Catalysis Today xxx (xxxx) xxx–xxx

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Dual origins of photocatalysis: Light-induced band-gap excitation of
zirconium oxide and ambient heat activation of gold to enable 13CO2
photoreduction/conversion
Hongwei Zhanga, Takaomi Itoib, Kaori Nikia, Takehisa Konishia, Yasuo Izumia,*ab

Department of Chemistry, Graduate School of Science, Chiba University, Yayoi 1-33, Inage-ku, Chiba, 263-8522, Japan
Department of Mechanical Engineering, Graduate School of Engineering, Chiba University, Yayoi 1-33, Inage-ku, Chiba, 263-8522, JapanA R T I C LE I N FO

A B S T R A C T

Keywords:
CO2 photoconversion
Zirconium oxide
Gold
Localized surface plasmon resonanceAmbient heat
Extended X-ray absorption ﬁne structure

Photoconversion of CO2 into fuels completes the carbon neutral cycle in a sustainable society. To exclude thecontribution of adventitious carbon, monitoring the time course of 13CO2 conversion into 13C-fuel is essential,
but has been rarely reported. In the present work, a composite of Au nanoparticles with ZrO2 was found to beeﬀective in converting 13CO2 into 13CO at a rate of 0.17 μmol h−1 gcat−1 in the presence of H2 and UV–vis light.
The detected 12CO as a minor byproduct (11.9 %) was identiﬁed as due to adsorbed 12CO2 from the air. The 12C
ratio in the total amount of CO2 was evaluated based on a 13CO2 photoexchange reaction (8.7 %). The discrepancy between these values suggested a slower exchange reaction step between the chemisorption site forCO2 reduction and the physisorption site for CO2 compared to the reduction step to CO. Furthermore, based on
in-proﬁle kinetic studies using sharp-cut ﬁlters and control reactions in the dark, the contribution ratio for CO2
conversion was determined to be via charge separation at the band-gap of ZrO2 (λ < 320 nm): 69 % and via
1ambient heat ( 2 kT ): 31 %. Localized surface plasmon resonance (LSPR) absorption of Au and infrared absorption
in the range of λ > 320 nm did not promote catalysis. The LSPR absorption was further investigated by Au L3edge extended X-ray absorption ﬁne structure analysis. Ambient heat on the Au nanoparticles should havepromoted H2 activation enough, supplying protons to the CO2 reduction sites over ZrO2; however, a temperature
increase of 26 K on the Au surface was marginal for further H2 activation. CO2 photoconversion with added
moisture was also attempted; the CO formation rate using ZrO2 under these conditions was 0.15 μmol h−1
gcat−1. However, 47 % was characterized as 12CO originating from chemisorbed 12CO2, and H2 was also formed
at a comparable rate of 0.14 μmol h−1 gcat−1 from a competing reaction. The addition of Au to ZrO2 was foundto suppress CO formation and promote H2 formation, and Mg2+ addition to Au–ZrO2 eﬀectively suppressed H2formation directing to the CO formation.

1. Introduction
The conversion of CO2 into fuels utilizing sustainable energy sources
completes the carbon neutral cycle in an ideal, sustainable society.
Solar fuels, in particular, have been identiﬁed as promising alternatives
to fossil fuels and have thus been widely investigated [1]. However,solar fuel generation from CO2 is a highly endothermic, unfavorable
process [2], and developing a reaction pathway from CO2 to fuels

without the contribution of adventitious carbon is essential for making
this approach viable [3]. Zirconium oxide (ZrO2) has been shown tosuccessfully promote the photoconversion of 13CO2 into 13CO, enabled
by both ultraviolet (UV) light-induced band-gap excitation and heatinduced promotion eﬀects converted from localized surface plasmon
resonance (LSPR) via Ag doping [3].
13
C-labeled studies were carried out to conﬁrm 13CO2 conversion
via gas chromatography-mass spectrometry (GC–MS) analyses.

Abbreviations: EXAFS, extended X-ray absorption ﬁne structure; fcc, face-centered cubic; FTIR, Fourier transform infrared; GC, gas chromatography; HAADF, highangle annular dark ﬁeld; HR-TEM, high-resolution transmission electron microscopy; LSPR, localized surface plasmon resonance; MS, mass spectrometry; NMR,
nuclear magnetic resonance; PET, polyethene terephthalate; PTFE, polytetraﬂuoroethene; rpm, rotations per minute; SHE, standard hydrogen electrode; STEM,
scanning transmission electron microscopy; TEM, transmission electron microscopy; XRD, X-ray diﬀraction⁎
Corresponding author.
E-mail addresses: zhw9590@yahoo.co.jp (H. Zhang), itoi@faculty.chiba-u.jp (T. Itoi), niki@chiba-u.jp (K. Niki), konishi@faculty.chiba-u.jp (T. Konishi),yizumi@faculty.chiba-u.jp (Y. Izumi).
https://doi.org/10.1016/j.cattod.2020.02.040
Received 27 July 2019; Received in revised form 5 February 2020; Accepted 28 February 20200920-5861/ © 2020 Elsevier B.V. All rights reserved.

Please cite this article as: Hongwei Zhang, et al., Catalysis Today, https://doi.org/10.1016/j.cattod.2020.02.040

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analysis, and the photocatalytic role of Au was clariﬁed using Au L3edge extended X-ray absorption ﬁne structure (EXAFS) spectroscopy.
2. Experimental section
2.1. Catalyst synthesis/preparation
2.1.1. Au–ZrO2
ZrO2 (JRC-ZRO-3, Catalysis Society of Japan; major monoclinic andminor tetragonal phase, speciﬁc surface area = 94.4 m2 g−1) powder
was immersed in 50 mL of puriﬁed water (< 0.055 μS cm−1), andhydrogen tetrachloroaurate trihydrate (> 99 %, Wako Pure Chemical,
Japan; 0.064 or 0.11 g, 0.16 or 0.28 mmol) was added into the suspension. The mixture was agitated by ultrasound (430 W, 38 kHz) for
10 min and magnetically stirred at a rate of 900 rotations per minute(rpm) for 2 h. Sodium borohydride (> 95 %, Wako Pure Chemical; 1.9
or 3.4 mmol) dissolved in puriﬁed water (20 mL) was added andmagnetically stirred at a rate of 900 rpm. Then, the suspension was
ﬁltered using a polytetraﬂuoroethene (PTFE)-based membrane ﬁlter(Omnipore JVWP04700, Millipore, Burlington, MA, USA; pore size 0.1μm) and washed by puriﬁed water (50 mL) ﬁve times. The resultant
powder was dried at 373 K overnight. The obtained purple powder isdenoted as Au–ZrO2. The loading of Au was 3.0 and 5.0 wt %.
2.1.2. Mg–Au–ZrO2
Au (5.0 wt %)–ZrO2 (0.20 g) and magnesium nitrate hexahydrate(2.6 mg, > 99.5 %, Wako Pure Chemical) were mixed in 30 mL of
puriﬁed water and agitated by ultrasound. The water was distilled andthe resultant powder was dried at 373 K overnight. The obtained purple
powder is denoted as Mg–Au–ZrO2. The molar ratio of Mg:Au was 1:5.
2.2. Photocatalytic conversion of 13CO2
The photocatalyst (0.100 g) was placed in a quartz photoreactor andevacuated at 295 K for 2 h while connected to a Pyrex glass circulation
system (206.1 mL) and both rotary and diﬀusion pumps (10−6 Pa)[2,15,20]. For 13CO2 photoexchange tests, 0.67 kPa of 13CO2 (13C 99.0%, 17O 0.1 %, 18O 0.7 %, chemical purity > 99.9 %, Cambridge Isotope
Laboratories, Inc., Tewksbury, MA, USA) were introduced (test i) [3].
For 13CO2 photoreduction tests, 2.3 kPa of 13CO2 and 21.7 kPa of H2(> 99.99 %) were introduced (test ii) [3,20]. Separately, 2.3 kPa of
13
CO2 and 2.7 kPa of H2O were introduced for 13CO2 photoconversion
tests using water as the reductant (test iii) [3]. For tests i–iii, the reactor
containing the catalyst was irradiated with UV–vis light using a 500 W
Xe arc lamp (Model OPM2 − 502, Ushio, Japan). The distance between
the UV–vis light source and the photocatalyst was 20 mm. The lightintensity was 90.2 mW cm−2 at the center of the photocatalyst. The
intensity wavelength distribution of the Xe arc lamp was measured
using a spectroradiometer (Model USR45DA, Ushio, Japan) at a distance of 20 mm from the UV–vis light source [3]. In-proﬁle kinetic data
were collected as a function of the light’s excitation wavelength byinserting a sharp-cut ﬁlter (2.5 mm thick) at the lighthouse(OPM2−502) exit. UV32 and L42 (Hoya, Japan) models were used topass light of wavelengths λ > 320 nm and λ > 420 nm, respectively.
Control tests with exposure to 13CO2, H2, and no light were performedby completely wrapping the reactor with Al foil. Control tests were also
performed with exposure to only H2 gas and UV–vis light.
A packed column of 13X-S molecular sieves (3 m length, 3 mm internal diameter; GL Sciences, Inc., Japan) for 13CO2 photoreduction/
conversion tests ii and iii and a packed column of polyethene glycol6000/Flusin P support column (3 m length, 3 mm internal diameter; GL
Sciences, Inc.) for 13CO2 exchange tests i were employed for online
GC–MS analyses (Model JMS-Q1050GC, JEOL, Tokyo, Japan). Helium(purity > 99.9999 %) was used as the carrier gas at 0.40 MPa. 4 mL ofsampling loops composed of a Pyrex glass system were kept under vacuum using rotary and diﬀusion pumps (10−6 Pa) connected to theFig. 1. (A) Mass spectrum @11’22” and (B) mass chromatogram @m/z = 29
sampled during a test under 13CO2 (2.3 kPa), H2 (21.7 kPa), and UV–vis lightusing the Au (5.0 wt %)–ZrO2 photocatalyst.

However, very limited data are available in the literature regardingthese characterizations: (i) a mass spectrum (Fig. 1A) at a certain retention time in the GC and at only one timepoint during/after thephotocatalytic test [4–7], (ii) a mass chromatogram (Fig. 1B) at only
one timepoint during/after the photocatalytic test [8,9], and (iii) both amass spectrum and a mass chromatogram at only one timepoint during/after the photocatalytic test [10–12]. 1H and/or 13C nuclear magnetic
resonance (NMR) spectroscopic analyses at one timepoint during/after
the photocatalytic test have also been reported [13]. Although such
limited data exist, time course monitoring of reactants and products via
isotopic labeling, mass chromatography, and/or NMR spectroscopy willbe essential for understanding the 13CO2 conversion reaction mechanism [3].
The LSPR eﬀects of Au and Ag have been intensively investigated[14]. The resonance peak position is red-shifted for Au in the wavelength of visible light in comparison to that for Ag, promising for use ofsolar energy [15,16]. In the present work, Au particles were doped into
ZrO2 as a potential catalyst for CO2 reduction [15–17]. As reductant, H2
and water were tested [2,18]. Using water, CO2 reduction and water
reduction to H2 are competitive reactions, and the Mg2+ addition to
Ag–ZrO2 photocatalyst was eﬀective to direct to the CO2 reduction by
the anchoring of CO2 with alkaline [3,19]. The Mg2+ addition was also
tried to Au–ZrO2. The reaction progress was monitored via GC–MS
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edge background was approximated with a modiﬁed Victoreen function:

GC–MS via 1.5 m deactivated fused silica tubes (No. 160-2845-10,
Agilent, Santa Clara, CA, USA; internal diameter 250 μm), which weremaintained at 393 K during analysis to avoid gas adsorption.

C2C+ 1 + C0E2
E2.3. Characterizations

where E is the photon energy. The background for post-edge oscillation,
μx, was approximated with a smoothing spline function and was calculated for a particular number of data points:

The surface species were monitored with a single-beam Fouriertransform infrared (FTIR) instrument (JASCO, Tokyo, Japan; Model FT/
IR-4200) equipped with a mercury–cadmium–tellurium-M detector at aconstant temperature of 77.4 K. A 20 mm-Φ self-supporting disk of Au(5.0 wt %)–ZrO2 disk (65 mg) was placed in a quartz photoreaction cell
equipped with NaCl windows on both sides. The photoreaction cell wasconnected to the Pyrex glass circulation system as well as the GC–MS toenable simultaneous monitoring of surface species via FTIR and isotope
distribution in the gas with the GC–MS. The photocatalyst disk was
evacuated (10−6 Pa) at 295 K for 2 h prior to FTIR and GC–MS measurements.
In situ FTIR measurements were performed at 295 K in a range from4000 to 650 cm−1. The sample disk was irradiated with UV–vis lightfrom a 500 W Xe arc lamp using a quartz ﬁber light guide (Optel,
Tokyo, Japan; Model 1.2S15-1000F-1Q7-SP-RX(400)). The distance
between the ﬁber light exit and sample disk was 46 mm. The light intensity at the center of sample was 90 mW cm−2. The spectrometer’s
energy resolution was 1 cm−1. Data accumulation was 512 scans (approximately 2 s per scan).
X-ray diﬀraction (XRD) patterns were observed using a D8
ADVANCE diﬀractometer (Bruker, Billerica, MA, USA) at the Center for
Analytical Instrumentation, Chiba University, at a Bragg angle (θB) of
2θB = 10–60° with a scan step of 0.02° and a scan rate of 1 s per step.
The measurements were performed at 40 kV and 40 mA using Cu Kαemission (wavelength λ =0.15419 nm) [21] and a nickel ﬁlter. Crystallite sizes (t) were estimated using the Scherrer equation [16,22].

t=

0.9λ
Peak width× cos θB(2)

Data Points∑
i=1(μx i − backgroundi )2≤ smoothing factor
exp( −0.075ki2 )(3)where k is the angular photoelectron wavenumber.
Multiple-shell curve-ﬁt analyses were performed on the Fourierﬁltered k3-weighted EXAFS data in k- and R-space (R: interatomic distance) based on the plane-wave approximation for amplitude Ai(k),coordination number Ni, backscattering amplitude fi, Debye–Waller
factor σi, and mean free photoelectron path λ for shell i using an XDAP
code [24],Ai (k ) =NiR|f (k )| exp ⎡−2 ⎛σi2 k 2 + i ⎞ ⎤, i= Au
λ ⎠⎦kR i2 i⎣ ⎝(4)

in which the empirical amplitude extracted from the EXAFS data for
the Au2O3 powder and Au metal foil (6 μm thick) was used. The R
values for the Au–O and Au − Au interatomic pair were set to 0.201 3
nm with an N value of 4 [25] and 0.288 4 nm with an N value of 12,
respectively [26]. We assumed that the many-body reduction factor,
S02, was identical for both the sample and reference. The Debye temperature (θD, 165 K) was used for bulk Au [27] while a “surface Debye
temperature” [θD(Surf,∞)] of 83 K was used to account for the motion ofvertical freedom of translation of face-centered cubic (fcc) Au(1 1 1) orthe (1 1 0) surface [28].
EXAFS analysis was also performed by dividing the amplitude bythat of the reference EXAFS data (log-ratio method) using the Athena
Demeter package (version 0.9.26) based on the following:(1)

UV–visible spectra were recorded on a double-beam model V-650
spectrophotometer using D2 and halogen lamps below and above 340
nm equipped with a photomultiplier tube and an integrated ISV-469
sphere (JASCO, Tokyo, Japan) for diﬀuse-reﬂectance detection within
the wavelength range of 200–800 nm. Data were transformed using the
Kubelka–Munk function. A PTFE plate was used as the references[3,15,20].
Au L3-edge EXAFS spectra were measured at 290 K in transmission
mode at the Photon Factory Advanced Ring, High Energy Accelerator
Research Organization (KEK, Tsukuba, Japan) on the NW10A beamline
and also at the Photon Factory, KEK on the 9C beamline [15,16]. On the
NW10A beamline, a Si(3 1 1) double-crystal monochromator and a Ptcoated focusing bent cylindrical mirror were inserted into the X-ray
beam path. On the 9C beamline, a Si(1 1 1) double-crystal monochromator and Rh-coated focusing bent cylindrical mirror were inserted
into the X-ray beam path. On both beamlines, a Piezo transducer wasused to detune the X-ray to two-thirds of the maximum intensity tosuppress higher harmonics. The Au L3-edge absorption energy was calibrated at 11 921.2 eV [23] using the X-ray spectrum of a Au metal foil
(6 μm thick).
A disk (Φ =10 mm) of the Au (5.0 wt %)−ZrO2 photocatalyst (80mg) was set in a Pyrex glass reactor equipped with a Kapton ﬁlm(Dupont, Wilmington, DE, USA; 50 μm thick) for X-ray transmission anda polyethene terephthalate (PET) ﬁlm (Teijin, Japan, G2; 50 μm thick)for both UV–vis light and X-ray transmission. The reactor was ﬁlledwith 2.3 kPa of CO2 and 21.7 kPa of H2. The sample was irradiated with
UV–vis light from a Xe arc lamp through the PET ﬁlm at the beamline.
X-rays transmitted the disk perpendicularly while incident angle ofUV–vis light was 45°. The distance between the light exit of quartz ﬁberlight guide and the sample was 5 cm. The obtained Au L3-edge EXAFS
data were analyzed using the XDAP software package [24]. The pre-ln

Ai (k )Ni2
) k 2 + f (Ri , Ri,Ref )= ln− 2(σi2 − σi,Ref
Ai,Ref (k )Ni,Ref
Ni
2
) k2
≈ ln
− 2(σi2 − σi,Ref
Ni,Ref(5)

The EXAFS amplitude of the Au–ZrO2 photocatalyst before lightirradiation was used as the reference Ai,Ref(k) data. The k3-weighted
EXAFS function was Fourier transformed and inversely Fourier transformed in a ﬁltered range of 0.20–0.32 nm (phase shift uncorrected) for
Au–Au bonds (i = Au in Eq. (5)). Then, the function was ﬁt to data in a
k-range of 0–130 nm−1 based on Eq. (5).
The sample temperature changes irradiated under UV–vis light werereconﬁrmed by IR thermography using a model T650sc (FLIR,
Wilsonville, OR, USA) equipped with a lens of instantaneous ﬁeld ofview =25 μm. The position and the incident angle of UV–vis light werecommon to the conditions for Au L3-edge EXAFS measurements, but the
sample was in ambient air in the IR thermography monitoring.
The transmission electron microscopy (TEM) investigations were
performed using a JEM-2100 F (JEOL) equipped with a ﬁeld emission
gun at an acceleration voltage of 200 kV at the Center for Analytical
Instrumentation [3,22]. The samples were mounted on Cu mesh (250mesh per inch) coated with carbon and a copolymer ﬁlm of poly(vinyl
alcohol) and formaldehyde (Formvar, Monsanto, St. Louis, MO, USA).
High-angle annular dark-ﬁeld (HAADF) scanning TEM (STEM) andhigh-resolution (HR) TEM images were also observed using the JEM2100 F model. Chemical compositions and elemental distributions wereanalyzed using energy dispersive spectra equipped with a Si(Li) detector in the TEM.
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1-ii, iii) than CeO bond dissociation reaction of CO2 to form CO(Scheme 1-iii, vi), and thus the 12CO ratio of the total CO formed did
not reach the equilibrium isotope ratio of 13CO2: 12CO2 (7.3 %) [3].
Using Au (3.0 wt %)–ZrO2 as the photocatalyst, the formation ratesof 13CO and 12CO were constant for 30 and 48 h, respectively (Fig. 3Ab, Right panel). Between 30 and 50 h of reaction time, the 13CO formation rate decreased by 5.5 %. The initial constant formation rates
were 0.12 and 0.017 μmol, and the 12CO ratio based on total CO was
12.2 % (Table 1A-b). The 12CO ratio, based on total CO formed using Au(3.0 wt %)–ZrO2 as the photocatalyst dramatically decreased from thevalue under ZrO2 photocatalytic conditions (49.2 %), but did notcompletely reach the 12C ratio in the exchange equilibrium of CO2 (7.3
% for ZrO2 [3] and 8.7 % for Au (5.0 wt %)–ZrO2, Fig. 2). The increaseof CO formation rate by a factor of 3.9 times compared to that using
ZrO2 (Table 1A-a, b) suggested the activation of chemisorbed CO2
species over ZrO2 in proximity of Au (Scheme 1-iii), facilitating both thetransformation from physisorbed to chemisorbed CO2 species (ii, iii)
and further decomposition to CO and H2O (iii, vi).
The formation rates of 13CO and 12CO were 0.17 and 0.022 μmol
h−1 gcat−1 using Au (5.0 wt %)–ZrO2 (Table 1A-c), greater by factors of
1.38 and 1.34, respectively, compared to the corresponding values
using Au (3.0 wt %)–ZrO2 (Fig. 3A-b, c). The 12CO ratio in total CO
formed was 11.9 %, eﬀectively equivalent to the Au (3.0 wt %)–ZrO2
conditions (12.2 %, Table 1A-b, c), but still greater than the observed
12
C isotopic ratio in the exchange reaction of 13CO2 (8.7 %, Fig. 2). The
formation rate was constant for 13CO and 12CO for 29 and 48 h of reaction time, respectively, whereas the formation rate of 13CO decreased
by 5.0 % after 47 h (Fig. 3A-c, Right panel). The Au (5.0 wt %)–ZrO2
photocatalyst was most eﬃcient at reducing CO2 into CO (0.19 μmol
h−1 gcat−1) in the present work (Table 1A-c). The relatively stablechemisorption site of the photocatalyst (Scheme 1-iii) was an eﬀective
photoreduction site for CO2, and the slower exchange rate between
chemisorbed CO2 and physisorbed CO2 (Scheme 1-ii, iii) both on ZrO2
led to minor diﬀerences in the 12C isotopic population in formed CO(11.9 %, Table 1A-c) and in equilibrated gaseous CO2 with the photocatalyst (8.7 %, Fig. 2).
In relation to the reaction mechanism, CO formation rate was as low
as 0.004 0 μmol h−1 gcat−1 in the presence of H2 (21.7 kPa) and UV–vis
irradiation using Au (5.0 wt %)–ZrO2 photocatalyst (Table 1A-c””). The
rate was only 2.1 % of that in the presence of 13CO2 and H2 (Table 1Ac), and the 12CO formation rate was 18 % of the corresponding value inthe presence of 13CO2 and H2. Thus, the amount of chemisorbed 12CO2derived species (Scheme 1-iii) decreased in the equilibrium at surface
because physisorbed 13CO2-derived species (Scheme 1-ii) was absent in
the photocatalytic test under H2 only.
To compare to the photocatalytic reactions under 13CO2 and fullUV–vis light (Fig. 3A-a–c), studies under a 13CO2/H2 mixture and a
UV–vis light ﬁlter at λ > 320 nm and at λ > 420 nm were also carriedout using the Au (5.0 wt %)–ZrO2 photocatalyst (Fig. 3A-c’, c”). The
total CO formation rates decreased by 70 % when using a ﬁlter at
λ > 320 nm (Table 1A-c, c’) and by 78 % when using a ﬁlter at λ > 420
nm (Table 1A-c”). The rate diﬀerence based on diﬀerence in the sharpcut wavelength was marginal. The formation rates of 13CO and total CO
in a control test in the dark (0.050 and 0.055 μmol h−1 gcat−1;
Table 1A-c’’’ and Fig. 3A-c’’’) were essentially identical to the corresponding values in the ﬁltered test at λ > 320 nm (0.052 and 0.055
μmol h−1 gcat−1; Table 1A-c’). Thus, visible and IR light were not effective energy sources for the Au–ZrO2 photocatalysts under CO2 andH2. The contribution ratio to CO2 conversion was charge separation at1
the band-gap of ZrO2 (λ < 320 nm): 69 % and ambient heat ( 2 kT ): 31%.
Unexpectedly, the LSPR absorption of Au (see the following section
on UV–vis absorption) and IR absorption for wavelengths λ > 320 nm
did not activate H2 over Au. The decrease of 12CO ratio in total produced CO was evident in the presence of Au (Table 1A-a–c) due to theactivation of chemisorbed CO2 species in the proximity of Au

Fig. 2. Time course exchange reaction of 13CO2 (0.67 kPa) irradiated by UV–vislight using the Au (5.0 wt %)–ZrO2 catalyst (0.100 g).

3. Results and discussion
3.1. The 13CO2 exchange reaction
We performed a 13CO2 (0.67 kPa) exchange reaction with the Au(5.0 wt %)–ZrO2 photocatalyst under UV–vis light irradiation (Fig. 2).
The exchange reaction proceeded with 12CO2, which was adsorbed fromthe atmosphere and remained after pretreatment under vacuum. The
exchange reaction reached equilibrium between the adsorbed 12CO2
and gas phase 13CO2 after 2 h. We assumed that the exchange reaction
followed ﬁrst-order kinetics and that the rate constants, kr and kr',corresponded to the exchange reactions between gas-phase 13CO2 withadsorbed 12CO2 and gas-phase 12CO2 with adsorbed 13CO2, respectively.

dP13CO2dt= −k r P13CO2 + k r ′P12CO2(6)P13CO2 + P12CO2 = P13CO2 (initial)(7)

P12CO2 = P12CO2 (equilibrium) {1 − e−(kr + kr ′) t }(8)

On the basis of the ﬁt of Eq. (8) to the data of CO2 evolution(Fig. 2, ◻), the sum of the rate constants (kr + kr') required to attain an
exchange equilibrium was 2.8 h−1. The evolution of 12CO2 means theuptake of 13CO2 on surface with the same equilibrium amount (2.9μmol) and rate constant (2.8 h−1; Fig. 2, ○). Furthermore, a simple
adsorption reaction of 9.8 μmol 13CO2 along the free sites of the ZrO2
surface was substantially faster than 13CO2/12CO2 exchange (2.9 μmol)
with a rate constant of 6.3 h−1. The converged 12CO2 partial pressure
based on total CO2 was 8.7 %.
12

3.2. Photoconversion in CO2 and H2
The ZrO2 and Au–ZrO2 photocatalysts selectively formed CO in thepresence of 13CO2 and H2 (Fig. 3A). Using ZrO2 as the catalyst, 13CO
and 12CO were formed at the same rates (0.018 μmol h−1 gcat−1),which were constant for more than 50 h of reaction time (Table 1A-aand Fig. 3A-a). In the 13CO2 (0.67 kPa) exchange reaction with adsorbed 12CO2, the converged 12CO2 partial pressure based on total CO2
was 7.3 % for ZrO2 [3]. During the initial 5 h of the photoreduction
reaction, 12CO formation was more favorable compared to 13CO formation, which started after 1.5 h of reaction time (Fig. 3A-a, Leftpanel). This observation suggests that the 12CO source was actually preadsorbed 12CO2 on ZrO2 from the atmosphere. The exchange equilibrium between adsorbed 12CO2 at the photoreduction site of ZrO2 (CO2chemisorbed site) and 13CO2 in the gas phase should be slower (Scheme
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H. Zhang, et al.

Fig. 3. (A) Time course formation of 13CO and 12CO during exposure to 13CO2 (2.3 kPa) and H2 (21.7 kPa) using (a) ZrO2 (0.100 g), (b) Au (3.0 wt %)–ZrO2 (0.100 g),(c) Au (5.0 wt %)–ZrO2 (0.100 g) irradiated under full UV–vis light, and (c’, c”, c’’’) Au (5.0 wt %)–ZrO2 (0.100 g) irradiated under ﬁltered light at (c’) λ > 320 nm
and (c”) λ > 420 nm and (c’’’) under dark conditions. (B) Time course formation of 13CO, 12CO, and H2 during exposure to 13CO2 (2.3 kPa) and H2O (2.3 kPa) using(a) ZrO2 (0.100 g), (b) Au (5.0 wt %)–ZrO2 (0.100 g), and (c) Mg–Au (5.0 wt %)–ZrO2 (0.100 g; Mg:Au = 1:5) irradiated under full UV–vis light.

nanoparticles (Scheme 1-iii). However, the 12CO ratio increased again
in the photocatalytic test using a ﬁlter at λ > 420 nm (Table 1A-c”),
suggesting LSPR eﬀects at Au surface suppressed the activation ofchemisorbed CO2 species. Although spectroscopic evidence is notavailable for the eﬀects, one possibility is that hot electrons originatingfrom LSPR combined with protons at the interface between Au
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Table 1
Kinetic Data on Photoreduction/Conversion of CO2 using the ZrO2-Based Photocatalyst under UV–vis Light.
(A) 13CO2 (2.3 kPa) and H2 (21.7 kPa) except for c”” (H2 21.7 kPa)
entry

incident wavelength

photocatalyst

formation rate
(μmol h−1 gcat−1)
13afull lightbcc’
λ > 320 nmc”
λ > 420 nmc’’’
in the darkc””*1
full light(B) 13CO2 (2.3 kPa) and H2O (2.3 kPa)afull light
b
c

CO

12CO
Σ CO
12

CO

(%)

ΣCO

formation rate
(μmol h−1 gcat−1)
H2

ZrO2
Au (3.0 wt %)–ZrO2
Au (5.0 wt %)–ZrO2

0.018
0.12
0.17
0.052
0.034
0.050
< 0.002

0.018
0.017
0.022
0.004 70.006 20.004 90.004 0

0.036
0.14
0.19
0.057
0.041
0.055
0.004 0

49.2
12.2
11.9
8.3
15.3
8.9
> 95.2

–
–
–
–
–
–
–

ZrO2
Au (5.0 wt %)–ZrO2
Mg–Au (5.0 wt %)–ZrO2 (Mg:Au = 1:5)

0.078
0.001 50.001 60.068
0.008 5
0.020

0.15
0.010
0.022

46.6
85.1
92.6

0.14
0.23
0.040

*1 Using H2 only.

Furthermore, a competing photoreduction reaction to form H2 also
proceeded at nearly the same rate (0.14 μmol-H2 h−1 gcat−1; Table 1B-aand Fig. 3B-a).
The photocatalytic product distribution drastically changed using
Au (5.0 wt %)–ZrO2 as the photocatalyst (Fig. 3B-b). The CO formation
rate was extremely suppressed (6.8 %) when compared to the value
using ZrO2 (Table 1B-a and b). The 12C isotopic ratio among total CO
produced increased to 85.1 % in comparison to 46.6 % using ZrO2.
Conversely, the H2 formation rate increased to 0.23 μmol h−1 gcat−1,
which was 1.61 times higher than the rate obtained using ZrO2. O2
formation was not detected above the detection limit of the GC–MS,
suggesting that H2 formation was associated with the oxygen vacancy
site of ZrO2.
The eﬀects of Mg2+ addition on the 13CO2 photoconversion reaction
were then investigated using a Mg–Au (5.0 wt %)–ZrO2 catalyst(Table 1B-c). Due to the binding eﬀect of CO2 by the Mg2+ site [3,19],
the photocatalysis favored CO2 reduction rather than H+ reduction; theH2 formation rate was suppressed by 83 %, whereas the CO formation
rate increased by a factor of 2.17 in comparison to the value using Au(5.0 wt %)–ZrO2 (Fig. 3B-b, c). However, the molar ratio of CO versus
H2 formation, 0.54: 1, was still smaller in comparison to the ratio obtained when using ZrO2, 1.02: 1 (Table 1B-a, c).

Scheme 1. Proposed reaction pathway (i–vi) for the conversion of CO2 and H2
to CO during photocatalytic CO2 reduction.

nanoparticle and ZrO2 and directed to H2 formation as the reverse reaction (Scheme 1-iv, v). Such a negative eﬀect of LSPR is in contradiction to recent progress of the utilization of visible/infrared light[29,30].
It should be noted that the 12CO ratio in total CO formed was 8.3 %using a ﬁlter at λ > 320 nm and 8.9 % in the dark (Table 1A-c’, c’’’),
which are eﬀectively the same as that obtained for 12C isotopic population in equilibrated gaseous CO2 with the photocatalyst (8.7 %,Fig. 2). When the CO formation rates were suppressed (0.055–0.057
μmol h−1 gcat−1), the exchange reaction step between physisorbed CO2
and chemisorbed (and active site) CO2 was not slow enough to control
the overall CO2 reduction into CO.

3.4. Monitoring the surface species during photoconversion of CO2 and H2
by FTIRFTIR spectra were measured for the Au (5.0 wt %)–ZrO2 pretreatedunder vacuum (10−6 Pa) at 295 K for 2 h. At 2.3 kPa of 13CO2 and 21.7
kPa of H2 for 2 h, peaks at 1588, 1389, and 1220 cm−1 appeared(Fig. 4A1) that were assigned to antisymmetric and symmetricstretching vibration of OCO [νas(OCO), νs(OCO)] and bending vibration of
OH (δOH) for bicarbonate (monodentate or bridging) [3,33] and peaks
at 1518 and 1304 cm−1 also appeared that were assigned to νas(OCO)and νs(OCO) for carbonate [3,34]. The peak positions were essentially
identical to those for 13C-bicarbonate and 13C-carbonate species observed for ZrO2 and Ag–ZrO2 [3], demonstrating the formation of these
species over ZrO2 surface.
Corresponding to the spectra in the wavenumber region of
1800–1100 cm−1 above, stretching vibration peak of OH (νOH) for bicarbonate appeared at 3615 cm−1 under 2.3 kPa of 13CO2 and 21.7 kPaof H2 (Fig. 4A2). The compensation of νOH for hydroxy group was notclear due to skewed background level probably due to combinational
excitation of IR and visible light [3]. Furthermore, a broad peak appeared at 2905 cm−1 in the wavenumber region of 3000–2800 cm−1
(Fig. 4A3). This wavenumber is too high as formate species [22,35],
and would be due to formed CHx species and/or CHx group in

3.3. Photoconversion reactions in the presence of CO2 and moisture
The CO2 photoconversion with water as the reductant was evaluated in comparison to CO2 photoreduction with H2 (Section 3.2).
Using ZrO2 as the catalyst, formation rates of both 13CO and 12CO in thepresence of 13CO2 and moisture were higher by 4.3 and 3.9 times, respectively, than the corresponding values under 13CO2 and H2(Table 1A-a and B-a). This result seems contradictory if the latter reaction is regarded as the second step of the former reaction, i.e. waterphotosplitting and CO2 reduction with H+ and e− (or H2) [31,32]. In
fact, the activation of H2 was diﬃcult over ZrO2, whereas water could
be activated at the acid–base sites of ZrO2. The total CO formation rate
under these conditions was 0.15 μmol-CO h−1 gcat−1 (Fig. 3B-a).
6

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Fig. 4. FTIR spectra of Au (5 wt%)–ZrO2 (65 mg). (A) Under 13CO2 (2.3 kPa) and H2 (21.7 kPa) for 2 h, (B) Under 13CO2 (2.3 kPa), H2 (21.7 kPa), and UV–vis light for
2 h, and (C) under vacuum and UV–vis light for 2 h.

hydrocarbons.
We then monitored FTIR changes associated with UV–vis light irradiation (Fig. 4B). At 2.3 kPa of 13CO2 and 21.7 kPa of H2 irradiated by
UV–vis light for 2 h, peak intensity at 1588 and 1389 cm−1 signiﬁcantly
decreased by the eﬀects of UV–vis light, suggesting the conversion ofbicarbonate species into CO and/or decomposition to CO2 and hydroxygroup by light. In contrast, peaks at 1518 and 1304 cm−1 became
prominent (Fig. 4B1) because carbonate species were inert even irradiated under UV–vis light. Accordingly, the νOH peak intensity at 3615
cm−1 due to bicarbonate decreased and negative peak of νOH for hydroxy group at 3700 cm−1 appeared demonstrating that bicarbonate
species partially remained. The νCH peak due to CHx species/group
totally disappeared under irradiation of UV–vis light (Fig. 4B2,3).
During the UV–vis light irradiation for 2 h, we conﬁrmed formationsof 13CO and 12CO by GC–MS at rates of 1.8 μmol h−1 gcat−1 and 0.085μmol h−1 gcat−1, respectively. The decrease of 13C-bicarbonate peak in
FTIR and predominant formation of 13CO (95 mol%) by GC–MS suggested the transformation of 13C-bicarbonate into 13CO. In contrast, a
broad peak at 2905 cm−1 completely disappeared under 13CO2, H2, andUV–vis light (Fig. 4B3), and no C-containing products were detected
other than CO above the detection limit of GC–MS, indicating the peakat 2905 cm−1 was not related to CO2 photoconversion in this study.
Later than 2 h of light irradiation, the 13CO2 and H2 gas were
evacuated for 30 s at 295 K while the UV–vis light irradiation was
continued (Fig. 4C). Negative peaks at 1624 and 1389 cm−1 appeared
while peaks at 1518 and 1322 cm−1 remained. Based on the harmonic
oscillation approximation

v͠ =

1
2πc

k
μ

v͠ 13CO=v͠ 12CO

1
1
+ 16
13
1
1
+ 16
12

= 0.97778(10)

the νas(OCO) and νs(OCO) peaks for C-bicarbonate (1588 and 1389cm−1) should shift to 1624 and 1421 cm−1, respectively, for 12C-bicarbonate. The discrepancy (32 cm−1) for the wavenumber of νs(OCO)
peaks would be due to complex overlap of positive and negative FTIR
peaks in the region. The 12C-bicarbonte should be chemisorbed fromair, but decomposed under vacuum and UV–vis light. Due to the same
reason, the wavenumber of νs(OCO) peaks (1322 cm−1) assigned to inert
13
C-carbonate apparently shifted by 18 cm−1 from 1304 cm−1 under13
CO2 and H2 (Fig. 4A1, C1) while the νas(OCO) peak perfectly remainedat the same wavenumber (1518 cm−1). The negative peak at 3700
cm−1 and neighboring positive peak at 3673 cm−1 (Fig. 4C2) should be
due to formed water and/or hydroxy converted from CO2 and H2 reduced former isolated hydroxy and increased H-bonded one.
13

3.5. Characterization by XRD and UV–vis spectroscopy
XRD patterns were collected for the ZrO2 and Au–ZrO2 samples. The
XRD peaks appeared at 2θB = 17.5°, 24.3°, 28.3°, 31.5°, 34.3°, 35.3°,
38.6°, 41.0°, 45.1°, 50.3°, 54.2°, and 55.6° (Fig. 5A), which were ascribed to the 0 0 1, 0 1 1, 111, 1 1 1, 0 2 0, 0 0 2, 1 2 0, 112 , 202, 0 2 2, 0
0 3, and 3 1 0 reﬂections, respectively, of monoclinic ZrO2 [36,37]. No
peaks due to Au metal or Au2O3 nanoparticles were observed for the Au(3.0 and 5.0 wt %)–ZrO2 samples (Fig. 5B, C). In contrast, for the Au
nanoparticle samples supported on layered double hydroxide, a Au
(111) reﬂection was reported at 2θB = 38.3° [16], suggesting that the
Au metal or Au2O3 nanoparticles were dispersed and small (< 5 nm)(9)7Catalysis Today xxx (xxxx) xxx–xxx

H. Zhang, et al.

with a previous report [3].
The LSPR peak appeared centered at 525 nm for samples doped with
3.0 and 5.0 wt % of Au (arrows in Fig. 6B, C), which is similar to thereported resonance for Au supported on TiO2 (∼550 nm) [38]. Electrontransfer from the LSPR Au nanoparticles to the conduction band of TiO2
within 240 fs was conﬁrmed by transient absorption spectroscopy. In
contrast, no transient absorption rise was observed for the Au–ZrO2
sample, and electron transfer from LSPR Au to the conduction band of
ZrO2 was not detected [39]. Similar to the ﬂat absorption feature thatwas detected below 450 nm for a colloidal solution of Au nanorods(mean 10 nm), the intermediate absorption between 250 and 450 nm
for Au–ZrO2 (Fig. 6B, C) was due to the intraband electronic transition
within Au. Notably, this feature is diﬀerent from the hot electrontransfer from Au to CdSe nanorods [40] because the conduction band of
ZrO2 positions its potential much higher (–1.0 V @standard hydrogenelectrode (SHE)) than CdSe (−0.2 V @SHE). The intermediate absorption between 250 and 450 nm detected for Au, the LSPR absorption
on Au, and the IR absorption all negligibly contributed to the CO2
photoreduction (Table 1A-c’, c’’’).

Fig. 5. XRD patterns of the ZrO2 (A) and Au–ZrO2 samples (B, C). The Au
content was 3.0 (B) and 5.0 wt % (C).

3.6. Characterization by TEM
The TEM image for the Au (5.0 wt %)–ZrO2 sample is shown in
Fig. 7A. Relatively dark spherical images were found and the size was
between 2 and 7 nm. Darker parts also appeared near the center of theimage primarily due to the primary particles being stuck in the direction of the electron injection probe.
High-resolution transmission electron microscopy (HR-TEM) images
for the Au (5.0 wt %)–ZrO2 sample were then measured and showed theformation of various lattice fringes. Intervals between 0.278–0.284 nm
were found for 4–6 nm sized nanoparticles (Fig. 7B, D), corresponding
to the Au 1 1 0 lattice (theoretically 0.288 nm based on fcc metal) [26].
For the 4 nm nanoparticles (Fig. 7C), narrower intervals between
0.190–0.224 nm were also detected, corresponding to the Au 2 0 0
lattice (theoretically 0.204 nm). Conversely, at the basal part (a relatively light color compared to the Au nanoparticle part) of the sample,
which supported the Au nanoparticles, intervals between 0.305–0.314
nm were detected (Fig. 7B–D). Based on the XRD peak position of themonoclinic ZrO2 phase (Fig. 5) [36,37], the lattices were ascribed to themonoclinic ZrO2 111 lattice.
HAADF-STEM images were also observed for the Au (5.0 wt%)–ZrO2 photocatalyst. The Au particles were much more emphasized(see the arrows in Fig. 7F) based on the brilliance dependence on

Fig. 6. Diﬀuse-reﬂectance UV–vis spectra for ZrO2 (A) and Au–ZrO2 samples (B,C). The Au content was 3.0 (B) and 5.0 wt % (C).

enough for this study.
UV–visible spectra were measured for the ZrO2 and Au–ZrO2 samples (Fig. 6). A sharp absorption edge appeared at the same position in
the UV region for all samples, which was extrapolated to 248 nm fromthe x-axis. The evaluated band-gap value of 5.0 eV for ZrO2 is consistentFig. 7. (A) TEM, (B–D) HR-TEM, and (E, F) HAADF-STEM images for the Au (5.0 wt %)–ZrO2 photocatalyst and (G, H) the histogram of Au nanoparticle size based on(G) HR-TEM and (H) HAADF-STEM.
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atomic number Z2 (Z for Au 79, Zr 40), and they were positioned on thebundled ZrO2 nanoparticles (Fig. 7E, F), which is consistent with the
HR-TEM images (Fig. 7A–D). The Au nanoparticle size distribution was
summarized in the histogram based on HR-TEM and HAADF-STEM
analyses (Fig. 7G, H), suggesting a mean size of 4.2 nm and 4.5 nm,
respectively.
3.7. In-situ Au L3-edge EXAFS measurements
Light-induced changes in the Fourier transform function of the
EXAFS data were quantitatively evaluated using a curve-ﬁt analysis
based on Eq. (4). For the Au (5.0 wt %)–ZrO2 photocatalyst, the N value
was 10.8 ± 0.5 before light irradiation (Fig. 8A), 10.2–11.1 (mean
10.7) during light irradiation, and 10.5–11.1 after the light was turnedoﬀ (mean 10.8). Based on the mean particle size range of 4.2–4.5 nm
obtained for Au in this study (HR-TEM, HAADF-STEM; Fig. 7), thedispersion of the 4.3 nm Au particle was calculated to be 0.31 [41]. The
dispersion of Au was thus assumed to be constant at 0.31 in the subsequent analyses.
The σ value was calculated to be 0.009 968 nm for Au metal at 290
K using the correlated Debye model [42,43] with the ab initio multiplescattering calculation code, FEFF8 [44], and the Debye temperature for
Au (165 K) [27]. The XDAP code provides an experimental diﬀerence
for the σ2 value from that of the Au metal (model) based on Eq. (4). The
initial σ value of 0.010 54 nm for Au (5.0 wt %)–ZrO2 before lightirradiation quickly increased to 0.011 29 nm (10 min light irradiation),
remained nearly constant between 0.011 24 and 0.011 34 nm after 70
min of irradiation, and progressively increased to 0.011 40 nm after
110 min of light irradiation (Fig. 8B). Subsequently, this value quickly
decreased to 0.010 72 nm after the light was turned oﬀ after 120 min of
irradiation. The value further decreased gradually to 0.010 55 nm at
190 min, which was essentially the same value as that before light irradiation.
A change in the major peak intensity at 0.24 nm (phase shift uncorrected) was not completely in accord with the change in the σ value
during irradiation of UV–vis light for 120 min (Fig. 8B, C). The peakintensity progressively decreased during light irradiation for 110 min,whereas the σ value increased only during the ﬁrst 10 min but essentially remained constant between 10–110 min of light irradiation.
Conversely, the N(Au–Au) value clearly exhibited a decreasing trend
during the entire 110 min of light irradiation (Fig. 8A).
To reconﬁrm the curve-ﬁt analysis results described above, log-ratio
method analyses were performed for the EXAFS data of Au (5.0 wt%)–ZrO2 (Fig. 9). In the log-ratio plots, the y-intercept and slope wererelated to the N and σ2 values, respectively, and compared to corresponding values for the reference Ai,Ref(k) data before light irradiation
based on Eq. (5). During light irradiation (Fig. 9A), the y-intercept remained nearly zero (between −0.07 and 0.04), but it decreased after 30
min and 90 min of photoirradiation, which was similar to the N value
decrease detected after 10 min and 90 min via curve-ﬁt analyses(Fig. 8A). Conversely, the slope was also always negative and demonstrated an increase in the σ2 value compared to the reference value. In
stark contrast, after the light was turned oﬀ (Fig. 9B), N (y-intercept)
values were mostly zero or positive after 30 min of irradiation and thenegative σ2 (slope) values increased, ﬁnally reaching zero after 120 minof darkness (Fig. 9B-f). This result demonstrates that these Au nanoparticle features were restored to their original values before light irradiation, which is in agreement with the curve-ﬁt analyses based on
Eq. (4) (Fig. 8).
Taken together, the combined eﬀects of the N(Au–Au) and σ valuechanges resulted in a gradual decrease in the peak intensity at 0.24 nm(phase shift uncorrected), i.e. the initial sudden increase of the σ value
and subsequent gradual, slow decrease of the N value.
Furthermore, we evaluated the temperature at the Au site based onthe σ values. The temperature dependence of σ value is derived fromFEFF8 combined with the correlated Debye model [42,43] for both bulk

Fig. 8. Time course changes of (A) N values, (B) σ values, and (C, D) Fouriertransform obtained from k3-weighted Au L3-edge EXAFS χ-function for Au (5.0wt%)–ZrO2 under CO2 (2.3 kPa) and H2 (21.7 kPa) (C) irradiated by UV–vislight for 120 min followed by (D) dark conditions for 120 min. (E) The correlation between the σ value and temperature for bulk sites (circle, ◦) and surface
sites (vertical motion; square, □) in/on the Au metal generated by the correlated Debye model using an FEFF8 code.
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Fig. 9. Log-ratio analyses for EXAFS amplitudes of the Au (5.0 wt %)–ZrO2 photocatalyst irradiated by UV–vis light (A) and after the light was turned oﬀ (B). Thechanges under light after 120 min (A-a–f) and in the dark for 120 min (B-a–e).

and surface Au sites using the bulk Debye temperature θD(Bulk) andsurface Debye temperature θD(Surf,⊥) (Fig. 8E-a and b, respectively). We
assume preferable exposure of thermodynamically stable fcc(1 1 1) or(1 1 0) face for the latter value (83 K) [28]. We also approximated themean Au nanoparticle temperature as the arithmetic mean temperature
based on θD(Surf,⊥) weighted by 1/2·1/3D [D: dispersion of nanoparticles(0.31), for an eﬀective vertical degree of freedom at a free hemispheresurface] and that based on the θD(Bulk) weighted by (1 – D) + 1/2D +1/2·2/3D (bulk site, non-free hemisphere in contact with ZrO2 and twolateral degrees of freedom at a free hemisphere surface).
As a result, the initial temperature of 295 K before light exposure
rose to 316 K after 10 min of irradiation, remained nearly constant
between 314 and 318 K after 70 min of irradiation, and progressively
increased to 321 K after 110 min of light irradiation (Fig. 8B). The
temperature quickly dropped to 301 K after the light was turned oﬀ.
The temperature gradually decreased further to its initial value before
light irradiation (295 K) after 190 min. Such rise/drop in temperature ispossibly due to LSPR heat transformation [3].

Under the common irradiation conditions of UV–vis light fromquartz ﬁber light guide at 5 cm and incident angle of 45° to the ZrO2
and Au (5.0 wt %)–ZrO2 samples (65 mg each), the temperaturechanges were monitored using IR thermography (Fig. S1,
Supplementary material). The temperature of center of light spot on
Au–ZrO2 quickly increased from 295 K to 318 K within less than 1 min
and then gradually increased to 328 K at 5 min of irradiation (Fig. S1B)
in consistent with the Au temperature change (321 K) monitored by
EXAFS (Fig. 8B). In contrast, the temperature of center of UV–vis lightspot on ZrO2 disk increase to only 299 K at 5 min of irradiation (Fig.
S1A) and negligible increase was found later than that. Thus, Au nanoparticles absorbed mainly visible light as LSPR resulting in heat for
the temperature increase of Au–ZrO2 sample.
A control EXAFS experiment was performed for the Au (5.0 wt%)–ZrO2 photocatalyst under Ar and UV–vis light (Fig. 10). The ﬁtparameters, N and σ, followed similar trends to those obtained under
CO2 and H2 (Fig. 8). The initial N value for light irradiation (11.8)
showed a decreasing trend, but the decrease was less than 0.5 and the10Catalysis Today xxx (xxxx) xxx–xxx

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Fig. 11. Time course changes of (A) N values, (B) σ values, and (C) Fouriertransform obtained from angular photoelectron wavenumber k3-weighted Au
L3-edge EXAFS χ-function for Au (5.0 wt%)–ZrO2 under CO2 (2.3 kPa) and H2(21.7 kPa) irradiated by visible light (> 715 nm) for 40 min.

Fig. 10. Time course changes of (A) N values, (B) σ values, and (C) Fouriertransform obtained from angular photoelectron wavenumber k3-weighted Au
L3-edge EXAFS χ-function for Au (5.0 wt%)–ZrO2 under Ar, (C) irradiated by
UV–vis light for 80 min followed by (D) dark conditions for 80 min.

value was restored after the light was turned oﬀ (Fig. 10A). Conversely,
the initial σ value (0.010 3 nm) rapidly increased after 10 min of lightirradiation, remained at high values (∼0.011 6 nm) during light irradiation, and quickly dropped to the level before light irradiation in thedark (∼0.010 4 nm). Based on the correlated Debye model, the temperature of the Au surface was evaluated to 283 K before light irradiation, 320–324 K during light irradiation, and back down to 290–287
K when the light was turned oﬀ (Fig. 10B). The heated temperature
range was similarly between 314–321 K under CO2 and H2 conditions(Fig. 8B). CO2 reduction into CO using H2 as the reductant is an endothermic process (Δreaction H⦵ =41.16 kJ mol−1) [3], but the temperature of the system should decrease only by 0.29 mK min−1 basedon the enthalpy change and the standard heat capacity of ZrO2 (Cp,m⦵= 56.123 J K−1 mol−1) and Au (Cp,m⦵ = 25.42 J K−1 mol−1) [45,46].
Thus, the reactant gas did not aﬀect the changes of the N or σ values.

Scheme 2. Energetic diagram of Au or Ag combined with ZrO2 under CO2, H2,
and UV–vis light.

The gradual decrease of the Au–Au peak intensity (Fig. 10C) was primarily due to the increase in the σ value and was also associated withthe gradual decrease of the N value during light irradiation. As
11

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(5.0 wt %)–ZrO2, suggesting that the photocatalytic contribution wasband-gap charge separation in ZrO2 (absorption edge 248 nm): 69 %1
and ambient heat ( 2 kT ): 31 %. H2 was eﬀectively activated over a Au
surface and spilled over H coupled with a hole to form a proton andcombined with CO2 over ZrO2 to generate CO. The greater work function of Au in comparison to Ag resulted in electron accumulation at Au,
which interfered with H spillover and proton formation. The temperature of Au was monitored based on correlated Debye model analysis of
EXAFS. The lower Debye temperature of Au (165 K) compared to Ag(225 K) resulted in greater heat capacity and the temperature rise via
LSPR was smaller, 26 K under CO2, H2, and UV–vis light using Au (5.0
wt %)–ZrO2 in comparison to 106 K under similar conditions using Ag(5.0 wt %)–ZrO2. ZrO2 was eﬀective in promoting both CO and H2formation under CO2, moisture, and UV–vis light conditions, but Audoped ZrO2 directed the products toward H2 production (96 %) via areverse reaction step. During this pathway, a proton is coupled with anelectron, and H2 formation occurs over the Au surface. The addition of
Mg2+ to Au–ZrO2 mitigated the selectivity toward CO, aﬀording 35 %of CO and 65 % of H2.

observed previously, these values were soon restored after the light was
turned oﬀ (Fig. 10D).
Another control experiment was performed under CO2 (2.3 kPa) andH2 (21.7 kPa) conditions using a λ > 715 nm ﬁlter. Upon exposure tolight for 30 min, no changes in the N or σ values or the Fourier transform were observed (Fig. 11A–C). Thus, visible light of λ > 715 nm andIR light do not heat the photocatalyst.
An LSPR-induced thermal eﬀect on phenol decomposition has beensuggested using Ag nanoparticles on Bi2WO6 based on comparisons tocontrol photocatalytic reaction tests conducted at 278–343 K [47]. The
conversion of LSPR into heat was monitored using various techniques,
including surface-enhanced Raman scattering of Au coated with Pt
using p-aminothiophenol as the probe molecule [48], Raman scatteringof the Au thin ﬁlms via electric polarization by a 532 nm laser [49], andabsorption spectroscopy for Au nanoparticles irradiated by the laser[50].
The energy diagram of Au–ZrO2 was drawn in comparison to that of
Ag–ZrO2 [3] for CO2 photoreduction (Scheme 2). The band-gap excitation in ZrO2 by UV light irradiation generates excited electrons in its
conduction band for CO2 reduction at –0.11 V @SHE [2]. When combined with the Ag nanoparticles, H2 was eﬀectively activated by LSPRinduced heat at 392 K via the irradiation of visible light. The activatedhydrogen was spilled over ZrO2 and reacted with the hole to form a
proton [3] (Scheme 2, left). In stark contrast, Au nanoparticles weredisadvantageous for CO2 photoreduction; the rate under these conditions (0.19 μmol−CO h−1 gcat−1; Table 1A-c) was much slower than
the rate obtained using the Ag (5.0 wt %)–ZrO2 catalyst (0.57 μmol-CO
h−1 gcat−1) [3]. This discrepancy is likely due to two major reasons: (i)the heating eﬀect (321 K, Fig. 8B) via LSPR was not suﬃcient because
the Debye temperature was lower for Au (165 K), and thus the heat
capacity was greater under UV–vis light in comparison to the Ag (Debye
temperature 225 K, enabled 392 K under UV–vis light) (Scheme 2) and(ii) a deeper Fermi level led to collect electrons from the conduction
band of ZrO2 to cancel the hole oxidation of activated H over Au(Scheme 2, right). The reason ii also explains why the light ﬁltered at
λ > 320 nm did not contribute to convert CO2 (Table 1Ac’, c”, c’’’).
The promotion mechanism by LSPR has been reported to involve (i)charge excitation to an unoccupied adsorbate state, (ii) hot electron
injection that originates from LSPR to the support, (iii) electron trapping from the support to the Schottky barrier, (iv) plasmonic resonant
energy transfer, and (v) transformation to heat [3]. Mechanism v wasplausible using Ag–ZrO2, whereas the plasmonic eﬀect was completely
hindered by electron injection from the ZrO2 conduction band to a
deeper Au level (Scheme 2, right). The work functions for Au and Ag are
signiﬁcantly diﬀerent: 5.31–5.47 eV versus 4.52–4.74 eV, respectively[14–18]. In the present work, Au activated H2 under solely thermal
conditions at 295 K to boost photocatalytic CO formation from 0.036
μmol-CO h−1 gcat−1 (ZrO2) to 0.19 μmol-CO h−1 gcat−1 (Au (5.0 wt%)–ZrO2 (Table 1A-a, c).

Declaration of Competing Interest
I declare no conﬂict of interest.
Acknowledgments
The authors are grateful for ﬁnancial support from the Grant-in-Aid
for Scientiﬁc Research C (17K05961) from the Japan Society for the
Promotion of Science, Grant for Academic Research from the Japan Gas
Association (2018–2019), and Leading Research Promotion Program(2015–2019) from the Institute for Global Prominent Research, Chiba
University. X-ray absorption experiments were performed with theapproval of the Photon Factory Proposal Review Committee(2016G577, 2019G141).
Appendix A. Supplementary data
Supplementary material related to this article can be found, in the
online version, at doi:https://doi.org/10.1016/j.cattod.2020.02.040.
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4. Conclusions
The photoexchange reaction under 13CO2 revealed that the exchange rate constant between gas-phase CO2 and chemisorbed CO2 was
2.8 h−1, whereas adsorption of CO2 on the free site over ZrO2 wassigniﬁcantly faster (6.3 h−1). The former site was determined to be theactive site for photocatalytic reduction of CO2 into CO. Au nanoparticles promoted CO2 photoreduction and Au (5.0 wt %)–ZrO2 was
identiﬁed as the most eﬃcient catalyst for CO production at a rate of
0.19 μmol h−1 gcat−1. 11.9 % of 12CO was detected in the product,
which is believed to originate from adsorbed 12CO2 from the air. It wasdetermined that exchange between chemisorbed and physisorbed CO2
was slower because the ratio did not reach the equilibrated ratio of
13
CO2/adsorbed 12CO2 in the photoexchange reaction (8.7 %). The COformation rates using a sharp-cut ﬁlter at λ > 320 nm and a control
kinetic test in the dark were identical, 0.055 μmol h−1 gcat−1 using Au
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