Photocatalytic reduction of CO2 with H2O: Significant enhancement of the activity of Pt-TiO2 in CH4 formation by addition of MgO

Article abstract of DOI:10.1039/c3cc00107e

Photocatalytic activity in the reduction of CO₂ with H ₂O to CH₄ was significantly enhanced by simply adding MgO to TiO₂ loaded with Pt. A positive correlation between CH₄ formation activity and basicity was observed. The interface between TiO ₂, Pt and MgO in the trinary nanocomposite played a crucial role in CO₂ photocatalytic reduction.

Full text of DOI:10.1039/c3cc00107e

ChemComm
View Article Online
View Journal | View Issue
COMMUNICATION
Photocatalytic reduction of CO2 with H2O: significant
enhancement of the activity of Pt–TiO2 in CH4
formation by addition of MgO†
Cite this: Chem. Commun., 2013,
49, 2451
Received 6th January 2013,
Accepted 7th February 2013
Shunji Xie, Yu Wang, Qinghong Zhang,* Wenqing Fan, Weiping Deng and
Ye Wang*
DOI: 10.1039/c3cc00107e
www.rsc.org/chemcomm
Photocatalytic activity in the reduction of CO2 with H2O to CH4 was
Enhanced adsorption of CO2 on basic sites may promote its
significantly enhanced by simply adding MgO to TiO2 loaded with Pt. subsequent photocatalytic reduction. Photocatalytic conversion of
A positive correlation between CH4 formation activity and basicity was CO2 to CO was reported over solid bases such as MgO, an insulator,
observed. The interface between TiO2, Pt and MgO in the trinary in the presence of H2 or CH4.13 Very recently, layered double
nanocomposite played a crucial role in CO2 photocatalytic reduction.
hydroxides, typical solid bases, were reported to promote the photo-
decomposition of CO2 to CO and O2 or the photoreduction of CO2 to
The diminishing fossil resources and the growing concerns over CH4 with H2O with deep ultraviolet (UV) radiation (l o 200 nm).14,15
the emissions of CO2 have stimulated research activities on the These reactions, however, do not involve the semiconductor catalyst
utilization of CO2.1 The activation of CO2 is one of the most and have been proposed to proceed via different mechanisms.
challenging themes in chemistry since CO2 is a highly stable To the best of our knowledge, there is no study on the effect of
molecule. Typically, a large energy input or co-feeding of a high- basic additives on the performance of a semiconductor-based
energy reactant such as H2 is required for the activation and photocatalyst in the reduction of CO2 with H2O under UV or visible
conversion of CO2. The photocatalytic conversion of CO2 using light irradiation. Herein, we report a significant enhancement of
solar energy, i.e., the artificial photosynthesis, is one of the most photocatalytic activity of TiO2-based catalysts by simply adding a
attractive routes for the utilization of CO2. Inoue et al. reported a basic metal oxide. Our work provides a novel strategy for the design
pioneering study on the reduction of CO2 in aqueous suspensions of efficient semiconductor-based nanocomposite photocatalysts for
containing semiconductor powders.2 Semiconductor-based photo- the reduction of CO2 with H2O.
catalytic reduction of CO2 with H2O has attracted much attention in
Fig. 1 displays the photocatalytic performances of several TiO2-
recent years.3–6 Various semiconductors such as TiO2,3–6 Ga2O3,7 based photocatalysts in the reduction of CO2 in the presence of H2O
ZnGe2O4,8 ZnGa2O4 have been reported for the photocatalytic (see the ESI† and Fig. S1 for experimental details). CO and CH4 were
9
conversion of CO2. Several strategies that are usually used for the two main products. TiO2 (Degussa P25) alone provided CO and
promoting the photocatalytic H2 evolution,6 e.g., the addition of CH4 with amounts of 0.24 and 0.07 mmol, respectively. The loading
noble or coinage metals or metal oxides onto the semiconductor,10
the preparation of semiconductors with different crystalline struc-
tures or morphologies,11 and the preparation of semiconductor–
semiconductor nanocomposites,12 have also been exploited for CO2
reduction. The photocatalytic activity is, however, still low.
Undoubtedly, new strategies are required to further promote the
photocatalytic reduction of CO2 with H2O.
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: wangye@xmu.edu.cn, zhangqh@xmu.edu.cn; Fax: +86-592-2183047;
Tel: +86-592-2186156
† Electronic supplementary information (ESI) available: Experimental details,
Fig. 1 Amounts of CO and CH4 formed in the photocatalytic reduction of CO2 in
the presence of H2O. Reaction conditions: catalyst, 0.020 g; CO2 pressure, 2 MPa;
H2O, 1.0 mL; irradiation time, 10 h.
comparison of CH4 formation in the presence and absence of CO2, effect of
various basic metal oxide modifiers on photocatalytic performances of Pt–TiO2.
See DOI: 10.1039/c3cc00107e
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 2451--2453 2451

View Article Online
Communication
ChemComm
Table 1 Effect of the content of MgO on photocatalytic performances of the
of Pt nanoparticles onto TiO2 with a content of 0.5 wt% using a
photodeposition method increased the amount of CH4 to 1.0 mmol.
The increase in CH4 formation because of a noble metal co-catalyst
was also reported by other groups.10,16 This is attributable to the
efficient electron–hole separation caused by the Pt nanoparticles.10b
The addition of 1.0 wt% MgO onto TiO2, followed by loading of Pt
nanoparticles, further significantly increased the amount of CH4 to
2.2 mmol. The amount of CH4 increased almost linearly with reaction
time, whereas almost no CH4 was observed in the absence of CO2
(Fig. S2, ESI†), confirming that CH4 was formed by the photocatalytic
conversion of CO2. The formation of O2 over the Pt–MgO/TiO2 was
confirmed, and the molar ratio of O2/CH4 was approximately 2.5/1,
slightly higher than the stoichiometric ratio (2/1) expected from the
photocatalytic conversion of CO2 with H2O to CH4 and O2 (see the
ESI†). On the other hand, the addition of 1.0 wt% MgO onto TiO2
without loading Pt only slightly increased the amounts of CO and
CH4. The 0.5 wt% Pt–MgO was almost inactive in photocatalytic
reduction of CO2, confirming that the reduction of CO2 arises
from the photogenerated electrons from TiO2. These observations
indicate the significant role of MgO in photocatalytic formation of
CH4 and the importance of co-existence of Pt nanoparticles.
Pt–MgO/TiO2 in the reduction of CO2 in the presence of H2Oa
Product
amount
(mmol)
c
SBET
Mean size Chemisorbed
Photocatalystb
Pt–TiO2
(m2
g
À1) of Pt (nm) CO2 (mmol gÀ1) CO
CH4
55
4.2
3.8
4.1
4.0
4.2
4.1
4.2
5.0
7.4
10
12
15
17
—
0.22 1.0
Pt–0.25% MgO/TiO2 41
0.058 1.3
0.006 1.6
0.006 2.2
0.006 1.8
0.004 1.3
0.28 0.70
Pt–0.5% MgO/TiO2
Pt–1.0% MgO/TiO2
Pt–2.0% MgO/TiO2
Pt–3.0% MgO/TiO2
Pt–TiO2 + MgO/TiO2
43
43
42
44
—
d
a
Reaction conditions: catalyst, 0.020 g; CO2 pressure, 2 MPa; H2O,
b
1.0 mL; irradiation time, 10 h. Pt loading in each catalyst was
0.50 wt%. BET surface area. The physical mixture with the same
composition as the 0.5 wt% Pt–1.0 wt% MgO/TiO2 photocatalyst.
c
d
The Pt–MgO/TiO2 catalyst exhibited the best photocatalytic per-
formance in CH4 formation among the modified Pt–TiO2 catalysts
examined in our work. We further investigated the effect of the
content of MgO on the photocatalytic performance. Table 1 shows
that the amount of chemisorbed CO2 increases monotonically with
the content of MgO. However, there is an optimum content of MgO
for CH4 formation; the amount of CH4 increases with increasing
content of MgO up to 1.0 wt%, and then decreases with a further
increase in the content of MgO. The BET surface area and the size of
Pt nanoparticles did not undergo significant changes with an
increase in the content of MgO from 0.25 wt% to 3.0 wt%. Thus,
there may exist other factors controlling the photocatalytic perfor-
mance in the Pt–MgO/TiO2 trinary nanocomposite.
We performed transmission electron microscopy (TEM)
studies of the Pt–MgO/TiO2 catalysts with MgO contents of
1.0 wt% and 3.0 wt%. As displayed in Fig. 3, MgO existed as
amorphous thin layers located on the surfaces of TiO2 crystal-
lites in both catalysts. Over the 0.5 wt% Pt–1.0 wt% MgO/TiO2
photocatalyst, the interface between TiO2, Pt and MgO could be
clearly observed in the enlarged image (Fig. 3b). Moreover,
because the MgO layer is thin, Pt nanoparticles could be easily
The modifying effect of other metal oxides on the photocatalytic
activity of the 0.5 wt% Pt–TiO2 catalyst was further investigated. We
found that the modification of the Pt–TiO2 using basic oxides such as
SrO, CaO, BaO, La2O3 and Lu2O3 could also increase the amount of
CH4 (Table S1, ESI†). Over these modified catalysts, the formation of
CO was inhibited. The mean sizes of Pt nanoparticles over these
modified catalysts were 3.8–4.1 nm, close to that (4.2 nm) over the
Pt–TiO2 catalyst without modification. The Brunauer–Emmett–Teller
(BET) surface areas of the modified catalysts measured using N2
physisorption were 43–51 m2 gÀ1, slightly lower than that (55 m2 gÀ1
)
of the Pt–TiO2 catalyst. Thus, the increase in the photocatalytic activity
as a result of modification with basic oxides should not be due to the
changes in the size of Pt nanoparticles or the surface area of
photocatalysts. We evaluated the catalyst basicity by measuring the
amount of CO2 chemisorption.17 We found a linear correlation
between the amount of CH4 formed and the amount of chemisorbed
CO2 over a series of basic oxide-modified Pt–TiO2 photocatalysts
(Fig. 2). This allows us to propose that the catalyst basicity contributes
to the photocatalytic reduction of CO2 with H2O to CH4.
Fig. 2 Correlation between the amount of CH4 formed and the amount of CO2
chemisorbed on the Pt–TiO2 photocatalysts modified with various basic metal
oxides. Reaction conditions: catalyst, 0.020 g; CO2 pressure, 2 MPa; H2O, 1.0 mL;
irradiation time, 10 h.
Fig. 3 TEM micrographs of 0.5 wt% Pt–1.0 wt% MgO/TiO2 (a and b) and 0.5 wt%
Pt–3.0 wt% MgO/TiO2 (c and d). (b and d) Enlarged images of (a) and (c).
c
2452 Chem. Commun., 2013, 49, 2451--2453
This journal is The Royal Society of Chemistry 2013

View Article Online
ChemComm
Communication
significantly enhanced by simply adding MgO, a basic oxide,
onto TiO2 in the presence of a Pt co-catalyst. A positive correla-
tion between the activity of CH4 formation and the catalyst
basicity has been observed in the trinary nanocomposites
containing TiO2, Pt and a basic metal oxide. The interface
between TiO2, Pt and MgO plays a crucial role in the photo-
catalytic reaction. The functions of MgO are proposed to be
mainly enhancing the density of CO2 on the catalyst surface
and destabilizing CO2 molecules, which are subsequently
reduced by the photogenerated electrons enriched on the
nearby Pt nanoparticles from TiO2.
This work was supported by the National Basic Research
Program of China (2013CB933100), the NSF of China
(21033006), the Program for Changjiang Scholars and Innova-
tive Research Team in University (No. IRT1036).
Fig. 4 Proposed functioning mechanisms of the MgO layer and Pt nanoparticles
over TiO2 for photocatalytic reduction of CO2 in the presence of H2O.
accessed by the reactants such as H2O (or protons) during the
reaction. On the other hand, relatively thicker MgO layers were
observed over the 0.5 wt% Pt–3 wt% MgO/TiO2 photocatalyst.
The thicker MgO layer could cover Pt nanoparticles (Fig. 3d).
This may lead to the decrease in the photocatalytic activity.
Thus, the proper location of the three components in the
photocatalyst should also determine the photocatalytic perfor-
mance. This has further been confirmed by the experimental
fact that the physical mixture of Pt–TiO2 and MgO/TiO2 with the
same composition as in the 0.5 wt% Pt–1.0 wt% MgO/TiO2
photocatalyst exhibited much inferior performances (Table 1).
Based on the results described above, we propose functioning
mechanisms of MgO and Pt co-catalysts for accelerating the
formation of CH4 in Fig. 4. CO2 is first chemisorbed on the
MgO layer on TiO2 crystallites. The chemisorbed CO2 molecule
becomes destabilized and its reactivity is believed to be higher
than that of the linear CO2 molecule.14,17 On the other hand,
photogenerated electrons on TiO2 can be easily trapped by the Pt
nanoparticles because of the lower Fermi energy level of the noble
metal. It is known that the formations of CO and CH4 require two
and eight electrons, respectively. The enriched electron density on
Pt nanoparticles would favour the formation of CH4, which is
thermodynamically more feasible than the formation of CO.10b
Our result has demonstrated that Pt enhances the photocatalytic
reduction of CO2 to CH4 but not to CO (Fig. 1). The synergistic
effect between MgO, which enhances the density of destabilized
CO2 molecules on the catalyst surface, and Pt nanoparticles with
Notes and references
1 For reviews on the transformations of CO2, see for examples:
(a) C. S. Song, Catal. Today, 2006, 115, 2–32; (b) M. Aresta and
A. Dibendetto, Dalton Trans., 2007, 2365–2387; (c) G. Centi and
S. Perathoner, Catal. Today, 2009, 148, 191–205; (d) J. Ma, N. Sun,
X. Zhang, N. Zhao, F. Xiao, W. Wei and Y. Sun, Catal. Today, 2009,
115, 2–32; (e) W. Wang, S. Wang, X. Ma and J. Gong, Chem. Soc. Rev.,
2011, 40, 3703–3727; ( f ) M. Cokoja, C. Bruckmeier, B. Rieger,
W. A. Herrmann and F. E. Ku¨hn, Angew. Chem., Int. Ed., 2011, 50,
8510–8537.
2 T. Inoue, A. Fujishima, S. Konishi and K. Honda, Nature, 1979, 277,
637–638.
3 V. P. Indrakanti, J. D. Kubicki and H. H. Schobert, Energy Environ.
Sci., 2009, 2, 745–758.
4 S. C. Roy, O. K. Varghese, M. Paulose and C. A. Grimes, ACS Nano,
2010, 4, 1259–1278.
5 K. Mori, H. Yamashita and M. Anpo, RSC Adv., 2012, 2, 3165–3172.
6 W. Fan, Q. Zhang and Y. Wang, Phys. Chem. Chem. Phys., 2013, 15,
2632–2649.
7 H. Park, J. H. Choi, K. M. Choi, D. K. Lee and J. K. Kang, J. Mater.
Chem., 2012, 22, 5304–5307.
8 (a) Q. Liu, Y. Zhou, J. Kou, X. Chen, Z. Tian, J. Gao, S. Yan and
Z. Zou, J. Am. Chem. Soc., 2010, 132, 14385–14387; (b) S. Yan, L. Wan,
Z. Li and Z. Zou, Chem. Commun., 2011, 47, 5632–5634.
9 (a) S. C. Yan, S. X. Ouyang, J. Gao, M. Yang, J. Y. Feng, X. X. Fan,
L. J. Wan, Z. S. Li, J. H. Ye, Y. Zhou and Z. Zou, Angew. Chem., Int.
Ed., 2010, 49, 6400–6404; (b) S. Yan, H. Yu, N. Wang, Z. Li and
Z. Zou, Chem. Commun., 2012, 48, 1048–1050.
10 (a) O. K. Varghese, M. Paulose, T. J. LaTempa and G. A. Grimes, Nano
Lett., 2009, 9, 731–737; (b) W. N. Wang, W. J. An, B. Ramalingam,
S. Mukherjee, D. M. Niedzwiedzki, S. Gangopadhyay and P. Biswas,
J. Am. Chem. Soc., 2012, 134, 11276–11281.
11 L. Liu, H. Zhao, J. M. Andino and Y. Li, ACS Catal., 2012, 2,
1817–1828.
enriched electron density can further accelerate the reduction of 12 (a) C. Wang, R. L. Thompson, J. Baltrus and C. Matranga, J. Phys.
Chem. Lett., 2010, 1, 48–53; (b) S. I. In, D. D. Vaughn and
R. E. Schaak, Angew. Chem., Int. Ed., 2012, 51, 3915–3918.
13 K. Teramura, T. Tanaka, H. Ishikawa, Y. Kohno and T. Funabiki,
CO2 to CH4. Clearly, such a synergistic effect requires the intimate
contact of Pt between both TiO2 and MgO, strengthening the key
role of the interface.
J. Phys. Chem. B, 2004, 108, 346–354.
14 K. Teramura, S. Iguchi, Y. Mizuno, T. Shishido and T. Tanaka,
Angew. Chem., Int. Ed., 2012, 51, 8008–8011.
Besides the role of basicity, MgO layers on TiO2 surfaces
might also capture the holes and facilitate the separation of
photogenerated electrons and holes. Actually, such a role of Al2O3
thin layers coated on Fe2O3 surfaces was once reported by Formal
et al.18 Moreover, the coverage of TiO2 surfaces with MgO might
hinder the direct contact of CO or CH4 with TiO2 surfaces,
reducing the reoxidation possibilities for CO and CH4.
In conclusion, we have demonstrated for the first time that
the photocatalytic reduction of CO2 with H2O to CH4 can be
´
15 F. Sastre, A. Corma and H. Garcıa, J. Am. Chem. Soc., 2012, 134,
14137–14141.
16 (a) O. Ishitani, C. Inoue, Y. Suzuki and T. Ibusuki, J. Photochem.
Photobiol., A, 1993, 72, 269–271; (b) T. Yui, A. Kan, C. Saitoh,
K. Koike, T. Ibusuki and O. Ishitani, ACS Appl. Mater. Interfaces,
2011, 3, 2549–2600.
17 K. Teramura, S. Okuoka, H. Tsuneoka, T. Shishido and T. Tanaka,
Appl. Catal., B, 2010, 96, 565–568.
´
¨
18 F. L. Formal, N. Tetreault, M. Cornuz, T. Moehl, M. Gratzel and
K. Sivula, Chem. Sci., 2011, 2, 737–743.
c
This journal is The Royal Society of Chemistry 2013
Chem. Commun., 2013, 49, 2451--2453 2453