Journal of Catalysis 413 (2022) 588–602

Contents lists available at ScienceDirect

Journal of Catalysis
journal homepage: www.elsevier.com/locate/jcat

Anchoring and reactivation of single-site Co–porphyrin over TiO2 for theefficient photocatalytic CO2 reduction q
Chao Zhang a, Jiwon Yang a, Keisuke Hara a, Rento Ishii a, Hongwei Zhang a, Takaomi Itoi b, Yasuo Izumi a,⇑ab

Department of Chemistry, Graduate School of Science, Chiba University, Yayoi 1-33, Inage-ku, Chiba 263-8522, Japan
Department of Mechanical Engineering, Graduate School of Engineering, Chiba University, Yayoi 1-33, Inage-ku, Chiba 263-8522, Japana r t i c l e

i n f oArticle history:
Received 22 March 2022
Revised 6 July 2022
Accepted 6 July 2022
Available online 16 July 2022
Keywords:
CO2 photoreduction
CobaltPorphyrin
TiO2
Reactivationa b s t r a c t
Porphyrin and its derivatives have been often used for electrochemical and photocatalysis combined with
first-row transition metals for sustainable applications. Cobalt–tetrakis(4-carboxyphenyl)porphyrin (Co–
TCPP) was covalently anchored onto TiO2. Under CO2, H2, and UV–visible light irradiation, Co–TCPP
anchored on TiO2 produced CO at a rate of 2.5 lmol h1 (using 100 mg of catalyst and UV–visible lightof 118 mW cm2) when the Co–TCPP content was 2.5 wt %; however, the CO formation rate dropped to
0.3 lmol h1 (using 100 mg of catalyst and the same intensity of light). The photoactivity drop to 12 %was solved by removing the hydroxy ligands coordinated to the central Co ions through ethanol treatment for 5 min of the Co–TCPP–TiO2 photocatalysts. This resulted in an increased CO photoformation rate
to 7.3 lmol h1 (using 100 mg of catalyst and the same intensity of light) because the dispersion of the Jand/or H-aggregates to the isolated Co–TCPP unit proceeded more with ethanol treatment and under UV–
visible light. Conversely, to use ethanol as reductant was not effective because it reduced and deactivated
Co–TCPP.
Ó 2022 Elsevier Inc. All rights reserved.

1. Introduction
Photocatalytic conversion of CO2 into renewable energy sources
or high value-added compounds using homogeneous or heterogeneous catalysts is a feasible solution to simultaneously addressenergy shortage and environmental pollution [1-3]. The conversion
of CO2 into methane, methanol, and CO, among others, has beenreported using composites of noble metals and semiconductors[4-6], metal complexes [7], and metal–organic frameworks [8].
Among various catalysts, semiconductors, especially TiO2, havebeen extensively studied because of their inexpensive price, stability, and superior photooxidation activity, often combined with
noble metal nanoparticles or metal complexes [9].
Metal complexes are usually applied as homogeneous catalysts,
allowing precise control of the catalytic properties. Nevertheless,they suffer from quick deactivation within a few hours of photoreaction [10,11]. Accurate molecular design of metal complexes andfurther appropriate combination with semiconductors enablehigher catalytic activity while keeping the robust structure.
Although various novel metal complex–semiconductor compositesq

Dedicated to Takeo Izumi (1936–2022).

⇑ Corresponding author.

E-mail address: yizumi@faculty.chiba-u.jp (Y. Izumi).
https://doi.org/10.1016/j.jcat.2022.07.006
0021-9517/Ó 2022 Elsevier Inc. All rights reserved.

have been designed and showed high catalytic activities [12-14],
the chemical environment around the active single atom site,
e.g., the coordination around the active site and the adsorption ofreactants, has been rarely studied as well as the time-course monitoring of isotope-labeled 13C productions from 13CO2 under UV–
visible light irradiation [4-6].
In this study, detailed active structure of single Co site and thereactivation were intensively studied as the central problems forsupported/unsupported molecular photocatalysts [1,12-14]. A
cobalt–tetrakis(4-carboxyphenyl)porphyrin (Co–TCPP) complex
and a TiO2 semiconductor were combined to prepare a photocatalyst that exhibits high activity of reducing CO2 to CO. The site coordination and electronic structure of the active Co sites weremonitored via UV–visible absorption spectroscopy and extended
X-ray absorption fine structure (EXAFS), and the intermediate surface species during CO2 reduction were monitored via Fouriertransform infrared (FTIR) spectroscopy.
This study mainly evaluated the reaction in Eq. (2) that follows
water photooxidation reaction (Eq. (1)) [4-6,15,16]. The efficientphotoreduction of CO2 via Eq. (2) proceeded on a Co–TCPP–TiO2
composite. Overall reaction of Eq. (3) was also tried using wateras reductant. Nevertheless, the rate decreased to 12 % of that using
a fresh photocatalyst after 9 h of photoreaction using H2 as alreadyreported in the literature as a typical weak point of metal–
porphyrin electrochemical catalysts [17-20] and photocatalysts

Journal of Catalysis 413 (2022) 588–602

C. Zhang, J. Yang, K. Hara et al.

[21-25]. To overcome the drawback, photocatalyst reactivation
was attempted and the overall reaction of ‘‘artificial photosynthesis” was tested to form CO as indicated in Eq. (3), which is the summation of Eqs. (1) and (2). Assuming a system of constant pressure:

H–TCPP (2.5 wt %)–TiO2 was prepared using a similar procedure
to that of Co–TCPP–TiO2. Similarly, most of H–TCPP was supported
on TiO2 after the filtration and washing. The color of obtained powder was purple.

2H2 O ! O2 + 2H2 (or 4Hþ + 4e ),2.2. Photocatalytic CO2 reduction

Dr H = 483.64 kJ mol1ð1Þ

CO2 + H2 (or 2Hþ + 2e ) ! CO + H2 O,

The composite photocatalyst (0.100 g) was placed in a quartz
photoreactor (bottom plate area 23.8 cm2, volume 45.5 mL)[1,15,27] connected to a Pyrex glass circulation system(206.1 mL) and evacuated using rotary and diffusion pumps(106 Pa). The photocatalyst was treated in vacuum for 1 h. Then,
2.7 kPa of 13CO2 (13C 99.0 %, 17O 0.1 %, 18O 0.7 %, chemical
purity > 99.9 %; Cambridge Isotope Laboratories, Inc., Tewksbury,
MA, USA) and 20.7 kPa of H2 (purity > 99.99 %) were introduced.
The photocatalyst was irradiated with UV–visible light upwardfrom a 500 W xenon arc lamp (Model OPM2-502; Ushio, Inc.,Tokyo, Japan). The distance between the light exit and the sample
was 20 mm. The maximum temperature reached under the lightirradiation conditions was 328 K [1,15,27]. The light intensity distribution of the xenon (Xe) arc lamp on wavelength was measured
using a spectroradiometer (Model USR45DA, Ushio, Inc., Tokyo,
Japan) at a distance of 20 mm from the UV–visible light source,
and the light intensity at the center of the photocatalyst was measured as 118 mW cm2. Control photocatalytic tests were performed in a similar way using H–TCPP (2.5 wt %)–TiO2, Co–TCPP,
or TiO2 (0.100 g each). A reaction using 13CO2 (2.7 kPa) and H2O(1.7 kPa) was also performed under similar conditions.
In-profile kinetic data were collected as a function of the light
excitation wavelength by inserting a sharp-cut filter (thickness
2.5 mm; Hoya, Tokyo, Japan) at the exit of a lamp house. Model
UV32 (wavelength k > 320 nm), U330 (245 nm < k < 386 nm, andk > 686 nm), and L38 (k > 380 nm) filters were used to pass lightof specific wavelength. The absorbance of filters was noted in Supplementary materials [S1–S3]. Reactivation of the used photocatalysts was tried for Co–TCPP (2.5 wt %)–TiO2 that was used for a
photocatalytic test for 48 h under H2 (20.7 kPa, 21 h), O2(9.7 kPa, 21 h), or ethanol (2.7 kPa, 5 min) all under the irradiationof UV–visible light.
The analytical conditions of online gas chromatography (GC)–mass spectrometry (MS) measurements were described in detailelsewhere [4-6]. Briefly, a packed column of 13X-S molecularsieves (length 3 m, internal diameter 3 mm; GL Sciences, Inc.,Tokyo, Japan) for the CO2 photoreduction tests or polyethene glycol
6000/Flusin P support column (length 3 m, internal diameter
3 mm; GL Sciences, Inc., Tokyo, Japan) for the CO2 exchange tests
was applied in GC–MS (Model JMS-Q1050GC, JEOL, Tokyo, Japan)
using helium (purity > 99.99995 %) as the carrier gas.

Dr H = 41.16 kJ mol1ð2Þ

CO2 + H2 O ! CO + O2 + H2 ,Dr H = 524.80 kJ mol1
ð3Þ

2. Experimental
2.1. Photocatalyst synthesis
4-Formylbenzoic acid (purity > 97 %), pyrrole (purity > 98 %),cobalt(II) chloride hexahydrate (CoCl26H2O, purity > 98 %), andN,N-dimethylformamide (DMF, purity > 99.8 %) were purchasedfrom Sigma-Aldrich, St. Louis, MO, USA. Propionic acid, methanol(purity > 99.8 %), and ethanol (purity > 99.5 %) were purchasedfrom Wako Pure Chemical, Japan. TiO2 [major anatase phase,
purity > 93.4 %, mean particle size 7.4 nm, specific surface area(SA) 308 m2 g1; JRC-TIO-14, Catalysis Society of Japan (CSJ)] andTiO2 (rutile phase, chemical purity > 99 %, mean particle size
15 nm, specific SA 110 m2 g1; JRC-TIO-6, CSJ) were provided by
Ishihara Sangyo Kaisha, Ltd., Osaka, Japan and Sakai Chemical
Industry Co., Ltd, Japan, respectively. JRC-TIO-14 was mostly usedin this study and JRC-TIO-6 was tested as a reference.
H–TCPP compound was synthesized via a previously reported
procedure [26]. 4-Formylbenzoic acid (6.08 g, 40.5 mmol) was dissolved in propionic acid (150 mL), and pyrrole (2.9 mL, 40.5 mmol)
was added dropwise within 20 s. The mixed solution was ultrasonicated (430 W, 38 kHz) for 10 min and was refluxed at 423 K for
2 h. Then, the solution was added to methanol (200 mL) and stirred
at a rate of 2,000 rotations per minute (rpm) for 30 min at 273 K,
resulting in a dark purple precipitate. The precipitate was filtratedusing a polytetrafluoroethene (PTFE)-based membrane filter (poresize 0.1 lm; Omnipore JVWP04700, Millipore, Burlington, MA,
USA) and washed with methanol (200 mL) and deionized water(100 mL, <0.055 lS cm1 supplied by an RFU424TA system, Advantec, Tokyo, Japan). After drying in vacuum at 353 K for 6 h, H–TCPP
was obtained as a purple powder with a yield of 15 %.
The as-prepared H–TCPP (0.24 g, the molecular weight 790.77,
0.30 mmol) and CoCl26H2O (0.36 g, 1.5 mmol) were dissolved in
DMF (15 mL), followed by ultrasonification for 10 min. Then, themixed solution was refluxed at 433 K for 5 h. The red precipitate
was washed with deionized water three times (60 mL each) anddried in vacuum at 333 K for 5 h. The obtained sample was brickcolor powder with a yield of 99.9 % and was denoted as Co–TCPP(molecular weight 847.69).
TiO2 (0.5 g) was suspended in ethanol (60 mL) with stirring at
700 rpm for 10 min. Then, Co–TCPP was added into the suspension
and was kept stirred at 700 rpm for 4 h. The precipitate was filteredusing JVWP04700, washed with ethanol (20 mL), and dried in vacuum at 298 K for 3 h. The Co–TCPP content in the composite wasvaried at 1.0, 2.5, 5.0, and 7.5 wt %. The filtrate was almost colorlessdemonstrating that most of the introduced Co–TCPP was supported on TiO2. The obtained powder was denoted as Co–TCPP–
TiO2. As the Co–TCPP content in the catalyst was increased, thecolor of the catalyst changed from light orange (1.0 wt %), to darkorange (2.5 wt %), and then finally to deep orange (7.5 wt %).

2.3. Characterizations
UV–visible absorption spectra were obtained in diffuse–reflectance (DR) mode for solid samples and in transmission modefor samples dissolved in ethanol in the wavelength range from
200 to 800 nm using a spectrophotometer (Model V-650; JASCO,Tokyo, Japan) equipped with D2 and halogen lamps of wavelength
below and above 340 nm, respectively, and with an integrated ISV469 sphere in DR mode. A formed PTFE plate was used as a reference in diffuse–reflectance mode, and the data were transformed
into corresponding absorbance spectra using the Kubelka–Munkfunction [15,27].
The surface species on the catalysts were monitored using asingle-beam FTIR instrument (Model FT/IR-4200; JASCO, Tokyo,
Japan) equipped with a mercury–cadmium–tellurium–M detector
at a temperature of 77.4 K. A self-supporting disk (diameterU = 20 mm) of Co–TCPP (2.5 wt %)–TiO2 (10 mg) diluted with589

C. Zhang, J. Yang, K. Hara et al.

Journal of Catalysis 413 (2022) 588–602

TiO2 (60 mg) was placed in a quartz photoreaction IR cell equippedwith NaCl windows on both sides. The cell was connected to a
Pyrex glass circulation system also connected to GC–MS to enable
simultaneous monitoring of the surface species and products. The
sample disk was treated in vacuum for 1 h before themeasurements.
FTIR measurements were performed at 298 K in a wave number
range of 4000–650 cm1. The sample (Co–TCPP–TiO2 or TiO2) disk
was irradiated with UV–visible light from a 500 W Xe arc lamp via
a Y-shaped quartz fiber light guide (40-cm-long fiber and 80-cmlong branches, Model 1.2S15-1000F-1Q7-SP-RX; Optel, Tokyo,
Japan). The distance between the fiber light exit and sample disk
was 46 mm. The light intensity at the sample center was 150
mW cm2. The energy resolution of the spectrometer was
1 cm1, and the data accumulation included 128 scans (2 s perscan). The spectrum of the fresh sample treated in vacuum wasused as a background for subtraction from the spectra during reaction tests.
The binding between Co–TCPP and TiO2 was studied using DR
FTIR using KBr pellets. The data were subtracted by the data ofTiO2 to elucidate the change in Co–TCPP before and after combinedwith TiO2.
Transmission electron microscopy (TEM) was performed using
JEM-2100F (JEOL, Tokyo, Japan) equipped with a field emission
gun (acceleration voltage of 200 kV) at the Center for Analytical
Instrumentation. The samples were mounted on a Cu mesh (250meshes per inch) coated with carbon and a copolymer film ofpoly(vinyl alcohol) and formaldehyde (Formvar, Monsanto, St.
Louis, MO, USA). High-angle annular dark-field scanning TEM andhigh-resolution (HR) TEM images were observed. The chemicalcompositions and elemental distributions were analyzed using
energy-dispersive spectra with a Si (Li) detector equipped in
JEM-2100F.
Co K-edge X-ray absorption fine structure (XAFS) measurements and the spectral analyses were described in detail elsewhere[4-6]. In brief, the spectra were measured at 298 K in the transmission mode at the Photon Factory, High Energy Accelerator Research
Organization (Tsukuba, Japan) on a 9C beamline equipped with a Si
(1 1 1) double-crystal monochromator, an Rh-coated focusing bent
cylindrical mirror, and a piezotransducer. The Co K-edge absorption energy was calibrated at 7709.54 eV [28] using the X-ray spectrum of Co metal (thickness 10 lm).
A Co–TCPP (25 wt %)–TiO2 (U = 10 mm, 30 mg) disk was set in a
Pyrex glass reactor equipped with a Kapton film (thickness 50 lm;
Dupont, Wilmington, DE, USA) on both sides for X-ray transmission
using 2.7 kPa of CO2, 20.7 kPa of H2, and UV–visible light from a Xe
arc lamp via branches of a quartz fiber light guide 1.2S15-1000F1Q7-SP-RX. The samples were irradiated under UV–visible lighton the beamline. The distance between the light exit of the quartz
fiber light guide and the sample was 50 mm. In comparison, a sample under argon (Ar) was also measured under similar conditions.
The Co K-edge XAFS spectra for Co–TCPP (2.5 wt %)–TiO2(170 mg) sample in a glass cell equipped with Kapton windows
was measured in fluorescence detection mode using Lytle detector[29].
The obtained Co K-edge EXAFS data were analyzed using an
XDAP software package [30]. Multiple-shell curve-fit analyses
were performed on the Fourier-filtered angular photoelectron
wave number k3-weighted EXAFS data in k- and interatomic distance R-space on the basis of the plane-wave approximation for
amplitude Ai(k), coordination number Ni, backscattering amplitudefi, Debye–Waller factor ri, and mean free photoelectron path k forshell i:Ai ðkÞ ¼

 

Ri
2
f
ðkÞ
2
r
k
þ
; i ¼ Co
j
jexp
i
i
k
kR2i
Ni

The empirical parameters were extracted from the EXAFS data
for the Co metal (thickness 10 lm), CoO powder, and the Co–TCPP
powder. The R and N values of the Co–Co interatomic pair in Co
metal (hexagonal closed packed) were set to 0.2502 nm and 12,
respectively [31]. Those values of the Co–O interatomic pair in
CoO powder (cubic) were set to 0.2131 nm and 6, respectively[32]. Those values of the Co–N interatomic pair in Co–TCPP crystal
were set to 0.196 nm and 4, respectively [13]. The many-body
reduction factor S20 was assumed to be identical for both the sample
and reference.
2.4. Calculations
Density functional theory (DFT) calculations were performed
using Gaussian 09 W [33] through a mixed basis set of lanl2DZ
for the Co metal and 6–31 + G(d) for the other elements (H, C, N,
and O) and the hybrid-GGA functional (U)B3LYP. The most stablemolecular structure and energy of each orbital were obtained on
the basis of structural optimization. The band gap value was evaluated on the basis of the difference between the highest occupied
molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO).
3. Results and discussion
3.1. Photocatalytic CO2 reduction using H2
The photocatalytic tests were performed using Co–TCPP–TiO2
composites, 13CO2, H2, and UV–visible light irradiation instead of
typical sacrificial reagents, e.g., triethanolamine [7,34]. Among
the Co–TCPP (1.0–7.5 wt %)–TiO2 photocatalysts, the maximum
initial 13CO formation was obtained using Co–TCPP (2.5 wt %)–Ti
1O2 at a rate of 17 lmol h1 g1; Fig. 1A). For convecat (1.7 lmol hnience, the value is noted as the rate per catalyst weight. However,the photocatalytic test conditions were commonly using 100 mg ofcatalyst and UV–visible light of 118 mW cm2. Although the linearity is not always guaranteed between catalyst amount and photocatalytic formation rate, especially for catalysis at the interface ofliquid (reactants)–solid (catalyst), we use the unit of lmol h1 g1cat
in this paper based on the linearity between 20 and 100 mg of photocatalyst (Fig. S1) [15,27]. 12CO was also constantly formed at a
rate of 7.1 lmol h1 g1cat (Fig. 1C). The total CO formation rate12
was 25 lmol h1 g1
C ratio in the initially formed CO
cat, andwas 29 % (entry a in Table 1), suggesting that the preadsorbed
12
CO2 from the air on the photocatalyst remained and was photoreduced to 12CO. The formed 13CO/12CO ratio was essentially constant throughout the photocatalytic test (Fig. 1C), while the 12CO2
ratio in gas phase during 13CO2 (0.67 kPa) photoexchange reaction
due to partial decomposition of Co–TCPP was 16 % at the maximum(Fig. S2, Supplementary materials), and the 12CO2 ratio during CO2(2.7 kPa) photoreduction tests using H2 was less. Thus, the products were preferably formed via the 12CO2-preadsorbed site and13CO2- and 12CO2-derived species were equilibrated during CO2
photoreduction [4-6].
The 13CO formation rate increased by 9.1 times in Co–TCPP (2.
5 wt %)–TiO2 compared with Co–TCPP (1.0 wt %)–TiO2 (Fig. 1A,
entries a and b in Table S1), suggesting a synergistic reaction mechanism by the combination of Co–TCPP with TiO2 surface. Conversely, the 13CO formation rate gradually decreased when the
Co–TCPP content increased to >2.5 wt % (Fig. 1A, entries b–e inTable S1).
The 12CO ratio for all the total CO formations rapidly increased
as the photocatalytic activity decreased (Tables 1 and S1) because
preadsorbed 12CO2 from the air preferably adsorbed on the Co–
TCPP active site when the overall photocatalytic CO2 reduction rate

ð4Þ
590

Journal of Catalysis 413 (2022) 588–602

C. Zhang, J. Yang, K. Hara et al.

effects of light by Co–TCPP. The aggregation of porphyrin resulted
in lower photocatalytic activity [35,36].
Next, the dependence on light wavelength was tested for themost active Co–TCPP (2.5 wt %)–TiO2 (Fig. 1B). Under light irradiation of 245 nm < k < 386 nm and k > 686 nm, i.e., UV and mostly IR
light, the 13CO formation rate moderately decreased to 64 % of that
under full UV–visible light irradiation (entries a and aʹ in Table 1).
The 13CO formation rate under light irradiation of k > 320 nm further decreased to 22 % of that under full light (entry a‘‘ in Table 1).
Then, the 13C formation rate under light irradiation of k > 380 nm,
i.e., visible and IR light, decreased to only 6.1 % of that under fulllight irradiation (entry a000 in Table 1) because the band gap ofanatase-phase TiO2 can be excited by light with k < 387 nm [16]and almost all the effective light was filtered (k > 380 nm). Hence,
the UV light with k < 380 nm was essential to 13CO2 photoreduction using Co–TCPP–TiO2 (Scheme 2A). Based on the result and also
the fact that one third of UV light was absorbed in Co–TCPP–TiO2
sample, the quantum yield for the highest 13CO formation rate inthis paper (63 lmol h1 g1cat, Table 2g) was 1.6 % [4,15,16,27].
On the anode of dye-sensitized solar cells, metal complexes
and/or dye molecules provide photoexcited electrons to the conduction band (CB) of semiconductors, e.g., TiO2, and the electrons
are transferred to the cathode [34,37]. As the effects of band gap
excitation of TiO2 on photocatalytic CO2 reduction into CO on photocatalysis was predominant based on the critical dependence of
CO formation rate on the wavelength of excitation light(Table 1a, aʹ, a‘‘, a000 ), two possibilities existed. One is electron flowfrom CB of TiO2 into Co–TCPP and subsequent HOMO–LUMO excitation resulted in CO2 reduction (Scheme 2A). The excited electronsin Co–TCPP should be used for photoreduction of CO2, and photoexcited electrons at CB in TiO2 compensate the holes in HOMO
of Co–TCPP.
The other possibility is CO2 reduction by the electrons from CB
of TiO2, electron flow from LUMO of Co–TCPP to TiO2, hole flowfrom valence band (VB) of TiO2 to HOMO/HOMO–1 of Co–TCPP,
and H2 oxidation at Co–TCPP (Scheme 2A). The second scenario
is difficult to explain the oxygen source at Co–TCPP, critical CO formation rate dependence on the ratio of Co–TCPP (Table S1), andpreferable CO2 adsorption on Co site (see 3.6 XAFS section). Thus,
we propose the first scenario. The activation of H2 would proceed
via hole injection to hydrogen at the interface between Co–TCPP
and TiO2 and the resultant proton combines with hydroxy species
on TiO2 (see 3.4 FTIR section; Scheme 2A).
Using H–TCPP (2.5 wt %)–TiO2 that is free from Co2+ ions, the13
CO formation rate was only 0.022 lmol h1 g1cat (entry b in
Table 1), whereas 13CO was not detected above the detection limit
of GC–MS using Co–TCPP or TiO2 (entries c and d in Table 1),
demonstrating that the composite of Co–TCPP and TiO2 was essential for the photocatalytic formation of 13CO.
Critically, it can be observed from the time-course trend that
there is an initial quick formation of CO and decelerated formation

Fig. 1. Time-course formation of photocatalytic (A–F) 13CO and (C–F) 12CO and thesum of 13CO and 12CO (C–F) during exposure to 13CO2 (2.7 kPa) and H2 (20.7 kPa)
using (A) Co–TCPP (1.0, 2.5, 5.0, and 7.5 wt %)–TiO2 under UV–visible light; (B) Co–
TCPP (2.5 wt %)–TiO2 irradiated under full light and filtered light of k > 320 nm;
k > 380 nm; and 245 nm < k < 386 nm and k > 686 nm; and (C) Co–TCPP (2.5 wt %)–TiO2 under UV–visible light. (D, E) Co–TCPP (2.5 wt %)–TiO2 used for the test in
panel C was treated with ethanol (2.7 kPa) for 5 min and irradiated under UV–
visible light under (D) 13CO2 (2.7 kPa) and H2 (20.7 kPa) and (E) 13CO2 (2.7 kPa). (F)Time-course formation of photocatalytic 13CO and 12CO and the sum of 13CO and
12
CO during exposure to 13CO2 (2.7 kPa) and C2H5OH (2.7 kPa) using Co–TCPP(2.5 wt %)–TiO2 under UV–visible light. Photocatalyst amount was 100 mg and theUV–visible light intensity was 118 mW cm2.

became slower. In contrast, partial decomposition rate of Co–TCPP
irradiated under UV–visible light should be always constant in 10 hof reaction (Fig. S2). The maximum of CO formation rate was at
much lower loading of Co-TCPP (2.5 wt %) for clarity comparedto the total area of Co–TCPP molecules (Scheme 1, 1 nm2 permolecule; 30 wt %) to cover the TiO2 surface, clearly neglecting flatorientation of Co–TCPP to the TiO2 surface and suggesting a specificinteraction of Co–TCPP with TiO2 and also significant blocking

Table 1
Initial CO formation rates during the first 5 h of reaction using Co–TCPP (2.5 wt %)–TiO2, H–TCPP (2.5 wt %)–TiO2, Co–TCPP, and TiO2 (100 mg each) under 13CO2 (2.7 kPa), H2(20.7 kPa), H2O (1.7 kPa), and UV–visible light irradiation. The UV–visible light intensity was 118 mW cm2.
Entry

Catalyst

Reactant

Formation rate (lmol h1 g1cat)

Irradiated light

a
aʹ
a‘‘
a000
a‘‘”
b
c
d

Co–TCPP (2.5 wt %)–TiO2

13

CO2 and H2

13

H–TCPP (2.5 wt %)–TiO2
Co–TCPPTiO2

CO2 and H2O
CO2 and H2

UV–visible
245 nm < k < 386 nm,
k > 686 nm
k > 320 nm
k > 380 nm
UV–visible

13

591

12

CO ratio (mol %)

CO

O2

R CO

17
11

7.1
3.6

–

25
15

29
25

3.8
1.1
1.8
0.022
<0.0014<0.00141.1<0.002
4.9
0.82
0.38
<0.002

4.8
1.1
6.6
0.84
0.38
<0.003

22
<0.19
74
97
>99
–

13

CO

12

<0.005–

C. Zhang, J. Yang, K. Hara et al.

Journal of Catalysis 413 (2022) 588–602

Scheme 1. Synthesis, impregnation on TiO2, and transformation under CO2, H2, and UV–visible light of the Co–TCPP–TiO2 composites.

3.2. Photocatalytic CO2 reduction using H2O

of CO at 7–9 h of reaction for all the Co–TCPP–TiO2 photocatalysts(Co–TCPP 1.0–7.5 wt %, Fig. 1A). Using the Co–TCPP (2.5 wt %)–TiO2
photocatalyst, the initial 13CO formation rate of 17 lmol h1 g1catdecreased to 12 % (2.3 lmol h1 g1cat) after 9 h of photoreaction (entries a and b in Table 2). Analogously, the initial 12CO formation
1 1rate of 7.1 lmol h1 g1gcat)
cat decreased to 8.8 % (0.63 lmol hafter 7 h of photoreaction. We tested the reactivation of used
Co–TCPP (2.5 wt %)–TiO2 at the end of this article.

CO2 photoreduction tests were also performed using Co–TCPP(2.5 wt %)–TiO2 under 13CO2, moisture, and UV–visible light irradiation (Fig. S3A). Good CO formation rate of 6.6 lmol h1 g1cat (entrya‘‘” in Table 1) was confirmed as a part of reaction in Eq. (3), andthe rate was lower by 27 % compared with that using 13CO2 andH2 (entry a in Table 1, Eq. (2)). Nevertheless, the 12CO ratio was592Journal of Catalysis 413 (2022) 588–602

C. Zhang, J. Yang, K. Hara et al.

Scheme 2. Frontier energy levels for the Co–TCPP–TiO2 composites and orbital distribution of LUMO + 1, LUMO, HOMO, and HOMO – 1 [38,39].

Table 2
Initial and deactivated CO formation rates during the first 5 h of reaction using fresh Co–TCPP (2.5 wt %)–TiO2 (100 mg) under 13CO2 (2.3 kPa), H2 (21.7 kPa), and UV–visible lightirradiation (a, b), under 13CO2 (2.3 kPa), C2H5OH (2.7 kPa), and UV–visible light irradiation (c, d), and reactivated rates (e–l) under similar conditions to entries a (e–j) and under13
CO2 (2.3 kPa) and UV–visible light irradiation (k, l) after treatment with H2 (20.7 kPa; e, f), O2 (9.7 kPa; g, h), ethanol (2.7 kPa; i–l), and UV–visible light. The UV–visible lightintensity was 118 mW cm2.
Entry

Test no.

Reactants

Time-course period

Formation rate (lmol h1 g1
cat)
13

a
b
c
d
e
f
g
h
i
j
k
l

First test

13

CO2 + H213

CO2 + C2H5OH

Regeneration test (H2)13

CO2 + H2

Regeneration test (O2)
Regeneration test (C2H5OH)
13

CO2

1–7 h of reaction
15–48 h of reaction
1–3 h of reaction
3–20 h of reaction
1–3 h of reaction
15–48 h of reaction
1–3 h of reaction
15–48 h of reaction
1–3 h of reaction
15–48 h of reaction
1–6 h of reaction
20–26 h of reaction

higher (74 %) because gas-phase 13CO2 was in equilibrium with therelatively stronger CO2 adsorption site on TiO2 [4-6] and the partial
photodecomposition of Co–TCPP to form 12CO2 was also not negligible (Fig. S2). The activation step(s) was slower using moisturecompared with that using H2. Similar to the time course using
H2, the turning point of the CO formation rate was also observed
at 9 h of reaction (Fig. S3A). Unfortunately, O2 was not detected
above the detection limit of GC–MS using Co–TCPP (2.5 wt %)–Ti
O2 (Table 1a‘‘”), indicating oxidative product could not be desorbed
from TiO2 (anatase phase) surface [16]. One of the plausible reactions was.

CO2 + H2 O !CO + OH (surface)

CO

17
2.3
16
1.5
2.3
2.3
<0.0014<0.0014
63
18
8.7
4.6

12

12

CO

RCO

7.1
0.63
26
15
<0.0020.044<0.002
0.018
9.8
2.1
3.3
1.3

25
3.0
42
16
2.3
2.4
<0.003
0.018
73
21
12
5.9

CO ratio (mol %)

29
21
61
90
<0.085
1.8
–
>93
13
10
28
23

phase) were lower than those using Co–TCPP (2.5 wt %)–TiO2 (anatase phase) exclusively used in this paper (Fig. S3), leaving further
catalyst design of Co–TCPP moiety anchored to preferable crystalline face of TiO2 (anatase phase) in the proximity to preferable
crystalline face of TiO2 (rutile phase) in view of effective catalysis(Scheme 2A).

3.3. UV–visible spectra
Fig. 2 depicts UV–visible absorption spectra for H–TCPP compound and Co–TCPP complex in ethanol measured in transmission
mode (panel A) and for each composite with TiO2 measured in DR
mode (panels B, C).
The major peaks centered at 415 and 427 nm in Fig. 2A originated from electronic transition from a1u(p) to eg*(p) states (Soretband) of H–TCPP and Co–TCPP, respectively (Scheme 2). The red
peak shift for Co–TCPP compared to H–TCPP was consistent withthe literature [42]. The a1u(p) state is with C4 symmetry and populates on the N atoms of pyridine ring and connecting C atoms,

ð5Þ

Co–TCPP (2.5 wt %)–TiO2 was also prepared using JRC-TIO-6(rutile phase) and tested for a photocatalytic test using 13CO2(2.7 kPa) and H2O (1.7 kPa). In clear contrast to Fig. S3A, CO andO2 were comparably formed (Fig. S3B) owing to the effective desorption of O2 from rutile-phase TiO2 [40,41]. On the other hand,the photocatalytic rates using Co–TCPP (2.5 wt %)–TiO2 (rutile593

C. Zhang, J. Yang, K. Hara et al.

Journal of Catalysis 413 (2022) 588–602

Fig. 2. UV–visible absorption spectra of (A) H–TCPP and Co–TCPP (1 mg each) dissolved in ethanol (5.6 mL; 0.23 and 0.21 mmol L1, respectively). (B) TiO2, H–TCPP(2.5 wt %)–TiO2, and Co–TCPP (2.5 wt %)–TiO2 composites and (C) Co–TCPP (2.5 wt %)–TiO2 before and after photoreduction test under 13CO2, H2, and UV–visible lightirradiation. CO formation rates using filtered light (entries a–a000 in Table 1) in log scale are also drawn in panel B.

whereas the eg*(p) state is with C2 symmetry and populates also on
the N atoms and connecting C atoms on the basis of DFT calculations (Scheme 2B). The major peaks suggested stacking and aggregation of porphyrin units in which the Co center and N atom of thenext Co–TCPP unit are connected in a head-to-tail fashion [36](Scheme 1B) and/or H-aggregation (Scheme 1C) in which the Co
centers are weakly bound and connected in a straight fashion[34,35,42] for H–TCPP and Co–TCPP.
The shoulder peak (396 and 413 nm, Fig. 2A) was attributed toeach monomer, which is in agreement with the literature assigning
the Soret peaks centered at 417, 427, and 430 nm to monomers, Haggregates, and J-aggregates, respectively, for Zn-TPP [35] and thered shift to the aggregation of monomers for Co–TCPP [36].
The four weaker peaks that appeared at 512, 546, 589, and646 nm were due to a2u(p) to eg*(p) transitions (Q bands,
Scheme 2) for H–TCPP, whereas the four peaks merged into two
peaks at 539 nm (broad) and 585 nm (weak) for Co–TCPP, suggesting a higher symmetry for the porphyrin ring for Co–TCPP to preserve the degeneracy of the a2u(p) orbital [43]. The a2u(p) state iswith C4 symmetry and populates on the C atoms of pyridine ring,
but not on the N atoms. The peak wavelength at 585 nm indicatesa HOMO–LUMO gap of 2.12 eV.
Upon combination of porphyrins with TiO2, the absorption edgeof TiO2 appeared in the UV region between 380 and 330 nm(Fig. 2B), which is in accordance with the band gap for anatasetype TiO2 (3.2 eV). Interestingly, the UV absorption edge wasaccording to the plot of the CO formation rates in the CO2 photoreduction tests (green squares in Fig. 2B, and entries a–a000 in Table 1),
suggesting the critical role of charge separation of the TiO2 band
gap for the photoreduction of CO2 into CO.
The Soret band for the H- and J-aggregates of H–TCPP on TiO2
appeared at the same position as that in ethanol (415 nm), but it
was significantly broadened by the interaction with the TiO2 sur-face (Fig. 2B). The H- and/or J-aggregates were still predominant,and a minor shoulder peak due to monomers was not well resolved
because of the broadening. By contrast, the Soret band for the Hand/or J-aggregates of Co–TCPP shifted by + 4 nm, and that formonomers shifted by  10 nm (Fig. 2A, B), strongly suggesting
the interaction of Co–TCPP with the TiO2 surface [44].
Upon interaction with the TiO2 surface, the four Q band positions of H–TCPP red-shifted by 3–9 nm, and the major Q band for
Co–TCPP shifted by 3 nm. The relative intensity compared with
the Soret band significantly increased (Fig. 2B), suggesting a preferable interaction between the bridging C atoms connected to thecarboxyphenyl group of H–TCPP and Co–TCPP [a1u(p), Scheme 2]
and TiO2 surface (edge interaction, Scheme 1D).
The fluorescence signal due to band gap emission was confirmed at 376 nm in the emission spectra (Fig. S4), and the major
peak intensity progressively decreased as the Co–TCPP content in
the composite was increased, suggesting the charge transfer from
TiO2 to Co–TCPP (Scheme 2A). One-electron and two-electron
reductions to LUMO were also confirmed in the cyclic voltammetry(CV) of Co–TCPP (Fig. S5).
After the photocatalytic test using Co–TCPP (2.5 wt %)–TiO2
under 13CO2, H2, and UV–visible light irradiation for 48 h, the Soretband showed a clear change from a peak pattern due to the majorJ- and/or H-aggregates (a peak at 431 nm), accompanied with asmaller shoulder peak due to the Co–TCPP monomer (a shoulder
peak at 403 nm) for the fresh photocatalyst, to a major peak at
412 nm due to the Co–TCPP monomer, accompanied with a weakshoulder peak due to the J- and/or H-aggregates at 435 nm(Fig. 2C).
The absorption in UV region apparently decreased after photocatalytic test under 13CO2, H2, and UV–visible light (Fig. 2C)
although it critically depended on the measurement conditions ofUV–visible spectra (Fig. 2B). The decrease suggested reductive594

Journal of Catalysis 413 (2022) 588–602

C. Zhang, J. Yang, K. Hara et al.

immobilized
structureof
Co–tetraphenylporphyrins
inScheme 1D was reported on carbon surface via an organic synthetic route through diazocation on the phenyl group with C surface [19] as well as dehydration between carboxys of Co–TCPP
with monolayer TiO2 on carbon [34]. By contrast, J- and/or Haggregates were the major species on TiO2 for the fresh Co–
TCPP–TiO2 composites in this study. This comparison suggests
the activation of hydroxy species with photogenerated hole (h+)
to form hydroxy radical (HO) on TiO2 [16]. In view of photocatalysis, HO radical should be related to H2 activation, leading to CO2
reduction on the Co sites by the following reaction mechanism (seethe following Eqs. (10)–(16)).
Fig. 4 depicts the in situ FTIR spectra for TiO2 and Co–TCPP (2.
5 wt %)–TiO2 composite under 13CO2 (2.3 kPa), H2 (21.7 kPa), andUV–visible light irradiation at 295 K. The isotopic shift is calculated
assuming harmonic oscillation on the basis of Eqs. (7) and (8):decomposition of Co–TCPP during the test in addition to simple
photodecomposition of Co–TCPP monitored in 13CO2 gas only
(Fig. S2).
3.4. FTIR spectra
The binding mode of Co–TCPP on the TiO2 surface was investigated via FTIR. First, Co–TCPP crystals were thoroughly mixed with
TiO2 at a ratio of 2.5 wt % Co–TCPP. Co–TCPP exhibited peaks at
1628 and 1408 cm1 (curve A in Fig. 3), which were characteristicsof the m(C = O) and m(C–O) stretching vibrations of the carboxy
group in TCPP, respectively (Scheme 1A) [45].
By contrast, upon supporting 2.5 wt % Co–TCPP on TiO2 fromethanol suspension (see Section 2, Experimental), the peak intensity at 1628 cm1 became weaker than that for the physical mixture, whereas a new peak centered at 1531 cm1 appeared(curve B in Fig. 3), which was ascribed to the asymmetric mas(CO2)stretching vibration of carboxylate (Scheme 1D). The peak at
1386 cm1 for Co–TCPP (2.5 wt %)–TiO2 was assigned to the symmetric ms(CO2) stretching vibration of carboxylate overlapping withthe m(C–O) peak (1408 cm1), which is due to the remaining carboxy groups among the four carboxy groups for the original Co–
TCPP (Scheme 1D).
The peaks at 1006 and 1176 cm1 were attributed to the m(Co–N) and C–C skeleton vibration of the Co–TCPP benzene rings,
respectively (curve A in Fig. 3) [46]. These peak positions did notchange upon complexation with TiO2 (curve B in Fig. 3), but theintensities became 30 % of those for the physical mixture. In contrast to the isotropic orientation of the physical mixture of Co–
TCPP molecules with TiO2 (curve A in Fig. 3), incident IR light transmits TiO2 disk and the Co–TCPP moiety would prefer to orient vertically (Scheme 1D). Thus, the vibration of the Co–N and C–C
skeleton in the plane of the porphyrin ring is nearly perpendicular
to the electric field vector of the incident IR light, and the peakintensity of these vibrations was relatively suppressed.
Based on UV–visible, fluorescence, and FTIR spectra (Figs. 2, 3,
and S4), the neighboring two of the four carboxy groups of TCPP
are dissociatively adsorbed on TiO2 [43,47]:(HO2 C—)2 Porphyrin(—CO2 H)2 + 2Ti—OH (surface)! ðHO2 C—Þ2 Porphyrinð—CO2 Þ2 Ti2 ðsurfaceÞ þ 2H2 O

m ¼

1

sﬃﬃﬃﬃ
k

2p c

l

ð7Þ

qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ

þ 16
m13CO
13
¼ qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ ¼ 0:97778
m12CO
1
þ 1
1

1

12

16

ð8Þ

where m is the wave number, c is the speed of light, k is the force
constant, and l is the reduced mass.
For TiO2 under 13CO2 (2.3 kPa) and H2 (21.7 kPa) for 30 min (redcurve of subpanel i in Fig. 4A), an intense peak at 1471 cm1 and ashoulder peak at 1519 cm1 were assigned to mas(OCO) of carbonates (monodentate and/or bidentate), whereas another intense
peak at 1330 cm1 and a shoulder peak at 1400 cm1 wereassigned to their companion ms(OCO) peaks at 1635 and1575 cm1 were assigned to mas(OCO) of bicarbonates (monodentate and/or bidentate), whereas the peaks at 1388 and
1220 cm1 were assigned to their companion ms(OCO) and d(OH)bending vibration [48,49].
The broad negative peak at 3700 cm1 (red curve of subpanel ii
in Fig. 4A) was a characteristic of m(OH) of the hydroxy groups on
the TiO2 surface [50], which were involved in the formation ofbicarbonate combined with CO2.

ð6Þ

CO2 + HO—Ti (surface) ¢ HO—CO2 —Ti (surface)

Here, a question arises why dehydrative immobilization of TCPP
proceeded under CO2, H2, and UV–visible light irradiation, in whichwater was a product associated with CO formation. A similar

ð9ÞThe FTIR spectrum for the Co–TCPP (2.5 wt %)–TiO2 composite(red curve of subpanel i in Fig. 4B) was similar to that of TiO2.
The CO2 adsorption on Co–TCCP crystal was very limited (datanot shown) because of their small heat of adsorption and smaller
surface area compared with TiO2.
Despite the UV–visible light irradiation, the peak intensities at
1519, 1471, and 1330 cm1 that are attributed to carbonates did
not significantly change both for TiO2 (green curve of subpanel iin Fig. 4A) and Co–TCPP (2.5 wt %)–TiO2 (green curve of subpanel
i in Fig. 4B). By contrast, on the basis of the sharp d(OH) peak at
1220 cm1, bicarbonate species slowly decreased by 80 % in 15 husing TiO2, whereas the peak quickly decreased by 80 % in 8 husing Co–TCPP (2.5 wt %)–TiO2. As the m(OH) peak did not recoverfor both catalysts (subpanel ii in Fig. 4A, B) that were consumed via
Eq. (9) (Scheme 1E, F), the bicarbonate did not decompose as thereverse reaction of Eq. (9) under UV–visible light irradiation. Nevertheless, it reacted with H2 to form 13CO2 and water for TiO2 and/or transformed to formate and water (Scheme 1F, G) for Co–TCPP(2.5 wt %)–TiO2 as evidenced by the mas(OCO) peak at 1560 cm1owing to formate [6] (subpanel i in Fig. 4B).
The absorbance in the area around 1628, 1531, and 1386 cm1owing to terminal ligands of TCPP (Fig. 3) in general decreased for
Co–TCPP–TiO2 sample under UV–visible light irradiation (Fig. 4B-i)

Fig. 3. FTIR spectra of (A) Co–TCPP (2.5 wt %) crystal physically mixed with TiO2and (B) Co–TCPP (2.5 wt %)–TiO2 composite.
595

C. Zhang, J. Yang, K. Hara et al.

Journal of Catalysis 413 (2022) 588–602

Fig. 4. FTIR spectra of (A) TiO2 and (B) Co–TCPP (2.5 wt %)–TiO2 under 13CO2 (2.3 kPa), H2 (21.7 kPa), and UV–visible light irradiation.

to more extent in comparison to the monitoring for TiO2 sample
under similar conditions (Fig. 4A-i). This contrast suggested simple(Fig. S2) and/or reductive decomposition of Co–TCPP (Fig. 2C).
The negative peak of isolated hydroxy species (3700 cm1) andpositive peaks of not isolated hydroxy species (3674–3671 and3650 cm1) were commonly observed both on TiO2 and Co–
TCPP–TiO2 (Fig. 4A-ii, B-ii) [51]. In contrast, the hydroxy peak at
3686 cm1 was detected only for Co–TCPP–TiO2 (Fig. 4B-ii) [51],suggesting reactive hydroxy species for Co–TCPP–TiO2, e.g. at theinterface between TiO2 and Co–TCPP (Scheme 2A).
We recall the hydroxy activation of TiO2 by UV light as suggested in the discussion on the binding mode analysis of Co–TCPP(Fig. 3) for H2 activation:

TiO2 + 3hm ! 3 hþ + 3eð10Þ

2HO—Ti + 2 hþ !2HO+ 2(*)Ti

ð11Þ2HO+ 2H2 + hþ !2H2 O + H+ Hþ

ð12Þ

CO2 + H+ (*)Co + e !HCO2 —Co

ð13Þ

HCO2 —Co ! HO—Co + CO

ð14Þ

HO—Co + Hþ !(*)Co + H2 O (this proceeded slowly)

ð15Þ

(*)Ti + O—Ti + H2 O ! 2HO—Ti

ð16Þ

where the asterisk denotes surface vacant site. The formates in
Eqs. (13) and (14) could be formed also via bicarbonates (Fig. 4B).
The different reactivity of bicarbonate on TiO2 and Co–TCPP (2.5
wt %)–TiO2 was because Co–TCPP-anchored formate was formed
from bicarbonate on TiO2 (Scheme 1F, G). The overall reaction is
obtained by summing Eqs. (10)–(16).

CO2 + H2 + H2 + O—Ti !CO + H2 O + (*)Ti + H2 O + 2eð17ÞThus, the photoreduction of CO2 into CO proceeded while thereduction of surface O of TiO2 should be irreversible.
Then, the light was turned off and the samples were evacuatedfor 30 min. On TiO2, the bicarbonates were totally removed, andthe m(OH) peak recovered to the original level (subpanel ii in
Fig. 4A), suggesting exclusive decomposition of bicarbonate backto 13CO2 and hydroxy group. By clear contrast, the m(OH) negative
peak at 3700 cm1 essentially remained the same under vacuumfor 30 min using Co–TCPP (2.5 wt %)–TiO2 (subpanel ii in
Fig. 4B), suggesting Co-anchored formate species were stable undervacuum (Scheme 1G and Eq. (13)). Similarly, the physisorption of
CO on Cu or Fe atoms through van der Waals forces results in theformation of a strongly bound formate by overcoming an activationenergy barrier [52]. The carbonate peaks decreased by 30 % for TiO2
under vacuum for 30 min, suggesting the reverse reaction of Eq.
(18), whereas the decrease was only 5 % for Co–TCPP (2.5 wt %)–TiO2 probably because of the effects of Co–TCPP species.

CO2 + O (surface) ¢ CO3 (surface)

596

ð18Þ

Journal of Catalysis 413 (2022) 588–602

C. Zhang, J. Yang, K. Hara et al.

3.5. HR-TEM

Finally, as shown in Fig. 4B (subpanel i), two negative peaks
appeared at 1121 and 1070 cm1 under continuous light irradiation, which were not found in either TiO2 moiety or Co–TCPP crystal probably because of the irreversible deformation of the Co–
TCPP–TiO2 composite.
In summary, for the Co–TCPP (2.5 wt %)–TiO2 composite, CO2
was mainly adsorbed on TiO2, and a different reactivity on the formation of formate via bicarbonates was suggested by the ligation
to Co. In the absence of the stabilization effect of Co, bicarbonates
were easily decomposed into 13CO2 and water by H2.

HR-TEM images were observed for Co–TCPP crystal. The latticefringes with the intervals of 0.348 nm (image A1 in Fig. 5) and1.34 nm (image A2 in Fig. 5) were observed, demonstrating the
Co–TCPP stacking interval and the interval between stacked Co–
TCPP, respectively [53].
In the images for Co–TCPP (2.5 wt %)–TiO2 composite, latticefringes with the intervals of 0.210, 0.232, 0.239, 0.241–0.250, and
0.336–0.350 nm were observed because of the diffractions by theFig. 5. HR-TEM images of (A1, A2) Co–TCPP and (B1–B4) Co–TCPP (2.5 wt %)–TiO2.
597

C. Zhang, J. Yang, K. Hara et al.

Journal of Catalysis 413 (2022) 588–602

for CoO and Co–TCPP–TiO2, the valence state of Co in Co–TCPP
should be close or equal to 2+, which is consistent with the CV
study for Co–TCPP (Fig. S5).
Fig. 6 (panel B) depicts the time-course change of whiteline
peak intensity of Co–TCPP (25 wt %)–TiO2 under CO2, H2, andUV–visible light irradiation. When the light was turned on, theintensity quickly became weaker by 4.1 % under Ar and by 6.4 %under CO2 and H2, indicating the electron excitation to CB of TiO2
because of UV light energy, followed by the transfer to the Co sites(Scheme 2A). In the time course, the intensity was lower by 2.4 %–6.1 % in the presence of CO2 and H2 compared with that under Ar,
suggesting that the ligand-to-metal charge transfer from coordinated CO2 p orbital(s) to empty 3d orbital(s) of Co further reducedCo.
After the UV–visible light was turned on, the white line peakintensity soon reached an equilibrium either under Ar or under
CO2 and H2 (panel B in Fig. 6), inferring an equilibrium of electrontransfer from TiO2 to Co and charge recombination. Nevertheless, agradual increase of the intensity by 2.3 % during 70 and 150 min ofreaction was observed because the major species on the Co sitetransformed from formate into hydroxy (Scheme 1G, H) using
the Co–TCPP (25 wt %)–TiO2 photocatalyst because the hydroxygroup withdraws electrons more effectively than formate.
The major J- and/or H-aggregates of Co–TCPP molecules were
gradually separated by the reaction of surface hydroxy of TiO2

(2 0 0), (1 1 2), (0 0 4), (1 0 3), and (1 0 1) faces (theoretical values of
0.1893, 0.2333, 0.2379, 0.2431, and 0.3517 nm, respectively) (seeSupplementary materials for assignment) of anatase-phase TiO2
(images B1–B4 in Fig. 5). Predominant lattices observed in the
HR-TEM images were in consistent with exclusive X-ray diffraction
peaks of anatase-phase TiO2 (Fig. S6).
3.6. XAFS spectra
The coordination of the central Co atom in TCPP during CO2
photoreduction was monitored by X-ray absorption near-edge
structure (XANES) and EXAFS under photocatalytic conditions.
Co–TCPP (25 wt %)–TiO2 was used because the X-ray absorbanceof the most active Co–TCPP (2.5 wt %)–TiO2 was insufficient forquantitative EXAFS data analysis measured in transmission mode.
Nevertheless, the angular photoelectron wave number k3 multiplied by normalized EXAFS v function was qualitatively common
between the two samples (Fig. S7), and the coordination information obtained from EXAFS analyses can be adapted also for themost active Co–TCPP (2.5 wt %)–TiO2 photocatalyst.
The pre-edge peaks at 7,710 and 7,716 eV were attributed to1 s–3d and 1 s–4p transitions irrelevant to the presence or absenceof reactants (panel A in Fig. 6), reflecting the square planar coordination of the Co–N4 center with D4h symmetry (Schemes 1A and 2)[12]. Based on relatively similar absorption edge energy position

Fig. 6. (A) Normalized XANES spectra of Co metal, CoO, and Co–TCPP (25 wt %)–TiO2 under Ar and under CO2 (2.7 kPa) and H2 (20.7 kPa). (B) Time course of normalized whiteline intensities of Co–TCPP (25 wt %)–TiO2 under Ar (blue) and under CO2 and H2 (red). (C, Cʹ) Fourier transform of EXAFS spectra at Co K-edge of the Co metal foil, CoO, Co–
TCPP (25 wt %)–TiO2 under Ar, and Co–TCPP (25 wt %)–TiO2 under CO2 and H2. Time course of (D) coordination number and (E) interatomic distance of Co–N under Ar and Co–
O and/or Co–N under CO2 and H2 for the Co–TCPP (25 wt %)–TiO2 photocatalyst.
598

Journal of Catalysis 413 (2022) 588–602

C. Zhang, J. Yang, K. Hara et al.

ditions and form J- and/or H-aggregates. However, major reasons
were not only the decomposition of monomeric Co–TCPP hydroxyspecies but also the oxidation of the reduced Ti(*) site in Eq. (17)also affected the reactivation of the Co–TCPP–TiO2 photocatalyst.

prompted by light activation (Eq. (6)). Hence, formate generation
was accelerated via Eq. (13) and/or via reductive transfer of bicarbonate on TiO2 (subpanel i in Fig. 4B and Scheme 1F), which was
related to the decrease of the N value of Co–N of Co–TCPP units
over TiO2: from 4.7 before light irradiation to 3.4 under Ar andUV–visible light irradiation (blue squares of panel D in Fig. 6).
However, the decrease may be also related to the partial photodecomposition of Co–TCPP at carboxy sites (2.9 CO2 per Co–TCPP in
10 h of UV–visible light irradiation, Fig. S2 and Scheme 1).
In the Fourier transform of k3-weighted EXAFS v oscillation(panels C and C0 in Fig. 6), the peak at 0.142 nm (phase shift uncorrected) for Co–TCPP (25 wt %)–TiO2 under Ar was attributed to Co–
N interatomic pair [13,51], which is almost at the same distance asthat for the peak of Co–TCPP crystals (panel C0 in Fig. 6). The bestfit result was N value of 4.7 (with an error of ± 0.2), which is associated with an interatomic distance (R) value of 0.192 nm (panels D
and E in Fig. 6). As the stacking of Co–TCPP units was observedwith an interval of 0.348 nm (subpanel A1 in Fig. 5), the interunit
coordination between Co and N was by far longer than the Co–N
coordination in porphyrin. Hence, the variation of the N value
was mostly due to the axial coordination to Co due to the increased
population of monomer unit over TiO2 (Scheme 1F). A significant
portion of the Co–TCPP units became the monomer on TiO2 under
the photocatalytic reaction conditions, and the coordination to Co
by water and/or CO2 from air should increase the N value.
Under CO2 and H2, the peak at 0.142 nm under Ar shifted to
0.163 nm (phase shift uncorrected, panel Cʹ in Fig. 6). The best-fit
result was an R value of 0.203 nm associated with N of 5.5 (withan error of ± 0.1, panels D and E in Fig. 6). This increase suggested
CO2 coordination to Co in a significant portion of the monomeric
Co–TCPP unit because no formate peak appeared under CO2 andH2 in the absence of UV–visible light irradiation (subpanel i in
Fig. 4B).
When Co–TCPP–TiO2 under Ar was irradiated by UV–visible
light, the N value for Co–N interatomic pair gradually decreased
from 4.6 to 3.6 in 70 min and the R value for Co–N interatomic pair
was 0.193–0.195 nm in 40 min (panels D and E in Fig. 6), suggesting coordinated water/CO2 desorption under light irradiation frommonomeric Co site and/or partial photodecomposition of Co–TCPP
at carboxy sites (Scheme 1) affected the coordination of central Co
cite.
Under CO2, H2, and UV–visible light irradiation, the N value for
Co–N increased from 5.5 to 5.8–6.3 in 70–140 min (panels D and E
in Fig. 6), suggesting formate and/or hydroxy formation on Co(Scheme 1G, H) via Eqs. (13) and (14) and/or reductive transferof bicarbonate on TiO2 as suggested by FTIR (subpanel i in
Fig. 4B), and the first shell was actually both Co–N and Co–O interatomic pairs. Accordingly, the R value for Co–N and Co–O interatomic pairs gradually decreased from 0.203 to 0.196 nm after
80 min of light irradiation (panel E in Fig. 6).
In the UV–visible absorption spectra of the sample used for 48 hof photoreduction test under 13CO2, H2, and UV–visible light irradiation (Fig. 2C), the peak at 431 nm that is attributed to the J- and/or
H-aggregates became weaker, and the peak at 403 nm that is
attributed to the monomeric Co–TCPP unit became major, supporting covalent anchoring of the Co–TCPP unit on TiO2 via Eq. (6) by
the effects of very reactive HO radical under the irradiation of
UV–visible light in consistent with EXAFS results.
Unexpectedly, when the used Co–TCPP–TiO2 photocatalyst
under the UV–visible light irradiation (k > 320 nm) was exposed
to air for 5 days, the UV–visible absorption spectrum became similar to that of the fresh sample (Fig. S8A), and the CO formation rate
under 13CO2, H2, and UV–visible light irradiation (k > 320 nm) inthe second test was 1.62 lmol h1 g1cat (Fig. S8B), which was higher
by 1.54 times than the rate after 9 h of reaction in the first test
(Fig. S8B). The monomeric Co–TCPP would hydrate in ambient con-

3.7. Reactivation of Co–TCPP (2.5 wt %)–TiO2 catalyst
Finally, we attempted to reactivate the Co–TCPP (2.5 wt %)–TiO2
photocatalyst that was used in a test under 13CO2, H2, and UV–visible light irradiation for 48 h on the basis of partial reactivation
under ambient air (Fig. S8) and also on the mechanistic knowledge
previously discussed. The color of Co–TCPP (2.5 wt %)–TiO2 slightly
changed from dark orange similar to that of its fresh state to dark
saffron after 48 h of the photoreaction test. When the catalyst was
treated with H2 and UV–visible light, the 13CO formation rate wasvery similar to relatively deactivated one after 9 h of a photoreaction in the first test (panel C of Fig. 1, entries b, e, and f in Table 2).
With H2 treatment, 12CO formation became negligible because H2
treatment under UV–visible light irradiation removed CO2 on therelatively stronger adsorption site.
When the catalyst was treated with O2 and UV–visible light, thecolor of the catalyst changed from dark saffron to light gray, andCO formation was below the detection limit of GC–MS or negligible(entries g and h in Table 2). Contrary to the expected results, the O2
treatment completely decomposed the Co–TCPP units.
By contrast, when the catalyst was treated with ethanol(2.7 kPa) and UV–visible light, the 13CO formation rate increasedby 3.6 times compared with the initial rate in the first test (panels
C and D of Fig. 1; entries a and i in Table 2). The turnover of CO2
into CO until 3 h of reaction in the second test was 7.4. Interestingly, in the second test, the 13CO formation rate did not change
during 8 h of reaction. In the regeneration test, after 8 h of reaction,
the 13CO formation rate became 18 lmol h1 g1cat, which was comparable with the initial rate in the first test (17 lmol h1 g1cat;
entries a and j in Table 2). The CO formation rate increased to12
73–21 lmol h1 g1
CO ratio decreased to 13–
cat, and the
10 mol % (entries i and j in Table 2) because the reaction step(s)later than the equilibrium between 13CO2 gas and relatively strong
adsorption site of CO2 on TiO2 proceeded faster because of the contribution of the monomeric Co–TCPP units on TiO2. As the partial
photodecomposition of Co–TCPP essentially completed until 10 hof the first photoreaction test (Fig. S2), the contribution of 12CO2
derived from the decomposition is low for the 12CO formation.
The reason of reactivation of Co–TCPP (2.5 wt %)–TiO2 wasinvestigated using Co K-edge EXAFS (Fig. 7A). The coordination at
single Co site basically retained during the reactivation tests using
ethanol and UV–visible light. In a closer look, Co–N coordination
number (4) for fresh catalyst decreased to 3.5 after the photocatalytic test (a peak at 0.14 nm, phase shift uncorrected, Fig. 7B-b)
mostly due to the deformation and/or partial/reductive decomposition of Co–TCPP (Fig. S2 and Scheme 1G, H). The coordination
number further decreased to 3.1 after the reactivation using ethanol and UV–visible light (a peak at 0.14 nm, phase shift uncorrected, Fig. 7B-c) due to the formation of unsaturated Co site by
the removal of hydroxy group using ethanol (Scheme 1G, H)
enabling the highest 13CO formation rate: 63 lmol h1 g1cat.
The Co K-edge XANES spectra were also compared for fresh Co–
TCPP (2.5 wt %)–TiO2 (Fig. 6A and 7C-a), the catalyst under CO2 andH2 before (Fig. 6A), and after 17-h irradiation of UV–visible light(Fig. 7C-b), and the catalyst of Fig. 7C-b reactivated under ethanol
and UV–visible light for 5 min (Fig. 7C-c). These XANES patternbasically retained very similar pattern, demonstrating the retention of single Co site coordinated by TCPP ligand (Scheme 1). In acloser look, the intensity of pre-edge peak at 7712 eV progressively
decreased by CO2 photoreduction test (Fig. 7C-b) then reactivation
under ethanol and UV–visible light (Fig. 7C-c), suggesting gradual599

C. Zhang, J. Yang, K. Hara et al.

Journal of Catalysis 413 (2022) 588–602

to the dropped rates for fresh catalyst (panels A and C of Fig. S9 andTable 2b) due to the removal of hydroxy attached to Co and partial
deformation of central Co site. By the careful reactivation, supported porphyrin photocatalysts would be utilized sustainably.
Control tests were also performed to verify the hydrogen source
for CO2 reduction reaction. After the first 48-h test and reactivation
using ethanol of UV–visible light for 5 min, a test was done under13
CO2 and UV–visible light. The initial 13CO formation rate waslower by 50 % compared to corresponding rate of fresh catalyst(Fig. 1C and E and Table 2a and k) and 14 % of corresponding rateof reactivation test under 13CO2, H2, and UV–visible light (Fig. 1D
and E and Table 2i and k). Thus, the contribution of adsorbed ethanol and/or ethanol-derived species to photocatalytic CO formation
as hydrogen source was minor.
Based on similar point of view, a control a test was also performed using fresh Co–TCPP (2.5 wt %)–TiO2 catalyst under13
CO2, C2H5OH, and UV–visible light (Fig. 1F). The initial 13CO formation rate (16 lmol h1 g1cat) was essential equivalent to one oftest under 13CO2, H2, and UV–visible light (17 lmol h1 g1cat;
Table 2a and c). Furthermore, the 13CO formation quickly deactivated at 3 h of reaction, and the rate after 3 h of reaction was verylow (1.5 lmol h1 g1cat; Table 2d). Thus, ethanol was found to be adouble-edged sward for Co–TCPP (2.5 wt %)–TiO2 photocatalyst.
While ethanol was equivalent reducing agent to H2 in view of initial 13CO formation, it also quickly decomposed TCPP ligand as evident from the quick 12CO formation derived from TCPP at rates of
26–15 lmol h1 g1cat (Table 2c and d).
Ethanol effectively removed the ligand blocking the central Co
site and supplied sufficient electrons to the Co–TCPP moiety. However, TCPP framework slowly decomposed under relatively harsh
reductive conditions. To avoid this problem, to enforce the porphyrin ligand by even greater conjugate system should improve
the stability of the catalyst in this study [54-56].
Recently, several selective photocatalysts, e.g. Ni–ZrO2 and Co–
ZrO2, to convert CO into value-added hydrocarbons were reported[6,57]. Co–TCPP/TiO2 that was selective to CO formation in thisstudy will be more valuable when metal nanoparticles (Ni or Co)
were mixed or accommodated in the catalyst.

4. Conclusions
Using Co–TCPP (2.5 wt %)–TiO2 under 13CO2, H2, and UV–visible
light irradiation, CO was generated at a maximum rate of
2.5 lmol h1 (using 100 mg of catalyst and UV–visible light of
118 mW cm2). The photocatalyst amount and light intensity werecommon in this study. The transformation of the J- and/or Haggregates of Co–TCPP to isolated monomers anchored on TiO2
proceeded via dehydration between the carboxy group and thehydroxy group of TiO2 by the effects of hydroxy radicals on TiO2,
which were activated under UV light. The uncoordinated Co site
was essential for the stabilization of formate that will be further
reduced to CO as the product. Moreover, the remaining hydroxy
and/or O after the CO evolution on Co and simple and/or reductive
partial decomposition of Co–TCPP caused the serious deactivationof the catalyst. Unexpectedly, partial reactivation of the used photocatalysts was possible by leaving the used catalyst in the air for
5 days because the partially irreversibly reduced TiO2 surface wasagain oxidized by reaction with O2 in the air. However, reactivationof the reduced TiO2 surface was still partial. Ethanol treatment of
the used Co–TCPP (2.5 wt %)–TiO2 photocatalyst under UV–visible
light irradiation was conducted for further reactivation, resulting
in 3.6 and 3.0 times higher 13CO and total CO formation rates compared with values using the fresh photocatalyst, and the turnover
was 7.4 during 3 h of photocatalytic reaction. Reactivation underethanol and UV–visible light must be within short period because

Fig. 7. (A) k3v and (B) its associated Fourier transform of Co K-edge EXAFS of Co–
TCPP (2.5 wt %)–TiO2 as fresh, 1 h evacuated (a), after photoreduction test using CO2(2.7 kPa) H2 (20.7 kPa) and UV–visible light for 17 h (b), and after subsequent
treatment using C2H5OH (2.7 kPa) and UV–visible light for 5 min (c).

reduction of single Co ion site. The reductive decomposition of
TCPP was still minor under CO2, H2, and light for 17 h and/or underethanol and light for 5 min. When the reductive decomposition
became even serious under ethanol and light, serious deactivationof the Co–TCPP–TiO2 photocatalyst was observed (see below).
We further tried repeated reactivation using ethanol (2.7 kPa)
and UV–visible light irradiation (Fig. S9). After the second reactivation, the highest 13CO formation rate (67 lmol h1 g1cat) was notachieved but essentially the same rate as the test for fresh catalyst
was obtained (18 lmol h1 g1cat). The rate later than 8 h of reaction
was 5.5 lmol h1 g1
cat that was higher by 1.8–2.4 times compared600

Journal of Catalysis 413 (2022) 588–602

C. Zhang, J. Yang, K. Hara et al.

ethanol removed not only coordinated hydroxy on Co site but proceeded significant reductive decomposition of TCPP demonstrated
as an evolution of 12CO when ethanol was utilized as a reductant
instead of H2. This study paves the way to provide a general
method to reactivate single-site metal complex photocatalysts
anchored on semiconductors.

[10] B. Ma, G. Chen, C. Fave, L. Chen, R. Kuriki, K. Maeda, O. Ishitani, T.-C. Lau, J.
Bonin, M. Robert, Efficient Visible-Light-Driven CO2 Reduction by a Cobalt
Molecular Catalyst Covalently Linked to Mesoporous Carbon Nitride, J. Am.
Chem. Soc. 142 (2020) 6188–6195.
[11] K.E. Dalle, J. Warnan, J.J. Leung, B. Reuillard, I.S. Karmel, E. Reisner, Electro- andSolar-Driven Fuel Synthesis with First Row Transition Metal Complexes, Chem.
Rev. 119 (4) (2019) 2752–2875.
[12] J.-D. Yi, R. Xu, G.-L. Chai, T. Zhang, K. Zang, B. Nan, H. Lin, Y.-L. Liang, J. Lv, J. Luo,
R. Si, Y.-B. Huang, R. Cao, Cobalt Single-atoms Anchored on Porphyrinic
Triazine-based Frameworks as Bifunctional Electrocatalysts for Oxygen
Reduction and Hydrogen Evolution Reactions, J. Mater. Chem. A. 7 (3) (2019)
1252–1259.
[13] A. Zitolo, N. Ranjbar-Sahraie, T. Mineva, J. Li, Q. Jia, S. Stamatin, G.F. Harrington,
S.M. Lyth, S. Mukerjee, E. Honda, F. Jaouen, Identification of Catalytic Sites in
Cobalt-Nitrogen-Carbon Materials for the Oxygen Reduction Reaction, Nat.
Commun. 8 (2017) 957.
[14] X. Zhao, X. Liu, M. Yu, C. Wang, J. Li, The Highly Efficient and Stable Cu Co, Znporphyrin–TiO2 Photocatalysts with Heterojunction by Using Fashioned Onestep Method, Dyes Pigm. 136 (2017) 648–656.
[15] N. Ahmed, Y. Shibata, T. Taniguchi, Y. Izumi, Photocatalytic Conversion of
Carbon Dioxide into Methanol Using Zinc–Copper–M(III) (M = Aluminum,
Gallium) Layered Double Hydroxides, J. Catal. 279 (1) (2011) 123–135.
[16] H. Zhang, S. Kawamura, M. Tamba, T. Kojima, M. Yoshiba, Y. Izumi, Is Water
More Reactive Than H2 in Photocatalytic CO2 Conversion into Fuels Using
Semiconductor Catalysts under High Reaction Pressures?, J Catal. 352 (2017)452–465.
[17] A.N. Marianov, A.S. Kochubei, T. Roman, O.J. Conquest, C. Stampfl, Y. Jiang,
Resolving Deactivation Pathways of Co Porphyrin-Based Electrocatalysts for
CO2 Reduction in Aqueous Medium, ACS Catal. 11 (6) (2021) 3715–3729.
[18] S. Lin, C.S. Diercks, Y.-B. Zhang, N. Kornienko, E.M. Nichols, Y. Zhao, A.R. Paris,
D. Kim, P. Yang, O.M. Yaghi, C.J. Chang, Covalent Organic Frameworks
Comprising Cobalt Porpyrins for Catalytic CO2 Reduction in Water, Science
349 (2015) 1208–1213.
[19] A.N. Marianov, Y. Jiang, Covalent Ligation of Co Molecular Catalyst to Carbon
Cloth for Efficient Electroreduction of CO2 in Water, Appl. Catal. B 244 (2019)881–888.
[20] X.-M. Hu, M.H. Rønne, S.U. Pedersen, T. Skrydstrup, K. Daasbjerg, Enhanced
Catalytic Activity of Cobalt Porphyrin in CO2 Electroreduction upon
Immobilization on Carbon Materials, Angew. Chem. Int. Ed. 56 (23) (2017)
6468–6472.
[21] C. Zheng, X. Qiu, J. Han, Y. Wu, S. Liu, Zero-Dimensional-g-CNQD-Coordinated
Two-Dimensional Porphyrin MOF Hybrids for Boosting Photocatalytic CO2
Reduction, ACS Appl. Mater. Interfaces 11 (45) (2019) 42243–42249.
[22] S. Tian, S. Chen, X. Ren, Y. Hu, H. Hu, J. Sun, F. Bai, An Efficient Visible-Light
Photocatalyst for CO2 Reduction Fabricated by Cobalt Porphyrin and Graphitic
Carbon Nitride via Covalent Bonding, Nano Resear. 13 (10) (2020) 2665–2672.
[23] J. Xu, X. Liu, Z. Zhou, M. Xu, Photocatalytic CO2 Reduction Catalyzed by
Metalloporphyrin: Understanding of Cobalt and Nickel Sites in Activity andAdsorption, Appl. Surf. Sci. 513 (2020) 145801.
[24] J. Xu, X. Liu, Z. Zhou, L. Deng, L. Liu, M. Xu, Visible Light-Driven CO2
Photocatalytic Reduction by Co-Porphyrin-Coupled MgAl Layered DoubleHydroxide Composite, Energy Fuels 35 (19) (2021) 16134–16143.
[25] N. Sadeghi, M. Sillanpää, High Selective Photocatalytic CO2 Conversion into
Liquid Solar Fuel over a Cobalt Porphyrin-Based Metal-Organic Framework,
Photochem. Photobio. Sci. 20 (3) (2021) 391–399.
[26] L. Wang, P. Jin, J. Huang, H. She, Q. Wang, Integration of Copper(II)-Porphyrin
Zirconium Metal-Organic Framework and Titanium Dioxide to Construct ZScheme System for Highly Improved Photocatalytic CO2 Reduction. ACS
Sustainable, Chem. Eng. 7 (2019) 15660–15670.
[27] Y. Yoshida, Y. Mitani, T. Itoi, Y. Izumi, Preferential Oxidation of Carbon
Monoxide in Hydrogen Using Zinc Oxide Photocatalysts Promoted and Tuned
by Adsorbed Copper Ions, J. Catal. 287 (2012) 190–202.
[28] J.A. Bearden, X-Ray Wavelengths, Rev. Modern Phys. 39 (1) (1967) 78–124.
[29] F.W. Lytle, R.B. Greegor, D.R. Sandstrom, E.C. Marques, J. Wong, C.L. Spiro, G.P.
Huffman, F.E. Huggins, Measurement of Sof X-Ray Absorption Spectra with a
Fluorescent Ion Chamber Detector, Nuclear Instrum. Meth. Phys. Resear. 226(1984) 542–548.
[30] M. Vaarkamp, H. Linders, D. Koningsberger, XDAP Version 3.2.8, XAFS Services
International, Woudenberg, The Netherlands, 2021.
[31] https://www.webelements.com/cobalt/crystal_structure.html (checked on
February 27, 2022).
[32] W. Jauch, M. Reehuis, H.J. Bleif, F. Kubanek, P. Pattison, Crystallographic
Symmetry and Magnetic Structure of CoO, Phys. Rev. B 64 (2001) 052102.
[33] M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R.
Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H.
Nakatsuji, et al. Gaussian 09, Revision A. 2, Gaussian, Inc., Wallingford CT,
USA, 2016.
[34] H. Gao, J. Wang, M. Jia, F. Yang, R.S. Andriamitantsoa, X. Huang, W. Dong, G.
Wang, Construction of TiO2 Nanosheets/Tetra (4-Carboxyphenyl) Porphyrin
Hybrids for Efficient Visible-light Photoreduction of CO2, Chem. Eng. J. 374(2019) 684–693.
[35] S. Devaramani, M.I. Shinger, X. Ma, M. Yao, S. Zhang, D. Qin, X. Lu, Porphyrin
Aggregates Decorated MWCNT Film for Solar Light Harvesting: Influence of Jand H-aggregation on the Charge Recombination Resistance, Photocatalysis,and Photoinduced Charge Transfer Kinetics, Phys. Chem. Chem. Phys. 19(2017) 18232–18242.

Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgments
The authors are grateful for the financial support from theGrant-in-Aid for Scientific Research B (20H02834) and C(17 K05961) from the Japan Society for the Promotion of Science.
X-ray absorption experiments were performed with the approvalof the Photon Factory Proposal Review Committee (2020G676
and 2018G649).
Appendix A. Supplementary material
The following files are available free of charge:
The Experimental methods section provides details on photocatalytic 13CO2 reduction, the absorption–fluorescence spectra,
CV, and XRD measurements. (file type, MS Word).
Results and discussion covers the dependences of CO formation
rate on the amount of photocatalysts and also on the Co–TCPP content in photocatalysts, time-course profile of the 13CO2 exchange
test, time-course profiles of the photocatalytic tests under 13CO2
and H2O, fluorescence spectra of photocatalysts, CV of Co–TCPP,XRD of the photocatalysts, information for HR-TEM, EXAFS comparison depending on the Co–TCPP content in the photocatalysts,
and results on the reuse of the photocatalysts after reactivation.
(file type, MS Word).
Supplementary data to this article can be found online athttps://doi.org/10.1016/j.jcat.2022.07.006.
References[1] Y. Izumi, Recent Advances in the Photocatalytic Conversion of Carbon Dioxide
to Fuels with Water and/or Hydrogen Using Solar Energy and Beyond, Coord.
Chem. Rev. 257 (2013) 171–186.
[2] A. Goeppert, M. Czaun, J.-P. Jones, G.K. Surya Prakash, G.A. Olah, Recycling of
Carbon Dioxide to Methanol and Derived Products – Closing the Loop, Chem.
Soc. Rev. 43 (23) (2014) 7995–8048.
[3] J. Albero, Y. Peng, H. García, Photocatalytic CO2 Reduction to C2+ Products, ACS
Catal. 10 (2020) 5734–5749.
[4] H. Zhang, T. Itoi, T. Konishi, Y. Izumi, Dual Photocatalytic Roles of Light: Charge
Separation at the Band Gap and Heat via Localized Surface Plasmon Resonanceto Convert CO2 into CO over Silver-Zirconium Oxide, J. Am. Chem. Soc. 141(2019) 6292–6301.
[5] H. Zhang, T. Itoi, K. Niki, T. Konishi, Y. Izumi, Dual Origins of Photocatalysis:
Light-induced Band-gap Excitation of Zirconium Oxide and Ambient Heat
Activation of Gold to Enable 13CO2 Photoreduction/Conversion, Catal. Today
356 (2020) 544–556.
[6] H. Zhang, T. Itoi, T. Konishi, Y. Izumi, Efficient and Selective Interplay Revealed:
CO2 Reduction to CO over ZrO2 by Light with Further Reduction to Methane
over Ni0 by Heat Converted from Light, Angew. Chem. Int. Ed. 60 (2021) 9045–
9054.
[7] D.-I. Won, J.-S. Lee, Q. Ba, Y.-J. Cho, H.-Y. Cheong, S. Choi, C.H. Kim, H.-J. Son, C.
Pac, S.O. Kang, Development of a Lower Energy Photosensitizer for
Photocatalytic CO2 Reduction: Modification of Porphyrin Dye in Hybrid
Catalyst System, ACS Catal. 8 (2018) 1018–1030.
[8] R. Li, W. Zhang, K. Zhou, Metal–Organic-Framework-Based Catalysts for
Photoreduction of CO2, Adv. Mater. 30 (2018) 1705512.
[9] H. Huang, J. Lin, G. Zhu, Y. Weng, X. Wang, X. Fu, J. Long, A Long-LivedMononuclear Cyclopentadienyl Ruthenium Complex Grafted onto Anatase
TiO2 for Efficient CO2 Photoreduction, Angew. Chem., Int. Ed. 55 (29) (2016)
8314–8318.
601

C. Zhang, J. Yang, K. Hara et al.

Journal of Catalysis 413 (2022) 588–602[47] J. Rochford, D. Chu, A. Hagfeldt, E. Galoppini, Tetrachelate Porphyrin
Chromophores for Metal Oxide Semiconductor Sensitization: Effect of theSpacer Length and Anchoring Group Position, J. Am. Chem. Soc. 129 (2007)
4655–4665.
[48] L. Mino, G. Spoto, A.M. Ferrari, CO2 Capture by TiO2 Anatase Surfaces: A
Combined DFT and FTIR Study, J. Phys. Chem. C. 118 (43) (2014) 25016–25026.
[49] L.F. Liao, C.F. Lien, D.L. Shieh, M.T. Chen, J.L. Lin, FTIR Study of Adsorption andPhotoassisted Oxygen Isotopic Exchange of Carbon Monoxide, Carbon Dioxide,
Carbonate, and Formate on TiO2, J. Phys. Chem. B. 106 (2002) 11240–11245.
[50] U. Tumuluri, J.D. Howe, W.P. Mounfield, M. Li, M. Chi, Z.D. Hood, K.S. Walton,
D.S. Sholl, S. Dai, Z. Wu, Effect of Surface Structure of TiO2 Nanoparticles on
CO2 Adsorption and SO2 Resistance, ACS Sustainable Chem. Eng. 5 (2017)
9295–9306.
[51] M.-Y. He, J.G. Ekerdt, Infrared Studies of the Adsorption of Synthesis Gas on
Zirconium Dioxide, J. Catal. 87 (1984) 381–388.
[52] F. Huber, J. Berwanger, S. Polesya, S. Mankovsky, H. Ebert, F.J. Giessibl,
Chemical Bond Formation Showing a Transition from Physisorption to
Chemisorption, Science 366 (6462) (2019) 235–238.
[53] N. Keller, M. Calik, D. Sharapa, H.R. Soni, P.M. Zehetmaier, S. Rager, F. Auras, A.
C. Jakowetz, A. Görling, T. Clark, T. Bein, Enforcing Extended Porphyrin JAggregate Stacking in Covalent Organic Frameworks, J. Am. Chem. Soc. 140(48) (2018) 16544–16552.
[54] K.S. Min, R.S. Kumar, J.H. Lee, K.S. Kim, S.G. Lee, Y.-A. Son, Synthesis of new
TiO2/porphyrin-based composites and photocatalytic studies on methylene
blue degradation, Dyes Pigments 160 (2019) 37–47.
[55] C. Liu, K. Liu, C. Wang, H. Liu, H. Wang, H. Su, X. Li, B. Chen, J. Jiang, Elucidating
heterogeneous photocatalytic superiority of microporous porphyrin organic
cage, Nat. Commun. 11 (2020) 1047.
[56] D.-Il Won, J.-S. Lee, Q. Ba, Y.-J. Cho, H.-Y. Cheong, S. Choi, C. H. Kim, H.-J. Son, C.
Pac, S. O. Kang, Development of a Lower Energy Photosensitizer for
Photocatalytic CO2 Reduction: Modification of Porpyrin Dye in Hybrid
Catalyst System, ACS Catal. 8 (2018) 1018–1030.
[57] T. Loumissi, K. Hara, H. Zhang, T. Oyumi, R. Ishii, R. Hirayama, K. Niki, T. Itoi, Y.
Izumi, manuscript in preparation.

[36] E.H.A. Beckers, S.C.J. Meskers, A.P.H.J. Schenning, Z. Chen, F. Würthner, P. Marsal,
D. Beljonne, J. Cornil, R.A.J. Janssen, Influence of Intermolecular Orientation on
the Photoinduced Charge Transfer Kinetics in Self-Assembled Aggregates ofDonorAcceptor Arrays, J. Am. Chem. Soc. 128 (2) (2006) 649–657.
[37] B. Zhuang, X.Q. Li, R.Y. Ge, S.Z. Kang, L.X. Qin, G.D. Li, Assembly and ElectronTransfer Mechanisms on Visible Light Responsive 5,10,15,20-meso-tetra(4Carboxyphenyl)Porphyrin/Cuprous Oxide Composite for Photocatalytic
Hydrogen Production, Appl. Catal. A. 533 (2017) 81–89.
[38] Q. Xiang, J. Yu, M. Jaroniec, Enhanced Photocatalytic H2-Production Activity ofGraphene-Modified Titania Nanosheets, Nanoscale 3 (2011) 3670–3678.
[39] W. Hou, W.H. Hung, P. Pavaskar, A. Goeppert, M. Aykol, S.B. Cronin, Photoatalytic
Conversion of CO2 to Hydrocarbon Fuels via Plasmon-Enhanced Absorption and
Metallic Interband Transitions, ACS Catal. 1 (2011) 929–936.
[40] K. Maeda, Direct Splitting of Pure Water into Hydrogen and Oxygen Using Rutile
Titania Powder as a Photocatalyst, Chem. Commun. 49 (2013) 8404–8406.
[41] R. Li, Y. Weng, X. Zhou, X. Wang, Y. Mi, R. Chong, H. Han, C. Li, Achieving
Overall Water Splitting Using Titanium Dioxide-Based Photocatalysts ofDifferent Phases, Energy Environ. Sci. 8 (8) (2015) 2377–2382.
[42] H. Shahroosvand, S. Zakavi, A. Sousaraei, M. Eskandari, Saddle-Shaped
Porphyrins for Dye-Sensitized Solar Cells: New Insight into the Relationshipbetween Nonplanarity and Photovoltaic Properties, Phys. Chem. Chem. Phys.
17 (2015) 6347–6358.
[43] R. Giovannetti, The Use of Spectrophotometry UV–Vis for the Study ofPorphyrins, Macro Nano, Spectrosc. 1 (2012) 87–108.
[44] F. Koch, M. Kullmann, U. Selig, P. Nuernberger, D.C.G. Götz, G. Bringmann, T.
Brixner, Coherent Two-dimensional Electronic Spectroscopyin the Soret Bandof a Chiral Porphyrin Dimer, New J. Phys. 15 (2013) 025006.
[45] J. Roales, J.M. Pedrosa, M.G. Guillén, T. Lopes-Costa, P. Castillero, A. Barranco, A.
R. González-Elipe, Free-Base Carboxyphenyl Porphyrin Films Using a TiO2
Columnar Matrix: Characterization and Application as NO2 Sensors, Sensors 15(2015) 11118–11132.
[46] X. Guo, B. Guo, T. Shi, The Photochemical and Electrochemical Properties of
Chiral Porphyrin Dimer and Self-aggregate Nanorods of Cobalt(II) Porphyrin
Dimer, Inorganica Chim. Acta 363 (2010) 317–323.

602