pubs.acs.org/JPCC

ArticleAdsorbed CO2‑Mediated CO2 Photoconversion Cycle into Solar Fuelat the O Vacancy Site of Zirconium Oxide
Keisuke Hara, Misa Nozaki, Rumiko Hirayama, Rento Ishii, Kaori Niki,* and Yasuo Izumi*
Cite This: J. Phys. Chem. C 2023, 127, 1776−1788

Downloaded via CHIBA UNIV on February 2, 2023 at 11:01:37 (UTC).
See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

ACCESS

Metrics & More

Read Online

Article Recommendations

sı Supporting Information*

ABSTRACT: ZrO2 photoreduces 13CO2 under ultraviolet−visible
light to 13C-product(s) negligibly affected by adventitious carbon.
The dual-site reaction pathway was theoretically clarified from CO2
to CO using monoclinic ZrO2, followed by multiple hydrogenation
steps to methane over Ni nanoparticles. The oxygen vacancy (V••O)
played essential roles, including the M-shaped CO2 adsorption over••the V••
O site and the direct occupation of the VO site by the
dissociated O and/or hydroxy group from the hydroxycarbonyl
species favorably on the ZrO2(111) surface. The rate-limiting step
was for the regeneration of the V••
O site with an activation energy(Eact) of 2.6 eV, but the water desorption energy was greatlycompensated by the CO2 adsorption energy at the V••
O site, incontrast to the first-row transition-metal oxides. The COH and/or
CO species transfer from ZrO2 to Ni in a concerted mechanism was energetically favorable, and the apparent Eact value fromhydroxycarbonyl species to methane was reduced to 0.67 eV.

1. INTRODUCTION
CO2 conversion into fuels utilizing natural light completes a
new C neutral cycle.1,2 To achieve such solar fuels as apotential renewable energy source in a sustainable society, theunderstanding of reaction routes using good photocatalysts forsolar fuel production is essential to improve the efficiency to beeconomically viable. Recently, selective methane photogeneration using a Ni−zirconium oxide photocatalyst was reported at
a rate of 0.98 mmol h−1 gcatalyst−1.3 The rate was one of thehighest solar fuel production rates confirmed by the online
isotopic monitoring of 13CO2 and 13C-products.4
A pioneering study was reported for the photocatalytic CO2
reduction pathway with an anatase-phase TiO2(101) surface
based on density functional theory (DFT) calculations using
the climbing image-nudged elastic band (CI-NEB) method topropose a hydrogenation route maintaining the C−O unit
(formaldehyde pathway) over the oxygen vacancy (V••O ) site of
TiO2.5 The other possibility is the carbene pathway that is via
dissociated C and carbene (CH2).5 However, an essential
question remains as to why ZrO2 can photoconvert 13CO2 into
13
CO, while the other semiconductors, e.g., TiO2, ZnO, andCuxO (x = 1, 2), often significantly form 12CO derived fromadventitious C.6 To achieve sustainable energy and create a
new carbon neutral cycle, the reason why ZrO2 exclusively
forms 13C-labeled products using 13CO2, not affected byadventitious C,1,2,7−14 except for CO2 strongly adsorbed fromthe air was investigated.3,4 This paper reports the unexpectedcatalytic role of VO•• sites on ZrO2 and the detailed© 2023 American Chemical Society

photocatalytic reaction routes to CO and/or COH and further
to CH4 with the aid of Ni nanoparticles.

2. METHODS
2.1. Experiments. Twenty milligrams of ZrO2 [JRC-ZRO3, Catalysis Society of Japan; major monoclinic and minor
tetragonal phase; specific surface area (SA), 94.4 m2 g−1]
powder was placed in a quartz U-tube connected to a Pyrex
glass circulation system (206.1 mL) and treated in a vacuumfor 1 h (10−6 Pa). H2 gas (20 kPa; purity > 99.99%) was
introduced, and the temperature was increased to 723 K,
maintained for 10 min, and treated in a vacuum. Thereafter,
13
CO2 (2.3 kPa; 13C 99.0%, purity > 99.9%, Cambridge
Isotope Laboratories, Inc., Tewksbury, MA) and H2 (21.7 kPa)
were introduced into the system and irradiated by UV−visible
light using a 500 W Xe arc lamp (SX-UID502XAM, Ushio,
Japan) through a quartz fiber light guide (Optel, Tokyo, Japan;
Model 1.2S15-1000F-1Q7-SP-RX) for 48 h. The reaction gas
was monitored by online gas chromatography-mass spectrometry (GC-MS; JMS-Q1050GC, JEOL, Tokyo, Japan).
Received: August 24, 2022
Revised: December 7, 2022
Published: January 19, 2023

1776

https://doi.org/10.1021/acs.jpcc.2c06048
J. Phys. Chem. C 2023, 127, 1776−1788

The Journal of Physical Chemistry C

pubs.acs.org/JPCC

Chart 1. Monoclinic-Phase ZrO2(111), (001), and (1̅1̅1)
Surfaces Comprising (2 × 2 × 2) Element Unit Cellsa

Subsequently, the reaction gas was evacuated for 1 min;natural CO2 (0.67 kPa; 12CO2 98.9%) was introduced into thesystem, and the pressure changes of 12CO2 and 13CO2 gases
were monitored by online GC-MS. In comparison, aphotocatalytic test was performed under 13CO2 and UV−
visible light irradiation in the absence of a reductant.
2.2. Theoretical Calculations. Spin-polarized periodic
DFT calculations were conducted using the Vienna Ab initio
Simulation Package (VASP) code version 6.2.115−18 computed
on a VT64 Server XS2-2TI comprising four units of Intel Xeon
Platinum 9242 Processors (2.3 GHz, 48 cores; Visual
Technology, Tokyo, Japan). The projector-augmented wave
method was employed at the DFT-D3 level19 to incorporate
the van der Waals interaction. The generalized gradientapproximation (GGA)-revised Perdew−Burke−Ernzerhof(RPBE) exchange−correlation functional20 was employed by
setting a plane-wave energy cutoff at 500 eV. Separately, each
optimized structure at the D3 level employing the GGA−
RPBE functional was further optimized by adding U (U = 4.0eV) to the original functional. The geometry of each species
starting from CO2 to CO evolution and hydroxy on Zr and toCO evolution and hydroxy to occupy the V••
O site negligiblychanged. The activation energy (Eact) values were essentiallypreserved independent of the functional difference. The
detailed discussion on the charge distribution of transition
state (TS) and the localized density of states is reported
elsewhere (unpublished results).
The criterion convergence was set at 10−6 eV for the selfconsistent field cycle, and optimizations were regarded asconverged when the forces on all atoms were smaller than 1.0
eV nm−1. All of the atoms were fully relaxed at the structural
optimization. The Brillouin zone was sampled with a 3 × 3 × 1
wave number vector k-point grid. The transition states were
pursued using the CI-NEB method.21 Based on the wave
functions obtained from the DFT calculations, the molecularorbital-projected density of states (MOPDOS) was simulatedusing the MOPDOS@VASP code22 compatible with the VASP
package.
The monoclinic-phase ZrO2(111), (001), and (1̅ 1̅ 1)
surfaces comprising (2 × 2 × 2) element unit cells werechosen as a slab model with a vacuum spacing of 1.0 nm
between the slabs (Chart 1a, c, and e). The surface oxygen
vacancy site on ZrO2 (V••O ) was modeled by removing an O
atom at the top layer of the ZrO2(111), (001), and (1̅1̅1)
surfaces (Chart 1b, d, and f).
The crystal phase and exposed faces of these slab models
were chosen based on the results of high-resolution transmission electron microscopy and X-ray diffraction measurements of the ZrO2 photocatalyst combined with Ni (10 wt %)via the liquid-phase reduction method.3 Hemispherical Ni19
clusters exposing the (111) face based on the face-centeredcubic (fcc) structure of the Ni metal were used. The (111)
surface of fcc Ni with (2 × 2 × 2) element unit cells waschosen as the slab model allowing a vacuum spacing of 1.0 nm
between the slabs (Chart 2).
The formation energy (Eformation) of oxygen vacancy sites andthe adsorption energy (Eads) were calculated based on eqs 1
and 2, respectively
Eformation = E VO

Eslab +

Eads = Emol/slab

Emol

O

(1)

Eslab

(2)

Articlea(a) Monoclinic ZrO2(111) faces with parameters of a = 1.35 nm, b =1.464 nm, c = 1.488 nm, α = 114.68°, β = 61.79°, γ = 116.44°, and
192 atoms comprising 64 ZrO2 units with a 1.0 nm vacuum layer
introduced in the direction of the x-axis. Green, Zr atom; red, O atom.
(b) V••
O sites on the ZrO2(111) face. Silver, O atom to be removed
bound to two Zr atoms, thereby forming (μ-Zr)2 and (η-O)2
adsorption orientations of CO2. Yellow, O atom to be removed
bound to three Zr atoms, thereby forming (μ-Zr)3 and (η-O)2
adsorption orientations of CO2. (c) ZrO2(001) faces with parametersof a = 1.03 nm, b = 1.05 nm, c = 1.07 nm, α = 90°, β = 99.25°, γ =90°, and 96 atoms comprising 32 ZrO2 units with a 1.0 nm vacuum
layer introduced in the direction of the z-Axis. (d) V••
O sites on theZrO2(001) face. silver, O atom to be removed bound to two Zr
atoms, thereby forming (μ-Zr)2 and (η-O)2 adsorption orientations ofCO2. (e) ZrO2(1̅1̅1) facets with parameters of a = 1.639 nm, b =1.489 nm, c = 1.469 nm, α = 53.82°, β = 57.95°, γ = 56.94°, and 192
atoms comprising 64 ZrO2 units with a 1.0 nm vacuum layer
introduced into the x-axis. (f) V••
O sites on the ZrO2(1̅ 1̅1) face. Silver,
O atom to be removed bound to two Zr atoms, thereby forming (μZr)2 and (η-O)2 adsorption orientations of CO2. Yellow, O atom tobe removed bound to three Zr atoms, thereby forming (μ-Zr)3 and(η-O)2 adsorption orientations of CO2 depicted using the VESTA
Version 3.5.7.23

Chart 2. Ni(111) Face with Parameters of a = 0.859 nm, b =0.496 nm, and c = 0.607 nm and 32 Atoms and a 1.5 nm
Vacuum Layer Introduced into the z-Axis Depicted Using
the VESTA Version 3.5.723

where Emol/slab, EVO, Emol, Eslab, and μO are the total energy of
the adsorbate on the slab model, the energies of the surfaced1777

https://doi.org/10.1021/acs.jpcc.2c06048
J. Phys. Chem. C 2023, 127, 1776−1788

The Journal of Physical Chemistry C

pubs.acs.org/JPCC

ArticleScheme 1. (A) 13CO2 and (B) 12CO2 Exchange Reaction
Mechanisms over the Oxygen Vacancy at the ZrO2 Surface

comprising the O vacancy site, the isolated molecule in the gas
phase and surface, and the chemical potential of O,
respectively.

3. RESULTS AND DISCUSSION
3.1. 13CO2 Photoreduction. The photoreduction of
13
CO2 into 13CO has been reported using 13CO2, H2, ZrO2,
and UV−visible light irradiation.3,4 Here, the formation rate
was compared with that using 13CO2, ZrO2, and UV−visible
light irradiation.
The 13CO formation rate was 0.80 μmol h−1 gcat−1 using
13
CO2 and H2 and 0.28 μmol h−1 gcat−1 using 13CO2 within 5 hof the reaction (Figure 1). Although the formation continued
H2, and UV−visible light irradiation was desorbed at a rate
constant of 3.0 h−1, and the amount was 2.8 μmol (Figure 2).
Further, 13CO2 desorption was significantly slower at a rate
constant of 0.02 h−1, and the amount was 3.0 μmol. Thisshould be the exchange of chemisorbed 13CO2 on the V••
O sitesduring the photocatalytic test with gas-phase 12CO2 (Scheme
1B) comparable to a rate constant of 0.07 h−1 for 12CO2
desorption from the V••
O sites for the fresh catalyst with gasphase 13CO2 (Scheme 1A).3
Based on the specific SA of ZrO2 (94.4 m2 g−1), the densityof such a chemisorption site of CO2 on the V••
O sites wascalculated to be 0.96 in an area of 1 nm2 at the ZrO2 surface.
Zhang et al. previously assigned 0.655 μmol of 12CO2
exchanged during the 13CO2 uptake and exchange test related
to O vacancy sites (Scheme 1A),3 which was only 22% of 3.0
μmol of the 13CO2 exchanged (Figure 2). The exchanged
12
CO2 originated from the 12CO2 adsorbed from air on the V••
O
site of ZrO2,3,4 but it was not fully occupied (Scheme 1A, top).
In contrast, the ZrO2 photocatalyst was only evacuated for 1
min after the 13CO2 photoreduction test, and most of thechemisorbed 13CO2 was preserved for this uptake andexchange test (3.0 μmol; Scheme 1B).
••
3.3. Formation Energy of V••
O Sites. First, VO sites wereformed on monoclinic ZrO2(001), (111), and (1̅1̅1) faces(Chart 1, b, d, and f). Based on eq 1, the formation energy wascalculated to be 5.9−6.4 eV (Table 1). The major part of V••
O

Figure 1. Time-course reaction of 13CO2 photoreduction irradiatedwith UV−visible light using ZrO2 reduced at 723 K under 13CO2 (2.3
kPa) and H2 (21.7 kPa) (circles) and 13CO2 (2.3 kPa) (squares). The
amount of ZrO2 was 0.020 g.

for 48 h using 13CO2 and H2, the 13CO formation ceased at∼15 h of the reaction using 13CO2, suggesting that the reaction
was not truly catalytic but just stoichiometric, e.g., theirreversible reaction with the V••
O site.
3.2. Isotope-Labeled CO2 Uptake and Exchange. The
time course of the 12CO2 uptake and exchange that wasmonitored after the photocatalytic test using 13CO2 and H2(Figure 1) is illustrated in Figure 2. The experimental reaction
steps are illustrated in Scheme 1. Within 1 h of the reaction,
12
CO2 was rapidly adsorbed on the ZrO2 surface at a rate
constant of 8.0 h−1. The amount was 8.2 μmol per 20 mg of
ZrO2. Corresponding to the physisorption of 12CO2, thephysisorbed 13CO2 during the photocatalytic test under 13CO2,

Table 1. Formation Energy of Oxygen Vacancy Sites at (μZr)2 and (μ-Zr)3 Sites of the Monoclinic ZrO2 Surface
entry

surface

Eformation (eV)

a
b
c
d
e

(μ-Zr)2 site on ZrO2(001)(μ-Zr)2 site on ZrO2(111)(μ-Zr)3 site on ZrO2(111)(μ-Zr)2 site on ZrO2(1̅1̅1)(μ-Zr)3 site on ZrO2(1̅1̅1)

6.2
6.4
6.0
5.9
5.9

sites was formed during the production process of the ZrO2
sample at the supplier (Daiichi Kigenso Kagaku Kogyo Co.,
Osaka, Japan),4 and the population increased by 47% with theH2 treatment at 723 K.3
3.4. CO2 Adsorption Geometry. First, V••
O sites wereformed on monoclinic ZrO2(001), (111), and (1̅1̅1) faces(Table 1). Fast methane photoformation was achieved using amajor monoclinic-phase ZrO2 combined with Ni0 nanoparticles.3 Typically, three types of initial geometries exist for
the adsorption structure optimization using VASP:15−18 the (i)vertical orientation of the CO2 molecule axis versus the ZrO2
surface within the proximity of the V••
O site, (ii) bonding from

Figure 2. Time-course uptake and exchange reaction of natural CO2(12CO2 98.9%, 0.67 kPa) irradiated with UV−visible light using ZrO2
reduced at 723 K under 13CO2 (2.3 kPa) and H2 (21.7 kPa) underUV−visible light for 48 h (Figure 1, circles). The amount of catalyst
was 0.020 g.
1778

https://doi.org/10.1021/acs.jpcc.2c06048
J. Phys. Chem. C 2023, 127, 1776−1788

The Journal of Physical Chemistry C

pubs.acs.org/JPCC

ArticleScheme 2. (A) Proposed Reaction Pathways and (B) Energetics of CO2 Photoreduction Energetically Facilitated by the
Oxygen Vacancy at the ZrO2 Surface

laboratory, and a CO2 adsorption step on the V••
O site wasenough exothermically (Table 2). If we consider the
combination of the V••
O site formation energy with the CO2
adsorption energy (Tables 1 and 2), the total energy changes
were 3.6, 2.5, 2.8, 3.1, and 3.4 eV for the (μ-Zr)2 sites on ZrO2(001) and ZrO2(111), the (μ-Zr)3 site on ZrO2(111), and (μZr)2 and (μ-Zr)3 sites on ZrO2(1̅1̅1), respectively, demonstrating that the combined steps were endothermic in accord withthe thermodynamic stability of CO2.
Similarly, the removal of two-, three-, and four-coordinatedO sites is discussed to create V••
O sites at the tetragonal
ZrO2(101) surface.25 Furthermore, similar M-shaped CO2
adsorption was recently reported at various V••
O sites at an
anatase TiO2(101),5,26 Ca2+-doped monoclinic ZrO2(1̅11),27
and Bi2WO6(010) surfaces.28
For the most stable (μ-Zr)2 and (η-O)2 orientations on
ZrO2(111), the C atom was relatively reduced based on the
Bader charge (1.05; Table 2b), while the C atom at the most
unstable (μ-Zr)3 and (η-O)2 orientations on ZrO2(1̅1̅1) waseven more reduced (Bader charge 1.02; Table 2e), comparedwith that of the other orientation and/or ZrO2 crystal face
(1.05−1.10; Table 2a−d). The O−C−O angle was 116.87−
132.06° with an M-shaped drawing. Although it is believed that
the O−C−O angle and/or Bader charge correlated well with
the Eads value,29 based on Table 2, the number of supporting
Zr sites more closely correlated with the Eads value; the (μ-Zr)2
site led to a greater Eads value rather than that of the (μ-Zr)3
site.

two surface Zr atoms (μ-Zr)2 and two O atoms of CO2 (η-O)2
orientations maintaining the CO2 molecular axis parallel to theZrO2 surface and the C atom was within the proximity of theV••
O site, and (iii) (μ-Zr)3 and (η-O)2 orientations maintaining
the CO2 molecular axis parallel to the ZrO2 surface and the C
atom was within the proximity of the V••
O site. Geometries (ii)and (iii) were associated with the V••
O site created by removing
an O atom of coordination numbers to Zr, 2 and 3 (Chart 1b,
silver and yellow positions, respectively).
Among them, the (μ-Zr)2 and (η-O)2 orientations of CO2
starting from the initial geometry (ii) on the monoclinic
ZrO2(111) surface were significantly stable (Scheme 2A(b)and Chart 3b), originating from the V••
O site coordinated totwo Zr atoms (Chart 1b, silver, Scheme 2A(a), and Chart 3a).
The energy change upon adsorption (Eads) was −3.9 eV (Table
2b and Scheme 2B(a,b)). The value was compared with −3.2
eV for the (μ-Zr)3 site on ZrO2(111) (Table 2c), originatingfrom the V••
O site coordinated to three Zr (Chart 1b, yellow);−2.6 eV for the (μ-Zr)2 site on ZrO2(001) (Table 2a),originating from the V••
O site coordinated to two Zr (Chart 1d,silver); −2.8 eV for the (μ-Zr)2 site on ZrO2(1̅1̅1) (Table 2d),
originating from the V••
O site coordinated to two Zr (Chart 1f,
silver); and −2.5 eV for the (μ-Zr)3 site on ZrO2(1̅1̅1) (Table2e), originating from the V••
O site coordinated to three Zr(Chart 1f, yellow). The (μ-Zr)3 adsorption of CO2 wasgeometrically inhibited at the ZrO2(001) surface (Chart 1c,d).
On all of the assumed V••
O sites, the Eads values for CO2
adsorption were between −2.5 and −3.9 eV. This is because
the V••
O sites exist in the fresh ZrO2 sample supplied to our1779

https://doi.org/10.1021/acs.jpcc.2c06048
J. Phys. Chem. C 2023, 127, 1776−1788

The Journal of Physical Chemistry C

pubs.acs.org/JPCC

ArticleChart 3. Three-Dimensional Illustration of Surface Species (Scheme 2A) during CO2 Photoreduction to CO over theZrO2(111) Surface Depicted Using the OVITO Version 3.7.8.24 Panels (a)−(o) Correspond to Species a−o, Respectively, in
Scheme 2A

3.5. Associative Mechanism. Next, the proton addition
step and the effect of light irradiation were evaluated during
the CO2 dissociation processes starting from the most stableM-shaped (μ-Zr)2 and (η-O)2 orientations of CO2 on theZrO2(111) surface (Scheme 2A(b)). The adsorbed CO2transformed into a hydroxycarbonyl species, OCOH (Scheme
2A(d) and Chart 3d), through H addition via the reaction of aphotogenerated hole with split H from H2 on ZrO2 and/or
spillover from the Ni0 nanoparticle site (Scheme 2A(c) and
Chart 2c). Similar H spillover from the Ag site toward theanatase-phase TiO2(001) and (101) surfaces has beenreported.4,30
The charge transfer was suggested during the reaction steps
from CO2 adsorption to hydroxycarbonyl species formation

Table 2. Adsorption Energy, the Bader Charge, and the O−
C−O Angle of (μ-Zr)2 and (μ-Zr)3 M-Shaped CO2
Adsorption on Oxygen Vacancy Sites at the Monoclinic
ZrO2 Surface
entry

surface

Eads
(eV)

a
b
c
d
e

(μ-Zr)2 site on ZrO2(001)(μ-Zr)2 site on ZrO2(111)(μ-Zr)3 site on ZrO2(111)(μ-Zr)2 site on ZrO2(1̅1̅1)(μ-Zr)3 site on ZrO2(1̅1̅1)

−2.6
−3.9
−3.2
−2.8
−2.5

Bader
chargeof C

O−C−O angle

1.08
1.05
1.10
1.10
1.02

132.06°129.72°116.87°128.70°123.34°

Table 3. Change of the Summation of Bader Charges on Surface Species on (μ-Zr)2 Oxygen Vacancy Sites and Two μ-Zr
Atoms at the Monoclinic ZrO2 (111) Surface in the Absence/Presence of Ni Nanoparticle
entry

surfaceaV••
O site on ZrO2bV••
O (μ-Zr)2 site on ZrO2 with the Ni nanoparticle

none
adsorbed

two μ-Zr
atoms

Schemes2A(a)and3A(a)4.22
Schemes 4A(a′) and 3B(B(a′)
5.26
1780

CO2
adsorbed

two μ-Zr
atoms

OCOH
adsorbed

two μ-Zr
atoms

Schemes 2A(b) and 3A(b)
−1.16
5.12
Schemes 4A(b′) and 3B(b′)

Schemes 2A(d) and 3A(d)
−1.02
4.90
Schemes 4A(d′) and 3B(d′)

−1.33

−0.72

5.32

5.14

https://doi.org/10.1021/acs.jpcc.2c06048
J. Phys. Chem. C 2023, 127, 1776−1788

The Journal of Physical Chemistry C

pubs.acs.org/JPCC

ArticleScheme 3. Extent of Charge Transfer during the Reaction Steps from CO2 Adsorption to the Formation of O−C−O−H
Species (Schemes 2A(d) and 4A(d′)) Using (A) ZrO2 and (B) Ni−ZrO2, Respectively(Scheme 2A(a−d),B(a−d)). Upon the CO2 adsorption, a net
1.16 negative charge based on the Bader charge was transferredfrom the ZrO2 (111) surface to one molecule of CO2 (Table
3a and Scheme 3A(b)), followed by the hydrogen addition andelectron transfer forming hydroxycarbonyl species. The species
was still charged by −1.02 based on the Bader charge owing to
the charge transfer from the supporting V••
O site (Table 3a andScheme 3A(d)). The net one negative charge transfer strongly
implies the direction of the separated electron injection fromthe accumulated V••
O site and/or the O 2p orbital of ZrO2 by
the UV light irradiation to CO2-derived surface species. Such aphotocatalytic scenario is further discussed below based on the
MOPDOS of hydroxycarbonyl species rather than via heatreaction mechanism.
The direction of charge transfer from the V••
O site to CO2
based on Bader charges was also confirmed by the DOS••
distribution for V••
O -ZrO2 comprising the VO site below theFermi energy level (EF; Supporting Information, Chart S1a)and the appearance of CO2 lowest-unoccupied molecularorbital (LUMO) just below the EF level by the CO2 adsorption(Chart S1b).
MOPDOS (Chart 4A) was obtained by projecting the wave
function of OCOH−ZrO2 onto the wave function of theisolated OCOH (Scheme 2A(d)) to extract the frontier
orbitals of OCOH species among the total DOS. The total
DOS (Chart 4B) is the sum of the DOS of all atoms in thesystem and was used to narrow the range for extracting theband structure described below. The band structure (Chart
4C) was derived using the unfolding method31 to correctly
extract the supercell band structure. Partial charge density(Chart 4D) was obtained based on the valence band (VB)
maximum, conduction band (CB) minimum, and twointermediate levels determined from the total DOS and band
structure. The molecular orbitals of OCOH species hybridized
with those of ZrO2 were determined from Gaussian-based
molecular orbitals32 (Chart 4E).
The energy interval was approximately 1.4 eV between thehighest occupied molecular orbital (HOMO) of the OCOH
species (Chart 4A,B) and CB of ZrO2. The LUMO and
HOMO were the transformation from antibonding (π*) e1u
and nonbonding e1g orbitals of the straight CO2 molecule by
the presence of the H atom and the surface (Chart 4E(i,ii),
respectively). Regarding the electronic configuration, the
LUMO and HOMO of the OCOH species effectively
overlapped with the 4d orbital of the right-hand side Zr(Chart 4D(ii),E(i,ii)), which is also the major component of1781

https://doi.org/10.1021/acs.jpcc.2c06048
J. Phys. Chem. C 2023, 127, 1776−1788

The Journal of Physical Chemistry C

pubs.acs.org/JPCC

ArticleChart 4. (A) Molecular Orbital-Projected (MOP) Density of States (DOS), (B) Total DOS, (C) Band Structure Around
Valence Band Maximum (VBM) and Conduction Band Minimum (CBM), and (D) Partial Charge Density of OCOH−
ZrO2(111) (Scheme 2A(d)). (E) Corresponding MO of OCOH Species Geometry-Optimized on ZrO2(111) Surface Using
Gaussian 0932

the CB of ZrO2 (Chart 4D(i)), suggesting the electron transferfrom the right-hand-side Zr atom to the C atom of the OCOH
species as evident from the smaller Bader charge on the C(Table 2b and Scheme 3A(d)).
The key reaction steps of photocatalysis are redox reactions
by the separated electrons and holes in the semiconductor
under the irradiation of ultraviolet (UV) and/or visible light.1,2
The above-suggested charge transfer from the V••
O site to theCO2-derived surface species (Scheme 3A and Chart 4) predicts
the direction of the separated electron injection in clear
contrast to the basically directionless motion of thermal
electrons.33
The plausible electron injection routes to hydroxycarbonyl
species (Schemes 2A(d) and 3A(d)) should be (i) from VB to
CB (gap 4.2 eV; Chart 4B) and then to LUMO, (ii) from VB
to LUMO (gap 4.8 eV; Chart 4A,B), (iii) from the V••
O site to
CB (gap 1.4 eV) and then to LUMO, (iv) from the V••
O site to
LUMO (gap 2.0 eV), and (v) from HOMO to CB (gap 1.4eV) and then to LUMO (Chart 4A−C). The DOS of the V••
O
site effectively mixed with that of hydroxycarbonyl HOMO

(Chart 4A), and the plausible route v requires electroncompensation from ZrO2 to the HOMO of surface species.
The energy gaps were calculated to be 1.4−4.8 eV,
effectively exceeding thermal activation barrier, whereas
concentrated UV light photon energy (3.1−6.2 eV) from theXe arc light source used in the photocatalytic tests for ZrO2
and Ni−ZrO23,4 enabled to exceed the energy gaps of 1.4−4.8
eV in accord with the photocatalytic scenario in this study
rather than thermal catalysis. Thus, the MOPDOS approachassociated with DFT calculations in this study nicely extracts
the geometry and electronic structure of surface intermediate
species in the reaction coordinate of the CO2 photoreduction,
probably similar to that disclosed by the state-of-the-art
spectroscopy combined with DFT calculations.34
The O(HO)C···Zr-coordinated structure (Scheme 2A(d))
should further change to interact more when the excitedelectron from the VB to CB flows into the LUMO of thesurface state under the irradiation of UV light. As a plausible
route, the C−O bond of the O−COH species dissociates tohydroxy species and CO evolution (Scheme 2A(d−f)). The
1782

https://doi.org/10.1021/acs.jpcc.2c06048
J. Phys. Chem. C 2023, 127, 1776−1788

The Journal of Physical Chemistry C

pubs.acs.org/JPCC

ArticleThe direct participation of the V••
O site in the CO2 reduction
has rarely been reported to occupy the V••
O site by the O
derived from CO2 (Scheme 2A(k,m)), followed by theregeneration of the V••
O site (see the next section). Instead,the contribution of the interband state between the band gap
to the CO2 reduction step was discussed regarding the V••
O siteat the Bi2WO6 (010) surface.28
The bicarbonate species (Scheme 2A(g)) was predominantly detected by Fourier transform infrared (FTIR)
spectroscopy under CO2 photoreduction conditions.3 Mostof the bicarbonate species detected by FTIR should be
separated from the V••
O site and accumulated, rather than form
species h because the concentration of the V••
O site was 0.96 in
an area of 1 nm2 at the ZrO2 surface (see the Isotope-Labeled
CO2Uptake and Exchange section).
••
3.8. Regeneration of V••
O Sites. The VO regenerationroute was evaluated starting from the common hydroxy species
m occupying the V••
O site independent of the CO2 reduction
route, associative, hopping, and shortcut associative routes(Scheme 2A(m) and Chart 3m).
First, the H species was supplied neighboring the hydroxyspecies via the reaction of the photogenerated hole anddissociated/spillover H (Scheme 2A(n),B(n) and Chart 3n).
Water formation reaction from the species required the Eact of
2.61 eV (Scheme 2A(n−o),B(n−o)). Thus, this step was ratelimiting among the overall catalytic steps of CO2 photoreduction using monoclinic ZrO2. Fortunately, in view ofcatalysis, this route did not require the water desorption energyof as much as 5.60 eV (Scheme 2A(o,a),B(o,a) and Chart
3o,a) to regenerate the original species a. Instead, theadsorption energy of CO2 on the V••
O site (3.90 eV) effectively
compensated for the water desorption energy (5.60 eV). Thus,
the water desorption required just 1.70 eV to restart thecatalytic cycle from species b but not species a (Scheme
2B(a,b,o) and Chart 3a−o).
3.9. Theoretical and Experimental Evaluation of V••
O
Sites. The concentration of the V••
O sites at the ZrO2 surface
heated at 723 K under H2 and evacuated at 295 K for 1 h was
0.655 μmol per 20 mg of ZrO2 based on the 13CO2 exchange
reaction (Scheme 1A).3 In contrast, the concentration of
13
CO2 chemisorbed on the V••
O sites was monitored at theZrO2 surface heated at 723 K under H2, exposed to 13CO2, H2,
and UV−visible light irradiation for 48 h, evacuated for 1 min,and exposed to 12CO2. The 13CO2 amount desorbed (3.0μmol) per 20 mg of ZrO2, which was 4.6 times the simple
12
CO2 desorption under 13CO2 gas,3 corresponded to the full2
concentration of the 0.96 V••
O sites in 1 nm at the ZrO2 surface
13
during the CO2 photoreduction test (Figure 2 and Scheme
1B). This scenario correlated with the proposed reaction
mechanism illustrated in Scheme 2A that skipped vacantspecies a, but the CO2 adsorption energy always compensatedfor the V••
O site regeneration to desorb water.
The 13CO2 photoreduction test was performed using ZrO2
and the irradiation of UV−visible light in the absence of the
reductant (H2 or H2O). The 13CO formation ceased at ∼15 hof reaction and 0.085 μmol of the 13CO formation, whereas
constant 13CO formation was observed at a rate of 0.80 μmol
h−1 gcat−1 using 13CO2, H2, ZrO2, and UV−visible lightirradiation (Figure 1). Thus, the reaction path of the direct
dissociation of CO2 in the absence of hydrogen (Scheme
2A(b,p),B(b,p)) was negligible.
Alternatively, the Eact value for water formation from species
n to o (Scheme 2A) drastically decreased to 1.6 eV if the

TS was the square O−C−O−H species (Scheme 2A(e) and
Chart 3e) resulting in hydroxy species over the left-hand-side
Zr atom (Scheme 2A(f) and Chart 3f). The Eact value of thisreaction path was 1.37 eV (Scheme 2B(d,e)). This path wasenabled by the photogenerated electron excitation to the VB
and then the injection to the OCOH species.
The associative route was favorable starting from CO2 on
the (μ-Zr)2 site using ZrO2(111), owing to the greater Eads
value of −3.9 eV, compared with that of −2.5 to −3.2 eVstarting from CO2 on the other site and/or on the other ZrO2
crystal face (Table 2a−e).
The final state in the associative route left the hydroxyspecies neighboring the V••
O site (Scheme 2A(f),B(f) and Chart
3f) that differed from the original V••
O site that lacked thehydroxy species at the beginning of the reaction (Scheme
2A(a)). If secondary CO2 reacted, the insertion reaction of thesecondary CO2 into the O−H bond to form bicarbonate(Schemes 2A(g) and 2B(g) and Chart 3g), and the filling of
the V••
O site by the terminal O atom of bicarbonate (Schemes
2A(h) and 2B(h) and Chart 3h), proceeded nearly
spontaneously, compared with the C−O dissociation steps inwhich the Eact value was significant, 1.37 eV (Scheme 2B(d,e)).
Thereafter, similar to the primary CO2 dissociation (Scheme
2A,B(e−f)), a square geometry comprising O−C−O−H wasformed with the Eact value of 1.44 eV, transforming intohydroxy and OCO species vertically filling the V••
O site(Scheme 2A(i),B(i) and Chart 3i). However, the dissociationof the C−O bond of the OCO species to desorb CO (Scheme
2A(j),B(j) and Chart 3j) required as much as an Eact of 2.43 eV(Scheme 2B(j)). Thus, the secondary CO2 dissociation route
was implausible.
3.6. Hopping Route of CO 2 Dissociation. The
possibility of the lateral dissociative reaction route was
evaluated in comparison to the associative route in theprevious section. The key was the interatomic distance
between the Zr and O atoms: 2.015 nm on the ZrO2(111)
surface shorter by 0.064 nm (Chart 1a,b), compared with
2.079 nm on the ZrO2(001) surface around the V••
O site (Chart
1c,d). The two routes were distinguished whether the H atom
combined with the O atom on the other side before the O−C
dissociation (associative route; Scheme 2A(e)) or the O−C
bond in the OCOH species directly dissociated (hopping
route; Scheme 2A(k),B(k)) and the left-hand-side O atom
filled the V••
O site, leaving the COH species. The COH species
was stabilized on the bridging site between the O and Zr atoms
by hopping from the terminal site on the Zr atom. The Eact
value of the first O−C dissociation step was 1.30 eV, which
was smaller, compared with 1.37 eV for the associative route(Scheme 2B(d−f,k)). Following the COH species in the
hopping route, the H atom approached the C atom, thereby
forming a formyl group, then hydroxy, and CO (Scheme
2A(l,m),B(l,m) and Chart 3l,m).
3.7. Shortcut in the Associative Route. The hopping
mechanism appeared to be more plausible, compared with theassociative mechanism based on the energy comparison of the
O−C dissociation step, 1.30 eV (Scheme 2B(d,k)) versus 1.37
eV (Scheme 2B(d−f)), respectively. However, a shortcut wasobserved in the associative route following species d, e, and m
via the direct V••
O site occupation by the hydroxy species(Scheme 2A(d,e,m) and Chart 3d, e, and m). The Eact value ofthe step was minimal, 1.18 eV (Scheme 2B(d,e,m)). Thus, thisshortcut associative route was the most plausible for CO2
photoreduction on the ZrO2 photocatalyst to CO.3
1783

https://doi.org/10.1021/acs.jpcc.2c06048
J. Phys. Chem. C 2023, 127, 1776−1788

The Journal of Physical Chemistry C

pubs.acs.org/JPCC

ArticleScheme 4. (A) Proposed Reaction Pathways and (B) Energetics of CO2 Photoreduction Energetically Facilitated by the
Oxygen Vacancy at the ZrO2(111) Surface Neighboring to the Ni NanoparticleaaEnergy changes for the simple ZrO2(111) surface free from the Ni nanoparticle (Scheme 2) are also drawn in panel B for comparison.

original V••
O site was on the edge between the ZrO2(111) and(001) faces (figure not shown), suggesting the catalytic role ofspecific V••
O sites (unpublished results).
3.10. Interface Effects between ZrO2 Surface and Ni0
Nanoparticle. The CO2 adsorption and activation were also
evaluated at the interface between the ZrO2(111) surface andNi(111) surface (Scheme 4). The orientation of the Ni cluster
and V••
O site on the ZrO2(111) surface is depicted in Chart
S2A. Further details of the position of the (μ-Zr)2-V••
O site andthe orientation of the Ni nanoparticle are also shown in Chart
S2B. The Eads value of CO2 became smaller to −1.9 eV by theeffect of the neighboring Ni nanoparticle compared to −3.9 eV
for CO2 on the ZrO2 (111) surface free from Ni (Schemes
2B(a,b) and 4B(a′,b′) and Chart S3a′,b′).
The energy levels during the following reaction steps tohydroxycarbonyl and OCH species (Scheme 4A(a′−d′) and
Chart S3a′−d′) were essentially in parallel separated by thedifference of Eads value (2.0 eV, Scheme 4B(a−d),(a′−d′) and
Chart S3(a′−d′)); however, the Eact value from hydroxycarbonyl and OCH species (Scheme 4(d′,s) and Chart S3(d′,s))was 0.32 eV at the interface between ZrO2 and Ni nanoparticle(Scheme 4B(d′,s)), significantly smaller compared to 1.37 eV
in the associate route, 1.30 eV in the hopping route, and 1.18
eV in the associative, shortcut route (Scheme 2A), because
COH species was significantly more stable on the Ni(111)
surface [Scheme 4A(s) and Chart S3-TS from d′−s,s]compared to that on the ZrO2(111) surface (Scheme 2A(k)),
and the intersection of potential curves for OCOH and COH
species in the reaction coordinate was lowered more on the Ni(111) surface. The above discussion strongly suggested thecritical photocatalytic role of the interface site between the
ZrO2 surface and Ni for the CO2 photoreduction.
In contrast to the charge transfer from the V••
O site at theZrO2 (111) surface to CO2 (Scheme 3A), a net 1.04 negative
charge transferred from the V••
O site at the ZrO2 surface to the
Ni cluster (Table 3a,b and Scheme 3B(a′)). Then, a net 1.33
negative charge based on the Bader charge was transferredfrom the Ni cluster to one molecule of CO2 (Table 3b andScheme 3B(b′)), followed by the hydrogen addition andelectron transfer forming hydroxycarbonyl species (Scheme
3B(c′,d′)). The species was still charged by −0.72 based on

the Bader charge owing to the charge transfer from the Nicluster; however, the species almost combined with the Ni
nanoparticle (Table 3b and Scheme 3B(d′)).
As the OCOH species almost shared electrons with the Ni
nanoparticle, the charge on Ni relatively increased compared tothe state that CO2 adsorbed on the ZrO2 surface (Scheme
3B(b′,d′) and Table 3b). The DOS distribution between the••
EF level and −2.5 eV for V••
O -ZrO2 comprising VO (Chart S1c)increased by the CO2 adsorption in the proximity of the Ni
nanoparticle (Chart S1d), supporting the electron transferfrom the V••
O site to the Ni nanoparticle and then to CO2derived species. Thus, the photocatalytic reaction mechanism
was partially tuned by the presence of Ni in the proximity of
the V••
O site.
MOPDOS was also obtained by projecting the wave
function of OCOH−Ni−ZrO2 onto the wave function ofisolated OCOH (Scheme 4A(d′)) to extract the frontier
orbitals of OCOH species among the total DOS. The total
DOS (Chart 5B) is the sum of the DOS of all atoms in thesystem. Smaller DOS of isolated OCOH, especially in thesimilar energy range to the DOS of the Ni cluster, wassuccessfully projected (Chart 5A). Because the hydroxycarbonyl species effectively shared electrons with the Ni cluster(Table 3b and Chart 5C), the OCOH LUMO was at 1.2 eV(Chart 5A) compared to the OCOH LUMO (Up spin) at 1.8
eV on the ZrO2(111) surface free from Ni (Chart 4A).
Partial charge density (Chart 5C) was obtained based on the
VB maximum, CB minimum, and two intermediate levels
determined from the total DOS and band structure. Similar to
the results for the OCOH−ZrO2 system (Chart 4D,E), the
LUMO and HOMO were the transformation from antibonding (π*) e1u and nonbonding e1g orbitals of the straight CO2
molecule by the presence of the H atom and the surface (Chart
5C(i,ii), respectively). As was evident in the charge-transfer
evaluation (Scheme 3B(d′)), the LUMO and HOMO of the
OCOH species effectively overlapped with the 3d orbital of the
Ni cluster.
3.11. Hydrogenation of CO/Formate over Ni0 Nanoparticle. The 13CO formation rate using 13CO2, H2, ZrO2 (20mg), and UV−visible light irradiation (0.016 μmol h−1, Figure1) and that using 13CO2, ZrO2 (20 mg), and UV−visible light1784

https://doi.org/10.1021/acs.jpcc.2c06048
J. Phys. Chem. C 2023, 127, 1776−1788

The Journal of Physical Chemistry C

pubs.acs.org/JPCC

Articleirradiation (0.0057 μmol h−1 until 15 h of reaction; Figure 1)
was quite low if 3.0 μmol of V••
O sites were utilized per 20 mg
of ZrO2 (Scheme 1). This fact does not contradict the efficient
reaction path from CO2 to CO, as revealed in Scheme 2A, e.g.,
via the M-shaped adsorption of CO2 (label b), H-assisted O−
C bond dissociation through the square intermediate of
OCOH (labels c, d, and e), and occupation of the V••
O site byhydroxy species (label m).
The H addition on the O atom neighboring the M-shaped
adsorbed CO2 (Scheme 2A(c) and Chart 3c) should have been
accelerated by the H spilled over from the Ni nanoparticle thateasily activated H2.3 Thus, the 13CH4 formation rate using
13
CO2, H2, Ni0 (10 wt %)−ZrO2 (20 mg), and UV−visible
light irradiation (20 μmol h−1)3 accounted for one turnoverper 9.2 min over the V••
O site of the ZrO2 surface to CO,formate, and/or COH that were rapidly converted intomethane using the Ni0 nanoparticle surface.
In this context, the transfer of evolved CO (Scheme 2A(e−f,i−j,l−m), and from e to m) from the ZrO2 surface to the Ni0
nanoparticle was evaluated assuming the thermodynamicallymost stable and active Ni(111) surface.35 Because theevaluation results (geometry, energy, and charge distributionof the surface intermediate species) negligibly changed
whether on a simple Ni(111) surface (Scheme 5A(q−z),B(q−z) and Chart S4q−z) or the Ni(111) neighboring to
the ZrO2(111) surface (Scheme 4A(s−x),B(s−x) and Chart
S3s−x), the calculated results on the simple Ni(111) surface
were basically enough. The formation rates of hydrocarbons,e.g., methane, from CO2 using pristine transition-metal oxide
semiconductors are insufficient for practical applications.1−3,7Thus, the understanding of the latter part of the reduction
route, e.g., from CO/COH to hydrocarbons utilizing
combined noble metals and/or first-row transition metal, isChart 5. (A) MOPDOS, (B) Total DOS, and (C) Partial
Charge Density of OCOH−Ni−ZrO2(111) (Scheme
4A(d′))

Scheme 5. (A) Proposed Reaction Mechanism and (B) Energetics of CO2 Photoreduction over the Ni(111) Surface Combinedwith ZrO2

1785

https://doi.org/10.1021/acs.jpcc.2c06048
J. Phys. Chem. C 2023, 127, 1776−1788

The Journal of Physical Chemistry C

pubs.acs.org/JPCC

essential, rather than slower photocatalysis exclusively usingtransition-metal oxides, e.g., the formaldehyde pathway5
compared to their composites with noble metals and/or firstrow transition metal (Figure 1). It should be noted that theformyl species was extremely unstable and easily transformed
into carbonyl and H species over the Ni surface similar to thedecomposition over the ZrO2(111) surface (Scheme 2A(l,m)).
Such a reactant transfer at the surface between a semiconductor metal oxide and supported metal nanoparticle wasreported.36
Using the Ni(111) surface (Chart 2 and Scheme 5A(q),B(q),B(q)), H2 activation to H atoms was essentiallyspontaneous;35 however, the H addition to the O atom of
the adsorbed CO to form COH species (Scheme 5A(r,s))
required a maximum Eact value of 1.72 eV (Scheme 5B(r,s)).
As the reverse reaction discussed above for formyl species
decomposition, the H addition to the C atom of adsorbed CO(Scheme 5A(r)) to form formyl species hardly occurred.
In contrast to the fact that the C−O bond dissociation of
COH species to C and hydroxy species and then CH andhydroxy species (Scheme 5A(s,y,z)) were disadvantageous dueto the Eact value of the step 2.06 eV (Scheme 5B(s,y,z)), thetransformation from COH species into hydroxymethin(Scheme 5A(s,t)) easily proceeded with an Eact value of 0.72
eV (Scheme 5B(s,t)). The Eact value from hydroxymethin tomethin (Scheme 5A(t,u)) was 1.45 eV (Scheme 5B(t,u)). The
alternative route from hydroxymethin to hydroxycarbene(hydrogenation) and then to methin (dehydration; Scheme
5A(t,u′,u)) was essentially energetically the same as the directroute from hydroxymethin to methin (Scheme 5B(t,u′,u)). On
the other hand, the route from hydroxymethin to hydroxycarbene and then to carbene (hydrogenative dehydration;
Scheme 5A(t,u′,v)) was implausible due to the greater Eact
value for the latter step of 1.77 eV (Scheme 5B(t,u′,v)).
Experimentally, CO and methyl species (Scheme 5A(r,w))
were observed on the metallic Ni0 surface using FTIR.3
Therefore, it is reasonable to consider the following

transfer of hydroxycarbonyl on ZrO2 to COH species on Ni(Scheme 4B(d′,s)). The apparent Eact value on Ni0 sites also
decreased from 1.09 eV (eq 3) to 0.67 eV (eq 4) over theNi(111) surface in the absence and presence of neighboring Ni
sites (Scheme 5B(q,t,u,u′) and 4B(d′,t,u)), respectively,
followed by the subsequent progressive hydrogenation fromcarbene to methane (Schemes 4A(v−x),B(v−x) and 5A(v−x)B(v−x)).
In other words, the transfer of hopped COH species(Scheme 2A(k) and Chart 3k) and/or the intermediate COH
species that can be formed from the square HOCO
intermediate to fill the O atom in the V••
O site via the shortcut
associative route (Scheme 2A(e,m)) could explore a further
shortcut to methane from Scheme 2A(k) and/or between
species e and m to Scheme 5A(s), reducing the first hill of 1.72
eV over the Ni(111) surface (Scheme 5B(r,s)) to 0.32 eV at
the interface between the ZrO2(111) surface and Ni(111)
surface (Scheme 4B-d′,s and TOC Graphic).

4. CONCLUSIONSThe presence of V••
O sites over the ZrO2 surface was verified by
the isotopic exchange reaction of gaseous CO2 under theirradiation of UV−visible light. Using 13CO2 gas and ZrO2 (20mg), 0.655 μmol of 12CO2 was exchanged at a rate constant of
0.07 h−1, suggesting that CO2 adsorbed from air to the V••O
sites. The population was 0.21 in the area of 1 nm2 at the ZrO2
surface. Conversely, isotopic exchange reaction of 12CO2 wasalso performed under the irradiation of UV−visible light using
ZrO2 photocatalyst (20 mg) just after the photocatalytic test
under 13CO2, H2, and UV−visible light. 13CO2 (3.0 μmol) was
exchanged at a rate constant of 0.02 h−1. Thus, the population2of V••
O sites was 0.96 in the area of 1 nm at the ZrO2 surface.
The photocatalytic reduction of CO2 did not proceed using
CO2 only as the reactant, but reducing agent, e.g., H2, wasindispensable.
The overall photocatalytic reaction pathway was evaluatedfrom CO2 to methane utilizing the Ni−ZrO2 composite. The
V••
O sites as confirmed by experiments, particularly at the
monoclinic ZrO2(111) surface, were effective for the stable
adsorption of CO2 as an M shape. The adsorption energy was
between −2.5 and −3.9 eV depending on the site and ZrO2
crystal face; however, the total steps of the formation of theV••
O site as well as the CO2 adsorption was in fact endothermic.
The associate route via hydroxycarbonyl (OCOH) species
forming a square intermediate to fill the V••
O site by hydroxyspecies was the most energetically favorable. The regenerationof the V••
O site required the greatest Eact (2.61 eV); however,
the greater adsorption energy of CO2 on the site partiallycompensated for the water desorption energy, different fromthe photocatalytic energetics using first-row transition-metaloxides. The O−C bond dissociation of hydroxycarbonyl evendecreased from 1.18 eV via associate, shortcut route over theZrO2(111) surface to 0.32 eV via the interface transfer ofhydroxycarbonyl on the ZrO2(111) surface to COH species
stabilized more on the Ni(111) surface. The apparent Eact
value on Ni0 sites also decreased from 1.09 to 0.67 eV over theNi(111) surface in the absence and presence of theneighboring Ni nanoparticle, respectively. Thus, the interface
transfer of CO/COH species was the key to the whole reaction
pathway from CO2 to methane over the Ni−ZrO2 photocatalyst.

apparent Eact(1.09 eV)= (E of the TS between species t and u and/or species u
and u)(E of species q)(3)Furthermore, the apparent Eact value needs to be consideredat the interface site between ZrO2 and Ni (Scheme 4B)
apparent Eact(0.67 eV)= (E of the TS between species t and u)(E of species d )Article(4)

The value over the Ni surface (1.09 eV) decreased to 0.67 eVat the interface between ZrO2 and Ni, close to the apparent Eact
value for methane formation from CO2 and H2 (0.58 eV)
measured in the absence of light irradiation.3 In this sense, theevaluated reaction routes (Scheme 5) were not purely
activated by Ni only but also affected by the ZrO2 surface(Scheme 4).
In the whole reaction pathway from CO2 to methane, theV••O site recovery step was rate-limiting (Scheme 2A(n,o),B(n,o),B(n,o)); however, the Eact value of the first O−C bonddissociation step for hydroxycarbonyl species reduced from
1.18 eV via associate, shortcut route over the ZrO2(111)
surface (Scheme 2B(d,e,m)) to 0.32 eV via the interface
1786

https://doi.org/10.1021/acs.jpcc.2c06048
J. Phys. Chem. C 2023, 127, 1776−1788

The Journal of Physical Chemistry C■

pubs.acs.org/JPCC

Further Reduction to Methane over Ni0 by Heat Converted from
Light. Angew. Chem., Int. Ed. 2021, 60, 9045−9054.
(4) Zhang, H.; Itoi, T.; Konishi, T.; Izumi, Y. Dual Photocatalytic
Roles of Light: Charge Separation at the Band Gap and Heat via
Localized Surface Plasmon Resonance To Convert CO2 into CO over
Silver−Zirconium Oxide. J. Am. Chem. Soc. 2019, 141, 6292−6301.
(5) Ji, Y.; Luo, Y. New Mechanism for Photocatalytic Reduction of
CO2 on the Anatase TiO2 (1 0 1) Surface: The Essential Role ofOxygen Vacancy. J. Am. Chem. Soc. 2016, 138, 15896−15902.
(6) Zhang, H.; Izumi, Y. Why Is Water More Reactive Than
Hydrogen in Photocatalytic CO2 Conversion at Higher Pressure?
Elucidation by Means of X-Ray Absorption Fine Structure and Gas
Chromatography-Mass Spectrometry. Front. Chem. 2018, 6, No. 408.
(7) Li, K.; Peng, B.; Peng, T. Recent Advances in Heterogeneous
Photocatalytic CO2 Conversion to Solar Fuels. ACS Catal. 2016, 6,7485−7527.
(8) Yang, C. C.; Yu, Y.-H.; van der Linden, B.; Wu, J. C. S.; Mul, G.
Artificial Photosynthesis over Crystalline TiO2-Based Catalysts: Fact
or Fiction? J. Am. Chem. Soc. 2010, 132, 8398−8406.
(9) Teramura, K.; Tanaka, T. Necessary and Sufficient Conditions
for the Successful Three-Phase Photocatalytic Reduction of CO2 by
H2O over Heterogeneous Photocatalysts. Phys. Chem. Chem. Phys.
2018, 20, 8423−8431.
(10) Wang, S.; Hou, Y.; Wang, X. Development of a Stable
MnCo2O4 Cocatalyst for Photocatalytic CO2 Reduction with Visible
Light. ACS Appl. Mater. Interfaces 2015, 7, 4327−4335.
(11) Kar, P.; Zeng, S.; Zhang, Y.; Vahidzadeh, E.; Manuel, A.;
Kisslinger, R.; Alam, K. M.; Thakur, U. K.; Mahdi, N.; Kumar, P.;
Shankar, K. High Rate CO2 Photoreduction Using Flame Annealed
TiO2 Nanotubes. Appl. Catal., B 2019, 243, 522−536.
(12) Ulagappan, N.; Frei, H. Machanistic Study of CO2 Photoreduction in Ti Silicalite Molecular Sieve by FT-IR Spectroscopy. J.
Phys. Chem. A 2000, 104, 7834−7839.
(13) Hameed, Y.; Gabidullin, B.; Richeson, D. Photocatalytic CO2
Reduction with Manganese Complexes Bearing a κ2-PN Ligand:
Breaking the α-Diimine Hold on Group 7 Catalysts and Switching
Selectivity. Inorg. Chem. 2018, 57, 13092−13096.
(14) Boston, D. J.; Xu, C.; Armstrong, D. W.; MacDonnell, F. M.
Photochemical Reduction of Carbon Dioxide to Methanol andFormate in a Homogeneous System with Pyridinium Catalysts. J. Am.
Chem. Soc. 2013, 135, 16252−16255.
(15) Kresse, G.; Hafner, J. Ab Initio Molecular Dynamics for Liquid
Metals. Phys. Rev. B 1993, 47, 558−561.
(16) Kresse, G.; Hafner, J. Ab Initio Molecular-Dynamics Simulationof the Liquid−Metal−Amorphous−Semiconductor Transition inGermanium. Phys. Rev. B 1994, 49, 14251−14269.
(17) Kresse, G.; Furthmüller, J. Efficiency of Ab-Initio Total Energt
Calculations for Metals and Semiconductors Using a Plane-Wave
Basis Set. Comput. Mater. Sci. 1996, 6, 15−50.
(18) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab
Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys.
Rev. B 1996, 54, 11169−11186.
(19) Grimme, S.; Ehrlich, S.; Goerigk, L. Effect of the Damping
Function in Dispersion Corrected Density Functional Theory. J.
Comput. Chem. 2011, 32, 1456−1465.
(20) Hammer, B.; Hansen, L. B.; Nørskov, J. K. Improved
Adsorption Energies within Density-Functional Theory Using Revised
Perdew−Burke−Ernzerhof Functionals. Phys. Rev. B 1999, 59, 7413−
7421.
(21) Henkelman, G.; Jonsson, H. Improved Tangent Estimate in the
Nudged Elastic Band Method for Finding Minimum Energy Paths
and Saddle Points. J. Chem. Phys. 2000, 113, 9978−9985.
(22) Lüftner, D.; Weiβ, S.; Yang, X.; Hardax, P.; Feyer, V.; Gottwald,
A.; Koller, G.; Soubatch, S.; Puschnig, P.; Ramsey, M. G.; Tautz, F. S.
Understanding the Photoemission Distribution of Strongly Interacting
Two-Dimensional Overlayers. Phys. Rev. B. 2017, 96, No. 125402.
(23) Momma, K.; Izumi, F. VESTA 3 for Three-Dimensional
Visualization of Crystal, Volumetric and Morphology data. J. Appl.
Cryst. 2011, 44, 1272−1276.

ASSOCIATED CONTENT

sı Supporting Information*

The Supporting Information is available free of charge athttps://pubs.acs.org/doi/10.1021/acs.jpcc.2c06048.

■

Density of states for ZrO2 comprising V••
O site and/or Ni
nanoparticle in the absence/presence of CO2 and threedimensional illustration of the Ni cluster and O vacancy
site as well as the surface species over the Ni−
ZrO2(111) surface or over the Ni(111) surface during
CO2 photoreduction, available free of charge (docx)(PDF)

AUTHOR INFORMATION

Corresponding Authors

Kaori Niki − Department of Chemistry, Graduate School of
Science, Chiba University, Chiba 263-8522, Japan;
Phone: +81-43-290-3699; Email: niki@chiba-u.jp
Yasuo Izumi − Department of Chemistry, Graduate School of
Science, Chiba University, Chiba 263-8522, Japan;
orcid.org/0000-0001-8366-1864; Phone: +81-43-2903696; Email: yizumi@faculty.chiba-u.jp; Fax: +81-43-2902783

Authors

Keisuke Hara − Department of Chemistry, Graduate School of
Science, Chiba University, Chiba 263-8522, Japan
Misa Nozaki − Department of Chemistry, Graduate School of
Science, Chiba University, Chiba 263-8522, Japan
Rumiko Hirayama − Department of Chemistry, Graduate
School of Science, Chiba University, Chiba 263-8522, Japan
Rento Ishii − Department of Chemistry, Graduate School of
Science, Chiba University, Chiba 263-8522, Japan

Complete contact information is available at:https://pubs.acs.org/10.1021/acs.jpcc.2c06048
Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS
The authors are grateful for the financial support from theGrant-in-Aid for Scientific Research B (20H02834, Y.I.) and C(17K05961, Y.I.; 20K05643, K.N.) from the Japan Society for
the Promotion of Science and from the Establishment of
University Fellowships toward the Creation of Science andTechnology Innovation from Japan Science and Technology
Agency (JPMJFS2107, K.H.&M.N.). Part of the calculations
was executed at the Supercomputer Center, the Institute for
Solid State Physics, the University of Tokyo, Japan.

■

ArticleREFERENCES(1) Izumi, Y. Recent advances in photocatalytic conversion of carbon
dioxide into fuels with water and/or hydrogen using solar energy andbeyond. Coord. Chem. Rev. 2013, 257, 171−186.
(2) Izumi, Y.Recent Advances (2012−2015) in the Photocatalytic
Conversion of Carbon Dioxide to Fuels Using Solar Energy:
Feasibility for a New Energy, in ACS Books ″Advances in CO2
Capture, Sequestration, and Conversion″. In Advances in CO2 Capture,
Sequestration, and Conversion; Jin, F.; He, L.-N.; Hu, Y. H., Eds.; ACS
Symposium Series; ACS Publications, 2015; Vol. 1194, pp 1−46.
DOI: 10.1021/bk-2015-1194.ch001.
(3) Zhang, H.; Itoi, T.; Konishi, T.; Izumi, Y. Efficient and Selective
Interplay Revealed: CO2 Reduction to CO over ZrO2 by Light with1787

https://doi.org/10.1021/acs.jpcc.2c06048
J. Phys. Chem. C 2023, 127, 1776−1788

The Journal of Physical Chemistry C

pubs.acs.org/JPCC(24) Stukowski, A. Visualization and Analysis of Atomistic
Ssimulation Data with OVITO−the Open Visualization Tool. Modell.
Simul. Mater. Sci. Eng. 2010, 18, No. 015012.
(25) Puigdollers, A. R.; Illas, F.; Pacchioni, G. Structure andProperties of Zirconia Nanoparticles from Density Functional Theory
Calculations. J. Phys. Chem. C 2016, 120, 4392−4402.
(26) Li, Z.; Zhang, L.; Huang, W.; Xu, C.; Zhang, Y. Photothermal
Catalysis for Selective CO2 Reduction on the Modified Anatase TiO2
(1 0 1) Surface. ACS Appl. Energy Mater. 2021, 4, 7702−7709.
(27) Souza, E. F.; Appel, L. G. Oxygen Vacancy Formation and
Their Role in the CO2 Activation on Ca Doped ZrO2 Surface: An AbInitio Study. Appl. Surf. Sci. 2021, 553, No. 149589.
(28) Liu, T.; Li, H.; Gao, J.; Ding, S.; Liu, X.; Jia, H.; Xue, J. Effect ofOxygen Vacancies on the Photocatalytic CO2 Reduction Performanceof Bi2WO6: DFT and Experimental Studies. Appl. Surf. Sci. 2022, 579,
No. 152135.
(29) Nakamura, S.; Hatakeyama, M.; Wang, Y.; Ogata, K.; Fujii, K.A
Basic Quantum Chemical Review on the Activation of CO2. In
Advances in CO2 Capture, Sequestration, and Conversion; Jin, F.; He, L.N.; Hu, Y. H., Eds.; ACS Books: Washington, D. C.1194; Chapter 5,
pp 123−134.
(30) Hu, J.; Kim, E. M.; Janik, M. J.; Alexopoulos, K. Hydrogen
Activation and Spillover on Anatase TiO2-Supported Ag Single-AtomCatalysts. J. Phys. Chem. C 2022, 126, 7482−7491.
(31) Medeiros, P. V. C.; Tsirkin, S. S.; Stafström, S.; Björk, J.
Unfolding Spinor Wave Functions and Expectation Values of General
Operators: Introducing the Unfolding-Density Operator. Phys. Rev. B
2015, 91, No. 041116.
(32) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.;
Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci,
B.; Petersson, G. A.; Nakatsuji, H.et al.Gaussian 09, revision A. 2;
Gaussian, Inc.: Wallingford CT, USA, 2016.
(33) Clavero, C. Plasmon-Induced Hot-Electron Generation at
Nanoparticle/Metal-Oxide Interfaces for Photovoltaic and Photocatalytic Devices. Nat. Photonics 2014, 8, 95−103.
(34) Li, X.; Wang, S.; Li, L.; Sun, Y.; Xie, Y. Progress and Perspectivefor In Situ Studies of CO2 Reduction. J. Am. Chem. Soc. 2020, 142,
9567−9581.
(35) Kresse, G. Dissociation and Sticking of H2 on the Ni (1 1 1), (10 0), and (1 1 0) Substrate. Phys. Rev. B 2000, 62, 8295−8305.
(36) Yang, J.-J.; Zhang, Y.; Xie, X.-Y.; Fang, W.-H.; Cui, G.
Photocatalytic Reduction of Carbon Dioxide to Methane at the PdSupported TiO2 Interface: Mechanistic Insights from Theoretical
Studies. ACS Catal. 2022, 12, 8558−8571.

ArticleRecommended by ACS
Electron Paramagnetic Resonance Quantifies Hot-ElectronTransfer from Plasmonic Ag@SiO2 to Cr6+/Cr5+/Cr3+
Constantinos Moularas, Yiannis Deligiannakis, et al.
JANUARY 19, 2023
THE JOURNAL OF PHYSICAL CHEMISTRY C

READ

Facet-Dependent Ba Dissolution of Tetragonal BaTiO3 Single
Crystal Surfaces
Hee-Joon Chun, Seong-Chan Park, et al.
JANUARY 19, 2023
THE JOURNAL OF PHYSICAL CHEMISTRY C

READControlling Energy Transfer in Plasma-Driven Ammonia
Synthesis by Adding Helium Gas
Rusen Zhou, Patrick J. Cullen, et al.
JANUARY 25, 2023
ACS SUSTAINABLE CHEMISTRY & ENGINEERING

READ

Oblique-Incidence-Excited Localized Hot Spots in Plasmonic
Particle-on-Film Nanocavities
Zulhumar Turup, Min Gao, et al.
DECEMBER 29, 2022
THE JOURNAL OF PHYSICAL CHEMISTRY C

READ

Get More Suggestions >1788

https://doi.org/10.1021/acs.jpcc.2c06048
J. Phys. Chem. C 2023, 127, 1776−1788