Structure-dependent iron-based metal-organic frameworks for selective CO2-to-CH4photocatalytic reduction

Article abstract of DOI:10.1039/d0ta10278d

Visible-light driven reduction of carbon dioxide (CO2) to methane (CH4) is a challenge in the photocatalytic reaction system. Herein, two Fe-based metal-organic frameworks (MOFs) MIL-100(Fe) and MIL-101(Fe) with single-trimetallic clusters but distinct organic ligands were employed to explore the structure-dependent CO2 conversion performance. Compared with MIL-101(Fe), MIL-100(Fe) exhibits superior catalytic activity and selectivity for CH4 generation under visible-light irradiation via a solvent-free route. This is the first report that MIL-100(Fe) finishes the photoreduction of CO2 into CH4. Importantly, the plausible conversion pathways of the reaction were given by density functional theory (DFT) calculations. This work furnishes new direction for constructing MOFs to achieve CO2-to-CH4 conversion.

Full text of DOI:10.1039/d0ta10278d

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J Po lue ran sael od fo Mn aot te rai ad l js u Cs ht emm ai rs gt ri yn sA
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DOI: 10.1039/D0TA10278D
ARTICLE
Structure-dependent iron-based metal-organic frameworks for
selectively CO -to-CH photocatalytic reduction†
2
4
‡
‡
Xiao-Yao Dao, Jin-Han Guo, Xiao-Yu Zhang, Shi-Qing Wang, Xiao-Mei Cheng and Wei-Yin Sun*
Received 00th January 20xx,
Accepted 00th January 20xx
Visible-light driven reduction of carbon dioxide (CO
Herein, two Fe-based metal-orgnic frameworks (MOFs) MIL-100(Fe) and MIL-101(Fe) with single-trimetallic clusters but
distinct organic ligands were employed to explore the structure-dependent CO conversion performance. Compared with
MIL-101(Fe), MIL-100(Fe) exhibits superior catalytic activity and selectivity for CH generation under visible-light irradiation
via a solvent-free route. This is the first report that MIL-100(Fe) finishes photoreduction of CO into CH . Importantly, the
plausible conversion pathways of the reaction were given by density functional theory (DFT) calculations. This work furnishes
new direction for constructing MOFs to achieve CO -to-CH conversion.
2 4
) to methane (CH ) is a challenge in the photocatalytic reaction system.
DOI: 10.1039/x0xx00000x
2
4
2
4
2
4
NENU-605/606, NNU-13/14 and PCN-601.30-34 Undeniable, it is
a challenge to develop the diversity of photocatalysts, which is
remarkable and attractively promising for facilitating efficiency
and selectivity of CO conversion into CH .
2 4
Different series of MOFs with CUS and trimetallic nodes
emerge in the MOFs such as MIL-88B(Fe), MIL-100(Fe, Cr), MIL-
Introduction
Photoreduction of CO to high value-added products utilizing
2
sustainable solar energy as the reaction impetus is a pursuable
way to accomplish CO
and energy crisis.1 However, CO
2
recycling, alleviating greenhouse effect
has high C=O bond energy,
-3
2
1
01(Fe, Cr), PCN-250 (Fe
3
, Fe
2
Mn, Fe
The trimetallic clusters with CUS make
these MOFs as a charming platform for redox catalysis. For
instance, trimetallic cluster-based PCN-250 (Fe , Fe Co, and
Fe Ni) exhibit superior reactivity towards CO -to-CO
transformation. The mixed-metal-containing MOFs of NNU-31
Fe Co, Fe Ni, Fe Zn) display excellent photocatalytic CO
reduction performance to achieve high efficiency for CO -to-
2 2
Co, and Fe Ni), NNU-31
CO
to dissociate the C=O bond. Generally, the photon-assisted CO
reduction process can produce a series of products from two-
2
photoreduction has to overcome such high energy barriers
3
5-42
2 2 2
(Fe Co, Fe Ni, Fe Zn).
4
2
5
-6
3
2
electron to eight-electron products.
The eight-electron
2
2
product of CH is the desirable hydrocarbon fuels in the
4
4
1
photocatalytic system, but at the same time, is harder to
achieve than the two-electron products of formic acid (HCOOH)
and carbon oxide (CO).
Since Lin and co-workers reported that CO
was realized by incorporating Re (CO)
framework of UiO-67. Metal-organic frameworks (MOFs),
consisting of metal nodes and flourishing types of organic
linkers, are considered as a kind of promising photocatalysts for
performance regulation.
high-density metal nodes provide abundant coordinatively
unsaturated metal sites (CUS) that have attracted mounting
(
2
2
2
2
7
2
4
2
HCOOH conversion. Additionally, single-metal-based NH
MIL-101(Fe) and NH
organic linkers display superior selectivity for conversion of CO
2
-
2
photoreduction
(5,5’-cbpy)Cl into the
2
-MIL-88B(Fe) with amino-functionalized
I
3
8
2
2
5,29
to HCOOH or CO.
MOFs with mixed-trimetallic clusters have
been synthesized and thoroughly studied for catalytic reactions,
further reveal the relationship of the metal node composition
9
-17
Furthermore, MOFs containing
4
2-44
on the catalytic performance.
However, few reports reveal
the relationship of the distinct ligands with single-trimetallic
2
0,45-48
clusters on the photocatalytic performance,
CO photoreduction.
especially for
1
8,19
attention for potential redox activity.
flourishing types of multidentate organic ligands can be
In addition, the
2
Herein, MOFs of MIL-100(Fe) and MIL-101(Fe) with single-
trimetallic clusters but distinct ligands were employed for
studies on photocatalytic performance. These characters make
MOFs as the desired platform to explore the impact of ligand on
the catalytic reactivity and to uncover structure-performance
correlation. The results confirm the superior reactivity of MIL-
rationally designed and/or modified to regulate the catalytic
performance.2
0-22
To date, the majority of studies on MOF
materials has been reported with primary focus on HCOOH/CO
production.2
3-29
Only several of pristine MOFs present the
generation, such as MOF-525, Zn/PMOF,
selectivity for CH
4
1
2 4 4
00(Fe) towards CO photoreduction into CH . The CH
Coordination Chemistry Institute, State Key Laboratory of Coordination Chemistry,
School of Chemistry and Chemical Engineering, Nanjing National Laboratory of
Microstructures, Collaborative Innovation Center of Advanced Microstructures,
Nanjing University, Nanjing 210023, China. Email address: sunwy@nju.edu.cn; Tel:
production yield of MIL-100(Fe) exceeds at least 16.5 times of
MIL-101(Fe). This is the first report that MIL-100(Fe) finishes
photoreduction of CO into CH . Furthermore, the plausible
2 4
reduction pathways of the optimal photocatalyst were
confirmed by density functional theory (DFT) calculations.
+86 25 89683485
†
2
Electronic Supplementary Information (ESI) available: SEM, CO uptake, M-S plots
1
H NMR, GC-MS, and other tables and figures. See DOI: 10.1039/x0xx00000x
These authors contributed equally to this work.
‡
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J Po lue ran sael od fo Mn aot te rai ad l js u Cs ht emm ai rs gt ri yn sA
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Journal Name
+
-
ΔG[*CO] = G[*CO] + G[H
2
O] + 7 × G[H + e ] – (G[*] + G[CO
2
] +
Experimental
Density functional theory (DFT) calculations
View Article Online
+
-
DOI: 10.1039/D0TA10278D
8
×G[H + e ])
+
-
ΔG[CH
2
O] = G[*] + G[CH
2
O] + G[H
2
O] + 4 × G[H + e ] – (G[*] +
+ e ])
ΔG[*OCH ] = G[CH O*] + G[H O] + 3 × G[H
G[CO ] + 8×G[H + e ])
2
ΔG[CH OH*] = G[CH OH*] + G[H O] + 2 × G[H
All spin-polarized DFT computations were performed by using
Vienna ab-initio simulation package (VASP) based on projector
augmented wave (PAW) method.
G[CO ] + 8×G[H
+
-
2
+
-
3
3
2
+ e ] – (G[*] +
4
9-51
+
-
Electron-ion interactions
were described using standard PAW potentials with valence
+
+ e ] – (G[*] +
-
3
3
2
2
6
2
6
2
2
2
4
+
-
configurations of 2s 2p 3s 3d for Fe, 2s 2p for C, 2s 2p for O,
2
G[CO ] + 8×G[H + e ])
1
52,53
+
-
and 1s for H.
A plane-wave basis set was employed to
ΔG[*OH] = G[*OH] + G[H O] + G[CH ] + G[H + e ] – (G[*] +
2
4
expand the smooth part of wave functions with a cut-off kinetic
energy of 480 eV. For electron-electron exchange and
correlation interactions, the functional parameterized by
G[CO ] + 8×G[H
+
+ e ])
-
2
ΔG[CH +2H O] = G[*]+ G[CH ] +2 × G[H O] – (G[*] + G[CO ] +
4
2
4
2
2
+
-
8×G[H
+ e ])
5
4
Perdew-Burke-Ernzerhhof (PBE), a form of general gradient Electrochemical measurements
approximation (GGA), was used throughout. Due to insufficient Photocurrent measurements were performed on a CHI 730E
consideration of the on-site Columbic repulsion, DFT might fail electrochemical work station (Chenhua Instrument, Shanghai,
to describe the electronic structure of the MIL-100(Fe) and MIL- China) in a standard three-electrode system with photocatalyst-
1
01(Fe). To overcome this shortcoming, the GGA+U approach coated ITO as working electrode, Pt plate as counter electrode,
5
5
was used with U – J = 6.0 eV for Fe atoms. Since the unit cells and Ag/AgCl as reference electrode. CH CN solution of 0.1 M (n-
3
of MIL-100(Fe) and MIL-101(Fe) contain more than 10,000 Bu) N(PF ) was used as electrolyte. The as-synthesized sample
4
6
atoms, the calculation on such big systems was beyond the (5 mg) was added into 20 µL Nafion and 2.0 mL ethanol solution,
computational power of current supercomputer. Considering and the working electrode was prepared by dropping the
that the building block of MIL-100(Fe) and MIL-101(Fe) is a suspension (200 µL) onto the surface of an ITO plate. The
super tetrahedral cell, which includes Fe O(COO) trimers working electrode was dried at room temperature, and the
3 6
connected by BTC or BDC ligands, the active reaction center for photo-responsive signals of the samples were measured under
MIL-100(Fe) and MIL-101(Fe) is 5-coordinated metal ions in chopped light at 0.5 V.
trimers for adsorption of CO
2
. A finite-size cluster model Fe
3
O
The electrochemical impedance spectroscopy (EIS) was
trimer, which is cut from the periodic structure, was adopted to performed on the Zahner electrochemical workstation in a
approximate the structure. The Brillouin-zone integrations were standard three-electrode system with the photocatalyst-coated
performed using the Gamma-point only grid. When the carbon paper as working electrode, Pt plate as counter
geometries were optimized, all atoms were allowed to relax. electrode and Ag/AgCl as reference electrode. A 0.2 M Na SO4
2
And the atomic structures were optimized until the residual solution was used as electrolyte. The working electrode was
forces were below 0.02 eV/Å. Gas-phase molecules were prepared by dropping the ink (200 µL) onto the surface of the
treated using the ideal gas approximation, whereas adsorbates carbon paper. The working electrode was dried at room
were treated using a harmonic approximation. The DFT- temperature, and then EIS measurements were performed with
was corrected by + 0.45 eV,5 a usual a bias potential of -0.5 V in the dark with a frequency range from
6
calculated energy for CO
2
adjustment to correct the overestimation by DFT. H
2
O was 10-2 to 10
5
Hz under nitrogen atmosphere. The Mott-Schottky
calculated under 0.035 atm. The relative free energies were test was performed on the Zahner electrochemical workstation
calculated based on the computational hydrogen electrode in a standard three-electrode system with the photocatalyst-
(
CHE) model.57
coated glassy carbon as working electrode, Pt plate as counter
The Gibbs free energy (ΔG) of each reaction step is defined electrode and Ag/AgCl as reference electrode. A 0.2 M Na SO
2
4
as the difference between the free energy of the reaction solution was used as electrolyte. The working electrode was
initiation and termination states, and the general formula is prepared by dropping the ink (50 µL) onto the surface of glassy
described as
carbon and dried at room temperature. The measurements
were carried out with different frequencies of 1000, 2000 and
∆퐺 = ∆퐸퐷퐹푇 + ∆퐸푍푃퐸 ― 푇∆푆
where EDFT is the electronic energy obtained from DFT 3000 Hz.
calculations of the catalysts, ΔEZPE is the change of zero-point Photocatalytic reduction of CO experiments
2
energy, TΔS is the entropy contribution. Zero-point energies The photocatalytic reduction of CO experiments were carried
2
and entropies of photocatalytic CO
were computed from the vibrational frequencies.
Therefore, free energy changes relative to an initial state of The photocatalyst (2 mg) was uniformly dispersed into a circular
gaseous CO
ΔG[*COOH] = G[*COOH] + 7 × G[H + e ] – (G[*] + G[CO
2
reduction of intermediates out in a batch-type reaction system (CEL-SPH2N-D9, CeAulight,
China) equipped with a homemade gas-solid reactor (Fig. S1†).
2
free above an empty surface can be displayed by: glass-fibre membrane (0.22 μm pore diameter) with an
+
-
2
2
] +
illuminated area of about 2.8 cm . The membrane activated in
+
-
8
×G[H + e ])
vacuum at 150 °C for 12 h. Subsequently, the membrane was
installed in the reactor with the distance from the light source
to the sample was 11 cm and TEOA (2mL) was added into the
reactor. The reaction system was completely vacuumed and
2
+
-
ΔG[*CO] = G[*CO] + G[H
2
O] + 7 × G[H + e ] – (G[*] + G[CO ] +
2
+
-
8
×G[H + e ])
2
O] + 5 × G[H + e ] – (G[*] + G[CO ]
+
-
ΔG[*CHO] = G[*CHO] + G[H
8×G[H + e ])
2
+
-
+
pumped with CO gas. A 300 W xenon arc lamp (Sirius-300P,
2
| J. Name., 2012, 00, 1-3
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Zolix Instruments Co., Ltd., China) with a 400-780 nm filter was by gas chromatography (GC-9860, Luchuang Instrument,
View Article Online
1
3
DOI: 10.1039/D0TA10278D
used as the light source. The optical power density remained at China). The CO
00 mW cm-2 measured by an optical power meter (CEL- gas chromatography-mass spectrometer (Trace GC Ultra,
NP2000-2, CeAulight, China). The gas products were analyzed ThermoFisher Scientific).
2
isotopic labelling was determined by using a
4
Fig. 1 Schematic illustration of MIL-100(Fe) and MIL-101(Fe) with Fe
3
O clusters and BTC and BDC ligands. Color code: O, red; C, gray; Fe, green.
morphology (Fig. S2†). The as-obtained samples reveal well-
distinguished diffraction peaks that matched well with the
simulated ones, demonstrating the framework structures were
3
6,38
achieved successfully (Fig. 2a and 2b).
X-ray photoelectron
spectroscopy (XPS) was further employed to determine the
composition and chemical environment of MIL-100(Fe) and
MIL-101(Fe). From the survey scanning spectrum (Fig. S3†), it
can be found the existence of Fe, C, and O in both samples. The
high-resolution spectra of Fe 2p of MIL-100(Fe) and MIL-101(Fe)
are exhibited in Fig. 2c. Compared with MIL-100(Fe), the peaks
of MIL-101(Fe) display tiny negative shifts, indicating the
4
5
3
electron-density on Fe O clusters is enhanced. This result
further confirms that the bridging-ligands influence the local
electron-density of the metal nodes. The porosity of MIL-
1
2
00(Fe) and MIL-101(Fe) were determined by N adsorption-
desorption isotherms (Fig. 2d) with BET surface areas of 2147.6
2
and 3663.5 m /g, respectively. Compared with MIL-101(Fe),
3
MIL-100(Fe) displays higher absorption capacity (97.2 cm /g at
2
73 K) and larger adsorption enthalpy (39.4 kJ/mol) for CO
adsorption-desorption (Figs. S4, S5 and S6†), indicating the relatively more robust CO
framework interactions in MIL-100(Fe). The results imply that
2
Fig. 2 XRD patterns of MIL-100(Fe) (a) and MIL-101(Fe) (b). XPS spectra of MIL-
1
00(Fe) and MIL-101(Fe): Fe 2p spectra (c) and N
2
2
-
isotherms of MIL-100(Fe) and MIL-101(Fe) (d).
5
8
2
MIL-100(Fe) has kinetically advantage for CO transformation.
UV-vis diffuse reflectance spectroscopy (UV-DRS) was
employed to evaluate the optical-response and the results
display that both samples have optical absorption covering the
UV-visible region owing to the presence of μ-O-Fe clusters (Fig.
Results and discussion
MOFs MIL-100(Fe) and MIL-101(Fe) were synthesized and
characterized in a conventional way (Fig. 1). The morphologies
of the as-prepared samples display monodispersed
nanocrystals and present typical octahedral-shaped
5
9
3
a). The bandgap of MIL-100(Fe) and MIL-101(Fe) are
identified to be about 2.65 and 2.08 eV, respectively (Fig. S7†).
This journal is © The Royal Society of Chemistry 20xx
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Mott-Schottky (MS) measurements were performed to catalytic process (Fig. S12†). Clearly, MIL-100(Fe) exhibits
View Article Online
DOI: 10.1039/D0TA10278D
determine the conduct band (LUMO) of MIL-100(Fe) and MIL- superior catalytic activity with CO and CH
4
generation.
1
01(Fe). The positive slopes imply typical n-type Importantly, the MIL-100(Fe) achieves high selectivity of about
4
semiconductor-like character. The flat-band potentials of MIL- 98% for CH formation (relative to the total number of reacted
1
00(Fe) and MIL-101(Fe) are determined to be -0.98 and -1.04 electrons) (Fig. S13†). Meanwhile, MIL-101(Fe) displays limited
V vs. normalized hydrogen electrode (NHE), respectively (Fig. catalytic activity with CO and CH production. Thus, MIL-100(Fe)
S8†). The potentials of these samples are more negative than possesses superior selectivity and activity for CO conversion
the redox potential of CO to CO (-0.53 V vs. NHE) as well as to into CH . In addition, solar-to-methane (CH ) conversion
CH (-0.24 V vs. NHE), accordingly, these materials have efficiency (ɳ%) and apparent quantum efficiency (AQY%) over
capability for CO photoreduction. Besides, to assess the MIL-100(Fe) are 0.023% and 0.943%, respectively. It is
4
2
2
4
4
4
2
separation efficiency of photo-excited charge carriers, noticeable that MIL-100(Fe) gives high AQY compared with the
photocurrent response was measured with the illumination of MIL-101(Fe) and reported heterogeneous photocatalysts (Table
visible-light. As illustrated in Fig. 3b, MIL-100(Fe) possesses S1†). To further illustrate the photocatalytic activity, the
higher photocurrent than MIL-101(Fe), reflecting that MIL- turnover frequencies (TOF) of these catalysts were determined
1
00(Fe) has more efficient interface charge separation. by the amount of metal active sites, and corresponding TOF
Additionally, electrochemical impedance spectroscopy (EIS) for values of these photocatalysts are displayed in Fig 4a.
MIL-100(Fe) implies low resistance in the Nyquist plots (Fig. 3c),
also indicating the fast interface charge transfer. Furthermore,
the time-resolved photoluminescence (PL) decay spectra
suggest that the average PL lifetimes of MIL-100(Fe) and MIL-
1
3
01(Fe) are calculated to be 0.46 and 0.25 ns, respectively (Fig.
d). The prolonged lifetime further proves that the free charges
in MIL-100(Fe) may transfer for more extended until
recombination. On account of the efficient charge separation
and prolonged charge lifetime, MIL-100(Fe) can be considered
2
as a desired photocatalyst for CO reduction.
Fig. 4 (a) Photocatalytic CO
values and (b) compassion of CO (left) and CH
photocatalysts.
2
into CO and CH
4
production and corresponding TOF
4
(right) generation rate of varied
Fig. 3 Diffuse reflectance UV/vis spectra (a), transient photocurrents (b), EIS
Nyquist plots (c) and time-resolved fluorescence spectra (d) of MIL-100(Fe) (black-
line) and MIL-101(Fe) (red-line).
In order to verify the carbon origins of the target products of
was conducted CO and CH4, 13C-labeled 13CO was applied for the photocatalytic
Visible-light driven photoreduction of CO
via a solvent-free reaction route by utilizing triethanolamine test. The reaction products 13CO and 13CH were confirmed by
2
2
4
(TEOA) as sacrificial agent. As displayed in Fig. 4, two samples mass spectrometry (Fig. S14†), ensuring that both goal products
exhibit distinct yields of transformation CO to CO and CH in resulted from CO2 reduction. To further explore the
2
4
five hours under the visible-light irradiation (Fig. S9† with photocatalytic activity of MIL-100(Fe), we compared the
calibration curves and Fig. S10†). In order to monitor the liquid photocatalytic activity of MIL-100(Fe) with additional catalysts
1
products, H NMR spectral measurements were conducted, and (Fig. 4a) such as MIL-100(Cr, Al), MIL-88B(Fe) and MIL-53(Fe). All
no liquid product was detected in the catalytic process (Fig. samples exhibit weak photocatalytic activity. Additionally, the
S11†), which may be due to the employment of solvent-free constituents of MIL-100(Fe), including iron net and BTC, give an
reaction system.2
6,29,31
2
Additionally, no H -production was insignificant photocatalytic effect (Fig. 4), thus indicating the
detected by gas chromatography (GC) during the whole importance of the MIL-100(Fe) skeleton for the photocatalytic
4
| J. Name., 2012, 00, 1-3
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ARTICLE
reaction. According to the previous work,29 NH
possesses a superior selectivity for conversion of CO
More importantly, the order of CH -yield is MIL-100(Fe) > MIL-
01(Fe) > MIL-88B(Fe) >NH -MIL-101(Fe). Meanwhile, the order
of CO-yield is NH -MIL-101(Fe) > MIL-101(Fe) > MIL-88B(Fe) >
-MIL-101(Fe)
to CO.
2
View Article Online
DOI: 10.1039/D0TA10278D
2
4
1
2
2
MIL-100(Fe) (Table S2†). These results demonstrate that the
photocatalytic performance of Fe-based MOFs was improved by
changing organic ligand with identical Fe-oxo clusters. In
addition, the different performance of these photocatalysts
may also be attributed to their unique framework structures.
MIL-100(Fe) and MIL-101(Fe)/NH
Fe-oxo clusters and four μ -O-bridged Fe
constitute a tetrahedron as structural unit (Fig. S15†). For MIL-
01(Fe) or NH -MIL-101(Fe), terephthalate (BDC) or amino-
substituted terephthalate (NH -BDC) ligands as the edge of
tetrahedron connect with Fe O clusters, while MIL-100(Fe) with
tricarboxylate (BTC) one as the facet of tetrahedron coordinates
with the Fe O clusters. Comparatively speaking, the structural
unit of MIL-100(Fe) is more closely arranged than the one of CH
2
-MIL-101(Fe) possess uniform
3
3
O clusters together
1
2
2
3
3
Fig. 5 DFT calculations. Free energy diagram of photocatalytic reduction CO
for MIL-100(Fe) (a) and MIL-101(Fe) (b). Optimal pathways to CH and the
three limited potential steps are shown by ΔV , ΔV , and ΔV . The black line
-MIL-101(Fe) (Fig. S16†), in which closely indicates a path with lower limited potential, and the blue line indicates a
competitive path. * The active surface sites for adsorption and reaction.
2
-to-
4
4
1
2
3
MIL-101(Fe) or NH
packed tetrahedron structure is conducive to the migration of
electrons. Furthermore, MIL-100(Fe) has greater Fe O cluster
density than MIL-101(Fe) within the same photocatalytic area
Fig. S17†), in which the CO photoreduction occurs. As for MIL-
8B(Fe), with the triangular bipyramidal arrangement of five
2
3
for both catalysts. The *COOH intermediate can react with
proton-electron and forming *CO, which is connected with the
Fe-O clusters. Subsequently, the protonation reaction of *CO
can produce two potential intermediates: *CHO or *COH. The
relative potentials between *CO and *COH are too high to be
realized. This fact indicates that the *CHO intermediate is
preponderant in this step. From Fig. 5, the reaction process of
(
8
2
Fe
3
O clusters (Fig. S18†), the structure unit is loosely arranged
than the tetrahedron as the structural unit, and further results
poor photocatalytic activity than MIL-100(Fe) and MIL-101(Fe).
More significantly, the electrophilicity of coordinated organic
*
CHO to CH
reduction process of CH
demands. Therefore, the optimal reaction path for CO
on MIL-100(Fe) and MIL-101(Fe) follows CO → *COOH → *CO
*CHO → CH O(g) → *OCH → *CH OH → *OH → CH (g). The
4
are thermodynamically favorable, except that the
O(g) to *OCH requires to meet energy
-to-CH
ligands follows the order: NH
00(Fe) displays more
delocalization. Hence, the excellent photocatalytic activity
2
-BDC < BDC < BTC, and MIL-
2
3
1
a
efficient inter-cluster
2
4
4
5,60
2
of MIL-100(Fe) can be attributed to the closely arranged
structural units, BTC with high electrophilicity and further
reduced electron-density of the Fe O clusters. Furthermore, a
3
→
2
3
3
4
stable structures corresponding to the optimal reaction path
are displayed in Figs. S23 and S24†. Between the samples, the
ΔV of the corresponding endothermic processes on MIL-100(Fe)
is lower than that on MIL-101(Fe) (Table S4†), which
demonstrated that the MIL-100(Fe) would be a more efficient
photocatalyst for CO reduction. Besides, the CO temperature-
2
programmed desorption (TPD) measurement was employed to
reveal the CO desorption process on the photocatalyst surface
series of control tests were conducted in the vacuum or dark or
without photocatalyst or TEOA. The target products were
undetectable in these catalytic systems (Table S3†).
Importantly, the photostability of MIL-100(Fe) was evaluated
using recycling experiments. As shown in Fig. S19†, MIL-100(Fe)
exhibits the long-term photocatalytic stability after multiple
uninterrupted cycles. Corresponding SEM image, PXRD pattern
and XPS spectra after the photocatalysis were shown in Figs.
S20-S22†. The post-catalyst retained the octahedral
morphology after multiple photocatalytic test, except that its
surface became rough. These results demonstrate that the
morphology, crystalline-structure and oxidation state of MIL-
(Fig. S25†). The results suggest that the stronger interaction
between CO and MIL-100(Fe) can more efficiently prevent CO
to escape from the Fe-O clusters, which benefits for further
hydrogenation of CO to generate CH
agreement with the photocatalytic experiment, in which MIL-
00(Fe) shows superior selectivity and reactivity for CO -to-CH
The choice of CO, HCHO and CH OH feeds was based on the
calculational results that all of them may constitute reactive
4
. This result is in
1
2
4
.
1
00(Fe) have no noticeable alteration, confirming the structural
3
robustness.
DFT calculations were performed to get further insight into
3
3, 61, 64
intermediates of the catalytic process.
the optimal catalyst of MIL-100(Fe), the photocatalytic
experiment under CO, HCHO, and CH OH were applied
separately as feed reactants (Fig. S26†), the result proved that
CO, HCHO, and CH OH are the intermediates for CH formation,
which is in line with the DFT predictions.
Therefore, taking
4
, 61-63
the CO
assuming converted proton-electron transfer and evaluated at
V vs. RHE. The Gibbs free energy diagrams of the reaction
2
-to-CH
4
reaction process.
The computations are
3
0
pathways are illustrated in Fig 5, the formation of *COOH on
MIL-100(Fe) and MIL-101(Fe) are endothermic processes with
energy barriers of 1.906 and 2.005 eV, respectively. And the
formation of *COOH is considered as the rate-determining step
3
4
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ARTICLE
Journal Name
On the basis of the above results, a possible reaction pathway Nanjing University for conducting the DFT calculation in this
View Article Online
DOI: 10.1039/D0TA10278D
of visible-light-driven CO
proposed. Compared with MIL-101(Fe), MIL-100(Fe) not only
possesses superior CO adsorption capacity because there are
more interaction sites on the surface, but also displays
acceleration of electron transfer owing to the closely packed
structure units. In addition, the flat-band potentials of MIL-
2
photoreduction over MIL-100(Fe) was work on its Flex cluster system.
2
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also supported by a Project Funded by the Priority Academic
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