Supplementary materials

Anchoring and reactivation of single-site Co–porphyrin over TiO2 forthe efficient photocatalytic CO2 reduction

Chao Zhang,a Jiwon Yang,a Keisuke Hara,a Rento Ishii,a Hongwei Zhang,a Takaomi Itoi,b andYasuo Izumi a,*a

Department of Chemistry, Graduate School of Science, Chiba University, Yayoi 1-33, Inage-ku,

Chiba 263-8522, Japanb

Department of Mechanical Engineering, Graduate School of Engineering, Chiba University,

Yayoi 1-33, Inage-ku, Chiba 263-8522, Japan

1. Experimental
1.1. Photocatalytic CO2 reduction
The absorbance of Model UV32 (wavelength λ > 320 nm) [S1], U330 (245 nm < λ < 386 nm,and λ > 686 nm) [S2], and L38 (λ > 380 nm) filters [S3] are illustrated in each product website.

1.2. Absorption–fluorescence spectra
The absorption–fluorescence spectra were recorded on an FP-8600 spectrofluorometer(JASCO, Tokyo, Japan; Chiba Iodine Resource Innovation Center) equipped with a 150 W Xe
arc lamp and a photomultiplier tube in the range of excitation light from 200 to 380 nm and therange of fluorescence light from 200 (or 400) to 800 nm. Before the measurements, the sample
powder (2.0 mg) was poured into deionized water (3.0 mL) in a quartz cell and ultrasonicated(430 W, 38 kHz) for 30 min.
S1

1.3. CV measurements
CV measurements were performed to monitor the reduction and oxidation potential of the Co–
TCPP complex. A glassy carbon (Φpolyetheretherketone 6.0 mm, ΦC 3.0 mm, Model 002012), platinum
wire (Model 002222), and Ag/AgCl (Model RE-1B; all three models from BAS Inc., Tokyo,Japan) were used as a working electrode (WE), counter electrode, and reference electrode,respectively. Dimethyl sulfoxide (DMSO) was used as a solvent and the cobalt–tetrakis(4carboxyphenyl)porphyrin (Co–TCPP) concentration was 1.0 mmol L−1. Tetrabutylammonium
perchlorate (0.1 mol L−1) was added as a supporting electrolyte, and 1.0 mmol L−1 of ferrocene
was added as an internal reference by monitoring the redox of ferrocene/ferrocenium (Fc/Fc+).
The WE voltage was swept between −2.5 and 1.0 V versus standard hydrogen electrode (SHE) ata rate of 50 mV s−1 using a potentiostat/galvanostat (VersaSTAT 3–100; Princeton Applied
Research, Oak Ridge, TN, USA).

1.4. X-ray diffraction (XRD) patterns
XRD patterns were observed using a D8 ADVANCE diffractometer (Bruker, Billerica, MA,USA) at the Center for Analytical Instrumentation at a Bragg angle (θB) of 2θB = 5°–80° with ascan step of 0.01° and at a rate of 1 s per step. The measurements were performed at 40 kV and40 mA using Cu Kα emission (wavelength λ = 0.15419 nm) and a Ni filter.

2. Results and discussion
2.1. Photocatalytic 13CO2 reduction and 13CO2 exchange
13CO formation rate was 6.3 µmol h−1 (Table 2i) using Co–TCPP–TiO2 (100 mg) under theirradiation of UV–visible light (118 mW cm−2; Model OPM2-502, Ushio, Inc.). We previously

S2

reported the light intensity of the light source, and the light intensity to excite anatase-type TiO2,i.e. λ < 387 nm, was 10.4 mW cm−2 [S4]. Therefore, the light intensity at the light exit (φ = 5.0cm) was 0.205 J s−1, and if we assume a photon of wavelength of 387 nm, the number was 3.99 ×1017 photons s−1. The obtained rate of 63 µmol h−1 gcat−1 using 0.100 g of catalyst corresponds to1.05 × 1015 molecules-CO s−1. We also considered the light absorbed in Co–TCPP–TiO2 (0.100g) was one third [S5]. Thus, the quantum yield for the two electron reduction reaction wasestimated to 2 × 1.05 × 1015 / ( 1 × 3.99 × 1017 photons s−1) = 1.6%.
3Fig. S1. The dependence of 13CO formation rates (µmol h−1) using 13CO2, H2, Co–TCPP (2.5 wt %)–TiO2,and UV–visible light on the photocatalyst amount used. The UV–visible light intensity was 118 mW cm−2.

S3

Table S1
Initial CO Formation rates during the first 5 h of reaction using Co–TCPP–TiO2 (100 mg) under 13CO2 (2.7 kPa), H2(20.7 kPa), and UV–visible light irradiation. The UV–visible light intensity was 118 mW cm−2.
Entry

Co–TCPP loading(wt %)

abcde

1.0
2.5
5.0
7.5
25Formation rate (µmol h−1 gcat−1)
13CO

1.9
17
7.0
5.7
0.27

12COΣ CO0.45
7.1
6.53.30.272.425
14
9.0
0.53

12COratio(mol%)
1929483650

Fig. S2. Time-course uptake of 13CO2 (0.67 kPa) and partial decomposition of Co–TCPP irradiated underUV–visible light using Co–TCPP (2.5 wt %)–TiO2. Photocatalyst used was 0.100 mg and the UV–visiblelight intensity was 118 mW cm−2.

S4

Fig. S3. Time-course formation of photocatalytic 13CO and 12CO during exposure to 13CO2 (2.7 kPa) andH2O (1.7 kPa) using (A) Co–TCPP (2.5 wt %)–TiO2 (anatase phase, JRC-TIO14) and (B) Co–TCPP(2.5 wt %)–TiO2 (rutile phase, JRC-TIO6) under UV–visible light irradiation. Photocatalyst amount was
100 mg and the UV–visible light intensity was 118 mW cm−2.

2.2. Absorption–fluorescence spectra
The fluorescence spectra of the Co–TCPP–TiO2 composite with the excitation at 200 nm areshown in Fig. S2A. The major peak at 376 nm was attributed to the band gap emission (3.2 eV)of anatase-phase TiO2. As the Co–TCPP content in the sample was increased, the major peak as
well as a shoulder peak at 396 nm progressively diminished, demonstrating effective electrontransfer from the conduction band of TiO2 into the HOMO of Co–TCPP. Three weak peaks at452, 468, and 529 nm for TiO2 (curve A in Fig. S4) were due to the electron transition from or tothe middle-gap trapping states on/in TiO2, e.g., surface O vacancy [S6]. The intensity of thesepeaks also decreased with an increase of Co–TCPP content, suggesting electron transfer from orto such trap states to or from Co–TCPP electronic orbital state (Scheme 2A).

S5Fig. S4. Fluorescence spectra of (A) Co–TCPP–TiO2 composite with various amounts of Co–TCPP (0,1.0, 2.5, 5.0, and 7.5 wt %) with the excitation wavelength at 200 nm and (B) Co–TCPP (2.5 wt %)–TiO2composite with the excitation wavelength at 200 and 380 nm.

When the excitation wavelength was changed to 380 nm, the fluorescence intensity became
essentially zero in the wavelength range of 400–800 nm (Fig. S2B), indicating that thefluorescence from the composites exclusively originated from the relaxation of excitations inTiO2 rather than the HOMO–LUMO relaxation of porphyrins.

2.3. CV results
The reduction and oxidation of Co–TCPP were monitored by CV (Fig. S5). In comparison
with the Fc/Fc+ pair at mean 0.52 V (versus SHE) that is close to the value in literature (0.422 V)[S7], the redox pair at mean −0.79 V can be assigned to the redox at LUMO [eg*(π), Scheme 2],
dispersing over the porphyrin ring: on the N atoms of pyridine rings and C atoms of connecting
CH part rather than localized at Co+/Co2+ [S8].
S6The redox pair at mean −1.89 V can be assigned to LUMO + 1 [eu*(π)], dispersing over thebenzene rings of porphyrin (Scheme 2B) rather than localized Co0/Co1+ [S8]. On the basis of theQ band and Soret band peak positions (Fig. 2B), HOMO [a2u(π)] and HOMO − 1 [a1u(π)] levels
were determined (Scheme 2A).

Fig. S5. CV for Co–TCPP dissolved in DMSO.

2.4. XRD patterns
XRD patterns of Co–TCPP (0, 1.0, 2.5, 5.0, and 7.5 wt %)–TiO2 composites (Fig. S6) weremeasured. The peaks at 25.2°, 37.6°, 47.7°, 54.5°, 62.4°, 69.1°, and 75.0° can be assigned to thediffraction in the 1 0 1, 0 0 4, 2 0 0, 2 1 1, 2 0 4, 1 1 6, and 2 1 5 planes, respectively, of anatasetype TiO2 [S9]. No obvious peaks due to Co–TCPP appeared, suggesting that the stacking
repetition of J- and/or H-aggregation is limited which could not result in a crystal-like structureon the TiO2 surface.

S7

Fig. S6. XRD patterns of the Co–TCPP–TiO2 composites with various amounts of Co–TCPP (0, 1.0, 2.5,5.0, and 7.5 wt %).

2.5. HR-TEM
The crystal parameters for anatase-phase TiO2 were based on ref. S10 for the assignment oflattice fringes in HR-TEM (Fig. 5).

S8

2.6. EXAFS spectra

Fig. S7. (a) Comparison of angular wave number k3-weighted EXAFS χ oscillation between (a) fresh Co–TCPP (2.5 wt %)–TiO2 measured in fluorescence detection mode and (b) fresh Co–TCPP (25 wt %)–TiO2
measured in transmission mode both at the Co K-edge.

2.7. Reactivation of Co–TCPP (2.5 wt %)–TiO2 catalyst

Fig. S8. (A) UV–visible absorption spectra of Co–TCPP (2.5 wt %)–TiO2 used for reaction under theirradiation of UV–visible light of λ > 320 nm, before reaction (black), after reaction (red), and set aside
for 5 days (blue). (B) Time course of 13CO formation using Co–TCPP (2.5 wt %)–TiO2 under λ > 320 nm
in the first (black) and second photoreduction tests of 13CO2 (red). Photocatalyst amount was 100 mg.

S9Fig. S9. Time-course formation of photocatalytic 13CO and 12CO during exposure to 13CO2 (2.7 kPa) andH2 (20.7 kPa) using (A) Co–TCPP (2.5 wt %)–TiO2 under UV–visible light, (B) Co–TCPP (2.5 wt %)–TiO2 used for the test in panel A was treated with ethanol (2.7 kPa) for 5 min and irradiated under UV–
visible light and was reused, and (C) Co–TCPP (2.5 wt %)–TiO2 used for the test in panel B was treatedwith ethanol (2.7 kPa) for 5 min and irradiated under UV–visible light and was reused. Photocatalystamount was 100 mg and the UV–visible light intensity was 118 mW cm−2.

References[S1] http://www.newportglass.com/hoyuv32.htm (checked on February 27, 2022).
[S2] https://www.hoyacandeo.co.jp/japanese/products/eo/color/10.html (checked on February 27,2022).
[S3] https://www.hoyacandeo.co.jp/japanese/products/eo/color/01.html (checked on February 27,2022).
[S4] 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.

S10

[S5] Y. Yoshida, Y. Mitani, T. Itoi, Y. Izumi, Preferential Oxidation of Carbon Monoxide inHydrogen Using Zinc Oxide Photocatalysts Promoted and Tuned by Adsorbed Copper Ions,
J. Catal. 287 (2012) 190–202.
[S6] D. Pan, N. Zhao, Q. Wang, S. Jiang, X. Ji, L. An, Facile Synthesis and Characterization ofLuminescent TiO2 Nanocrystals, Adv. Mater. 17 (2005) 1991–1995.
[S7] G. Pu, Z. Yang, Y. Wu, Z. Wang, Y. Deng, Y. J. Gao, Z. Zhang, X. Lu, Investigation into
the Oxygen-Involved Electrochemiluminescence of Porpyrins and Its Regulation by
Peripheral Substituents/Central Metals, Anal. Chem. 91 (2019) 2319–2328.
[S8] A. N. Marianov, A. S. Kochubei, T. Roman, O. J. Conquest, C. Stampfl, Y. Jiang,
Resolving Deactivation Pathways of Co Porphyrin-Based Electrocatalysts for CO2Reduction in Aqueous Medium, ACS Catal. 11 (2021) 3715–3729.
[S9] K. Thamaphat, P. Limsuwan, B. Ngotawornchai, Phase Characterization of TiO2 Powder by
XRD and TEM. Kasetsart J. (Nat. Sci.) 42 (2008) 357–361.
[S10] B. G. Hyde, S. Andersson, Inorganic Crystal Structures, John Wiley & Sons, New York,USA, 1989, p 14.

S11