Addressing the Reproducibility of Photocatalytic Carbon Dioxide Reduction

Article abstract of DOI:10.1002/cctc.201901686

Reproducibility of photocatalytic reactions, especially when conducted on small scale for improved turnover numbers with in situ formed catalysts can prove challenging. Herein, we showcase the problematic reproducibility on the example of attractive photocatalytic CO₂ reduction utilizing [FeFe] hydrogenase mimics. These Fe complexes, well-known for their application in proton reduction reactions, were combined with a heteroleptic Cu photosensitizer and produced CO/H₂/HCO₂H mixtures of variable constitution. However, the reactions indicated a poor reproducibility, even when conducted with well-defined complexes. Based on our experience, we make suggestions for scientists working in the field of photocatalysis on how to address and report the reproducibility of novel photocatalytic reaction protocols. In addition, we would like to highlight the importance of studying reproducibility of novel reaction protocols, especially in the fields of photocatalytic water splitting and CO₂ reduction, where TONs are widely used as the comparable measure for catalytic activity.

Full text of DOI:10.1002/cctc.201901686

Accepted Article
Title: Addressing the Reproducibility of Photocatalytic Carbon Dioxide
Reduction
Authors: Maximilian Marx, Andrea Mele, Anke Spannenberg, Christoph
Steinlechner, Henrik Junge, Philippe Schollhammer, and
Matthias Beller
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10.1002/cctc.201901686
ChemCatChem
CONCEPT
Addressing the Reproducibility of Photocatalytic Carbon Dioxide
Reduction
Maximilian Marx,[a] Andrea Mele,[b] Anke Spannenberg,[a] Christoph Steinlechner,[a] Henrik Junge,[a]
Philippe Schollhammer[b] and Matthias Beller*[a]
Abstract: Reproducibility of photocatalytic reactions, especially
when conducted on small scale for improved turnover numbers with
in situ formed catalysts can prove challenging. Herein, we showcase
the problematic reproducibility on the example of attractive
photocatalytic CO2 reduction utilizing [FeFe] hydrogenase mimics.
These Fe complexes, well-known for their application in proton
enhanced interest in those research areas are justified. The
ambition of scientists to compete with or even outperform
previously reported methods and to aim for new record results
for those transformations is certainly the foundation for future
improvement. However, the stride towards better catalyst
performance can just as well become ambivalent when pursued
at all costs. This becomes obvious by the recent trend of
lowering catalyst loadings to achieve higher turnover numbers
(TONs) which are then reported as the prime measure of
catalyst activity. In the case of synthetic applications, product
yield and substrate conversion are direct indicators of the
applicability of those lowered catalyst loadings and a reduced
amount of catalyst giving similar yields is undoubtedly preferred.
In contrast, photocatalytic CO2 reduction reactions or water
splitting processes are usually far from reaching full conversion
under batch reaction conditions. Hence, lower catalyst loadings,
albeit feigning higher catalytic activity, diminish the applicability
of these systems, since intricate upscaling of those reactions is
most often not performed!
reduction reactions, were combined with
a heteroleptic Cu
photosensitizer and produced CO/H2/HCO2H mixtures of variable
constitution. However, the reactions indicated a poor reproducibility,
even when conducted with well-defined complexes. Based on our
experience, we make suggestions for scientists working in the field
of photocatalysis on how to address and report the reproducibility of
novel photocatalytic reaction protocols. In addition, we would like to
highlight the importance of studying reproducibility of novel reaction
protocols, especially in the fields of photocatalytic water splitting and
CO2 reduction, where TONs are widely used as the comparable
measure for catalytic activity.
Introduction
Photocatalysis is arguably one of the most active fields in
nowadays catalysis research.[1] Hence, numerous researchers
all over the world investigate photocatalytic water splitting[2] or
reduction of CO2 to potential energy carriers.[3] In addition,
photocatalysis is intensely applied for producing more value
added compounds, such as fine chemicals or life science
molecules.[4] The strong interest in this technology can readily be
explained by the intrinsic advantage offered by direct utilization
of sunlight as an abundant and sustainable energy source. Not
surprisingly, the number of publications in these fields has
skyrocketed over the last decade, outlined by a more than 10-
fold increase in publication numbers in the field of photocatalytic
CO2 reduction (Figure 1). With respect to the major challenges
mankind has to face in the upcoming decades, which are
probably the man-made climate change as a result of extensive
usage of fossil fuels and an increasing demand in energy and
food eventuating from the on-going demographic growth,[5] the
Figure 1. Number of publications in the area of photocatalytic CO2 reduction,
(compiled using SciFinder searching for “Photocatalysis CO2 reduction” on
August 12, 2019).
[a]
[b]
M. Marx, Dr. A. Spannenberg, Dr. H. Junge, Prof. Dr. M. Beller
Leibniz Institute for Catalysis at the University of Rostock
Albert-Einstein-Straße 29a, 18059 Rostock (Germany)
E-mail: Matthias.Beller@catalysis.de
A. Mele, Prof. Dr. P. Schollhammer
UMR CNRS 6521 CEMCA
Faculté des Sciences et Techniques, University of Brest
6 Avenue Victor le Gorgeu, 29238 Brest (France)
E-mail: Philippe.Schollhammer@univ-brest.fr
In addition, the impact of impurities on the catalyst performance
becomes more pronounced at lower catalyst concentrations,
potentially resulting in poor reproducibility for the described
reactions.[6] Combined with elaborate reaction setups used for
photocatalytic CO2 reduction reactions and the crucial influence
7]
of the light source (position and constant radiant flux),[2h,
Supporting information for this article is given via a link at the end of
the document.
reproduction of reported systems and further improvement of
literature-known CO2 reduction protocols can prove challenging.
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10.1002/cctc.201901686
ChemCatChem
CONCEPT
Interestingly, the reproducibility of photocatalytic CO2 reduction
reactions is seldom addressed and often no information on the
number of reaction runs utilized for the calculation of the TONs
is given.
Herein, we showcase the importance of these factors on the
example of photocatalytic CO2 reduction using [FeFe]
hydrogenase mimics. Based on this example, we would like to
encourage researchers in the field of photocatalysis to pay
Heteroleptic Cu complexes, such as 2, are well-known to exist in
an equilibrium with the homoleptic [Cu(N^N)2]+ complex in
solution.[11-12] This equilibrium is depending on the solvent, the
illumination conditions and the amount of ancillary ligands
11]
present in the reaction mixture.[9a,
Since no excess
bisphosphine ligand is present in the well-defined 2, enhanced
formation of the [Cu(N^N)2]+ complex could occur in this case.
Even though this might account for the decreased catalytic
special attention to the reproducibility of novel reaction protocols. activity with respect to CO formation of the well-defined CuPS 2,
the plain difference compared to the in situ CuPS could also
result from a more complex equilibrium that includes the [FeFe]
hydrogenase mimic itself.
For optimizing this equilibrium, we investigated the influence of
Results and discussion
the [Cu(CH3CN)4]PF6/bathocuproine/xantphos ratio on the
overall catalytic activity of the catalyst system (Table S2). The
Recently, Wang and co-workers reported the electrocatalytic
CO2 reduction to CO, hydrogen and formate utilizing [FeFe]
hydrogenase mimics [Fe2(μ-bdt)(CO)6] (1, bdt = benzene-1,2-
dithiolate) and [Fe2(μ-edt)(CO)6] (edt = ethane-1,2-dithiolate).[8]
In parallel to their study, we investigated the photocatalytic CO2
reduction using complex 1 and similar hydrogenase mimics. We
started our investigations based on 1 in combination with a
heteroleptic copper photosensitizer (CuPS) 2 previously utilized
for photocatalytic CO2 reduction in our group.[9] CuPS 2 can be
readily formed in situ from [Cu(MeCN)4](PF6), bathocuproine and
xantphos in a 1:1:3 ratio.[9] At a wavelength of 415 nm and a
radiant flux of 1.00 W, we were pleased to find simultaneous
CO2 and proton reduction with a total TON for CO, HCO2H and
H2 of 614 (average of 7 reactions, detailed results are presented
in Table S1) at a CO/H2/HCO2H ratio of 1.8/1.2/1 after 3 h of
illumination in 5:1 N-methylpyrrolidinone (NMP) / triethanolamine
(TEOA) using BIH (1,3-dimethyl-2-phenylbenzo[d]imidazole) as
the sacrificial electron donor (Scheme 1). This correlates with
the reported results for the electrocatalytic CO2 reduction
utilizing complex 1, where HCO2H was the main product.[8]
However, we already observed a low reproducibility for the
HCO2H TONs, which we at first attributed partially to direct CO2
reduction by BIH.[10] Interestingly, conducting the same reaction
with the isolated heteroleptic copper complex CuPS 2 instead of
the in situ prepared photosensitizer resulted in a total TON of
865 and a CO/H2/HCO2H ratio of 1/7/3 (average of 2 reactions,
for single experiment results see Table S1). Since previously
reported systems developed in our group showed a good
correlation between the utilization of isolated 2 and the in situ
generated CuPS,[9a, 11] this was an unexpected result. In addition,
we already observed a significant difference between the TONs
for H2 formation of the single experiments (476 vs. 623).
optimum
Cu/bathocuproine/xantphos ratio with a total TON of 756 at a
CO/H2/HCO2H ratio of 1.7/1.1/1 (average of reactions).
composition
was found
to be
a
1/2/5
5
However, we witnessed a diminished reproducibility under these
conditions, which became even more obvious when optimizing
the illumination conditions by switching to a broader 400-500 nm
irradiation at an increased radiant flux of 1.50 W (Table S3).
While at first a comparably high catalytic activity of 880 TON
with a CO/H2/HCO2H ratio of 2/1/1 (average over 3 reactions)
was obtained and the reproducibility for CO was within 10%
range (TON(CO): 445, 417, 466), this reproducibility dramatically
decreased after repeating the same reaction six months later (all
results are presented in Table S4). After changing all the
employed reagents and the solvent, we were unable to identify
the cause of the poor reproducibility observed for this reaction
(Figure 2). Notably, a five-fold increase in the utilized catalyst
amount did not facilitate complete reproducibility, with an almost
200% difference in TON(CO) for reactions conducted within a
timeframe of one month (Table S5).
Figure 2. Single experiment results for the photocatalytic CO2 reduction
conducted with in situ formed CuPS 2 and Fe catalyst 1 underscoring the poor
reproducibility.
Scheme 1. General photocatalytic CO2 reduction utilizing [Fe2(μ-bdt)(CO)6] (1)
and in situ formed CuPS 2.
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10.1002/cctc.201901686
ChemCatChem
CONCEPT
To evaluate, whether this poor reproducibility occurs only in the
single case of [Fe2(μ-bdt)(CO)6] (1), we also evaluated the
photocatalytic CO2 reduction using [Fe2(μ-pbdt)(CO)6] (3),
[Fe2(μ-Cl-bdt)(CO)6] (4), and simple [Fe2(CO)9] (5). In all cases,
moderate to poor reproducibility was observed with respect to
CO, HCO2H and H2 formation under the previously optimized
conditions, even though general trends in the product mixture
composition were observable (Figure 3, for detailed results see
Table S6).
As mentioned earlier, Cu complexes, such as 2, exist in an
equilibrium with the homoleptic [Cu(N^N)2]+ complex.[11-12] Since
CO replacement by bidentate N or P donor ligands is well-known
for [FeFe] hydrogenase mimics,[13] we supposed that a reaction
of the Fe carbonyl complexes employed as catalysts with the in
situ CuPS might occur under our reaction conditions. This would
further complicate the equilibria within the mixture and might
eventually cause the poor reproducibility observed.
Thus, we reacted 1 with an equimolar amount of either
bathocuproine or xantphos in toluene under illumination (415 nm,
1.00 W). To our delight, this resulted in the formation of novel
diiron complexes 6 and 7 in mediocre to good yield (64% for 6;
76% for 7) (Scheme 2).
Scheme 2. Synthesis of the new [FeFe] complexes 6 and 7 via photolytic CO
replacement by xantphos and bathocuproine.
Crystals suitable for X-ray crystallographic analysis were
obtained from CH2Cl2/hexane and enabled indisputable
elucidation of their solid state structures (Figures 4 & 5). In both
cases, the dithiolate bridge remained unaltered and one
molecule of the bidentate ligand is coordinated in a chelating
fashion to one of the two Fe centers. This is in accordance with
previously reported compounds of this type.[13a-j]
Our result also emphasized the universality of the synthesis of
chelated diiron dithiolate complexes via irradiation of the
13g, 13l]
hexacarbonyl precursor and the bidentate ligand.[13a,
Interestingly, we were able to perform this reaction without the
explicit utilization of UV light (maximum at 415 nm), which
indicates that CO dissociation for the [Fe2(μ-bdt)(CO)6] can
indeed be achieved using visible light i.e. our reaction
conditions.[14]
Having isolated these new complexes, we evaluated their
activity in the photocatalytic CO2 reduction. We hoped that these
isolated complexes in combination with either the in situ or the
well-defined CuPS 2 might give entirely reproducible results.
Combining complex 6 with the isolated CuPS 2 gave a low TON
for CO of 10, a TON of 159 for HCO2H, and a TON for H2 of 340
(average of two runs, Figure 6 a), detailed results are given in
Table S8 & S9), which seemed fairly reproducible. However, the
combination of 6 with the in situ formed CuPS resulted once
again in poor reproducibility (Figure 6, b)). Similar observations
Figure 3. Reproducibility of the photocatalytic CO2 reduction utilizing in situ
generated CuPS 2 and various dinuclear Fe complexes. a) [Fe2(μ-pbdt)(CO)6]
(3). b) [Fe2(μ-Cl-bdt)(CO)6] (4). c) [Fe2(CO)9] (5).
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10.1002/cctc.201901686
ChemCatChem
CONCEPT
were made for the combination of 7 and the in situ generated or
the isolated CuPS 2 (Figure 6 c) & d)).
Figure 4. Molecular structure of 6. Hydrogen atoms and solvent molecules in
the crystal structure are omitted for clarity (thermal ellipsoids set to 30%
probability).
Figure 6. Photocatalytic CO2 reduction utilizing the [FeFe] complexes 6 and 7
reported herein in combination with in situ formed or well-defined CuPS 2. a)
Fe complex 6 in combination with well-defined CuPS 2. b) Complex 6 in
combination with in situ generated CuPS 2.. c) Fe complex 7 in combination
with well-defined CuPS 2. d) In situ formed CuPS 2 in combination with
complex 7.
Conclusions
In summary, we demonstrated the importance of assessing the
reproducibility of a novel catalytic system for photocatalytic
reduction of carbon dioxide. Here, significant reproducibility
issues arose, arguably due to one of the following factors: 1.
specific problems of photocatalysis (i.a. light source,
transmission), 2. dynamic catalyst systems (ligand exchange),
and 3. gas-liquid reaction system. If one of these or related
problems exist, reproducibility issues become especially
important.
The results obtained for our photocatalytic CO2 and proton
reduction system based on [FeFe] hydrogenase mimics and a
CuPS clearly indicate the unreliability of single experiments. In
this specific case it might be partially due to a complex
equilibrium between the CuPS and the [FeFe] complexes,
Figure 5. Molecular structure of 7. Hydrogen atoms are omitted for clarity
(thermal ellipsoids set to 30% probability).
Again, this outlines that the observed reproducibility is not a
result of the employed pre-catalyst. Interestingly, 7 appeared to
yield higher TONs for CO2 conversion compared to 6. This might
indicate a privileged role of 7 in the formation of an active CO2
reduction catalyst, which would be similar to the effect of 2,2’-
bipyridine in the photocatalytic CO2 reduction using
[Fe3(CO)12].[15] However, the poor reproducibility prohibits a
conclusive statement based exclusively on these experimental
results.
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10.1002/cctc.201901686
ChemCatChem
CONCEPT
suggested by the isolation of two new dinuclear Fe complexes
formed via photolytic CO dissociation. However, utilization of the
isolated complexes in combination with a well-defined CuPS still
resulted in low reproducibility. Therefore, an important lesson
from this study is that application of isolated complexes or well-
structured nanomaterials does not guarantee reproducibility.
Nevertheless, reproducibility constitutes the fundamental base
for any research, especially for cutting-edge transformations,
such as CO2 reduction or water oxidation, which mostly rely on
the catalytic activity reported in terms of turnover frequencies or
TONs and not product yield/conversion as their quality index. In
addition, the exact number of conducted reactions and the
deviance of those single experiments are seldom stated for
novel catalytic transformations.
Hence, we would like to make the following recommendations
for scientists working in such fields as catalyst and/or method
development:
1. The number of independent experiments should always be
stated when reporting a novel catalytic transformation or a new
catalyst system. This is especially the case for results obtained
from single experiments.
Keywords: CO2 reduction • Hydrogenase • Iron • Photocatalysis
• Reproducibility
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This work was supported by the European Union (H2020-
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10.1002/cctc.201901686
ChemCatChem
CONCEPT
CONCEPT
Reproducibility for photocatalytic
CO2 reduction has been evaluated
on the example of [FeFe]
hydrogenase mimics in combination
with a heteroleptic Cu
Maximilian Marx, Andrea Mele, Anke
Spannenberg, Christoph Steinlechner,
Henrik Junge, Philippe Schollhammer
and Matthias Beller*
Page No. – Page No.
photosensitizer. Based on these
results, we highlight the importance
of testing reproducibility for new
photocatalytic reaction protocols.
Furthermore, we provide
Addressing the Reproducibility of
Photocatalytic Carbon Dioxide
Reduction
suggestions on how to ensure
reproducibility of those
transformations.
This article is protected by copyright. All rights reserved.