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ChemieHow to cite: Angew. Chem. Int. Ed. 2021, 60, 9045 – 9054
International Edition:
doi.org/10.1002/anie.202016346
German Edition:
doi.org/10.1002/ange.202016346

Photoreduction

Efficient and Selective Interplay Revealed: CO2 Reduction to CO overZrO2 by Light with Further Reduction to Methane over Ni0 by Heat
Converted from Light
Hongwei Zhang, Takaomi Itoi, Takehisa Konishi, and Yasuo Izumi*
Abstract: The reaction mechanism of CO2 photoreduction intomethane was elucidated by time-course monitoring of the mass
chromatogram, in situ FTIR spectroscopy, and in situ extended
X-ray absorption fine structure (EXAFS). Under 13CO2, H2,
and UV/Vis light, 13CH4 was formed at a rate of
0.98 mmol h@1 gcat@1 using Ni (10 wt %)-ZrO2 that was effectiveat 96 kPa. Under UV/Vis light irradiation, the 13CO2 exchange
reaction and FTIR identified physisorbed/chemisorbed bicarbonate and the reduction because of charge separation in/on
ZrO2, followed by the transfer of formate and CO onto the Ni
surface. EXAFS confirmed exclusive presence of Ni0 sites.
Then, FTIR spectroscopy detected methyl species on Ni0,
which was reversibly heated to 394 K owing to the heat
converted from light. With D2O and H2, the H/D ratio in theformed methane agreed with reactant H/D ratio. This study
paves the way for using first row transition metals for solar fuelgeneration using only UV/Vis light.

Introduction
Photocatalytic CO2 fuel conversion has been extensively
examined to initiate a carbon neutral cycle using homogeneous or heterogeneous catalysts, which includes the use ofsolar fuel unlike the irreversible fossil fuel consumption[1, 2]
and on-site fuel supply on planets.[3] The conversion of CO2
into methane, methanol, or formate can be achieved using
metal complex-based photocatalysts; however, they requirea sacrificial reductant[4, 5] and in most cases deactivate in a few
hours.[6] However, the CO2 conversion performed using
heterogeneous photocatalysts is often stable and does not
require a sacrificial reductant. Nevertheless, the productobtained is CO, which is an essential raw chemical but nota fuel,[1, 2, 7–9] unlike the ethene or ethanol produced if
electricity is applied.[10–12]
Previously, studies rarely clarified a steady-state reaction
mechanism for converting CO2 to solar fuel especially using[*] Dr. H. Zhang, Prof. Dr. T. Konishi, Prof. 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

13

C-labelled reactant, which led to a misunderstanding of
nonsteady-state reactions.[1, 13, 14] Although the mass spectrometry (Supporting Information, Figure S1A) of the photocatalytic product or both mass spectrometry and mass
chromatography (Figure S1B) using 13CO2 have been extensively reported,[15–17] these data are available at one timepoint
in a kinetic study and the steady-state information has rarely
been reported.[18]The operating mechanism of reaction steps comprising
photocatalysis is often uncertain. This study demonstrates
a steady-state photoconversion of CO2 into methane using
a composite of ZrO2 and Ni0 nanocrystals through the timecourse monitoring of online mass chromatogram (time courseof Figure S1B) of an isotope-labeled solar fuel (13CH4)
produced from 13CO2 using only UV/Vis light. We reported
CO2 photoreduction to CO over ZrO2 surface assisted by H2
activation and spillover from Ag[19] or Au nanoparticles.[20]Moreover, photocatalytic CO formation from CO2 wasreported using ZrO2[21–23] and Cu-ZrO2.[24]However, a Ni-ZrO2 photocatalyst converts CO2 to COover a ZrO2 surface owing to a charge separation by UV/Vis
light followed by hydrogenation of CO into methane overa Ni nanoparticle surface using heat (394 K) converted fromlight energy. Such an interplay in photocatalyst composites
has been speculated for various photocatalytic reactions but
has been revealed for the first time through the in situ
temperature monitoring of Ni sites under UV/Vis light byextended X-ray absorption fine structure (EXAFS). The
concerted mechanism does not proceed by heat only, i.e., in
the absence of the first photoreduction step from CO2 into
CO over ZrO2.
Among reaction steps from CO2 to methane [Eqs. (1)–(3)], we first evaluate the reaction rate of the exothermic Step
2 [Eq. (2)], which follows the photo-oxidation of water in Step
1 (eq. 1). Finally, we attempt the photoconversion of 13CO2
into methane using moisture (Step 3).
2 H2 O ! O2 þ 2 H2 ðor 4 Hþ þ 4 e@ Þ, Dr HA ¼ 483:64 kJ mol@1

ð1Þ

CO2 þ 4 H2 ðor 8 Hþ þ 8 e@ Þ ! CH4 þ 2 H2 O, Dr HA ¼ @164:94 kJ mol@1
ð2Þ
@1

CO2 þ 2 H2 O ! CH4 þ 2 O2 , Dr HA ¼ 802:34 kJ mol

ð3Þ

Prof. Dr. T. Itoi
Department of Mechanical Engineering
Graduate School of Engineering, Chiba University
Yayoi 1–33, Inage-ku, Chiba 263-8522 (Japan)

Results and Discussion

Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
https://doi.org/10.1002/anie.202016346.

We examined the samples for 13CO2 photoreduction(Table 1 a–e). ZrO2 previously reduced in H2 at 723 K

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ChemieTable 1: Kinetic data on photoconversion of 13CO2 using H2 and the H2-pretreated ZrO2- and SiO2-based photocatalysts (0.020 g).[a]Entry Catalyst

13

Formation rate of C-containing products [mmol h@1 gcat@1]

CO2 H2
Light irradiated[kPa] [kPa]
13

CO

a
b

ZrO2-Reduced
Ni (5.0 wt %)-ZrO2-Reduced

c
c’
c’’
c’’’
Ni (10 wt %)-ZrO2-Rec’’’’c’’’’’ ducedc’’’’’’
c’’’’’’’
c’’’’’’’’defNi (15 wt %)-ZrO2-Reduced
Ni (30 wt %)-ZrO2-Reduced
Ni (10 wt %)-SiO2-Reduced

2.3

21.7

4.6
9.2

43.4
86.8

2.3

21.7

12

CO

13

CH4

12

CH4

S C-containing products

1.9
0.086 < 0.002 < 0.002
0.027 < 0.002 21
0.44
Full lightl > 320 nm
l > 520 nm
l > 715 nm
No light
Full light, 295 K
bath

0.15
0.54
0.38
0.37
0.049
0.007
< 0.002< 0.002< 0.002

Full light< 0.002< 0.002< 0.002< 0.002< 0.002< 0.002< 0.002< 0.002< 0.002

C product
ratio[mol %]

4.3
2.0

14
340
79
1060
47
780
30
730
1.5
71
1.1
19
0.049
0.29
0.027
0.080
0.014
0.16

4.3
7.5
6.0
4.1
2.1
5.7
29
34
8.5

0.084 < 0.002 130

4.8

3.5

0.023 < 0.002

1.3

0.054

1.3

4.0

0.27

0.26

0.035

0.57

6.2

< 0.002

320
980
740
700
69
18
0.24
0.053
0.15

2.0
22

12

140

[a] Irradiated under UV/Visible light @142 mWcm@2 except for c’, c’’, c’’’, and c’’’’’’’’ (186 mWcm@2).

(ZrO2-Reduced) exclusively formed CO (Figure 1 A). The
CO formation remained steady for ca. 18 h and the rate
decreased by 14 % at 28 h (Figure 1 A, right).
In clear contrast, Ni-ZrO2 previously reduced in H2 at
723 K (Ni-ZrO2-Reduced) samples mostly produced CH4
with a molar ratio of > 98 % (Table 1 b–e) and CO as theminor product. The switching of the photoproduct from CO
to methane by typically doping noble metals on TiO2 has beenreported,[1, 2] where the valence state of metals under photocatalytic reaction conditions is the key factor rather than thework function of metals suggested in the literature.[2, 25]
By varying the Ni content between 5.0 and 30 wt % in NiZrO2-Reduced, the formation rate of C-containing products
reached a maximum with Ni 10 wt % (340 mmol h@1 gcat@1;
Figures 1 C and S3), which was higher than the rate obtainedusing ZrO2-Reduced by a factor of 170 times (Table 1 a,c).
The reason why the rate drew a maximum curve is discussed
in the Supporting Information, EXAFS section. Using NiZrO2-Reduced photocatalysts, the methane formation rates
remained constant until 27–48 h and decreased by 0–13 % at
48–49 h of the reaction (Figure 1 B–E). The equilibrium of theexothermic reaction [Eq. (2)] was preferably balanced to the
products thermodynamically, and the 33 % decrease of rate
after 48 h of reaction using Ni (10 wt %)-ZrO2-Reduced did
not result from photocatalyst deactivation but from thealmost total consumption of the reactant 13CO2 (Figure 1 C).
At 723 K, the pretreatment under H2 was essential forCO2 photoreduction using Ni-ZrO2-Reduced (Table 1 b–e).
The methane formation rate using Ni (10 wt %)-ZrO2(3.2 mmol h@1 gcat@1, Table S1b), i.e., not H2-treated, was only
0.94 % of that using Ni (10 wt %)-ZrO2-Reduced (Figure 1 C
and Table 1 c) owing to the difference in NiII versus Ni0 states.

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The 12C ratio in C-containing compounds generated using
ZrO2-Reduced and Ni (5.0–30 wt %)-ZrO2-Reduced was 2.0–
4.3 mol % (Table 1 a-e), which was greater than the ratio of
12
C contained in the 13CO2 reaction gas (1.0 %). To understandthe inconsistency, 13CO2 uptake/exchange tests were conducted under UV/Vis light irradiation. Using ZrO2-Reduced and13
CO2 (56.7 mmol), 9.9 mmol of CO2 gas very quickly adsorbed
at a rate constant of 8.0 h@1 with the 13CO2 :12CO2 molar ratioof 99:1 (Figure 2 A). Then, a slower 13CO2/12CO2 exchange
reaction proceeded at a rate constant of 0.07 h@1, illustrated
by the gradual increase in 12CO2 for 24 h (Figure 2 A).
Moreover, the 13CO2 photo-uptake/exchange was tested
using Ni (10 wt %)-ZrO2-Reduced (Figure 2 B). The initial
uptake amount of CO2 reduced by 40 % (5.9 mmol) comparedto ZrO2-Reduced (Figure 2 A). Furthermore, the rate constant was smaller (3.0 or 0.4 h@1) than that of 8.0 h@1 obtainedusing ZrO2-Reduced, suggesting both a physical block andkinetic interference by Ni nanocrystals for CO2 adsorption on
ZrO2.
Based on the 12CO2 ratio (2.4 and 2.1 mol %) in theequilibrated CO2 gas (Figure 2), the 12CO2 amounts existingprior to the photouptake/exchange test on ZrO2-Reduced andNi (10 wt %)-ZrO2-Reduced were 0.79 and 0.64 mmol (Supporting Information). Therefore, the equilibrated 12CO2 ratios
during the photocatalytic reduction tests using 2.3 kPa of
13
CO2 (193 mmol), including an impurity of 12CO2 (1.9 mmol),were 1.4 and 1.3 mol %, respectively (Supporting Information). However, the 12C ratios in the CO produced using ZrO2Reduced and CH4 produced using Ni (10 wt %)-ZrO2-Reduced were both 4.3 mol % (Table 1 a, c) and negligiblychanged during the test (Figure 1 A, C). Thus, a relatively
stronger adsorption site for CO2 over ZrO2 was an active site

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ChemieFigure 2. Time-course uptake and exchange reaction of 13CO2(0.67 kPa) irradiated by UV/Vis light using A) ZrO2-Reduced and B) Ni(10 wt %)-ZrO2-Reduced. The amount of catalyst was 0.020 g.

Figure 1. Time course of A) 13CO and 12CO and B–E) 13CH4 and 12CH4
formation during the photocatalytic test exposed to 13CO2 (2.3 kPa)
and H2 (21.7 kPa) using A) ZrO2-Reduced, B) Ni (5.0 wt %)-ZrO2Reduced, C) Ni (10 wt %)-ZrO2-Reduced, D) Ni (15 wt %)-ZrO2-Reduced, and E) Ni (30 wt %)-ZrO2-Reduced. The amount of catalyst was
0.020 g.

for the reduction to CO (Scheme 1 A-c).[19, 20] Previously, O
vacancy site (Zr3+) was suggested for the reduction to CO
based on electron paramagnetic resonance.[22] The O vacancy
sites increased by 1.46 times by the H2 pretreatment of ZrO2
at 723 K, accounting for the increase of CO formation rate by
1.16 times (Tables 1 a and S1a; Supporting Information).
Pressure dependence was evaluated by increasing thelight intensity (142 mW cm@2) applied in all the experiments
to 186 mW cm@2 (Table 1 c’, c’’, c’’’). By increasing the pressureof 13CO2 and H2 by two and four times, methane formation
rates decreased by 26 % and 31 %, respectively, but theformation was always steady until the 13CO2 was consumed.
Thus, the effect of adsorption inhibition by CO2 for thetransfer of formate/CO to Ni0 was moderate, and Ni-ZrO2Reduced was applicable near the atmospheric pressure.
Angew. Chem. Int. Ed. 2021, 60, 9045 – 9054

Scheme 1. A) Proposed intermediate species during CO2 exchange andphotocatalytic CO2 reduction, B) the energetics, and C) the ratecomparison to heat reactions.

Approximately zero order kinetics were reported for AgZrO2 under similar pressure range.[19]Next, the effects of the excitation light wavelength were
investigated using the most active Ni (10 wt %)-ZrO2-Reduced photocatalyst (Figure S4A-C and Table 1 c’’’’, c’’’’’,
c’’’’’’). The formation rates of C-containing products irradiatedby light at wavelengths l > 320 nm, l > 520 nm, and l >715 nm (Figure S6) were 21 %, 5.6 %, and 0.086 % of that
irradiated by full light, respectively (Figure 1 C and Table 1 c).
Furthermore, the methane formation rate in the kinetic test
under dark conditions at 298 K was even smaller: 0.024 %(Table 1 c’’’’’’’) of that irradiated by full light. The dark

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reaction was not effectively catalytic because the CH4formation slowed down after 10 h of the reaction to deactivate (Figure S4D, right) owing to the oxidation of the Ni
surface by CO2. Thus, the contribution of light for CO2
conversion was 79 % (l < 320 nm), 15 % (320 < l < 520 nm),
5.5 % (520 < l < 715 nm), 0.06 % (l > 715 nm), and 0.02 %(heat reaction).
The gradual increase of 12C-product ratio from 4.3 to34 mol % as the minimum wavelength of irradiated lightincreases (Table 1 c, c’’’’, c’’’’’, c’’’’’’, c’’’’’’’) indicates that thelight of shorter wavelength mostly activates CO2 strongly
adsorbed on ZrO2 (12CO2 in early stage of reaction; Scheme 1 A-c) via major band-gap and minor mid-band excitations (Figure 5; Supporting Information).
For comparison, the Ni (10 wt %)-SiO2 catalyst previously
reduced in H2 at 723 K [Ni (10 wt %)-SiO2-Reduced] wastested for CO2 photoconversion under 13CO2, H2, and full UV/
Vis light (Figure S5A). CO and methane were formed at 0.27
and 0.30 mmol h@1 gcat@1, respectively; however, the total rate
was 0.17 % of that obtained using Ni (10 wt %)-ZrO2Reduced (Table 1 c, f). Other control kinetic tests were
performed using Ni (10 wt %)-ZrO2-Reduced in dark conditions at 373–413 K. The obtained Arrhenius plot is shown in
Scheme 1 C. The formation rates of the C-containing products, which were mostly methane at 373, 393, and 413 K were
1.4 %, 6.6 %, and 18 %, respectively, as a percentage of thatobtained on irradiation by full light at 142 kW cm@2.
The photocatalytic methane formation was reported from
CO2 and H2 using Ni-SiO2·Al2O3 with no heat supply fromsurroundings.[26] The transformation of light energy into heaton Ni appeared to be similar; however, the contribution of
charge separation in ZrO2 is in contrast to that in SiO2·Al2O3.
To obtain further insight into the mechanism of CO2
photoreduction to methane, Fourier transform infrared(FTIR) spectra were measured for Ni (10 wt %)-ZrO2Reduced. 13CO2 was used to distinguish the species formed
from atmospheric 12CO2.[19, 20] The isotopic shift was calculatedusing harmonic oscillation from Equations (4)–(6):
1
~n ¼
2pc

sﬃﬃﬃ
k
m

ð4Þ

qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
1
1~n13CH13 þ 1¼ qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ ¼ 0:99704~n12CH1
1
12 þ 1

ð5Þ

qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ
1
1~n13CO
13 þ 16¼ qﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃﬃ ¼ 0:97778~n12CO1
1
12 þ 16

ð6Þ

where ~n is the wave number, c is the speed of light, k is theforce constant, and m is the reduced mass.
With the adsorption of 13CO2 (2.3 kPa) and H2 (21.7 kPa)
at 295 K for 1 h, peaks appeared at 1588, 1389, and 1220 cm@1
and were assigned to the frequency modes of antisymmetricstretching vibration nCO2 ðasÞ , symmetric stretching vibrationnCO2 ðsÞ , and bending vibration dOH of bicarbonate species,
respectively (monodentate or bridging; Figure S7A). The
peaks at 1624, 1421, and 1220 cm@1 correspond to 12C-

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Chemiebicarbonate species according to eq. 6, and agree with theresults available in literature.[19, 20, 27, 28] Simultaneously, the nOH
peak intensity at 3700 cm@1 of ZrO2 decreased owing to thereaction with CO2 (Scheme 1 A-a, b), whereas the nOH peakintensity of the frequency mode of bicarbonate increased at3615 cm@1 (Figure S7A).
Weaker shoulder peaks, owing to the frequency modes ofnCO2 ðasÞ and nCO2 ðsÞ of the carbonate species, appeared at 1509
and 1304 cm@1, respectively (Figure S7A). These positions
were in agreement with the 12CO3 species based on eq. 6 (1543
and 1334 cm@1), as well as with literature.[19, 20, 28, 29] However,no distinguishable peaks were observed in the nCO or nCH
region (Figure 3 A).
We then monitored the FTIR spectroscopy changes
associated with UV/Vis light irradiation (Figures 3 B and
S7B). Two intense peaks (2940 and 2857 cm@1) and a weak
peak (2750 cm@1) gradually grew in the nCH region (Figure 3 B). The values corresponded to 2949, 2865, and2758 cm@1 for 12CH stretching vibration, calculated using
Equation (5), and could be assigned to nCH(as) and nCH(s) of
13
CH3 on Ni0 (Scheme 1 A-f) and nCH of 13C-formate.[28, 30]
Because formed 13CH4 diffuses both in IR cell and Pyrex
glass circulation system (Supporting Information), the IR
active nCH (3010 cm@1) and dCH (1302 cm@1) peaks[31] for 13CH4
were not clearly detected. Instead, CH4 was quantified by gas
chromatography-mass spectrometry (GC-MS, see below).
In the nCO region, a peak gradually appeared at 2017 cm@1
(Figure 3 B, right) and was assigned to CO on Ni0 (Scheme 1 A-d).[32] Any peaks in nCH and nCO regions (Figure 3 B)
did not appear in the FTIR spectra for ZrO2 (Figure S8),
demonstrating that the methyl and CO species were over Ni0.
Formate was formed at the interface between the ZrO2 andNi0 surfaces by a two-electron reduction of bicarbonate(Scheme 1 A-b, c), followed by transformation to CO on theNi0 surface and the hydroxy group of ZrO2. Based on the12
CH4 ratio (2.0–4.3 mol %) using Ni-ZrO2-Reduced irradiated under full light significantly increased from the 12C ratio
in the reactant (1.0 mol %), methane formation starting fromCO2 exclusively over Ni0 surface is not plausible because 12C
in 12CH4 was derived from ZrO2 surface, not from clean Ni0
surface. Furthermore, heat reaction at 393 K under dark
conditions mostly over Ni0 surface was very slow compared tolight reaction (see the final part and Scheme 1), demonstrating that CO2 reduction to CO mostly occurred over ZrO2
surface.
In the nCO2 region under UV/Vis light, new peaks at 1530
and 1340 cm@1 gradually increased (Figure S7B, right) incompensation of the bicarbonate peaks at 1588, 1389, and1220 cm@1 (Figure S7A, right) decreased. The new peaks
could be assigned to nCO2 ðasÞ and nCO2 ðsÞ of a bidentate formate.
The calculated values (1565 and 1370 cm@1, respectively) for
12
C-formate based on eq. 6 were in agreement with theliterature.[27] Then, with the light turned off and undervacuum, the peaks ascribed to formate became dominant
and that at 1382 cm@1 was resolved owing to the dCH offormate, which corresponded to 1386 cm@1 for the 12Cformate species based on Equation (5).[28]The gradual increase in intensity for the nCH(s) peak of
13
CH3 on Ni0 under light (Figure 3 B) corresponded to the

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Chemiecombined with H occurred on the Ni0 surface (Figure 3 E
and Scheme 1 A-e, f).
In the transmission electron microscopy (TEM) image ofNi (10 wt %)-ZrO2, ca. 10 nm of crystal was observed (Figure 4 A). On further inspection, a 5–10 nm crystal showing
a lattice fringe with
E an
C interval of 0.194 nm, assignable tomonoclinic ZrO2 (202 plane, was connected to ca. 20 nm of
crystal, showing a lattice fringe with an interval of 0.299 nm,
assignable to the monoclinic ZrO2 ð111Þ plane (theoretical
0.285 nm, Figure 4 B). The other crystal showing a latticefringe with anof 0.314 nm, assignable to the monoE intervalC
clinic ZrO2 (1(11 plane (theoretical 0.318 nm), was further
connected. Similarly, crystals showing lattice fringes withintervals of 0.275–0.280, 0.314, and 0.258 nm wereE observed
C(Figure 4 C), assignable to monoclinic ZrO2 ð111Þ, (1(11 , andð200Þ planes [theoretical 0.255 nm for ð200Þ], respectively.[33, 34]

Figure 3. A)–D) FTIR spectra of Ni (10 wt %)-ZrO2-Reduced (10 mg)
for 3000–2700 cm@1 (left) and 2200–1900 cm@1 (right). The spectrum
for Ni (10 wt %)-ZrO2 just reduced in H2 at 723 K was subtracted fromeach data. A) Under 13CO2 (2.3 kPa) and H2 (21.7 kPa) for 1 h.
B) Under 13CO2, H2, and UV/Vis light for 20 h. C) Under 13CO2, H2, anddarkness for 2 h. D) Under vacuum for 1 min, and then UV/Vis lightfor 20 h. E) Correlation between methane formed by GC-MS and nCH(s)peak area owing to CH3 species by FTIR.

amount of formed methane simultaneously monitored by
GC-MS (Figure 3 E). This fact suggests that the hydrogenation of the methyl species on the Ni0 surface (Scheme 1 A-f)
was a relatively slow step. The methyl species slowly
decreased when the light was turned off (Figure 3 C,E).
Under irradiation by UV/Vis light in vacuum for 1 min,a significant portion of the methyl, CO, formate, andbicarbonate species decomposed (Figures 3 C,D and S7C,D)
and a slower methane formation from methyl species
Angew. Chem. Int. Ed. 2021, 60, 9045 – 9054

Figure 4. A) TEM, B),C) HR-TEM, and D) HAADF-STEM images observed for Ni (10 wt %)-ZrO2-Reduced sample. E) Associated energydispersive spectrum mapping for X-ray fluorescence of Ni La, Zr La,
and O Ka. F) Histogram of Ni nanoparticle size based on TEM andHAADF-STEM. The crystal face indexes for ZrO2 are of monoclinic
phase.

Furthermore, smaller spheres/spheroids were found attached on ZrO2 crystals as darker and brighter images in TEM
and higher-angle annular dark-field (HAADF)-scanning
TEM (STEM), respectively (the arrows in Figure S9A, B).
Then, a spherical crystal with a lattice fringe interval of
0.245 nm, assignable to the Ni ð110Þ plane (theoretical
0.249 nm)[35] connected to the monoclinic ZrO2 ð111Þ crystals,
was visible (Figure 4 C). It was relatively difficult to identify
Ni crystals despite the high content (10 wt %), indicating thatmost Ni crystals were smaller than the ZrO2 crystals, whichcorresponds to a weak X-ray diffraction (XRD) peakassigned to Ni ð111Þ (Figure S10). Moreover, this corresponds

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to the distribution of Ni, Zr, and O in the X-ray fluorescence
mapping (Figure 4 E) compared to the spatial distribution of
crystals in HAADF-STEM (Figure 4 D). Based on highresolution (HR)-TEM and HAADF-STEM images, the Ni
particle size distribution was obtained with a mean of 2.2 :0.8 nm (Figure 4 F).
The fluorescence spectra were measured for Ni-ZrO2Reduced samples at an excitation wavelength of 200 nm,
which is below the band gap wavelength (248 nm) of ZrO2
(Figure S6).[19, 20] The broad fluorescence band centered at
370 nm with a half width of 120 nm agreed with the datapreviously reported for ZrO2 nanocrystals (mean 2.9 nm).[36]Moreover, a sharp peak at 396 nm appeared in the ZrO2
spectrum (Figure 5 A). The emission in this range is attributed
to the short-wavelength excitation on the near band-edgetransitions, whereas the peaks at 530 and 625 nm are
attributed to the excitation to the mid-gap trap states, e.g.,surface O vacancy.[37]As the Ni content gradually increased, the progressive
decrease in fluorescence intensity was much quicker for Ni(0–30 wt %)-ZrO2-Reduced samples (Figure 5 A) than thatfor the non-H2-pretreated samples (Figure S11A). A part of
the reasons of the fluorescence peak decrease was the chargetransfer from conduction band of ZrO2 (@1.0 V)[19] to Ni(Fermi level 0.60–0.91 V).[35] The accelerated hydrogenation
steps over Ni0 can be expected owing to the transferred
electrons from ZrO2 (Scheme 1 A-d-f).
Figure 5 B shows the excitation spectra for ZrO2 and Ni(10 wt %)-ZrO2-Reduced, where the dependence is essentially equivalent for the fluorescence peaks at 370 (red line)
and 396 nm (blue line). The emission below 248 nm was
attributed to the relaxation of the band gap excitation;9050

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Chemiehowever, the weaker fluorescence above 248 nm was causedby the low concentration of O vacancy level between the
ZrO2 band gaps. As already shown for the excitation at
200 nm (Figure 5 A), the fluorescence was extensively suppressed by 10 wt % Ni and became negligible in the excitation
wavelength range of > 248 nm.
Xe arc lamps and quartz sample cells enabled UV light
below 248 nm in 13CO2 kinetic tests and fluorescence spectroscopy (Figures 1, 5, S2, and S11; Experimental section,
Supporting Information), leading to the highest 13CH4 formation rate based on ZrO2 band gap (Table 1 c). In contrast,
the light absorption above 248 nm due to interband states,e.g., surface O vacancy of ZrO2[37] and subsequent electrontransfer to Ni were indirectly confirmed based on the
excitation spectra (Figure 5 B), leading to the 13CH4 formation rate under light of > 320 nm, 22 % of the rate under fulllight.
The importance of this study is steady CO2 conversion
into methane using Ni-ZrO2-Reduced and UV/Vis light, noheat applied from surroundings (see below). However, therate is suppressed to 20–30 % (see below for the reaction
using water) using sunlight reached on the earth surface,
filtered by O2 and ozone layer.[38, 39] To fully utilize theperformance of Ni (10 wt %)-ZrO2-Reduced (Table 1 c, c’),the applications in the area affected by O3 layer depletion thatallows UV light below 248 nm,[39–41] at 25–50 km high in the O3
layer (CO2 < 1 Pa,[42] H2O < 0.03 Pa[43]) to increase O2 via
Equation (1) to mitigate O3 depletion by the O2 photodissociation, and as on-site fuel supply on Mars (43 % ofsunlight flux compared to the earth[38, 44]) where UV light
below l < 300 nm is negligibly filtered due to the compositionof atmosphere (95 % CO2, 3 % N2, and a few water).[45]Next, the variation in the Ni K-edge EXAFS for Ni(10 wt %)-ZrO2-Reduced photocatalyst, irradiated by UV/
Vis light, was monitored. The Fourier transform showeda typical pattern for metallic Ni sites before light irradiation(Figure 6 A). However, the major peak intensity at 0.21 nm(phase shift uncorrected) quickly decreased to 78 % within
20 min of irradiation and essentially remained the same for
130 min. The peak intensity ratio did not change throughout
the monitoring and no new peaks appeared (Figure 6 A, B),
demonstrating the Ni0 state unchanged (Scheme 1 A). Then,the reason for the decrease in peak intensity was investigated.
The time course changes of the major fit parameters,
coordination number N and Debye-Waller factor s, for the
Ni-Ni interatomic pair are illustrated in Figures 6 C and D,
respectively.
The s value was calculated as 0.00492 nm for Ni metal at
295 K based on the correlated Debye model[46, 47] using abinitio multiple-scattering calculation code FEFF8.4[48] and the
Debye temperature of Ni [qDebye(bulk) 450 K].[49] The XDAP
code[50] provided the experimental difference for the s2 valueof the Ni metal (model) based on Equation (7).
Aj ðkÞ ¼

Figure 5. Fluorescence spectra for Ni (0–30 wt %)-ZrO2-Reduced withthe excitation at 200 nm (A) and excitation spectra for ZrO2 and Ni(10 wt %)-ZrO2-Reduced (B); 2 mg of the sample powder was suspended in 3 mL of water.

Angewandte

+ .
-*
4
Nj 44
Rj
2 2
4
f
ðkÞ
exp
@2
s
k
þ
; j ¼ NickeljlkR2j jð7Þ

in which Aj(k), fj, and Rj are amplitude, backscattering
amplitude, and interatomic distance, respectively, for shell j, k

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Research Articles

Figure 6. Ni K-edge EXAFS for Ni (10 wt %)-ZrO2-Reduced (0.12 g)under CO2 (2.3 kPa) and H2 (21.7 kPa). A),B) Time-course change in
Fourier transform of k3c irradiated by light from Xe arc lamp (A) andunder dark (B). C),D) N (C) and s values (D) obtained by curve-fit
analyses of the Ni-Ni shell at 0.21 nm in (A) and (B). E) Temperaturedependence of s of the bulk site (a) and surface site for vertical andhorizontal motions to the surface (b, c) calculated using the FEFF8.4
code.[48]is the angular photoelectron wavenumber, and l is the mean
free photoelectron path.
Furthermore, we consider that the contribution of structural disorder for the s value[51] is the difference of the s2
value from the Ni metal.

Angewandte

ChemieTo adjust the theoretical temperature estimated for the Ni(10 wt %)-ZrO2 sample measured at 295 K, the sdisorder value
was evaluated as 0.00313 nm.
Apparently, the N(Ni-Ni) value negligibly changed within
8.3 : 0.2 (Figure 6 C) during light irradiation, whereas thes(Ni-Ni) value quickly increased from 0.00585 nm before
irradiation to 0.00745 nm within 20 min of irradiation, andremained at 0.00751 : 0.00008 nm (Figure 6 D).
Then, the UV/Vis light was turned off. Although theFourier transform and the N(Ni-Ni) value remained approximately the same, at 8.4 : 0.2 for 111 min (Figure 6 B, C), thes(Ni-Ni) value quickly decreased to 0.00587 nm within 10 min
and remained within 0.00589 : 0.00004 nm for 111 min (Figure 6 D). Separately, the N(Ni-Ni) value was 8.4 : 0.2 basedon the analysis of the Ni (10 wt %)-ZrO2-Reduced sample
after 48 h of reaction under 13CO2, H2, and UV/Vis light,
which suggests that the Ni particle size only changes
negligibly after a two-day reaction.
We evaluated the temperature at Ni sites based ons values. The s value temperature dependence was derived
from FEFF8.4,[48] combined with the correlated Debye
model,[46, 47] for Ni sites using the values of qDebye (bulk)
450 K,[49] qDebye (Surf,?) 208 K, and qDebye (Surf,k) 348 K[52] for bulk
sites and the motion of vertical and horizontal freedom oftranslation, respectively, assuming face-centered cubic (fcc)
Ni ð111Þ surface sites (Figure 6 E).
Based on the mean N(Ni-Ni) value (8.3) for Ni (10 wt %)ZrO2 (Figure 6 C) and assuming a fcc nanocluster model
exposing the preferred ð111Þ face,[53, 54] a mean particle size of
1.7 nm and a dispersion (D) of 0.64 were evaluated. Moreover, we approximated the mean Ni nanoparticle temperature as the arithmetic mean value based on qDebye(Surf,?)
weighted by 1/2·1/3D and qDebye(Surf,k) weighted by 1/2·2/3Dfor vertical and horizontal translational motions at a freehemisphere surface and qDebye (bulk) weighted by 1@1/2D (bulksite and non-free hemisphere in contact with ZrO2).
Thus-evaluated temperature of Ni is indicated in the timecourse of Figure 6 D. The initial temperature of Ni sites(295 K) quickly increased to 389 K within 20 min after the
UV/Vis light irradiation began and remained essentiallyconstant (389–394 K) during light irradiation for 130 min.
When the UV/Vis light was turned off, the temperaturesuddenly decreased to 296 K within 10 min.
The methane formed during the initial 20 min of full lightirradiation over the Ni (10 wt %)-ZrO2 photocatalyst (Table 1 c and Figure 6 D) was 3.4 mmol, corresponding to theheat of reaction of 0.56 J (Eq. (2)]. Based on the molar heat
capacity of Ni and ZrO2 (Cp,mA = 26.060[35] and56.123 J K@1 mol@1,[55] respectively), the temperature rise wascalculated as < 41 K when we consider the heat dissipation toreactor, which is insufficient for the monitored value of 94 K
(Figure 6 D). As the temperature remained approximately
constant during irradiation and only dropped after the light
was turned off and during the control tests being irradiated byfiltered light (Figure S14), the temperature increase resulted
from the transformation of light energy to heat at the Ni0
surface, and quickly reached the equilibrium of heating anddissipation.

s2 ¼ scorrelated Debye 2 þ Dðsdisorder 2 Þ þ DðsXDAP 2 Þð8ÞAngew. Chem. Int. Ed. 2021, 60, 9045 – 9054

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Based on the temperature reached at the Ni sites underfull light (394 K), filtered light of l > 320 nm (387 K), andfiltered light of l > 715 nm (334 K; Table S2 b–d), the light of
320 < l < 715 nm was primarily converted to heat by absorption in accordance with the absorption spectra (Figure S6).
CO2 reduction that occurs partially owing to the contributionof warmed Ni0 sites converted from light energy wascompared to thermal CO2 reduction under dark conditions(Scheme 1 C). At 394 and 387 K, the formation rates of Ccontaining products under full light were higher than thecorresponding ones in dark conditions (heater used) bya factor of 15 and 5.8 times, respectively, which demonstrates
the contribution of charge separation in ZrO2 at band gap (byUV) and O vacancy (by visible light; Scheme 1 A-a–d).
However, IR light had no effect on CO2 reduction (Scheme 1 C).
Arrhenius-type dependence for CO2 reduction was reported under both light and heat using ZrO2 at the maximum
formation rate of 4 mmol h@1 gcat@1, for which a thermal
reaction step was suggested.[23] The present study elucidates
that Ni0 paves the thermal route of CO to methane by the1
activation of H2 and the 2 kT value exceeding the minimum
activation barrier from CO to CH4 (Scheme 1 A-d–f and B)
combined with the reduction of CO2 to CO over the ZrO2
surface, which is driven by charge separation at the band gap
and the O vacancy sites by light (Scheme 1 A-a–d). Chemisorbed bicarbonate analyzed by FTIR and 12C ratio in
products (2.0–4.3 mol %) support such efficient interplay inclear contrast to the exclusive heat reaction on the Ni surface
under dark conditions. The combined promoting effects oflight and heat for Ru-Cu-MgO for NH3 decomposition may
be through a similar mechanism.[56]Then, the interplay of light and heat steps was reconfirmed by the comparisons of kinetic tests under full light (noheat added), at 393 K under dark, and at 393 K under fulllight. Note that the light intensity (142 mW cm@2) applied in
all the experiments increased to 186 mW cm@2 in this comparison. In the absence of applied heat, methane formation rateincreasedfrom
330 mmol h@1 gcat@1(Figure 1 C)to@1
@11060 mmol h gcat (Figure 7 A) under full light (Table 1 c,c’). Then, the methane formation rate at 393 K under dark
(Figure 7 B) was only 1.6 % of that under UV/Vis light(Figure 7 A). In contrast, when light irradiation started at
393 K, the rate increased by 190 times (Figure 7 B). This
demonstrates that light irradiation is essential for methane
formation to activate CO2 over ZrO2 surface. However,methane formation rate under both light and external heat
was higher by a factor of 3.2 times compared to that underlight (Figure 7 A,B). One of the reasons is that the reactor was
homogeneously at 393 K by heater in Figure 7 B while Ni0
nanoparticles were locally at 393 K by the heat converted
from light (Figure 6 D) but ZrO2 parts in catalyst and reactor
was less than 393 K due to heat dissipation. The essence of“photothermal catalysis”[57, 58] would be the interplay between
photoexcited step with thermally excited step as in Scheme 1 A. The photocatalysts in this paper do not need the heatfrom surroundings and are advantageous because light energy
is converted into heat as in Scheme 1 A-d–f (Figure 7 A).

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Angewandte

ChemieFigure 7. Time course of 13CH4 and 12CH4 formation during thecatalytic test exposed to 13CO2 (2.3 kPa) and H2 (21.7 kPa) (A) under
UV/Vis light and (B) at 393 K, under dark (first 24 h) followed by under
UV/Vis light (6 h) and exposed to (C) 13CO2 (2.3 kPa), H2O (2.3 kPa),
and H2 (21.7 kPa) under UV/Vis light and (D) at 393 K, under dark(first 24 h) followed by under UV/Vis light (6 h) both using Ni(10 wt %)-ZrO2-Reduced (0.020 g). The UV/Vis light intensity was
186 mWcm@2.

Conversely, kinetic test was performed under 13CO2, H2,
and full UV/Vis light through water bath to maintain the Ni(10 wt %)-ZrO2-Reduced photocatalyst at 295 K (Table 1 c’’’’’’’’). Methane was exclusively formed, however, therate was only two times of that under dark (Table 1 c’’’’’’’),
suggesting that Ni0 sites were cooled at ca. 295 K andnegligibly proceed the hydrogenation of CO/formate (Scheme 1 A-d–f). The reason why the rate was even lower thanthat using ZrO2-Reduced under full light (Table 1 a) would be
the blocking of ZrO2 surface and the inhibition of formed CO
to desorb both by Ni nanocrystals.
Finally, we tested CO2 photoconversion using water, H2,
and Ni (10 wt %)-ZrO2-Reduced catalyst [Eq. (3)]. With theaddition of water and H2 to the photoreactor, the metallic Ni0
state was preserved and 13CO2 was photoconverted into 13CH4
using H2O (Figure S16B) in dramatic contrast to test in theabsence of H2 (Supporting Information, Figure S16A). The
total formation rate of C-containing products was
130 mmol h@1 gcat@1 (Table 2 b), which was 38 % of the rate
obtained using only H2 (Table 1 c). The isotopic labeling of D2
confirmed that both water and H2 photoreduced CO2 intomethane (Table 2 c). A parent and a major fragment peak andmajor 13C-products were considered in the population analysis (Table S3). Under 13CO2 (2.3 kPa), H2 (21.7 kPa), andD2O (2.3 kPa), 13CH4 was the major product, and 13CH4,
13
CH3D, 13CH2D2, 13CHD3, and 13CD4 were all steadily
produced for 48 h (Figure S16C). The ratio of the formation
rates was 31 300:12 500:1740:95:1, demonstrating that 8.9 %of hydrogen in methane originated from water (Table 2 c) inagreement with D ratio in reactant (9.6 %) among D2O andT 2021 Wiley-VCH GmbH

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Research Articles

ChemieTable 2: Kinetic data on the photoconversion of 13CO2 (2.3 kPa) using H2O/D2O (2.3 kPa) and/or H2 (21.7 kPa), and Ni (10 wt %)-ZrO2-Reduced
photocatalyst (0.020 g).[a]Entry

Reactant

Formation rate [mmol h@1 gcat@1]

Light irradiated
13

a
b
b’
b’’
c

13

CO2 and H2O

13CO2, H2O, and H2

13

CO2, D2O, and H2

full lightl > 320 nm
full light

CO

0.12
0.22
0.28
0.28
0.20

12

CO

< 0.002< 0.002< 0.002< 0.002< 0.002

13

CH4

0.071
120
210
59
130[b]

12

12

CH4

H2

S products

0.60
9.2
15
5.7
–[c]

3.8
–
–
–
–

4.6
130
220
65
130

C product ratio[mol %]
76
7.2
6.7
8.8
–[c]

[a] Irradiated under UV/Vis light @142 mWcm@2 except for b’ (186 mWcm@2). [b] Including all 13C-methane forms (13CH4, 13CH3D, 13CH2D2, 13CHD3,
and 13CD4). [c] Owing to the overlap of mass numbers, we neglected the minor 12C-methane formation.

H2. Under these conditions, Ni0 was the major site rather thanNiII, as confirmed by EXAFS monitoring (Figure S15A, B).
The formation rate of C-containing products irradiated bylight at wavelength l > 320 nm was 30 % of that irradiated byfull light (Table 2 b, b’), allowing the applications for conversion of CO2 and H2O using sunlight reached on the earth
surface filtered by O2 and O3 layer.[38, 39] by the effects of ZrO2
O vacancy (Scheme 1 A-a–c). Furthermore, this methane
formation directly from CO2 and water is much morefavorable than water electrolysis and subsequent Sabatier
reaction by NASA on Mars.[3]The interplay of light and heat reaction steps wasreconfirmed under 13CO2, H2, H2O, and UV/Vis light(186 mW cm@2). In the absence of applied heat, methane
formation rate increased from 130 mmol h@1 gcat@1 (Figure S16B) to 220 mmol h@1 gcat@1 (Figure 7 C) under full light(Table 2 b, b’). Then, the methane formation rate at 393 K
under dark was essentially zero (Figure 7 D). In contrast,
when light irradiation started at 393 K, the rate increased by2000 times (Figure 7 D). This demonstrates that light irradiation is essential for methane formation using water as
reductant.

Conclusion
Using 13CO2, H2, Ni (10 wt %)-ZrO2-Reduced, and UV/
Vis light, 13CH4 was constantly generated at a rate of
320 mmol h@1 gcat@1 @142 mW cm@2 for two days, and the rateincreased to 980 mmol h@1 gcat@1 @186 mW cm@2. The prereduction of the catalyst in H2 at 723 K was essential for the
reduction of Ni species to active Ni0. Using FTIR spectroscopy, CO2 was adsorbed as a bicarbonate over ZrO2 surface
and transformed to formate and then CO under the effects ofcharge separation in ZrO2. CO was reduced to methyl, andthen methane on an average of 1.7 nm Ni0 nanocrystals. Basedon EXAFS, Ni0 was reversibly heated to 394–387 K by theenergy transformed from UV and/or visible light and activated H2 and CO. The CO2 conversion rates under UV and/or
visible light was 15 and 5.8 times higher, respectively, than thecorresponding temperature under dark conditions while therate when the photoreactor was maintained in water bath@295 K was essentially equivalent to that in the dark reaction@298 K, demonstrating that the interplay of charge separation in/on ZrO2 by light with the hydrogenation over Ni0
Angew. Chem. Int. Ed. 2021, 60, 9045 – 9054

surface by heat converted from light (no external heatapplied) was essential for the catalytic methane formation.
Using 13CO2, H2, D2O, Ni-ZrO2-Reduced, and UV/Vis light,
13
CH4 was constantly generated at a rate of 210 mmol h@1 gcat@1
for two days, including 8.9 % of D derived from D2O (molar D
ratio in reactant 9.6 %). This study paves the way for theutilization of first row transition metals in a reduced stateutilizing sunlight reached on earth (l > 300 nm) for solar fuelgeneration and ideally utilizing full spectrum of sunlightthrough O3 hole or at/near stratosphere to mitigate O3
deletion and on-site fuel supply on planets, for example, Mars.

Acknowledgements
The authors are grateful for the financial support obtained
from the Grant-in-Aid for Scientific Research B (20H02834)
and C (17K05961) from the Japan Society for the Promotionof Science and the Hirose Foundation (2020). X-ray absorption experiments were performed with the approval of the
Photon Factory Proposal Review Committee (2020G676,
2019G141, 2018G649). The authors would like to thankEnago (http://www.enago.jp) for the English language review.

Conflict of interest
The authors declare no conflict of interest.
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Manuscript received: December 9, 2020
Accepted manuscript online: January 20, 2021
Version of record online: March 8, 2021

T 2021 Wiley-VCH GmbH

Angew. Chem. Int. Ed. 2021, 60, 9045 – 9054