Direct Coupling of Thermo- and Photocatalysis for Conversion of CO2–H2O into Fuels

Article abstract of DOI:10.1002/cssc.201701472

Photocatalytic CO₂ reduction into renewable hydrocarbon solar fuels is considered as a promising strategy to simultaneously address global energy and environmental issues. This study focused on the direct coupling of photocatalytic water splitting and thermocatalytic hydrogenation of CO₂ in the conversion of CO₂–H₂O into fuels. Specifically, it was found that direct coupling of thermo- and photocatalysis over Au?Ru/TiO₂ leads to activity 15 times higher (T=358 K; ca. 99 % CH₄ selectivity) in the conversion of CO₂–H₂O into fuels than that of photocatalytic water splitting. This is ascribed to the promoting effect of thermocatalytic hydrogenation of CO₂ by hydrogen atoms generated in situ by photocatalytic water splitting.

Full text of DOI:10.1002/cssc.201701472

Accepted Article
Title: Direct coupling of thermo- and photo-catalytic conversion of CO₂-
H₂O to fuels
Authors: Li Zhang, Guoguo Kong, Yaping Meng, Jinshu Tian, Lijie
Zhang, Shaolong Wan, Jingdong Lin, and Yong Wang
This manuscript has been accepted after peer review and appears as an
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of the final Version of Record (VoR). This work is currently citable by
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the VoR from the journal website shown below when it is published
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To be cited as: ChemSusChem 10.1002/cssc.201701472
Link to VoR: http://dx.doi.org/10.1002/cssc.201701472
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10.1002/cssc.201701472
ChemSusChem
COMMUNICATION
Direct coupling of thermo- and photo-catalytic conversion of CO2-
H2O to fuels
Li Zhang,[a] Guoguo Kong,[a] Yaping Meng,[a] Jinshu Tian,[a] Lijie Zhang,[a] Shaolong Wan,[a,b,c] Jingdong
Lin,*[a,b] and Yong Wang [a,b,c,d]
Abstract: Photocatalytic CO2 reduction into renewable hydrocarbon
solar fuels is considered as a promising strategy to simultaneously
address the global energy and environmental issues. This article
focuses on the direct coupling of photocatalytic water splitting and
thermo-catalytic hydrogenation of CO2 in the conversion of CO2-H2O
to fuels. Specifically, it was found that direct coupling of thermo- and
photo-catalysis over Au-Ru/TiO2 leads to 15 times higher activity
(358 K, with ~99% CH4 selectivity) in the conversion of CO2-H2O to
fuels than that of photo-catalytic water splitting. This is ascribed to
the promoting effect of thermo-catalytic hydrogenation of CO2 by
hydrogen atoms generated in situ by photo-catalytic water splitting.
hydrogenation of CO2 reactions into a twin reactor to mimic
photosynthesis process, which is more efficient than that of the
single photocatalyst system.[5-7] Guan et al. directly combined
Fe-based
or
Cu/ZnO
catalysts
for
thermo-catalytic
hydrogenation of CO2 with photo-catalytic water splitting.[8]
Recently, MacDonnell et al. reported a one-step, gas-phase
photo-thermocatalytic process for the synthesis of hydrocarbons
from CO2-H2O in a flow photo reactor using a Co/TiO2 catalyst
under UV illumination.[9] These preliminary and encouraging
results show that the photo-thermocatalytic process is efficient in
enhancing the conversion of CO2-H2O to fuels using solar
energy, and is worthy of further extensive studies.
We note that thermo-catalytic hydrogenation of CO2 to
alkanes, known as “Sabatier reaction”, is a mature technology
for energy storage.[10] Sabatier reaction can be readily realized
under mild conditions. For example, CO2 conversion and CH4
selectivity can reach ~100%, respectively, over Ni[11] or Ru[12]
based catalysts at atmospheric pressure and a low reaction
temperature (~160 oC). However, hydrogen is needed in
Sabatier reaction. Currently, approximately 95% of total H2
production is obtained from steam reforming of natural gas and
petroleum, which emits unwanted CO2.[8b ] “Green” conversion of
CO2 to fuels requires that hydrogen is produced from renewable
sources such as solar energy. Simply using the H2 that is
produced from a separate and standalone photo-catalytic water
splitting in conventional Sabatier plants could cause efficiency
losses and result in more capital and operating costs. Direct
coupling of hydrogen production from photo-catalytic water
splitting and photo-reduction of CO2 is limited by lower efficiency
of latter. To mitigate these issues, coupling of photo-catalytic
water splitting for hydrogen production and thermo-catalytic
hydrogenation of CO2 could be more efficient than direct photo-
reduction of CO2, especially considering the fact that bifunctional
catalysts such as Ru/TiO2 or Ni/TiO2 are efficient for both photo-
catalytic water splitting[13] and thermo-catalytic hydrogenation of
CO2.[11]
Conversion of CO2 to fuels by means of an environmentally
friendly solar energy manner not only provides a long-term
alternative to the depletion of fossil fuels, but also reduces the
net emission of CO2.[1] Since the first report on the photo-
reduction of CO2 to organic compounds dating back to the mild
1970s,[2] enormous efforts have been undertaken to the
development of effective photo-catalysts for CO2 reduction.[3]
However, it is rather difficult to efficiently convert CO2 to fuels
only through photo-catalysis due to the kinetic limitations of
multiple e-/H+ transfer process and the thermodynamically stable
CO2.[4] Photo-catalysis coupled with other reactions is one of the
most effective and distinctive strategies to improve the efficiency
of photo-catalysis. For example, natural photosynthesis includes
two steps: (a) water splitting to generate protons using solar
energy; (b) CO2 reduction by the generated protons to produce
hydrocarbons in Calvin cycle.[5] Inspired by photosynthesis, the
ideal route for CO2 reduction would be the direct utilization of H2
produced from photo-catalytic water splitting under solar
radiation. Wu et al. combined photocatalyitc water splitting and
[a]
L. Zhang, G. Kong, Y. Meng, J. Tian, L. Zhang, S. Wan, J. Lin
and Y. Wang
Department of Chemistry
College of Chemistry and Chemical Engineering
Xiamen 361005, China
E-mail: jdlin@xmu.edu.cn (J. Lin)
S. Wan, J. Lin and Y. Wang
National Engineering Laboratory for Green Chemical
Productions of Alcohols-Ethers-Esters
Xiamen 361005, China
S. Wan and Y. Wang
Collaborative Innovation Center of Chemistry for Energy
Materials
[b]
[c]
[d]
Xiamen 361005, China
Prof. Dr. Y. Wang
Voiland School of Chemical Engineering and Bioengineering,
Washington State University
Scheme 1. Direct coupling of photo-catalytic water splitting with thermo-
catalytic hydrogenation for the conversion of CO2-H2O to fuels.
Pullman, Washington 99164, United States
Supporting information for this article is given via a link at the
end of the document
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10.1002/cssc.201701472
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COMMUNICATION
Herein, as shown in Scheme 1, we report that direct coupling
of photo-catalytic water splitting and thermo-catalytic
hydrogenation of CO2 on a bifunctional Au-Ru/TiO2 catalyst is
efficient for the conversion of CO2-H2O to fuels. A CH4 yield as
high as 27.1 μmol g-1 h-1 can be achieved via such a direct
photo-thermocatalytic coupling process (DPTCP), which is much
higher than that of photo-catalytic water splitting (7.2 μmol g-1 h-
1). The synergetic effects of photo-catalysis and thermo-
catalysis are hereby discussed.
nanoparticles confirmed by the energy dispersive spectroscopy
(EDS) mapping (Figure 2 (c-g)). UV-vis DRS and TEM results
indicate that Au-Ru/TiO2 catalyst consists of Au and Ru NPs
which are in intimate contact.
A bifunctional catalyst with both active sites for photo-
catalytic water splitting and thermo-catalytic hydrogenation of
CO2 is required for DPTCP (Figure S1-S3 for experimental
details). In this study, due to the high stablility and relatively high
photocatalytic performance, TiO2 (P25) provided by Evonik
Degussa Co., which contains 80% anatase and 20% rutile, is
used for photocatalysis. Ru is used as an active site for thermo-
catalytic hydrogenation of CO2. In order to improve the light
absorption of TiO2, Au nanoparticles (NPs), which is well known
for surface plasmon resonance (SPR), is also deposited on TiO2.
Au-Ru/TiO2 was synthesized using
a facile impregnation-
reduction method (ESI for experimental details).
Figure S5 shows the X-ray powder diffraction (XRD) patterns
of the prepared samples TiO2, Au/TiO2, Ru/TiO2 and Au-Ru/TiO2.
All the samples exhibit a similar XRD pattern as the parent TiO2
support. The diffraction peaks at 2θ = 27.5°, 36.2°, 41.2° are
assigned to (110), (101) and (111) planes of rutile, while other
peaks at 2θ = 25.3°, 37.8°, 48.0°, 53.9°, 55.1°, 62.7°, 70.3° and
75.0° are ascribed to anatase. No Au-derived or Ru-derived
peak is observed, due to the low loading amount of Au and Ru.
The diffraction peaks ascribed to TiO2 are almost unchanged
after Au and Ru loading, suggesting that the presence of Au and
Ru do not influence the crystalline structure of TiO2. The XRD
pattern of the Au-Ru/TiO2 sample after thermo-photocatalytic
reaction of CO2-H2O is identical to that of the fresh one, which
reveals the stability of the catalyst Au-Ru/TiO2 (Figure S6).
Figure 1. UV-vis DRS spectra of TiO2, Au/TiO2, Ru/TiO2 and Au-Ru/TiO2.
Figure 1 shows the UV-visible diffuse reflection spectra (UV-
vis DRS) spectra of TiO2, Au/TiO2, Ru/TiO2 and Au-Ru/TiO2. All
four samples show a strong absorption edge at about 390 nm,
corresponding to the band gap absorption of TiO2. Apparently,
loading of Au and Ru does not affect the band gap of TiO2,
which suggests that the deposition of Au and Ru does not lead
to their lattice doping in TiO2. In contrast to the pure TiO2, the
Au/TiO2 sample exhibits obvious enhancement on light around
570 nm, which is due to the SPR of Au NPs.[14] Meanwhile, after
deposition of Ru on Au/TiO2, the band ascribed to SPR of Au
NPs is slightly red-shifted to around 580 nm, implying an
interaction between Ru and Au.[15]
The morphologies of the prepared samples were
characterized by transmission electron microscopy (TEM). As
shown in Figure 2, TEM image of the as-obtained Au-Ru/TiO2
reveals that TiO2 NPs were of around 20-30 nm and the well-
dispersed metal nanoparticles (NPs) can be clearly
distinguished from TiO2. Figure 2 (b) also shows the interplanar
distances of 0.233 nm, 0.206 nm and 0.360 nm which
Figure 2. (a) TEM image, (b) High-resolution TEM (HRTEM) image, (c) SEM
image and (d-g) EDS elemental mapping of the prepared Au-Ru/TiO2.
correspond to the lattice of Au(111), Ru(101) and TiO2(101)
16]
planes, respectively,[14,
as well as Au and Ru metal
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10.1002/cssc.201701472
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COMMUNICATION
Table 1 summarizes the thermo-catalytic conversion activities reported.[20] Liu et al. suggested that light illumination could lower
of CO2 with H2 over TiO2, Au/TiO2, Ru/TiO2 and Au-Ru/TiO2 the activation energy of thermo-catalytic hydrogenation of CO2,
[20]
catalysts (Figure S1 for experimental details). No product was which is beneficial to thermo-catalytic conversion CO2 with H2.
detected over pure TiO2 without active sites for thermo-catalytic
Table 2. Catalytic activities of thermo-catalytic conversion of CO2 with H2 over
Au-Ru/TiO2 under light illumination.[a]
hydrogenation of CO2. After the loading of Au, the activities of
thermo-catalytic hydrogenation of CO2 are quite low. While for the
Ru/TiO2 catalyst, CH4 and C2H6 were observed at the reaction
temperature as low as 323 K and their yields increase with
reaction temperature, reaching 2.69*104 μmol g-1 h-1 CH4 and 405
μmol g-1 h-1 C2H6 at 423 K for thermo-catalytic hydrogenation of
CO2 with H2. In contrast to Ru-based catalysts, Au/TiO2 shows
quite low CO2 conversion and extremely high selectivity to CO.
However, almost no methanation reaction is observed, which is
consistent with the low methanation activity on Au as previously
reported.[17-19] These results confirm that Ru is essential for
thermo-catalytic conversion of CO2 with H2. For Au-Ru/TiO2
catalyst, CH4 and C2H6 were also observed at the reaction
temperature as low as 323 K and their yields reaching 1.60*104
μmol g-1 h-1 CH4 and 243 μmol g-1 h-1 C2H6 at 423 K. Compared to
Ru/TiO2, the activities decrease slightly after addition of Au on
Ru/TiO2, which is due to partial blocking of Ru by Au as shown in
TEM image (Figure 2). These results indicate that Au-Ru/TiO2 is
an active catalyst for the thermo-catalytic conversion of CO2 with
H2 to alkanes.
Formation rate
(μmol g-1h-1)
Temperature
Catalyst
（K）
CH4
82.3
C2H6
11.4
22.9
54.5
386
CO
nd
nd
nd
nd
323
358
905
Au-Ru/TiO2
373
2.28*103
3.06*104
423
[a] Reaction conditions: atmospheric pressure, catalyst: 0.20 g, light source:
Hg lamp (150 mW/cm2), illumination area: 0.8 cm2, reactor: fixed bed, feed
gas: CO2 (18.02%) + Ar (8.01%) + H2 (73.97%), flow rate: 2 mL/min, nd: no
detected
It is well known that TiO2-based catalysts are also effective
for H2 production by photo-catalytic water splitting. As shown in
Table 3 (Figure S1 for experimental details), under continuous-
flow of H2O/Ar (2 mL/min), 6.3 μmol g-1 h-1 H2 was produced at a
low temperature of 323 K over Au-Ru/TiO2 catalyst. As the
temperature increased to 358 K, a higher H2 production rate of
7.2 μmol g-1 h-1 H2 was achieved. However, H2 production rate
decreased to 1.9 μmol g-1 h-1 when the temperature further
increased to 373 K. These results indicate that Au-Ru/TiO2
catalyst is also efficient for H2 production from water splitting
under illumination, but the reaction temperature has a great
influence on the H2 production from photo-catalytic water
splitting. In general, elevated temperature does not favor H2
production from photo-catalytic water splitting due to the
increasing rate of charge recombination in the photocatalyst at
high temperature.[3] In addition, adsorption of H2O is another key
factor to the performance of H2 production from water splitting
under illumination in a continuous-flow reactor. Bazzo et al.
reported that the adsorption or even condensation of H2O and its
sufficient concentration on the catalyst surface at low
temperatures (< 373 K) are crucial for the stable production of
H2 under light illumination, and the active sites for H2 production
at high temperature such as 423 K were irreversibly consumed.[3]
Table 1. Catalytic activities of thermo-catalytic conversion of CO2 with H2 over
TiO2, Au/TiO2, Ru/TiO2 and Au-Ru/TiO2.[a]
Formation rate
Catalyst
Temperature
(μmol g-1h-1)
C2H6
nd
(K)
323~423
323
CH4
nd
CO
nd
nd
nd
8.2
440
nd
nd
nd
nd
nd
nd
nd
nd
TiO2
nd
nd
358
6.1
7.0
Au/TiO2
Ru/TiO2
373
7.0
5.4
423
2.1
nd
323
82.5
998
1.87*103
2.69*104
37.3
356
30.7
56.1
96.5
405
358
373
423
323
24.0
31.7
47.3
243
358
As mentioned above, Au-Ru/TiO2 is not only an excellent
catalyst for thermo-catalytic conversion of CO2 with H2, but also
effective for H2 production from photo-catalytic water splitting
under the similar temperature. Apparently, the rate of thermo-
catalytic conversion of CO2 with H2 is much higher than that of
photo-catalytic water splitting for H2 production. Direct coupling
of thermo-catalytic CO2 hydrogenation with renewable energy for
hydrogen production, e.g., photo-catalytic water splitting over the
bifunctional Au-Ru/TiO2 catalyst, represents a viable path for
highly efficient conversion of CO2-H2O to fuels. In order to
confirm this hypothesis, experiments were carried out using a
continuous flow fixed-bed reactor with a quartz window under
illumination at different temperatures. It is well known that
extreme care should be taken in photochemical CO2 reduction
due to the possible carbon residues on catalysts, which are
formed from carbon containing precursors.[21] Prior to the test of
DPTCP for the conversion of CO2 and H2O, the Au-Ru/TiO2 was
Au-Ru/TiO2
373
972
1.60*104
423
[a] Reaction conditions: atmospheric pressure, catalyst: 0.20 g, without light
illumination, feed gas: CO2 (18.02%) + Ar (8.01%) + H2 (73.97%), flow Rate: 2
mL/min, nd: no detected.
In order to study the effects of light illumination on thermo-
catalytic conversion of CO2-H2, the activities of conversion CO2
with H2 over Au-Ru/TiO2 catalyst under light illumination at
different temperature were measured and are summarized in
Table 2 (Figure S1 for experimental details). As shown in Table 1
and Table 2, thermo-photocatalysis activity is much higher than
thermo-catalytic activity at the same temperature over Au-
Ru/TiO2 catalyst for the conversion of CO2 with H2. These results
indicate that light illumination indeed facilitates the thermo-
catalytic conversion of CO2 with H2 to alkane as previously
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pretreated with H2 and then purged with He without CO2 under
illumination at various temperatures (323-423K), no any carbon
containing compound was detected, indicating that the carbon
containing compounds are indeed originated from CO2 in
DPTCP. In order to exclude the possible hydrogen residues,
CO2 was then purged without light illumination until no CH4, CO,
C2H6 and H2 were detected at all temperatures studied.
Furthermore, there was no carbon containing compound and H2
detected under H2O or CO2/H2O flow without light illumination.
The above control experiments ensure that the carbon
containing compounds and H/H2 products are originated from
CO2 and photocatalytic water splitting.
generated electrons and holes on catalyst surface.[22] The
efficiency of electron-hole pair generation mainly depends on the
intrinsic structure of photocatalyst. Meanwhile, effective capture
and quick consumption of electrons by reduction reactions will
speed up the migration of photogenerated electrons, lower the
recombination rate of photogenerated electrons and holes,
resulting in a high photocatalytic activity. It is well known that
cocatalysts, such as Pt, Ni etc., can effectively capture
photogenerated electrons and act as active sites for reduction
reactions by the captured electrons.[14, 23] For example, Li et al.
reported that Pt cocatalyst on CdS can capture electrons
efficiently and catalyze proton reduction, i.e., H2 evolution.[24] As
mentioned above, photo-catalytic reduction of CO2 is more
difficult than photo-catalytic reduction of H2O. Therefore, in the
case of DPTCP under CO2-H2O flow, it is very likely that most of
the captured electrons will preferentially reduce H+ rather than
direct reduction of CO2. If there is no CO2 present, reduction of
H+ will lead to H2 formation with the help of cocatalyst. However,
H atoms from H+ reduction may directly react with CO2 to form
CO or alkanes under CO2-H2O flow. Our results show that the
Au-Ru/TiO2 catalyst is highly efficient for thermo-catalytic
hydrogenation of CO2 to alkanes. In addition, illumination
facilitates thermo-catalytic hydrogenation of CO2 (also see Table
2), as reported by literature.[20] Meanwhile, comparison with
thermo-catalytic conversion of CO2 direct with H2, it is evident
that CO2 can direct react with highly active hydrogen atoms,
which are in situ generated by photo-catalysis. Although the
detailed mechanisms on how the reactive intermediates react
with photo-catalytic generated hydrogen atoms are still not clear,
we believe that the photo-catalytic generated hydrogen could
directly react with CO2 due to the highly efficient thermo-catalytic
hydrogenation of CO2 over Au-Ru/TiO2. Rapid consumption of
the generated hydrogen by direct thermo-catalytic hydrogenation
of CO2 to produce alkanes greatly accelerates the migration of
photogenerated electrons, thus lowers the recombination rate of
photogenerated electrons and holes, providing a synergetic
effect between photo-catalysis and thermo-catalysis in the direct
thermo-photocatalytic reduction of CO2-H2O. Recently,
MacDonnell et al. reported a similar photo-thermochemical
process using Co/TiO2 for both photo-catalytic water splitting
and Fischer–Tropsch synthesis.[9] They suggested that the
products by photo-catalysis can be consumed in thermal
reactions.
As shown in Table 3, in the case of CO2-H2O flow, H2 and
CH4 are simultaneously produced at the reaction temperature
from 323 K to 423 K under light illumination, except that the
production of H2 is much lower than those via photo-catalytic
water splitting (flowing with only H2O in Table3) at the same
conditions. Theoretically, splitting one mole H2O forms one mole
H2 and half mole O2 (H2O—H2 + 1/2O2). Unfortunately, even
using He as carrier gas for thermal conductivity detector (TCD),
O2 cannot be detected via TCD in the photocatalytic water
splitting and DPTCP, due to the low sensitivity of TCD for O2
detection. Because no extra H2 or reductants provided, the
reduction of H2O to H2 or CO2 to CH4 can only be originated from
photo-catalysis. In the control experiments, there was no H2 and
alkane obtained over Au-Ru/TiO2 without light illumination or
under illumination without photocatalyst at 323 K to 423 K,
further suggesting the electrons for the reduction of water to H2
or CO2 to CH4 are indeed originated from photo-catalysis. For
example, reduction of CO2 with two photo-generated electrons
yields CO (CO2 + 2H+ + 2e- – CO + H2O); reduction of CO2 with
eight photo-generated electrons yields CH4 (CO2 + 8H+ + 8e- –
CH4 + 2H2O); reduction of H+ with two photo-generated
electrons yields H2 (2H+ + 2e- – H2). In order to provide a
quantitative comparison, we used photo-generated electrons for
the reduction reactions (R-electrons) to estimate the catalytic
activity of different reactions.
R-electrons = 2r (CO) + 8r (CH4) + 2r (H2)
where r (CO), r (CH4), and r (H2) are the formation rates of CO,
CH4, and H2, respectively.
As shown in Table 3, 59 μmol g-1 h-1 R-electrons are reached
at 323 K in DPTCP. The R-electrons increase to 218 μmol g-1 h-1
at 358 K, and decrease to 46 and 33 μmol g-1 h-1 with further
increasing the reaction temperature to 373 K and 423 K,
respectively. However, it can be seen from Table 3, the R-
electrons of DPTCP (218 μmol g-1 h-1) are ~15 times higher than
that of photo-catalytic water splitting (15 μmol g-1 h-1) under the
same intensity of illumination at 358 K. On the other hand, the
trend of changing in R-electrons with temperature in DPTCP is
the same with that of photo-catalytic water splitting, which
implies photo-catalytic water splitting and thermo-photocatalytic
conversion of CO2-H2O are very different from each other, but
they also have some similarities.
Table 3. Activities of photo-catalytic water splitting and the direct coupling of
thermo-catalytic hydrogenation of CO2 and photo-catalytic water splitting for
conversion of CO2-H2O over Au-Ru/TiO2.[a]
Tempera
ture
Formation rate
R-electrons
(μmol g-1 h-1)
AQE(%)
Feed gas
(μmol g-1 h-1)
(K)
H2
CH4
--
--
--
--
7.30
27.1
5.60
4.05
CO
H2O
H2O
H2O
323
358
373
423
323
358
373
423
6.30
7.20
1.90
--
0.21
0.27
0.49
--
--
--
--
--
--
--
--
--
13
15
4
0.54
0.62
0.17
--
H2O
--
CO2+H2O
CO2+H2O
CO2+H2O
CO2+H2O
59
218
46
33
2.45
9.05
1.91
1.31
Generally, photo-catalysis mainly includes three steps:
generation of electron-hole pairs, separation and migration of
photo-generated electrons and holes, redox reactions by photo-
[a] Reaction conditions: atmospheric pressure, catalyst, 0.20 g, light source,
Hg lamp, light intensity: 150 mW/cm2, illumination area: 0.8 cm2, flow Rate: 2
mL/min
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In order to further confirm the synergetic effects between
photo-catalysis and thermo-catalysis in thermo-photocatalytic
reduction of CO2-H2O, photocatalytic water splitting and thermo-
photocatalytic conversion of CO2-H2O on pure TiO2 catalysts
without thermo-catalytic active sites, such as Ru, were studied.
Due to the low phtotocatalytic activity over pure TiO2, a reactor
with greater illumination area (12.5 cm2) was used (Figure S2 for
experimental details). As shown in Table 4, there is no
noticeable difference in R-electrons between photo-catalytic
water splitting and DPTCP conversion of CO2-H2O under the
same conditions, implying the important role of the thermo-
catalytic hydrogenation of CO2 to accelerate the migration of
photogenerated electrons. This result further confirms the
synergetic effect between photo-catalysis and thermo-catalysis
in the direct thermo-photocatalytic reduction of CO2-H2O.
hydrogen atoms generated in situ by photo-catalysis, which can
greatly reduce the recombination rate of photogenerated
electrons and holes. This example presents a conceptually
promising process for alkane production using CO2 and solar
energy. Furthermore, the direct coupling of photo-catalysis with
thermo-catalysis process presented herein may be further
extended to other hydrogenation reactions, such as ammonia
synthesis or COx to methanol etc., with hydrogen in situ derived
from solar energy.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (No. 21673189, No. 91545114, No.
21576227 and No. 21373168), the National Thousand Talents
Program of P. R. China, and the Fundamental Research Funds
for the Central Universities (No. 20720160032).
Table 4. Activities of photo-catalytic water splitting and direct coupling of
thermo-catalytic hydrogenation of CO2 and photo-cataly
for the conversion of CO2-H2O over TiO2.
tic water splitting
[a]
Formation rate
Feed gas
Temperature
R-electrons
(μmol g-1 h-1)
(μmol g-1 h1)
(K)
323
358
373
423
323
358
373
423
H2
6.1
8.7
9.1
12.0
1.3
2.1
2.6
2.0
CH4
--
--
CO
--
--
Keywords: CO2 conversion • photo-catalysis • coupling •
thermo-catalysis • hydrogenation
H2O
H2O
H2O
13
18
19
24
13
25
34
53
--
--
H2O
--
--
[1]
[2]
S. Neatu, J. A. Macia-Agullo, P. Concepcion, H. Garcia, J. Am. Chem.
Soc. 2014, 136, 15969.
CO2+H2O
CO2+H2O
CO2+H2O
CO2+H2O
0.6
1.9
3.2
5.7
2.7
2.6
1.3
1.5
T. Inoue, A. Fujishima, S. Konishi, K. Honda, Nature (Lond.), 1979, 277,
637.
[a] Reaction conditions: atmospheric pressure, catalyst: 0.60 g, light source:
[3]
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Entry for the Table of Contents
COMMUNICATION
An activity of direct coupling thermo-
and photo-catalysis over Au-Ru/TiO2
leads to 15 times higher activity (358
K, with ~99% CH4 selectivity) in the
conversion of CO2-H2O to fuels than
that of photo-catalytic water splitting,
which is ascribed to the promoting
effect of thermo-catalytic
Li Zhang, Guoguo Kong, Yaping Meng,
Jinshu Tian, Lijie Zhang, Shaolong Wan,
Jingdong Lin,* and Yong Wang
Page No.1 – Page No.6
Direct coupling of thermo- and photo-
catalytic conversion of CO2-H2O to
fuels
hydrogenation CO2 with hydrogen
atoms generated in situ by photo-
catalytic water splitting.
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