Immobilization of Mn(i) and Ru(ii) polypyridyl complexes on TiO2 nanoparticles for selective photoreduction of CO2 to formic acid

Article abstract of DOI:10.1039/c9cc05129e

TiO₂ nanoparticles are successively functionalized with [Mn(κ²N¹,N²-Ttpy)(CO)₃Br] as catalyst and [Ru(bpy)₃]²⁺ as photosensitizer to yield Ru^II/TiO₂/Mn^I. Under continuous irradiation at 470 nm and in the presence of a sacrificial electron donor, this triad reduces CO₂ to HCOOH (TON_max = 27) with 100% selectivity.

Full text of DOI:10.1039/c9cc05129e

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Immobilization of Mn(I) and Ru(II) polypyridyl
complexes on TiO2 nanoparticles for selective
photoreduction of CO2 to formic acid†
Cite this: Chem. Commun., 2019,
55, 13598
Received 4th July 2019,
Accepted 21st October 2019
ab
Long Le-Quang,
Matthew Stanbury,a Sylvie Chardon-Noblat,a
Jean-Marie Mouesca,b Vincent Maurelb and Jerome Chauvin
*
a
´ ˆ
DOI: 10.1039/c9cc05129e
rsc.li/chemcomm
TiO2 nanoparticles are successively functionalized with [Mn(j2N1,N2- catalyst on ZrO2 NPs for photocatalytic CO2 reduction. Under
ttpy)(CO)3Br] as catalyst and [Ru(bpy)3]2+ as photosensitizer to yield visible light and in the presence of ascorbic acid as a sacrificial
RuII/TiO2/MnI. Under continuous irradiation at 470 nm and in the electron donor, RuII/ZrO2/NiII system reduced CO2 to a mix of
presence of a sacrificial electron donor, this triad reduces CO2 to CO and H2. Meanwhile, the solution of homogeneous complexes
HCOOH (TONmax = 27) with 100% selectivity.
RuII and Ni(cyclam)2+ only produced traces of CO. The authors
concluded that the proximal immobilization of the photosensitizer
Carbon dioxide (CO2) is arguably one of the most notorious and the catalyst molecules on ZrO2 surface enhanced the kinetics
greenhouse gases that contribute to global warming1 and rises in sea of photo-induced electron transfer reaction between RuII* and
level.2 In recent years, novel molecular catalysts able to reduce CO2 Ni(cyclam)2+.
into more value-added products have been extensively investigated
Semiconducting nanoparticles can also be used as both a
to replace the prototypical [Re(bpy)(CO)3Cl] (bpy = 2,20-bipyridine) substrate and an electron relay for the photo-induced electron
catalyst.3–5 Those based on earth-abundant elements such as Fe, Ni, transfer process between the photosensitizer and the catalyst.
Co, Cu and Mn are of particular interest,6–8 not only for academics In particular, long lived charge separation state can be reach
but also industrialists for potential upscaling.9 Moreover, CO2 in nanostructured dye-sensitized TiO2 photoelectrode thus
conversion to produce useful compounds must be economically facilitating multi-electron reduction reactions of CO2.16 For
viable and photochemical reduction of CO2, using renewable instance, Kang and co-workers17,18 grafted an organic dye and
light energy source could be one of the solutions to solve the [Re(bpy)(CO)3Cl] catalyst on TiO2 NPs. Under visible light and
equation.
in the presence of a sacrificial electron donor, the dye/TiO2/ReI
A general strategy to develop photoreduction system is to system efficiently and selectively reduced CO2 to CO with maximum
covalently link a photosensitizer with a molecular catalyst.1,10,11 turnover number (TONmax) surpassing 570 after 30 hours. In this
It, however, often requires challenging multi-step syntheses to work TiO2 played the role of electronic relay between the excited dyes
obtain the desired supramolecules. An alternative approach, and the Re(I) catalytic center.
that combined also the advantage of heterogeneous catalysis,12
As an alternative for rhenium complexes, manganese complexes
consists to graft the photosensitizer and the catalyst onto the have been extensively investigated in recent years. The prototype
surface of nanoparticles (NPs) as they offer great surface area to [Mn(bpy)(CO)3Br] is an efficient electrocatalyst19 and was shown to
accommodate the molecules.7,13,14 This approach usually requires work in association with RuII in a photocatalytic cycle20 for the
significantly less complicated syntheses than the supramolecular reduction of CO2 to CO and HCOOH. Immobilization of this
homogeneous approach and allows easy control of the surface complex onto a (semi)conducting surface allowed for better product
occupancy and of the ratio between photosensitizer and catalyst. selectivity.21–24 However, other Mn polypyridyl complexes than
For instance, Neri et al.15 grafted a [Ru(bpy)3]2+ photosensitizer (RuII) the bipyridines are still underexplored.6,8 We recently reported
and a Ni(cyclam)2+ (cyclam = 1,4,8,11-tetraazacyclotetradecane) the synthesis and characterization of
a related complex
[Mn(k2N1,N2-ttpy)(CO3)X]n+ (ttpy = 4-tolyl-2,20:60-200-terpyridine)
(X = Br and n = 0 or X = MeCN and n = 1).25 Herein we present
(i) the homogeneous electrocatalytic and photocatalytic reduction
of CO2 using [Mn(k2N1,N2-ttpy)(CO)3Br] catalyst (MnI), and RuII
photosensitizer and (ii) the immobilization of MnI and RuII on TiO2
NPs (RuII/TiO2/MnI, Scheme 1) for heterogenous photocatalytic CO2
reduction.
a Department of Molecular Chemistry, UMR CNRS 5250,
University of Grenoble-Alpes, CS 40700, 38058 Grenoble Cedex 9, France.
E-mail: jerome.chauvin@univ-grenoble-alpes.fr
b Univ. Grenoble Alpes, CEA, CNRS, IRIG, SyMMES, F-38000 Grenoble, France.
E-mail: vincent.maurel@cea.fr
† Electronic supplementary information (ESI) available. See DOI: 10.1039/
c9cc05129e
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at both potentials, as already observed for [Mn(terpy)(CO)3Br]26
and [Mn(phen)(CO)3Br]27 (phen = 1,10-phenanthroline).
Controlled-potential electrolysis at ꢀ1.7 V produced CO as
the only product (TONmax = 12, faradaic efficiency (FE) = 100%
after 4 h). This result shows remarkable improvement on
catalytic activity and stability compared to [Mn(terpy)(CO)3Br]
(TONmax (CO) = 4.1, FE = 129%)26 where such an anomalously
high FE could be a consequence of the decarbonylation of the
complex as side reaction.
Scheme 1 Multi-step route for RuII/TiO2/MnI triad.
Photocatalytic CO2 reduction experiments were carried out
in DMF/TEOA mixed solution (TEOA: triethanolamine, CTEOA = 1 M)
in the presence of RuII as photosensitizer, MnI as catalyst and
1-benzyl-1,4-dihydronicotinamide (BNAH) as sacrificial electron
donor. TEOA is added in the media as a base to suppress reverse
electron transfer reaction after oxidation of BNAH by RuII*.28
Excitation wavelength was chosen at 470 ꢁ 40 nm to selectively
activates the photosensitizer and not the catalyst (Fig. S2, ESI†),
as this catalyst can undergo a photo-induced decarbonylation
reaction.29 After 16 h, the amounts of CO and H2 in the gas phase
and HCOOH in the liquid phase were measured by GC and HPLC,
respectively (Table 1, entries 1–5, see ESI† for detail on the sample
treatment and product quantification). Neither CO nor HCOOH was
produced in the absence of MnI or light. We investigated various
MnI :RuII ratios while keeping the RuII concentration unchanged so
that light absorption by the photosensitizer remained the same in all
the series. All the MnI :RuII ratios produced both CO and HCOOH
The MnI complex was synthesized and characterized following a
reported procedure25 (see ESI† for more information).
Cyclic voltammograms (CVs) of MnI were recorded in MeCN
under argon (Fig. 1, black line). The CV is decoupled in Fig. S1
(ESI†). It is in accordance with the behavior of [Mn(terpy)(CO)3Br]
(terpy = 2,20:60-200-terpyridine)26 and the interpretation is based on
relevant literature.8,26,27 The first irreversible peak at Epc1 = ꢀ1.44 V
is assigned to the Mn+/0 one electron reduction process. This
process leads to the leaving of Brꢀ and coordination of the free
pyridyl of terpyridine to form a tridentate linkage between the ttpy
ligand and Mn center (reaction (S1)–(S3), ESI†). This reduction
event is associated with the release of a CO group and the
formation of a Mn0–Mn0 dimer through metal–metal bonding
(eqn (1)).
[Mn(k2-ttpy)(CO)3Br] + 1eꢀ - 1/2[Mn0(ttpy)(CO)2]2 + CO + Brꢀ while no H2 was detected. Prolonging the irradiation time did not
(1)
increase the product amounts. Table 1 shows that the decrease in
the MnI : RuII ratio leads to a higher total TON. That may be due
to a better protection of the MnI catalyst against photo-induced
decarbonylation by RuII as an inner filter which absorbs more
strongly visible light at 470 nm (e (Mꢀ1 cmꢀ1) = 2300 for MnI and
8400 for RuII) (Fig. S2, ESI†). This property was already observed
for similar molecular photocatalytic systems.11,15 In the series,
the best MnI : RuII ratio is determined at 1 : 10 where the total
TON reaches the highest value of 50.
On the reverse scan, the anodic peak at ꢀ0.89 V corresponds
to the oxidation of the dimer (reaction (S4), ESI†). Further
sweeping to more negative potentials leads to a second one-
electron irreversible peak at Epc2 = ꢀ1.67 V that is ascribed to the
reduction of the Mn0–Mn0 dimer to form [MnꢀI(ttpy)(CO)2]ꢀ
species (reaction (S5) and (S6), ESI†). The peak at ꢀ1.67 V is
associated with a small oxidation peak at ꢀ1.45 V corresponding
to the oxidation of the newly formed [MnꢀI(ttpy)(CO)2]ꢀ to
regenerate the dimer species (reaction (S7) and (S8), ESI†).
Under CO2 and in the presence of water (5% v/v) as a proton
source, the CV of MnI shows significant enhancement in
current magnitude on both reduction peaks at ꢀ1.44 V and
ꢀ1.67 V (Fig. 1, red line). Without water, the CV shows
negligible difference to the CV recorded under Ar. These results
hence suggest that the electrocatalytic CO2 reduction may occur
We then prepared the triad RuII/TiO2/MnI following a three-
step procedure (Scheme 1, see ESI† for detail), in which phos-
phonic acid was chosen to anchor the complexes on anatase
Table 1 Results of the photocatalytic CO2 reduction process. Solution:
DMF/TEOA (1 M) and BNAH (0.1 M). Irradiation was achieved by using a Xe
lamp (3 ꢂ 10ꢀ4 W cmꢀ2, 5 cm apart), a UV-hot filter and a 470 ꢁ 40 nm
bandpass filter. Results were obtained after 16 h. TON uncertainties ꢁ2
HCOOH
CO
Ratio of
b
b
c
Entry MnI : RuII a n (mmol) TONmax n (mmol) TONmax TONtotal
(a) For homogeneous solution of MnI and RuII free complexes
1
2
3
4
5
1 : 1
9.8
3.9
2.2
71.4
88.9
14
28
31
20
13
5.6
1.4
1.3
18.9
35.0
8
10
19
5
22
38
50
25
17
5 : 10
1 : 10
5 : 1
10 : 1
5
(b) For RuII/TiO2/MnI
6
1 : 10
0.8
27
—
—
27
Fig. 1 CVs of MnI (1 mM) in MeCN + 0.1 M TBAPF6 under Ar (black) and
a
b
RuII concentration is unchanged (0.14 mM). Turnover number
under CO2 in the presence of 5% H2O (red). n = 0.1 V sꢀ1
.
c
(TON) versus Mn catalyst. TONtotal = TONmax (HCOOH) + TONmax (CO).
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TiO2 NPs.30 It is noted that RuII was functionalized with only one Therefore, we grafted MnI on a conductive fluorine-doped Tin
phosphonic acid group in order to avoid it to be grafted on Oxide (FTO) electrode to investigate the electrochemical behavior of
different TiO2 NPs, which could lead to aggregation in colloidal the immobilized MnI complex. FTO was chosen as phosphonic acid
solution. The ratio MnI : RuII = 1 : 10 was chosen following results is a suitable anchoring group on this surface,35 and it is rather
in Table 1. After the immobilization of each complex, modified redox-inactive up to ꢀ2 V. The CV of FTO/MnI electrode (Fig. S6,
nanoparticles were separated by centrifugation and washed ESI†) shows a cathodic peak at Epc = ꢀ1.43 V which is at the same
thoroughly with corresponding solvents. Maximum surface potential as the first cathodic peak of the free complex in solution.
loading of MnI and RuII complexes was estimated at 14 mmol gꢀ1 However, on the reverse scan the associated anodic peak appears at
and 140 mmol gꢀ1 ꢁ 20% respectively, by measuring the UV-visible a very different potential (Epa = ꢀ1.16 V) than the peak recorded for
absorbance of supernatant solutions after each centrifugation step. the homogeneous complex in solution (ꢀ0.89 V), which has been
MnI complex was anchored first so that it can presumably be well assigned to the oxidation of the Mn0–Mn0 dimer formed after the
distributed on TiO2 surface and surrounded by RuII molecules. The first reduction step. Since MnI is immobilized, the formation of
presence of MnI on TiO2 was confirmed by IR spectroscopy (Fig. S3, Mn0–Mn0 dimer should be prohibited. This assumption is sup-
ESI†). The merging of two antisymmetric CRO stretching bands ported by the lack of a second reduction process which is assigned
after MnI has been grafted on TiO2 is consistent with literature for to the reduction of the Mn0–Mn0 dimer in solution at ꢀ1.67 V.
Re31 and Mn21,22 triscarbonyl bipyridine complexes immobilized Scanning to potentials more negative than ꢀ2 V lead to the
on TiO2 NPs. The presence of RuII on TiO2 was confirmed by degradation of the FTO surface itself. Therefore, we conclude that
UV-visible absorption and emission spectroscopies (Fig. S4, ESI†). the reduced form of immobilized complex is monomeric
The absorption maximum at around 450 nm of RuII/TiO2/MnI is [Mn0(ttpy)(CO)3] or [Mn0(ttpy)(CO)2(MeCN)] and not the dimer
attributed to the singlet metal-to-ligand charge transfer (1MLCT) observed for the complex in solution. The monomeric form of a
absorption band of RuII.32 In addition, its emission spectrum related complex, [Mn0(bpy)(CO)3Br], anchored on a metal–organic
shows a maximum at 618 nm, which is characteristic of the 3MLCT framework36 or on TiO2 particules22 was proposed to be the
luminescence of [Ru(bpy)3]2+* state.32 Meanwhile, TiO2/MnI does intermediate to access 100% selectivity for HCOOH.
not emit light in the 500–850 nm range under excitation at 450 nm.
Previous works on the mixture of RuII and [Mn(bpy)(CO)3Br]
RuII/TiO2/MnI NPs were then tested in a photocatalytic CO2 in solution and using BNAH as sacrificial electron donor have
reduction experiment under the same conditions as described shown that, under irradiation, RuII is first reductively quenched
above for the mix of component complexes. The concentration by BNAH to produce RuI and then RuI transfers an electron to
of the NPs was kept at 0.4 g Lꢀ1 as higher concentrations lead to the catalyst.20 In the triad RuII/TiO2/MnI, reduction of MnI may
quick precipitation and light blocking. Surprisingly this system proceed via a similar process on surface between adjacent
produced only HCOOH with a quantum yield of 0.17% and complexes, but more probably after population of the CB of
TONmax = 27 (Table 1, entry 6), while CO and H2 were not TiO2 from the RuII* excited state, since the injection of charge
detected. Control experiments using TiO2 or TiO2/RuII instead from RuII* is known to be very fast (kinj 4 1011 ꢀ1).37 To
s
of RuII/TiO2/MnI (amounts of TiO2 and RuII were kept understand whether the TiO2 matrix participate to the electron
unchanged) produced no CO, H2 or HCOOH, suggesting that transfer process and if the electrons injected in the CB of TiO2
HCOOH was formed only in the presence of MnI catalyst. Under can reduced the MnI catalyst, we performed CO2 reduction
Ar, the irradiation of RuII/TiO2/MnI in the presence of BNAH experiments using a mix of free MnI, BNAH and SiO2/RuII or
does not produce HCOOH either. When TiO2/RuII NPs and free TiO2/RuII NPs in DMF/TEOA mixed solution. Both systems
MnI complex were used instead of the triad, the results were contained the same concentration of MnI and BNAH in
comparable to the free complexes in homogeneous solution solution. They give rise to different results (Table S1, ESI†),
(i.e. production of both CO and HCOOH) (Table S1, ESI†). These with an enhancement of the production of HCOOH and CO
experiments suggest that the immobilization of the MnI with TiO2 (TONtotal = 44) instead of SiO2 (TONtotal = 12) as
catalyst on TiO2 is related to the suppression of CO formation. support for the RuII complex. We postulate then that the
The excellent yield and selectivity of HCOOH in this study electron injected in the CB of TiO2 from RuII* participates in
surpass those of CO production from [Re(bpy)(CO)3Cl]33 and the reduction of the MnI. The large surface of the TiO2 NPs
HCOOH production from [Mn(bpy)(CO)3Br]34 anchored on favours the reduction of the MnI complex in solution leading to
non-conducting supports.
an increase in the TON compared to the SiO2/RuII based
In order to elucidate the selectivity enhancement for HCOOH system. In the latter the charge injection, in the NPs is not
when MnI is anchored on TiO2, we first used cyclic voltammetry possible and the reduction of MnI only occurs from specific site
to probe redox properties of immobilized MnI. A sample of TiO2/ of RuI transient species on surface (energy diagrams in
MnI NPs powder was pressed inside the cavity of a platinum Schemes S2 and S3 summarize the photoinduced electron
microelectrode and used as working electrode. Its CV recorded transfer events, ESI†). EPR spectroscopy was also performed
under Ar did not show a clear reduction signal upon cathodic on TiO2/RuII and RuII/TiO2/MnI under irradiation with BNAH:
scanning due to the population of electrons into the conduction besides signals due to BNAꢃ free radicals, typical signals of Ti3+
band (CB) of TiO2 from around ꢀ1.4 V vs. Ag/AgNO3 0.01 M sites confirmed the charge injection into TiO2. For TiO2/RuII
(Fig. S5, ESI†), which occurs at similar potential than the the spectrum is the superimposition of the signal of Ti3+ sites
reduction of MnI (ꢀ1.44 V for free complex in solution). and BNAꢃ in similar intensity. For RuII/TiO2/MnI, a lower
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intensity is obtained for the signal of Ti3+ sites compared to
BNAꢃ. This suggests that, most of the electrons transferred
from the RuII* photosensitizers do not remain on TiO2, but are
further transferred to the grafted MnI catalysts (see Fig. S7 and
discussion underneath, ESI†).
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To sum up, MnI tolylterpyridine derivative appears as promising
catalyst for the reduction of CO2. When immobilized onto TiO2
together with RuII as photosensitizer, the system efficiently and
selectively reduces CO2 to HCOOH with a quantum yield of 0.17%
under visible irradiation. Electron transfer from RuII* to MnI goes
through the CB of TiO2. Cyclic voltammetry suggests that mono-
meric Mn0 is the catalytically active sites in the RuII/TiO2/MnI
hybride system whereas in a mixed solution of RuII and MnI, a
Mn0–Mn0 dimer is formed as a precatalyst. Heterogenization
prevents the formation of dimers and achieves 100% selectivity
toward HCOOH production.
The authors thank the LabEx ARCANE (ANR-11-LABX-0003-01)
and O3H-EUR-GS (ANR-17-EURE-0003) for financial support.
Conflicts of interest
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27 J.-X. Zhang, C.-Y. Hu, W. Wang, H. Wang and Z.-Y. Bian, Appl. Catal.,
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There are no conflicts to declare.
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