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

Article abstract of DOI:10.1021/jacs.8b13894

Confirmation of ¹³CO₂ photoconversion into a ¹³C-product is crucial to produce solar fuel. However, the total reactant and charge flow during the reaction is complex; therefore, the role of light during this reaction needs clarification. Here, we chose Ag-ZrO₂ photocatalysts because beginning from adventitious C, negligible products are formed using them. The reactants, products, and intermediates at the surface were monitored via gas chromatography-mass spectrometry and FTIR, whereas the temperature of Ag was monitored via Debye-Waller factor obtained by in situ extended X-ray absorption fine structure. With exposure to ¹³CO₂, H₂, and UV-visible light, ¹³CO selectively formed, while 8.6% of the ¹²CO mixed in the product due to the formation of ¹²C-bicarbonate species from air that exchanged with the ¹³CO₂ gas-phase during a 2 h reaction. By choosing the light activation wavelength, the CO₂ photoconversion contribution ratio was charge separated at the ZrO₂ band gap (λ < 248 nm): 70%, localized at the Ag surface plasmon resonance (LSPR) (330 < λ < 580 nm): 28%, and characterized by a thermal energy of 295 K: 2%. LSPR at the Ag surface was converted to heat at temperatures of up to 392 K, which provided an efficient supply of activated H species to the bicarbonate species, combined with separated electrons and holes above the ZrO₂, which generated CO at a rate of 0.66 μmol h^-1 g_cat^-1 with approximately zero order kinetics. Photoconversion of ¹³CO₂ using moisture was also possible. Water photo-oxidation step above ZrO₂ was rate-limited, and the side reactions that formed H₂ above the Ag were successfully suppressed instead to produce CO via the Mg²⁺ addition to trap CO₂ at the surface.

Full text of DOI:10.1021/jacs.8b13894

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Article
Dual Photocatalytic Roles of Light: Charge Separation at
the Band Gap and Heat via Localized Surface Plasmon
2
Resonance to Convert CO into CO over Silver-Zirconium Oxide
Hongwei Zhang, Takaomi Itoi, Takehisa Konishi, and Yasuo Izumi
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 27 Mar 2019
Downloaded from http://pubs.acs.org on March 27, 2019
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Journal of the American Chemical Society
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Dual Photocatalytic Roles of Light: Charge Separation at
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onance to Convert CO into CO over Silver–Zirconium Oxide
2
†
‡
†
†,
Hongwei Zhang, Takaomi Itoi, Takehisa Konishi, and Yasuo Izumi *
†
‡
Department of Chemistry, Graduate School of Science, Chiba University, Yayoi 1-33, Inage-ku, Chiba 263-8522, Japan. Depart-
ment of Mechanical Engineering, Graduate School of Engineering, Chiba University, Yayoi 1-33, Inage-ku, Chiba 263-8522, Japan.
13
13
ABSTRACT: Confirmation of CO
2
photoconversion into a C-product is crucial to produce solar fuel. However, the total reactant and
charge flow during the reaction is complex; therefore, the role of light during this reaction needs clarification. Here, we chose Ag–ZrO pho-
2
tocatalysts because beginning from adventitious C, negligible products are formed using them. The reactants, products, and intermediates at
the surface were monitored via gas chromatography-mass spectrometry and FTIR, whereas the temperature of Ag was monitored via Debye–
1
3
13
Waller factor obtained by in situ extended X-ray absorption fine structure. Exposure to CO
2
, H
2
, and UV–visible light, CO selectively
1
2
12
13
formed while 8.6% of the CO mixed in the product due to the formation of C-bicarbonate species from air that exchanged with the CO
2
gas-phase during a 2 h reaction. By choosing the light activation wavelength, the CO
rated at the ZrO band gap (λ < 248 nm): 70%, localized at the Ag surface plasmon resonance (LSPR) (330 < λ < 580 nm): 28%, and charac-
terized by a thermal energy of 295 K: 2%. LSPR at the Ag surface was converted to heat at temperatures of up to 392 K, which provided an
efficient supply of activated H species to the bicarbonate species, combined with separated electrons and holes above the ZrO , which gener-
using moisture was also possible.
above the Ag were successfully suppressed
2
photoconversion contribution ratio was charge sepa-
2
2
−
1
−1
13
ated CO at a rate of 0.66 μmol h gcat with approximately zero order kinetics. Photoconversion of CO
Water photo-oxidation step above ZrO was rate-limited and the side reactions that formed H
2
2
2
2
+
instead to produce CO via the Mg addition to trap CO at the surface.
2
+
−
o
−1
CO
CO
2
2
+ H
+ H
2
2
(or 2H + 2e ) → CO + H
2
O, Δ
r
H = 41.16 kJ mol (2)
1. INTRODUCTION
o
−1
O → CO + O
2
+ H
2
, Δ
r
H = 524.80 kJ mol
(3)
Numerous studies have investigated photocatalytic CO fuel
2
We analyzed the photocatalytic mechanism in step 2, and we also
conversion to initiate a carbon-neutral cycle, which includes the use
of solar fuel in contrast to the irreversible consumption of fossil
1
3
attempted the photoconversion of CO using moisture (step 3).
2
1
,2
fuels. However, pitfalls exist in these previous investigations that
mistake methane and/or C-containing products converted from
2
. EXPERIMENTAL METHODS
An aqueous Ag nitrate (>99.8%, Wako Pure Chemical, Japan)
solution was reduced in a liquid phase using NaBH (>95%, Wako
1
,3–6
catalyst impurities as products converted from CO
2
.
Therefore,
(standard for-
mation enthalpy = −393.5 kJ mol ) using only sustainable energy
4
finding an effective photocatalyst to convert CO
2
−
1
Pure Chemical) in the presence of ZrO (JRC-ZRO-3, Catalysis
2
2
1
3
Society of Japan; tetragonal phase, specific surface area = 94.4 m
while confirming labeled CO conversion, not adventitious C, is
2
−
1
17
1
,3,6,7–12
g ). We varied the sample Ag content between 0.50 and 10
weights (wt.) %. The sample is denoted as Ag–ZrO . Mg nitrate
>99.5%, Wako Pure Chemical) was impregnated into the Ag–
essential.
2
Previous studies have rarely used time course monitoring for iso-
(
1
3
tope labeled- CO
2
to confirm CO
2
conversion. This study moni-
13
ZrO aqueous suspension. The molar ratio of Mg:Ag was 1:1. The
2
1
3
tors the CO
2
photoreduction time course and investigates the C
water was distilled at 358 K, and the powder product was dried at
1
2
and C product origins. Based on this mechanistic investigation,
3
73 K for 40 h.
we observe and clarify the dual roles that UV–visible light has for
1
3–15
The photocatalyst (0.100 g) was placed in a quartz photoreactor
the first time using ZrO -based photocatalysts : (i) charge sepa-
2
and evacuated at 295 K for 2 h while connected to a Pyrex glass
circulation system (206.1 mL) and both rotary and diffusion
ration at the band gap (BG) and (ii) heat transformed via localized
surface plasmon resonance (LSPR) via extended X-ray absorption
fine structure (EXAFS). Temperature monitoring of the Ag via
Debye–Waller factor, which was obtained with in situ EXAFS, was
direct in comparison with surface-enhanced Raman techniques that
use adsorbed probe molecules.
−6
16
13
13
pumps (10 Pa). We then introduced 2.3 kPa of CO
9
2
( C
17
18
9.0%, O 0.1%, O 0.7%, chemical purity >99.9%, Cambridge
Isotope Laboratories, Inc., Tewksbury, MA, USA) and 21.7 kPa of
1
3
H
2
(>99.99%). For comparisons, the pressure of CO
2
and H was
2
varied in the ranges 1.1–6.7 kPa and 21.7–66.7 kPa, respectively.
The reactor catalyst was irradiated with UV–visible light from a 500
W xenon arc lamp (Model OPM2-502, Ushio, Japan). The distance
between the UV–visible light exit and the photocatalyst was 20 mm.
First, this study evaluates the rates of step 2 that follows water
1
6–18
photo-oxidation in step 1 below.
+
−
o
−1
2
H
2
O → O
2
+ 2H
2
(or 4H + 4e ), Δ
r
H = 483.64 kJ mol (1)
1
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−
2
The light intensity was 90.2 mW cm at the center of the photo-
catalyst. The intensity distribution of the Xe arc lamp was measured
using a spectroradiometer (Model USR45DA, Ushio, Japan) at a
position 20 mm apart from the UV–visible light exit (Figure S1,
Supporting Information). A small portion of distribution lower than
Silver K-edge EXAFS spectra were measured at 290 K in trans-
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
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3
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5
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5
5
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5
5
6
mission mode at the Photon Factory Advanced Ring, High Energy
Accelerator Research Organization (KEK, Tsukuba, Japan) on the
2
5
NW10A beamline. A Si(3 1 1) double-crystal monochromator
and a Pt-coated focusing cylindrical mirror were inserted into the
path of the X-ray beam. A Piezo transducer was used to detune the
X-ray to two thirds of the maximum intensity to suppress higher
harmonics. The Ag K-edge absorption energy was calibrated at
λ = 248 nm exists enabling the BG excitation of ZrO . In-profile
2
kinetic data were collected as a function of the light’s excitation
wavelength by inserting a sharp-cut filter (2.5 mm thick) at the
lighthouse exit. We used the UV32 and O58 (Hoya, Japan) types to
pass light with λ > 320 nm and λ > 580 nm, respectively. Control
, and no light were performed by
completely wrapping the reactor with Al foil. We also performed
25516.5 eV using the X-ray spectrum of a Ag metal foil (40 μm
1
9,20
26
thick). A disk (Φ = 10 mm) of a Ag (3.0–5.0 wt. %)–ZrO
2
photo-
1
3
tests with exposure to CO
2
, H
2
catalyst (125 mg) was set in a Pyrex glass reactor equipped with a
Kapton film (Dupont, Wilmington, DE, USA; 50 μm thick) for X-
ray transmission and a polyethylene terephthalate (PET) film (Tei-
jin, Japan, G2; 50 μm thick) for both UV–visible light and X-ray
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
3
control tests with exposure CO gas only and UV–visible light.
13
2
The exchange reaction with 0.67 kPa of CO
using a similar procedure.
2
was also performed
transmission filled with 2.3 kPa of CO
2
and 21.7 kPa of H . The
2
sample was irradiated with UV–visible light from a Xe arc lamp
through the PET film at the beamline.
A packed column of 13X-S molecular sieves (3 m length, 3 mm
internal diameter; GL Sciences, Inc., Japan) was employed for on-
line gas chromatography-mass spectrometry analyses (GCMS;
18,24,27,28
The obtained Ag K-edge EXAFS data were analyzed using the
2
1–23
29
Model JMS-Q1050GC, JEOL, Japan).
Helium (purity >
XDAP software package. The pre-edge background was approxi-
2
9
9.9999%) was used as the carrier gas at 0.40 MPa. The sampling
mated with a modified Victoreen function, i.e., C
2
/E + C
1
/E + C ,
0
loop comprised a Pyrex glass system kept under vacuum using ro-
tary and diffusion pumps (10 Pa) connected to the GCMS 1.5 m
where E is the photon energy. The background for post-edge oscil-
lation, μx, was approximated with a smoothing spline function and
was calculated for a particular number of data points:
−6
deactivated fused silica tubes (No. 160-2845-10, Agilent, Santa
Clara, CA, USA; internal diameter 250 μm), which were main-
tained at 393 K during analysis to avoid gas adsorption.
Data Points
2
(
µx − background )
i
i
≤ smoothing factor
∑
2
exp −0.075k
i=1
(
i
)
The surface species were monitored with a single-beam Fourier
transform infrared (FTIR) instrument (JASCO, Japan; Model
FT/IR-4200) equipped with a mercury–cadmium–tellurium-M
(4)
Where k is the angular photoelectron wave number.
Multiple-shell curve fit analyses were performed on the Fourier-
24
detector at a constant temperature of 77.4 K. A 20 mm-Φ self-
3
filtered k -weighted EXAFS data in k- and R-space (R: interatomic
supporting disk of ZrO
2
or Ag (5.0 wt. %)–ZrO disk (50 mg) was
2
distance) using the empirical amplitude extracted from the EXAFS
data for the Ag metal foil (40 μm thick). The R and coordination
number (N) values for the Ag–Ag interatomic pair were set to
placed in a quartz photoreaction cell equipped with NaCl windows
on both sides. The photoreaction cell was connected to the Pyrex
glass circulation system as well as the GCMS to enable simultane-
ous surface species monitoring via FTIR and isotope distribution in
the gas with the GCMS. The photocatalyst disk was evacuated
3
0
0
.288 9 nm and 12. We assumed that the many-body reduction
2
factor, S , is identical for both the sample and reference.
0
−
6
The high-resolution transmission electron microscopy (HR-
TEM) investigations were performed using a JEM-2100F (JEOL)
equipped with a field emission gun at an acceleration voltage of 200
(10 Pa) at 295 K for 2 h prior to FTIR and GCMS measure-
1
6
ments.
In situ FTIR measurements were performed at 295 K in a range
2
7
kV. The samples were mounted on a Cu mesh (250 mesh per
inch) coated with a copolymer film of poly(vinyl alcohol) and for-
maldehyde (Formvar, Monsanto, St. Louis, MO, USA) and coated
with carbon. Chemical compositions and elemental distributions
were analyzed using energy dispersive spectra equipped with a
Si(Li) detector in the TEM.
−
1
from 4000 to 650 cm . The sample disk was irradiated with UV–
visible light from a 500 W Xe arc lamp using quartz fiber light guide
(San-ei Electric Co., Japan; Model 5Φ-2B-1000L). The distance
between the fiber light exit and sample disk was 50 mm. The light
−
2
intensity at the center of sample was 88 mW cm . The spectrome-
−
1
ter’s energy resolution was 1 cm . A 10%-cut filter was inserted in
front of the photoreaction cell. Data accumulation was between
3. RESULTS AND DISCUSSION
1
28 and 256 scans (approximately 2 s per scan).
We first compared the reactions exposed to CO
2
and H
2
using
UV–visible spectra were recorded on a double-beam model V-
several ZrO -based photocatalysts doped with varying amounts of
2
6
50 spectrophotometer using D and halogen lamps below and
2
Ag under UV–visible light irradiation (Table 1A-a–e and Figure
S2). The major product using these catalysts was always CO. The
above 340 nm equipped with a photomultiplier tube and an inte-
grated ISV-469 sphere (JASCO) to diffuse reflectance detection
−1
−1
formation rate using 5.0 wt. % Ag–ZrO
higher by a factor of 3.9 than when using ZrO
2
(0.57 μmol h
g
cat ) was
1
6,23
within the range from 200 to 800 nm.
Data was transformed
−1
−1
2
(0.15 μmol h gcat ;
using the Kubelka–Munk function. A formed polytetrafluoroeth-
ylene plate was used as a reference. Absorption–fluorescence spec-
tra were recorded on model FP-8600 (JASCO; Chiba Iodine Re-
source Innovation Center, Chiba University) using 150-W Xe arc
lamp equipped with a photomultiplier tube within the excitation
range from 200 and 300 nm and fluorescence range from 300 to
Table 1A-a, d). When the photocatalyst Ag content varied between
0.50 and 10 wt. %, we were able to maximize the total CO for-
mation rate by using Ag (5.0 wt. %)–ZrO (Table 1A-a–e and Fig-
2
ure S3). We confirmed the predominant photocatalytic formation
13
13
12
of CO, which was derived from CO
2
. However, minor CO also
formed continuously (Figures 1A and S2). Caution should be exer-
8
00 nm.
−1
cised for formation rates of CO that are of the order of μmol h
−
1
−1
−1
g
cat ; the rates of tens of mmol h
g
cat have been reported for
2
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methane formation.
The differences of reaction conditions,
considered; however, this study isotopically clarifies the mecha-
nism from CO to CO activated by light in the course of time.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
products, and their associated free energy change must be carefully
2
TABLE 1. Summary of kinetic data on photoconversion of CO
2
using the ZrO -based photocatalyst under UV–visible light.
2
1
3
(A) CO
2
(2.3 kPa) and H
2
(21.7 kPa)
Formation rate (μmol h−1
cat−1)
g
Entry
Catalyst
Light irradiated
13CO
12CO
a
Full light
0.10
0.045
ZrO
2
a'
λ > 320 nm
0.018
0.20
0.0084
0.11
b
c
Ag (0.50 wt. %)–ZrO
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
Ag (3.0 wt. %)–ZrO
Ag (5.0 wt. %)–ZrO
2
Full light
0.38
0.079
0.049
0.017
0.0034
0.065
d
0.52
d'
d”
e
2
λ > 320 nm
λ > 580 nm
Full light
0.15
0.0093
0.46
Ag (10 wt. %)–ZrO
2
13
(
(
B) CO
2
(2.3 kPa) and H
2
O (2.7 kPa)
Formation rate (μmol h−1
g
cat−1)
Entry
Catalyst
13CO
12CO
H
2
a
b
c
ZrO
2
0.018
0.16
0.074
0.15
Ag (5.0 wt. %)–ZrO
2
0.0031
0.0010
0.0049
0.092
2+
Mg –Ag (5.0 wt. %)–ZrO
2
<0.002
1
3
C) Pressure dependence of CO formation rate using CO
2
, H
)
)
2
, and Ag (5.0 wt. %)–ZrO
pressure (kPa)
21.7 43.8
2
13
−1
−1
Formation rate of CO (μmol h
Formation rate of CO (μmol h
g
g
cat
H
2
12
−1
−1
cat
13
CO
2
pressure (kPa)
0
66.7
0
.39
1
.1
0
.045
<
0.0009
0.52
0.59
0.52
2.3
4.8
6.7
<
0.001
0.049
0.049
0.033
0
.61
0
.034
0
.50
0.62
0
.035
0.036
13
12
The CO and CO formation rates using Ag (5.0 wt. %)–ZrO
2
Based on the fit (Figure 2B), the sum of the rate constants (k +
r
−1
−1
−1
−1
−1
were 0.52 μmol h gcat and 0.049 μmol h gcat , respectively (Fig-
k ’) required to attain an exchange equilibrium was 2.0 h . A
r
12
12
13
ure 1A). The C ratio in reactant CO
2
was 1.0%, whereas the C
quicker reaction of 13.5 μmol CO
2
along the free sites of the Ag
−1
ratio in formed CO was 8.6% (Table 1A-d). To understand this
(5.0 wt. %)–ZrO surface had a rate constant of 9.0 h in compari-
2
13
13
12
inconsistency, we performed a CO
with Ag (5.0 wt. %)–ZrO under UV–visible light irradiation (Fig-
ure 2B). The exchange reaction proceeded with CO
2
(0.67 kPa) exchange reaction
son to the CO
2
/ CO
2
exchange (3.5 μmol). The converged
was 9.3%, which is in good
1
2
2
CO
2
partial pressure in total CO
2
1
2
12
2
, which was
agreement with the C ratio that formed in the CO (8.6%). We
adsorbed from air and remained after pretreatment under vacuum.
The exchange reaction reached equilibrium after 2 h. We assumed
that this reaction followed the first-order kinetics and that the rate
also performed the exchange test when exposed to dark conditions
1
3
(Figure S4B). The total CO
2
3
uptake changed negligibly. However,
1
12
the amount of exchangeable CO
2
/ CO was greater by a factor of
2
1
3
constants, k
with adsorbed CO
respectively.
r
and k
r
’, are the exchange between gas-phase CO
2
1.46 when exposed to light (Figures S4B and 2B), which suggests
1
2
12
13
13
2
and gas-phase CO
2
with adsorbed CO
2
,
that the CO
2
-derived species is light activated. CO began to form
after an induction period of 0.5 h due to an exchange delay with the
1
2
C surface species (Figure S2, Left), as well as the Ag activation via
heating by light (see below).
Using ZrO (Figure 2A), the converged partial pressure of CO
in the total CO
as high as 27% (Table 1A-a and Figure S2A), which suggests the
existence of an irreversible CO -derived species leading to CO
dP13 CO
2
=
−k P13
+ k 'P12
r
CO2
r
CO2
dt
(
(
5)
6)
1
2
2
2
P
13
+ P12 CO = P13 CO
12
CO
2
2
2
(initial)
2
was 7.3%, whereas the C ratio in the total CO was
−
r r
(k +k ')t
P
12
= P12 CO
{
1−e
}
CO
2
2
(eqilibrium)
2
ꢀ
ꢁ
(7)
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formation associated with O vacancy in ZrO
2
(2.3 μmol, Figure 2A
3
2,33
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
and Scheme 1c). The uptake of O atoms at the O vacancy site in
ZrO correlated well with the intensity ratio between the Zr 3d and
O 1s peaks in the X-ray photoelectron spectra.
was more reactive in Ag–ZrO when exposed to UV–visible light
The rate dependence on reactant pressure approaching atmos-
pheric pressure was also studied using Ag (5.0 wt. %)–ZrO cata-
lysts (Table 1C). Extremely weak dependence on both the pressure
of CO and H was found with a maximum CO formation rate of
.66 μmol h gcat at 6.7 kPa CO
approximately zero order kinetics due to the strong adsorption of
CO and H.
FTIR spectra were measured for the ZrO
uum at 295 K for 2 h. With the adsorption of CO
2
2
3
4,35
The O vacancy
2
2
2
due to H activation by Ag (3.5 μmol, Figure 2B), whereas we ob-
served no Ag effects when exposed to dark conditions (Figure S4A,
−1
−1
0
2
and 66.7 kPa H , demonstrating
2
B). The number of total CO
surface was 1.3 CO molecules per nm , which decreased to 1.1
CO molecules per nm of Ag–ZrO
that CO -derived species were above the ZrO
above Ag.
2
-derived sites on the ZrO (0.100 g)
2
2
2
2
2
2
pretreated under vac-
at 295 K for 2 h,
2
2
(Figure 2). This demonstrates
surface and not
2
1
2
2
−
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
peaks appeared at 1,624; 1,421; and 1,220 cm , which were as-
signed to bicarbonate species (monodentate or bridging; Figure
3
6
−1
3A1, Left). Simultaneously, the ZrO
decreased due to a reaction with CO
2
hydroxy peak at 3,700 cm
while the bicarbonate hy-
2
−
1
droxy peak increased at 3615 cm (Figure 3A1, Right). Much
weaker shoulder peaks, due to carbonate species, also appeared at
,553 and 1,334 cm . When exposed to vacuum for 30 s and 7
min, the peaks, due to the bicarbonate species, decrease by 1/2–
/3 (Figure 3A2, 3, Left). Since carbonate species were present in
the pretreated sample under vacuum (data not shown), the changes
under both CO and vacuum were minimal and we, therefore, re-
−
1 37
1
2
2
gard carbonate as inert.
We then monitored FTIR changes associated with UV–visible
1
3
light irradiation (Figure 3B). At 2.3 kPa of CO
2
and 21.7 kPa of
H
2
irradiated by UV–visible light for 2 h, the background level, e.g.,
−
1
the level in the wavenumber region of 3650–3500 cm , increased
as a result of the IR excitation of UV–visible-excited electrons
trapped beneath the conduction band of ZrO (Figure 3B2, 3)
2
7,38
2
while the vibrational peaks of surface species showed negligible
change before and after irradiation by UV–visible light. A similar
trapping state in CdSe during IR excitation and further hot electron
Figure 1. Time course formation of photocatalytic 13CO and 12CO
39
during exposure to 1 CO
2
injections from LSPR Au nanoparticles were reported. The anti-
2
(2.3 kPa) and H
2
(21.7 kPa) using Ag
symmetric and symmetric stretching vibration peaks at 1,624 and
(
5.0 wt. %)–ZrO (0.100 g) irradiated by (A) full UV–visible light
2
−1
−1
1
,421 cm for bicarbonate and at 1,553 and 1,334 cm for car-
and (B) filtered light at λ > 320 nm.
bonate in natural CO (Figure 3A, Left) shifted to 1,588 and 1,389
2
−
1
−1
13
cm and 1,518 and 1,304 cm in CO
2
and H (Figure 3B, Left),
2
respectively, based on the following equation for harmonic oscilla-
2
4
tion :
1
k
ν! =
2
πc
µ
(
(
8)
9)
1
1
16
1
+
+
ν! 13CO
ν! 12CO
13
1
=
= 0.97778
1
2
16
where ν is the wave number, c is the speed of light, k is the force
−
1
constant, and μ is the reduced mass. We chose a peak at 1389 cm
to evaluate the amount of change in the bicarbonate species. Under
vacuum for 90 s, the bicarbonate peaks decreased to one third of
13
their intensity under CO
2
and H
2
(Figure 3B1, 2), which corre-
13
12
sponds to the amount of exchange between CO
2
and CO
2
(2.3
μmol per 0.100 gcat; Figure 2A). The chemisorbed bicarbonate
species, which were exchangeable in gaseous CO , became bridged
while the bicarbonate species desorbed under vacuum since CO
2
2
3
6
monodentately coordinates to the Zr site (Scheme 1b, c). Similar-
ly, isotope labeled bicarbonate exchanged with CO , which was
2
Figure 2. Time course exchange reaction of 13CO
(0.67 kPa) irra-
2
monitored using attenuated total reflectance IR. Previous studies
have suggested these as an intermediate to CO formation above
diated by UV–visible light using (A) ZrO
ZrO . Photocatalyst used was 0.100 g.
2
and (B) Ag (5.0 wt. %)–
4
0
2
Au.
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3
We also confirmed that the amount of exchange of CO
2
and rate
versus CO detected by GCMS (0.001 9 μmol). Only 1.1% of the
1
1
3
3
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
constants (simple adsorption, exchange) negligibly varied between
the powder (Figure 2) and disk samples (Figure 3). When exposed
to vacuum and UV–visible light for 24 h, the bicarbonate peaks
further decreased to 85% of their intensity under vacuum for 90 s
(Figure 3B2, 3). The decreased amount was:
C-bicarbonate species along the ZrO
CO because of the simultaneous occurrence of a reverse reaction
to CO
umn, desorbed CO
GCMS and the total mass balance starting from bicarbonate was
not confirmed. The amount of reduction of CO
be related to that of surface O vacancies in ZrO
apparatus-based IR light on photocatalysis should be negligible
because no reduction in CO was observed, even in the presence of
(see the following wavelength dependence section, Table 1A-
surface photoconverted to
2
1
3
2
. Due to the limitations of the 13X-S molecular sieve col-
13
2
could not be quantitatively evaluated using
1
3
13
2
to CO should
0
0
.050
.100
13
2
.3 µmol×
×0.15 = 0.17 µmol- CO H
3
2
. The effects of
(
10)
2
Scheme 1. Proposed intermediate species starting from CO
2
and
H
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
H
2
to CO during CO exchange and photocatalytic CO reduction.
2
2
d”) nor was significant amount of heat induced in comparison to
LSPR-induced heat as a result of UV–visible irradiation (see the
following EXAFS section, Table S2-e).
We also monitored the behavior between CO
wt. %)–ZrO photocatalyst using FTIR. The peak position and
change in intensity during CO adsorption at 295 K and subsequent
evacuation (Figure 3C) were similar to the changes observed when
using ZrO (Figure 3A), which demonstrates that both the bicar-
bonate and carbonate species were above the ZrO surface, not the
2
and the Ag (5.0
2
2
2
2
Ag surface. A relatively greater portion of bicarbonate species re-
mained (3/5–1/3) under vacuum for 30 s and 7 min (Figure 3C)
in comparison with the ratio using the ZrO
the amounts of exchanged (chemisorbed) CO
μmol using ZrO and Ag (5.0 wt. %)–ZrO , respectively (Figure 2).
was irradiated using UV–
visible light for 2 h (Figure 3D), we observed a perfect isotope shift
similar to the shift observed using ZrO (Figure 3B and equation 9).
However, the intensity of the bicarbonate peak decreased by ap-
2
(Figure 3A) based on
2
, i.e., 2.3 and 3.5
2
2
13
At 2.3 kPa of CO
2
and 21.7 kPa of H
2
2
13
proximately 50% when exposed to CO
with CO , which suggests the presence of H
spillover onto the ZrO surface (Scheme 1d). When exposed to
2
and H
2
in comparison
2
2
activation on Ag and
2
vacuum and UV–visible light for 24 h, the bicarbonate peaks fur-
ther decreased to 89% of their intensity observed under vacuum for
90 s (Figure 3D2, 3). The decreased amount was:
0
0
.050
.100
13
3
.5 µmol×
×0.11= 0.19 µmol- CO H
3
(
11)
1
3
versus CO detected by GCMS (0.029 μmol). Fifteen percent of
1
3
13
the CO
2
at the Ag–ZrO
2
surface photoconverted to CO. Clearly,
Ag promoted intermediate species reduction to CO associated with
H supplied from Ag and O vacancy sites in comparison with the
ratio (1.1%) using ZrO associated with only O vacancy sites.
2
Figure 3. FTIR spectra of ZrO
ZrO (50 mg; C, D). (A, C) Under CO
um for 30 s (2) and 7 min (3). (B, D) Under UV–visible light,
2
(50 mg; A, B) and Ag (5.0 wt. %)–
2
2
for 2 h (1) and under vacu-
1
3
CO
2
(2.3 kPa), and H (21.7 kPa) for 2 h (1) and under vacuum
2
Figure 4. Diffuse reflectance UV–visible spectra for the Ag–ZrO
samples. The Ag contents were 0, 0.50, 3.0, 5.0, and 10 wt. %.
2
for 90 s (2) and 24 h (3).
5
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In the UV–visible spectra for the Ag–ZrO
2
samples, a peak cen-
pressed, i.e., by as much as 31%, during a 100 min period of light
irradiation (Figure 5C). On the contrary, peak intensity quickly
recovered when the light was turned off.
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
tered at 428 nm was dominant between an Ag composition of 0.5
and 3.0 wt. %, whereas shoulder features centered at 368 and/or
470 nm increased at elevated Ag contents, i.e., 5.0–10 wt. % (Figure
4). These peaks are due to LSPR induced by UV–visible light
where the Ag particle size distribution became wider with increased
Ag content. The absorption edge was always at 248 nm (Figures 4
Such light-induced changes in the FT were quantitatively evalu-
ated using a curve fit analysis based on the plane-wave approxima-
tion for amplitude A
factor σ, and mean free photoelectron path λ for shell i using a
XDAP 2.2.7 code:
i
(k), backscattering amplitude f, Debye–Waller
i
43
i
and S1), which indicates that the ZrO
2
BG is 5.0 eV.
29
As the response to UV light absorption, ZrO
2
samples showed
broad and sharp fluorescence peaks at 370 and 396 nm, respective-
ly, with excitation at 200 nm (Figure S5A-a), whereas the peaks
were almost extinguished with excitation at 240 nm (Figure S5A-b).
The excitation spectra for the two peaks (Figure S5B) and the
wavelength dependence of the Xe arc lamp (Figure S1) suggested
contribution of light at 200 < λ < 248 nm to charge separation in
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
ZrO
nm-ZrO
tion wavelength was ascribed to the transition involving extrinsic
2
(Scheme 1). A fluorescence peak was reported for mean 4
2
with excitation at 300 nm and an extension of the excita-
4
1
states. Such an effect involving O vacancy states is also plausible
under the conditions using H
study (Scheme 1c).
2
and UV–visible light shown in this
Formation rates of CO using ZrO
2
and Ag (5.0 wt. %)–ZrO ir-
2
−
1
−1
radiated under full light (0.15 and 0.57 μmol h gcat ; Table 1A-a,
−
1
−1
d) decreased to 0.027 and 0.17 μmol h
g
cat , respectively, under
light at λ > 320 nm (Table 1A-a’, d’). The rate using Ag (5.0
wt. %)–ZrO
.013 μmol h
2
irradiated by light at λ > 580 nm further decreased to
−1
−1
0
g
cat (Table 1A-d”). Therefore, for ZrO
2
, the CO
formation rate decreased by 82% with the filtration of the excitation
light at λ > 320 nm in comparison to a photocatalytic test irradiated
under full UV–visible light (Table 1A-a, a’), i.e., UV light at λ < 248
nm to separate the charges at the ZrO
significant photocatalytic role in the reduction of CO
the Ag (5.0 wt. %)–ZrO , the CO formation rate decreased by 70%
2
BG (Figure 4) played a
2
(82%). For
2
and 98% with the filtration of the excitation light at a λ > 320 nm
and λ > 580 nm, respectively, in comparison with tests under full
UV–visible light (Table 1A-d, d’, d”). UV light at λ < 248 nm sepa-
rated the charges at the ZrO
2
BG and UV–visible light at 330 < λ <
5
80 nm induced Ag LSPR (Figures 4 and S1), which also played a
significant role in the photocatalytic reduction of CO
and 28%, respectively).
2
(i.e., 70%
Total CO formation rates in the blank tests when exposed to
1
3
−1
−1
CO
ZrO
se rates proceeded thermally (295 K,
2
, H
2
, and no light were 0.0062 and 0.013 μmol h gcat using
2
and Ag (5.0 wt. %)–ZrO
2
, respectively (Table S1A-a, b). The-
1
2
RT =1.2 kJ mol−1 ) but were
only 4.2% and 2.2%, respectively, of the corresponding rates under
1
3
UV–visible light (Table 1A-a, d). The other blank test under CO
and UV–visible light using Ag (5.0 wt. %)–ZrO formed no prod-
ucts above GCMS detection limits (Table S1B-a).
Ag K-edge EXAFS spectra were monitored for Ag (3.0 or 5.0
wt. %)–ZrO under CO (2.3 kPa) and H (21.7 kPa) irradiated
2
2
Figure 5. The time course changes of (A) coordination number N
values, (B) Debye–Waller factor σ values, and (C) FT obtained
2
2
2
3
from angular photoelectron wave number k -weighted Ag K-edge
using UV–visible light at beamline. In the Fourier transform (FT;
Figure 5C), a peak due to a Ag–Ag interatomic scattering of photo-
electrons dominated at 0.28 nm (phase shift uncorrected), which
EXAFS χ-function for Ag (3.0 wt. %)–ZrO
and H (21.7 kPa) irradiated by UV–visible light for 100 min fol-
lowed by dark conditions for 20 min. (D) The change in the FT of
the Ag K-edge EXAFS for fresh Ag–ZrO : 3.0, 5.0, and 10 wt. % Ag.
E) The correlation between the σ value and temperature for bulk
2
under CO
2
(2.3 kPa)
2
25
demonstrates that the Ag nanoparticles were exclusively metallic.
2
In fact, in the HR-TEM images, we observed the metallic Ag(1 1 0)
lattice fringes (i.e., the interval 0.287 nm versus the theoretical
(
3
0
sites (circle, ꢀ) and surface sites (vertical motion; square, ꢁ)
in/on the Ag metal generated by the correlated Debye model using
a FEFF8 code.
0
.289 nm ) to be in contact with the tetragonal ZrO
2
(1 0 0) lattice
fringe (i.e., the interval 0.364 nm versus the theoretical 0.364 nm;
4
2
Figure 6). The EXAFS FT peak intensity was significantly sup-
6
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1
heat transformation. Similarly to the result using Ag (3.0 wt.%)–
ZrO above, the initial temperature increased to 363 K after a 90
min UV–visible light irradiation and quickly dropped to 297 K
once the light was off using the Ag (5.0 wt. %)–ZrO sample (Table
1
2
3
4
5
6
7
8
9
2
2
S2c). Related to the EXAFS method, the nanoparticle temperature
was also monitored based on surface-enhanced Raman scattering
5
2
for the NꢁC stretching vibration of adsorbed probe molecules.
The method based on the σ value used in this study via EXAFS is
common and can directly probe for all nanoparticles to expose
LSPR, e.g., Au and Ag. Three-dimensional mapping of Ag nanopar-
ticles was performed via electron energy loss spectroscopy as a
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
5
3
function of energy loss.
Furthermore, we also observed increases or decreases in temper-
ature under argon and UV–visible light (367 K at 50 min of light,
3
02 K 10 min after light had been turned off; Figure S6B and Table
S2b), demonstrating that the ambient gas was not a major factor for
temperature up/down. The σ value/temperature negligibly
changed until 329 K when the incident light wavelength was fil-
tered at more than 580 nm (Table S2e), whereas an essentially
identical rise/drop during full light (365 K at 90 min, 298 K when
light was off) for the σ value/temperature was observed when λ was
more than 320 nm (Table S1d), demonstrating the heating effect
by infrared portion of light were marginal in comparison to that by
light whose wavelength was in the range 320 < λ < 580 nm via
LSPR of Ag (Figures 4 and S1). Reaction heat was not a factor in
Figure 6. HR-TEM image of Ag (5.0 wt. %)–ZrO .
2
Ni
⎡
⎛
2
2
R ⎞⎤
i
Ai (k) =
f (k) exp −2⎜σ k + ⎟ , i = Ag
i i
⎢ ⎥
2
kRi
⎣
⎝
λ ⎠⎦
(
12).
The N value was 9.0 ± 0.7 before light irradiation and did not
change significantly during both light irradiation (8.3–10.0) and
after the light was off (10.1 ± 0.7; Figure 5A). N values of 9–10
correspond to a particle size (d) of 2.5–3.7 nm assuming the spher-
ical face-centered cubic (fcc) nanoparticle model and that surface
this study because the reaction that reduces CO
) is endothermic. The heating and reaction promotion was re-
ported for the exothermic formation of methane from CO using
2
to CO (equation
2
2
3
1
Ni-based photocatalysts. The temperature dropped to the same
level (i.e., 290–302 K) essentially in 10–35 min after UV and/or
visible light turned off (Table S2a–e) in comparison to 286 K be-
fore UV and/or visible light irradiation. This fact guarantees that
the heat originating from probe synchrotron X-ray was negligible in
this study.
44,45
dispersion (D) is 0.54–0.36 (mean value = 0.45).
The σ value was calculated to be 0.00995 nm for Ag metal at 290
4
6,47
K using the correlated Debye model with the ab initio multiple-
48
scattering calculation code, FEFF8 and the Debye Ag tempera-
49
ture [θD(Bulk) 225 K]. The XDAP code provides an experimental
2
Ag nanoparticles were always metallic for the Ag–ZrO
2
samples
difference for the σ value from that of the Ag metal (model) based
(Figure 5D), and N values for the fresh Ag–ZrO samples gradually
2
on equation 12. Initial σ values of 0.0101 nm for Ag (3.0 wt. %)–
increased from 9.0 ± 0.7, 9.6 ± 0.6, and 10.7 ± 0.8 for 3.0, 5.0, and
ZrO before light irradiation quickly increased to 0.0107 nm (10
2
10 wt. % Ag, respectively, which corresponds to (d, D) sets of (2.5
min irradiation) and progressively increased to 0.0117 nm (90 min
light irradiation, shown in Figure 5B). Then, the value quickly de-
creased to 0.0101 nm after the light was turned off at 112 min.
4
4,45
nm, 0.54), (3.1 nm, 0.43), and (4.2 nm, 0.28). Thus, Ag surface
−
1
atoms increase from 0.15 to 0.20 and finally to 0.26 mmol gcat
,
respectively, whereas the CO formation rates reached maximum
values at 5.0 wt. % Ag (Table 1A and Figure S3), which indicates
Furthermore, we evaluated the temperature at the Ag site based
on the σ values. The σ value temperature dependence derives from
the FEFF8 combined with the correlated Debye model
4
6,47
that the ZrO
2
sites were primarily responsible for CO reduction
2
for both
and were assisted by heated neighboring Ag sites via LSPR that
activated and supplied H to move forward equation 2 (Scheme 1d).
bulk and surface Ag sites using the bulk and surface Debye temper-
ature (Figure 5E-a and b, respectively). We assumed that the ther-
modynamically stable fcc(1 1 1) face had preferable exposure for
The close correlation between Au LSPR and H
2
activation was
reported using the Au–SiO
2
photocatalyst due to the charge excita-
5
0
latter value [θD(Surf,ꢀ) 155 K]. We also approximated the mean Ag
54
tion to the untibonding H
2
orbital from LSPR.
H
2
activation via
31
nanoparticle temperature as the arithmetic mean temperature
the Ni–H species above the Ni/SiO
2
·Al photocatalyst relates
2
O
3
based on the θD(Surf,ꢀ) weighted by 1/2·1/3D [D: dispersion of na-
noparticles (0.45), for an effective vertical degree of freedom at a
free hemisphere surface] and that based on the θD(Bulk) weighted by
(1–D) + 1/2D + 1/2·2/3D (bulk site, non-free hemisphere in con-
to the mechanisms discussed in this study, i.e., endothermic CO
31
formation, versus exothermic methane formation.
In the Ag–ZrO
2
UV–visible absorption spectrum, peaks appear
centered at 428 and 368 nm (2.90–3.37 eV) due to a LSPR at Ag
tact with ZrO , and two lateral degrees of freedom at a free hemi-
2
5
5
(
Figure 4). The Ag work function is 4.52–4.74 eV, i.e., a Fermi
level (E ) of 0.08–0.30 V for a standard hydrogen electrode versus
the reduction potential from CO
sphere surface) (see Supporting Information for the detail).
F
As a result, the initial temperature of 286 K before irradiation ro-
se to 325 K after 10 min of irradiation and progressively increased
to 392 K after 90 min of light irradiation (Figure 5B and Table S2a).
The temperature quickly dropped to 290 K after the light was
turned off. Such rise/drop in temperature is possibly due to LSPR
1
2
to CO (−0.11 V) or ZrO con-
2
13
duction band minimum (−1.0 V). For nanoparticles to exhibit
LSPR behavior, previous reported that the following mechanisms
need to occur: (i) charge excitation to an unoccupied adsorbate
54,56
state,
(ii) hot electron injection that originates from LSPR for
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9,57–60
support,
(iii) electron trapping from the support to the
In this study, the Ag (5.0 wt. %)–ZrO
2
photocatalyst was the
and UV–visible light
5
9
13
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
3
3
3
4
4
4
4
4
4
4
4
4
4
5
5
5
5
5
5
5
5
5
5
6
Schottky barrier, and (iv) plasmonic resonant energy transfer
most active during CO
at a rate of 0.66 μmol h
reduction using H
2
−1
2
5
9–61
−1
(PRET).
how holes remained react after electron activation/reaction be-
cause the O moiety that derives from CO must transfer to the
LSPR nanoparticles for oxidation. In this study, H oxidizes based
on its association with the reduction of bicarbonate species above
However, for cases i–iii it is often difficult to explain
g
cat . The product included 5.5–8.6%
1
2
CO. However, this CO derives from preadsorbed CO from the
2
1
3
−1
2
air that exchanged with gas-phase CO
FTIR spectra demonstrated bridging and monodentate bicar-
bonate species via ZrO surface hydroxy group consumption under
photoreaction CO and/or H conditions. The bridging bicar-
bonate species were exchangeable with the gas-phase CO and O
vacancy on the ZrO surface should have participated in its for-
mation. Under H and UV–visible light with the expense of bridg-
ing bicarbonate, 15% was directed to CO formation whereas the
2
at a rate constant of 2.0 h .
2
2
+
ZrO
2
. Therefore, holes react with H
react with O that derives from CO
surface. Thus, Ag thermally contributes for CO generation when
heated to 325 K–392 K (Figure 5B), i.e., converted from LSPR (H
easily dissociates on the heated Ag surface and spills over the ZrO
surface) (Scheme 1d, e).
2
to form H and must further
2
2
2
to form water above the Ag
2
2
2
2
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
1
3
2
1
3
remaining proportion reversely desorbed CO
2
.
The conversion of LSPR over Ag and Au owing to heat preceded
phenol decomposition and thiosulfate oxidation, whereas previ-
The charge separation contribution at the ZrO BG and the Ag
2
6
2
63
contribution characterized by LSPR were evaluated based on in-
profile kinetic data measurements using sharp-cut filters: 70 and
28%, respectively. We further investigated the contribution of the
mean 2.5–3.7 nm Ag nanoparticles using in situ EXAFS. A rise in
temperature from 286 to 392 K when exposed to UV–visible light
irradiation and a rapid drop to 290 K under dark conditions were
directly monitored based on the Debye–Waller factor change for a
ous studies considered that short-lived heat converted from LSPR
6
4
over Ag promotes reactant diffusion in the aqueous solution. This
study is the first to report the conversion of CO assisted by heat
2
that was converted via LSPR. Peaks due to formate species were
not detected in our FTIR spectra, and we are unable to find evi-
14,65
dence that the formate was an intermediate to CO.
The H and
bicarbonate species coupled with the hole and electron, respective-
ly, and combine to form CO and water, which is enhanced by a
factor of 3.9 due to Ag (Table 1A-a, d). The rise in the Ag tempera-
Ag–Ag interatomic pair interference. H
association with bicarbonate species reduction above ZrO
evidence for available O on Ag was found in this study. Thus, the
heated Ag surface activated H , and the H atoms spill over to the
bicarbonate species above ZrO rather than hot electron injection
or PRET to/with ZrO
Such dual roles for light during charge separation at the BG and
heat via LSPR were remarkably effective when using CO and H
Instead, we found that using CO and moisture disadvantageously
yielded reversely formed H over Ag. However, H formation was
redirected toward CO formation via the addition of Mg as the
CO -anchor sites. The combination of alkaline [earth] metal ion–
LSPR metal (Ag, Au)–ZrO is required for artificial photosynthesis
2
should oxidize based on its
2
, but no
ture in the ~0.5 h experiment via LSPR (Figure 5B) and H
2
activa-
2
tion effects (Scheme 1) agree with the ~0.5 h induction period for
CO formation using Ag-containing photocatalysts (Figure S2, Left).
2
2
.
Finally, we tested the photocatalytic conversion of CO
2
using
(2.3 kPa), moisture (2.7
kPa), and UV–visible light, the CO generation rate increased by a
13
moisture (equation 3). Using ZrO
2
, CO
2
2
2
.
2
1
3
factor of 1.2 compared with using CO
However, the molar ratio of the newly formed CO:H
rather than 1:1 based on equation 3. Since 90% of the newly
2
and H
2
(Table 1A-a, B-a).
2
2
2
+
2
was 1:0.41
2
1
2
formed CO was CO, the bidentate bicarbonate species c was
more stable above ZrO (Scheme 1), forming CO gas and a hy-
droxy group. Conversely, using the Ag (5.0 wt. %)–ZrO photo-
catalyst, the CO formation rate was suppressed by 96% compared
with the formation rate when using ZrO (Table 1B-a, b) and by
9% when using CO and H (Table 1A-d). Instead, H preferably
formed at 0.15 μmol h gcat . Based on the effects of Ag, H selec-
2
2
free from C-impurity problems and using the dual roles of light:
charge separation and heat via LSPR.
2
2
ASSOCIATED CONTENT
Supporting Information
Supporting Information includes intensity distribution of Xe arc lamp,
detailed data from the photocatalytic tests and data from control exper-
iments, absorption–fluorescence spectra, theoretical details and syn-
chrotron X-ray experiments under Ar, a summary of the Ag tempera-
ture evaluation based on the Debye–Waller factor, and time course as
9
2
2
2
−
1
−1
2
tivity in the reduced products increased from 29% to 95% with
equation 1 (water splitting) rather than equation 3 due to the pref-
erential adsorption of water versus CO
2
on ZrO
2
and H desorption
2
above Ag (Scheme S1). Water that blocks the O vacancy site is also
plausible. Once reaction intermediates, i.e., H and bicarbonate
well as the proposed reaction mechanisms under CO and moisture.
2
form under light, CO and H are slowly formed via thermal energy
2
The Supporting Information is available free of charge on the ACS
Publication website.
12
under dark at 295 K (Table S1C-a’). In contrast that CO was
preferably formed among total CO under light (61%; Table 1B-b
13
and Figure S7, Left column), CO became a major product (68%,
AUTHOR INFORMATION
Corresponding Author
* To whom correspondence should be addressed. E-mail yizu-
mi@faculty.chiba-u.jp.
Table S1C-a’ and Figure S7, Right column) under dark due to the
1
2
gradual consumption of preadsorbed bicarbonate ( C 98.9%) that
1
3
is in the equilibrium with gas-phase CO
2
(Figure 2B). The addi-
tion of Mg onto the Ag (5.0 wt. %)–ZrO to attract CO to the
surface mitigated the water blocking problem. Therefore, the CO
2
+
2
2
Author Contributions
The authors declare no competing financial interest. YI, HZ, TI, and
TK contributed equally. The manuscript was written by all.
−
1
−1
formation rate increased from 0.0080 to 0.093 μmol h gcat , i.e., a
factor of 11.7, via the addition of Mg similar to the MgO–Pt–
2
+
6
6
TiO
2
photocatalysts (Table 1B-c).
ACKNOWLEDGMENTS
4
. CONCLUSIONS
8
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Journal of the American Chemical Society
The authors are grateful for financial support from the Grant-in-Aid for
(16) Ahmed, N.; Shibata, Y.; Taniguchi, T.; Izumi, Y. Photocatalytic
Conversion of Carbon Dioxide into Methanol Using Zinc–Copper–M(III)
M = Aluminum, Gallium) Layered Double Hydroxides. J. Catal. 2011,
79, 123–135.
17) Kawamura, S.; Zhang, H.; Tamba, M.; Kojima, T.; Miyano, M.; Yo-
shida, Y.; Yoshiba, M.; Izumi, Y. Efficient Volcano-type Dependence of
Photocatalytic CO Conversion into Methane Using Hydrogen at Reaction
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
3
3
3
3
3
3
3
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4
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4
4
4
4
4
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5
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5
5
5
6
Scientific Research C (17K05961) from the Japan Society for the Pro-
motion of Science, Grant for Academic Research from the Japan Gas
Association (2018), and Leading Research Promotion Program
(
2
(
(
2015–2019) from the Institute for Global Prominent Research, Chiba
University. X-ray absorption experiments were performed with the
approval of the Photon Factory Proposal Review Committee
2
Pressures up to 0.80 MPa. J. Catal. 2017, 345, 39–52.
(18) Zhang, H.; Kawamura, S.; Tamba, M.; Kojima, T.; Yoshiba, M.;
(
2016G577, 2018G649). The authors thank Dr. K. Niki, Department
of Chemistry, Chiba University, for helpful discussion on LSPR. The
authors would like to thank Enago (www.enago.jp) for the English
language review.
Izumi, Y. Is Water More Reactive Than H
2
in Photocatalytic CO Conver-
2
sion into Fuels Using Semiconductor Catalysts under High Reaction Pres-
sures? J. Catal. 2017, 352, 452–465.
0
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19) Tian, Y.; Tatsuma, T. Mechanism and Applications of Plasmon-
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