Enhancement of CO Evolution by Modification of Ga2O3 with Rare-Earth Elements for the Photocatalytic Conversion of CO2 by H2O

Article abstract of DOI:10.1021/acs.langmuir.7b03191

Modification of the surface of Ga₂O₃ with rare-earth elements enhanced the evolution of CO as a reduction product in the photocatalytic conversion of CO₂ using H₂O as an electron donor under UV irradiation in aqueous NaHCO₃ as a pH buffer, with the rare-earth species functioning as a CO₂ capture and storage material. Isotope experiments using ¹³CO₂ as a substrate clearly revealed that CO was generated from the introduced gaseous CO₂. In the presence of the NaHCO₃ additive, the rare-earth (RE) species on the Ga₂O₃ surface are transformed into carbonate hydrates (RE₂(CO₃)₃·nH₂O) and/or hydroxycarbonates (RE₂(OH)_2(3-x)(CO₃)_x) which are decomposed upon photoirradiation. Consequently, Ag-loaded Yb-modified Ga₂O₃ exhibits much higher activity (209 μmol h^-1 of CO) than the pristine Ag-loaded Ga₂O₃. The further modification of the surface of the Yb-modified Ga₂O₃ with Zn afforded a selectivity toward CO evolution of 80%. Thus, we successfully achieved an efficient Ag-loaded Yb- and Zn-modified Ga₂O₃ photocatalyst with high activity and controllable selectivity, suitable for use in artificial photosynthesis.

Full text of DOI:10.1021/acs.langmuir.7b03191

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Article
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Enhancement of CO Evolution by Modification of GaO with Rare-
2
2
earth Elements for the Photocatalytic Conversion of CO by HO
Hiroyuki Tatsumi, Kentaro Teramura, Zeai Huang, Zheng Wang,
Hiroyuki Asakura, Saburo Hosokawa, and Tsunehiro Tanaka
Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03191 • Publication Date (Web): 16 Nov 2017
Downloaded from http://pubs.acs.org on November 17, 2017
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Enhancement of CO evolution by modification of Ga
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Enhancement of CO Evolution by Modification of Ga O3
2
with Rareꢀearth Elements for the Photocatalytic
Conversion of CO by H O
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†
†,‡*
†
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Hiroyuki Tatsumi , Kentaro Teramura , Zeai Huang , Zheng Wang , Hiroyuki
†,‡
†,‡
†,‡*
Asakura , Saburo Hosokawa , Tsunehiro Tanaka
†
Department of Molecular Engineering, Graduate School of Engineering, Kyoto
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University, Kyotodaigaku Katsura, Nishikyo–ku, Kyoto 615–8510, Japan
‡
Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University, 1ꢀ30
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GoryoꢀOhara, Nishikyoꢀku, Kyoto 615ꢀ8245, Japan
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Abstract
Modification of the surface of Ga O with rareꢀearth elements enhanced the
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evolution of CO as a reduction product in the photocatalytic conversion of CO using
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H O as an electron donor under UV irradiation in aqueous NaHCO as a pH buffer, with
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the rareꢀearth species functioning as a CO capture and storage material. Isotope
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experiments using CO as a substrate clearly revealed that CO was generated from the
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introduced gaseous CO . In the presence of the NaHCO additive, the rareꢀearth (RE)
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species on the Ga O surface are transformed into carbonate hydrates
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(RE (CO ) •nH O) and/or hydroxycarbonates (RE (OH)
(CO ) ) which are
3 x
2
3 3
2
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2(3ꢀx)
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decomposed upon photoirradiation. Consequently, Agꢀloaded Ybꢀmodified Ga O3
2
−1
exhibits much higher activity (209 µmol h of CO) than the pristine Agꢀloaded Ga O .
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The further modification of the surface of the Ybꢀmodified Ga O with Zn afforded a
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selectivity toward CO evolution of 80%. Thus, we successfully achieved an efficient
Agꢀloaded Ybꢀ and Znꢀmodified Ga O photocatalyst with high activity and controllable
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selectivity, suitable for use in artificial photosynthesis.
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Introduction
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Currently, fossil fuels such as coal, oil, and natural gas are being consumed at a
frightening pace, and their inevitable depletion necessitates the expeditious development
of sustainable and clean energy production systems. Through its voluminous emissions
of CO into the atmosphere, fossil fuel combustion contributes significantly to global
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warming. To rectify both our energy and environmental problems, artificial
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ꢀ8
photosynthesis has received significant attention of late. Photosynthesis may be
categorized into five distinct process steps—(1) light harvesting, (2) charge separation,
(3) oxidation of H O, (4) reduction of H O, and (5) reduction of CO —which means
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that solar fuels such as H and CH can be produced by using sunlight and abundant
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H O and/or CO . The photocatalytic conversion of CO in the presence of H O as a
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coꢀreactant involves three of these processes (steps (2), (3), and (5)), and has the
potential to mitigate both our energy and environmental problems. On this topic, very
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few papers have been published to date, compared to those concerned with overall water
9ꢀ17
splitting to produce pure H and O .
This is because water splitting occurs relatively
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easily over the appropriate photocatalysts, and H is predominantly generated even in
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the presence of CO because the redox potential of CO/CO (E = −0.11 V vs SCE at pH
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with rareꢀearth elements for the photocatalytic conversion of CO by H O
+
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= 0) is lower than that of H /H . In these cases, CO functions as an inert gas, and H
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and O are evolved as the reduction and oxidation products, respectively. This means
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that it is very difficult for the electrons generated by charge separation to be supplied to
+
CO in preference to H . Thus, we were motivated to enhance the selectivity of the
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generated electrons toward the reduction of CO , to afford products such as CO,
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HCOOH, HCHO, CH OH, and CH , which would essentially achieve the photocatalytic
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conversion of CO using H O as an electron donor. In this process, a stoichiometric
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amount of O evolution should be observed, which means that the number of holes (h )
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−
generated and consumed is consistent with that of electrons (e ). If CO is evolved as the
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reduction product, the selectivity toward CO evolution and the balance between the
−
+
consumed e and h can be expressed as follows:
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Selectivity toward CO evolution (%) = 100 × 2RCO / (2RCO + 2RH2
)
−
+
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Consumed e /h = (2RCO + 2RH2) / 4RO2
where RCO, RH2, and RO2 represent the rates of CO, H
2
, and O
2
formation, respectively.
Strictly speaking, the successful photocatalytic conversion of CO into CO using H O as
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an electron donor requires (1) greater than 50% selectivity toward CO evolution and (2)
−
+
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the generation of a stoichiometric amount of O (e /h = 1).
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Kudo and his colleagues published a cuttingꢀedge paper in the history of the
photocatalytic conversion of CO by H O, in which Agꢀloaded ALa Ti O (A = Ca, Sr,
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Ba) exhibited a higher formation rate of CO than H and evolved a stoichiometric
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amount of O . In this case, more of the CO reduction product was generated than H .
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On the other hand, our research group consistently focused on Ga O as a photocatalyst.
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It has alreadly reported that overall water splitting has been shown to occur over a plain
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Ga O photocatalyst.
Sakata et al. demonstrated that Znꢀdoped Ga O is one of the
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best photocatalysts for water splitting under UV irradiation.
Znꢀdoped Ga O3
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modified by Rh Cr O as a coꢀcatalyst exhibited H and O formation rates of 21.0
0
.5 1.5
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2
2
−1
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and 10.5 mmol h under UV irradiation, respectively. The DFT calculations provided
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the potential that CO is adsorbed and activated on the surface of Ga O . Recently, we
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found that Agꢀloaded Znꢀmodified Ga O demonstrated high conversion and good
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selectivity toward CO evolution in the photocatalytic conversion of CO with H O as an
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electron donor, compared to several other recently reported photocatalysts.
XꢀRay
diffraction (XRD) and Xꢀray absorption spectroscopy (XAS) analyses revealed that
ZnGa O was formed on the surface of the Ga O , where it suppressed only the
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2
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evolution of H derived from water splitting. Actually, we reported that Agꢀloaded
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Enhancement of CO evolution by modification of Ga
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ZnGa O also exhibited photocatalytic activity with high selectivity toward CO
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evolution. The selectivity for CO evolution was dependent on the loading of the
ZnGa O , and reached nearly 100% in the case of 3.0 mol% Znꢀmodified Ga O with
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1.0 wt% Ag coꢀcatalyst. The rate of H formation decreased exponentially with
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increased ZnGa O loading, although the rate of CO formation was not changed. Thus,
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the next objective in this field is to enhance the rate of CO formation.
In our 20 years of research into the photocatalytic conversions of CO , we have
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found that CO can be generated as a result of the photocatalytic conversion of CO over
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25ꢀ27
28ꢀ29
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the solid base materials ZrO2
, MgO
, ATaO (A = Li, Na, and K) , and
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Ga O
in the presence of H or CH as a reductant. We achieved the highest
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conversion of CO , 7.8%, after 48 h photoirradiation over Ga O . Thus, we have
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already demonstrated that the solid base materials have an important role in the
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surfaceꢀcapture of CO molecules for the photocatalytic conversion reaction. It is
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wellꢀknown that rareꢀearth (RE) oxides react with H O and CO to form carbonate
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species. This behavior was reported by Bernal and coworkers and applied to a CO gas
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sensor.
La species doped into SnO functioned as active sites for the adsorption of
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CO , suggesting their strong basicity. Moreover, both the strength and number of base
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sites were found to change by the addition of RE oxides.
On the basis of these very
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interesting features of RE elements, in this study, we used REs to modify the surface of
Ga O in order to more effectively capture and enhance the concentration of CO near
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the surface for subsequent photocatalytic reduction.
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Experimental Section
Catalyst preparation
Ga O was obtained by the calcination of gallium hydroxide (Ga(OH) ),
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prepared via the hydrolysis of Ga(NO ) with an aqueous solution of NH , at 1273 K for
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9ꢀ20, 22ꢀ23, 41
6 h. REꢀmodified Ga O was prepared by an impregnation method.
Ga O3
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2
3
was introduced to an aqueous solution of 0.1–10 mol% RE(NO ) (RE = Y, La, Ce, Pr,
3
3
Nd, Sm, Gd, Dy, Ho, Er, Yb, or Lu). The suspension was stirred at room temperature for
1 h with ultrasonication, and then gradually evaporated to dryness at 323 K. The sample
was calcined from 1023 to 1423 K for 6 h in air. The fabricated photocatalysts (0.5 g)
were modified with 1.0 wt% of a Ag coꢀcatalyst by a typical photodeposition method:
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the photocatalysts were dispersed in an aqueous solution of AgNO and then illuminated
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in an Ar flow. In this paper, samples are designated according to their elemental
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composition as follows: 1.0 wt% Agꢀloaded x mol% REꢀloaded Ga O is referred to as
2 3
Ag/x RE/Ga O .
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Catalyst characterization
The Xꢀray diffraction (XRD) patterns of the REꢀmodified Ga O were recorded
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using a Rigaku Ultima IV powder diffractometer. The UV–Vis diffuse reflectance
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spectra (UV–vis DRS) were measured by a JASCO Vꢀ670 spectrometer equipped with
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an integrating sphere. Spectralon , which was supplied by Labsphere Inc., was used as a
standard reflection sample. The XPS measurement was acquired using an Xꢀray
photoelectron spectrometer (ESCA 3400, Shimadzu Corp.). The Ag Kꢀedge (25.5 keV)
and Yb L ꢀedge (8.94 keV) Xꢀray absorption fine structure (XAFS) measurements were
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obtained in transmission mode and fluorescence mode at the BL01B1 beamline of the
SPringꢀ8 synchrotron radiation facility (Hyogo Pref., Japan). The ring energy used was
8 GeV, and the stored current was 99.5 mA. A double crystal monochromator (Si (311))
were used to obtain the monochromatic Xꢀray beam. The photon energy of the Xꢀrays
was calibrated using a foil or typical oxide as reference. Data reduction was performed
using the Athena ver. 0.9.21 software package. BrunauerꢀEmmettꢀTeller (BET) surface
1
1
12
area was measured by N adsorption at 77 K using a volumetric gas adsorption
2
1
1
1
1
1
3
4
5
6
7
apparatus (BELmini, BEL Japan, Inc.). SEM images were obtained with a Hitachi
SUꢀ8220 field emission scanning electron microscope (SEM). Inductively coupled
plasma (ICP) analysis was performed with an iCAP 7400 ICPꢀOES DUO system
(Thermo Fischer Scientific Inc.).
2 2
Photocatalytic activity test for the conversion of CO with H O
9
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Page 10 of 37
Tatsumi et al.
Enhancement of CO evolution by modification of Ga
2
with rareꢀearth elements for the photocatalytic conversion of CO by H O
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
2
O
3
2
1
2
3
4
5
6
7
8
9
0
1
The photocatalytic conversion of CO using H O, including the Ag coꢀcatalyst
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
modification of the photocatalysts, was carried out in a quartz innerꢀirradiation type
reaction vessel (1.0 L) in a quasiꢀflow batch system. The synthesized photocatalyst (0.5
g) was dispersed in an aqueous solution of NaHCO (0.1 M). CO gas (99.999%) was
3
2
−
1
bubbled into the solution at a flow rate of 30 mL min . The suspension was irradiated
with a 400 W highꢀpressure mercury lamp through a quartz filter equipped with a
cooling water system. The CO, H , and O2 products were analyzed by gas
2
chromatography with thermal conductivity detection (GCꢀTCD) on a Shimadzu GCꢀ8A
chromatograph equipped with a molecular sieve 5A column (carrier gas: Ar) and by GC
with flame ionization detection (FID) with a methanizer and a Shincarbon ST column
1
1
13
(carrier gas: N ). In an experiment with isotopically labeled CO , the formation of
2
2
1
3
12
12
CO and CO was observed by mass spectroscopy by using a quadrupoleꢀtype mass
1
1
3
4
spectrometer (BEL Japan, BEL Mass).
10
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Langmuir
Tatsumi et al.
Enhancement of CO evolution by modification of Ga
2
with rareꢀearth elements for the photocatalytic conversion of CO by H O
1
2
3
4
5
6
7
8
9
2
O
3
2
1
2
3
4
5
6
7
8
9
0
1
Results and Discussion
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
Table 1 shows the rates of product formation in the photocatalytic conversion of
CO in an aqueous solution of 0.1 M NaHCO over Ag/Ga O with or without
2
3
2
3
modification by 3.0 mol% RE under UV irradiation. As seen the XRD patterns of all the
RE/Ga O (Figure S1), RE species could be observed in addition to the Ga O . As
2
3
2
3
shown in the UVꢀVis DRS of REꢀmodified Ga O (Figure S2), the band gap energies of
2
3
the Ga O and RE/Ga O (except Ce/Ga O ) are nearly the same. This indicates that the
2
3
2
3
2
3
RE species do not influence the band structure of Ga O . In all cases, CO and H are
2
3
2
+
evolved as the reduction products from CO and H by the generated electrons,
2
1
1
respectively; in addition, stoichiometric amounts of O are observed. Over all the
2
REꢀmodified Ag/Ga O catalysts, except La and Ce, the CO formation rate is higher
2
3
12
than that over Ag/Ga O , and the selectivity for CO evolution exceeds 50%. With Ce
2 3
1
1
1
1
3
4
5
6
modification, the rates of H , O , and CO formation decline compared to unmodified
2 2
Ga O , whereas only a slight decrement is observed in the case of La. Relatively high
2
3
rates of CO formation are obtained after modification with Yb, Dy, and Sm, although
the other RE elements also enhance the rate (Figure S3). With the exceptions of Ce and
La, RE modification largely enhances the activity of the photocatalyst, affording CO
17
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Page 12 of 37
Tatsumi et al.
Enhancement of CO evolution by modification of Ga
2
with rareꢀearth elements for the photocatalytic conversion of CO by H O
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
2
O
3
2
1
2
3
4
formation rates ~2.0–2.6 times that of Ag/Ga O . Thus, the modification with RE
2
3
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
elements effectively enhances the evolution of CO.
a
Table 1 Rates of product formation in the photocatalytic conversion of CO
2
Rate of gas evolved
Selectivity toward CO
−
1
−
+
Photocatalyst
/ µmol h
O2
e /h
evolution (%)
H2
141
169
118
123
4.09
29.5
84.8
131
110
169
141
93.0
144
144
CO
4.8
Ga
2
O
3
67.3
116
3.3
1.08
1.08
1.03
1.28
1.52
1.28
1.08
1.11
1.04
0.98
1.04
0.95
1.03
1.01
Ag/Ga
2
O
3
80.7
161
51.7
32.6
182
167
200
172
197
156
194
209
160
32.3
57.7
29.6
88.9
86.1
66.3
60.4
61.0
53.8
52.5
67.6
59.2
52.6
Ag/3.0 Y/Ga
Ag/3.0 La/Ga
Ag/3.0 Ce/Ga
2
O
3
136
2
O
3
68.3
12.1
82.9
117
2
O
3
Ag/3.0 Pr/Ga
Ag/3.0 Nd/Ga
Ag/3.0 Sm/Ga
2 3
O
2
O
3
2
O
3
149
Ag/3.0 Gd/Ga
Ag/3.0 Dy/Ga
Ag/3.0 Ho/Ga
2
2
2
O
O
O
3
3
3
136
187
143
Ag/3.0 Er/Ga
2
O
3
151
Ag/3.0 Yb/Ga
2
O
3
171
Ag/3.0 Lu/Ga
2
O
3
151
a
−1
5
6
7
8
9
Photocatalyst weight: 0.5 g; CO
2
flow: 30 mL min ; solution volume: 1.0 L; additive:
0.1 M NaHCO
mol%.
3
; light source: 400 W Hg lamp; Ag loading: 1.0 wt%, RE loading: 3.0
Among the investigated RE elements, the rate of CO formation was improved
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Langmuir
Tatsumi et al.
Enhancement of CO evolution by modification of Ga
2
with rareꢀearth elements for the photocatalytic conversion of CO by H O
1
2
3
4
5
6
7
8
9
2
O
3
2
1
2
3
4
5
6
7
8
9
0
1
after Ga O modification with all except La and Ce, which can be explained as follows.
2 3
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
For 3.0 La/Ga O , peaks attributable to La (CO ) •8H O do not disappear upon
2
3
2
3 3
2
photoirradiation, indicating that it is stable under the reaction conditions. In the case of
3.0 Ce/Ga O , peaks due to CeO were observed in the XRD pattern (Figure S1).
2
3
2
Neither cerium carbonate hydrate nor the hydroxycarbonate is formed because CeO is
2
13
too stable to react with NaHCO . We followed the time course of the reaction of CO
3
2
1
2
over Ag/3.0 Yb/Ga O , and initially observed a tiny amount of CO, which decreased
2
3
with prolonged photoirradiation time (Figure S4), although it was not observed over
22ꢀ23
Ag/ZnGa O /Ga O in the previous work.
In addition, CO was hardly evolved even
2
4
2
3
1
1
in the presence of NaHCO when inert Ar gas was bubbled instead of CO , which
3 2
−
means that CO as a product is derived not from HCO , but rather, from CO dissolved
3
2
41
12
from the gas phase. The Yb4d XPS (Figure S5) and Yb L ꢀedge XANES spectra
3
3
+
1
1
1
1
3
4
5
6
(Figure S6) clearly represent that the Yb species kept trivalent cation (Yb ) before and
12
after the reaction. We expect that CO is initially released from ytterbium carbonate
2
hydrates (Yb (CO ) •nH O) and/or ytterbium hydroxycarbonate (Yb (OH)
(CO ) )
3 x
2
3 3
2
2
2(3ꢀx)
1
3
and reduced by the Ag coꢀcatalyst, freeing the Yb species to react with CO and
2
1
3
13
17
reform the isotopically modified Yb ( CO ) •nH O and/or Yb (OH)
( CO ) . Note
2
3 3
2
2
2(3ꢀx)
3 x
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Tatsumi et al.
Enhancement of CO evolution by modification of Ga
2
with rareꢀearth elements for the photocatalytic conversion of CO by H O
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
2
O
3
2
1
2
3
4
5
6
7
8
9
0
1
that Pr (CO ) •nH O and/or Pr (OH)
(CO ) are generated on the surface of Ga O in
2(3ꢀx) 3 x 2 3
2
3 3
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
case that Prꢀmodified Ga O is prepared in an aqueous solution of NaHCO in the same
2
3
3
4
2
manner. We conclude that the RE (CO ) •nH O and/or RE (OH)
(CO ) function
2
3 3
2
2
2(3ꢀx)
3 x
as CO capture and storage materials, although peaks attributable to crystalline
2
RE (CO ) •nH O and/or RE (OH)
(CO ) were not always observed by XRD.
2(3ꢀx) 3 x
2
3 3
2
2
In our approach, to achieve high CO evolution, five system components are
necessary: the photocatalyst, photoirradiation, CO flow, the Ag coꢀcatalyst, and the
2
NaHCO additive. We examined the photocatalytic conversion of CO by H O over
3
2
2
Ag/3.0 Yb/Ga O with the systematic omission of each component, as shown in Figure
2
3
1
1
1. No product is obtained in the absence of photoirradiation. Without a photocatalyst,
small amounts of H and O are evolved due to the photolysis of H O under UV
2
2
2
12
irradiation. When a flow of Ar gas is introduced instead of CO , H is mainly evolved
2 2
1
1
1
1
3
4
5
6
from water splitting, along with a small amount of CO due to the decomposition of
−
bicarbonate ion (HCO ). Without the Ag coꢀcatalyst or NaHCO additive, H is mainly
3
3
2
evolved. This is explained as follows. In the photocatalytic reaction, electrons and holes
are generated by the absorption of photons in the conduction and valence bands of the
catalyst, respectively. The photoexcited electrons in the conduction band subsequently
17
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Langmuir
Tatsumi et al.
1
2
3
4
5
6
7
8
9
2
Enhancement of CO evolution by modification of Ga O3
with rareꢀearth elements for the photocatalytic conversion of CO by H O
2
2
+
1
2
3
4
5
6
7
8
9
0
1
migrate to surface active sites, where they are used to reduce CO
2
and H . In contrast,
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
the holes in the valence band are used to oxidize H O to O . It is known that the
2
2
2
electrochemical reduction of CO to CO occurs selectively on a Ag metal electrode in
43ꢀ44
aqueous solution.
2 3
Similarly, the metallic Ag particles on the Ybꢀmodified Ga O
2
function as active sites to capture the photoexcited electrons for the reduction of CO .
The positive effects of various additives including NaHCO
3
have been reported in the
45ꢀ46
case of water splitting.
Here, NaHCO
3
2
does not accelerate the rate of H formation
but rather, that of CO. The pH of the medium during the photocatalytic reaction
3 2
decreases without NaHCO . This indicates that the solubility of CO as a substrate is
1
1
3
dramatically changed by adding NaHCO .
1
1
2
3
2 2
Figure 1 Formation rates of H (gray), O (white), and CO (black) in the absence of
15
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Page 16 of 37
Tatsumi et al.
Enhancement of CO evolution by modification of Ga
2
with rareꢀearth elements for the photocatalytic conversion of CO by H O
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
2
O
3
2
1
2
3
4
5
6
7
8
9
0
1
each component in the photocatalytic conversion of CO by H O over Ag/3.0 Yb/Ga O .
2 2 2 3
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
Photocatalyst weight: 0.5 g, CO flow: 30 mL min , solution volume: 1.0 L, additive:
2
0.1 M NaHCO , light source: 400 W Hg lamp.
3
To clarify the source of the carbon products generated in this photocatalytic
reaction system and ensure that CO is the substrate that is reduced, we carried out the
2
13
reaction with CO . Figure 2 shows the gas chromatogram and mass spectra (m/z = 28
2
1
3
and 29) for the products from the photocatalytic conversion of CO by H O over
2
2
Ag/3.0 Yb/Ga O . We introduced the sampling gas into the mass spectrometer after
2
3
1
1
separation by GCꢀTCD. Peaks attributable to CO were detected at 820 s in both the GC
1
3
chromatogram and mass spectrum. The principal reaction product is CO, with a minor
1
2
12
amount of CO. Thus, we have demonstrated that CO introduced from the gas phase
2
1
1
3
4
functions as the substrate and is reduced to CO during the photocatalytic conversion of
CO over Ag/3.0 Yb/Ga O .
2
2
3
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Langmuir
Tatsumi et al.
1
2
3
4
5
6
7
8
9
2
Enhancement of CO evolution by modification of Ga O3
with rareꢀearth elements for the photocatalytic conversion of CO by H O
2
2
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
1
2
Figure 2 Gas chromatogram and mass spectra (m/z = 28 and 29) in the photocatalytic
1
3
3
4
conversion of CO
2
by H
2
O over Ag/3.0 Yb/Ga
2
O
3
after 2 h. Photocatalyst weight: 0.5
; light
−1
g; CO flow: 30 mL min ; solution volume: 1.0 L; additive: 0.1 M NaHCO
2
3
5
source: 400 W Hg lamp.
6
7
To optimize the catalyst performance, we varied the Yb loading from 0.1 to 10
mol% and the calcination temperature from 1023 to 1423 K. With respect to the RE
8
9
2 3 2 3
loads, the XRD patterns of the Ga O and x Yb/Ga O are consistent, which indicates
10
that the Yb species are dispersed in the amorphous phase (Figure S7). The peaks at
15.2°, 29.8°, 30.8°, 33.7°, 43.4°, 52.9°, and 56.7° increase with the increased loading of
1
1
1
2
2 3
Yb, meaning that bulk Yb species form on the Ga O surface. The SEM images
17
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Tatsumi et al.
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
2
Enhancement of CO evolution by modification of Ga O3
with rareꢀearth elements for the photocatalytic conversion of CO by H O
2
2
1
2
3
4
5
6
7
8
9
0
revealed that the particle size and surface morphology of the catalysts do not change as
compared to Ga (Figure S8). Figure 3 shows the effect of the Yb loading on the rates
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
2
O
3
2 2 2 2
of H , O , and CO formation for the photocatalytic conversion of CO by H O over Ag/x
2
−1
2 3
Yb/Ga O . Although the specific surface areas of all samples remained at ~10 m g ,
the formation rate of CO was greatly changed. The lowest Yb loading (0.1 mol%)
−1
dramatically improved the evolution of CO. The formation rate of CO (135 µmol h )
2 3
was much higher than that without that over bare Ga O with Ag cocatalyst (80.7 µmol
−1
h ), and then increased with increasing Yb content, reaching a maximum and
decreasing thereafter, which indicates that appropriate amount of Yb contributes to the
enhancement of CO evolution.
1
1
1
1
2
Figure 3 Effects of Yb loading on the formation rate of CO (closed circles) and
13
2 2
specific surface area (open diamond) in the photocatalytic conversion of CO by H O
1
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with rareꢀearth elements for the photocatalytic conversion of CO by H O
1
2
3
4
5
6
7
8
9
2
O
3
2
1
2
3
4
5
6
7
8
9
0
1
over 1.0 wt% Agꢀloaded Ybꢀmodified Ga O . Reaction conditions are the same as those
2 3
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
in Table 1.
We varied the calcination temperature from 1023 to 1423 K. The XRD patterns
of Ga O and 3.0 Yb/Ga O calcined at various temperatures (Figure S9) show only the
2
3
2
3
typical βꢀGa O phase. No peaks attributable to Yb species are observed at lower
2
3
temperatures, indicating that the amorphous Yb species are spread on the surface of
Ga O . The peaks at 15.2°, 29.8°, 30.8°, 33.7°, 43.4°, 52.9°, and 56.7° increase with the
2
3
calcination temperature, as similarly observed when the Yb content was increased.
Figure 4 shows the effect of the calcination temperature on the rates of H , O , and CO
1
1
2
2
formation in the photocatalytic conversion of CO by H O over Ag/3.0 Yb/Ga O . In
2
2
2
3
12
terms of these rates, the photocatalytic performance of Ag/3.0 Yb/Ga O calcined at
2 3
1
1
1
1
3
4
5
6
1023 K is higher than that of Ag/Ga O . The rate of CO formation steadily increases
2 3
with the calcination temperature, and then plummets at 1423 K. The highest rates of CO
formation are obtained using Ag/3.0 Yb/Ga O calcined at 1173 and 1223 K. The
2
3
specific surface area gradually decreases with the increase in calcination temperature,
2
−1
2
−1
17
from 10.7 m g at 1023 K to 7.3 m g at 1423 K. According to the SEM images, the
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2
3
4
5
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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
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2
Enhancement of CO evolution by modification of Ga O3
with rareꢀearth elements for the photocatalytic conversion of CO by H O
2
2
1
2
3
4
Ga
2
O
3
particles become aggregated upon calcination at 1423 K; this calcination
(Figure S10).
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
temperature is much higher than that used in the preparation of Ga
2
O
3
Thus, the catalyst that exhibits the highest rate of CO formation is prepared with a
2 3
composition of 1.0 wt% Ag and 3.0 mol% Yb on Ga O and calcined at 1223 K.
5
6
Figure 4 Effect of calcination temperature on the formation rate of CO (closed circles)
7
8
9
0
1
2
and specific surface area (open diamond) in the photocatalytic conversion of CO
O over Ag/3.0 Yb/Ga . Reaction conditions are the same as those in Table 1.
2
by
H
2
2 3
O
1
1
1
Figure 5 shows the time course of H
2 2
, O , and CO formation for the reaction
over Ag/3.0 Yb/Ga calcined at 1223 K. In this experiment, the reaction was purged
2
O
3
with Ar gas after 7 and 14 h of photoirradiation for overnight storage, and then resumed
13
2
the next day. After 39 h photoirradiation, we obtained 4.5 mmol CO, 7.3 mmol H , and
2
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2
Enhancement of CO evolution by modification of Ga O3
with rareꢀearth elements for the photocatalytic conversion of CO by H O
2
2
1
2
3
4
2
5.4 mmol O . The turnover number (TON) for the CO evolution, which is estimated on
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
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0
1
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9
0
1
2
3
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7
8
9
0
1
2
3
4
5
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8
9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
the basis of the loading amount of Ag coꢀcatalyst as an active site, is ~96.2, indicating
that the conversion proceeds photocatalytically. Unfortunately, the rate of CO formation
2
gradually decreases with photoirradiation time, although that of H hardly changes.
5
6
2 2
Figure 5 Time course of H (triangle), O (square), and CO (circle) evolution for the
7
8
photocatalytic conversion of CO
2 2 2 3
by H O over Ag/3.0 Yb/Ga O calcined at 1223 K.
Reaction conditions are the same as those in Table 1.
9
10
11
It is wellꢀknown that the particle size and oxidation state of a coꢀcatalyst loaded
on the surface of a photocatalyst affect the photocatalytic reaction. Figure 6 shows the
1
1
2
3
FTs of the Ag Kꢀedge EXAFS spectra of Ag/3.0 Yb/Ga
2 3
O before and after 7 h
photoirradiation. The Ag peak positions of the Agꢀloaded catalyst are consistent with Ag
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1
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2
2
2
2
2
2
2
2
2
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3
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2
Enhancement of CO evolution by modification of Ga O3
with rareꢀearth elements for the photocatalytic conversion of CO by H O
2
2
1
2
3
4
foil as a reference, indicating that the Ag particles loaded on the surface of the 3.0
2 3
Yb/Ga are metallic. As a result, the Ag–Ag bond shell between the Ag/3.0 Yb/Ga O
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
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9
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
5
6
7
8
9
0
2
O
3
before and after the reaction is nearly the same. This means that the Ag particle size
does not change during photoirradiation.
5
6
Figure 6 FT of Ag Kꢀedge EXAFS spectra of Ag/3.0 Yb/Ga
2 3
O before (a) and after
7
8
photoirradiation for 7 h (b).
9
Table 2 shows the ICP analysis results in which the amounts of Ag, Yb, and Ga
dissolved into the solution were measured over time. The time point “0 h” represents the
time after the photodeposition of the Ag coꢀcatalyst. Very little Ag and Ga are dissolved
after 39 h photoirradiation, whereas Yb gradually dissolves over time, with more than
30% of the Yb dissolving after 21 h. It is known that Ag is not stable under
10
11
1
1
2
3
2
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Enhancement of CO evolution by modification of Ga
2
with rareꢀearth elements for the photocatalytic conversion of CO by H O
1
2
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4
5
6
7
8
9
2
O
3
2
+
1
2
3
4
5
6
photoirradiation, but in our system, we assume that Ag leached in the solution as Ag is
1
1
1
1
1
1
1
1
1
1
2
2
2
2
2
2
2
2
2
2
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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
redeposited on the surface of Ga O , because the catalyst is dispersed in the solution and
2
3
not the gas phase.
Table 2 Extent of catalyst dissolution during the photocatalytic conversion of CO
2
over Ag/3.0 Yb/Ga O . Reaction conditions are the same as those in Table 1.
2
3
Irradiation time
Rate of CO evolved
Amount dissolved (%)
−
1
/
h
/µmol h
Ag
0.0
0.0
0.2
0.1
Yb
1.4
14
31
31
Ga
0.7
0.5
1.2
1.0
a
0
209
7
151
96.9
91.7
2
1
3
9
a
7
8
9
0
1
2
Irradiation time: 1 h
2 3 2 3
Figure 7 shows the normalized XRD patterns of Ga O and Ag/3.0 Yb/Ga O
1
1
1
before the reaction and after photoirradiation for different periods. The peak intensities
were normalized against the peak of the (111) plane of Ga . The peak at 33.6°,
2
O
3
2 3
assigned to Yb O , decreases with the photoirradiation time. The ICP analysis and XRD
13
2 3
results clearly reveal that Yb species are leached from the surface of the Ga O into the
1
4
2
solution. As mentioned above, the Yb species function as CO capture and storage
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2
Enhancement of CO evolution by modification of Ga O3
with rareꢀearth elements for the photocatalytic conversion of CO by H O
2
2
1
2
3
4
materials and can absorb CO
2
molecules as the Yb carbonate hydrates. The amount of
0
1
2
3
4
5
6
7
8
9
0
1
2
3
4
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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
absorbed CO decreases with the declining surface Yb content, resulting in the
2
degradation of CO evolution. This is why the rate of CO formation gradually decreases
over time.
5
6
2 3 2 3
Figure 7 Normalized XRD patterns of Ga O (a) and Ag/3.0 Yb/Ga O before reaction
7
8
(b), and after photoirradiation for 7 h (c), 21 h (d), and 39 h (e). Patterns were
normalized using the (111) plane of Ga
2
3
O .
9
10
11
In this work, we have shown that the modification of Ga
2 3
O with REs clearly
enhances the evolution of CO. However, H is also produced from water splitting that
2
1
1
2
3
occurs concomitantly. We previously reported that ZnGa
2
O
4
on the surface of Ga
2 3
O
23
effectively suppresses the evolution of H
2
. Figure 8 shows the rates of H
2
, O , and CO
2
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2
Enhancement of CO evolution by modification of Ga O3
with rareꢀearth elements for the photocatalytic conversion of CO by H O
2
2
1
2
3
4
5
6
7
8
9
0
1
formation and the selectivity toward CO evolution over Ag/3.0 Yb/Ga
2 3
O modified with
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
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5
5
5
5
5
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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
various amounts of Zn. The loading of Yb was fixed at 3.0 mol% and that of the Zn
species was varied from 0.5 to 5.0 mol%. As the amount of Zn increases, the rate of H
2
2 4
formation is clearly suppressed, which indicates that ZnGa O can control the amount of
2 2
H evolved in the presence of the Yb species. The rate of H formation becomes stable
−1
2 3
above 2.0 mol% Zn. In the case of Ag/3.0 Ybꢀ3.0 Zn/Ga O , 150 ꢁmol CO is
−
1
2
generated, compared to the very low amount of H , 37.6 ꢁmol , affording 80%
2
selectivity toward CO evolution. The stoichiometric rate of O formation is 103 µmol
−1
2 3
h . Thus, the modification of the surface of Ga O with Zn efficiently suppresses the
1
1
production of H
2
, and further surface modification with Yb simultaneously enhances the
evolution of CO.
1
2
13
Figure 8 Formation rates of H
2
(gray), O
2
(white), and CO (black) and selectivity
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Enhancement of CO evolution by modification of Ga
2
with rareꢀearth elements for the photocatalytic conversion of CO by H O
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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
2
O
3
2
1
2
3
toward CO evolution (circles) over Ag/3.0 Yb/Ga O with various amounts of Zn
2 3
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
species. Reaction conditions are the same as those in Table 1.
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Enhancement of CO evolution by modification of Ga
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with rareꢀearth elements for the photocatalytic conversion of CO by H O
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2
O
3
2
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0
1
Conclusion
The modification of the surface of Ga O with rareꢀearth (RE) elements
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
2
3
improved the rate of CO formation in the photocatalytic conversion of CO using H O
2
2
13
as an electron donor. An experiment using isotopically labeled CO and five blank
2
tests clarified that the CO introduced in the gas phase is reduced to CO under
2
photoirradiation. It is expected that RE (CO ) •nH O and/or RE (OH)
(CO ) , which
3 x
2
3 3
2
2
2(3ꢀx)
are generated in the aqueous solution of NaHCO , function as CO capture and storage
3
2
materials and supply the CO molecules to the active sites of the Ag coꢀcatalyst. As a
2
consequence, Agꢀloaded/Ybꢀmodified Ga O showed a higher rate of CO formation
2
3
1
1
than Agꢀloaded Ga O , evolving 4.5 mmol of CO after 39 h photoirradiation. The
2 3
further modification of the surface of the Ybꢀmodified Ga O with Zn efficiently
2
3
12
suppressed the production of H . Thus, we succeeded in designing a novel system for
2
1
1
1
3
4
5
the selective photocatalytic conversion of CO using H O as an electron donor over
2 2
Agꢀloaded Ybꢀ and Znꢀmodified Ga O .
2
3
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Enhancement of CO evolution by modification of Ga
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with rareꢀearth elements for the photocatalytic conversion of CO by H O
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1
1
1
1
1
1
1
1
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2
2
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2
O
3
2
1
2
3
4
5
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8
9
0
1
Acknowledgement
0
1
2
3
4
5
6
7
8
9
0
1
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0
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9
0
1
2
3
4
5
6
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8
9
0
1
2
3
4
5
6
7
8
9
0
This study was partially supported by a GrantꢀinꢀAid 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)
program, supported by the Japan Science and Technology Agency (JST); and the
Program for Elements Strategy Initiative for Catalysts & Batteries (ESICB),
commissioned by the MEXT of Japan. The Xꢀray absorption experiments were
performed with the approval of the Japan Synchrotron Radiation Research Institute
(JASRI) (Proposal No. 2015A1764).
1
1
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O
3
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1
Reference
1. Lewis, N. S.; Nocera, D. G., Powering the planet: chemical challenges in solar
energy utilization. Proc. Natl. Acad. Sci. U. S. A. 2006, 103 (43), 15729ꢀ15735.
2. Morris, A. J.; Meyer, G. J.; Fujita, E., Molecular Approaches to the
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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
Photocatalytic Reduction of Carbon Dioxide for Solar Fuels. Acc. Chem. Res. 2009, 42
(12), 1983ꢀ1994.
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