Iron(iii)-bipyridine incorporated metal-organic frameworks for photocatalytic reduction of CO2with improved performance

Article abstract of DOI:10.1039/d0dt03500a

Metal-organic frameworks (MOFs) represent an emerging class of platforms to assemble single site photocatalysts for artificial photosynthesis. In this work, we report a new CO2 reduction photocatalyst (UiO-68-Fe-bpy) based on a robust Zr(iv)-MOF platform with incorporated Fe(bpy)Cl3 (bpy refers to the 4′-methyl-[2,2′-bipyridine] moiety) via amine-aldehyde condensation. We show that this hybrid catalyst can reduce CO2 to form CO under visible light illumination with excellent selectivity and enhanced activity with respect to its parent MOF and corresponding homogeneous counterpart. Using steady state and transient absorption (TA) spectroscopy, we show that the enhanced photocatalytic activity of UiO-68-Fe-bpy is attributed to the elongated excited state lifetime of Fe(bpy)Cl3 after being incorporated to the UiO-68-NH2 platform. This work demonstrates the great potential of MOFs as a next generation platform for solar fuel conversion.

Full text of DOI:10.1039/d0dt03500a

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Iron(III)-bipyridine incorporated metal–organic
frameworks for photocatalytic reduction of CO2
with improved performance†
Cite this: DOI: 10.1039/d0dt03500a
a
b
a
a
b
Yuan-Ping Wei,‡ Sizhuo Yang,‡ Peng Wang, Jin-Han Guo, Jier Huang * and
a
Wei-Yin Sun
*
Metal–organic frameworks (MOFs) represent an emerging class of platforms to assemble single site
photocatalysts for artificial photosynthesis. In this work, we report a new CO reduction photocatalyst
UiO-68-Fe-bpy) based on a robust Zr(IV)-MOF platform with incorporated Fe(bpy)Cl (bpy refers to the
’-methyl-[2,2’-bipyridine] moiety) via amine–aldehyde condensation. We show that this hybrid catalyst
can reduce CO to form CO under visible light illumination with excellent selectivity and enhanced activity
2
(
3
4
2
with respect to its parent MOF and corresponding homogeneous counterpart. Using steady state and
transient absorption (TA) spectroscopy, we show that the enhanced photocatalytic activity of UiO-68-Fe-
Received 9th October 2020,
Accepted 30th November 2020
bpy is attributed to the elongated excited state lifetime of Fe(bpy)Cl
UiO-68-NH platform. This work demonstrates the great potential of MOFs as a next generation platform
for solar fuel conversion.
3
after being incorporated to the
DOI: 10.1039/d0dt03500a
2
rsc.li/dalton
and stability is necessary in order to convert CO
chemical fuels.
2
to useful
Introduction
With soared energy demand and intensified global warming,
the search for effective conversion of CO into valuable chemi- ion or metal cluster nodes interconnected with organic
cals is an important but challenging task.
nature, chemists express strong interest in the systems that and have been explored for a variety of potential applications
Metal–organic frameworks (MOFs), constructed from metal
2
1
–3
Inspired by ligands, are a class of crystalline micro-/meso-porous materials
1
5,16
17–19
can photocatalytically reduce CO
2
to energy carrier products including drug delivery,
gas storage,
In particular, MOFs have shown great
include homogeneous transition-metal complexes, hetero- potential for photocatalytic CO2 reduction by incorporating
chemical separ-
4
20–26
such as CH , CH OH, HCOOH and CO. These systems ation and catalysis.
4
3
5
6
27–37
geneous metal-incorporated zeolites,
and inorganic catalytic sites into a porous and robust MOF matrix.
However, the homogeneous transition- Among the systems reported, one valuable strategy is to incor-
metal complexes are often not recyclable although many of porate organometallic species into MOFs by post-synthetic
them show high efficiency in reducing CO in the presence of modification (PSM), where the organometallic species serve as
7
–12
semiconductors.
2
1
3
photosensitizers. On the other hand, the metal-incorporated both a photosensitizer and a CO -reduction catalyst such that
2
zeolites and inorganic semiconductors are robust but often the photocatalytic system can take advantage of both hom-
1
4
38–42
suffer poor efficiency for the photocatalytic reduction of CO
2
.
ogenous and heterogeneous catalysts.
For example, mole-
Therefore, developing new photocatalysts with high efficiency cular complexes based on noble metals such as platinum,
ruthenium and iridium have been incorporated into MOFs,
which demonstrate catalytic activity for photocatalytic CO
2
a
Coordination Chemistry Institute, State Key Laboratory of Coordination Chemistry,
43–45
reduction.
Molecular complexes based on cost effective
School of Chemistry and Chemical Engineering, Nanjing National Laboratory of
Microstructures, Collaborative Innovation Center of Advanced Microstructures,
Nanjing University, Nanjing 210023, China. E-mail: sunwy@nju.edu.cn
and earth abundant transition metals including iron, cobalt,
manganese and nickel have also been reported in recent
b
46–48
Department of Chemistry, Marquette University, Milwaukee, Wisconsin 53201, USA.
years.
While these examples demonstrate the great poten-
E-mail: jier.huang@marquette.edu
tial of MOFs as single site photocatalysts for CO reduction,
the selectivity and efficiency of most of these systems need to
be improved.
Among the reported MOFs, UiO-series (UiO = University of
Oslo) have high thermal and chemical stability, which are
2
†
Electronic supplementary information (ESI) available: Synthesis and character-
ization of UiO-68-NH
2
and UiO-68-Fe-bpy, photocatalytic experiments, UV-Vis,
1
MS, IR, HNMR, XPS, TGA, EXAFS, femtosecond TA spectra, XRPD patterns, SEM
images and gas adsorption experiments. See DOI: 10.1039/d0dt03500a
‡
These authors contributed equally.
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essential requirements of photocatalysts. The reports on modi- lected by centrifugation and washed with abundant MeOH
−
1
fied UiO-68 for photoreduction are far fewer than those on (three times) and dried in a vacuum. IR (KBr pellet, cm
UiO-66 and UiO-67.
)
4
9,50
However, the larger cavity and better 3429 (m), 1700 (m), 1600 (vs), 1533 (m), 1420 (s), 1182 (w),
CO adsorbability imply that UiO-68 could be the ideal plat- 1104 (w), 785 (m), 713 (w), 657 (m), 567 (w), 467 (w).
2
form for the photoreduction of CO . On the other hand, iron
2
complexes have been shown to be efficient molecular catalysts
5
1
Photochemical CO reduction experiments
2
for CO
idine ligand can serve as an electron reservoir and is expected The photocatalytic activities of all samples were carried out in
to promote proton-coupled electron transfer (PCET) reactions batch-type reaction system (CEL-SPH2N-D9, CeAulight,
2
-to-CO conversion. Besides, the redox active 2,2′-bipyr-
a
5
2
by combining with iron. Therefore, it is meaningful that the China) equipped with a homemade photoreactor and a home-
Fe-bpy complex and UiO-68 MOF are selected to assemble an made circulating water filter to avoid the photothermal effect.
efficient photocatalyst for CO
2
reduction.
2 mg of sample was dispersed in 1 mL of acetone and ultra-
Herein, we report a new MOF-based single site photo- sonicated for 5 minutes, and then the suspension was uni-
catalyst, i.e. UiO-68-Fe-bpy (bpy refers to 4′-methyl-[2,2′-bipyri- formly dispersed onto a glass fiber film (Φ 25 mm, 0.22 μm
dine] moiety), synthesized based on UiO-68-NH
2
and 4′- pore diameter) drop by drop under an infrared lamp. The film
methyl-[2,2′-bipyridine]-4-carbaldehyde (bpy-CHO) using an was activated in a vacuum at 120 °C for 12 h. Then the film
amine–aldehyde condensation and post-synthetic chelated was placed in the reaction cell, and 2 mL of triethanolamine
metalation with FeCl
nation, we show that UiO-68-Fe-bpy can reduce CO
CO with excellent selectivity and demonstrate much better the reaction system (no O or N could be detected), 80 kPa of
3
(Scheme 1). Under visible light illumi- (TEOA) was added into the reactor, and the reactor was main-
2
to form tained at 50 °C using an oil bath. After complete evacuation of
2
2
performance than its homogeneous counterpart with the pure CO
same Fe(bpy)Cl moiety. The enhanced photocatalytic per- airtight system. A 300 W xenon arc lamp (Sirius-300P, Zolix
formance of UiO-68-Fe-bpy can be attributed to the prolonged Instruments Co., Ltd, China) with a 400–780 nm filter
excited state lifetime of the Fe(bpy)Cl moiety in UiO-68-Fe- (CeAulight, China) was used as the light source. The optical
bpy as confirmed by femtosecond transient absorption (TA) power density was set at 200 mW cm− , which was measured
2
gas (99.999%, Tianhong Gas) was injected into the
3
3
2
spectroscopy.
using an optical power meter (CEL-NP2000-2, CeAulight,
China). The gas products were monitored using gas chromato-
graphy (GC-9860, Luchuang Instrument, China), equipped
with a TCD to measure H , and an FID detector with a
methane conversion oven to convert CO and CO into CH
2 4
2
Experimental
Synthesis of UiO-68-Fe-bpy
before entering the FID. The gas was separated by a 1 m
TDX-01 column with N (99.999%) as the carrier gas, and the
2
The as-synthesized UiO-68-NH
pended in MeOH (5 mL) under ultrasound conditions, and
then bpy-CHO (0.81 mmol, 0.16 g) was dissolved in MeOH
2
(0.03 mmol, 0.08 g) was sus-
temperatures for the TCD, FID, column and methane converter
were 100, 150, 100 and 360 °C, respectively. The retention
times of CO, CH4 and H2 were 1.290, 2.540 and 0.578 min,
respectively. The calibration curves for gases were established
separately.
(5 mL) followed by the addition of acetic acid (HOAc) (25 μL).
The mixture was refluxed at 70 °C for 24 h. Afterwards, the pre-
cipitate was collected by centrifugation and washed with abun-
dant MeOH (three times) and dried in air. The new MOF par-
ticles were dispersed in MeOH (10 mL), and then FeCl3
Femtosecond transient absorption (TA) spectroscopy
(1.54 mmol, 0.25 g) was added, and the mixture was heated at
The Femtosecond TA setup is based on a regenerative ampli-
fied Ti-Sapphire laser system (Solstice, 800 nm, <100 fs
FWHM, 3.5 mJ per pulse, 1 kHz repetition rate). The tunable
7
0 °C for an additional 12 h. The resulting precipitate was col-
(
235–1100 nm) pump was generated by TOPAS from seventy-
five percent of the Solstice output and was chopped at 500
Hz. The remaining twenty-five percent of the Solstice output
was used to generate white light in a sapphire crystal
(
420–800 nm) in a Helios ultrafast spectrometer (Ultrafast
Systems LLC). To make thin films for TA measurements, 1 mg
of sample was added into 0.5 mL of Nafion (5% w/w in water
and 1-propanol) in a glass vial. The hybrid was stirred for 2 h
and then distributed uniformly on a piranha-etched glass.
The samples were dried in air. The film samples were con-
stantly translated in the case of heating and permanent
degradation. The pump power at 440 nm is 0.2 mW for all
measurements.
Scheme 1 Synthesis of UiO-68-Fe-bpy.
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The average integration ratio of the signals that originated
Results and discussion
Synthesis of UiO-68-Fe-bpy
from bpy-CHO and NH -TPDC as observed for the digested
2
UiO-68-bpy sample (Fig. S2b, ESI†) is 0.62, implying that the
Briefly, the yellowish crystalline UiO-68-NH
2
was obtained by a functionalization yield of bpy-CHO is 62% agreeing with the
solvothermal reaction of amino-triphenyldicarboxylic acid ICP result of the Fe–Zr ratio, which further confirms the suc-
NH -TPDC) and ZrCl in DMF in the presence of HOAc as a cessful combination of bpy-CHO with UiO-68-NH
modulator. UiO-68-NH was then treated with bpy-CHO Thermogravimetric analysis (TGA) was performed to examine
excess) and HOAc in MeOH at 70 °C for 24 h to afford UiO-68- the thermal stability of the frameworks. The TG curves of
bpy via an amine–aldehyde condensation. UiO-68-Fe-bpy was UiO-68-NH and UiO-68-Fe-bpy show that they have similar
(
2
4
2
.
5
3,54
2
(
2
achieved by metalation of UiO-68-bpy using FeCl
The powder X-ray diffraction (PXRD) patterns of the as-syn-
thesized UiO-68-NH
well with the simulated pattern of UiO-68-NH
ing that the crystallinity of UiO-68-NH is retained during the thesized by the reaction of dimethyl ester of amino-triphenyl-
3
(Scheme 1).
thermal stability (Fig. S3, ESI†).
In addition to the bulk structure, the local geometry of the
2
and post-modified UiO-68-Fe-bpy agree Fe center was examined by steady-state X-ray absorption
(Fig. 1a), imply- spectra (XAS). The control sample was TPDC-Fe-bpy syn-
2
2
2 3
PSM reaction. The SEM images show that UiO-68-NH particles dicarboxylic acid (amino-TPDC), bpy-CHO and FeCl using the
are discrete octahedral crystals with dimensions of 2 to 3 μm method for the preparation of UiO-68-Fe-bpy (Fig. S4, ESI†).
per side (Fig. 1b). The post-modified UiO-68-Fe-bpy shows a Fig. 2a shows the XANES (X-ray absorption near edge structure)
similar morphology to its parent MOF (Fig. 1c), which further spectra of UiO-68-Fe-bpy and TPDC-Fe-bpy measured at the Fe
supports that UiO-68-NH is chemically stable during the PSM. edge. The XANES spectrum of the reference sample (FeCl with
2 3
The inductively coupled plasma (ICP) measurement of the octahedral geometry) is also shown in Fig. 2a. The main
Zr : Fe ratio of the digested UiO-68-Fe-bpy is 1 : 0.59 (calcd feature observed at 7121.13 eV is assigned to the dipole
1
: 1), where the functionalization yield is 59%, which is allowed 1s–4p transition. Additionally, the spectrum of FeCl
3
5
4
reasonable as reported previously. We tried by varying the exhibits a sharp pre-edge feature corresponding to the quadru-
reactant ratio, reaction temperature and time to improve the pole allowed 1s–3d transition and is forbidden for a perfect
functionalization yield, and finally got the highest yield of centrosymmetric octahedral geometry, which suggests the dis-
5
9%. The limited modification yield may be ascribed to the torted octahedral geometry of the Fe center in FeCl . The
3
heterogeneous PSM reaction.
XANES spectrum of TPDC-Fe-bpy is similar to that of FeCl
3
.
The FT-IR spectra of UiO-68-NH
range of 4000–400 cm shown in Fig. S1, ESI† demonstrate UiO-68-Fe-bpy and TPDC-Fe-bpy, including the intensity
2
and UiO-68-Fe-bpy in the However, there are some differences between the spectra of
−
1
the formation of a covalently modified compound. The FT-IR change of the so called B-feature (dashed line in Fig. 2a), and
shows bands at 830 and 1580 cm−
1
the different shape and intensity of the absorption peak at
spectrum of UiO-68-NH
2
corresponding to the primary amino in- and out-of-plane region C compared to those of FeCl and TPDC-Fe-bpy. These
3
5
5
bending vibrations, respectively. After the amine–aldehyde differences were established as a structure sensitive multiple
5
7,58
condensation, the intensity of these characteristic amino scattering resonance previously,
implying that the Fe
peaks decreases. In contrast, the intensity of the peak at center in the UiO-68-Fe-bpy sample likely adopts a more dis-
610 cm− associated with the NvC stretching vibration torted octahedral geometry. Moreover, the edge energy of both
1
1
5
6
2
increases. These changes imply that UiO-68-NH was suc- the samples, as shown in the first derivative spectra (the inset
cessfully modified by amine–aldehyde condensation to gene- of Fig. 2a), is in good agreement with the FeCl3 reference
1
rate UiO-68-bpy. In addition, H NMR spectral measurements sample, suggesting that the Fe center retains the +3 oxidation
in DMSO-d
6 2
for digested UiO-68-NH (Fig. S2a, ESI†) and state after incorporation.
UiO-68-bpy (Fig. S2b, ESI†) with aqueous HF were performed.
To gain more insight into the coordination environment,
we quantitatively analyzed the extended X-ray absorption fine
Fig. 2 (a) XANES and (b) Fourier-transform EXAFS spectra of UiO-68-
Fe-bpy and TPDC-Fe-bpy at the Fe K-edge. The inset in (a) illustrates the
first derivative of the XANES edges. The solid lines and open circles in (b)
are experimental and fitted results, respectively.
Fig. 1 (a) PXRD patterns of simulated and measured UiO-68-NH
2
and
UiO-68-Fe-bpy. SEM images of (b) UiO-68-NH and (c) UiO-68-Fe-bpy.
2
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structure (EXAFS) spectra (Fig. S5, ESI†) using the FeFF model. absorb visible light (Fig. S14, ESI†). Indeed, the color change
Fig. 2b shows the EXAFS spectra of UiO-68-Fe-bpy and from yellow to dark red after metalation was observed due to
TPDC-Fe-bpy in R-space. Compared to the spectrum of the coordination of Fe(III) with N atoms of bpy and the pres-
III
TPDC-Fe-bpy, the intensity of the peak corresponding to the ence of ligand-to-metal (bpy→Fe ) charge transfer (LMCT),
Fe-ligand first shell (e.g. Fe–N and Fe–Cl) in the spectrum of from which the corresponding electronic edge energy (E ) of
g
6
0
UiO-68-Fe-bpy is reduced, which can be either attributed to the LMCT transitions can be estimated. UV-Vis diffuse-reflec-
decrease in the Fe coordination number or the increase in dis- tance spectra (DRS) show that the synthesized UiO-68-NH (ca.
2
order. It is unlikely that the coordination number of Fe 2.67 eV) and UiO-68-Fe-bpy (ca. 1.87 eV) have a similar absorp-
changes. As a result, the decreased amplitude for the Fe-ligand tion band in the region of 250–380 nm (Fig. S14 and S15,
first shell peak can be attributed to the increased disorder of ESI†), which can be ascribed to the absorption of the Zr–O
6
1
the Fe coordination environment in UiO-68-Fe-bpy, consistent cluster. However, UiO-68-Fe-bpy exhibits an extra absorption
with the XANES results. In addition, slight reduction of the band with the edge extending to around 700 nm in the visible
bond distance of the Fe-ligand first shell in UiO-68-Fe-bpy was light region, in agreement with the dark red color. These
observed compared to that of TPDC-Fe-bpy. These results are results suggest that UiO-68-Fe-bpy exhibits a better absorption
further supported by the fitting results. The best fitting to the capacity of visible light after the PSM.
EXAFS experimental data is also shown in Fig. 2b with the
fitting parameters being listed in Tables S1 and S2, ESI,† from gated by cyclic voltammetry (Fig. S16, ESI†) in a 0.1 M
which we found that the Fe–N distance of UiO-68-Fe-bpy n-Bu NPF /DMF supporting electrolyte saturated with CO
2.07 Å) is reduced by 0.04 Å compared to that of TPDC-Fe-bpy When the electrochemical potential is scanned in the reduc-
2.11 Å). In contrast, while the Fe–Cl distance remains the tive direction, three distinct redox waves at −0.012 (process 1),
The electrochemical properties of TPDC-Fe-bpy were investi-
4
6
2
.
(
(
same (2.18 Å) for both samples, the Debye–Waller factor −0.626 (process 2) and −0.971 (process 3) V versus SCE were
increases slightly in the UiO-68-Fe-bpy sample with respect to observed, which can be attributed to Fe(III/II), Fe(II/I) and Fe(I/
6
2
TPDC-Fe-bpy, consistent with the assignment of the increase 0). As shown in Fig. S17, ESI,† compared to a N atmosphere,
2
in disorder. These results together suggest that the coordi- the redox couple Fe(II/I) becomes irreversible, which suggests
nation geometry for the Fe center in UiO-68-Fe-bpy is largely that Fe(I) can have interaction with CO
similar to that of TPDC-Fe-bpy except for the slightly reduced excited state potential (E*) of UiO-68-Fe-bpy was estimated
Fe–N distance and increased disorder for Fe–Cl. using the equation, E* = E(FeII/I) + E , where E(FeII/I) is the
X-ray photoelectron spectroscopy (XPS) was employed to ground state potential and E is the above-mentioned elec-
2
(Scheme S1†). The
g
g
explore the elemental composition of UiO-68-Fe-bpy (Fig. S6, tronic edge energy. E* was calculated to be about 1.24 V, imply-
ESI†). The peaks at 710.1, 531.6, 400.1, 284.5 and 184.5 eV ing that the electrons can be transferred from UiO-68-Fe-bpy to
6
3
correspond to the Fe 2p, O 1s, N 1s, C 1s and Zr 3d, respect- CO
2
.
The results together suggest that UiO-68-Fe-bpy has the
ively. Furthermore, the peak-fitted N 1s core-line spectrum of appropriate energetics and redox properties to catalyze the
UiO-68-Fe-bpy (Fig. S7, ESI†) shows the presence of three photocatalytic reduction of CO to form CO.
2
resolved peaks at 400.4, 399.6 and 398.9 eV, corresponding to
2
Photochemical CO conversion experiments
the imine (–CvN–), amino (–NH ) and pyridine N, respect-
2
5
5
ively. Furthermore, the energy dispersive spectroscopy (EDS) With the successful modification of Fe(bpy)Cl
spectra (Fig. S8, ESI†) of UiO-68-Fe-bpy disclosed the presence MOF UiO-68-NH and its feasibility as a photocatalyst for CO
of metal element and the elemental mapping indicates that Fe reduction, we evaluated the activity and selectivity of UiO-68-
was modified uniformly throughout the MOF. The atomic ratio Fe-bpy for photocatalytic CO reduction in the presence of
of Zr and Fe from EDS is estimated to be 1 : 0.51, which is TEOA as an electron donor upon visible light irradiation. As
3
into the robust
2
2
2
close to the ICP result.
shown in Fig. S18, ESI,† the FID, which monitors CO and CH4,
The gas adsorption properties of the guest-free UiO-68-NH
2
showed that CO was detected with a retention time of about
and UiO-68-Fe-bpy were examined to ensure the architectural 1.290 min while no CH
4
(retention time: 2.540 min) was
stability and porosity after PSM. N and CO adsorption iso- detected. On the other hand, the TCD, which monitors H2
2
2
2
therms are shown in Fig. S9–S13, ESI.† As expected, the results generation, showed that no H (retention time: 0.578 min) was
1
indicate a decrease in porosity after PSM (Fig. S13, ESI†) with detected. Besides, the H-NMR spectrum was used to detect
Brunauer–Emmett–Teller (BET) surface areas of 2589.7 and the liquid product of the photocatalytic reaction and the
2
−1
1
2
372.1 m g for UiO-68-NH (Fig. S9, ESI†) and UiO-68-Fe- results show that no liquid product is produced in the reaction
bpy (Fig. S10, ESI†), respectively, which can be attributed to system (Fig. S19, ESI†). These results together suggest that the
the introduction of an Fe(bpy)Cl moiety to the functional selectivity of CO2 photoreduction to CO is 100%. Such high
3
5
9
ligand of UiO-68-NH
With the PSM of the Fe(bpy)Cl
is expected that UiO-68-Fe-bpy can improve the absorption electron and proton donors and is oxidized to 2-(bis(2-hydro-
2
.
selectivity may be due to the solvent-free photocatalytic reac-
moiety into UiO-68-NH , it tion system as reported previously, in which TEOA acts as
2
64
3
6
5
capacity of visible light compared to UiO-68-NH
absorption region of TPDC-Fe-bpy extends to 750 nm, which experiment with the Xe lamp off shows that CO
means that TPDC-Fe-bpy (ca. 1.69 eV) has a great capacity to occur, suggesting that this reaction is light-driven. When the
2
since the xyethyl)amino)acetaldehyde. As shown in Fig. 3a, the control
2
to CO did not
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photocatalytic reactions, suggesting that irradiation does not
lead to the photodegradation of the catalyst within 18 hours.
The results of each cycle obtained by ICP are shown in
Table S4, ESI.†
Femtosecond transient absorption (TA) spectroscopy
characterization
While the capability of UiO-68-Fe-bpy MOFs as efficient photo-
, TPDC-Fe-bpy catalysts for CO2 reduction has been successfully demon-
Fig. 3 (a) Photocatalytic CO evolution of UiO-68-NH
2
and UiO-68-Fe-bpy (2 mg photocatalyst). (b) Recycling performance of
UiO-68-NH and UiO-68-Fe-bpy.
strated by the above photocatalytic experiments, it is necessary
to study the excited state (ES) dynamics in order to figure out
the specific roles that MOFs play in catalytic reactions.
Femtosecond TA spectroscopy was used to investigate the ES
2
system was operated in the presence of UiO-68-NH2 under dynamics of UiO-68-Fe-bpy MOFs. Fig. 4a and b shows the TA
visible-light illumination, CO was split into CO at a reaction spectra of TPDC-Fe-bpy on an Al O film and UiO-68-Fe-bpy
2
2 3
−
1
evolution of 10.01 μmol g for six hours. Compared to the MOFs following 400 nm excitation, respectively. The Al O film
2
3
poor activity of UiO-68-NH , UiO-68-Fe-bpy exhibits much was used as a control to illustrate the intrinsic effect of the
2
better photocatalytic efficiency (about 10 times that of UiO-68- porous surface on ES dynamics. As shown in Fig. 4a, the spec-
NH
2
, Fig. 3a), suggesting that the incorporated Fe(bpy)Cl
3
trum of TPDC-Fe-bpy exhibits a negative band centered at
plays a critical role in improving its photocatalytic activity. It is 570 nm, which can be assigned to the ground state bleach
noteworthy that the photocatalytic efficiency of UiO-68-Fe-bpy (GSB). At an early time period (<2 ps), two positive features
is better than those of some typical MOFs and inorganic identified as ES absorption were observed at 460–500 nm and
materials (Table S3, ESI†). Moreover, the photocatalytic per- 650–800 nm regions. The ES signal at 460–500 nm was quickly
formance of the physical mixture of TPDC-Fe-bpy and UiO-68- replaced by a negative feature, which can be attributed to the
2
NH was evaluated under the same catalytic conditions. As overlap of ES absorption with GSB and the faster decay of ES
shown in Fig. 3a, the system stopped generating CO after three than the GSB recovery. Compared to TPDC-Fe-bpy, UiO-68-Fe-
hours upon visible-light illumination, while UiO-68-Fe-bpy can bpy shows similar GSB centered at 600 nm, which is consistent
steadily generate CO for at least 6 hours, suggesting that with the UV-Vis ground state absorption spectrum. However,
UiO-68-NH2 can improve the stability of Fe(bpy)Cl , and the distinct differences were observed between the GSB recovery
3
CO
2
reduction activity of UiO-68-Fe-bpy is not a simple sum of kinetics of two samples. As shown in Fig. 4c, the recovery of
activities. The covalent linkage of GSB in UiO-68-Fe-bpy is slower than TPDC-Fe-bpy on the Al O
Fe-bpy and UiO-68-NH
2
2
3
CvN between UiO-68-NH2 and bpy could provide the ideal film, which suggests that the recovery of ES molecules to the
stability of UiO-68-Fe-bpy after the PSM, which can prevent the ground state was inhibited in UiO-68-Fe-bpy and thus a longer
Fe complex from decomposing under visible light illumina- lived ES of the Fe complex in UiO-68-Fe-bpy. One possible
6
6
tion.
A proposed mechanism for photocatalytic CO2 explanation for the elongated ES lifetime is that the rigid
reduction over the UiO-68-Fe-bpy under visible light is shown framework plays an important role in stabilizing the charged
in Scheme S1.† Furthermore, the recycling experiments show separated excited state and suppressing the undesired charge
that a negligible change was observed in the CO-production recombination, which has also been observed in previously
6
7
yield during the three runs for 18 h (Fig. 3b). PXRD patterns published work. Therefore, the prolonged ES lifetime of a
Fig. S20, ESI†) and SEM results (Fig. S21, ESI†) confirm that photocatalyst is beneficial for charge transfer from the sacrifi-
(
the crystallinity and structural integrity were retained after the cial donor to the catalyst or from the catalyst to reduce CO2,
Fig. 4 Femtosecond TA spectra of (a) TPDC-Fe-bpy and (b) UiO-68-Fe-bpy. (c) Comparisons of the ground state bleach (GSB) recovery between
TPDC-Fe-bpy and UiO-68-Fe-bpy.
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which well explains the enhanced catalytic performance of
6 Y. Huang, Z. Guo, H. Liu, S. Zhang, P. Wang, J. Lu and
UiO-68-Fe-bpy with respect to TPDC-Fe-bpy for CO reduction.
Y. Tong, Adv. Funct. Mater., 2019, 29, 1903490–1903499.
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611.
2
Conclusions
1
In summary, we have successfully synthesized a MOF photo-
catalyst by incorporating Fe(bpy)Cl using PSM, which shows
an enhanced light-absorbing ability and undergoes highly
9
S. Habisreutinger, L. Schmidt-Mende and J. Stolarczyk,
3
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0 Y. Tamaki, T. Morimoto, K. Koike and O. Ishitani, Proc.
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efficient CO
visible-light irradiation. Using transient absorption spec-
troscopy, we show that Fe(bpy)Cl in UiO-68-NH has longer-
2
reduction to form CO with 100% selectivity under
3
2
lived ES and slower recovery of GSB, which is beneficial for
boosting the efficiency of photocatalytic CO production. In
addition, the robust nature of UiO-68 ensured the stability of
the photocatalyst (UiO-68-Fe-bpy), enabling at least three
cycles (18 h) of the photocatalytic process. This work not only
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2
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perfect selectivity for CO generation but also elucidates that
PSM is an effective strategy to introduce molecular metal com-
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photocatalysis.
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4
There are no conflicts to declare.
9 R. Matsuda, R. Kitaura, S. Kitagawa, Y. Kubota,
R. Belosludov, T. Kobayashi, H. Sakamoto, T. Chiba,
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0 J. Lang, Q. Xu, R. B. Yuan and B. F. Abrahams, Angew.
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We gratefully acknowledge the National Basic Research
Program of China (grant no. 2017YFA0303504) for financial
support of this work. This work was also supported by a
Project Funded by the Priority Academic Program
Development of Jiangsu Higher Education Institutions. The
use of the Advanced Photon Source at Argonne National
Laboratory was supported by the U. S. Department of Energy,
Office of Science, Office of Basic Energy Sciences, under Award
No. DE-AC02-06CH11357. S. Y. acknowledges the John J. Eisch
fellowship during the 2018–2019 academic year.
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