A ZnTa2O6 photocatalyst synthesized: Via solid state reaction for conversion of CO2 into CO in water

Article abstract of DOI:10.1039/c6cy00271d

Because of the environmental problems and the resulting exigent demand for CO₂ recycling processes, great attention is being paid to the photocatalytic conversion of CO₂ into useful chemicals such as CO, HCOOH, HCHO, CH₃OH, and CH₄. We have previously reported that the Ag-loaded, Zn-modified Ga₂O₃ photocatalyst exhibits excellent photocatalytic activity required for the conversion of CO₂ into CO by using H₂O as a reductant and that the Ag particles that exist together with the Zn species act as good cocatalysts for the selective formation of CO. In this study, we demonstrated the photocatalytic activity of ZnTa₂O₆ under UV light irradiation, which was prepared via solid-state reaction, for the conversion of CO₂ in an aqueous NaHCO₃ solution. A Ag cocatalyst-loaded ZnTa₂O₆ photocatalyst evolved CO as a reduction product of CO₂ with 46% selectivity toward CO evolution among the reduction products. In contrast, when Pt and Au were introduced as cocatalysts, the ZnTa₂O₆ photocatalyst evolved H₂ with high selectivity (>99.9%).

Full text of DOI:10.1039/c6cy00271d

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ZnTa2O6 photocatalyst synthesized via solid state reaction for
conversion of CO2 into CO in water †
Shoji Iguchi,a Kentaro Teramura,*ab Saburo Hosokawa,ab and Tsunehiro Tanaka*ab
Received 00th January 20xx,
Accepted 00th January 20xx
Because of the environmental problems and the resulting exigent demand for CO2 recycling processes, great attention is
being paid to the photocatalytic conversion of CO2 into useful chemicals such as CO, HCOOH, HCHO, CH3OH, and CH4. We
have previously reported that the Ag-loaded, Zn-modified Ga2O3 photocatalyst exhibits excellent photocatalytic activity
required for the conversion of CO2 into CO by using H2O as a reductant, and that the Ag particles that exist together with
the Zn species act as good cocatalysts for the selective formation of CO. In this study, we demonstrated the photocatalytic
activity of ZnTa2O6 under UV light irradiation, which was prepared via a solid-state reaction, for the conversion of CO2 in an
aqueous NaHCO3 solution. Ag cocatalyst-loaded ZnTa2O6 photocatalyst evolved CO as a reduction product of CO2 with 46%
of the selectivity toward CO evolution among the reduction products. In contrast, when Pt and Au were introduced as
cocatalysts, ZnTa2O6 photocatalyst evolved H2 with high selectivity (> 99.9%).
DOI: 10.1039/x0xx00000x
www.rsc.org/
We previously reported that ZrO2,8 11 MgO,12,13 and
−
Ga2O3,14 which can act as solid base materials, were active for
the photocatalytic conversion of CO2 in the presence of H2 as a
reductant. Detailed analyses of the adsorbed species formed
Introduction
Artificial carbon cycling systems based on the catalytic
conversion of CO2 into useful carbon sources such as CO,
HCOOH, HCHO, CH3OH, and CH4 are receiving a great deal of
attention because of environmental issues such as global
on the surface of these photocatalysts showed that the
adsorption of a CO2 molecule on the photocatalyst material is
an essential step for the photocatalytic conversion of CO2.13,14
Moreover, the photocatalytic activities of layered double
hydroxides (LDHs), which have high water-tolerant base sites,
for the conversion of CO2 in water were found.15,16 In
parꢀcular, the use of a Ni−Al LDH enables the selecꢀve
evolution of CO as a reduction product of CO2.17 Another
method for the selective conversion of CO2 is the introduction
of a metal cocatalyst onto the photocatalyst. Our research
group also found that the Zn-modified Ga2O3 photocatalyst
exhibited excellent activity for the conversion of CO2 into CO
and O2 in the presence of Ag as a cocatalyst, with
stoichiometric amounts of O2 being formed as an oxidation
product of H2O.18,19 We presume that Ag particles, in
combination with the zinc species, can act as an effective
cocatalyst for the conversion of CO2 to CO, because the
consumption of the photogenerated electrons by H+ derived
from H2O was completely suppressed. These results indicate
that a Ag cocatalyst in combination with a Zn-based mixed
oxide semiconductor photocatalyst should enable the
conversion of CO2 with high selectivity under photoirradiation.
In this study we focused on the zinc tantalum mixed oxide
as a photocatalyst for the conversion of CO2 in water. A series
warming and natural resource depletion.1 3 In heterogeneous
−
catalysis, it is generally necessary to introduce a reducing
reagent such as H2 or a hydrocarbon at a high temperature in
order to reduce CO2.3,4 However, H2O is a more favorable
reductant, because it is harmless and is an abundant source of
protons. The photocatalytic conversion of CO2 by means of
H2O, also known as artificial photosynthesis, is considered an
uphill reaction involving photoexcited electrons and positive
charge holes.5,6 H2 evolution via reduction of protons (H+)
takes place competitively during the photocatalytic conversion
of CO2 in water, because the standard reduction potential of
CO2 (E°(CO2/CO) = −0.11 V vs. SHE)7 is more negative than that
of H+ (E°(H+/H2) = 0.0 V vs. SHE).7 In order to convert CO2 to CO,
rather than H2 evolution from H+, the photocatalyst surface
should be design in such a way that it helps improve the
selectivity of photogenerated electrons toward CO2 reduction.
The primary strategy for designing such a photocatalyst is to
provide suitable surface properties to enable the absorption of
CO2.
of isostructural zinc tantalum mixed oxides: ZnTa2O6,20 22
−
a.Department of Molecular Engineering, Graduate School of Engineering,
Kyoto University, Kyotodaigaku Katsura, Nishikyo-ku, Kyoto 615−8510, Japan.
E-mail: teramura@moleng.kyoto-u.ac.jp (K.T.), tanakat@moleng.kyoto-u.ac.jp (T.T.);
Fax: +81−75−383−2561; Tel: +81−75−383−2559
Zn3Ta2O8,23 and Zn4Ta2O9,24 have been successfully synthesized.
20
Moreover, the photocatalytic activities of ZnTa2O6
and
Zn3Ta2O8 23 have been reported. Kato and Kudo (1998) showed
the photocatalytic activity of ZnTa2O6 for water splitting in
b.Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University,
1-30 Goryo-Ohara, Nishikyo-ku, Kyoto 615−8245, Japan.
† Electronic Supplementary Information (ESI) available: See DOI: 10.1039/x0xx00000x
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their summary of various tantalate photocatalysts; NiO-loaded photodeposition, and chemical reduction methods. The
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ZnTa2O6 evolved H2 at a rate of 15 μmol h−1 and O2 at a rate of cocatalyst loading procedures of these methods are described
DOI: 10.1039/C6CY00271D
6 μmol h−1 from distilled water under UV light irradiation.25 As below based on the case of Ag. For the impregnation methods,
reviewed by Kudo et al., tantalum-based mixed oxides an aqueous AgNO3 solution including ZnTa2O6 powder was
(MxTayOz) have excellent photocatalytic abilities, where M = Li, thoroughly evaporated at 353 K, and calcined at 723 K for 2 h
Na. K, Mg, Ca, Sr, Ba, Ni, Rb, or La, and showed stoichiometric in air. For the photodeposition method, ZnTa2O6 powder was
decomposition of H2O into H2 and O2 (H2:O2 = 2:1).25 Takayama dispersed in 1.0 L of ultra-pure water containing a required
et al. reported that KCaSrTa5O15 photocatalyst, which shows amount of AgNO3, and the dissolved air in the solution was
good activity for the overall splitting of water using NiO as a completely degassed by a flow of He gas. The suspension was
cocatalyst, exhibited good activity for the reduction of CO2 in irradiated under a 400 W high-pressure Hg lamp with a quartz
water when Ag was loaded as a cocatalyst. 26 In contrast to the filter using an inner-irradiation-type reaction vessel with He
successful investigation of complete photocatalytic water gas flowing. For the chemical reduction method, an aqueous
splitting, the investigation on the photocatalytic conversion of solution of NaPH2O2 was added to a suspension of the ZnTa2O6
CO2 remains incomplete. In the present study, we demonstrate containing a given amount of AgNO3. After stirring at 353 K for
the photocatalytic activity of ZnTa2O6, synthesized via a solid- 2 h, the modified photocatalyst was filtered and washed with
state reaction method, for the conversion of CO2 to CO in an 1.0 L of ultra-pure water, and then dried at room temperature.
aqueous solution under UV irradiation.
Catalyst characterization
X-ray diffraction (XRD) patterns of prepared samples were
collected by using an X-ray diffractometer (Multi Flex, Rigaku),
Experimental Section
using Cu Kα radiation (λ = 0.154 nm) at a scan rate of 4 ° min−1.
Specific surface areas of the photocatalysts were estimated
from their N2 adsorption isotherms at 77 K using an adsorption
analyzer (BELSORP-miniII, BEL Japan, Inc.). Prior to the
measurements, each sample was evacuated at 673 K for 1 h
using a pretreatment system (BELPREP-vacII, BEL Japan, Inc.).
UV/Vis diffuse reflection spectra of synthesized materials were
measured using a UV-Visible Spectrometer (V-650, JASCO)
equipped with an integrated sphere accessory. Zn K-edge and
Ta LIII-edge XAFS spectra of ZnTa2O6 were collected at the
BL01B1 of SPring-8 with the approval of the Japan Synchrotron
Radiation Research Institute (JASRI). The spectra were
recorded in a transmittance mode at room temperature, using
Si(111) double crystal monochromator. The photon energy
was calibrated by using Zn foil. SEM images were captured
using a Field Emission Scanning Electron Microscope (FE-SEM,
SU-8220, Hitachi High-Technologies) at an acceleration voltage
of 3.0 kV. FT-IR spectra of adsorbed species on a ZnTa2O6 were
recorded with a FT-IR spectrometer (FT/IR-4000, JASCO) in a
transmission mode at room temperature. A ZnTa2O6 sample
(ca. 100 mg) was pressed into a wafer (diameter = 10 mm) and
placed in an in-situ IR cell equipped with CaF2 windows. The
cell enabled to perform evacuation, heating, O2 treatment,
introduction of CO2, and measurement of spectra in situ. Prior
to measurement, the sample was evacuated at 673 K. followed
by treatment with 5 kPa of O2 for 1 h and evacuation for 15
min at 673 K. The background spectrum was obtained after the
pretreatment at room temperature under evacuation. The
data from 128 scans were accumulated at a resolution of 4
cm−1.
Catalyst preparation
ZnTa2O6 photocatalyst was prepared via a solid state
reaction with referring the previous report.22 The calculated
amount of high purity Ta2O5 powder (Kojundo Chemical
Laboratory) was added into an aqueous solution of
Zn(NO3)2∙6H2O (Wako Pure Chemical) with molar ratio of Zn/Ta
= 0.5. An aqueous NaOH solution was dropped to the
suspension with vigorous stirring in order to adjust the pH at
around 10.0−10.3, and then the resulꢀng suspension was kept
stable for 30 min at room temperature. The white suspension
was aged at 363 K for 6 h with stirring by a magnetic stirrer.
The resulting cake was collected by a vacuum filtration, and
dried at room temperature under air condition. ZnTa2O6 was
obtained through the calcination of this precursor for 12 h
using an electric furnace at a specified temperature. ZnTa2O6
samples which were calcined at 1073, 1173, 1273, and 1373 K
were hereinafter called ZTO_1073, ZTO_1173, ZTO_1273, and
ZTO_1373, respectively. In addition, the polymerized complex
method was used to synthesize ZnTa2O6. The required amount
of TaCl5 (Wako Pure Chemical) methanol solution and
Zn(NO3)2∙6H2O (Wako Pure Chemical) were added into an
aqueous citric acid solution, and stirred at room temperature.
Ethylene glycol (Wako Pure Chemical) was added into this
solution, and then stirred at 353 K to form a gelatinous
solution. After the gel was heated at 723 K for 3 h with
vigorous mixing, the resulting powder was calcined at 1273 K.
ZnTa2O6 synthesized via the polymerized complex method
would be presented as ZTO_PC. As a reference material,
physical mixture of Ta2O5 and ZnO was prepared via a grinding
of Ta2O5 and ZnO powder using an agate mortar, in advance,
each of two powders was respectively calcined at 1273 K
under an air atmosphere. One of the metal species (Ag, Au, Pt,
Ni, and Cu) was loaded to the synthesized ZnTa2O6 as a
cocatalyst for the photocatalytic reaction by impregnation,
Photocatalytic conversion of CO2 in an aqueous solution
Photocatalytic reactions were performed by using an inner-
irradiation type reactor vessel under a flow of CO2 gas. 0.5 g of
the photocatalyst powder was dispersed to 1.0 L of an ultra-
pure water (if necessary, 0.1 M of NaHCO3 was added to the
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reaction solution as an additive), and the suspension was
thoroughly degassed by a flow of CO2 or He gas with a vigorous
stirring. The reaction solution including a photocatalyst
powder was illuminated by a 400 W high-pressure Hg lamp
through a quartz glass jacket equipped with a water cooling
system. Gas phase products were detected and quantified by
using an online gas chromatograph (Tracera GC-2010plus,
Shimadzu) equipped with a barrier discharge ionization
detector (BID) using He gas as a carrier. The selectivity toward
CO evolution among the reduction products was given by
formula (1) as below,
View Article Online
DOI: 10.1039/C6CY00271D
Formula (1)
Selectivity toward CO evolution (%) = 100 × RCO / (RCO + RH2)
where RCO and RH2 was formation rates of CO and H2,
respectively.
For an isotopic experiment using 13C-labeled CO2 as a
substrate, the photocatalytic reaction was conducted by using
a closed circulation system connected to a rotary vacuum
pump. 0.5 g of the photocatalyst powder was dispersed to 380
mL of an aqueous NaHCO3 solution in the inner-irradiation
type reaction vessel, and dissolved air in the reaction solution
was degassed under a vacuum condition. 7.6 mmol of 13CO2
(isotopic purity: 99 %, purchased from SI Science Co., Ltd.),
which was purified by a freeze distillation, was introduced to
the dead space. After the circulation for 30 min, the
suspension was illuminated from an inner of the reaction
vessel through the quartz filter by a 400 W high-pressure Hg
lamp. A thermal conductivity detector gas chromatograph (GC-
8A, Shimadzu) and a quadrupole mass spectrometer (BELMASS,
MicrotracBEL) were used to detect the CO evolved in the gas
phase.
Fig. 1 XRD patterns of (a) ZnO, (b) Ta2O5, (c) and (g) ZTO_1273, (d) reference pattern of
ZnTa2O6 (ICSD #36289), (e) ZTO_1073, (f) ZTO_1173, and (h) ZTO_1373.
Results and discussion
Figure 1 shows X-ray diffraction (XRD) patterns of ZnTa2O6
prepared via a solid-state reaction method; ZnTa2O6 was
calcined at various temperatures. When compared to a
reference pattern of ZnTa2O6 (Figure 1(d), ICSD #36289), our
observations showed that ZnTa2O6 was successfully
synthesized with no impurity phases, and that the diffraction
peaks corresponding to ZnO (Figure 1(a)) and Ta2O5 (Figure
1(b)) were not found at all. No diffraction peaks corresponding
to isostructural zinc tantalum mixed oxides such as Zn3Ta2O8
and Zn4Ta2O9 were observed in the XRD patterns of ZnTa2O6
(ZTO_1273, Figure 1(c)). In contrast to this, several impurity
phases were observed in the XRD patterns of ZTO_1073
(Figure 1(e)) and ZTO_1173 (Figure 1(f)). In the case of
ZTO_1173, a noticeably sharp diffraction peak was observed
around 30°, confirming the formation of ZnTa2O6; in contrast,
the XRD pattern of ZTO_1073 showed no clear peak in this
region. These results indicate that the formation of the
ZnTa2O6 starts at 1173 K and ends at 1273 K. As shown in
Figure S1, the characteristic diffraction peaks assigned to the
ZnTa2O6 structure appeared in the XRD patterns of ZTO_PC.
Hence, the polymerized complex method resulted in poor
crystallinity of ZTO_PC compared to ZTO_1273, which was
prepared via a solid-state reaction.
Fig. 2 SEM images of ZTO_1373 at magnification of (a) 30k and (b) 110k.
Figure 2 displays scanning electron microscopy images of
ZTO_1373. The micrometer-sized particles were observed to
have a very smooth surface, and were created by combining
several submicron-sized particles with each other. Figure 3
shows UV/Vis diffuse reflectance spectra of ZnTa2O6 prepared
at various temperatures and with different compositions of
Ta2O5 and ZnO, as described in the experimental section. The
results showed that an absorption edge of ZnTa2O6 was locates
around 270 nm, whereas the absorption edge corresponding
to Ta2O5 was found at 330 nm in the spectrum of the mixed
27
sample. Based on the Davis-Mott equation
using the
Kubelka-Munk function F(R∞) obtained from diffuse
reflectance spectrum, the energy band gap of the ZnTa2O6
photocatalyst was estimated to be 4.5 eV.
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Table 1 Formation rates of products evolved (H2, O2, and CO) and selectivity
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toward CO evolution in the photoacatalytic conversion of CO2 in an aqueous
DOI: 10.1039/C6CY00271D
NaHCO3 solution. Photocatalyst powder: 0.5 g, volume of reaction solution: 1.0 L,
additive: 0.1 M NaHCO3, amount of cocatalyst loaded: 1 wt. %, atmosphere: CO2 flow at
a flow rate of 30 mL min−1, light source: 400 W high-pressure Hg lamp.
a Formation rates of products / μmol h−1
b Selectivity toward
Photocatalyst
CO evolution (%)
H2
19.7
703.5
253.6
135.0
72.6
25.1
6.5
O2
9.3
CO
6.6
0.3
0.4
0.2
1.5
19.3
2.1
0.8
2.4
Bare ZTO_1273
c Pt/ZTO_1273
c Au/ZTO_1273
c Ni/ZTO_1273
c Cu/ZTO_1273
d Ag/ZTO_1273
d Ag/Ta2O5
25.1
0.04
397.6
80.8
67.6
45.2
18.6
3.3
0.2
0.1
Fig. 3 UV/Vis diffuse reflectance spectra of (a) ZTO_1073, (b) ZTO_1173, (c) ZTO_1273,
(d) ZTO_1373, and (e) mixture of ZnO + Ta2O5 (molar ratio Zn : Ta = 1 : 2). The inserting
figure is enlarged view of small y-axis range.
2.0
43.4
24.4
8.3
d, e Ag/(ZnO + Ta2O5)
d, f Ag/ZTO_PC
8.8
3.7
Formula (2)
−
hν hν
n
14.7
4.2
13.8
[
F
(R∞
)
]
=
A
(
− Eg)
where
h
,
v
, and A are Planck’s constant, frequency of vibration,
a After 1 h of photoirradiation. b Calculated in accordance with equation (1).
and proportional constant, respectively. The ZTO_1073
spectrum showed weaker absorption of ZnO than that of the
mixture sample. The absorption peak of ZnO completely
disappeared when the sample was calcined above 1173 K,
indicating that the zinc species was completely conjugated
with tantalum to form the ZnTa2O6 structure. Moreover, a
broad absorption peak was observed in the spectra of the Ag-
modified ZnTa2O6 samples, which can be ascribed to plasmonic
absorption of Ag particles. As shown in Figure S2,
Photoelectrochemical measurement also revealed that UV
photoirradiation at wavelengths below 290 nm is necessary to
induce the photocatalytic reaction over ZnTa2O6, because the
anodic photocurrent was effectively diminished when a UV-29
long-pass filter is used.
c
d
Cocatalyst was loaded via photodeposition method. Cocatalyst was
loaded via impregnation method. e ZnO + Ta2O5: physical mixture. f ZTO_PC:
synthesized via polymerized complex method.
toward CO evolution among the reduction products was
43.4% for the Ag/ZTO_1273 photocatalyst, whereas the
percentage for ZTO_1273 modified with other cocatalysts was
smaller by several percent points. The turnover number of
electrons consumed for reduction of CO2 into CO was 3.4 per
Ag atom for 5 h of photoirradiation. Ag particle is considered
to be an excellent cocatalyst for selective CO evolution in the
photocatalytic conversion of CO2, as it is recently known in the
field of photocatalysis. Moreover, the formation rate of CO for
ZTO_1273 was significantly higher than those for Ta2O5,
ZTO_PC, and the physical mixture of ZnO and Ta2O5, indicating
that the ZnTa2O6 photocatalyst, synthesized by the solid-state
reaction, is more suitable for the conversion of CO2 into CO in
an aqueous NaHCO3 solution under UV irradiation. The ratio of
consumed electrons/holes in the photocatalytic reactions did
not correspond to the stoichiometric value in the cases of
Au/ZTO_1273 and Cu/ZTO_1273. It can be estimated that the
counter anions of precursors of cocatalysts may cause
undesired reactions under UV light irradiation. For example,
chloride ions derived from HAuCl4, which was used as a
precursor of Au cocatalyst, should scavenge the
photogenerated holes.
Table 1 displays the formation rates of the products
evolved (H2, O2, and CO) and the selectivity toward CO
evolution during the photoacatalytic conversion of CO2 in an
aqueous NaHCO3 solution using various metal-loaded
ZTO_1273 and reference samples. Unmodified ZTO_1273 also
exhibited photocatalytic activity under UV irradiation and
generated reduction (H2 and CO) and oxidation products (O2).
However, no photocatalytic activity was observed when the
quartz jacket was replaced with a pyrex jacket. The
photocatalytic activity was evidently influenced by modifying
the ZnTa2O6 photocatalyst with a metal species as a cocatalyst.
Loading of Pt, Au, Ni, and Cu cocatalysts enabled the splitting
of water into H2 and O2 while simultaneously generating small
amounts of CO as a reduction product of CO2, indicating that
H2 evolution via a reduction of proton proceeded in preference
to a reduction of CO2. In particular, Pt/ZTO_1273, which was
prepared via photodeposition method, showed fairly high
overall water splitting activity and produced over 700 μmol of
H2 during 1 h of photoirradiation, whereas the formation rate
of CO was only 0.3 μmol h−1. In comparison, Ag-modified
ZTO_1273 exhibited good activity for the conversion of CO2
into CO. Although the total number of electrons consumed in
the photocatalytic reaction was less than a tenth of Pt/
ZTO_1273, the formation rate of CO (19.3 μmol h−1) was
significantly higher than that of other cocatalyst. The selectivity
Figure 4 shows the amount of product evolved in the
photocatalytic conversion of CO2 in an aqueous NaHCO3
solution using ZTO_1273 modified with Ag via impregnation
(IMP), chemical reduction (CR), and photodeposition (PD)
methods. The Ag-loading method affected both the
photocatalytic activity and selectivity toward the evolution of
CO. Ag/ZTO_1273, prepared via impregnation methods,
exhibited the largest amount of CO evolved as a reduction
product of CO2. In comparison to previously developed
compounds for the conversion of CO2, the influence of the Ag-
loading method on the photocatalytic activity and selectivity
toward CO evolution was not as obvious; these compounds
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DOI: 10.1039/C6CY00271D
Fig. 4 Amount of products evolved in the photocatalytic conversion of CO2 in an
aqueous NaHCO3 solution using ZTO_1273 photocatalyst modified with Ag via
impregnation (IMP), chemical reduction (CR), and photodeposition (PD) methods.
Photocatalyst powder: 0.5 g, volume of reaction solution: 1.0 L, additive: 0.1 M NaHCO3,
amount of Ag loaded: 3 wt. %, atmosphere: CO2 flow at a flow rate of 30 mL min−1, light
source: 400 W high-pressure Hg lamp. Red (fill): CO, blue (slash): H2, green (dot): O2,
and circle: selectivity toward CO evolution, reaction time: 5 h.
Fig. 5 Amounts of products evolved and the selectivity toward CO evolution in the
photocatalytic conversion of CO2 in an aqueous NaHCO3 solution using ZTO_1073,
ZTO_1173, ZTO_1273, and ZTO_1373 as photocatalysts. Photocatalyst powder: 0.5 g,
volume of reaction solution: 1.0 L, additive: 0.1 M NaHCO3, amount of Ag loaded: 3
wt. % (IMP), atmosphere: CO2 flow at a flow rate of 30 mL min−1, light source: 400 W
high-pressure Hg lamp. Red (fill): CO, blue (slash): H2, green (dot): O2, and circle:
selectivity toward CO evolution, reaction time: 5 h.
include Zn-modified Ga2O3,18,19 ZnGa2O4,28 and SrO-modified
29
Ta2O5
reported by our group, and BaLa4Ti4O15,30 and
26,31
KCaSrTa5O15
reported by Kudo et al. After 5 h of
photoirradiation, Ag/ZTO_1273, which was prepared by IMP,
CR and PD methods, showed similar absorption peaks that
corresponded to the plasmonic absorption of Ag in UV/Vis DR
spectra. Thus, Ag/ZTO_1273 prepared via impregnation was
used in the subsequent studies. As shown in Figure 5, the
calcination temperature of the ZnTa2O6 photocatalyst
drastically changed the photocatalytic activity for the
conversion of CO2 under UV irradiation. When ZnTa2O6 was
synthesized through the calcination process at less than 1173 K
(for ZTO_1073 and ZTO_1173), the main process was water
splitting into H2 and O2. In contrast, only small amounts of CO
were evolved as the reduction product of CO2. Here, the
selectivity toward CO evolution among the various reduction
products was equal to or less than 10%. However, the
Fig. 6 Effect of Ag loading amount on the photocatalytic conversion of CO2 in an
aqueous NaHCO3 solution using ZTO_1273 as a photocatalyst. Photocatalyst powder:
photocatalytic activity changed at 1273 K, at which the 0.5 g, volume of reaction solution: 1.0 L, additive: 0.1 M NaHCO3, Ag loading method:
impregnation (IMP), atmosphere: CO2 flow at a flow rate of 30 mL min−1, light source:
selectivity toward CO evolution greatly increased to ca. 40%
because H2 generation via reduction of H+ was effectively
400 W high-pressure Hg lamp. Red circle (fill): CO, blue circle (outlined): H2, green
diamond: O2, and square: selectivity toward CO evolution, reaction time: 5 h.
suppressed at 1273 K. The specific surface area (SBET) of as
synthesized ZnTa2O6 gradually decreased with the increase in
crystal structure of ZnTa2O6 is classified as a α-PbO2 structure,
in which Zn2+ species are in a six-fold coordination.34 This result
the calcination temperature; specifically, when ZnTa2O6 was
calcined at 1073, 1173, 1273, and 1373 K, the SBET values
indicates that Zn species in the precursor should be
became 2.5, 2.1, 1.8, and 1.4 m2 g−1, respectively. It is
transformed to six-fold coordination in order to construct the
estimated that the decrease in the surface area of ZnTa2O6 is
ZnTa2O6 structure via Zn3Ta2O8 phase during the calcination
one of the reasons why the total number of consumed
above 1273 K. Therefore, the Zn-K edge XANES spectrum of
electrons in the photocatalytic reaction noticeably decreased
ZTO_1173 was found to be between those of ZTO_1073 and
at high calcination temperatures. Furthermore, the Zn-K edge
ZTO_1273. As shown in Figure S3(f−j), in contrast, the XANES
XANES spectra of ZnTa2O6 materials showed that the structure
spectra of the Ta-LIII edge did not change on increasing the
was altered by the calcination temperature, as shown in Figure
calcination temperature. Hence, we can conclude that based
S3(a−e). Compared to the spectrum of the mixture sample
on the results of XRD and XAFS, the improvement in the
(ZnO + Ta2O5), which shows the characteristic Zn2+ peak in ZnO,
selectivity toward CO evolution was due to the structural
the Zn species in ZTO_1073 was different from ZnO; if anything,
change of Zn species, which in the ZnO phase were
it should be similar to a four-fold coordination of Zn2+, just like
incorporated into Ta2O5 to form a ZnTa2O6 structure when
ZnS.32 It is known that Zn3Ta2O8 phase contains tetrahedral
calcined above 1273 K, and because the potential of the
33
coordination of Zn2+. The XANES spectra of ZTO_1273 and
photogenerated electrons in the conduction band of ZnO was
ZTO_1373 were different from that of ZTO_1073, because the
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DOI: 10.1039/C6CY00271D
Fig. 8 GC-MS profile in the isotopic experiment for the photocatalytic conversion
of 13C-labeled CO2 using ZTO_1273 as a photocatalyst. (a) TCD-GC chromatogram,
Q-Mass profile of (b) m/z = 28, and (c) m/z = 29. Photocatalyst powder: 0.5 g,
volume of reaction solution: 380 mL, additive: 0.1 M NaHCO3, Ag loading: 3 wt. %
(IMP), 13C-labeled CO2: 7.6 mmol, light source: 400 W high-pressure Hg lamp.
Fig. 7 Result of control experiments for the photocatalytic conversion of CO2 using in
water Ag/ZTO_1273 as a photocatalyst. (A) no additive; He gas flow; dark, (B) no
additive; He gas flow; photoirradiation, (C) no additive; CO2 gas flow; photoirradiation,
(D) 0.1 M NaHCO3 additive; He gas flow; photoirradiation, (E) 0.1 M NaHCO3 additive;
CO2 gas flow; photoirradiation. Photocatalyst powder: 0.5 g, volume of reaction
solution: 1.0 L, Ag loading: 3 wt. % (IMP), flow rate of gas: 30 mL min−1, light source:
400 W high-pressure Hg lamp. Red circle (fill): CO, blue circle (outlined): H2, green amounts of H2, O2, and CO were formed in the gas phase
diamond: O2, and square: pH of the reaction solution.
stoichiometrically. The selectivity toward CO evolution was ca.
10% in region (D), indicating that the main photocatalytic
process was water splitting, which was accompanied by the
not enough to reduce CO2.35 The in situ FT-IR spectra of CO2
conversion of NaHCO3-derived CO2. With the start of CO2 gas
adsorbed species on ZTO_1173 and ZTO_1273 are presented
in Figure S4. Both of them showed absorption peaks
corresponding to the C-O stretching of the CO2 species
adsorbed on the metal oxide materials, with reference to the
review about the CO2 species adsorbed on other simple metal
flow in region (E), the evolution of CO noticeably improved.
Moreover, the selectivity toward CO evolution was over 40%
after 2 h of photoirradiation. The formation rate of CO
gradually decreased with the irradiation duration, whereas
that of H2 remained stable during the 12 h of photoirradiation.
It can be estimated that changes of Ag particles, such as
particle size and oxidation state, caused the deterioration of
the activity for CO evolution. Furthermore, the isotopic
experiment was performed using 13C-labeled CO2 in order to
confirm that carbon source of CO evolved in the
photocatalytic conversion of CO2 over Ag-modified ZTO_1273.
Figure 8 shows the result of GC-MS analysis for the isotopic
experiment. In the TCD-GC chromatogram, the clear peak at
10.5 min was assigned to CO, and the others peaks
corresponded to H2, O2, and contaminated N2. With a Q-Mass
profile of m/z = 29, one peak was found at the same retention
time to CO in the GC chromatogram, indicating that this peak
can be considered to originate from the 13C-labeled CO. In
contrast, no peak was observed in the profile of m/z = 28.
Hence, it can be concluded that the CO evolved in this study
originated from CO2. Therefore, other carbon sources such as
the contamination of organic residues did not influence this
photocatalytic reaction. Further investigation on the synthesis
procedure of photocatalyst and cocatalyst modification is
necessary, because the photocatalytic activity and selectivity
toward CO evolution have not been insufficiently investigated.
We found that the ZnTa2O6 photocatalyst is active for the
conversion of CO2 into CO under UV irradiation. The formation
of stoichiometric amounts of O2 indicates that H2O can act
both as an electron donor and a proton source for this reaction.
36
oxides; in contrast, it is difficult to determine the adsorbed
CO2 on the surface of Ta2O5. The absorption peaks observed at
1320, 1375, and 1632 cm−1 for both ZTO_1173 and ZTO_1273
can be assigned to the carbonate species.36 The peaks at 1450
36
and 1730 cm−1 corresponding to bicarbonate species were
found in ZTO_1273, indicating that the surface properties of
ZnTa2O6 were altered on changing the calcination temperature.
Hence, it can be considered that changes in the chemical
properties, which alter the adsorbed CO2 species, improve the
selectivity toward CO evolution.
Figure 6 displays the effect of the loading amount of Ag on
the photocatalytic activity for the conversion of CO2 in an
aqueous NaHCO3 solution using ZTO_1273 as a photocatalyst.
As the loading amount of Ag was increased to 3.0 wt.%, the
amount of CO evolved also increased, which was accompanied
with a decrease in H2 evolution. This is an indication that the
selectivity toward CO evolution clearly improved on increasing
the loading amount of Ag. As mentioned above, Ag particles
loaded on a ZnTa2O6 photocatalyst are considered to be
effective cocatalysts for the conversion of CO2 into CO.
The results of the control experiments using Ag-loaded
ZTO_1273 are presented in Figure 7. At region (A), no
photocatalytic reaction was observed under dark condition
under He gas flow. Small amounts of H2 and O2 were produced
in the He atmosphere under UV irradiation, as shown in region
(B). At region (C), small amounts of H2 and O2 were evolved
from pure water under photoirradiation and CO2 gas flow,.
Prior to starting the region (D), NaHCO3 (0.1 M) was added into
the reaction solution and the dissolved air was completely
degassed by the He gas flow. During illumination, small
6 | J. Name., 2012, 00, 1-3
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ARTICLE
12 Y. Kohno, H. Ishikawa, T. Tanaka, T. Funabiki and S. Yoshida,
Phys. Chem. Chem. Phys., 2001, 3, 1108-1113.
Conclusions
View Article Online
DOI: 10.1039/C6CY00271D
13 K. Teramura, T. Tanaka, H. Ishikawa, Y. Kohno and T. Funabiki,
J. Phys. Chem. B, 2004, 108, 346-354.
14 H. Tsuneoka, K. Teramura, T. Shishido and T. Tanaka, J. Phys.
Chem. C, 2010, 114, 8892-8898.
15 K. Teramura, S. Iguchi, Y. Mizuno, T. Shishido and T. Tanaka,
Angew. Chem. Int. Ed., 2012, 51, 8008-8011.
16 S. Iguchi, K. Teramura, S. Hosokawa and T. Tanaka, Catal.
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17 S. Iguchi, K. Teramura, S. Hosokawa and T. Tanaka, Phys.
Chem. Chem. Phys., 2015, 17, 17995-18003.
18 K. Teramura, Z. Wang, S. Hosokawa, Y. Sakata and T. Tanaka,
Chem. A Euro. J., 2014, 20, 9906-9909.
19 Z. Wang, K. Teramura, Z. Huang, S. Hosokawa, Y. Sakata and
We found that the Ag cocatalyst-loaded ZnTa2O6
photocatalyst showed activity for the conversion of CO2 into
CO in an aqueous NaHCO3 solution under UV irradiation. After
5 h of photoirradiation of Ag-loaded ZTO_1273 (3 wt.%), the
amount of CO evolved was 109.3 μmol and the selectivity
toward CO evolution among the reduction products was 45.8%.
The formation of stoichiometric amounts of O2 indicates that
H2O can act both as an electron donor and a proton source for
this reaction. In contrast, Pt/ZTO_1273, which was prepared by
the photodeposition method, showed good overall water
splitting capabilities and over 700 μmol of H2 was produced
over 1 h of photoirradiation. Through control and isotopic
experiments, we concluded that CO evolved during the
photocatalytic conversion of CO2 in an aqueous NaHCO3
solution using the Ag cocatalyst-loaded ZnTa2O6 photocatalyst
originated from the CO2 introduced as a substrate. Calcination
at high temperatures altered both the chemical and structural
properties of ZnTa2O6 and improved the selectivity toward CO
evolution.
T.
10.1039/C5CY01280E.
Tanaka,
Catal.
Sci.
Technol.,
2015,
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This study was partially supported by a Grant-in-Aid for
Scientific Research on Innovative Areas "All Nippon Artificial
Photosynthesis Project for Living Earth" (No. 2406) of the
Ministry of Education, Culture, Sports, Science, and
Technology (MEXT) of Japan, the Precursory Research for
Embryonic Science and Technology (PRESTO), supported by
the Japan Science and Technology Agency (JST), and the
Program for Element Strategy Initiative for Catalysts &
Batteries (ESICB), commissioned by the MEXT of Japan. Shoji
Iguchi thanks the JSPS Research Fellowships for Young
Scientists.
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Mater. Res., 2000, 15, 1320-1323.
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Catalysis Science & Technology
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DOI: 10.1039/C6CY00271D
Graphical abstract
Textual abstract
Photocatalytic activity of ZnTa2O6 for the conversion of CO2 using H2O as a reductant
was demonstrated. CO was produced as a reduction product of CO2 in the presence of
Ag cocatalyst, accompanied with the stoichiometric amount of O2 evolution.