Monometallic Catalytic Models Hosted in Stable Metal-Organic Frameworks for Tunable CO2 Photoreduction

Article abstract of DOI:10.1021/acscatal.8b04887

The photocatalytic reduction of CO₂ to energy carriers has emerged as one of the most promising strategies to alleviate the energy crisis and CO₂ pollution, for which the development of catalyst was considered as the determining factor for the accomplishment of this conversion process. In this study, three stable and isostructural metal-organic frameworks (denoted as MOF-Ni, MOF-Co, and MOF-Cu) have been synthesized and used as heterogeneous catalysts in photocatalytic CO₂ reduction reaction (CO₂RR). It is worth noting that the MOF-Ni exhibited very high selectivity of 97.7% for photoreducing CO₂ to CO, which has exceeded most of the reported MOF-based catalysts in the field. Significantly, the MOFs associated with a monometallic catalytic center offer a simple and precise structural model which allows us to understand more definitively the specific effects of different metal-ion species on photoreduction of CO₂ as well as the reactive mechanism.

Full text of DOI:10.1021/acscatal.8b04887

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Article
Monometallic Catalytic Models Hosted in Stable Metal-
Organic Frameworks for Tunable CO2 Photoreduction
Xiao-Kun Wang, jiang liu, Lei Zhang, Long-Zhang Dong,
Shun-Li Li, Yu-He Kan, Dongsheng Li, and Ya-Qian Lan
ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b04887 • Publication Date (Web): 15 Jan 2019
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Monometallic Catalytic Models Hosted in Stable Metal-
Organic Frameworks for Tunable CO2 Photoreduction
Xiao-Kun Wang†a, Jiang Liu†b, Lei Zhangb, Long-Zhang Dongb, Shun-Li Lib, Yu-He Kanc, Dong-Sheng
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Li*a, Ya-Qian Lan*b
[a] College of Materials and Chemical Engineering, Key Laboratory of Inorganic Nonmetallic Crystalline and Energy
Conversion Materials, China Three Gorges University, No. 8, Daxue Road, Yichang, 443002, P. R. China
[b] School of Chemistry and Materials Science, Jiangsu Key Laboratory of Biofunctional Materials, Nanjing Normal
University, Nanjing 210023, P. R. China
[c] Jiangsu Province Key Laboratory for Chemistry of Low-Dimensional Materials, School of Chemistry and Chemical
Engineering, Huaiyin Normal University, Huaian 223300, P.R. China
ABSTRACT: The photocatalytic reduction of CO2 to energy carriers has emerged as one of the most promising strategy to alleviate
the energy crisis and CO2 pollution, for which the development of catalyst was considered as the determining factor for the
accomplishment of this conversion process. In this study, three stably and isostructural metal organic frameworks (denoted as MOF-
Ni, MOF-Co and MOF-Cu), have been synthesized and used as heterogeneous catalysts applied into photocatalytic CO2 reduction
reaction (CO2RR). It is worth noting that the MOF-Ni exhibited a very high selectivity of 97.7% for photoreducing CO2 to CO, which
has exceeded most of the reported MOF-based catalysts in the field. Significantly, the MOFs associated with monometallic catalytic
centre offer a simply and precisely structural model which allows us to understand the specific effects of different metal ion species
on photoreduction of CO2 as well as reactive mechanism more intuitively.
KEYWORDS: Metal-Organic Frameworks, Isostructural, Photocatalytic CO2 Reduction, High Selectivity, Monometallic Catalytic
Model, Reactive Mechanism
photoreduction is somewhat harsh that commonly requires the
In recent years, the increased emission of anthropogenic CO2
catalyst that has highly structural stability in reaction solution. This
from the burning of fossil fuels is leading to serious issues such as
is actually a big challenge to the majority of the reported crystalline
global warming and energy crisis. Great efforts in finding efficient
MOF. In particular, the CO2RR carried out in relatively alkaline
strategies to solve these problems have been made.1-2 For instance,
system, which is beneficial for the dissolution of more CO2, has
considering that solar energy is a clean and renewable energy
additional requirements on the chemical stability of involved MOF.
source, visible-light driven photocatalytic CO2RR that converting
Additionally, the active centers of most MOF-based catalysts are
CO2 into carbon-based energy carriers (hydrocarbon fuels or
mainly homo-/hetero-metallic cluster as secondary building units
chemicals) has been considered as one of the most promising
(SBUs).42-44 Due to the collaborative contribution and interaction
between active centres within cluster on the photocatalytic
performance, the catalytic ability of single metal active site is hard
to evaluate such that the relevant reaction mechanism is still elusive
and intricate.45-47 Consequently, the construction of MOF-based
catalyst with single catalytic active center and high structural
robustness to overcome the aforementioned troubles is supposed to
be desirable.
solutions.3-6 However, the difficult activation process of CO2
molecule with intrinsically chemical inertness enables the reaction
to undergoing large thermodynamic barriers. Fortunately, the
development of efficient and selective catalysts has been proven to
be extremely vital in addressing above issue. Up to now, a variety
of semiconductors (e.g., TiO2, ZrO2, Bi2WO6 and WO3) and
nanocomposites have been used as photocatalysts to exert
CO2RR.7-11 Although most semiconductor-based nanomaterials
exhibit high photocatalytic performance, the complicated structural
components and indistinct active sites are always difficult to
productive in investigating the reactive mechanism.12-15
Consequently, how to develop an efficient photocatalyst with
precisely structural information in principle is the important
prerequisite for the explanation of photocatalytic mechanism of
CO2RR.16
With these considerations in mind, we successfully designed
and synthesized three stable and isomorphic MOFs,
{Cu3(TCA)2(dpe)3(H2O)3}n
{Co3(TCA)2(dpe)3(H2O)6}n
(MOF-Cu),
(MOF-Co)
and
{Ni3(TCA)2(dpe)3(H2O)6}n (MOF-Ni), which are used different
transition metal centers (CuII, CoII and NiII) and mixed organic
ligands [4,4',4''–nitrilotribenzoic acid ligands (TCA) and 1,2-di (4-
pyridyl) ethylene (dpe)]. It is noteworthy that these MOFs use
single active metal centre as node which implies a simple and
straightforward structural model to analyze the influence of
different transition metal centres on photocatalytic reduction of
CO2. As expected, the MOFs with different catalytic active centres
(CuII, CoII and NiII) were treated as catalysts applied in
heterogeneous photocatalytic CO2RR, which indeed resulted in
notable effects on the sort and selectivity of mainly reductive
products. Interestingly, MOF-Ni exhibited very high catalytic
Metal-organic framework (MOF) constructed by metal
ions/clusters and functionalized organic ligands is one kind of
crystalline material with well-defined structure. Because of the
structural tailorability and ultrahigh surface area, MOF has been
applied in many fields widely.17-31 Recently, many studies have
demonstrated that MOFs can serve as catalysts to reduce CO2 as
well as offer a good platform to study reaction mechanism from
molecular level, and the research heat of this study continues to
increase.32-41 However, the reaction condition of CO2
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selectivity (97.7%) of CO, which has surpassed most of the
independent of the frequency, the flat positions of MOF-Ni, MOF-
Co and MOF-Cu are determined to be -1.14, -1.34 and -1.28 V vs
Ag/AgCl, respectively. Thus the bottom of the conduction band
(LUMO) of MOF-Ni, MOF-Co and MOF-Cu are estimated to be
-0.94, -1.14 and -1.08 V vs the normal hydrogen electrode (NHE),
respectively.51 From the Tauc plot, the bandgaps of the MOF-Ni,
MOF-Co and MOF-Cu were estimated to be 1.50, 2.02 and 1.77
eV by Kubelka-Munk (KM) method (Figure S12, S13, S14). Then
the valence band (HOMO) positions of these three MOFs were
calculated to be 0.56, 0.88 and 0.69 eV versus NHE, respectively.
Since their LUMO positions are more negative than the reduction
potentials of CO2 to many products, it is theoretically feasible to
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reported MOF-based catalysts applied in photocatalytic CO2RR,
representing the most efficient MOF-based catalyst with NiII ion as
active centre. By contrast, MOF-Cu and MOF-Co showed high
selectivity (77.4%) of H2 and moderate selectivity (47.4%) of CO,
respectively. Notably, the corresponding theoretical calculations
are consistent with the photocatalytic prosperities and offer an
important insight into the influence of different monometallic
catalytic centres on photocatalytic CO2 conversion.
Single-crystal X-ray diffraction analysis reveals that MOF-Cu,
MOF-Co and MOF-Ni have almost identical host framework, all
of them crystallize in the trigonal system with R-3 space group. The
only difference is that five-coordinated CuII ion adopts tetragonal
pyramid geometry, while CoII and NiII ions have one more axial
coordination H2O molecule to form an octahedron environment
(Figure 1a, S1, S2). The coordination sphere of CuII ion is
surrounded by two carboxylate-O atoms from two TCA ligands,
two N atoms from two dpe ligands and one O atom from axial
coordination H2O molecule. Considering that MOF-Co and MOF-
Ni are isomorphic, MOF-Ni is selected to describe their structures
herein. MOF-Co and MOF-Ni embrace equivalent two couples of
N and O atoms as same as CuII ion in equatorial plane and two O
atoms from two axial coordination H2O molecules. The Ni-N/O
bond lengths in equatorial plane are in the range of 2.0-2.1 Å, while
the axial Ni-O band lengths are 2.0 Å.48-50 The carboxylate group
of TCA ligand adopts a μ1-η1:η0 coordination mode and each TCA
ligand connect three different NiII ions (Figure S3a). Notably, TCA
ligands are connected with Ni ions along the crystallographic c axis
to form 2D network having a twisted hexagon window (Figure S3b),
and 2D networks are three-fold interpenetrated into a layered
structure (Figure 1b). Interestingly, the adjacent 2D layers are
further pillared by the dpe ligands to form an overall 3D network
(Figure 1c and S4). Additionally, the framework of MOF-Ni and
MOF-Co can be described as 3,4-connected networks with the
Schläfli symbol {103}2{106}3 from topology (Figure S5).
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use these MOFs as catalysts for photoreducing CO2.12,
Additionally, the adsorption of CO2 is often believed to play a
crucial role in the catalytic performance of catalyst, so the
volumetric CO2 adsorption measurements were performed on the
activated samples at 298 K.53-54 As shown in Figure 2d, the CO2
uptakes at 298K were found to be 40.35, 38.87, 34.00 cm3 g-1 for
MOF-Co, MOF-Ni and MOF-Cu, respectively.
Figure 2. (a) PXRD patterns of MOF-Ni. (b) The UV-vis spectra
of MOF-Ni (blue), MOF-Cu (black) and MOF-Co (red). (c) Mott-
Schottky plots for MOF-Ni in 0.2 M Na2SO4 aqueous solution. (d)
CO2 adsorption behavior for MOF-Co (red) as well as MOF-Ni
(blue) and MOF-Cu (black) at 298 K.
Taking the above features of these MOFs into consideration,
the photocatalytic CO2RR was conducted under a pure CO2 (1.0
atm, 298 K) atmosphere in a mixed solution of MeCN/H2O (13:1)
with triisopropanolamine (TIPA) as an electron donor. Besides,
[Ru(bpy)3]Cl2·6H2O (bpy = 2’2-bipyridine) as an auxiliary
photosensitizer (PS) was added into the reaction system for
increasing visible-light absorption.12, 55 Because of the matched
LUMO positions between the PS and MOF-based catalysts (Figure
S15-S17), photo-generated electrons were allowed to migrate from
the PS to the MOFs.56-57 Based on the different optical and
electrochemical properties of these isomorphic MOFs, their
differences on the performance of CO2RR can be demonstrated
through a series of photocatalytic experiments. As shown in Figure
3a, increasing the generation of CO (22.3 mol, i.e., 371.6 mol
g-1 h-1) rather than H2 (0.5 mol, i.e., 8.3 mol g-1 h-1) were
observed when reducing CO2 with MOF-Ni as a photocatalyst
under visible light irradiation (λ ≥ 420 nm) (Figure 3b). By contrast,
MOF-Co displayed a dramatic promotion on the production of CO
(22.8 mol, i.e., 1140.0 mol g-1 h-1) and H2 (25.3 mol, i.e.,
1265.0 mol g-1 h-1), while only 1.7 mol (i.e., 68.0 mol g-1 h-1)
CO and 5.8 mol (i.e., 232.0 mol g-1 h-1) H2 by MOF-Cu at the
same reaction system. The TONs of these photocatalytic systems
were summarized in Table S3. Remarkably, the MOF-Ni exhibits
a higher selectivity of CO over H2 (97.7%) than MOF-Co (47.4%)
Figure 1. (a) Coordination environment of MOFs. (b) Schematic
view of the three-fold interpenetrating layer. (c) 3D channel
simulated diagram of MOF-Ni.
The purity of the as-synthesized crystals was verified by
powder X-ray diffraction (PXRD) pattern which showed well
match with the simulated one from crystal structure (Figure 2a, S7,
S8). It is found that these MOFs exhibit good chemical stabilities
that can keep their structures unchanged under broad pH value
variation ranges. Furthermore, their high thermal stabilities were
also verified by the thermogravimetric (TG) curves under O2 flow
(Figure S9).
The UV/vis spectra demonstrate that these three isostructural
MOFs show very broad absorption throughout the region of 450-
800 nm, indicating their potential to be catalyst applied to
photocatalysis (Figure 2b). In order to clarify the semiconductor
properties of these MOFs and the possibility of subsequent
photoreduction of CO2, Mott-Schottky measurements were
performed at frequencies of 500, 1000 and 1500 Hz. The results
indicate that these three MOFs are typical n-type semiconductors
(Figure 2c, and Figure S10, S11). Since the intersection point is
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and MOF-Cu (22.6%). Furthermore, among the reported of
impedance spectroscopy (EIS), which indicates that MOF-Co has
the smallest radius and the lowest resistance in charge
transportation, while MOF-Cu shows the biggest radius and the
largest resistance among them (Figure S27). Therefore, MOF-Co
and MOF-Ni possess higher charge-separation efficiency than
MOF-Cu.
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heterogeneous MOF-based catalysts applied in the photocatalytic
CO2RR that exhibited such a high selectivity towards CO have
been rarely seen. Gaseous CO and H2 were the main reaction
products detected by gas chromatography during the whole
photocatalytic process, only trace amounts of HCOOH were
produced in the aqueous solution as detected by ion
chromatography.
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Figure 4. (a) Geometry structures of CO2 adopted on three metal
sites, the Ni is present as dark blue and Co/Cu are in green. (b) The
free energy profile of CO2RR toward the production of CO. (c) The
free energy diagram of HER.
Density functional theory (DFT) calculations are performed to
understand the specific effects of different metal ion species on
photoreduction of CO2. We first investigate the binding between
CO2 and three metal ions, which plays an essential role in the
selectivity and reactivity of the following catalytic reactions. As
shown in Figure 4a, both Co and Cu present weak interaction
toward to CO2 with a long distance of 3.6 Å, while Ni and CO2
form strong coordination bond of 2.4 Å. The strong coupling
between Ni and CO2 is attributed to the high spin state of the Ni in
an octahedral coordination, as seen in the spin density plot (Figure
S30). However, the binding energy, calculated by EBE (*CO2) =
E(total) - E(5-coordination) - E(CO2), is determined to be a small
value due to the coupling between Ni and two O in the carboxyl
group (Figure S31). Free energy pathways of CO2 reduction to CO
on the metal sites of MOFs and the intermediate structures are
shown in Figure 4b and S32. Among the four elementary reaction
steps, the *COOH formation serves as the rate-limiting step and
follows the order of Cu (2.04 eV) > Ni (1.22 eV) > Co (0.95 eV).
The competition reaction of hydrogen evolution reaction (HER) is
considered for comparison and the free energy diagram obtains
0.67 eV for Co and 1.42 eV for Ni respectively (Figure 4c). The
calculation results suggest that MOF-Ni presents the best
selectively among the three complexes due to the strong binding
with CO2 and high HER free energy, and both CO2RR and HER
processes can readily occur for MOF-Co. These findings are in
good agreement with the aforementioned experiments.
Figure 3. (a) Photocatalytic production of CO and H2 catalyzed by
MOF-Cu, MOF-Co and MOF-Ni. (b) A mount of CO and H2
produced as a function of the time of visible-light irradiation over
MOF-Ni. (c) Mass spectra (m/z =29) analyse of the source of CO.
(d) Average yields of CO and H2 in fresh 7 h with MOF-Ni
catalysts in recycling tests.
Considering that MOF-Ni has better catalytic activity and
selectivity than MOF-Cu and MOF-Co in photocatalytic CO2RR,
a series of reference experiments with MOF-Ni as the example
were conducted to determine the importance role of the catalyst and
the experimental results are summarized in the Table S3. The
production of CO has a high selectivity of 97.7% over competing
H2 generation after 12 hours irradiation with visible light. This
selectivity is the highest among most of the reported MOF-based
photocatalysts for reducing CO2 to CO (Table S4). The calculated
quantum yield of the heterogeneous photocatalytic system was
5.3×10-3% under irradiation of 420 nm light (specific calculation
method in the ESI). To ascertain the source of the produced CO,
we performed an isotopic tracing experiment by replacing CO2
with 13CO2. The 13CO2 was used as the reactant under the same
photocatalytic reaction condition, and then the reaction product
was examined by gas chromatography-mass spectrometry. After
irradiation with visible light, the peak at 1.8 min with m/z 29 was
assigned to 13CO (Figure 3c). The results demonstrate that CO2 is
the main carbon source rather than the degradation of organics in
the reaction. Additionally, the total production of the reaction
products has no noticeable decrease after four cycles of 7 hours
reaction, suggesting the reservation of the original photocatalytic
activity of MOF-Ni (Figure 3d). Furthermore, there was no
noticeable alteration in their PXRD patterns and IR spectra
obtained before and after the photocatalytic reactions that
evidenced the structural robustness of the catalyst again (Figure
S23-S25).
In accordance with the above experimental results and
theoretical calculations, a possible photocatalytic mechanism was
proposed (Figure 5). First, the coordination water on the metal
center is easily detached to form exposed metal active site, where
CO2 molecules are adsorbed. Since the LUMO of MOF-M is lower
than that of [Ru(bpy)3]2+, the photo-generated electrons in the
LUMO of [Ru(bpy)3]2+ can be transferred to the surface of the
MOF-M.59-60 Second, the CO2 adsorbed on the metal active site
accepts an electron to form radical CO2¯ intermediate. Third, by the
proton-assisted two-electron transport process, the absorbed CO2
molecule was finally reduced to CO. Finally, the excited state of
photosensitizer was reductive quenching by the sacrificial electron
donor TIPA and the generated CO detached from the catalyst
surface.
To explore the reasons for the difference in photocatalytic
activity of the three catalysts, we first assume that charge
separation efficiency is an important factor.58 As proved by the
photocurrent characterization results, MOF-Ni and MOF-Co
reveal obviously more efficient separation of photogenerated
electron-hole pairs than MOF-Cu under the same conditions
(Figure S26). The fact is further supported by electrochemical
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heterogeneous photocatalytic CO2RR. It is significant that MOF-
Ni displays a very high selectivity of 97.7% for the CO2-to-CO
conversion, which has surpassed most of the reported MOF-based
catalysts in the field of CO2RR. Furthermore, the precisely and
simply structural models with single metal active site enable us to
understand the specific effects of different metal ion species on
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Figure 5. Proposed photocatalytic mechanism of MOFs for the
CO2 to CO conversion.
photoreduction of CO2 and reactive mechanism more intuitively.
Our findings are anticipated to providing more insights into the
development of more efficient, stable and selective catalysts for
photocatalytic CO2RR.
AUTHOR INFORMATION
Corresponding Author
* E-mail addresses: yqlan@njnu.edu.cn, lidongsheng1@126.com.
Author Contributions
†These authors contributed equally.
Notes
The authors declare no competing financial interest.
Supporting Information
This material is available free of charge via the Internet
at http://pubs.acs.org.
Crystallographic data for three MOFs.
Details of synthesis, more characterization, and theoretical
calculations.
ACKNOWLEDGMENT
This work was financially supported by NSF of China (21673127),
and the 111 project of Hubei Province (2018-19).
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