. Meyer and E. Fujita, Acc.
Chem. Res., 2009, 42, 1983–1994.
3 (a) J. Hawecker, J.-M. Lehn and R. Ziessel, Helv. Chim. Acta,
1986, 69, 1990; (b) J. Hawecker, J.-M. Lehn and R. Ziessel,
J. Chem. Soc., Chem. Commun., 1983, 536.
4 (a) A. Juris, V. Balzani, F. Barigelletti, S. Campagna,
P. Belser and A. von Zelewsky, Coord. Chem. Rev., 1988, 84,
85–277; (b) A. Ito and T. J. Meyer, Phys. Chem. Chem. Phys.,
2012, 14, 13731–13745; (c) K. A. King, P. J. Spellane and 10 (a) S.-P. Luo, E. Mejía, A. Friedrich, A. Pazidis, H. Junge,
R. J. Watts, J. Am. Chem. Soc., 1985, 107, 1431–1432;
(d) A. Kapturkiewicz and G. Angulo, Dalton Trans., 2003,
3907–3913; (e) P. J. Hay, J. Phys. Chem. A, 2002, 106, 1634–
1641.
5 (a) Y. Kuramochi, J. Itabashi, K. Fukaya, A. Enomoto,
M. Yoshida and H. Ishida, Chem. Sci., 2015, 6, 3063–3074;
(b) Y. Kuramochi, M. Kamiya and H. Ishida, Inorg. Chem.,
2014, 53, 3326–3332; (c) H. Ishida, T. Terada, K. Tanaka
and T. Tanaka, Inorg. Chem., 1990, 29, 905–911;
(d) J.-M. Lehn and R. Ziessel, J. Organomet. Chem., 1990,
382, 157–173; (e) H. Ishida, K. Tanaka and T. Tanaka,
Chem. Lett., 1988, 17, 339–342; (f) I. Hitoshi, T. Koji and
T. Toshio, Chem. Lett., 1987, 16, 1035–1038.
A.-E. Surkus, R. Jackstell, S. Denurra, S. Gladiali,
S. Lochbrunner and M. Beller, Angew. Chem., Int. Ed., 2013,
52, 419–423; (b) E. Mejía, S.-P. Luo, M. Karnahl,
A. Friedrich, S. Tschierlei, A.-E. Surkus, H. Junge,
S. Gladiali, S. Lochbrunner and M. Beller, Chem. – Eur. J.,
2013, 19, 15972–15978; (c) M. Karnahl, E. Mejía,
N. Rockstroh, S. Tschierlei, S.-P. Luo, K. Grabow, A. Kruth,
V. Brüser, H. Junge, S. Lochbrunner and M. Beller,
ChemCatChem, 2014, 6, 82–86; (d) S. Fischer, D. Hollmann,
S. Tschierlei, M. Karnahl, N. Rockstroh, E. Barsch,
P. Schwarzbach, S.-P. Luo, H. Junge, M. Beller,
S. Lochbrunner, R. Ludwig and A. Brückner, ACS Catal.,
2014, 4, 1845–1849.
Green Chem.
This journal is © The Royal Society of Chemistry 2017

View Article Online
Green Chemistry
Communication
11 A. J. J. Lennox, S. Fischer, M. Jurrat, S.-P. Luo, 13 M. D. Kärkäs, O. Verho, E. V. Johnston and B. Åkermark,
N. Rockstroh, H. Junge, R. Ludwig and M. Beller, Chem. –
Eur. J., 2016, 22, 1233–1238.
12 A. Rosas-Hernández, P. G. Alsabeh, E. Barsch, H. Junge,
Chem. Rev., 2014, 114, 11863–12001.
14 N. Armaroli, G. Accorsi, F. Cardinali and A. Listorri, Top.
Curr. Chem., 2007, 280, 69–115.
R. Ludwig and M. Beller, Chem. Commun., 2016, 52, 8393– 15 Y. Tamaki, K. Koike, T. Morimoto and O. Ishitani, J. Catal.,
8396.
2013, 304, 22–28.
This journal is © The Royal Society of Chemistry 2017
Green Chem.