RESEARCH ARTICLE
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.
Supporting information for this article is given via a link at the end of the document.

nanotube,[14] Au–Pd/TiO2 {1 0 1},[15] and TiON1−(O vacancy) for C2H6and
C2H4;[16]C/Cu2Onanorod,[17]Cu/TiO2,[18]Ag–Cnanotube@TiO2,[19] CuO/CuGaS2,[20] Bi2S3@In2S3,[21] FeCoS2,[22]In2.77S4/porous polymer,[23] N, S/Fe–MOF,[24] and Cuδ+/CeO2–
TiO2 for C2H4;[25] forming the corresponding alcohol, aldehyde,
and acid.[3,26–28]
The number of reports on the photocatalytic synthesis of C2
and C3 HCs from CO2 has dramatically increased since 2019
(Table S1a, b, g, i, j, and n–s), yet a comprehensive
understanding of the reaction pathways remains elusive,
hindering precise control. This study reports the switchover ofphotocatalytic pathways from CO2 to C1–3 paraffin versus fromCO to selective C2H4 and C3H6, using a Co0–ZrO2 catalyst(Supporting Information (SI), 1.2. Major Framework of This
Study).

Abstract: Photocatalytic reduction of CO2 into C2,3 hydrocarbons
completes a C-neutral cycle. The reaction pathways of photocatalytic
generation of C2,3 paraffin and C2H4 from CO2 are mostly unclear.
Herein, a Co0–ZrO2 photocatalyst converted CO2 into C1–3 paraffin,
while selectively converting CO into C2H4 and C3H6 (6.0 ± 0.6 μmol
h−1 gcat−1, 70 mol%) only under UV–visible light. The photocatalytic
cycle was conducted under 13CO and H2, with subsequentevacuation and flushing with CO. This iterative process led to anincrease in the population of C2H4 and C3H6 up to 61–87 mol%,
attributed to the accumulation of CH2 species at the interface
between Co0 nanoparticles and the ZrO2 surface. CO2 adsorbed
onto the O vacancies of the ZrO2 surface, with resulting COH
species undergoing hydrogenation on the Co0 surface to yield C1–3
paraffin using either H2 or H2O (g, l) as the reductant. In contrast, COadsorbed on the Co0 surface, converted to HCOH species, and thensplit into CH and OH species at the Co and O vacancy sites on ZrO2,
respectively. This comprehensive study elucidates intricate
photocatalytic pathways governing the transformation of CO2 into
paraffin and CO to olefins.

Results and Discussion
Photocatalytic 13CO2 Reduction Using H2. The
photocatalytic reduction tests of 13CO2 were first performed
using ZrO2-823R, Co–ZrO2-823R (where 823 denotes the
pretreatment temperature (K) of the photocatalysts with H2 andR represents reduced), and a Xe arc lamp guided through aquartz light conduit (142 mW cm−2; Table 1a–i). ZrO2 mostly
reflected/scattered 94.5% ± 0.3% of light (300 nm < wavelength
λ < 2800 nm), while Co–ZrO2-fresh, -723R, and -973R mostly
absorbed 93.2% ± 0.5%, 93.2% ± 0.3%, and 97.2% ± 0.2% oflight, respectively (SI, 2. Experimental Section). In contrast to
the sluggish formation of 13CO using ZrO2-823R (Table S3a), theutilization of Co–ZrO2-823R comprising 2.5–10 wt % of Co
resulted in the predominant formation of 13CH4 accompanied by
12
CH4, 13C2H6, 13CO, and 13C3H8 (Tables 1c and S3c–e and i).
13
CO emerged as the primary product within 5 h of the reaction,
followed by 13C1–3 paraffin formation (Figure 1). Among thesecatalysts, Co (7.5 wt %)–ZrO2-823R exhibited the highest total
formation rate of HCs and CO (Tables 1c and S3e and Scheme
S1A). This study confirmed a similar distribution of C1–3 paraffin
distribution using Co–Cu/TiO2 (Table S1a)[7] via monitoring of
13
C-isotopic time-course dynamics.

Introduction
In contrast to the irreversible consumption of fossil fuels andraw materials, the conversion of CO2 reduction into fuels and/or
valuable chemicals using a sustainable energy represents apivotal step toward establishing a new carbon-neutral cycle.[1,2]
Photocatalytic CO2 reduction offers a direct and simple
approach; however, the range of products has been limited to
CO, CH4, and CH3OH,[1–3] unlike the electrochemical productionof a broad spectrum including formate,[4] C2H6, C2H4,[4,5]CH3CHO,[4] C2H5OH,[4–6] C3H8, C3H6, C3H7OH,[4,6] acetate,[5,6] andoxalate[5] from CO2 and/or CO, facilitated by concentrated
electrons supplied from electricity. The economically viable
nature that formed C2 and C3 hydrocarbons (HCs) derived fromphotocatalytic CO2 reduction (Table S1) has emerged as key
chemicals (0.9–8 $ kg−1) compared to CO and CH4 (0.06–0.18$ kg−1).[3] Various catalysts have been explored for this purpose,including Co–Cu/TiO2 for C2H6 and C3H8;[7] single Au/red P,[8]CdS/Cu–nanotube,[9] Nafion–Pd–TiO2,[10] Pt–graphene/TiO2−x,[11]graphene–TiO2,[12] and Au@Bi12O17Br2 for C2H6;[13] CuPt2/TiO2

1

RESEARCH ARTICLE
Table 1. Kinetic Data for Photoconversion of CO2 or CO Using Co (7.5 wt %)–ZrO2 Photocatalysts Irradiated under
UV–Visible Light.
Entry
a
b
c
d
e

C
oxide

13

CO2
(2.3
kPa)

f
g
h
i
j*5
k*5
l*5

13

CO
(2.3
kPa)
CO2
(95
kPa)

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

H2O (70 mL)

T (K)
under
H2
–
723
823
973
823

Light
intensity

823
973

CO
*3

142 mW
per 1 cm2cat*1

0.016
0.29*3
3.6*3
11*3
40
2.6
–

973

973

Formation rate (μmol h−1 gcat−1)

1390.2 mW
per flask*2
222 mW
per flask*2

CO

13

12

CH4

CH4

< 0.002
1.1
0.14
300
23
190
8.9
52
3.3

13

13

C2H4

C2H6

< 0.002
0.011
4.1
3.4
1.1

< 0.002

13

13

C3H6

C3H8

0.41
0.25
0.082

< 0.002

3.4

15

1.1

5.5*2

0.20
0.70

8.6

1.9

0.061

1.3

0.67

1.7

2.2

0.18

5.2*4

0.23

0.74*4

0.024

12

C2H6

O2

< 0.002

16

CH4

16

< 0.002

0.014
0.098

12

C3H8

< 0.002
O2

0.90

0.25

0.071

0.024

110

41
19

8.8
3.4

1.6
1.5

1.0
0.52

590
66

*1 Using a quartz reactor (Chart S1). *2 Using a Pyrex flask equipped with quartz window (Chart S2).
*3 Formation ceased within 1–4 h of reaction owing to the subsequent consecutive reactions.
*4 Selected consecutive reactions with the highest rates.
*5 Photocatalyst quantities were 20 mg each, except for entries j (9.0 mg), k (3.5 mg), and l (16 mg).

Scheme 1. (A, B) Proposed Mechanisms for CO2 Photoreduction to C1–3 Paraffin (A) and CO Photoreduction to C2,3 Olefins (B); (C) the
Energetics of First Half of Photocatalytic Reaction Steps from CO2 to CO and the Electron Flows; and (D) Comparisons of 13CO2 and 13CO
Reduction Rates between Photocatalytic (circle, ○) versus Thermal (square, □) Processes Using Co–ZrO2-823R for 13CO2 and Co–ZrO2-973R
for 13CO Reduction.

with previous findings on photocatalytic CO formation using
CO2 and Co3O4 nanoparticles, which necessitated a Ru
photosensitizer and triethanolamine.[29] Next, the reduction
temperature in H2 of the Co (7.5 wt %)–ZrO2 photocatalyst wasvaried between 723 and 973 K (Figure S2A–C and Table 1b–d).
The major 13CH4 formation rate increased by 280 times with
Co–ZrO2-823R compared to -723R. However, this rate dropped
to 63% with Co–ZrO2-973R compared to that using -823R,
owing to the progressive reduction from Co3O4 to metallic Co0
and the possibly superior reactivity of the face-centered cubic(fcc) Co0 surface versus hexagonal close-packed (hcp) one[30](see the following UV–visible spectra, high-resolution
transmission electron microscopy (HR-TEM), and X-ray
absorption near-edge structure (XANES) sections). More than
87% of the formation rates were retained in repeatedphotocatalytic tests (Figure 1), provided that adsorbent balanceon the Co0 surface was maintained (SI, 3.1. Kinetic Results).
The obtained molar ratio of C1–3 paraffin (13CH4, 12CH4,
13
C2H6, to 13C3H8) was almost constant at 100: 4.7–13: 0.98–
1.8: 0–0.14 (Table 1b–d), regardless of the reduction
temperature under H2. This consistency suggests a common
reaction pathway, independent of the reduction temperature(Scheme 1A). The considerably slow dissociation of the first C–
O bond in the CO2-derived intermediate on O vacancy (VO••)

Figure 1. Time–course formation of photocatalytic 13CH4, 12CH4,
13
CO, 13C2H6, and 13C3H8 during exposure to 13CO2 (2.3 kPa) andH2 (21.7 kPa) irradiated under UV–visible light (142 mW cm−2)
using Co (7.5 wt %)–ZrO2 (0.020 g) treated in H2 at 823 K. The
error bars for each product were evaluated based on three factors
described in SI, 3.1. Kinetic Results.

The performance of nonheated Co (7.5 wt %)–ZrO2 under H2
was even poorer than ZrO2 (Tables 1a and S3a). This aligns

2sites at the ZrO2 surface (Scheme 1A-b, c) is likely ratelimiting,[31] thereby determining the overall rates. We previously
reported the photocatalytic roles of VO•• sites on CO2
photoreduction.[31] Subsequent steps from COH involve
common progressive hydrogenation toward C1–3 paraffin overCo0 sites, resulting in a very similar paraffin ratio (Figure S2A–
C and Scheme 1A-c, d; see the following density functional
theory (DFT) calculations section).
Even when only visible light was utilized for irradiation, theC1–3 paraffin ratio remained consistent, suggesting a similar
reaction pathway involving the utilization of VO•• sites at theZrO2 surface (Table S3e, g, and h; SI, 3.1. Kinetic Results).

for CO2 electroreduction,[4] it has received limited attention in
CO2 photoreduction.[3]Furthermore, using the Co (7.5 wt %)–ZrO2-973R
photocatalyst, 13CO (2.3 kPa), H2 (2.3 kPa), and UV–visible
light (Figure 2) resulted in olefins as the major products: 13C2H4(61 mol%) and 13C3H6 (8.6 mol%), rather than 13CH4 (25 mol%;Table 1i and Scheme S2B). The formation rate (5.2 ± 0.5 μmol
h−1 gcat−1) and selectivity (61 mol%) of 13C2H4 are comparable tothose reported in 2023 (12–68 μmol h−1 gcat−1, 11–86 mol%;Table S1n–r).[20–24] However, the photocatalytic C3H6 formation(0.74 μmol h−1 gcat−1) from CO and/or CO2 is uncommon, with
13
C2H4 being formed first, followed by 13C2H6, then 13C3H6 and13
C3H8 in consecutive order (Figure 2). Such consecutive
reactions were well reproduced.
12
CH4 originated from 12CO2 in the air, which chemisorbed
onto VO•• sites at the ZrO2 surface.[31–33] This was confirmed by
X-ray photoelectron spectroscopy upon the introduction of CO2
and H2 to Co–ZrO2-823R (Figure S8A and B): the positive shiftof Zr 3d peak indicating more Zr4+ population (+0.14 eV, panelA) shifted from Zr3+ associated with VO•• site and the growth ofO (surface) peaks in the region 535.5–531.5 eV compared to O
(lattice) at 530.1 eV in O 1s region (panel B).
The ratio of 12CH4 among the total CH4 formed did not alignwith the impurity ratio of 12CO2 in the 13CO2 reagent used (1%)
but instead ranged from 4.5% to 12% during the tests using
13
CO2 and H2 (Table S4b–e). This suggests that gas-phase13
CO2 was in equilibrium with chemisorbed 12CO2 on VO•• sites
at the ZrO2 surface from the air (Scheme 1A-a). When 13CO
and H2 were used, the ratio of 12CH4 among the total CH4
formed was 6.7–18% (Table S4g–i). Thus, gas-phase 13CO
was also in equilibrium with chemisorbed 12CO2 on VO•• sites
via Eqs. 1 and 2.
13
CO + X (ZrO2 surface) ⇄13COX (ZrO2 surface)(1)
13
COX (ZrO2 surface) + 12CO2 (VO•• site) ⇄12
COX + 13CO2 (VO•• site)(2)
One possibility of such equilibrium is shown in Scheme 1Ba–c via HCOH species (X = 2H in Eq. 1). Such photocatalytic
reaction pathway is specific in contrast to thermal CO2
conversion, which predominantly occurs over the Co0 surface
at temperatures higher than 425 K (SI, 3.1. Kinetic Results).[3]Switching Photocatalytic 13CO Reduction. We attempted
to extract the first selective step of C2H4 and C3H6 formation inconsecutive photocatalysis using 13CO, H2, and the optimal Co(7.5 wt %)–ZrO2-973R photocatalyst (Scheme S1B) byswitching the reaction gases as follows: (i) under 13CO (2.3
kPa) and H2 (2.3 kPa) for 4 h, (ii) under vacuum for 1 h, then(iii) under 13CO (2.3 kPa) for 1 h. These steps were repeated(Figure 3). At step i of the first cycle, the molar ratio of olefin(13C2H4 and 13C3H6) formation was measured as 57 mol% after
4 h of reaction, similar to the initial stage of Figure 2.
The control key for the selective formation of C2H4 and C3H6
was the concentration of CH2 intermediate on the Co surface(Scheme 1B-e and Figure S4), which progressively decreased,
resulting in the switching from C2,3 olefin to paraffin formation
within 4 h of photoreaction. The HC intermediates on the Co
surface were effectively removed under vacuum conditions(Scheme 1B-e and f), followed by 13CO adsorption during steps
ii and iii (Figure 3).
In the second cycle, at step i, the total formation rate after 4
h of reaction was 84 mol% of that observed in the first cycle.
The molar ratio of olefin formation was 57 mol%. Subsequently,
in the third cycle, at step i, the total formation rate after 4 h ofreaction decreased to 69 mol% of the first cycle, with the olefin

Figure 2. Time–course formation of photocatalytic 13CH4, 12CH4,
13
C2H4, 13C2H6, 13C3H6, and 13C3H8 and the decrease of 13CO duringexposure to 13CO (2.3 kPa) and H2 (2.3 kPa) irradiated under UV–
visible light (142 mW cm−2) using Co (7.5 wt %)–ZrO2 (0.020 g)treated in H2 at 973 K. The error bars for each product were
evaluated based on three factors described in SI, 3.1. Kinetic
Results.

Photocatalytic 13CO Reduction. The CO photoreductions
were tested using the most active Co (7.5 wt %)–ZrO2-823R
compared to the CO2 photoreduction reactions (Scheme S1A).
When 13CO (2.3 kPa), H2 (2.3 kPa), and UV–visible irradiation
were used, 13C2H4 became the second major product (24mol%) following 13CH4 (67 mol%; Figure S3 and Table 1g).
13
C2H4 was the primary product, followed by secondary 13C2H6
evolution after 5 h of reaction, demonstrating a consecutive
first-order reaction kinetic model (Eq. S5 and Figure S4), with a
CH2 intermediate, followed by C2H4, then C2H6 generations.
The discrepancy between the amount of CO lost and theamount of products was mostly due to bidentate formate
formation on ZrO2 surface[32] (see Reaction Mechanism section
and Figure S16) because formic acid and C1–3 alcohols were
not found and negligible change of OH stretching vibration
region in Fourier transform infrared (FTIR) spectra under thephotocatalytic conditions. The molar ratio of C1–3 HC formation
rates, 13CH4, 12CH4, 13C2H4, 13C2H6, to 13C3H8, was determined
to 100: 7.2: 36: 4.6: 0.64 for photoreduction starting from 13CO
(2.3 kPa) and H2 (2.3 kPa), differing from the values of 100:
7.7: 0: 1.4: 0.14 for photoreduction starting from 13CO2 (2.3
kPa) and H2 (21.7 kPa) (Table 1c and g). In the photocatalytic
test using CO2 and H2, CO was negligibly identified as theintermediate, suggesting either its absence in the reaction
pathway or the presence of a specific active site that selectively
activates CO over CO2. While this aspect has been discussed

3formation molar ratio measured at 60 mol%. Furthermore, inthe fourth cycle, at step i, the total formation rate after 4 h ofreaction was 62 mol% of that observed in the first cycle, withthe olefin formation molar ratio of 61 mol%).
The switching olefin photoformation was nicely reproduced inseparated test under 13CO and H2, vacuum, then 13CO (FigureS5); olefin formation molar ratio gradually increased from 70 to
77, 83, and 87 mol% while total formation rate gradually
decreased from 100 to 99, 68, and 55 mol%. Due to the natureof consecutive reaction, olefin selectivity critically depended ontime exposed to 13CO, H2, and UV–visible light (4–10 h). Upon
repetition of the cycle, the ratio of 13C2H4 and 13C3H6 among all
the HCs increased owing to the gradual accumulation of CH2
and/or C2H4 intermediate species (Scheme 1B-e) over the Co0
surface compared to CO and CH3 species, even after flushingunder vacuum and exposure to 13CO between cycles (seefollowing FTIR spectroscopy section). Thus, the first step of theconsecutive photocatalytic CO reduction, i.e. C2,3 olefin
formation, using the Co (7.5 wt %)–ZrO2-973R photocatalyst
was successfully extracted.

Then, the CO2 adsorption sites were investigated using
CO2 uptake and exchange reactions with Co (7.5 wt %)–ZrO2-823R (Scheme S1A) and UV–visible light irradiation(Figure 4). At a rate constant of 4.7 h−1, the initial rapid uptakeof 13CO2 and 1.0% impurity 12CO2 (in total 19.0 μmol) were
attributed to physisorption on the ZrO2 surface. The additional
uptake (9.1 μmol) of CO2 compared to the 9.9 μmol-CO2 using
the same amount (20 mg) of undoped ZrO2 catalyst (Table S5a
and b)[32] corresponded to 36 mol% of the Co-site in the Co (7.5
wt %)–ZrO2-823R catalyst. Thus, rapid CO2 adsorption on Co0
was implausible, and the extra uptake was attributed to theformation of Co carbonate resulting from the reaction of CoO
with CO2.
Subsequent 13CO2 uptake on the Co (7.5 wt %)–ZrO2-823R
was also physisorption at a rate constant of 0.4 h−1 (Figure 4
and Table S5b), suggesting adsorption at the interface sites.
Conversely, the much slower 13CO2/12CO2 exchange reaction,with a rate constant of 0.2 h−1, was attributed to thechemisorption site, likely owing to the presence of VO•• sites. In
this mechanism, the 12CO2 adsorbed from the air is exchanged
with gas-phase 13CO2 (Scheme 1A-a). The concentration was
calculated to be one vacancy per a ZrO2 surface area of 61
nm2, in consistent with the total amount of 12CH4 formed using
N2, H2, and UV–visible light irradiation (Table S3f and SI, 3.1.
Kinetic Results).
13

Figure 3. Time–course formation of photocatalytic 13CH4, 13C2H4,
and 13C3H6 (i) during exposure to 13CO (2.3 kPa) and H2 (2.3 kPa)for 4 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 under UV–visible light (142 mW cm−2). The
cycle of steps (i)–(iii) was repeated four times. The error bars foreach product were evaluated based on three factors described inSI, 3.1. Kinetic Results.

Figure 4. Time–course uptake and exchange reaction of 13CO2(0.68 kPa) under UV–visible light irradiation (142 mW cm−2) using
Co (7.5 wt %)–ZrO2-823R and the fit equation curves based on firstorder kinetics. The amount of catalyst is 0.020 g.

Pressure Dependence of 13CO2/13CO Reduction and
CO2 Uptake/Exchange. To provide insights into reaction

13

Electronic Characterizations. Diffuse-reflectance UV–
visible spectra were measured to monitor effective electrons bychanging the reduction temperature of photocatalysts. In
addition to the absorption edge observed at 248 nm for ZrO2
(Figure S6A-a), two peaks appeared at 387 and 687 nm for the
fresh Co (7.5 wt %)–ZrO2 (spectrum b), attributed to the chargetransfer from O2− to Co2+ and O2− to Co3+, respectively, in
Co3O4.[34,35] For Co (7.5 wt %)–ZrO2-723R, a single peak
appeared at 612 nm owing to the charge transfer from O2− toCo2+ in CoO (spectrum c).[35] Based on the fitting of these
spectra to the equation proposed by Davis and Mott,[36]α × hν ! (hν − Eg)n,(3)where h is Planck constant, ν is the frequency of light, and α
is a constant, bandgap Eg values of 2.4 and 2.9 eV wereobtained for allowed direct transition (n = 1/2)[35] in Co3O4 andmechanism, the pressure dependence of the reactant wasconsidered. Using Co (7.5 wt %)–ZrO2-973R (Scheme S1B),
13
CO2 (2.3 kPa), and H2 (2.3–21.7 kPa), the formation rate ratioof 13CH4, 12CH4, 13C2H4, 13C2H6, 13C3H6, to 13C3H8 wasessentially constant: 100: 4.7–6.4: 0: 1.8–2.2: 0: 0.13–0.16(Table 1d and e), indicating no competition between CO2 andH2 for adsorption. In contrast, using CO (2.3 kPa) and H2 (2.3kPa), the ratio was skewed toward olefin selectivity, measuring
100: 8.6: 243: 11: 34: 1.1, which differs from the 100: 23:
0.71:15: 7.8: 20 observed with CO (2.3 kPa) and H2 (21.7 kPa;
Table 1h and i). This suggests that the surface concentration ofCH2 species critically depended on H concentration (Scheme
1B-e). Conversely, high H2 pressure decreased the 13CO
formation rate using 13CO2 and ZrO2 (Table S3a and b),
suggesting competitive adsorption of CO2 on VO•• sites and H
on neighboring Zr sites.[31]4CoO nanoparticles over ZrO2 (Figure S7A and B and Scheme1C), respectively.
The absorption in the range of 250–800 nm progressively
increased with the elevation of the reduction temperature to823 K and then to 973 K (Figure S6A-d and e), suggesting acomplete reduction to metallic Co0 at 973 K, in accordance withthe findings in the following XANES/extended X-ray absorption
fine structure (EXAFS) sections. However, the peak observed
at 612 nm attributable to CoO (15 mol%) (intensity ~0.05) was
not well resolved, as it overlapped with the absorption by Co0(85 mol%, intensity 2–3) for Co (7.5 wt %)–ZrO2-823R (seeXANES section).
The energetics of the Co–ZrO2 photocatalysts are
summarized in Scheme 1C. Concerning the valence band
maximum (VBM) of ZrO2 (4.0 V vs. normal hydrogen electrode,
NHE), the bandgap value was 5.0 eV based on the UV–visible
spectrum (Figure S6A-a), while the conduction band minimum(CBM) was at −1.0 V vs. NHE. The VBM values of CoO in Co(7.5 wt %)–ZrO2-723R and -823R photocatalysts were
calculated to 2.3 and 2.4 V vs. NHE (Scheme 1C), respectively(Figure S8C and SI, 3.2. Characterizations).
Based on the bandgap values for CoO (2.9 eV) shown in
Figure S7B, its CBMs were at −0.6 and −0.5 V (SI, 3.2.
Characterizations). Thus, UV and/or visible light-excitedelectrons to the CB of both ZrO2 and CoO could
thermodynamically reduce CO2 to CO (−0.11 V; Scheme 1C).
Thus, the presence of CoO adjacent to Co0 nanoparticles canboost CO2 photoreduction by leveraging visible light.
The efficiency of charge separation in photocatalysts by light
was evaluated by fluorescence spectroscopy (Scheme S1C).
The fluorescence peak intensity associated with the near bandedge and the midgap trap states was suppressed to one-tenth
to one-fifth for Co (7.5 wt %)–ZrO2 compared to corresponding
peaks for ZrO2 owing to the charge transfer of light-excitedelectrons to Co3O4, CoO, or Co0. The associated peaks inexcitation spectra were substantially suppressed owing to thecharge transfer effect to Co species in accordance withfluorescence spectra (Figure S6B and C; SI, 3.2.
Characterizations) and in-profile CO2 photoreduction tests
suggested an auxiliary role of vacancy/impurity level of ZrO2 (SI,3.1. Kinetic Results), suggesting the utilization of the charges
for photocatalysis.[1,2]Structural Characterizations. The morphology of the Co(7.5 wt%)–ZrO2-723R (Figure S9), -823R, and -973R samples(Figure 5) was observed using HR-TEM to provide structural
information on nano level and the changes of especially Co
sites owing to reduction temperature (Scheme S1C). The
monoclinic phase of ZrO2 crystals with 5–10 nm (Scheme 1) insize was preferably observed, exhibiting lattice fringes with theintervals of 0.274–0.296, 0.344–0.352, 0.246–0.257, 0.240,
0.264–0.267, and 0.308–0.313 nm corresponding to ZrO2 (1 1
1), (1 1 0), (2 0 0), (0 2 1), (0 0 2), and ( 1 1 1 ) (theoreticalvalues 0.285, 0.365, 0.255, 0.234, 0.264, and 0.318 nm),[32,33]
respectively, for these three samples.
In contrast to Co (7.5 wt %)–ZrO2-723R, where cubic CoO
was observed ((1 1 1) lattice interval = 0.247 nm, theoretical =0.245 nm; (0 0 2) lattice interval = 0.205–0.206 nm, theoretical= 0.212 nm[37]) (Figure S9), for Co (7.5 wt %)–ZrO2-823R, fcc
Co nanoparticles were observed ((0 0 2) lattice interval =0.178–0.179 nm, theoretical = 0.177 nm[37]) in the proximity to
CoO nanoparticles comprising (1 1 1) lattice with an interval of
0.240 nm,[38] as shown in Figure 5A2. Furthermore, for Co (7.5
wt %)–ZrO2-973R, hcp Co nanoparticles were frequently

observed with intervals of 0.215 and 0.196 nm corresponding
to the (0 1 0) and (0 1 1) lattices (Figure 5B1, 2; theoretical =0.217 and 0.192 nm[39]), respectively, while no CoO phase wasobserved. A few nanometer-sized Co nanoparticles could
exhibit fcc packing owing to the reduction at 823 K, while many
Co crystals (>5 nm) grown at 973 K transformed into a stable
hcp phase. The HR-TEM results were well consistent with the
X-ray diffraction (Figure S10A), which showed diffractions frommonoclinic ZrO2. However, only very weak traces of theshoulder owing to hcp Co (0 1 0) and (0 0 2) were identified(Figure S10B) because of their small size of a few nanometers.

Figure 5. HR-TEM images of Co (7.5 wt%)–ZrO2-823R (A1, A2)
and Co (7.5 wt %)–ZrO2-973R photocatalysts (B1, B2). Latticeintervals for monoclinic ZrO2 (all panels), CoO (A2), fcc Co (A1, A2),and hcp Co (B1, B2) were also drawn.

Co K-edge XANES spectra confirmed the speciation of Co:
Co3O4, CoO, and Co0 metal for fresh Co (7.5 wt %)–ZrO2, Co(7.5 wt %)–ZrO2-723R, and -973R, respectively (Figure S11A
and SI, 3.2. Characterizations), consistent with UV–visible
absorption spectroscopy (Figure S6A). For Co (7.5 wt %)–ZrO2-823R, the Co site comprised a mixture of CoO (15%) andCo0 (85%) (Figure S11B). The CoO amount based on rapid
CO2 physisorption (extra 9.1 μmol of CO2 corresponding to
CoO 36%; Table S5a and b) was greater than 15%, evaluatedthrough XANES, probably because slower CO2 physisorption at
the interface between Co and ZrO2 surface was also included.
In the comparison between Co state and photocatalytic CO2
reduction, while the fresh Co (7.5 wt %)–ZrO2 and Co (7.5
wt %)–ZrO2-723R photocatalysts comprising Co3O4 and CoO,
respectively (Figure S11A), exhibited poor activity for CO2
photocatalytic reduction (Tables 1a and b and Figure S2A), Co(7.5 wt %)–ZrO2-823R and -973R comprising metallic Co(Figure S11A) were the most and the second most active in theCO2 photocatalytic reduction (Scheme S1A and B, Table 1cand d, and Figures 1 and S2C), respectively. Thus, metallic Co
sites played an essential catalytic role combined with VO•• sites
at ZrO2 surface[31–33] as dual active sites, similar to our previous
reports for Ni–ZrO2 by experiments[32] and DFT calculations.[31]Three plausible reasons can be listed: (i) fcc Co0 surface wasmore conducive to dissociating COH species than hcp one,[30]5(ii) CoO could work as a promoter in contact with Co0 (Figure
5A2) owing to visible light absorption and charge separation
with an Eg value of 2.9 eV (Figures S6A-c and S7B and
Scheme 1C), and (iii) Co0 particle size may increase at 973 K.
However, the major reason cannot be identified because thesepotential changes occurred simultaneously at 973 K, and theCo0 particle size mixed with CoO at 823 K was difficult todetermine via EXAFS and HR-TEM (Figure 5A).

(1−D)+(1/2)D+(1/2·2/3)D for the bulk site, nonfree hemispherein contact with ZrO2 surface, and horizontal translational motion
at a free hemisphere surface.
Tnanoparticle = TSurface,⊥ × (1/6) D + TBulk × (1−(1/6) D)(4)The σ value for samples (σsample) was calculated using Eq. 5,
taking the contribution of structural disorder (σdisorder) and thedifference of the σsample value from the Co metal foil (σXDAP) into
account:
σsample2 = σCo metal, correlated Debye2 + Δ(σdisorder2) +Δ(σXDAP2) (5).
At 296 K, σsample and σCo metal, correlated Debye values were 0.007(28) and 0.007 (05) nm, respectively, for the nanoparticle
model above based on the correlated Debye model (Figure S1)and Eq. 4. The σXDAP value was given as −0.000 (32) nm. Thus,
σdisorder was evaluated as 0.001 (82) nm using Eq. 5.
Then, the temperature of the Co site was inversely evaluatedbased on the σsample value obtained from Eq. 5 and thearithmetic mean value based on Eq. 4 (Figure 6). This specificapproach directly monitored the local Co site temperature.[32,33]
The Co sites were initially at 296 K before exposure to UV–
visible light. Upon irradiation, the temperature of the Co sites
quickly increased to 362 ± 21 K, gradually decreasing once thelight was turned off after 138 min. The gradual drop in
temperature post-illumination cessation (Figure 6) is probably
attributed to the presence of a minor CoO layer between the
Co0 nanoparticle and the ZrO2 surface, which slows heatdissipation.
Compared with the critical change of σsample values triggered
by UV–visible light irradiation (Figure 6), the N(Co–Co) andR(Co–Co) values negligibly varied during UV–visible light
exposure and cessation, except for rapid initial quick changes
upon the light activation (Figure S13). This initial change may
result from the oxidation of minor CoII sites by the effect of CO2(Figure S11B), which were subsequently reduced under H2 andUV–visible light. The reduced state of CoII maintained a
dynamic equilibrium under these conditions, with N values
gradually decreasing upon light deactivation (Figure S13A).
The CH4 formation rate reached 196 μmol h−1 gcat−1 (Table1d) under UV–visible light irradiation leading to a temperaturerise of 11.9 K within 10 min thermodynamically. However, thisincrease was smaller than the initial observed increase of 66 K
within 10 min (Figure 6; SI, 3.2. Characterizations). Thus, thetemperature elevation in Figure 6 stemmed from thetransformation of light energy into heat at the Co0 surface,quickly reaching the heat equilibrium and dissipating into thereactor/EXAFS cell.
Consistent with this evaluation of warming of Co0 sites bylight under CO2, the time course of Fourier transform (Figure
S14), σ value, and Co0 site temperature under CO, H2, Co (7.5
wt %)–ZrO2-973R (Scheme S1B), and UV–visible lightirradiation (Figure S15) reached 350 ± 8 K and behaved very
similarly to Figure 6, irrelevant to reactants.
To identify the origin of photocatalytic CO reduction in thisstudy, control thermal reaction tests were performed using Co(7.5 wt %)–ZrO2-973R, 13CO, and H2 at a reaction temperature
between 363 and 423 K (Table S6B). The total formation rate
at 350 K was lower by a factor of 71 000 compared to that
under UV–visible irradiation (Scheme 1D and Table 1i),wherein the Co nanoparticles reached 350 K during the CO
photoreduction test (Figure S15). This indicates that simple
thermal catalysis did not occur solely owing to heattransformed from UV–visible light energy. Similarly, the total
formation rate at 362 K using Co (7.5 wt %)–ZrO2-823R as
evaluated for Co (7.5 wt %)–ZrO2-973R (Figure 6), 13CO2, andFigure 6. Time course of the Debye–Waller factor σ derived fromthe Co K-edge EXAFS analysis and the determined temperature of
Co nanoparticles in Co (7.5 wt %)–ZrO2-973R using CO2 (2.3 kPa),
H2 (21.7 kPa), and UV–visible light irradiation (142 mW cm−2),
based on the correlated Debye model. The error bars were drawn
based on data reproducibility in three runs and the fit errors.

Photothermal Monitoring and the Control Thermal
CO2/13CO Reduction. Related to the latter reaction steps of

13

CO2 photoreduction over Co sites followed by CO2 activation atVO•• sites on ZrO2 surface, the local electronic and geometricstructures associated with the thermal behavior of active
metallic Co sites essential for CO2 photoreduction weremonitored via Co K-edge EXAFS spectroscopy. The analysis
was conducted under UV–visible light irradiation using CO2 (2.3kPa), H2 (21.7 kPa), and Co (7.5 wt %)–ZrO2-973R. To
precisely analyze Co0 sites not mixed with CoO, the secondbest catalyst was chosen (Table 1d and Scheme S1B). In the
Fourier transform of EXAFS before UV–visible light irradiation,
the curve-fit analysis revealed an intense peak observed at
0.21 nm (uncorrected for phase shift; Figure S12). This peakdemonstrated a Co–Co interatomic distance (R) of 0.249 nmwith an associated coordination number (N) of 9.8.
Furthermore, the N and R values did not change considerably
during the photocatalytic reaction (Figure S13A and B). The N
value corresponds to a particle size of 2.9 nm, assuming a
spherical fcc nanoparticle model with a surface dispersion (D)was 0.43.[40]The Debye–Waller factor (σ) was calculated using thecorrelated Debye model[41,42] for bulk and surface Co sites(vertical motion versus surface),[32,33] considering the Debye
temperature for bulk θD(Bulk) (445 K)[43] and surface θD(Surface,⊥)for the vertical motion of freedom (211 K[44]; Figure S1). We
approximated the mean temperature of Co nanoparticle(Tnanoparticle) as the arithmetic mean value, consideringθD(Surface,⊥) weighted using 1/2·1/3 D for vertical translationalmotion at a free hemisphere surface and θD(Bulk) weighted using

6

H2 was lower by 200 times compared to that under UV–visible
irradiation (Table S6A and Scheme 1D). The C2,3 HC formation
was especially suppressed in the thermal reaction, suggesting
different reaction pathways under UV–visible light irradiation(SI, 3.1. Kinetic Results). The higher apparent activationenergy (Eact) from CO (151 kJ mol−1) compared to that fromCO2 (83.4 kJ mol−1) contradicts the possibility of higher Eact
from CO2 to CO that then follow common pathway. Instead, thefact suggests that CO followed a different thermal pathway
from that from CO2, related to the difference of photocatalytic
pathways between Scheme 1A and B.
Reaction Mechanism. FTIR spectroscopy investigated thespecific photocatalytic reaction mechanism, starting from theconversion of CO to C2H4 and C3H6 using the optimal Co (7.5
wt %)–ZrO2-973R photocatalyst (Scheme S1B). Analysis of the13
C/12C isotopic distribution of the photocatalytic products,
generated from 13CO2 and 13CO as starting materials (Table
S4b–i) and 13CO2 exchange reaction (Figure 4), revealed that apart of the 12CO2 initially adsorbed from the air onto VO•• sites at
the ZrO2 surface (one per the area of 61 nm2) remained after
pretreatment under H2. Subsequently, this 12CO2 was thenincorporated into the reaction pathway toward HCs (Scheme
1B-a). Migration of 12CO2 from VO•• sites to the surface of Co0
nanoparticles and its reaction with the surface (Eq. 6)
demonstrated by the presence of 12CO stretching vibration(ν12CO) peaks observed at 2142 and 2126 cm−1 (Figure 7A-a)
under 13CO and H2:
12
CO2 + 2 Co0 ⇄ O12C–CoIIO.
(6)
A minor part of the CO formed on the CoII site in Eq. 6
further moved laterally (Figure 5A2) to the Co0 site (ν12CO 2028
cm−1, Figure 7A-a). Because 13CO was not in direct equilibriumwith 12CO2 at the VO•• sites, species derived from 13CO were notobserved at 0 h under 13CO and H2 (Scheme 1B and Figure
7A-a and B-a).
When the UV–visible light irradiation started, leading to a
gradual decrease in the ν12CO peak intensity on CoII sites, thecorresponding ν12CO peak on the Co0 site observed at 2028
cm−1 progressively grew (Figure 7A-b and c). It was followed bya progressive increase in the isotopically-labeled terminal andbridging ν13CO peaks observed at 1978, 1921, 1874, and 1839
cm−1 (Figure 7A-c and d),[45] demonstrating that the 12CO
species derived from 12CO2 adsorbed at VO•• site when the light
was turned on (0 h) were gradually replaced by gas phase13
CO (0–12 h), with 13CO on Co0 sites becoming the major
species at 34 h under light (Figure 7A-d and Scheme 1B-b).
However, the isotopic ratio of the ν12CO/ν13CO peak intensity
in FTIR (Figure 7A-b and c) and the ratios of 12C2H4/(12C2H4 +13
C2H4) and 12CH4/(12CH4 + 13CH4) (7.9 mol%, Tables 1i andS4i; Figure 7A-c and d) obtained from gas chromatography–
mass spectrometry (GC–MS) were significantly greater than12
CO ratio in reagent (1%). This discrepancy can be explained
if the 13CO adsorbed on Co0 sites was in equilibrium with 12CO2
on VO•• site at ZrO2 surface (Scheme 1B-a; Eq. 6 and Figure
7A-a–d), and the CO adsorbed on Co0 sites should be theintermediate species toward HCs (Scheme 1B-b).
Synchronized with the growth of the ν12CO peak (Figure 7Ab–d), the antisymmetric and symmetric vibration of 12CH2(νas12CH2 and νs12CH2) peaks increased at 2936 and 2858 cm−1,
respectively, accompanied by weak vibration peaks for 12CH3(νas12CH3 2958 cm−1; νs12CH3 2876 cm−1) under 13CO, H2, andUV–visible light (Figure 7B-c). Then, following the isotopic
replacement by multiple ν13CO peaks (Figure 7A-b–d), theCH2/CH3 vibrational pairs of νas12CHn and νs12CHn (n = 2, 3)

progressively shifted to the corresponding 13C-isotopic pairs ofνas13CHn and νs13CHn (2927 and 2850 cm−1 when n = 2 and2948 and 2869 cm−1 when n = 3, respectively, Figure 7B-d).
Although the stretching vibration peak intensity ratio of
CH2/CH3 in FTIR was almost constant at 3:1 during the 34-hourphotocatalytic test (Figure 7C), the major product obtainedusing GC–MS transformed consecutively transformed from13
C2H4 to 13CH4 (Figure 7D). This indicates that the ratio of CHx
to H adsorbed at a specific local site, e.g., the interface site
between Co0 and the ZrO2 surface, determined the productselectivity (Scheme 1B-b–e). In contrast, the 12CO3 at the ZrO2
surface was replaced by 13CO3 species (1513 and 1308 cm−1),
while bidentate 13C-formate at the ZrO2 surface gradually
increased (1523 and 1344 cm−1)[32] until 34 h of reaction under13
CO, H2, and UV–visible light (Figure S16), both irrelevant to
HC generation over the Co0 surface.
Then, the Co (7.5 wt%)–ZrO2-973R photocatalyst subjected
to vacuum and the UV–visible light irradiation. In contrast to thequick disappearance of all the weakly adsorbed CO peaks(Figure 7A-e and f), approximately half of the reaction
intermediate CH2 and CH3 species remained at 2 h undervacuum (Figure 7B-e, f and C), consistent with the HC
preference using this photocatalyst.

Figure 7. FTIR monitoring of reaction intermediates using the Co(7.5 wt %)–ZrO2-973R photocatalyst (71 mg) in the (A) carbonyland (B) C–H stretching vibration regions and the peak assignments.
The reaction 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). (C) The time course of CH2 and CH3 stretchingvibration peak intensity (including convolution of 13C and 12C peaks)
by FTIR and (D) the corresponding time course of photocatalytic
products monitored using GC–MS.

Spin-polarized periodic DFT+U calculations were conductedusing the Vienna Ab initio Simulation Package code version
6.2.1[46] (SI, 2. Experimental Section). CO2 was adsorbed at theVO•• site (Scheme 1A-a and Figure S8A and B).[31] When H wasprovided, the adsorbed CO2 transformed to OCOH with an Eact7

of 1.3 eV (Scheme 2A-b). If the VO•• site neighbored a Co0 site,
the terminal O atom of the OCOH group occupied the VO•• site
(species b), and the COH moiety hopped onto the metallic Co0
atom with an Eact of 0.28 eV (species c). A very similar reaction
mechanism was also proposed for Pd–ZrO2.[47] The populationof VO•• sites adjacent to the Co0 nanoparticle was anticipated tobe relatively higher compared to the mean population of oneVO•• per ZrO2 surface area of 61 nm2 (Table S5b). This isbecause the Fermi level of Co (work function 5.0 eV)[48] is lower
than the energy level of VO•• in ZrO2,[31] enabling electron
acceptance,[49] thereby facilitating the generation of VO•• sites
neighboring to Co0 nanoparticles.
Throughout the pathway to CH4, the hydrogenation to OCOH
required the highest Eact of 1.3 eV, followed by the Eact of 0.93
eV for CH3 hydrogenation, and 1.2 eV for OH hydrogenation by
the elimination at the interface (Scheme 2A-a, b, d, and e,
Methyl route), thereby supporting the preferential formation of
CH4 from CO2 (Table 1b–e). The regeneration energy of theVO•• site can be minimized to 1.7 eV by replacing H2O with CO2
at the VO•• site.[31] C2H6 was formed via the Ethene route(Scheme 2Aʹ) as a byproduct, preferably via C2H4 (Eact of 0.70eV) rather than the coupling of two CH3 (Eact of 1.0 eV; Scheme
2A-d, Figure S4, and Eq. S1), different from the situation using
Ni–ZrO2 photocatalyst, where CH4 was exclusively formed.[31]The dissociation of HOCH2 into hydroxy and CH2 at theinterface was also suggested (Scheme 2A), in contrast to
considerably stable HOCH and dissociation on the Ni0
surface.[31]The reaction route of CO photoreduction, especially to C2H4,
was also investigated by DFT calculations. In contrast to CO2
adsorption at the VO•• site of the ZrO2 surface,[31–33] CO isadsorbed on the Co0 site (the adsorption energy (Eads) of 2.23
eV; Schemes 1B-b and 2B-b), followed by the formation of
CHO and HCOH species at the interface of the ZrO2 surface
and Co0 (Schemes 1B-c and 2B-c). It is energeticallyadvantageous when the hydroxy group fills the neighboring VO••
site to form CH (Schemes 1B-d and 2B-d). In the following
Methyl route, the barrier for CH3 hydrogenation to form CH4 ishighest (0.75 eV). In comparison, the Ethene route via thecoupling of CH2 was more favorable (Eact of 0.70 eV, Scheme
2B-e and f), which aligns with the specific formation of 13C2H4
and 13C3H6 from CO within the first 0–4 h of the reaction (Table
1g–i). The isotopic shuffling of 13C/12C was also plausible
between CO adsorbed on the Co0 surface and 12CO2 adsorbed
at the VO•• sites from the air (Scheme 1B-a–c).
The advantagenous CO adsorption in the first stagetransitions to competitive adsorption of CO (Eads of 1.36 eV)and H (Eads of 1.32 eV; Scheme 2B-b and bʹ) in the second
step of consecutive photocatalysis. The increased population of
H on Co0 sites kinetically explains the transition to form C1–3
paraffin and C3H6 after 4 h of the reaction (Figures 2 and S3).
In contrast, under CO2 and H2, H adsorption on Co0 is rarely
hampered by CO2, and CH2 is easily transformed to CH3 (Eactof 0.30 eV, Scheme 2A-d) throughout the photocatalytic tests(SI, 3.3. DFT Calculations).
Selective C2,3 olefin formation proceeded during intial 4 h ofreaction (Figures 2 and 3) while on later stage, CH4 was amajor product with minor C2H6, C3H6, and C3H8 (Figure 2). This
contrast was understandable if we assume the adsorbed CO/H
balance on Co0 surface decreased as the time elapsed(Scheme 2B-b and bʹ). Compared to the two different stages ofreaction starting from CO, COH species hopped from VO•• site
to Co0 surface followed by down-hill steps mostly to CH4(Scheme 2A-c and d). The consecutive reaction steps on
separated sites on ZrO2 and Co0 starting from CO2 proceeded
efficiently (Scheme 2A) compared to competitive steps mostly
on Co0 starting from CO (Scheme 2B and Figure 7).

Scheme 2. Energy Diagram over Monoclinic ZrO2 (1 1 1) Surface
Combined with Co19 Cluster Exposing hcp (0 1 0) Surface
Calculated (A) under CO2 and H2 and (B) under CO and H2. Ethene
Route Was Also Drawn in (Aʹ) and (B) Compared to Methyl Route.
Three-Dimensional Illustration of Each Species Is Presented in
Scheme S2.

Photocatalytic CO2 reduction has been proposed to proceed
via various ways, including the coupling of two CO molecules
to form C2H4 at VO•• sites of TiON,[16] In–(S vacancy)–Bi,[21] andCuδ+ sites,[25] the reaction of CO with CHOH to form OC–CHOH
and then to C2H4 at CuI–CuII sites,[20] and the reaction of COwith COH to form C2H4 at In2.77S4 surfaces[23] (Table S1j, n, o, q,
and s). However, herein, CO did not directly participate informing the C–C bond. Instead, the coupling of two species toform C2H4 at the FeCoS2 surface,[22] the coupling of two CHOs
to form CH–CHOH and then to C2H4 at the Fe–N surface,[24]and the coupling of CH3 with CH3 or C2H5 to form C2H6 andC3H8 at Cu2O[7] and Cu surfaces,[9] have been reported.
Additionally, the coupling of CH3 at TiO2 surfaces[11,12] and Au
surfaces[13] to form C2H6 has been reported via C–C formation,
involving oxygenate and/or HC intermediates (Table S1a, c, e–g, p, and r). In these reports, the combination of reduced active
sites with partially oxidized sites resembles the Co0 site and theneighboring VO•• site at the ZrO2 surface to form C2,3-olefinsfrom CO (Scheme 1B) and C1–3-paraffin from CO2 (Scheme
1A).
Photocatalytic CO2 Reduction Using H2O. Compared withthe photosynthesis of 13C2H6 and 13C3H8 from 13CO2 and H2(3.7–4.5 μmol h−1 gcat−1; Table 1c and d and Figure 8A) and thephotosynthesis of 13C2H4 and 13C3H6 from 13CO and H2 (5.5–5.9μmol h−1 gcat−1; Table 1g and i and Figure 8B), CO2
photoreduction was attempted using H2O as a one-stepsustainable reaction under reaction conditions similar to thosereported in Table S1.[7–25]
The key factors for paraffin synthesis were the metallic Co0
surface and the adsorbed H species on the Co0. Using the Co–
ZrO2-823R photocatalyst (Scheme S1A) and swiftly evacuating
gas-phase H2, H2O (2.2 kPa) served as a reductant for CO2

8

(2.3 kPa) under UV–visible light irradiation to form 13C2H6 and13
C3H8 at a rate of 0.21 μmol h−1 gcat−1 (Table 1f and Figures 8C
and S17). Conversely, O2 was not formed above the detection
limit because the formed O2 reacted with adsorbed H specieson the Co surface to regenerate H2O at the solid/gas interface.
This challenge was addressed using liquid H2O for thephotocatalytic CO2 conversion to release O2 from the surface.
C2H6 and C3H8 were photogenerated at rates of 2.0–2.7 μmol
h−1 gcat−1 at a steady state for 48 h (Figures S18A and B), withup to 60 mol% of C2,3 HC formation using H2 (Table 1c, k, and l
and Figure 8A and D), accompanied by O2 formation at therates higher than 66 μmol h−1 gcat−1 using Co–ZrO2-823R and 973R photocatalysts. The relatively high formation rates to C2,3
HCs compared to that to CH4 (31–60 mol%; Table 1k and l)
were attributed to competitive adsorption of C-species and H2O
on Co0, rather than the 1.4–1.8 mol% using H2 preferably
adsorbed on Co0 (Table 1c and d). However, sufficiently highlight intensity (222 mW cm−1) was needed for CO2
photoconversion in H2O (Tables 1j, l and S8a–e; SI, 3.4.
Photocatalytic Conversion of CO2 Using H2O).

13

CO2 (2.3 kPa) photoreduction using D2O (2.2 kPa), H2(21.7 kPa), and Co–ZrO2-823R formed 13C-methane with a D
ratio of 9.2 mol%, which agrees with a D ratio in the reactants(9.1 mol%; Figure S19 and Table S7b; SI, 3.5. Photocatalytic
Conversion of 13CO2 Using D2O). This suggests that D2O andH2 reached equilibrium more rapidly and were shuffled overCo0 more efficiently than the progressive hydrogenation steps
to C2,3 paraffin common under either H2 or H2O (Scheme 1A-a–d). Such a photocatalytic pathway from CO2 to C2,3 paraffinusing either H2 or H2O[7–15] and the photoformation from CO to
C3H6 have been rarely reported.
This study paves the way to precisely explore further active
photocatalysts to selectively produce C2,3-HCs utilizing
unsaturated/lower-dimensional semiconductors to regenerate
the VO•• sites with an Eact of < 2.6 eV (Scheme 2B). This can beachieved in combination with metal nanoparticles/single atoms
using either H2 or H2O, surpassing the capabilities of recent
photocatalysts (Table S1).[7–25]

Figure 8. Summary of major findings of C2,3 photosynthesis rates using (A, C) 13CO2, (B) 13CO, (D) CO2, (A, B) H2, (C, D) H2O, and Co (7.5
wt %)–ZrO2-823R (open symbol) or -973R (filled symbol) photocatalyst irradiated by UV–visible light at each intensity.

mol%) was achieved from CO after repeated tests for 4–10 h,followed by evacuation. The dual mechanism involved electron
donation from VO•• to OCOH/HOCH species using CO2/CO,
followed by consecutive hydrogenation steps on the Co0
surface at 362/350 K, facilitated by UV–visible light energy, at a
rate of 200/71 000 times higher than that observed via athermal reaction at 362/350 K, respectively, without light. CO2
photoreduction to C2H6 and C3H8 was possible in H2O (l) to
release O2 from the surface via a very similar pathway involving
CH3/2 activation on Co0.

Conclusion
The Co (7.5 wt %)–ZrO2-823R photocatalyst formed C1–3
paraffin at a total formation rate of 330 ± 20 μmol h−1 gcat−1
using CO2 and H2. In contrast, Co (7.5 wt %)–ZrO2-973R
formed C2H4 and C3H6 at a total formation rate of 6.0 ± 0.6μmol h−1 gcat−1 with an olefin selectivity of 70 mol% using CO
and H2. CO2 was adsorbed on the VO•• sites at the ZrO2 surface,
and the intermediate COH species hopped onto the Co0
surface, where it was progressively hydrogenated to form CH3
and/or CH2 species, which subsequently coupled to form C1–3
paraffin. Conversely, CO adsorbed on Co0 was hydrogenatedto form HOCH and was most effectively dissociated to CH at
the interface with the ZrO2 surface comprising the VO•• site.
Preferential CO adsorption and favorable CH2 coupling until 4 h
were followed by the competitive adsorption of CO and H on
Co0 sites, resulting in consecutive CH4 and C3H6 formation.
Predominant photocatalytic formation of C2H4 and C3H6 (61–87

Acknowledgements
The authors are grateful for the financial support from theGrant-in-Aid for Scientific Research B (24K01522, 20H02834,
YI) and C (20K05643, KN) from the Japan Society for the
Promotion of Science and from the Establishment of University
Fellowships toward the Creation of Science and Technology
Innovation from the Japan Science and Technology Agency

9

(JPMJFS2107, KH). X-ray absorption experiments were
performed with the approval of the Photon Factory Proposal
Review Committee (2022G527, 2021G546, 2020G676,
2019G141). A part of the computation was performed using thefacilities of Institute for Solid State Physics, University of Tokyo.
The authors thank Dr. Tetsuo Higuchi, JEOL for GC-MS
optimization and Yusuke Futamata, Shunsuke Watanabe, andMai Ichimura, Shimadzu for XPS measurements. The authors
would like to thank Enago (www.enago.jp) for the language
review.

[27] L. Liao, G. Xie, X. Xie, N. Zhang, J. Phys. Chem. C 2023, 127(6), 2766–
2781.
[28] M. Hu, J. Liu, S. Song, W. Wang, J. Yao, Y. Gong, C. Li, Y. Li, Y. Li, X.
Yian, Z. Fang, H. Xu, W. Song, Z. Li, ACS Catal. 2022, 12(5), 3238–3248.
[29] Y.-L. Zhang, L. Zhou, X.-C. Sun, Q. Zhang, H.-J. Yin, P. Du, X.-F. Yang,
Y.-W. Zhang, J. Phys. Chem. C 2022, 126(6), 3017–3028.
[30] J.-X. Liu, H.-Y. Su, D.-P. Sun, B.-Y. Zhang, W.-X. Li, J. Am. Chem. Soc.
2013, 135(44), 16284–16287.
[31] K. Hara, M. Nozaki, T. Hirayama, R. Ishii, K. Niki, Y. Izumi, J. Phys.
Chem. C 2023, 127(4), 1776–1788.
[32] H. Zhang, T. Itoi, T. Konishi, Y. Izumi, Angew. Chem. Int. Ed. 2021,
60(16), 9045–9054.
[33] H. Zhang, T. Itoi, T. Konishi, Y. Izumi, J. Am. Chem. Soc. 2019, 141(15),6292–6301.
[34] T. He, D. Chen, X. Jiao, Y. Wang, Y. Duan, Chem. Mater. 2005, 17(15),4023–4030.
[35] N. A. M. Barkakat, M. S. Khil, F. A. Sheikh, H. Y. Kim, J. Phys. Chem. C
2008, 112(32), 12225–12233.
[36] N. Ahmed, Y. Shibata, T. Taniguchi, Y. Izumi, J. Catal. 2011, 279, 123–
135.
[37] H. Putz, K. Brandenburg, Diamond version 3.2k; Crystal Impact: Bonn,
Germany, 2014.
[38] W. Jauch, M. Reehuis, H. J. Bleif, F. Kubanek, Phys. Rev. B 2001, 64,
052102.
[39] https://www.webelements.com/cobalt/crystal_structure.html (checked onJune 21, 2024).
[40] B. J. Kip, B. M. Duivenvoorden, D. C. Koningsberger, R. Prins, J. Catal.
1987, 105, 26–38.
[41] G. Beni, P. M. Platzman, Temperature and Polarization Dependence ofExtended X-Ray Absorption Fine-Structure Spectra. Phys. Rev. B 1976,
14, 1514–1518.
[42] E. Sevillano, H. Meuth, J. J. Rehr, Extended X-Ray Absorption Fine
Structure Debye–Waller Factors. I. Monoatomic Crystals. Phys. Rev. B
1979, 20, 4908–4911.
[43] American Institute of Physics Handbook, 3rd Ed; D. E. Gray, B. H.
Billings, H. P. R. Frederikse, D. F. Bleil, R. B. Lindsay, R. K. Cook, J. B.
Marion, H. M. Crosswhite, M. W. Zemansky, Eds.; McGraw-Hill: New York,USA, 1972; p 4-115.
[44] M. Hou, M. El Azzaoui, H. Pattyn, J. Verheyden, G. Koops, G. Zhang,Phys. Rev. B 2000, 62(8), 5117–5128.
[45] T. Liu, D. Xu, M. Song, X. Hong, G. Liu, ACS Catal. 2023, 13(7), 4667–
4674.
[46] G. Kresse, J. Furthmüller, Phys. Rev. B 1996, 54, 11169−11186.
[47] Y. Li, K. Zheng, Y. Shen, M. Huang, B. Liu, Y. Xu, X. Liu, J. Phys. Chem.
C 2023, 127(12), 5841–5854.
[48] CRC Handbook of Chemistry and Physics, 96th ed. (W. M. Haynes, Ed.),
CRC Press: Boca Raton, 2015; p 12-122.
[49] Y. Hinuma, T. Toyao, N. Hamamoto, M. Takao, K. Shimizu, T. Kamachi,
J. Phys. Chem. C 2020, 124(50), 27621–27630.

Keywords: Photocatalyst; Cobalt; Carbon dioxide; Paraffin;
Ethene[1] Y. Izumi, Coord. Chem. Rev. 2013, 257, 171–186.
[2] Y. Izumi, ACS Books "Advances in CO2 Capture, Sequestration, andConversion", F. Jin, L.-N. He, and Y. H. Hu, Eds., ACS Symposium
Series; ACS Publications, 2015; Volume 1194, Chapter 1, pp 1–46.
[3] J. Albero, Y. Peng, H. García, ACS Catal. 2020, 10, 5734–5749.
[4] S. Nitopi, E. Bertheussen, S. B. Scott, X. Liu, A. K. Engstfeld, S. Horch, B.
Seger, I. E. L. Stephens, K. Chan, C. Hahn, J. K. Nørskov, T. F. Jaramillo,
I Chorkendorff, Chem. Rev. 2019, 119, 7610–7672.
[5] H.-Q. Liang, T. Beweries, R. Francke, M. Beller, Angew. Chem. Int. Ed.
2022, 61(19), e202200723.
[6] Y. Lum, T. Cheng, W. A. Goddard, III, J. W. Ager, J. Am. Chem. Soc.
2018, 140(30), 9337–9340.
[7] N. Li, B. Wang, Y. Si, F. Xue, J. Zhou, Y. Lu, M. Liu, ACS Catal. 2019,9(6), 5590–5602.
[8] H. Ou, G. Li, W. Ren, B. Pan, G. Luo, Z. Hu, D. Wang, J. Am. Chem. Soc.
2022, 144(48), 22075–22082.
[9] H. Park, H.-H. Ou, A. J. Colussi, M. R. Hoffmann, J. Phys. Chem. A 2015,
119(19), 4658–4666.
[10] Kim, W.; Seok, T.; Choi, W. Nafion Layer-Enhanced Photosynthetic
Conversion of CO2 into Hydrocarbons on TiO2 Nanoparticles. Energy
Environ. Sci. 2012, 5(3), 6066–6070.
[11] S. Sorcar, J. Thompson, Y. Hwang, Y. H. Park, T. Majima, C. A. Grimes,
J. R. Durrant, Energy Environ. Sci. 2018, 11(11), 3183–3193.
[12] W. Tu, Y. Zhou, Q. Liu, S. Yan, S. Bao, X. Wang, M. Xiao, Z. Zou, Adv.
Funct. Mater. 2013, 23(14), 1743–1749.
[13] Y. Wang, J. Zhao, Y. Liu, G. Liu, S. Ding, Y. Li, J. Xia, H. Li, J. Colloid
Interf. Sci. 2022, 616, 649–658.
[14] X. Zhang, F. Han, B. Shi, S. Farsinezhad, G. P. Dechaine, K. Shankar,
Angew. Chem. Int. Ed. 2012, 51(51), 12732–12735.
[15] Q. Chen, X. Chen, M. Fang, J. Chen, Y. Li, Z. Xie, Q. Kuang, L. Zheng,
J. Mater. Chem. A 2019, 7(3), 1334–1340.
[16] Ni, B.; Zhang, G.; Wang, H.; Min, Y.; Jiang, K.; Li, H. Correlating
Oxidation State and Surface Ligand Motifs with the Selectivity of CO2
Photoproduction to C2 Products. Angew. Chem. Int. Ed. 2023, 62(6),e202215574.
[17] L. Yu, G. Li, X. Zhang, X. Ba, G. Shi, Y. Li, P. K. Wong, J. C. Yu, Y. Yu,
ACS Catal. 2016, 6(10), 6444–6454.
[18] K. Adachi, K. Ohta, T. Mizuno, Solar Ener. 1994, 53(2), 187–190.
[19] M. M. Gui, W. M. P. Wong, S.-P. Chai, A. R. Mohamed, Chem. Eng. J.
2015, 278, 272–278.
[20] S. Chakraborty, R. Das, M. Riyaz, K. Das, A. K. Singh, D. Bagchi, C. P.
Vinod, S. C. Peter, Angew. Chem. Int. Ed. 2023, 62(9), e202216613.
[21] K. Yan, D. Wu, T. Wang, C. Chen, S. Liu, Y. Hu, C. Gao, H. Chen, B. Li,ACS Catal. 2023, 13(4), 2302–2312.
[22] Y. Wu, Q. Chen, J. Zhu, K. Zheng, M. Wu, M. Fan, W. Yan, J. Hu, J. Zhu,
Y. Pan, X. Jiao, Y. Sun, Y. Xie, Angew. Chem. Int. Ed. 2023, e202301075.
[23] R. Das, R. Paul, A. Parui, A. Shrotri, C. Atzori, K. A. Lomachenko, A. K.
Singh, J. Mondal, S. C. Peter, J. Am. Chem. Soc. 2023, 145(1), 422–435.
[24] F. Guo, R.-X. Li, S. Yang, X.-Y. Zhang, H. Yu, J. J. Yuban, W.-Y. Sun,
Angew. Chem. Int. Ed. 2023, 62, e202216232.
[25] T. Wang, L. Chen, C. Chen, M. Huang, Y. Huang, S. Liu, B. Li, ACS
Nano 2022, 16(2), 2306–2318.
[26] Y. Wang, E. Chen, J. Tang, ACS Catal. 2022, 12(12), 7300–7316.

10

Entry for the Table of Contents

Pathways from CO2 to paraffin while from CO to C2H4 and C3H6
using Co–ZrO2 photocatalyst were switched. CO2 at the O
vacancy on ZrO2 transformed into COH and hopped onto Co0
to be paraffin while CO and H competed on Co0 leading topreferable coupling of CH2 to olefins.
Supporting information for this article is athttps://onlinelibrary.wiley.com/doi/10.1002/anie.2024*****.
Researcher X usernames: @AltmetricsYI

11