Photocatalytic conversion of carbon dioxide with water into methane: Platinum and Copper(I) oxide co-catalysts with a core-shell structure

Article abstract of DOI:10.1002/anie.201301473

Binary co-catalysts of Pt and Cu₂O with a core-shell structure significantly enhance the photocatalytic reduction of CO₂ with H ₂O to CH₄ and CO. The Cu₂O shell provides sites for the preferential activation and conversion of CO₂, whereas the Pt core extracts the photogenerated electrons from TiO₂. The deposition of Cu₂O shell on Pt nanoparticles markedly suppresses the reduction of H₂O to H₂ (see picture). Copyright

Full text of DOI:10.1002/anie.201301473

Angewandte
Chemie
DOI: 10.1002/anie.201301473
Heterogeneous Catalysis
Photocatalytic Conversion of Carbon Dioxide with Water to Methane:
Platinum and Copper(I) Oxide Co-catalysts with a Core–Shell
Structure**
Qingge Zhai, Shunji Xie, Wenqing Fan, Qinghong Zhang,* Yu Wang, Weiping Deng, and
Ye Wang*
The use of CO2 for the production of fuels and chemicals has
attracted much attention under the current background of the
depletion of fossil resources and the increase of emissions of
CO2. However, the activation of CO2, a very stable molecule,
is one of the biggest challenges in chemistry. In the long term,
the photocatalytic conversion of CO2 using solar energy, that
is, the artificial photosynthesis, is the most attractive route for
the transformation of CO2 to fuels and chemicals. Since the
pioneering work by Inoue et al.,[1] many studies have been
devoted to the semiconductor-based photocatalytic reduction
of CO2 with H2O and the pace has increased enormously in
recent years.[2] Several kinds of semiconductors such as
only very limited studies have been devoted to binary co-
catalyst systems for the photocatalytic conversion of CO2 to
hydrocarbons (mainly CH4).[2b,8,9] Pt and Cu bimetallic co-
catalysts coated on N-doped TiO2 nanotube arrays or periodi-
cally modulated multi-walled TiO2 nanotube arrays showed
promising promoting effects.[8,9] However, the functioning
mechanisms and even the chemical states of Pt and Cu in
these systems are ambiguous although it seems that Pt may
function for the activation of H2O, while Cu is responsible for
the activation of CO2 to CO.[8] It can be expected that the
reduction of CO2 through accepting the photogenerated
electrons trapped in the co-catalyst is competitive with the
reduction of H2O to H2. The insight for the rational design of
co-catalysts for the efficient reduction of CO2 is highly
deficient. Herein, we present a new strategy for the design
and preparation of efficient binary co-catalysts for the
preferential reduction of CO2 in the presence of H2O.
We prepared TiO2 (Degussa P25)-loaded Pt and Cu
binary co-catalysts by a stepwise photodeposition technique.
Pt nanoparticles were first introduced onto TiO2 by photo-
reducing H2PtCl6 in an aqueous suspension, providing Pt/TiO2
with a Pt content of about 0.9 wt% and a mean size of Pt
nanoparticles of 3.1 nm (see Figure S1 in the Supporting
Information). Then, copper was deposited onto Pt/TiO2 under
irradiation using CuSO4 as a precursor, and the samples
obtained were denoted as Cu/Pt/TiO2-xh, where xh was the
irradiation time for the photodeposition of Cu. We found that
the variation of the irradiation time changed the content of
Cu (Table S1). Figure 1 shows the high-resolution transmis-
sion electron microscope (HRTEM) images for the Cu/Pt/
TiO2-xh series of samples. For the Cu/Pt/TiO2-1h and the Cu/
Pt/TiO2-2h samples with lower Cu contents (0.59 and
1.1 wt%), although the Cu species were deposited on the Pt
nanoparticle, the covering of the Pt nanoparticle was incom-
plete. As the irradiation time reached 5 h (Cu content =
1.7 wt%), the Pt nanoparticle was covered by a shell of Cu
layers, forming a complete core–shell structure. The HRTEM
image displayed lattice fringes with an interplanar spacing of
0.211 nm for the shell (Figure 1c), which could be ascribed to
the (200) facet of Cu2O. This suggests that the shell may be in
the state of Cu2O. We have estimated the mean size of the
core–shell structured particles in the Cu/Pt/TiO2-5h sample,
and the value is 7.3 nm. It should be noted that the Cu species
were preferentially deposited on Pt nanoparticles over these
catalysts. We speculate that this is probably because the
photogenerated electrons are efficiently extracted onto Pt
from TiO2. For comparison, a sample with Pt and Cu contents
TiO2,[2] Ga2O3,[3] ZnGe2O4,[4] ZnGa2O4,[5] and BaLa4Ti4O15
[6]
have been reported for the photocatalytic conversion of CO2,
although the activity is still very low.
Co-catalysts are known to play crucial roles in the
semiconductor-based photocatalysis.[2f] Enhancing effects of
noble or coinage metal co-catalysts such as Pd, Pt, Au, Ag, or
Cu have been observed for the photocatalytic conversion of
CO2 with H2O to hydrocarbons over semiconductors such as
TiO2.[2,7] It is generally accepted that the noble or coinage
metal co-catalyst may facilitate the separation of photo-
generated electrons and holes by trapping electrons, enhanc-
ing the photocatalytic activity.[2,7] Because CO2 is a very stable
molecule, it is also highly important to provide catalytically
active sites for the activation and conversion of CO2. Thus, the
design of bifunctional co-catalysts with a proper structure is
crucial for obtaining high CO2 conversion activity. To date,
[*] Q. Zhai,[+] S. Xie,[+] W. Fan,[+] Prof. Dr. Q. Zhang, Y. Wang,
Dr. W. Deng, Prof. Dr. Y. Wang
State Key Laboratory of Physical Chemistry of Solid Surfaces
Innovation Center of Chemistry for Energy Materials
National Engineering Laboratory for
Green Chemical Productions of Alcohols, Ethers and Esters
College of Chemistry and Chemical Engineering
Xiamen University, Xiamen 361005 (China)
E-mail: zhangqh@xmu.edu.cn
wangye@xmu.edu.cn
[+] These authors contributed equally to this work.
[**] This work was supported by the National Basic Research Program of
China (grant numbers2010CB732303 and 2013CB933100), the NSF
of China (grant number21033006) and the Program for Changjiang
Scholars and Innovative Research Team in University (grant
numberIRT1036). We acknowledge Dr. Yanping Zheng and Ms.
Xiaobo Li for HS-LEIS and ICP measurements.
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201301473.
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Figure 2. HS-LEIS spectra for a) Cu/TiO2, b) Pt/TiO2, c) Pt-Cu/TiO2,
and d) Cu/Pt/TiO2-5h catalysts. A) 3 keV 4He+, B) 5 keV 20Ne+.
whereas the Ne+ ions can provide better sensitivities for
heavier elements although it is unsuitable for the measure-
ment of lighter elements. Figure 2B clearly shows that Pt and
Cu co-exist on the outmost surface of the Pt-Cu/TiO2 sample,
whereas only Cu is present on the outer surface of the Cu/Pt/
TiO2-5h sample. We further measured the HS-LEIS spectra
for the Cu/Pt/TiO2-xh series of samples and found that, with
an increase in the irradiation time, the intensity of the Cu
signal increased and that of the Pt signal decreased gradually
(Figure S7). When the irradiation time was ꢀ 5 h, the signal of
Pt disappeared. Therefore, our HS-LEIS results provide
further evidence for the formation of the core–shell structure
in the Cu/Pt/TiO2-5h and the Cu/Pt/TiO2-10h samples.
Figure 1. Typical HRTEM micrographs for the Cu/Pt/TiO2-xh series of
catalysts. a) Cu/Pt/TiO2-1h, b) Cu/Pt/TiO2-2h, c) Cu/Pt/TiO2-5h, d) Cu/
Pt/TiO2-10h.
of 0.90 and 1.6 wt%, which were similar to those in the Cu/Pt/
TiO2-5h, was also prepared by a hydrazine-reduction method.
TEM measurements suggested that this sample was com-
posed of Pt and Cu2O nanoparticles (denoted as Pt-Cu/TiO2
hereafter; Figure S2).
The chemical states of Pt and Cu in our samples were
further characterized by X-ray diffraction (XRD) and X-ray
photoelectron spectroscopy (XPS). Only diffraction peaks
ascribed to TiO2 were observed for each sample (Figure S3),
implying that no larger platinum or copper-related crystallites
were formed. XPS measurements revealed that the binding
energies of Pt 4f7/2 were about 70.9 eV for the Pt/TiO2, Cu/Pt/
TiO2-5h, and Pt-Cu/TiO2 samples (Figure S4), confirming that
Pt was in metallic state in these samples.[10] The binding
energies of Cu 2p3/2 for the Cu/TiO2, Cu/Pt/TiO2-5h, and Pt-
Cu/TiO2 samples were about 932.5 eV (Figure S5A), which
could be attributed to either Cu0 or CuI.[11] The positions of
the Cu L3VV Auger lines for these samples were centered at
916.6–917.1 eV (Figure S5B), corresponding to that of CuI.[11]
Considering that Cu0 may be easily oxidized into CuI after
exposure to air,[12] we further performed in situ CO-adsorbed
Fourier-transform infrared studies. The results suggested that
the Cu species in our catalysts after the irradiation (l = 320–
780 nm) existed in the state of CuI or Cu2O (Figure S6).
To further confirm the formation of the core–shell
structure in the sample prepared by the stepwise photo-
deposition technique, we have performed high-sensitivity
low-energy ion scattering (HS-LEIS) studies, which have
been proven to be powerful for providing the atomic
composition of the outmost atomic layer of the surface.[13]
The HS-LEIS spectra for the Cu/TiO2, Pt/TiO2, Pt-Cu/TiO2,
and Cu/Pt/TiO2-5h are displayed in Figure 2. It is known that
the He+ ions can give an overview of almost all elements on
the outer surface with poor sensitivity for heavier elements,
The photocatalytic reduction of CO2 with H2O over our
catalysts provided CO and CH4 as two main products
(Table 1). The formation of H2 was also observed at the
Table 1: Photocatalytic conversion of CO2 with H2O over TiO2-loaded
single or binary co-catalysts.[a]
Catalyst
Formation rate
[mmolgꢁ1 hꢁ1
Selectivity for CO2 reduction [%]
]
H2
CO
CH4
TiO2
Pt/TiO2
Cu/TiO2
Pt-Cu/TiO2
Cu/Pt/TiO2-5h
4.9
66
9.9
51
2.5
2.2
5.4
5.9
8.3
1.2
11
8.7
9.8
33
60
41
80
47
85
25
[a] Reaction conditions: photocatalyst, 0.020 g; CO2 pressure, 0.2 MPa;
H2O, 4.0 mL; light source, 320–780 nm; irradiation time, 4 h.
same time. We further confirmed the formation of O2 over our
catalysts (see the Supporting Information for experimental
details). These suggest the occurrence of the following
reductive reactions through photogenerated electrons and
the formation of O2 by photogenerated holes over our
catalysts [Eqs. (1)–(4)]:
CO2 þ 2 Hþ þ 2 eꢁ ! CO þ H2O
CO2 þ 8 Hþ þ 8 eꢁ ! CH4 þ 2 H2O
ð1Þ
ð2Þ
2
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2 Hþ þ 2 eꢁ ! H2
ð3Þ
ð4Þ
(Table 1). The rate of CH4 formation over this catalyst was
about 28 times higher than that over TiO2 alone, and was 3.0
and 3.8 times higher than those over the Pt/TiO2 and Cu/TiO2,
respectively. The formation of CO over the Cu/Pt/TiO2-5h
catalyst was also higher than that over Cu/TiO2 or Pt/TiO2. We
performed a blank reaction over the Cu/Pt/TiO2-5h catalyst.
In the absence of CO2, the formation of CO and CH4 was
negligible, confirming that CO and CH4 were formed from
CO2. The rate of H2 formation in the absence of CO2 was
32 mmolgꢁ1 hꢁ1, only slightly higher than that (25 mmolgꢁ1 hꢁ1)
in the presence of CO2. The transient photocurrent response
measurement showed that the Cu/Pt/TiO2-5h catalyst exhib-
ited a photocurrent similar to Pt/TiO2 (Figure 3), indicating
that the Pt@Cu2O core–shell structure could keep the higher
ability of Pt nanoparticles for trapping the photogenerated
electrons from TiO2. All these observations allow us to
exclude the possibility that CH4 is formed via the hydro-
genation of CO2 or CO adsorbed or formed on the Cu site by
the atomic H species generated on the Pt site.[2b,8] Instead, our
results clearly demonstrate that the covering of Pt by Cu2O
with a core–shell structure suppressed the reduction of H2O
to H2 and preferentially accelerated the reduction of CO2 to
CO and CH4. The selectivity for CO2 reduction reached 85%.
The photocatalytic behavior of the Cu/Pt/TiO2-xh series of
catalysts depended on the content of Cu, which had been
regulated by changing the irradiation time used for Cu
photodeposition. At a Cu content of < 1.7 wt%, the increase
in Cu content significantly decreased the formation of H2 and
increased that of CH4 and CO (Figure 4). At the same time,
the selectivity for CO2 reduction increased remarkably. TEM
H2O þ 2 hþ ! 1=2 O2 þ 2 Hþ
Table 1 shows that the loading of either Pt or Cu (existing
as Cu2O) single co-catalysts onto TiO2 can promote the
formation of CH4. The formation of CO was also improved by
Cu. The enhanced electron–hole separation through trapping
electrons by the noble or coinage metal co-catalyst with
a lower Fermi energy level is believed to be a main reason.
The increased electron density on the co-catalyst may favor
the formation of CH4, which requires eight electrons. Our
present work further clarified that the formation of H2 was
also enhanced by loading Pt or Cu onto TiO2, and such an
enhancement was much more significant in the case of Pt co-
catalyst. Considering that the reduction of H2O to H2 is
a competitive reaction with the reduction of CO2 to CO and
CH4,[2c] we have evaluated the selectivity for CO2 reduction
on an electron basis (Supporting Information, experimental
details). It becomes clear that the Pt co-catalyst decreases the
selectivity, whereas the selectivity for CO2 reduction increases
from 60 to 80% by loading Cu onto TiO2.
Although Cu (existing as Cu2O) is a more selective co-
catalyst for the reduction of CO2, the rate of CH4 formation
over Cu/TiO2 is lower than that over the Pt/TiO2. Our
transient photocurrent response measurements showed that
the photocurrent for Cu/TiO2 was significantly lower than
that for Pt/TiO2 (Figure 3), clarifying the lower efficiency of
Figure 3. Transient photocurrent responses for a) TiO2, b) Pt/TiO2,
c) Cu/TiO2, d) Pt-Cu/TiO2, and e) Cu/Pt/TiO2-5h catalysts.
Figure 4. Dependence of the photocatalytic behavior on the Cu content
in the Cu/Pt/TiO2-xh catalysts for the reduction of CO2 with H2O.
Reaction conditions: photocatalyst, 0.020 g; CO2 pressure, 0.2 MPa;
H2O, 4.0 mL; light source, 320–780 nm; irradiation time, 4 h.
the separation of photogenerated electron–hole pairs in the
Cu/TiO2 catalyst. The combination of Cu2O responsible for
the selective activation of CO2 and Pt capable of effectively
extracting the photogenerated electrons is expected to
increase the photocatalytic activity for CO2 reduction. How-
ever, the use of the Pt-Cu/TiO2 catalyst, which was composed
of Pt and Cu2O nanoparticles, did not show synergistic effects
for CO and CH4 formation. The rate of CH4 formation over
this catalyst was even lower than that over the Pt/TiO2. The
selectivity for CO2 reduction was also low over this catalyst.
On the other hand, the Cu/Pt/TiO2-5h catalyst containing
core–shell-structured Pt@Cu2O nanoparticles demonstrated
a significantly enhanced activity for the formation of CH4
images of these samples showed the gradual covering of Pt
nanoparticles by Cu layers. For the sample with a Cu content
of 1.7 wt%, over which the core–shell structure was formed,
the rates of CH4 and CO formation reached the maximum
values. A further increase in Cu content slightly decreased the
rates of CH4 and CO formation, suggesting that a too thick
Cu2O shell was unfavorable to the reduction of CO2. These
systematic changes further demonstrate the favorable role of
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the core–shell-structured Pt@Cu2O co-catalyst in the prefer-
ential reduction of CO2 by the photogenerated electrons.
It is worth mentioning that a few core–shell-structured co-
Keywords: carbon dioxide · co-catalysts ·
heterogeneous catalysis · nanostructures · photocatalysis
.
catalyst or catalyst systems such as Rh@Cr2O3,[14] Ni@NiO,[15]
[16]
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ential activation and conversion of CO2 molecules in the
presence of H2O, while the Pt core extracts the photogen-
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Pt markedly suppresses the reduction of H2O to H2, a com-
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further optimization of the chemical compositions of the core
and the shell may further improve the photocatalytic activity
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Experimental Section
The Cu/Pt/TiO2 catalyst was prepared by a stepwise photodeposition
method. Pt was first loaded onto TiO2 (Degussa P25) by photo-
reducing H2PtCl6 in an aqueous suspension, which was irradiated with
a 300 W Hg lamp for 1 h. The Pt/TiO2 was then dispersed in an
aqueous solution of CuSO4. After evacuation, the suspension was
exposed to irradiation with the Hg lamp to deposit Cu species onto Pt/
TiO2. The final product was washed thoroughly with deionized water
and dried overnight in vacuum. The content of Cu was regulated by
varying the irradiation time in the second step, and the obtained
sample was denoted as Cu/Pt/TiO2-xh. For comparison, a Pt-Cu/TiO2
sample was also prepared by a hydrazine-reduction method, in which
hydrazine was used for the reduction of the mixed aqueous solution of
Pt and Cu precursors in the presence of TiO2. The contents of Pt and
Cu in each sample were analyzed by inductively coupled plasma mass
spectrometry (ICP-MS). XRD, XPS, TEM, CO-adsorbed FT-IR, and
HS-LEIS techniques were used for catalyst characterizations. Photo-
electrochemical measurements were carried out with an Ivium
CompactStat (Holland). Photocatalytic reactions were performed in
a stainless-steel reactor with a quartz window on the top of the
reactor. The light source was a 200 W Xe lamp (l = 320–780 nm). The
photocatalyst was placed on a Teflon catalyst holder without being
immersed in water. Typically, 4.0 mL of H2O was charged in the
bottom of the reactor, and the pressure of CO2 was regulated to
0.2 MPa. The photocatalytic reaction was typically carried out at
323 K for 4 h. The products were analyzed by gas chromatography.
See the Supporting Information for the experimental details.
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G. Adami, M. Cargnello, S. Bernal, P. Fornasiero, J. Phys. Chem.
A 2010, 114, 3916 – 3925.
Received: February 20, 2013
Published online: && &&, &&&&
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Heterogeneous Catalysis
Binary co-catalysts of Pt and Cu2O with
a core–shell structure significantly
enhance the photocatalytic reduction of
CO2 with H2O to CH4 and CO. The Cu2O
shell provides sites for the preferential
activation and conversion of CO2,
whereas the Pt core extracts the photo-
generated electrons from TiO2. The dep-
osition of Cu2O shell on Pt nanoparticles
markedly suppresses the reduction of
H2O to H2 (see picture).
Q. Zhai, S. Xie, W. Fan, Q. Zhang,*
Y. Wang, W. Deng,
Y. Wang*
&&&& — &&&&
Photocatalytic Conversion of Carbon
Dioxide with Water to Methane: Platinum
and Copper(I) Oxide Co-catalysts with
a Core–Shell Structure
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