Elemental Boron for Efficient Carbon Dioxide Reduction under Light Irradiation

Article abstract of DOI:10.1002/anie.201701370

The photoreduction of CO₂ is attractive for the production of renewable fuels and the mitigation of global warming. Herein, we report an efficient method for CO₂ reduction over elemental boron catalysts in the presence of only water and light irradiation through a photothermocatalytic process. Owing to its high solar-light absorption and effective photothermal conversion, the illuminated boron catalyst experiences remarkable self-heating. This process favors CO₂ activation and also induces localized boron hydrolysis to in situ produce H₂ as an active proton source and electron donor for CO₂ reduction as well as boron oxides as promoters of CO₂ adsorption. These synergistic effects, in combination with the unique catalytic properties of boron, are proposed to account for the efficiency of the CO₂ reduction. This study highlights the promise of photothermocatalytic strategies for CO₂ conversion and also opens new avenues towards the development of related solar-energy utilization schemes.

Full text of DOI:10.1002/anie.201701370

Angewandte
Communications
Chemie
International Edition: DOI: 10.1002/anie.201701370
German Edition: DOI: 10.1002/ange.201701370
Heterogeneous Catalysis
Hot Paper
Elemental Boron for Efficient Carbon Dioxide Reduction under Light
Irradiation
Guigao Liu, Xianguang Meng, Huabin Zhang, Guixia Zhao, Hong Pang, Tao Wang, Peng Li,
Tetsuya Kako, and Jinhua Ye*
Abstract: The photoreduction of CO2 is attractive for the
production of renewable fuels and the mitigation of global
warming. Herein, we report an efficient method for CO2
reduction over elemental boron catalysts in the presence of
only water and light irradiation through a photothermocatalytic
process. Owing to its high solar-light absorption and effective
photothermal conversion, the illuminated boron catalyst
experiences remarkable self-heating. This process favors CO2
activation and also induces localized boron hydrolysis to
in situ produce H2 as an active proton source and electron
donor for CO2 reduction as well as boron oxides as promoters
of CO2 adsorption. These synergistic effects, in combination
with the unique catalytic properties of boron, are proposed to
account for the efficiency of the CO2 reduction. This study
highlights the promise of photothermocatalytic strategies for
CO2 conversion and also opens new avenues towards the
development of related solar-energy utilization schemes.
for decades, such processes for CO2 conversion still remain
very inefficient, largely owing to insufficient solar light
utilization, the high energy barrier for CO2 activation, and
the sluggish kinetics of the involved multiple eÀ/H+ transfer
processes.[1c,3] Therefore, aiming at promoting the reaction
rate of CO2 conversion, increasing the utilization of solar
energy, and also from the view of reducing CO2 emission, the
development of more convenient and effective approaches to
solar-light-driven CO2 reduction is urgently required.
The photothermal effect can be described as the con-
version of photon energy into heat and has been considered as
an emerging technology of great interest for solar energy
harvesting.[4] As this technology can enable a remarkable and
prompt increase in the local temperature at the nanometer
scale, it has found a broad range of applications in various
fields, including solar power generation, localized water
heating, seawater desalination, and catalysis.[4a,b,5] It is well
known that thermodynamically stable CO2 molecules are
more reactive at high temperatures.[6] This thought made us
aware of the benefits of the photothermal effect for CO2
conversion. Particularly, in our recent work, the photoreduc-
tion of CO2 into CO and CH4 with H2 gas as the reductant
proceeded at a remarkable reaction rate through a photo-
thermocatalytic pathway, demonstrating the important role of
the photothermal effect in CO2 photoreduction.[3b] However,
for this specific method, the employment of H2 constitutes
a major obstacle to potential applications owing to the well-
recognized problem of hydrogen storage and transportation,
which is due to the intrinsically very low volumetric density of
hydrogen.[7] New ideas are needed that make use of the
photothermal effect for CO2 conversion, which might lead to
distinct new possibilities towards the integration of the
efficient production of renewable fuels from CO2 and the
utilization of solar energy even though this comes with huge
challenges.
C
onverting carbon dioxide (CO2), an abundant and inex-
pensive C1 feedstock as well as a potent greenhouse gas, into
value-added carbon products provides an attractive and
promising strategy to meet global energy demands and to
simultaneously alleviate anthropogenic climate change.[1] The
ideal route to achieve this goal is to reduce CO2 catalytically
by a photocatalyst with the utilization of solar light as the only
input energy source.[2] However, despite having been studied
[*] Dr. G. Liu, Dr. X. Meng, Dr. H. Zhang, Dr. G. Zhao, H. Pang,
Dr. T. Kako, Prof. J. Ye
International Center for Materials Nanoarchitectonics (WPI-MANA)
National Institute for Materials Science (NIMS)
1-1 Namiki, Tsukuba, Ibaraki 305-0044 (Japan)
E-mail: Jinhua.YE@nims.go.jp
H. Pang, Prof. J. Ye
Graduate School of Chemical Sciences and Engineering
Hokkaido University, Sapporo 060-0814 (Japan)
Boron has recently gained wide attention owing to its
fascinating properties and various potential applications in
photocatalysis, superconducting devices, neutron detectors,
and hydrogen production from water splitting through self-
hydrolysis.[8] Herein, we attempted to further extend the
applicability of this promising material, and investigated the
photothermal conversion over elemental boron particles for
the first time. More importantly, it was found that upon light
irradiation, the boron particles are heated by the photo-
thermal effect, and enable the direct and efficient catalytic
reduction of CO2 into CO and CH4, with reaction rates
outperforming most reported state-of-the-art photocatalysis
systems (see the Supporting Information, Table S1). This was
attributed to 1) the excellent photothermal effect of the boron
Dr. P. Li, Prof. J. Ye
TU-NIMS International Collaboration Laboratory
School of Materials Science and Engineering
Tianjin University, Tianjin (P.R. China)
and
Collaborative Innovation Center of Chemical Science and Engineer-
ing (Tianjin), Tianjin 300072 (P.R. China)
Dr. T. Wang
Jiangsu Key Laboratory of Materials and Technology for Energy
Conversion, College of Materials Science and Technology
Nanjing University of Aeronautics and Astronautics
Nanjing 210016 (P.R. China)
Supporting information and the ORCID identification number(s) for
the author(s) of this article can be found under:
http://dx.doi.org/10.1002/anie.201701370.
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particles, which induces a high local temperature that
facilitates CO2 activation; 2) self-hydrolysis of the boron
particles driven by the photothermal effect, which in situ
produces H2 as the active proton source and electron donor
for CO2 reduction and boron oxides as promoters of CO2
adsorption; and 3) the unique catalytic properties of boron.
In this study, commercial amorphous boron powders were
used as the starting materials. As boron has a strong capability
to form stable covalently bonded molecular networks, even
amorphous boron has previously been demonstrated to
inevitably contain regular boron icosahedra.[9] Accordingly,
as seen from our X-ray diffraction (XRD) patterns (Fig-
ure S1), some diffraction peaks belonging to b-rhombohedral
boron (JCPDS 01-080-0323) were observed for the amor-
phous boron samples, which is consistent with previous
reports.[9,10] The TEM image in Figure 1a shows the network
morphology of amorphous boron. The amorphous nature was
confirmed by high-resolution TEM (HRTEM) imaging (Fig-
ure 1b) as no obvious lattice fringes were observed. It should
be noted that in agreement with the XRD results, some
crystalline boron particles were also found during our TEM
studies.
visible regions of the solar spectrum that also tail even into
the NIR region. The former absorption probably originates
from the interband transitions, and the latter might be related
to the absence of long-range order in the amorphous boron
particles.[13] The intense absorption of amorphous boron
implies its high capability for the utilization of solar energy.
Considering the benefits of high temperatures for CO2
activation, we investigated the photothermal effect of this
amorphous boron material. The changes in temperature upon
light irradiation at different intensities are depicted in Fig-
ure 1d. As expected, upon light irradiation, an instant and
remarkable increase in the temperature of the sample was
observed (reaching the maximum temperature within
10 min). Furthermore, a nearly linear relationship between
the increase in temperature and the light intensity was
observed for the present system (Figure S3). At a light
intensity of 456 mWcmÀ2 (full arc with a maximum input
power of 300 W), the temperature of the sample increased up
to 4628C. Taken together, these results clearly demonstrate
the good photothermal properties of the amorphous boron
material, which should be beneficial for the activation of CO2
and subsequent photoreduction.
Before investigating the CO2 photoreduction, the CO2
adsorption properties of the amorphous boron material were
studied. For comparison, commercial TiO2, a widely used
photocatalyst for CO2 reduction, was also tested. As shown in
Figure S4, although these two compounds possess nearly
equal specific surface areas, the CO2 uptake of the amorphous
boron sample was more than twice as large than that of TiO2.
This should be related to the presence of boron oxides on the
surface of the boron material (Figure S5),[14] which are usually
considered as trapping sites for CO2.[15] Considering the
superior CO2 adsorption capacity and excellent photothermal
properties, this amorphous boron material was anticipated to
display high CO2 photoreduction activity.
In the CO2 photoreduction measurements, water was used
as the hydrogen source, and no additional sacrificial agents or
metal cocatalysts were employed in the reaction system.
These features should be advantageous for potential applica-
tions. As expected, full arc irradiation of the amorphous
boron material led to continuous formation of CO and CH4 by
CO2 photoreduction with reaction rates of 1.0 and
2.5 mmolhÀ1, respectively (Figure 2a). Even under visible-
light irradiation (l > 400 nm), the use of the amorphous boron
catalyst still led to the formation of CO at 0.8 mmolhÀ1 and
CH4 at 1.9 mmolhÀ1 (Figure 2b and Figure S6). To the best of
our knowledge, this is the first example of CO2 photo-
reduction over amorphous boron. Moreover, it should be
noted that the boron material outperformed most photo-
catalysis systems for CO2 photoreduction (Table S1). Under
long-term irradiation, both CH4 and CO production increased
in a nearly linear fashion (Figure S7). This suggests that the
catalytic activity of the amorphous boron material for CO2
photoreduction is fairly stable. When experiments were
performed without light irradiation or in the absence of the
boron material, CO and CH4 were not detected (Figure 2b).
This result indicates the necessity for light and boron in this
CO2 reduction. To further confirm the origin of the CO and
CH4 products, isotope tracer analyses were conducted with
Figure 1. a) TEM and b) HRTEM images and c) UV/Vis/NIR spectrum
of the amorphous boron material. The inserted background in gray in
(c) is a solar irradiation spectrum. d) The increase in temperature of
amorphous boron samples under light irradiation at different light
intensities.
Owing to the rather negative potential required for CO2
reduction and also the fairly positive potential needed for the
oxidation of water or sacrificial agents,[3a,11] a photocatalyst
that might enable photocatalytic CO2 reduction must possess
a very large band gap (usually the photocatalyst presents in
white or light yellow color),[1c,2c,12] thus significantly limiting
the efficiency of solar light utilization (Figure S2). In sharp
contrast, the amorphous boron catalyst employed here has
a typical dark brown color (Figure S2b), implying that it has
a wide light absorption range. According to the absorption
spectrum in Figure 1c, the amorphous boron particles show
excellent absorption features (absorbance > 1) in the UV and
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To answer this question, the gas
products formed upon the addition
of water and also the possible sur-
face changes of the boron catalysts
during the reaction were carefully
explored. Aside from the CO and
CH4 obtained by CO2 reduction,
hydrogen was also detected during
the test (Figure S11), which should
result from the hydrolysis of boron
under the high-temperature condi-
tions induced by the photothermal
effect.[8b,17] Accordingly, B(OH)3,
the other product of boron hydrol-
ysis, was also observed (Figur-
es S12–S14). B(OH)3 is a volatile
compound,[8b,17b,18] and once it has
formed on the surface of the illumi-
nated boron sample, it will be
gasified owing to the high temper-
ature, thus keeping the boron cata-
lyst surface exposed to the gaseous
reaction species (e.g., H2O, CO2, or
H2). Indeed, after the test, B(OH)3
was not observed on the boron
powders (Figure 2d) but as a deposit
on the walls of the reaction cell
probably owing to the low surface
temperature of the walls (Fig-
Figure 2. a) Gas evolution (CO and CH4) over the amorphous boron catalyst under full arc irradiation
with a Xe lamp. b) Gas-evolution rates under different conditions. c) Time course of CO and CH4
generation upon irradiation of the amorphous boron catalyst before and after the addition of water to
the reaction system. d) High-resolution B 1s XPS spectra of an amorphous boron sample before and
after CO2 photoreduction.
13CO2 and D2O as the substrates on a GC-MS system under
identical catalysis conditions. As shown in Figures S8 and S9,
we observed the formation of 13CH4, 13CO, and CD4. These
results clearly demonstrate that the carbon and hydrogen
sources of the formed reduction products are CO2 and H2O.
Additionally, apart from CD4, small amounts of CDxH4Àx (x =
3, 2, 1, or 0) products were also formed (m/z = 19, 18, 17, 16,
15, 14, 13, and 12), which should arise from surface-adsorbed
H2O on boron.[16]
ure S12). Additionally, a comparison of the XPS spectra of
boron samples recorded before and after CO2 photoreduction
also indicated the generation of some boron suboxide
intermediates (BO and BxO with x > 1) in the test (Fig-
ure 2d),[8d] further corroborating the occurrence of a hydrol-
ysis reaction between boron particles heated by the photo-
thermal effect and water with H2 and gaseous B(OH)3 as the
products.
Based on these results and considering the high reactivity
of H2 for CO2 reduction, we speculated that the CO2
photoreduction observed upon water addition to the system
might depend on the presence of H2. However, using Al2O3 as
a reference substrate instead of boron, direct irradiation of
a gaseous H2/CO2 mixture (without boron) did not give rise to
any CH4 products (Figure S15; a 1000 W Xe lamp was
employed to guarantee that the substrate temperature
(3968C) is nearly equal to that (3788C) achieved under
normal conditions with boron as the substrate and a 300 W Xe
lamp as the light source; Figure S10). This result implies that
the boron material is uniquely able to catalyze CO2 reduction
with the in situ generated H2 acting as the proton source and
electron donor. To provide more evidence supporting this
conclusion, a control CO2 photoreduction experiment was
carried out with H2 instead of water. In Figure S16, the
continuous photoreduction of CO2 is shown, further suggest-
ing the capability of the boron material to catalytically reduce
CO2 in the presence of H2 and light irradiation. As this test
was performed under water-free conditions, boron oxides or
B(OH)3 will not be formed during the reaction, thus ruling
out that they have an effect on the CO2 reduction. Further-
To investigate the mechanism of the CO2 photoreduction
over the boron material, we first examined the CO2 photo-
reduction in the absence of H2O. To rule out any effects from
adsorbed water molecules, the boron material was intention-
ally pretreated in a long-term vacuum process (12 h) before
irradiation. Although the absence of water could result in
a higher temperature on the sample under irradiation
(relative to the normal conditions with water) and thus
benefit the activation of CO2 (Figure S10), only negligible
amounts of CO had been formed after five hours (CH4
formation is not reasonable because of the absence of
a hydrogen source in the reaction system; Figure 2b,c). This
result suggests that the boron material is incapable of
catalyzing CO2 reduction in the absence of water under
light irradiation. Indeed, once water had been introduced into
the reaction system, both CO and CH4 were detected, and
their amounts increased with irradiation time (Figure 2c).
This phenomenon indicated the critical role of water in this
boron-mediated CO2 photoreduction while ambiguity still
remained as to what reactions occurred over the illuminated
boron material after water addition.
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more, as shown in Figure S17, under identical irradiation
conditions, a lower reaction temperature resulted in signifi-
cantly decreased activity. This result emphasizes the profound
contribution of the photothermal effect of the boron catalyst
to efficient CO2 photoreduction.
To gain more insight into the details of the CO2 photo-
reduction over the boron particles, in situ FTIR experiments
were conducted. Figure 3 shows IR spectra for CO2 photo-
boron hydrolysis results in the in situ formation of boron
oxides, which could act as promoters for CO2 adsorption onto
the surface of the boron catalysts. 3) The in situ formed H2 is
a highly reactive proton source and electron donor for CO2
reduction. 4) The amorphous boron material is efficient in
solar light utilization and possesses unique catalytic proper-
ties for CO2 reduction. Additionally, crystalline boron also
showed good photothermocatalytic activity for CO2 reduction
(Figures S20 and S21). The reason for the difference in
activity between amorphous and crystalline boron is still
under investigation.
In conclusion, we have shown that elemental boron
enables the direct and efficient reduction of CO2 into CO
and CH4 in the presence of water and under light irradiation
(without any additional sacrificial agents and cocatalysts).
The strong absorption of the boron catalyst in the UV/Vis and
even IR region causes a remarkable local photothermal
effect, which not only favored the CO2 activation, but also
triggered localized self-hydrolysis of boron particles to
produce H2 as the active proton source and electron donor
for CO2 reduction as well as boron oxides as promoters of
CO2 adsorption. As a consequence of these synergistic effects
as well as the unique intrinsic catalytic properties of the boron
material, high CO2 conversion efficiencies were realized.
These findings highlight the great promise of elemental boron
for CO2 conversion through a one-step photothermocatalytic
process, and might lead to new possibilities for the develop-
ment of more efficient solar energy utilization schemes.
Figure 3. In situ FTIR spectra of CO2 photoreduction over the amor-
phous boron material recorded after various irradiation times.
reduction over amorphous boron particles for various irradi-
ation times; the band of the boron catalyst (in the presence of
CO2 and water) detected before illumination was used as the
background. The bands at 2341 and 2360 cmÀ1 are associated
with adsorbed CO2 (Figure 3 and Figure S18).[19] For longer
irradiation times, these band intensities gradually increased,
illustrating the increased adsorption of CO2 on the boron
catalysts. This interesting phenomenon could be explained by
the formation of boron oxides (Figure 2d), which have been
shown to promote CO2 adsorption.[15] Obviously, this
enhanced CO2 adsorption will benefit subsequent photo-
reduction. The band related to CO was detected at
2100 cmÀ1.[20] The increasing peak intensity indicates the
gradual formation of CO over the illuminated boron catalysts.
Meanwhile, the growing absorption band at 700–1700 cmÀ1
Acknowledgements
This work received financial support from the World Premier
International Research Center Initiative (WPI Initiative) on
Materials Nanoarchitectonics (MANA), MEXT (Japan), the
National Basic Research Program of China (973 Program,
2014CB239301), and the National Natural Science Founda-
tion of China (21633004).
Conflict of interest
=
À
À
was attributed to C O, C H, and C OH vibrations, indica-
tive of the formation of some intermediate products of CO2
reduction, such as aldehydes and bidentate carbonates.[19,21]
On the basis of the discussion above, we propose the
following mechanism for the efficient CO2 photoreduction
over amorphous boron particles. As shown in Figure S19,
under light irradiation, the boron particles are heated to
a high temperature (3788C) by photothermal conversion,
which enables the localized hydrolysis of boron with water,
leading to the in situ generation of H2 and boron oxides. Then,
the reduction of CO2 to CO and CH4 is catalyzed by the
illuminated boron particles with the in situ formed H2 as the
proton source and electron donor. It is noteworthy that the
high efficiency of the CO2 conversion might be due to several
factors: 1) The excellent photothermal effect of the boron
particles induces a high local temperature, which favors the
activation of CO2. 2) The high-temperature-induced localized
The authors declare no conflict of interest.
Keywords: boron materials · CO2 photoreduction ·
energy conversion · heterogeneous catalysis ·
photothermocatalysis
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Manuscript received: February 7, 2017
Revised: March 8, 2017
Final Article published: && &&, &&&&
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Communications
Heterogeneous Catalysis
Four in one: Elemental boron is an
efficient catalyst for direct CO2 reduction
into CO and CH4 in the presence of water
under light irradiation through a one-step
photothermocatalytic process. The ele-
mental boron material harvests the inci-
dent light, converts it into thermal energy,
generates hydrogen, and catalyzes the
overall process.
G. Liu, X. Meng, H. Zhang, G. Zhao,
H. Pang, T. Wang, P. Li, T. Kako,
J. Ye*
&&&& — &&&&
Elemental Boron for Efficient Carbon
Dioxide Reduction under Light Irradiation
6
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ꢀ 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2017, 56, 1 – 6
These are not the final page numbers!