RESEARCH ARTICLE (Supporting Information)
Exchange of CO2 with CO as Reactant Switches Selectivity in
Photoreduction on Co–ZrO2 from C1–3 Paraffin to Small Olefins
Tarik Loumissi,[a],# Rento Ishii,[a],# Keisuke Hara,[a],# Tomoki Oyumi,[a],# Ikki Abe,[a] Chongxu Li,[a]Hongwei Zhang,[a] Rumiko Hirayama,[a] Kaori Niki,[a] Takaomi Itoi,[b] and Yasuo Izumi*[a]
[a]

[b]

#

T. Loumissi, R. Ishii, Dr. K. Hara, T. Oyumi, I. Abe, C. Li, Dr. H. Zhang, R. Hirayama, Professor Dr. K. Niki, and Professor Dr. Y. Izumi
Department of Chemistry, Graduate School of Science
Chiba University
Yayoi 1-33, Inage-ku, Chiba 263-8522, Japan
E-mail: yizumi@faculty.chiba-u.jp
Professor Dr. T. Itoi
Department of Mechanical Engineering, Graduate School of Engineering
Chiba University
Yayoi 1-33, Inage-ku, Chiba 263-8522, Japan
T.L., R.I., and T.O. shared experiments and K.H. contributed for calculations equally to this paper all in the graduate course.

1. INTRODUCTION
1.1. Background of This Study.
Table S1. Reported Photocatalytic C2 and C3 HC Generation from CO2.
Entry

Catalyst

Light

a

Co–
Cu/TiO2

300-W XebSingleAu/red P

300-W Xe
(300 mW cm−2)

CdS/Cu–titanate
Nafion–Pd–TiO2
Pt–
graphene/
TiO2-xTiO2–graphene
Au@Bi12O
17Br2
CuPt2/TiO2
nanotube
Au–
Pd/TiO2{10 1}450-W Xe,>420 nm

c
d
e
f
g
h
i

100 W, AM1.5
filter (100 mW
cm−2)
300-W Xe
300-W Xe
AM1.5(100 mW cm−2)
300-W Xe(853 mW cm−2)

vacancy)

Xe, > 420 nm(40 mW cm−2)

k

C/Cu2Onanorod

350-W Xe>420 nml

Cu/TiO2

Xej

TiON1-(OSun, 36° north

Reactants
CO2(1.0 MPa)
KHCO3 +HCl →CO2 + H2O
+ KCl

Reaction timeProduct

Water(1 mL)

3h

C2H6
C3H8

–

Solvent/reactant–

1.3 μmol h−1 gcat−1, 96 mol%

2022

8

–

0.71 μmol h−1 gcat−1, 31 mol%

2015

9

1.1 μmol h−1, 9.0 mol%

2012

10

7h

11 μmol h−1, 23 mol%

2018

11

4h

17 μmol h−1 gcat−1, 68 mol%

CO2 flows
through H2O

CO2(101 kPa)
CO2(80 kPa)
CO2 flow(650 kPa)
CO2
(0.26
MPa)

Water
(0.4 mL)
Water(1 mL)
Water(15 μL)

20 h

Water
(0.1 mL)6h

The catalyst was
immersed inwater and driedin vacuum

CO2(101 kPa)
CO2(2.7 MPa)

2013

12

2022

13

20 μmol h−1 gcat−1; 14 mol%
10 μmol h−1 gcat−1; 7.1 mol%
0.79 μmol h−1 gcat−1, 3.2mol%

2012

14

0.73 μmol h−1 gcat−1; 2.9 mol%

2019

15

4h

1.9 μmol h−1 gcat−1, 2 mol%

2023

16

0.1 M KHCO3

12 h

0.016 μmol h−1, 56 mol%

2016

17

–

–

0.023 μmol h−1 gcat−1, 49 mol%

1994

18

7.5 h

0.048 μmol h−1 gcat−1, 5 mol%

2015

19

–

p

FeCoS2300-W Xe
AM 1.5-G filterqIn2.77S4/porouspolymeraq. NaOH (pH
12)
Water
(1 mL)
The catalyst
immersed inwater, dried at
333 K

450-W Xe(110 mW cm−2)r

N,S/Fe–MOF

sCuδ+/CeO2
–TiO2

CO2
(101 kPa)

C2H6

−1

gcat−1, 59 mol%

450-W XeUV cut filter
300-W Xe
(1-W cm−2)

300-W Xe(200 mW cm−2),>420 nmXe, 200 mW
cm−2

7

CO2 flow(101 kPa)

CO2 flows
through H2O

o

2019

6h

Visible light

n

Ref

Water
(25 mL)aq. Na2CO3,
1.5 gcat L−1

Ag–C
nanotube@TiO2
CuO/CuG
aS2
Bi2S3@In2
S3

m

Year

270 μmol h gcat−1, 45 mol%
10 μmol h−1 gcat−1, 1.7 mol%CO2(101 kPa)

Formation rate and selectivity

16 h

29 μmol h
C2H6
C2H4
C2H6

C2H4

−1

20 μmol h−1 gcat−1, 75 mol%
−120gcat−1, 86 mol%

5h

12 μmol h

10 h

20 μmol h−1 gcat−1, 83 mol%

Water
(20 mL)10 h

68 μmol h−1 gcat−1, 14 mol%

23

Water
(2 mL)

4h

18 μmol h−1 gcat−1, 11 mol%

24

Water
(30 mL)5h

4.5 μmol h−1 gcat−1, 47 mol%

S1

21

2023

2022

22

25

1.2. Major Framework of This Study.
Table S2. Summary of Kinetic Data and the Monitoring on Photoconversion of CO2 and/or CO Using the ZrO2 and Co–ZrO2
Photocatalysts in This Study.
Entr
y
a
b
c
d
e
f
g
h

Catalyst
ZrO2
Co (2.5 wt %)–ZrO2
Co (5.0 wt %)–ZrO2

PretreatT (K)under H2
823

Reactants
C oxide13

CO2 (2.3 kPa)

none
823
none
723

CO2 (95 kPa)

Reductant-2

Light irradiated/heat applied (T) *1

H2 (21.7 kPa)
H2 (2.3 kPa)
H2O (70 mL)

13

CO2 (2.3 kPa)

H2 (21.7 kPa)

13

CO2 (0.68 kPa)

none

S3

none

λ > 320 nm
λ > 520 nm
363 K
393 K
423 K

823
H2 (21.7 kPa)
Co (7.5 wt %)–ZrO2
D2O (2.2 kPa)

13

CO2 (2.3 kPa)

q

u

973

v
w
x
y

Co (10 wt %)–ZrO2z

Co (7.5 wt %)–ZrO2

823

H2 (2.3 kPa)
H2O (2.2 kPa)
CO2 (95 kPa)
13

CO2 (2.3 kPa)823

H2O (70 mL)
H2 (21.7 kPa)
H2 (2.3 kPa)
H2O (2.3 kPa)
H2 (21.7 kPa)

full light

2, S8
S3
2, S4

none

2, S4

13

973

S6A

2, S4

823

CO (2.3 kPa)
H2 (2.3 kPa)

α
β
γ

363 K
393 K
423 K

S2A
4
1, 8, S2B, S8c(the best for C1–3
paraffins)

S3

S7

973

r
s
t

2, S4
2, S3
S4, S7

N2 (2.3 kPa)p

FigureS8S5

CO2 (2.3 kPa)

j

Table
S3

full light

13

i

k
l
m
n
o

Reductant-1

Scheme 1D
S19;
S21 (EXAFS)
8, S2C;6, S11, S12,S13 (XAFS)(2nd best for C1–3
paraffins)
8, S178, S18(C1–3 paraffin
formation using
H2O)
8, S3, S4
S202, 3, 8, S5,
Scheme 1D;
S14, S15(EXAFS);7, S16 (FTIR)(the best for C2,3
olefins)

S6B

Scheme 1D*1 The photocatalyst was irradiated by light without any external heat applied or it was applied by external heat under dark.

Scheme S1. Correlation among Photocatalyst Composition, Photocatalytic Products, and Methods Utilized in This Study (Table S2i, q, t, u,
and z).

S2

Table S2 summarizes kinetic data and the characterization
results of this study. Among the tested photocatalysts, Co (7.5
wt %)–ZrO2-823R (where 973 denotes the pretreatment
temperature (K) of the photocatalysts with H2 and R represents
reduced) photocatalyst exhibited the highest activity for theformation of C1–3 paraffin using CO2, H2 and/or H2O, and UV–
visible light irradiation (Table S2i and t and Scheme S1A). We
investigated the photocatalytic reaction pathway by monitoring
isotope-labeled kinetic monitoring using 13CO2 and comparing
them with thermal reactions. To simplify the discussion for thepathway over Co species, the second active Co (7.5 wt %)–ZrO2-973R photocatalyst comprising only Co0 sites, was wellcharacterized via extended X-ray absorption fine structure(EXAFS) and density functional theory (DFT) calculations(Table S2q and Scheme S1B).
The Co (7.5 wt %)–ZrO2-973R photocatalyst exhibited thehighest activity for the formation of C2H4 and C3H6 formation
using CO, H2, and UV–visible light irradiation. We investigatedthe consecutive photocatalytic reaction pathway using 13COlabeled species and/or repeated kinetic tests, alongsidecomparison with thermal reaction, EXAFS, Fourier-transform
infrared (FTIR) spectroscopy, and DFT (Table S2z andScheme S1B).
All tested photocatalysts were compared using UV–visible
absorption/fluorescence/emission,
X-ray
photoelectron
spectroscopy (XPS), high-resolution transmission electron
microscopy (HR-TEM), X-ray diffraction (XRD), and X-ray
absorption near-edge structure (XANES) analyses (Scheme
S1C) to identify the key control factors influencing CO2/CO
photoreduction using Co–ZrO2 photocatalysts.

and 80 cm-long branches; Optel, Tokyo, Japan).[S1] The
distance between the fiber light exit and the photocatalyst was
2 cm (Chart S1A). The light intensity at the center of thesample was 142 mW cm−2 (Table 1a–i).
Light transmission was monitored using a photosensor(Model PCM-01, Prede, Tokyo, Japan) and a counter (Model
KADEC-UP, North One, Sapporo, Japan). The photosensor
was placed on the opposite side of the fiber light exit, with adistance of 2 cm between the photosensor (light entrance) andthe photocatalyst (Chart S1B and C). Approximately 60.4% ±
0.1% of light (300 nm < wavelength λ < 2800 nm) from one of
the fibers (71 mW cm−2) transmitted through both sides of thequartz reactor, resulting in 77.7% ± 0.1% transmission on one
side of the reactor. The transmitted light was through ZrO2, Co(7.5 wt %)–ZrO2-fresh (Chart S1B), Co (7.5 wt %)–ZrO2-723R,
and Co (7.5 wt %)–ZrO2-973R (Chart S1C) located in thereactor (each 20 mg) was 3.3% ± 0.2%, 4.1% ± 0.3%, 4.1% ±
0.2%, and 1.7% ± 0.1%, respectively. Thus, ZrO2 powder andCo (7.5 wt %)–ZrO2-fresh, -723R, and -973R samples
reflected/scattered/absorbed 94.5 ± 0.3%, 93.2% ± 0.5%,
93.2% ± 0.3%, and 97.2% ± 0.2% of light, respectively. Basedon the UV–visible absorption spectra (Figure S6A) and thecolor of samples, while ZrO2 powder mostly reflected/scattered
the light, Co3O4, CoO, and Co0 nanoparticles in dark green,
dark brown, and black Co (7.5 wt %)–ZrO2-fresh, -723R, and 973R samples, respectively, mostly absorbed the light.

2. EXPERIMENTAL SECTION
ZrO2 (0.500 g, JRC-ZRO-3, Catalysis Society of Japan,
predominantly monoclinic phase, specific surface area = 99.4
m2 g−1) was dispersed in deionized water (< 0.055 μS cm−1)
supplied by a model RFU424TA (Advantec, Japan), and0.0633–0.274 g of Co nitrate hexahydrate (purity > 99.5%,
Wako Pure Chemical, Japan) was added. The suspension was
ultrasonicated for 20 min (430 W, 38 kHz) and stirred at a rateof 900 rpm. NaBH4 (0.0658–0.285 g, NaBH4: Co molar ratio =8: 1) dissolved in an aqueous solution (20 mL) was added
within 1 min and stirred for 10 min at a rate of 900 rpm. The
resulting precipitate was filtered using a membrane filter (poresize = 0.1 μm; Omnipore JVWP04700, Merck–Millipore,
Darmstadt) and washed with deionized water (50 mL, fivetimes). The resulting green powder was dried at 373 K for 24 hand then heated in H2 (21.7 kPa) at 723 K for 10 min or at 823
or 973 K for 1 h. The prepared samples comprising x wt % (x =2.5–10) of Co reduced at T K under H2, were denoted as Co (xwt %)–ZrO2-TR.
The CO2 photoreduction tests at the gas/solid interface wereconducted using 20 mg of the photocatalysts. 13CO2 (2.3 kPa,
purity > 99.9%; 99.0% 13C, 0.1% 17O, 0.7% 18O, Cambridge
Isotope Laboratories, Inc., Tewksbury, MA, USA) or 13CO (2.3kPa, purity 99.5%; 99% 13C, < 2% 18O, Cambridge Isotope
Laboratories, Inc.) and H2 (2.3–21.7 kPa; purity > 99.99%)
and/or H2O (2.2 kPa) were used. The U-shaped quartz reactor
containing the catalyst (Chart S1A) was irradiated with UV–
visible light emitted from a 500 W Xe arc lamp (Model SXUID502XAM, Ushio, Japan) through a Y-shaped quartz lightguide (Model 1.2S15-1000F-1Q7-SP-RX; a 40 cm-long fiber

Chart S1. (A) Photocatalyst in Quartz Reactor Irradiated with UV–Visible Light from 500 W Xe Arc Lamp via Two Branches of Quartz
Light Guide. Light intensity 142 mW cm−2. (B, C) Transmitted Light
Intensity Detected by a Photosensor through (B) ZrO2 and Co (7.5
wt %)–ZrO2-fresh and (C) Co (7.5 wt %)–ZrO2-723R and -973R in
Quartz Reactor Irradiated via a Branch of Quartz Light Guide.

The CO2 photoconversion tests were also conducted using
CO2 (95 kPa) and H2O (70 mL) with NaHCO3 (0.202 g, purity >99.5%; Wako Pure Chemical, Japan) dissolved in it.
Photocatalyst samples (2.2–37 mg), pretreated under H2, weresealed using fire and transferred to this reactor using theSchlenk technique under the Ar atmosphere. CO2 (95 kPa) was
circulated within a closed Pyrex glass system connected to a
Pyrex flask equipped with a quartz window (Chart S2)
containing the photocatalyst (2.2–37 mg). The reactor was thenS3

irradiated with UV–visible light using either a 500 W Xe arc
lamp (Model OPM2-502, Ushio, Japan) through a light guidewith a diameter of 5 cm or a 300 W Xe arc lamp (Model MAX350, Asahi Spectra Co., Tokyo, Japan) through a quartz lightguide. The distance between the former light exit and thequartz window was 4 cm while that between the latter fiber light
exit and the photocatalyst was 2 cm (Chart S2). The lightintensity at the center of the sample was measured at 90.2 mW
per flask (Table 1j) and 222 mW per flask, respectively (Table
1k and l). The suspension was stirred at 1 000 rpm, and thereaction gas was bubbled using a gas circulation pumpconnected to the reactor during the photocatalytic reaction
tests (Chart S2).

with a packed column of 13X-S molecular sieves (length 3 m,internal diameter = 3 mm) using He (purity > 99.99995%) as
the carrier gas.
The photocatalytic products in 70 mL of H2O (liquid) werealso analyzed using a 500 MHz 1H nuclear magnetic resonance(NMR) spectrometer (Model JNM-ECA500, JEOL, Japan) at
the Center for Analytical Instrumentation of Chiba University,
Japan, employing the solvent suppression method. Before theexcitation with a rectangular 45° pulse with an irradiatedrotational magnetic field intensity B1 (16.7 kHz), the presaturation at 4.67 ppm (4 s, B1 = 25 Hz) was applied toattenuate the peak from H2O. A solution of 700 μL of
photocatalytic reaction in 1H2O solution was mixed with 35 μL
of D2O (D 99.8%, Wako Pure Chemical) solution containing 10
mM of dimethyl sulfoxide (> 99.0%, Wako Pure Chemical) as
the internal standard.
In-profile kinetic data were collected as a function of light
excitation wavelength by inserting sharp-cut filters (2.5 mm
thick) at the light fiber exits. The UV32 (or UV32N) and Y52
sharp-cut filters (Hoya, Japan) were used to pass light with λ >320 nm and > 520 mm, respectively.
UV–visible spectra were recorded on a double-beam model
V-650 spectrometer using D2 and halogen lamps below andabove 340 nm equipped with a photomultiplier tube and an
integrated ISV-469 sphere (JASCO, Tokyo, Japan) for diffusereflectance detection within the range from 200 to 800 nm.
Samples treated under H2 were transferred to an airtight cellwithout contact with air, and a formed polytetrafluoroethylene
plate was used as a reference.
XP spectra were measured using KRATOS ULTRA2(Shimadzu, Kyoto, Japan) equipped with an Al Kα light source
for Co (7.5 wt %)–ZrO2-723R, Co (7.5 wt %)–ZrO2-823R, andCo (7.5 wt %)–ZrO2-823R under conditions of 2.3 kPa of CO2
and 21.7 kPa of H2. The energy was calibrated to 285.0 eV for
C–H at C 1s.
The absorption–fluorescence spectra were recorded on an
FP-8600 spectrometer (JASCO; Chiba Iodine Resource
Innovation Center, Chiba, Japan) equipped with a 150 W Xe
arc lamp (UXL-159, Ushio) and a photomultiplier tube for
excitation at 200–450 nm within a fluorescence range of 300–
800 nm. The incident excitation light from the Xe lamp wasmonitored using a Si photodiode, and the monitored
fluorescence light emitted from the sample was normalizedbased on the incident light intensity at each wavelength. The
photocatalyst powder (2.0 mg) was mixed with deionized water(< 0.055 μS cm−1; 3.0 mL) and ultrasonicated (430 W, 38 kHz)for 30 min. All spectra were recorded for the suspensions in aquartz cell at 295 K.
HR-TEM was performed using a JEM-2100F (JEOL)
equipped with a field-emission gun (acceleration voltage of 200kV) at the Center for Analytical Instrumentation. The samples
were mounted on a Cu mesh (250 meshes per inch) coatedwith carbon and a copolymer film of poly(vinyl alcohol) andformaldehyde (Formvar, Monsanto, St. Louis, MO, USA).
XRD patterns were observed using a D8 ADVANCE
diffractometer (Bruker, Billerica, MA, USA) at the Center for
Analytical Instrumentation with a Bragg angle (θB) of 2θB = 20°–60°, employing a scan step of 0.02° and scan rate of 0.5 s per
step. The measurements were performed at 40 kV and 40 mA
using Cu Kα emission (λ = 0.15419 nm) with a Ni filter.
Cobalt K-edge XAFS spectra were measured in thetransmission mode at the Photon Factory, High Energy
Accelerator Research Organization (Tsukuba, Japan) on theChart S2. Photocatalytic Pyrex Flask Reactors Containing H2O (70mL) Equipped with Quartz Window on the Top Irradiated with UV–Visible Light from a 300 W Xe Arc Lamp through a Quartz Light
Guide. Light intensity 222 mW per flask.

For online gas chromatography–mass spectrometry (GC–MS; Model JMS-Q1050GC, JEOL, Japan) for photocatalytic
reaction tests, a packed column of 13X-S molecular sieves(length = 3 m, internal diameter = 3 mm; GL Sciences, Inc.,
Japan) was employed. Additionally, a packed column ofpolyethylene glycol-6000/Flusin P (length = 3 m, internal
diameter = 3 mm; GL Sciences, Inc.) was employed for HCO2H,
CH3OH, C2H5OH, (CH2OH)2, and iso-C3H7OH and 13CO2
photo-uptake/exchange reaction tests both using He (purity >99.99995%) as the carrier gas. The ionization energy ofseparated gas was 20 eV. Detection of 13CH4 and 12CH4 reliedon the mass number (m/z) of 17 and 16, respectively,
considering the fragment ratio of CH3+:CH4+ (0.708:1).[S1]
Similarly, 13CO, 13C2H6, 13C2H4, 13C3H8, 13C3H6, HCO2H, CH3OH,
C2H5OH, (CH2OH)2 , and iso-C3H7OH were evaluated based on
the fragment peak of m/z = 29 (13CO+), 30 (13C2H4+), 30(13C2H4+), 31 (13C2H5+), 44 (13C3H5+), 46 (HCO2H+), 31 (CH3O+),
31 (CH3O+), 62 ((CH2OH)2+), and 45 (C2H5O+), respectively, ineach mass chromatogram. Under the analysis conditions in thisstudy using the 13X-S molecular sieves packed column, themixture of 13CO and 13CO2 was well separated[S2] and no 13CO
(m/z = 29) was detected throughout the mass chromatogram
when pure 13CO2 was analyzed. Furthermore, all the reactants
and products were uniquely quantified based on the
predominant peak (GC retention time) in each mass
chromatogram.
For photocatalytic tests using H2O (70 mL), the products
were analyzed using an online GC-thermal conductivity
detector (Model GC-8AT, Shimadzu, Kyoto, Japan) equipped

S4

9C beamline using a Si (1 1 1) monochromator, a Pt-coatedmirror, and a piezo transducer.[S1,S3] The Co–ZrO2 samples
were treated in a quartz U-tube and transferred to a Pyrex cellfilled with reaction gases, which was equipped with apolyethylene terephthalate film (Toyobo, Japan, G2, 50 μm
thick) on both sides for both UV–visible light and X-ray
transmission. A 500 W Xe arc lamp (Model SX-UID502XAM)
was used as the light source, with the distance between thefiber light exit of the Y-shaped quartz light guide and thephotocatalyst set at 2 cm.
The obtained Co K-edge XAFS data were analyzed using
the XDAP software package version 3.2.9.[S4] The pre-edge
background was approximated with a modified Victoreen
function, C2/E2 + C1/E + C0, where E is the photon energy andC0, C1, and C2 are constants. Multiple-shell curve-fit analyses
were performed with the data obtained on the Fourier-filteredangular wave number k3-weighted EXAFS using the empirical
amplitude extracted from the EXAFS data for the Co metal foil
(10 μm thick) and CoO powder. The interatomic distances andcoordination number values for the Co–Co and Co–O
interatomic pairs were set to 0.2502 nm and 12[S5] and 0.2131
nm and 6,[S6] respectively.
The temperature dependence of the Debye–Waller factor
value was calculated for the bulk and surface sites of Co metal(Figure S1) using the correlated Debye model with the ab initio
multiple-scattering calculation code, FEFF version 8.4.[S7] Thebulk and surface Co site temperatures were determined (seethe main text, Photothermal Monitoring and the Control
Thermal 13CO2/13CO Reduction section).

6.2.1,[S8] computed on a VT64 Server XS2-2TI comprising four
units of Intel Xeon Platinum 9242 Processors (2.3 GHz, 48
cores; Visual Technology, Tokyo, Japan) and partially utilizing
the facilities of the Supercomputer Center at the Institute for
Solid State Physics, the University of Tokyo, Japan. The
projector-augmented-wave method was employed at the DFTD3 level[S9] to incorporate the van der Waals interactions. The
generalized gradient approximation-revised Perdew–Burke–
Ernzerhof exchange–correlation functional[S10] was employedwith a cutoff set at 500 eV, and Hubbard U parameter of 4.0 eV
for Zr was added.[S11,S12] Transition states were pursued using
the climbing image-nudged elastic band method.[S13]Hemispherical Co19 clusters exposing the (0 1 0) [= (0 1 1" 0)]face based on the hexagonal closest-packed (hcp) structure of
Co metal were used combined with monoclinic ZrO2 (1 1 1)
face comprising (2 × 2 × 2) element unit cells as a slab modelwith a vacuum spacing of 2.0 nm between the slabs in the
direction of the [1 1 1] axis of ZrO2.[S14]3. RESULTS AND DISCUSSION
3.1. Kinetic Results. The uncertainty in the amount of
product observed in the photocatalytic time course (Figures 1–
3 and S3) stemmed from several factors, and all the following
factors were taken into accounts to evaluate the data errors.
(a) The error of online GC–MS evaluation connected to acirculating system comprising a U-shaped quartz photoreactor(Chart S1A): 0.046%, 0.031%, 0.43%, 0.070%, 0.070%, 0.16%,
and 0.16% for 13CO, 13CH4, 12CH4, 13C2H6, 13C2H4, 13C3H8, and13
C3H6 using a packed column of 13X-S molecular sieves,based on calibration line.
(b) The error assuming first-order reaction kinetics due toreactant loss during sampling (1.8 volume-% each): 0.0051%
and 0.0794% using (13CO2 and H2) and (13CO and H2), basedon the comparison between five-point sampling and one-point
sampling for 5 h.
(c) The error among three photocatalytic kinetic tests: 0.57–
85%, 8.2–20%, 13–29%, 13–58%, and 18–101% for 13CO,
13
CH4, 12CH4, 13C2H6, and 13C3H8 formations using 13CO2 andH2 (Figure 1) and 3.3–9.0%, 0.96–60%, 15–46%, 10–90%, 14–87%, 21–143%, and 5.9–174% for 13CO, 13CH4, 12CH4, 13C2H4,
13
C2H6, 13C3H6, and 13C3H8 formations using 13CO and H2(Figures 2 and S3) owing to the differences in nonadjustable
reaction conditions, e.g., the storage duration ofCo(NO3)3·6H2O reagent under Ar at 273 K, the duration since
the synthesis of photocatalyst synthesized was kept under Ar,
and more critically, the nonlinear correlation between productformation rates and excitation light intensity.
More than 87% of the formation rates were consistent inrepeated photocatalytic tests under 13CO2 and H2 [(i) a set of
pretreatment and the 48-hour kinetic test was repeated threetimes, (ii) a 96-h kinetic test] except for a test conducted after a
48-hour kinetic test and a subsequent 1-hour evacuation. In thelatter reaction, the formation of 13CH4 and 13C2H6 formation
decelerated after 2 h of reaction owing to the presence of HC
intermediate remaining on the Co surface, while H wasremoved after evacuation, leading to an imbalance in thesurface population of CO/COH/HC/H species necessary for the
HC formation (see FTIR spectroscopy section in main text).
Next, in-profile CO2 reduction tests were performed. By
increasing the minimum wavelength for irradiation to 320 nm

Figure S1. Correlation between the Debye–Waller factor andtemperature for (a) bulk sites (circle, ○) and (b) surface sites(vertical motion; square, □) in/on the Co metal, calculated using
the correlated Debye model with a FEFF version 8.4 code.[S7]In situ FTIR spectroscopy measurements were performed at
293 K, ranging from 4000 to 650 cm−1, using a model FT/IR4200 instrument (JASCO, Tokyo, Japan). The Co–ZrO2
photocatalyst disk (71 mg) was heated under H2 in a quartz celland transferred to an FTIR cell using a glove box (UN6509LCIY, Unico, Japan). The sample disk was irradiated with
UV–visible light from a 500 W Xe arc lamp (Model SXUID502XAM) through a quartz fiber light guide. The distance
between the fiber light exit and the sample disk was 5 cm. Theenergy resolution of the spectrometer was set to 1 cm−1, anddata accumulation was performed for 512 scans (~2 s perscan).
Spin-polarized periodic DFT calculations were conductedusing the Vienna Ab initio Simulation Package code version

S5

and then 520 nm, the rate of 13CH4 formation gradually
decreased to 43% and 22% of that observed under full light(Table S3e, g, and h). The ratio of 13CH4, 12CH4, 13C2H6, to
13
C3H8, remained almost constant at 100: 4.6–7.7: 1.3–2.0:
0.14–2.6, regardless of the wavelength region of the irradiatedlight. This suggests that instead of band-gap excitation (5.0 eV;
Figure S6A), the vacancy/impurity level of ZrO2 (Figure S6B
and C) played an auxiliary role in charge separation for
reducing CO and subsequent reduction mechanisms over Co
sites to form HCs.

Figure S2. Time–course formation of photocatalytic 13CO, 13CH4,
12
CH4, 13C2H6, and 13C3H8 irradiated under UV–visible light (142mW cm−2) during exposure to 13CO2 (2.3 kPa) and H2 (21.7 kPa)
with Co (7.5 wt %)–ZrO2 (0.020 g) treated in H2 at (A) 723 K, (B)823 K, and (C) 973 K.

S6

Table S3. Kinetic Data for Photoconversion of CO2 and in the Absence of CO2 Using the H2-pretreated ZrO2 and Co–ZrO2
Photocatalysts Both at 823 K under UV and/or Visible Light Irradiation.
En
try

Reactant1

Catalyst

Reactant-2
H2(21.7 kPa)
H2
(2.3 kPa)

a
ZrO2
13

b
c
d
e

CO2
(2.3 kPa)

Co (2.5 wt %)–ZrO2
Co (5.0 wt %)–ZrO2

f

N2(2.3 kPa)

Co (7.5 wt %)–ZrO2

Formation rate
(μmol h−1 gcat−1)*2Light
irradiated

13

13

CO

12

CH4

13

CH4

C2H6

13

C3H8

Molar ratioof 12CH4/(13CH4 +12CH4) (%)

1.4< 0.002–

1.8

full light(142 mWcm−2)

H2(21.7 kPa)

4.1*1
6.1*1
3.6*1

42
64
300

3.1
7.1
23

< 0.0020.027*1

1.8*1

0.63
1.2
4.1

0.064
0.19
0.41
< 0.002g> 320 nm
3.5
130
6.0
2.6
3.4
13
CO2
h
> 520 nm
3.7
67
5.0
0.89
0.099
(2.3 kPa)i
Co (10 wt %)–ZrO2
full light
3.2*1
52
4.8
0.65
0.071
*1 The formation ceased/decreased in 1–4 h of reaction owing to the noncatalytic nature and/or subsequent consecutive reaction.
*2 13C2H4 and 13C3H6 were not found above the detection limit > 0.002 μmol h−1 gcat−1.

7.0
9.9
7.1
98.5
4.4
7.0
8.5

Table S4. 12C/13C Isotopic Ratio on the Photoconversion of CO2 or CO Using Co (7.5 wt %)–ZrO2 Photocatalysts under UV–
Visible Light Irradiation (142 mW cm−2; Associated Data with Table 1, the main text).
Entry
a
b
c
d
e
f
g
h
i

C oxideReductant

T (K) under H2

H2 (21.7 kPa)
CO2 (2.3 kPa)

CO (2.3 kPa)

CH4/(13CH4 + 12CH4) (mol-%)

–
723
823

13

13

12

–
12
7.1
4.5
6.0
17.7
6.7
18
7.9

973

H2 (2.3 kPa)
H2O (2.2 kPa)
H2 (2.3 kPa)
H2 (21.7 kPa)
H2 (2.3 kPa)

823
973

Table S5. Uptake and Exchange of CO2 Using ZrO2-723R and Co–ZrO2-823R under UV–Visible Light Irradiation (142 mWcm−2).*1
Physisorption

*1

Entry

Catalysta

ZrO2-723R[S1]bCo (7.5 wt %)–ZrO2-823R

Amount(μmol)
9.9
19
3.9
3.9

Rate
constant
(h−1)
8.0
4.7
0.4
0.4

Chemisorption
Amount(μmol)

Rate constant
(h−1)

0.66

0.07

0.050

0.2

The amount of photocatalyst was 0.020 g.

Figure S3. Time–course formation of photocatalytic 13CH4, 12CH4, 13C2H4, 13C2H6, and 13C3H8 and the decrease of 13CO under UV–visible light
exposure (142 mW cm−2) during the presence of 13CO (2.3 kPa) and H2 (2.3 kPa) with Co (7.5 wt %)–ZrO2 (0.020 g) catalyst treated in H2 at
823 K. The error bars for each product were evaluated based on three factors described in 3.1. Kinetic Results.

S7

Using CO and H2 as reactants, the consecutive reaction
kinetics of C2H4 followed by C2H6 formation (Figures 2, 3, andS3) was analyzed.
1
2
2CH 2 (surface) ⎯k⎯→ C 2 H 4 , C 2 H 4 + 2H(surface) ⎯k⎯→ C 2 H 6 (S1)x = [2CH2], y = [C2H4], z = [C2H6]x0 = a, y0 = 0, z0 = 0

(S2)
(S3)

dx
dy
dz
= −k1 x,
= k1 x − k2 y,= k2 y
dt
dt
dt

(S4)
⎛
⎞
k1a
k2 −k1t
k1
−k1t
−k2t
x = ae , y =(e − e ), z = a ⎜⎝1− k − k e + k − k e−k2t ⎟⎠k2 − k1
2
1
2
1
(S5)
The time–course change of 13C2H4 and 13C2H6 molar
amounts fitted well (Figure S4) when the parameters a, k1, andk2 were 0.31 μmol, 0.51 h−1, and 0.0077 h−1, respectively. The
amount of 13C2H6 formed was 0.26 μmol at 47 h ofphotoreaction (Figure 2), while the z value was 0.090 μmol
based on Eq. S5. Thus, 35% of C2H6 was generated via C2H4hydrogenation, i.e., Eq. S1, while 65% of C2H6 was formed via
coupling CH3 species with CH2 or CH3 species over the Co
surface.
In summary, selective C2H4 formation was enabled when the
k1 value was greater by a factor of 66 times than the k2 value,
while a nonconsecutive route directly forming C2H6 via CH3
coupling was also plausible.
−k1t

Figure S5. Time–course formation of photocatalytic 13CH4, 13C2H4,
and 13C2H6 (i) during exposure to 13CO (2.3 kPa) and H2 (2.3 kPa)for 4–10 h, followed by (ii) 1 h of evaluation and (iii) subsequent
exposure to 13CO (2.3 kPa) for 1 h using Co (7.5 wt %)–ZrO2-973R(0.020 g). Irradiated light intensity 142 mW cm−2. The cycle of steps(i)–(iii) was repeated four times.

A blank test using N2 (2.3 Pa), H2 (21.7 kPa), Co (7.5 wt %)–ZrO2-823R, and UV–visible light irradiation was also performed.
Only CH4 was formed, consistent with the natural abundance
ratio of 12C: 13C (98.9: 1.1; Table S3f). However, the timecourse exhibited noncatalytic behavior, with the formation rate
decreasing after 1 h of photoreaction (not shown). The total
amount of CH4 formed in 27 h was 0.06 μmol per 20 mg ofphotocatalyst, in consistent with the chemisorbed 12CO2
amount (0.050 μmol) determined using by 13CO2 exchange
reaction (Figure 4 and Table S5b).
Notably, the catalytic formation of 12CH4 using 13CO2 (13CO)
includes the contribution of 1.0% of impurity 12CO2 (12CO) in the13
CO2 (13CO) reagent (see 2. Experimental section, Tables 1,
S3a–e and g–i, S4, and S6).
Control thermal reaction tests were performed using Co (7.5
wt %)–ZrO2-823R, 13CO2 (2.3 kPa), and H2 (21.7 kPa) at areaction temperature between 363 and 423 K (Table S6A and
Scheme 1D). The formation rate ratio of 13CH4, 12CH4, 13C2H6,
to 13C3H8 was 100: 3.6–11: 0.56–1.1: 0–0.14 compared tothose observed in photocatalytic reaction, which were 100: 7.7:
1.4: 0.14 (Table 1c).
Control thermal reaction tests were also performed using Co(7.5 wt %)–ZrO2-973R, 13CO (2.3 kPa), and H2 (2.3 kPa) attemperatures between 363 and 423 K (Table S6B and Scheme
1D). At 423 K, the formation rate ratio of 13CH4, 12CH4, 13C2H4,
13
C2H6, 13C3H6, to 13C3H8 was 100: 16: 2.9: 12: 13: 4.5 (Table
S6B-c) compared to those observed in photocatalytic reaction,
which were 100: 8.6: 243: 11: 34: 1.1 (Table 1i). Notably, olefin
formation was especially suppressed in the thermal reaction,suggesting different reaction pathways under UV–visible lightirradiation compared to the traditional Fischer–Tropschmechanism.[S15,S16]

Figure S4. Time–course formation of photocatalytic 13C2H4 and13
C2H6 irradiated with UV–visible light (142 mW cm−2) duringexposure to 13CO (2.3 kPa) and H2 (2.3 kPa) with Co (7.5 wt %)–ZrO2 (0.020 g) treated in H2 at 823 K, along with the fit toconsecutive reaction via Eq. S5 and calculated amounts of CH2
species and C2H6 via C2H4.

S8

3.2. Characterizations.

midgap trap states at 468, 530, and 621 nm were observed.
The intensity of all the peaks was suppressed to one-tenth to
one-fifth for Co (7.5 wt%)–ZrO2 (Figure S6B-b–e) owing to thecharge transfer of light-excited electrons to Co3O4, CoO, or
metallic Co.
In the excitation spectra for ZrO2, fluorescence at 397, 468,
and 530 nm appeared from the excitation at 304 nm, while
fluorescence at 468 and 530 nm appeared from the excitation
at 320 nm. Additionally, fluorescence at 468, 530, and 621 nm
appeared from the excitation at 367 nm (Figure S6C-a), allattributed to the near band-edge and midgap trap states on/in
ZrO2. These peaks were substantially suppressed owing to thecharge transfer effect of Co species (Figure S6C-b, d, and e),
consistent with the trend observed in the fluorescence spectra(Figure S6B).

Figure S7. Davis–Mott plots for n = 1/2 (allowed direct transition) in
Eq. 3 depicting the UV–visible absorption spectra of (A) fresh Co(7.5 wt %)–ZrO2 (not pretreated with temperature) and (B) Co (7.5
wt %)–ZrO2-723R.

The valence band maximum (EVBM vs. normal hydrogenelectrode (NHE)) was calculated based on the work function of
the XPS apparatus (4.23 eV) and the XPS binding energy(Figure S8C).
4.23 + 2.54 − 4.44 = 2.33 V for Co (7.5 wt %)–ZrO2-723R
4.23 + 2.62 − 4.44 = 2.41 V for Co (7.5 wt %)–ZrO2-823R
The Eg value was 2.89 eV based on the Davis–Mott
equation[S18] (n = 1/2) for CoO (Figure S7B). Thus,
ECBM vs. NHE= EVBM vs. NHE – Eg
= 2.33 − 2.89 = −0.56 V for Co (7.5 wt %)–ZrO2-723R= 2.41 − 2.89 = −0.48 V for Co (7.5 wt %)–ZrO2-823R

Figure S6. (A) Diffuse-reflectance UV–visible absorption, (B)fluorescence, and (C) excitation spectra of ZrO2 (a), fresh Co (7.5
wt %)–ZrO2 (not pretreated with temperature) (b), and Co (7.5
wt %)–ZrO2 treated in H2 at 723 K (c), 823 K (d), and 973 K (e).
The excitation wavelength is 200 nm in the fluorescence spectra in(B) and the monitored fluorescence wavelength is 369, 397, 468,
530, and 621 nm in the excitation spectra in (C) for photocatalysts
a, b, d, and e.

The bandgap value (5.0 eV) obtained based on absorption
spectrum (Figure S6A-a) was within the range of reported
experimental values.[S17] In the fluorescence spectrum of ZrO2
excited at 200 nm (Figure S6B-a), the short-wavelength
excitation corresponding to near band-edge transitions at 367
and 397 nm was observed, along with excitation from/to the

S9Figure S8. XPS spectra in (A) Zr 3d, (B) O 2s, and (C) valence band regions measured for (a) Co (7.5 wt %)–ZrO2-723R, (b) Co (7.5 wt %)–ZrO2-823R, and (c) Co (7.5 wt %)–ZrO2-823R under 2.3 kPa of CO2 and 21.7 kPa of H2. Expanded view of peak tops is also drawn in (A).

Figure S9. HR-TEM images of Co (7.5 wt%)–ZrO2-723R photocatalyst. Lattice intervals for monoclinic ZrO2 and CoO were also drawn.

Table S6. Control Kinetic Data for Thermal Conversion at Reaction Temperature 363–423 K (A) Using 13CO2 (2.3 kPa), H2(21.7 kPa), and the H2-pretreated Co (7.5 wt %)–ZrO2 Catalyst (0.020 g) at 823 K and (B) Using 13CO (2.3 kPa), H2 (2.3 kPa),
and the H2-pretreated Co (7.5 wt %)–ZrO2 Catalyst (0.020 g) at 973 K in the Absence of UV–Visible Light Irradiation.
Entry

Treaction
(K)

13

a
b
c

363
393
423

CO
0.006*10.085*1
8.5*1

a
b
c

363
393
423

–
–

(A) 13CO2 (2.3 kPa), H2 (21.7 kPa), and Co (7.5 wt %)–ZrO2-823R
Formation rate
(μmol h−1 gcat−1)
13
12
13
13
13
13
CH4
CH4
C2H4
C2H6
C3H6
C3H8
1.6
0.18
0.009
<0.00213
0.48<0.0020.081<0.002
0.018
79
3.4
0.87
0.040
(B) 13CO (2.3 kPa), H2 (2.3 kPa), and Co (7.5 wt %)–ZrO2-973R<0.0020.026
0.0071<0.002
0.63
0.10
0.019
0.075
0.081
0.029

ΣC
1.8
14
91

0.035
0.94

Molar ratio of 12CH4/(13CH4 + 12CH4) (%)
10
3.5
4.2
–
21
14

*1 The formation ceased/decreased in 1–3 h of reaction owing to the noncatalytic nature and/or subsequent consecutive reaction.

S10

Figure S11. Normalized Co K-edge XANES spectra for (A) the
standard Co metal, CoO, and Co3O4 samples (dotted lines)
compared to the fresh Co (7.5 wt %)–ZrO2, Co (7.5 wt %)–ZrO2723R, Co (7.5 wt %)–ZrO2-823R, and Co (7.5 wt %)–ZrO2-973R
photocatalysts (solid lines), and (B) Co metal, CoO, and Co (7.5
wt %)–ZrO2-823R under H2 (21.7 kPa) only and under a mixture of
CO2 (2.3 kPa) and H2 (21.7 kPa) (solid lines), along with theconvolution of normalized XANES spectra for Co and CoO with themixing ratios of 0.85: 0.15 and 0.45: 0.55 (dotted lines).

The major CH4 formation rate reached 196 μmol h−1 gcat−1
during the initial 10 min of UV–visible light irradiation over theCo (7.5 wt %)–ZrO2-973R photocatalyst (20 mg), resulting in
the formation of 0.65 μmol of CH4. Calculating based on thestandard reaction enthalpy of CO2 hydrogenation to CH4(−164.94 kJ mol−1), the heat of the reaction was determined tobe 0.11 J. Based on the molar heat capacity of Co and ZrO2
(24.81 J K−1 mol−1 [S19] and 56.123 J K−1 mol−1,[S20] respectively),
the resulting temperature rise was calculated at 11.9 K.
However, this increase was notably smaller than the observed
initial temperature increase of 66 K in 10 min (Figure 6). Thus,
the temperature elevation was attributed to the transformationof light energy into heat at the Co0 surface, which quickly
reached the equilibrium (Figure 6), dissipating heat into thereactor/XAFS cell.

Figure S10. XRD patterns of (A) (a) ZrO2, Co (7.5 wt %)–ZrO2
treated in H2 at (b) 723 K, (c) 823 K, and (d) 973 K with peakassignments and (B) expanded pattern with peak assignments forthe data in panel A-d.

The Co K-edge XANES spectra were measured (Figure
S11A) to identify the Co species. The XANES pattern for freshCo (7.5 wt %)–ZrO2, Co (7.5 wt %)–ZrO2-723R, and Co (7.5
wt %)–ZrO2-973R resembled those of standard samples of
Co3O4, CoO, and Co metal, respectively.
For Co (7.5 wt %)–ZrO2-823R, the XANES pattern was fit
using the convolution of the XANES pattern by varying themixing ratio between the normalized XANES spectra of metallic
Co and CoO. The best fit was achieved with a ratio of 0.85:
0.15 (Figure S11B). Interestingly, the surface Co0 site (~40% of
total Co sites) of metallic Co nanoparticles slowly oxidized to
CoII in the presence of CO2 and H2 gas for 1 day without UV–
visible light irradiation (Figure S11B).

S11

Figure S14. Time–course evolution of the Fourier transform of theangular wave number k3-weighted EXAFS χ-function for the Co(7.5 wt %)–ZrO2-973R photocatalyst under CO (2.3 kPa) and H2(2.3 kPa) (A) during UV–visible light irradiation (142 mW cm−2) and(B) under dark.

Figure S12. Time–course evolution of the Fourier transform of theangular wave number k3-weighted EXAFS χ-function for the Co(7.5 wt %)–ZrO2-973R photocatalyst under CO2 (2.3 kPa) and H2(21.7 kPa) (A) during UV–visible light irradiation (142 mW cm−2)and (B) under dark.

Figure S15. Time course analysis of Debye–Waller factor derived
from Co K-edge EXAFS analysis and the corresponding
temperature evaluation based on the correlated Debye model of Co
nanoparticles in Co (7.5 wt %)–ZrO2-973R using CO (2.3 kPa), H2(2.3 kPa), and UV–visible light irradiation (142 mW cm−2) and underdark. The error bars were drawn based on data reproducibility inthree runs and the fit errors.

Figure S13. Time course analysis of (A) coordination number (N)and (B) interatomic distance (R) derived from Co K-edge EXAFS
analysis for Co (7.5 wt %)–ZrO2-973R using CO2 (2.3 kPa), H2(21.7 kPa), and UV–visible light irradiation (142 mW cm−2) using
curve-fit analyses and under dark. The error bars were drawn
based on data reproducibility in three runs and the fit errors.

S12

Figure S16. FTIR monitoring of the O–C–O stretching vibration
region and the peak assignments using a Co (7.5 wt %)–ZrO2973R photocatalyst (71 mg). The conditions were as follows: (a)under 13CO (2.3 kPa) and H2 (2.3 kPa) in the dark for 2 h, (b–d)
under 13CO (2.3 kPa), H2 (2.3 kPa), and UV–visible light irradiation
at 0 h (b), 12 h (c), and 34 h (d), and (e, f) under vacuum and UV–
visible light irradiation at 0 h (e) and 2 h (f).

3.3. DFT Calculations. The photocatalytic CO2 reduction
reaction using Co (7.5 wt %)–ZrO2 proceeded steadily (FigureS2), mostly owing to the effective separation of VO•• sites for
CO2
adsorption/activation
and
Co0
sites
for
H2
activation/hydrogenation (Schemes 1A and 2A). Notably, theactivation energy from CH2 to CH3 species decreased from0.55 eV in the absence of neighboring H atoms to 0.30 eV(Scheme 2A-d) in the presence of neighboring H atoms over
the Co0 surface.
Furthermore, the photocatalytic CO reduction reaction
proceeded consecutively (Figures 2, S3, and S4). The VO•• site
became occupied by hydroxy species and the concentration of
H species over Co0 sites gradually increased, making CO
adsorption on Co0 became unfavorable and leading tohydrogenation of C2H4 species (Scheme 2B-bʹ) ultimately
resulting in paraffin formation.

Scheme S2. Three-Dimensional Illustration of Surface Species(Scheme 2A, Aʹ, and B) over a Monoclinic ZrO2 (1 1 1) Surface,
Combined with a Co19 Nanocluster Exposing (0 1 0) Surface. The
Illustration Created Using the VESTA Version 3.5.7[S21] (Initial
State), and (A, Aʹ) under CO2 and H2 and (B) under CO and H2,Using the OVITO Version 3.7.8.[S22] Green, Zr Atom; Red, O Atom;Gold (the Panel of Initial State), O Atom to Be Removed Bound to
Two Zr Atoms;[S14] Gray, C Atom; White, H Atom; Purple, Co Atom.

S13

3.4. Photocatalytic Conversion of CO2 Using H2O. The
results of the CO2 photoreduction test in liquid H2O aresummarized in Table S8. Under UV–visible light irradiation(90.2 mW cm−2 per flask, Chart S2), CO2 photoconversion wasvery limited, e.g. 7.2 μmol h−1 gcat−1 of CO and CH4 formed in
total (Table S8c) or 0.095 μmol h−1 gcat−1 of C2H6 and C3H8
formed in total (Table S8d). In contrast, H2O photosplitting
proceeded well at the formation rates of 23–690 μmol h−1 gcat−1
using ZrO2 and Co (7.5 wt %)–ZrO2-973R photocatalysts(Table S8a–e).
With increase in light intensity from 90.2 to 222 mW cm−2 perflask (Chart S2), the formation rates of C1, C2, and C3 products
increased to 22–50, 1.5–1.6, and 0.52–1.0 μmol h−1 gcat−1,
respectively, using H2O (70 mL) and either Co (7.5 wt %)–ZrO2-823R or -973R photocatalyst (Table S8f and g and Figure
8D).
3.5. Photocatalytic Conversion of 13CO2 Using D2O. The
photoreduction of 13CO2 (2.3 kPa) using D2O (2.2 kPa), H2(21.7 kPa), and Co (7.5 wt %)–ZrO2-823R formed 13CH4,
13
CH3D, 13CH2D2, 13CHD3, to 13CD4 with a ratio of 100: 41: 6.2:
0.21: ~0 (9.2 mol%-D in these products), which agrees with thetheoretical H/D distribution: 100: 40: 6.1: 0.41: 0.010 inreactants (9.1 mol%-D in these products; Figure S19 and Table
S7b). This suggests that D2O and H2 reached equilibrium more
rapidly over Co0 more effectively than the progressivehydrogenation to C1–3 paraffin (Scheme 1A-a–d).
The methane formation rate was the 9.6% of that observed
using H2 (Table 1c, e) owing to the competitive adsorption of
OH on the Co0 surface and/or H2O adsorption at the VO•• site of
ZrO2. However, during photocatalytic CO2 reduction tests underH2O (2.3 kPa; Figure S20) via UV–visible spectroscopy andtests under H2O (2.3 kPa) and H2 (21.7 kPa; Figure S21)
through Co K-edge EXAFS, the CoII species could not be
detected. This demonstrates that the metallic Co surface
effectively activates H2O for C2,3 paraffin photosynthesis using
either H2, H2O (g), or H2O (l).
Based on the criteria for evaluating the fragment peak of m/z= 17 (13CH4+), 16 (12CH4+), 30 (13C2H4+), 30 (13C2H4+), 44(13C3H5+), and 31 (13C2H5+) for the formation of 13CH4, 12CH4,
13
C2H4, 13C2H6, 13C3H6, to 13C3H8 (see the 2. Methods section),
only the 13CH3+ fraction was considered for m/z = 16. The
contributions of the 12CH3D+ fraction for m/z = 17 (methane),
13
C2H2D+ for m/z = 30 (ethene), 13C2H2D+ and 13C12CH5+ for m/z= 30 (ethane), 13C3H3D+ and 13C212CH6+ for m/z = 44 (propene),and 13C2H3D+ and 13C12CH6+ for m/z = 31 (propane) were allneglected owing to the low D and 12C ratio in the reactant (D:
9.1 mol%) and the inability of our MS spectrometer (Model
JMS-Q1050GC, JEOL) to distinguish the mass number after
the decimal point. Thus, the tentatively obtained formation
molar ratio of (13CH4 + 13CH3D + 13CH2D2 + 13CHD3 + 13CD4),
12
CH4, 13C2H4, 13C2H6, 13C3H6, to 13C3H8 was 100: 10: ~0: 1.2:
~0: 1.7, similar to the ratio 100: 7.7: ~0: 1.4: ~0: 0.14 using
reactant 13CO2 (2.3 kPa) and H2 (21.7 kPa) (Tables 1c and S7a
and b).

Figure S17. Time–course formation of photocatalytic 13CO, 13CH4,
12
CH4, 13C2H6, and 13C3H8 during exposure to 13CO2 (2.3 kPa) andH2O (2.2 kPa) using Co (7.5 wt %)–ZrO2-823R, with rapid
evacuation of gas phase H2. Irradiated light intensity 142 mW cm−2.

Figure S18. Time–course formation of photocatalytic CO, CH4,
C2H6, and C3H8 during exposure to CO2 (95 kPa) and H2O (70 mL)
under UV–visible light irradiation (222 mW per flask; Chart S2)
using Co (7.5 wt %)–ZrO2-823R (3.5 mg; A) and -973R (16 mg; B).

Figure S19. Time–course formation of photocatalytic 13CH4,
13
CH3D, 13CH2D2, and 13CHD3 during exposure to 13CO2 (2.3 kPa),
D2O (2.2 kPa), and H2 (21.7 kPa) under UV–visible light irradiation(142 mW cm−2). using Co (7.5 wt %)–ZrO2-823R. Catalyst amount:
0.020 g.

S14

Figure S21. Time–course change of the Fourier transform of theangular wave number k3-weighted EXAFS χ-function for the Co(7.5 wt %)–ZrO2-973R photocatalyst under CO2 (2.3 kPa), H2O (2.3kPa), and H2 (21.7 kPa) (A) during irradiation under UV–visible light(142 mW cm−2) and (B) under dark.

Figure S20. Diffuse-reflectance UV–visible spectrum of Co (7.5
wt %)–ZrO2-823R photocatalyst (0.088 g) under UV–visible lightirradiation (142 mW cm−2) for 72 h, in the presence of CO (2.3 kPa)
and H2O (2.3 kPa). Peak assignment was based on corresponding
spectra in Figures S6A-b and e. The broad absorption in the visible
light region is attributed to metallic Co0 species.

Table S7. Kinetic Data for Photoconversion of CO2 in the Presence/Absence of D2O Using H2 and the H2-pretreated Co (7.5
wt %)–ZrO2 Photocatalyst (0.020 g) at 823 K under UV–Visible Light Irradiation (142 mW cm−2).
Entry

13

CH4

a
b

Formation rate
(μmol h−1 gcat−1)
13
13
CHD3
CD4

Reactants
13

CO2 (2.3 kPa)
+ H2 (21.7 kPa)
13
CO2 (2.3 kPa)
+ D2O (2.2 kPa)
+ H2 (21.7 kPa)

13

CH3D

13

CH2D2

300
13

–
5.4

0.82

0.028

< 0.002

C3H8

ΣC

D molarratio in13
CH4 (%)

4.1

0.41

303

–

0.23

0.32

20

9.2

13

C2H6

13

Table S8. Kinetic Data for Photoconversion of CO2 (95 kPa) Using H2O (70 mL) and ZrO2 or the H2-pretreated Co–ZrO2
Photocatalyst at 823 K or 973 K under UV–Visible Light Irradiation.
Catalyst

Entr
yCatalysta

ZrO2

Amount
(mg)
10

c

2.2

d
e
f
g

Co (7.5 wt %)–ZrO2-823R
Co (7.5 wt %)–ZrO2-973R

Light
intensity

5.0bCo (7.5 wt %)–ZrO2-973R

H2O

70 mL

90.2 mW
per flask

9.0
37
3.5
70 mL
16

222 mW
per flask

Formation rate
(μmol h−1 gcat−1)
CO

CH4

HCO2−

CH3OH

12

0.68

< 0.008

< 12

< 7.6

–

–

0.37

< 0.008

< 6.2

< 3,8

–

–

3.6

3.6

< 28

< 17

–

–

0.90
0.74
12
CO

0.25
0.064
12
CH4

< 6.9
< 1.7
HCO2−

< 4.2
< 1.0
CH3OH

0.071
–
12
C2H6

0.024
–
12
C3H8

41

8.8

< 18

< 11

1.6

1.0

19

3.4

< 3.9

< 2.4

1.5

0.52

C2H6

12

C3H8

H2
140
0
630
270
0
580
150
1
H2
130
0
500

O2
690
320
520
110
23
16
O2
590
66

K. Niki: 0000-0002-0795-1007
T. Itoi: 0000-0002-2864-6650
Y. Izumi: 0000-0001-8366-1864

Corresponding Author* yizumi@faculty.chiba-u.jp

ORCID
T. Loumissi: 0000-0002-4840-8625
R. Ishii: 0000-0001-7224-7107
K. Hara: 0000-0002-5014-9079
T. Oyumi: 0000-0002-8339-5161
I. Abe: 0009-0008-9732-6846
C. Li: 0009-0008-1494-2708
H. Zhang: 0000-0002-6773-8769
R. Hirayama: 0000-0002-9706-3010

Acknowledgements
The authors thank Daiki Mitsumoto, Chiba University for
NMR measurements.

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