CO2 capture, storage, and conversion using a praseodymium-modified Ga2O3 photocatalyst

Article abstract of DOI:10.1039/c7ta04918h

Praseodymium-modified gallium oxide (Pr/Ga₂O₃) was found to show enhanced activity and selectivity toward CO evolution in the photocatalytic conversion of CO₂ using H₂O as an electron donor in an aqueous solution of NaHCO₃ as compared to those of bare Ga₂O₃. The as-prepared Pr species, including Pr(OH)₃ and Pr₂O₂CO₃, on the surface of Ga₂O₃ were transformed into Pr hydroxycarbonates (Pr₂(OH)_2(3-x)(CO₃)_x) and Pr carbonate hydrates (Pr₂(CO₃)₃·8H₂O) in an aqueous solution of NaHCO₃, and then the Pr₂(OH)_2(3-x)(CO₃)_x was further transformed into Pr₂(CO₃)₃·8H₂O by CO₂ bubbling under photoirradiation. This indicates that CO₂ molecules dissolved in water can be captured and stored using Pr species in an aqueous solution of NaHCO₃ under CO₂ bubbling. More importantly, Pr₂(OH)_2(3-x)(CO₃)_x and Pr₂(CO₃)₃·8H₂O accumulated on the surface were decomposed to CO over the Ga₂O₃ photocatalyst with a Ag cocatalyst. Consequently, Ag/Pr/Ga₂O₃ exhibits much higher activity (249 μmol h^-1 of CO) than the pristine Ag-loaded Ga₂O₃ (136 μmol h^-1 of CO).

Full text of DOI:10.1039/c7ta04918h

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Kun Chang, Zhaorong Chang et al.
Bubble-template-assisted synthesis of hollow fullerene-like
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CO2 capture, storage, and conversion using a praseodymium-
modified Ga2O3 photocatalyst
Zeai Huang,a Kentaro Teramura,a,b* Hiroyuki Asakura,a,b Saburo Hosokawa,a,b Tsunehiro Tanakaa,b*
Received 00th January 20xx,
Accepted 00th January 20xx
Praseodymium-modified gallium oxide (Pr/Ga2O3) was found to show enhanced activity and selectivity toward CO
evolution in the photocatalytic conversion of CO2 using H2O as an electron donor in an aqueous solution of NaHCO3 as
compared to those of bare Ga2O3. The as-prepared Pr species, including Pr(OH)3 and Pr2O2CO3, on the surface of Ga2O3
were transformed into Pr hydroxycarbonates (Pr2(OH)2(3-x)(CO3)x) and Pr carbonate hydrates (Pr2(CO3)3·8H2O) in an aqueous
solution of NaHCO3, and then the Pr2(OH)2(3-x)(CO3)x was further transformed to Pr2(CO3)3·8H2O with CO2 bubbling under
photoirradiation. This indicates that CO2 molecules dissolved in water can be captured and stored using Pr species in an
aqueous solution of NaHCO3 under CO2 bubbling. More importantly, Pr2(OH)2(3-x)(CO3)x and Pr2(CO3)3·8H2O accumulated on
the surface were decomposed to CO over the Ga2O3 photocatalyst with a Ag cocatalyst. Consequently, Ag/Pr/Ga2O3
exhibits much higher activity (249 µmol h−1 of CO) than the pristine Ag-loaded Ga2O3 (136 µmol h−1 of CO).
DOI: 10.1039/x0xx00000x
www.rsc.org/
stoichiometric oxidation of H2O, which means that O2 is
evolved equivalently to the reduction products of H2O and
CO2, thus confirming the role of H2O as an electron donor. For
Introduction
CO2 is a major greenhouse gas and its atmospheric levels
have risen quickly during the last decades. This is largely
responsible for recent rapid changes in global climate. Carbon
capture and storage (CCS) using physical, chemical, and/or
biochemical methods is thought to be a safe, effective, and
environmentally friendly strategy to mitigate CO2 emissions.1-3
However, the development of methods for dealing with the
captured CO2 remains a significant challenge because the
desorption and compression of CO2 after capture is highly
energy demanding. Recently, carbon capture and utilization
(CCU) has been developed as an alternative approach to CCS.4-
instance, one mole of H2O is oxidized by holes to produce four
moles of electrons, four moles of protons, and one mole of O2.
Protons and electrons participate in the reduction of CO2.
Thus, the number of holes consumed should be equal to the
number of electrons consumed. In addition, the reduction
products of CO2 such as CO, HCOOH, HCHO, CH3OH, and CH4
should be the main products instead of H2. For example, when
CO2 is converted to CO, the selectivity toward CO evolution
and the balance between the consumed electrons and holes
can be expressed as follows:
6
Since the captured CO2 is converted to useful chemicals in
Selectivity toward CO (%) = 100 × 2
Consumed e−/h+ = (2
CO + 2 H2) / 4RO2
where RCO RH2, and RO2 represent the rates of CO, H2, and O2
RCO / (2RCO + 2RH2)
the CCU process, CCU has an obvious advantage over CCS. The
conversion of CO2 to chemical feedstocks and hydrocarbon
fuels such as carbon monoxide (CO),7 formic acid (HCOOH),8
formaldehyde (HCHO),9 methanol (CH3OH),10 and methane
(CH4),11 have been reported as examples of CCU processes.
Among CO2 conversion methods, photocatalytic conversion
of CO2 by H2O as an electron donor with heterogeneous
catalysts, which is also known as artificial photosynthesis, is a
promising strategy for CCU.12, 13 Semiconductors activation by
light leads to the formation of electrons (e−) and holes (h+),
which participate in photoreduction and photooxidation,
respectively. In such a process, it is important to achieve the
R
R
,
formation, respectively. Thus, the selectivity toward CO is
preferable when it is higher than 50%, and the consumed e−/h+
should be close to 1.0 when H2O is used as an electron donor.
Recently, several catalysts that exhibit activity for the
photocatalytic conversion of CO2 to CO using H2O as an
electron donor have been reported by our group and others.7,
14-20
Stoichiometric amounts of H2 and CO were evolved as
reduction products, and O2 was evolved as an oxidation
product in all these studies, which means that the number of
holes generated and consumed is consistent with that of
electrons. Therefore, we have sought to enhance the
conversion of CO2 and selectivity toward CO evolution in the
photocatalytic conversion of CO2 using H2O as an electron
donor.
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, tanakat@moleng.kyoto-u.ac.jp
b.Elements Strategy Initiative for Catalysts & Batteries (ESICB), Kyoto University, 1-
30 Goryo-Ohara, Nishikyo-ku, Kyoto 615-8245, Japan.
To achieve high efficiency for the photocatalytic conversion
of CO2, activation of CO2 by adsorption should be promoted as
Electronic Supplementary
DOI: 10.1039/x0xx00000x
Information
(ESI)
available.
See
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the first step.21 Generally speaking, alkali- and alkaline-earth- at 1273 K for 6 h. Pr-modified Ga2O3 (Pr/Ga2O3) was prepared
metal-related materials play important roles in the surface- by an impregnation method. Ga2O3 was introduced to an
capture of CO2 since these materials can be used as solid bases aqueous solution of 0.0–10.0 mol% Pr(NO3)3. The samples
(Lewis basic sites) for the adsorption of CO2. Alkali- and were labeled x Pr/Ga2O3, where x denotes the content of Pr as
alkaline-earth-metal-modified photocatalysts have been a molar percentage. The suspension was stirred at 353 K for 2
reported to show enhanced photocatalytic activities for CO2 h to evaporate the water. The sample was then calcined at
conversion using H2O as an electron donor.22, 23 CCS on the 1223 K for 6 h in air. Praseodymium hydroxide (Pr(OH)3) was
surface of photocatalysts is desirable for enhancing activity in fabricated by
the photocatalytic conversion of CO2 as it achieves capture, praseodymium nitrate (Pr(NO3)3) as a Pr precursor with an
storage, and conversion of CO2 in a single step. aqueous solution of NH3 at pH 11.0. Praseodymium
a
typical precipitation method using
Rare-earth (RE) metals can be alternatives to alkali and oxycarbonate (Pr2O2CO3) was prepared by calcining the as-
alkaline earth metals for CO2 adsorption.24, 25 Bernal et al.26, 27 prepared Pr(OH)3 at 773 K for 2 h under a CO2 atmosphere.
reported the behavior of Pr oxides when exposed to air at The fabricated photocatalysts (0.5 g) were modified with 1.0
room temperature. Pr oxides can be transformed into wt% of a silver (Ag) cocatalyst by a typical photodeposition
hydroxide and carbonate species on surfaces easily, which method, i.e., the photocatalysts were dispersed in an aqueous
indicates that Pr has the potential for CO2 capture and storage solution of AgNO3 and then illuminated for 2 h in an Ar flow.
at room temperature. Other kinds of rare-earth elements, such
Catalyst characterization
as La,28 Sm,29 Nd,30 Yb,29 and Y,31 showed similar abilities to
form hydroxide and/or carbonate species on surfaces when
exposed to air. More importantly, Borchert et al.32 reported were recorded using
X-ray diffraction (XRD) patterns of the Pr/Ga2O3 samples
Rigaku Ultima IV powder
a
that CO could not be adsorbed on Pr cations, which means diffractometer. The UV-Vis diffuse reflectance spectra (UV-vis
that Pr cations could promote CO desorption in the DRS) were measured with a JASCO V-670 spectrometer
photocatalytic conversion of CO2 to CO. On the basis of these equipped with an integrating sphere. Spectralon®, which was
interesting features, we expected that Pr would be a promising supplied by Labsphere Inc., was used as a standard reflection
material for the adsorption of CO2 and desorption of CO. Ga2O3 sample. The Pr L3-edge X-ray absorption fine structure (XAFS)
is reported to show good activity for the photocatalytic measurements were obtained in transmission and
conversion of CO2 using H2O as an electron donor, however, fluorescence modes at the BL01B1 beamline of the SPring-8
the selectivity toward CO evolution is not satisfied.7 Thus, the synchrotron radiation facility (Hyogo Pref., Japan). Scanning
combination of Pr with Ga2O3 is expected to be a good strategy electron microscopy (SEM) images were obtained using a field-
for the enhancement of CO evolution. In this work, we present emission scanning electron microscope (SU-8220, Hitachi High-
a new strategy for carbon capture, storage, and conversion Technologies) equipped with an energy-dispersive X-ray
(CCSC) as a new one-step CCU process in which Pr is used to spectroscopy (EDS) unit at an acceleration voltage of 15.0 kV.
enhance the CO2 capture and storage ability of a gallium oxide The Brunauer–Emmett–Teller (BET) surface area was
(Ga2O3) surface for subsequent photocatalytic conversion of measured by N2 adsorption at 77 K using a volumetric gas
CO2 to CO using H2O as an electron donor.
adsorption apparatus (BELmini, Bel Japan, Inc.). Temperature
programmed desorption of CO2 (CO2-TPD) profile was
recorded by a TPD-1-AT instrument supplied by the BEL Japan
Inc. The samples were heated at a rate of 10 K min−1 from 313
K to 1223 K under a He atmosphere.
Experimental
Catalyst preparation
Photocatalytic activity test for the conversion of CO2 with
H2O
Ga2O3 was obtained by the calcination of gallium oxide
hydroxides (GaOOH), prepared via the hydrolysis of gallium
nitrate (Ga(NO3)3) with an aqueous solution of ammonia (NH3),
The photocatalytic conversion of CO2 using H2O, including
Table 1 Surface area, CO2 stored, formation rates of product, selectivity toward CO evolution, and electron balance for the
photocatalytic conversion of CO2 using H2O as an electron donor.
Formation rate / μmol h−1[c]
Surface area /
m2 g−1[a]
CO2 stored /
μmol g−1[b]
Entry
Photocatalyst
Selec. (%)[d]
e−/h+
H2
O2
CO
136
215
bare Ga2O3
1.0 Pr/Ga2O3
3.0 Pr/Ga2O3
5.0 Pr/Ga2O3
10.0 Pr/Ga2O3
12
9.4
8.9
6.9
8.0
7.90
119
382
425
498
131
102
124
148
51.0
67.7
79.4
82.5
80.7
1.07
1.07
1.04
1.11
1.14
1
2
3
4
5
64.7
50.1
39.0
150.4
128.5
88.9
249
236
163
[a]BET surface area. [b]Measured by CO2-TPD. [c]Formation rate after 1 h of irradiation. [d]Selectivity toward CO evolution.
2 | J. Name., 2012, 00, 1-3
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the Ag cocatalyst modification of the photocatalysts, was
carried out in a quartz inner-irradiation type reaction vessel
(1.0 L) in
a quasi-flow batch system. The synthesized
photocatalyst (0.5 g) was dispersed in an aqueous solution of
NaHCO3 (0.1 M). CO2 gas (99.999%) was bubbled into the
solution at a flow rate of 30 mL min−1. The suspension was
irradiated with a 400 W high-pressure mercury lamp through a
quartz filter equipped with a cooling water system. The CO, H2,
and O2 products were analyzed by gas chromatography with
thermal conductivity detection (GC-TCD) on a Shimadzu GC-8A
chromatograph equipped with a 5Å molecular sieve column
(carrier gas: Ar) and by GC with flame ionization detection
(FID) with a methanizer and a Shincarbon ST column (carrier
gas: N2).
Fig. 1 XRD patterns of Ga2O3 with different amounts of Pr: (a) 0
mol%; (b) 1.0 mol%; (c) 3.0 mol%; (d) 5.0 mol%; and (e) 10.0 mol%.
Closed diamond: Pr-derived species. Open triangle: PrGaO3.
Results and discussion
The XRD patterns of Ga2O3 with Pr loadings from 0.0 to
10.0 mol% are shown in Fig. 1. The pattern of bare Ga2O3 (0.0
Table 1 shows the rate of product formation for the
photocatalytic conversion of CO2 in an aqueous solution of 0.1
M NaHCO3 over Ag/Ga2O3 with or without Pr modification. In
all cases, CO and H2 are evolved as the reduction products of
CO2 and H2O, respectively. Other reduction products, such as
CH4, HCOOH, HCHO, and CH3OH, were not detected by GC and
HPLC in the present study. In addition, almost stoichiometric
amounts of O2 are observed (electron balance was closed to
1.0), indicating the use of H2O as an electron donor. It should
be noted that all the Pr/Ga2O3 catalysts show higher CO
formation rates than that of bare Ga2O3 (0.0 Pr/Ga2O3).
Therefore, we succeeded in enhancing the photocatalytic
activity toward CO evolution using Pr modification. Moreover,
the selectivity toward CO evolution is significantly enhanced
with different amounts of Pr modification. Notably, 3.0
Pr/Ga2O3 shows the highest photocatalytic activity toward CO
evolution (249 µmol h−1) in Table 1, which is 1.8 times that
over bare Ga2O3 (136 µmol h−1). The selectivity toward CO
evolution was achieved at 79.4% using 3.0 Pr/Ga2O3, which is
much higher than using bare Ga2O3.
Pr/Ga2O3) corresponds to pure β-Ga2O3 phase (Fig. 1(a)). No
shifts in the diffraction peaks for different amounts of Pr are
observed relative to those of Ga2O3, therefore, the Pr ions do
not act as a dopant in the bulk Ga2O3. Several peaks are
observed in addition to those assigned to the β-Ga2O3 phase
when different amounts of Pr are loaded onto Ga2O3 (Fig. 1(b–
e)). Although the sample was calcined at high temperature in
air, these peaks are not consistent with those derived from Pr
oxides such as Pr2O3 and Pr6O11 (Fig. S3(a) and (b)). Instead, the
peaks at 26.3° and 27.5° are very closed to peaks assigned to
Pr2O2CO3 and Pr(OH)3, respectively (Fig. S3(c) and (d)). As
mentioned in the introduction section, Pr hydroxide and
carbonate species form readily when Pr oxide is exposed to
air.26, 27 Thus, it is reasonable that these phases of Pr(OH)3 and
Pr2O2CO3 are formed when a small amount of Pr (0.0–10.0
mol%) is loaded onto Ga2O3. Unlike 1.0–5.0 Pr/Ga2O3, 10.0
Pr/Ga2O3 shows additional peaks that can be easily identified
as PrGaO3 (Fig. 1(e)). The peak intensities of the Pr-derived
species in 10.0 Pr/Ga2O3 are decreased as compared with
those of 5.0 Pr/Ga2O3. Thus, a high concentration of Pr (10.0
mol%) in the precursor favors the formation of PrGaO3 rather
than Pr-derived hydroxide, and carbonate.
We should note that the surface area is not dependent on
the photocatalytic activity toward CO evolution, since the
surface areas of all the catalysts were decreased after the Pr
modification (Table 1). UV-Vis DR spectra of Pr/Ga2O3 showed
a slight red shift of absorption edge as compared to that of the
bare Ga2O3 (Fig. S1); however, it is hard to say that the change
of absorption edge affects the photocatalytic activity since the
red shift of absorption edge disappeared after reaction (Fig.
S2). On the other hand, we estimated amount of CO2 captured
and stored after reaction using the peak area in the CO2-TPD
profile as shown in Table 1. Note that the amount of CO2
captured and stored over bare Ga2O3 was very small (7.9 μmol
g−1); however, the Pr modification made it dramatically
enhanced. It is discussed that CO2 dissolved in water is easily Fig. 2 (A) Pr L3-edge XANES spectra and (B) EXAFS spectra of (a) 3.0
Pr/Ga2O3, (b) Pr(OH)3, (c) Pr2O2CO3, and (d) Pr6O11. (C) FTIR spectra
of (a) bare Ga2O3, (b) 3.0 Pr/Ga2O3, (c) Pr(OH)3, and (d) Pr2O2 CO3.
captured and stored by the Pr modification. The amount of
CO2 captured and stored did not depend on the photocatalytic
formation rate of CO. This phenomenon will be discussed later.
The Pr modification effectively enhances the conversion of CO2
and selectivity toward CO evolution in the photocatalytic
conversion of CO2 by H2O as an electron donor.
Fig. 2(A) and 2(B) show the Pr L3-edge normalized XANES
and the k3-weighted EXAFS spectra of 3.0 Pr/Ga2O3, with
Pr(OH)3, Pr2O(CO3)2, and Pr6O11 as references. The peak
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position of the white line in the Pr L3-edge XANES spectrum of Pr/Ga2O3 completely disappear after 5 h of photoirradiation. In
3.0 Pr/Ga2O3 is the same as those of Pr(OH)3, and Pr2O(CO3)2 addition, some new peaks derived from Pr2(CO3)3·8H2O appear
(Fig. 2(a), (b) and (c) in (A)), suggesting that the Pr species in in the XRD patterns of Pr/Ga2O3.34 This means that Pr(OH)3 and
these materials exist in the same average oxidation state, but Pr2O2CO3 react with other species in the solution to form
one that is slightly lower than that of Pr6O11 (Fig. 2(d) in (A)). Pr2(CO3)3·8H2O during the reaction. In the case of 5.0
This is easily understood because the state of Pr in Pr6O11 is Pr/Ga2O3, the peak at 31.2° assigned to the as-prepared Pr-
actually a mixture of Pr3+ and Pr4+, while only Pr3+ is contained derived species remains after 5 h of photoirradiation (Figs. 1(d)
in our catalysts (Fig. 2(a) in (A)). The k3-weighted EXAFS and 4(d)), and the intensities of the peaks assigned to
spectrum of the 3.0 Pr/Ga2O3 is not completely consistent with Pr2(CO3)3
any of the references; however, parts of the spectrum are indicating that a high concentration of Pr on the Ga2O3 hinders
similar to those of Pr(OH)3 and Pr2O2CO3 at low wavenumber the conversion of Pr(OH)3 and Pr2O2CO3 to Pr2(CO3)3 8H2O
(Fig. 2(B)). In the FTIR spectra of 3.0 Pr/Ga2O3 (Fig. 2(C)), he during the reaction. It was also found that PrGaO3 species do
·8H2O are much lower than that of 3.0 Pr/Ga2O3,
·
t
peak at 1381 cm−1 corresponding to carbonates was the same not readily change since all the peaks for PrGaO3 are still
as that of Pr(OH)3. As discussed in the introduction section, we present for 10.0 Pr/Ga2O3 and the related intensities of these
expected that Pr(OH)3 was reported to easily adsorb CO2 in the peaks are not significantly decreased (Figs. 1(e) and 4(e)).
air and then changed into carbonate.26, 27, 33 Moreover, the These results show that Pr(OH)3 and Pr2O2CO3 on Ga2O3 at low
peak at 1498 cm−1 is consistent with that of the Pr2O2CO3 concentration (1.0 and 3.0 mol%) tend to completely change
reference due to the ν3 mode of carbonate. However, in the to Pr2(CO3)3
case of Ga2O3, these peaks are rarely observed at this range concentration of Pr (5.0 and 10.0 mol%) do not completely
(Fig. 2(C)). These results indicated that our catalysts contain change to Pr2(CO3)3 8H2O after 5 h of photoirradiation (Fig.
many kinds of Pr-derived species, Pr(OH)3 and Pr2O2CO3 were 4(d) and (e)). Thus, it is important to use a suitably low amount
·8H2O (Fig. 4(b) and (c)), while those at high
·
formed on Ga2O3 during the preparation process.
of Pr on the surface of the Ga2O3 to enable efficient CO2
capture and storage.
Fig. 3 SEM images of (a) 3.0 Pr/Ga2O3, (b) one selected single crystal
and corresponding element mapping images for (c) Ga, (d) O, and
(e) Pr.
The SEM and related EDS mapping (Fig. 3) of 3.0 Pr/Ga2O3
indicate that its elliptical rod-like morphology is maintained as
compared to that of Ga2O3 (Fig. 3(a) and S4(a)). The amount of
Pr shows no obvious effect on the morphology and grain size
of the Ga2O3 (Figs. 3(a) and S4). The sizes of the Ga2O3 particles
with different amount of Pr are all around 1 μm. Thus, particle
size has no obvious effect on activity in the photocatalytic
conversion of CO2. The Ga and O elements were well mapped
with its morphology using a single crystal of 3.0 Pr/Ga2O3 (Fig.
3(b)–(d)). The EDS mapping confirms that Pr is well dispersed
on the Ga2O3 (Fig. 3(e)). Based on the XRD, XAFS, FTIR, and
SEM results, Pr(OH)3 and Pr2O2CO3 are found to be well loaded
on the surface of the Ga2O3 as a natural CO2 capture and
storage material, which should enhance its activity for the
photocatalytic conversion of CO2 by H2O.
Fig. 4 XRD patterns of Ga2O3 with different amounts of Pr after
photocatalytic conversion of CO2: (a) 0 mol%; (b) 1.0 mol%; (c) 3.0
mol%; (d) 5.0 mol%; and (e) 10.0 mol%. Closed circle:
Pr2(CO3)3·8H2O. Open triangle: PrGaO3. Closed diamond: Pr-derived
species.
To ascertain why Pr(OH)3 and Pr2O2CO3 are changed to
Pr2(CO3)3·8H2O after the reaction, the as-prepared 3.0
Pr/Ga2O3 was treated under various conditions. Considering
that NaHCO3 and CO2 are two crucial factors in the formation
of rare-earth carbonates,25, 26, 28 the treatment conditions for
samples in aqueous solutions were changed step by step. Fig. 5
denotes the XRD pattern of 3.0 Pr/Ga2O3 treated under various
conditions. When 3.0 Pr/Ga2O3 treated in water under flowing
Ar for 5 h, there is no change in the XRD patterns before and
after the treatment (Figs. 1(c) and 5(a)), indicating that Pr(OH)3
and Pr2O2CO3 are very stable in pure water. In contrast, the
peaks assigned to Pr(OH)3 and Pr2O2CO3 disappear completely
under flowing CO2 in water (Fig. 5(b)). Since basic Pr(OH)3
species exist in 3.0 Pr/Ga2O3 (Figs. 1(c) and 2), and the pH
changes from 7.21 under flowing Ar to 3.89 under flowing CO2
in water, it is not surprising that these species disappear after
The XRD patterns of Pr/Ga2O3 with different amounts of Pr
after 5 h of photoirradiation are shown in Fig. 4. There is no
difference before and after the photocatalytic conversion of
CO2 over bare Ga2O3 (0.0 Pr/Ga2O3) (Figs. 1(a) and 4(a)). This is
not surprising as β-Ga2O3 is very stable under photoirradiation.
Interestingly, all the peaks assigned to the as-prepared Pr-
derived species such as Pr(OH)3 and Pr2O2CO3 in 1.0 and 3.0
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reaction owing to the corrosion of these basic species under
acidic conditions.
It is important to confirm whether Pr2(OH)2(3-x)(CO3)x and
Pr2(CO3)3 8H2O formed on the surface of Ga2O3 can be
·
decomposed by photoirradiation. Thus, 3.0 Pr/Ga2O3 was
treated in NaHCO3 solution with flowing Ar or CO2 to obtain
Pr2(OH)2(3-x)(CO3)x and/or Pr2(CO3)3·8H2O-modified Ga2O3.
These catalysts were used for decomposition tests without
neither the addition of NaHCO3 nor flowing CO2, while the
other conditions were not changed. Fig. 6 shows the formation
rate of CO for the decomposition of Pr2(OH)2(3-x)(CO3)x and
Pr2(CO3)3·8H2O under photoirradiation using 3.0 Pr/Ga2O3 with
a Ag cocatalyst pretreated using different methods. Bare
Ga2O3 with and without pretreatment exhibits negligible CO
evolution (Fig. 6(a) and (b)). Conversely, the as-prepared 3.0
Pr/Ga2O3 is active for the decomposition of Pr-derived species,
and CO is observed during photoirradiation (Fig. 6(c)). As
discussed above, Pr2O2CO3 on Ga2O3 is a carbon-containing
species. Therefore, it is possible that CO can be formed with a
Ag cocatalyst during photoirradiation. Pr/Ga2O3 pretreated in
NaHCO3 with Ar shows higher CO formation rate than that of
as-prepared Pr/Ga2O3 (Fig. 6(c) and (d)). Furthermore, the CO
evolution rate is highest for Pr/Ga2O3 pretreated in NaHCO3
with flowing CO2 (Fig. 6(e)). We conclude that Pr2(OH)2(3-
Fig. 5 XRD patterns of 3.0 Pr/Ga2O3 dispersed for 5 h in water under
(a) Ar flow and (b) CO2 flow; and in 0.1 M NaHCO3 aqueous solution
under (c) Ar flow and (d) CO2 flow. Closed circle: Pr2(CO3)3·8H2O.
Closed square: Pr hydroxycarbonates (Pr2(OH)2(3-x)(CO3)x). Closed
diamond: Pr-derived species.
Interestingly, all the peaks of Pr(OH)3 and Pr2O2CO3 are
replaced by new peaks in an aqueous solution of NaHCO3
under a flow of gaseous Ar (Fig. 5(c)). These peaks could be
x)(CO3)x and Pr2(CO3)3·8H2O can be decomposed to CO during
assigned to Pr2(OH)2(3-x)(CO3)x and Pr2(CO3)3·8H2O. It has been
photoirradiation. From this result, it is clearly that only Ga2O3
shows no ability for the surface capture and storage of
aqueous CO2 molecules. In contrast, CO2 capture and storage
can be easily performed on Pr/Ga2O3, and it can be
decomposed to CO during photoirradiation.
reported that rare-earth hydroxycarbonates are easily formed
at pH > 6.0 and with a low concentration of CO2 released from
urea.35 Rare-earth carbonate hydrates are formed in an
aqueous solution of NaHCO3 using Pr(NO3)3 as a precursor.36
Thus, it is reasonable that both Pr2(OH)2(3-x)(CO3)x and
Pr2(CO3)3·8H2O are formed in the NaHCO3 solution under Ar
flow (pH = 8.3). Notably, 3.0 Pr/Ga2O3 treated in 0.1 M NaHCO3
under flowing CO2 show the same phases (a mixture of
Pr2(CO3)3
reaction (Figs. 4(c) and 5(d)), suggesting that the change of
Pr(OH)3 and Pr2O2CO3 to Pr2(CO3)3 8H2O is due to not
photoirradiation but the NaHCO3 solution with flowing CO2.
Pure Pr(OH)3 is also found to change to Pr2(CO3)3 8H2O in 0.1
·8H2O and Ga2O3) as those of 3.0 Pr/Ga2O3 after the
·
·
M NaHCO3 under flowing CO2 in 20 h (Fig. S5). Thus, there is no
doubt that new Pr species such as Pr2(OH)2(3-x)(CO3)x and
Pr2(CO3)3
of NaHCO3, and then Pr2(OH)2(3-x)(CO3)x is further transformed
to Pr2(CO3)3 8H2O alone under flowing CO2. We conclude that
·8H2O are generated on Ga2O3 in an aqueous solution
Fig.
6 Formation rate of CO using bare Ga2O3 (a) without
·
pretreatment, (b) pretreated in 0.1 M NaHCO3 under flowing CO2;
and using 3.0 Pr/Ga2O3 (c) without pretreatment, and pretreated in
0.1 M NaHCO3 under (d) flowing Ar and (e) flowing CO2. Weight: 0.5
g; Ar flow: 30 mL min-1; solution: 1.0 L pure water; light source: 400
W Hg lamp; Ag loading: 1.0 wt%.
Pr(OH)3 and Pr2O2CO3 can capture and store CO2 molecules
dissolved in an aqueous solution of NaHCO3, and then
Pr2(OH)2(3-x)(CO3)x and Pr2(CO3)3
surface of the Ga2O3.
·8H2O are generated on the
Scheme 1 Formation of Pr-species as a CO2 capture and storage material in an aqueous solution of NaHCO3.
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We proposed a probable mechanism for the capture,
Conclusions
storage, and conversion of CO2 over Ag/Pr/Ga2O3 in Scheme 1.
Pr(OH)3 and Pr2O2CO3 are formed on the surface of Ga2O3 after
calcination in air when a suitable amount of Pr is loaded onto
the Ga2O3. Pr(OH)3 and Pr2O2CO3 are partially converted into
Pr2(OH)2(3-x)(CO3)x and Pr2(CO3)3·8H2O in an aqueous solution of
NaHCO3. Accordingly, it is expected that Pr(OH)3 and Pr2O2CO3
function as CO2 capture and storage materials, i.e., Pr2(OH)2(3-
x)(CO3)x and/or Pr2(CO3)3·8H2O derived from Pr(OH)3, Pr2O2CO3,
NaHCO3, and CO2 on the surface of the Ga2O3 are decomposed
to CO with a Ag cocatalyst under photoirradiation. Therefore,
the photocatalytic activity and selectivity toward CO are
dramatically increased, and the H2 evolution is suppressed, as
compared to those of bare Ga2O3 due to the CO2 storage
ability of Pr2(OH)2(3-x)(CO3)x and/or Pr2(CO3)3·8H2O on the
surface of the Ga2O3.
The modification of Ga2O3 with Pr improved the rate of CO
formation in the photocatalytic conversion of CO2 using H2O as
an electron donor. Pr(OH)3 and Pr2O2CO3, which are formed on
the surface of Ga2O3 after the preparation process, function as
a CO2 capture and storage material and produce Pr2(OH)2(3-
x)(CO3)x and/or Pr2(CO3)3·8H2O in an aqueous solution of
NaHCO3. Pr2(OH)2(3-x)(CO3)x and/or Pr2(CO3)3·8H2O supply CO2
molecules to the active sites of the Ag cocatalyst and/or Ga2O3.
Consequently, Ag/Pr/Ga2O3 shows a much higher rate of CO
formation than Ag-loaded Ga2O3. Five blank tests verified that
the CO2 introduced in the gas phase is reduced to CO under
photoirradiation. Thus, we succeeded in designing a novel CCU
process in which CO2 is captured, stored on the surface of
Ga2O3, and converted into
photocatalytically.
a useful chemical feedstock
The release and storage of CO2 were performed stably with
three times repetition (Fig. S6). As mentioned in Fig. 3,
Pr2(CO3)3
·
8H2O was formed from Pr(OH)3 and Pr2O2CO3 during
8H2O
Acknowledgements
the reaction. The peaks corresponding to Pr2(CO3)3
·
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) from 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 Elements Strategy Initiative for Catalysts &
Batteries (ESICB), commissioned by the MEXT of Japan. The
XAFS experiments were performed at a public beamline
BL01B1 in SPring-8 with the approval of JASRI (Proposal No.
2015A1764). Zeai Huang thanks the State Scholarship of China
Scholarship Council, affiliated with the Ministry of Education of
the P. R. China.
appeared in the XRD pattern of 3.0 Pr/Ga2O3 (Fig. S6(a)), and
then vanished after the calcination at 1223 K for 1 h (Fig.
S6(b)). Interestingly, the peaks reappeared when 3.0 Pr/Ga2O3
calcined was retreated in an aqueous solution of NaHCO3
under CO2 bubbling (Fig. S6(c)). The phenomena were
observed approximately reversibly (Fig. S6(d) and (e)). The
small peak corresponding to PrGaO3 appeared because the
sample was regenerated at high temperature. Amounts of CO2
captured and stored as Pr2(OH)2(3-x)(CO3)x and Pr2(CO3)3·8H2O
were estimated using CO2-TPD profiles (Fig. S7). All the
samples after the reaction (the 1st run), and calcination and
treatment in an aqueous solution of NaHCO3 under CO2
bubbling (the 2nd and 3rd runs) were pretreated under the
vacuum at 373 K for 1 h before the CO2-TPD measurement.
The shapes of peaks were not different from each other. On
the other hand, the areas of peak slightly decreased with the
repetition treatments. The amounts of CO2 captured and
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