2558 Catal. Sci. Technol., 2012, 2, 2558–2568 This journal is c The Royal Society of Chemistry 2012 Cite this: Catal. Sci. Technol., 2012, 2, 2558–2568 Photocatalytic conversion of CO 2 and H 2 O to fuels by nanostructured Ce–TiO 2 /SBA-15 composites Cunyu Zhao, a Lianjun Liu, a Qianyi Zhang, a Jun Wang b and Ying Li* a Received 24th May 2012, Accepted 11th August 2012 DOI: 10.1039/c2cy20346d Cerium-doped titanium oxide (Ce–TiO 2 ) nanoparticles were prepared by a simple sol–gel method. Ce-doping decreased the crystal size of TiO 2 , increased the catalyst surface area, and inhibited the growth of rutile TiO 2 crystals. Ce–TiO 2 nanoparticles were also dispersed on SBA-15, mesoporous silica with one-dimensional pores, forming a Ce–TiO 2 /SBA-15 nanocomposite. The nanocomposite materials were well characterized and tested as photocatalysts to convert CO 2 and H 2 O to value-added fuels, mainly CO and CH 4 , under UV-vis illumination. Compared with pristine TiO 2 , TiO 2 doped by 1 or 3% Ce improved the production of CO by four times. The reason may be due to the facilitated charge transfer induced by the doped Ce ions, the higher surface area of the catalyst, as well as the stabilization of anatase phase. However, too high a Ce concentration reduced the catalytic activity, likely due to the formation of recombination centers. Compared with unsupported Ce–TiO 2 , Ce–TiO 2 supported on SBA-15 remarkably enhanced the CO 2 reduction rate. Ce–TiO 2 /SBA-15 with a Ti : Si ratio of 1 : 4 demonstrated 8-fold enhancement in CO production and 115-fold enhancement in CH 4 production. By contrast, amorphous silica as the substrate was much inferior to SBA-15. The findings in this work reveal a promising nanostructured catalyst material for solar fuel production using CO 2 and H 2 O as the feedstock. 1. Introduction Photocatalytic reduction of CO 2 using sunlight as the energy input is a promising way to reduce CO 2 level in the atmosphere and in the meantime produce alternative fuels or building blocks for industrial chemicals. Semiconductor photocatalysts such as WO 3 , ZrO 2 , Ga 2 O 3 , and TiO 2 1–4 have been studied for such applications, and among them, TiO 2 has been considered the most appropriate photocatalyst due to its high photo- sensitivity, non-toxic nature, low cost, and easy availability. 5–9 However, the photoefficiency of CO 2 reduction on TiO 2 is usually very low, mainly due to the fast recombination of photo-excited electron–hole (e–h) pairs and the wide band gap of TiO 2 (3.2 eV for anatase) that does not allow the utilization of visible light. 10,11 To enhance the photoefficiency of TiO 2 for CO 2 photo- reduction, several strategies have been reported in the literature by modifying the nanostructure of TiO 2 . The first strategy is to harvest visible light by tailoring the band gap of TiO 2 or introducing impurity level with non-metal ion doping (e.g. , I, N). 12,13 The second strategy is to lower the activation energy and to facilitate electron trapping by depositing noble metal nanoparticles (e.g., Pt, Rh, Ag) 14–17 on the TiO 2 surface. Finally, pairing of transition metal oxides (e.g., CuO, Cu 2 O, Fe 2 O 3 ) 4,11,18,19 with TiO 2 has been demonstrated to enhance CO 2 reduction by facilitating the separation of electrons and holes, although the exact mechanism has not been well understood. Rare earth element modified TiO 2 has demonstrated enhanced photocatalytic oxidation ability than bare TiO 2 . 7,20,21 Cerium (Ce) is one of the four most abundant rare earth elements, and composites of Ce–TiO 2 have shown enhanced photocatalytic activity for water splitting and degradation of organic compounds. 22–26 Ce-doping could result in smaller TiO 2 nanocrystals and thus enhance catalytic activity. 27 However, to the best of our knowledge, Ce-modified TiO 2 has not been studied for the photocatalytic CO 2 reduction with water. Besides modifying the properties and nanostructures of TiO 2 itself, many approaches have been used to immobilize TiO 2 nanoparticles on mesoporous substrates such as molecular sieve 5 A ˚ , amorphous silica, and SBA-15, 4,28–30 and the direct benefits include larger surface area and better dispersion of the nano-sized catalysts. It has been reported that Cu–TiO 2 , Ru–TiO 2 , and TiO 2 supported on amorphous silica showed higher CO 2 photoreduction rates than the catalysts without supports. 4,18,28 SBA-15, the well-known mesoporous silica with one-dimensional hexagonal pores, has been widely studied as a catalyst support and the ordered and uniform pore structures a Department of Mechanical Engineering, University of Wisconsin- Milwaukee, Milwaukee, WI, 53211, USA. E-mail: [email protected]; Fax: +1 414-229-6958; Tel: +1 414-229-3716 b Department of Environmental Engineering Sciences, University of Florida, Gainesville, FL, 32611, USA Catalysis Science & Technology Dynamic Article Links www.rsc.org/catalysis PAPER Downloaded on 06 January 2013 Published on 14 August 2012 on http://pubs.rsc.org | doi:10.1039/C2CY20346D View Article Online / Journal Homepage / Table of Contents for this issue
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
2558 Catal. Sci. Technol., 2012, 2, 2558–2568 This journal is c The Royal Society of Chemistry 2012
Photocatalytic conversion of CO2 and H2O to fuels by nanostructured
Ce–TiO2/SBA-15 composites
Cunyu Zhao,aLianjun Liu,
aQianyi Zhang,
aJun Wang
band Ying Li*
a
Received 24th May 2012, Accepted 11th August 2012
DOI: 10.1039/c2cy20346d
Cerium-doped titanium oxide (Ce–TiO2) nanoparticles were prepared by a simple sol–gel method.
Ce-doping decreased the crystal size of TiO2, increased the catalyst surface area, and inhibited the
growth of rutile TiO2 crystals. Ce–TiO2 nanoparticles were also dispersed on SBA-15, mesoporous
silica with one-dimensional pores, forming a Ce–TiO2/SBA-15 nanocomposite. The
nanocomposite materials were well characterized and tested as photocatalysts to convert CO2 and
H2O to value-added fuels, mainly CO and CH4, under UV-vis illumination. Compared with
pristine TiO2, TiO2 doped by 1 or 3% Ce improved the production of CO by four times. The
reason may be due to the facilitated charge transfer induced by the doped Ce ions, the higher
surface area of the catalyst, as well as the stabilization of anatase phase. However, too high a Ce
concentration reduced the catalytic activity, likely due to the formation of recombination centers.
Compared with unsupported Ce–TiO2, Ce–TiO2 supported on SBA-15 remarkably enhanced the
CO2 reduction rate. Ce–TiO2/SBA-15 with a Ti : Si ratio of 1 : 4 demonstrated 8-fold
enhancement in CO production and 115-fold enhancement in CH4 production. By contrast,
amorphous silica as the substrate was much inferior to SBA-15. The findings in this work reveal a
promising nanostructured catalyst material for solar fuel production using CO2 and H2O as the
feedstock.
1. Introduction
Photocatalytic reduction of CO2 using sunlight as the energy
input is a promising way to reduce CO2 level in the atmosphere
and in the meantime produce alternative fuels or building
blocks for industrial chemicals. Semiconductor photocatalysts
such as WO3, ZrO2, Ga2O3, and TiO21–4 have been studied for
such applications, and among them, TiO2 has been considered
the most appropriate photocatalyst due to its high photo-
sensitivity, non-toxic nature, low cost, and easy availability.5–9
However, the photoefficiency of CO2 reduction on TiO2 is
usually very low, mainly due to the fast recombination of
photo-excited electron–hole (e–h) pairs and the wide band gap
of TiO2 (3.2 eV for anatase) that does not allow the utilization
of visible light.10,11
To enhance the photoefficiency of TiO2 for CO2 photo-
reduction, several strategies have been reported in the literature by
modifying the nanostructure of TiO2. The first strategy is to harvest
visible light by tailoring the band gap of TiO2 or introducing
impurity level with non-metal ion doping (e.g., I, N).12,13 The
second strategy is to lower the activation energy and to facilitate
electron trapping by depositing noble metal nanoparticles
(e.g., Pt, Rh, Ag)14–17 on the TiO2 surface. Finally, pairing
of transition metal oxides (e.g., CuO, Cu2O, Fe2O3)4,11,18,19
with TiO2 has been demonstrated to enhance CO2 reduction
by facilitating the separation of electrons and holes, although
the exact mechanism has not been well understood.
Rare earth element modified TiO2 has demonstrated
enhanced photocatalytic oxidation ability than bare TiO2.7,20,21
Cerium (Ce) is one of the four most abundant rare earth
elements, and composites of Ce–TiO2 have shown enhanced
photocatalytic activity for water splitting and degradation of
organic compounds.22–26 Ce-doping could result in smaller TiO2
nanocrystals and thus enhance catalytic activity.27 However, to
the best of our knowledge, Ce-modified TiO2 has not been
studied for the photocatalytic CO2 reduction with water.
Besides modifying the properties and nanostructures of
TiO2 itself, many approaches have been used to immobilize
TiO2 nanoparticles on mesoporous substrates such as molecular
sieve 5 A, amorphous silica, and SBA-15,4,28–30 and the direct
benefits include larger surface area and better dispersion of the
nano-sized catalysts. It has been reported that Cu–TiO2,
Ru–TiO2, and TiO2 supported on amorphous silica showed
higher CO2 photoreduction rates than the catalysts without
supports.4,18,28 SBA-15, the well-known mesoporous silica with
one-dimensional hexagonal pores, has been widely studied as a
catalyst support and the ordered and uniform pore structures
aDepartment of Mechanical Engineering, University of Wisconsin-Milwaukee, Milwaukee, WI, 53211, USA. E-mail: [email protected];Fax: +1 414-229-6958; Tel: +1 414-229-3716
bDepartment of Environmental Engineering Sciences, University ofFlorida, Gainesville, FL, 32611, USA
CatalysisScience & Technology
Dynamic Article Links
www.rsc.org/catalysis PAPER
Dow
nloa
ded
on 0
6 Ja
nuar
y 20
13Pu
blis
hed
on 1
4 A
ugus
t 201
2 on
http
://pu
bs.r
sc.o
rg |
doi:1
0.10
39/C
2CY
2034
6DView Article Online / Journal Homepage / Table of Contents for this issue
2566 Catal. Sci. Technol., 2012, 2, 2558–2568 This journal is c The Royal Society of Chemistry 2012
study is very intriguing. Because eight protons are needed for
generation of one CH4 molecule in comparison with two
electrons for CO generation, the surface Si–OH groups or
active OH sites due to the presence of SBA-15 may be more
readily available for CO2 reduction and conversion to CH4.
Investigations on this high CH4 selectivity as well as the
potential synergies between the Ce species, TiO2 and the
SBA-15 support are in our future research plan.
Another important finding in this study was that Ce–TiO2
dispersed on SBA-15 was much more active than that dispersed
on amorphous mesoporous silica. As shown in Fig. 12, the CO
yield on the amorphous silica supported sample, 0.03Ce_1Ti_4aSi
was very small, even lower than the catalyst without silica
support. The CH4 yield of 0.03Ce_1Ti_4aSi was 1.5 mmol g�1,
higher than the catalyst without silica support but still much
lower than the catalyst supported on SBA-15. The overall CO2
reduction rate to CO and CH4 was more than 10 times higher
on 0.03Ce_1Ti_4Si than on 0.03Ce_1Ti_4aSi. Since the specific
surface area of the 0.03Ce_1Ti_4Si sample was only slightly
larger that of the 0.03Ce_1Ti_4aSi sample (Table 1), the much
higher activity of the SBA-15 supported sample should result
from other factors besides the surface area effect.
One possible reason is that ordered pores of SBA-15 are
more accessible to TiO2 precursors during the preparation
process, leading to a better dispersion of TiO2 nanoparticles.
Amorphous silica with random arrays of pores and shapes in
some cases are poor supports for functional agents because
not all the volume of irregular pores is accessible to the
incorporated species.58 The second possible reason could be
the stronger interactions between the SBA-15 and the TiO2
nanoparticles confined inside the ordered pores, which favor the
formation of interfacial active sites. Li et al.59 identified a
particular Ti–O–Si bond at the interface of nano-TiO2 embedded
in a mesoporous SiO2 through FTIR analysis, and they reported
that this Ti–O–Si bond attributed to a stronger light absorption
in the UV region compared with bare TiO2. Our UV-vis
absorption results (Fig. 8b) also showed a similar stronger
absorption for Ce–TiO2/SBA-15 samples at l o 350 nm,
suggesting a possible formation of a Ti–O–Si bond at the
TiO2–SBA-15 interface. The third possible reason may be
related to the larger CO2 adsorption capacity of SBA-1560
and easier diffusion of CO2 into the ordered pores compared
with the amorphous silica. The enhanced local concentration
of CO2 around the TiO2 nanoparticles could then promote the
subsequent CO2 reduction reaction.
4. Conclusion
A novel Ce–TiO2/SBA-15 nanocomposite was synthesized for
the first time in the literature and tested as a photocatalyst for
converting CO2 and H2O to fuels such as CO and CH4 under
photo-illumination. Modification of TiO2 with Ce significantly
stabilized the TiO2 anatase phase and increased the specific
surface area, which contributed to an improvement of CO
production from CO2 reduction. Dispersing Ce–TiO2 nano-
particles on the mesoporous SBA-15 support further enhanced
both CO and CH4 production. Particularly the CH4 production
was enhanced by up to 115 times compared with unsupported
Ce–TiO2. The superior catalytic activity may be related to the
partially embedded Ce–TiO2 nanoparticles in the ordered 1-D
pores in SBA-15 that form synergies between the different compo-
nents of the catalysts and enhance the diffusion and adsorption of
CO2. This mechanism also correlates well the results that using
SBA-15 as the support led to more than 10 times higher activity in
CO2 photoreduction than using amorphous silica as the support.
Findings in this work demonstrate the feasibility of solar fuel
production from CO2 and H2O using the prepared nanocomposite
photocatalysts. On-going research is being dedicated to further
improving the overall CO2 conversion rate and control of product
selectivity.
Acknowledgements
This work is supported by American Chemical Society –
Petroleum Research Fund (ACS–PRF, Grant #50631-
DNI10). The authors acknowledge Mr. Donald Robertson
at the Physics Laboratory for High Resolution Transmission
Electron Microscopy at UW-Milwaukee for his assistance in
TEM and HRTEM analyses. The authors also thank Dr.
Valentin Craciun at the Major Analytical Instrumentation
Center (MAIC) at the University of Florida for his assistance
in XPS analyses.
References
1 B. Aurian-Blajeni, M. Halmann and J. Manassen, Photoreductionof carbon dioxide and water into formaldehyde and methanol onsemiconductor materials, Sol. Energy, 1980, 25(2), 165–170.
2 Y. Kohno, T. Tanaka, T. Funabiki and S. Yoshida, Photoreduc-tion of CO2 with H2 over ZrO2. A study on interaction of hydrogenwith photoexcited CO2, Phys. Chem. Chem. Phys., 2000, 2(11),2635–2639.
3 H. Tsuneoka, K. Teramura, T. Shishido and T. Tanaka, AdsorbedSpecies of CO2 and H2 on Ga2O3 for the Photocatalytic Reductionof CO2, J. Phys. Chem. C, 2010, 114(19), 8892–8898.
4 Y. Li, W. N. Wang, Z. L. Zhan, M. H. Woo, C. Y. Wu andP. Biswas, Photocatalytic reduction of CO2 with H2O on mesoporoussilica supported Cu/TiO2 catalysts, Appl. Catal., B, 2010, 100(1–2),386–392.
5 O. K. Varghese, M. Paulose, T. J. LaTempa and C. A. Grimes,High-Rate Solar Photocatalytic Conversion of CO2 and WaterVapor to Hydrocarbon Fuels, Nano Lett., 2009, 9(2), 731–737.
Fig. 12 Product yields of CO and CH4 over Ce–TiO2/SBA-15 and
Ce–TiO2/aSiO2 catalysts under UV-vis illumination for 4 h.
This journal is c The Royal Society of Chemistry 2012 Catal. Sci. Technol., 2012, 2, 2558–2568 2567
6 H. M. Yang, K. Zhang, R. R. Shi and A. D. Tang, Sol–gel synthesisand photocatalytic activity of CeO2/TiO2 nanocomposites, J. Am.Ceram. Soc., 2007, 90(5), 1370–1374.
7 C. Wen, H. Deng, J. Y. Tian and J. M. Zhang, Photocatalyticactivity enhancing for TiO2 photocatalyst by doping with La,Trans. Nonferrous Met. Soc. China, 2006, 16, S728–S731.
8 C. S. Yuan, C. C. Lo, C. H. Hung and J. F. Wu, Photoreduction ofcarbon dioxide with H2 and H2O over TiO2 and ZrO2 in acirculated photocatalytic reactor, Sol. Energy Mater. Sol. Cells,2007, 91(19), 1765–1774.
9 A. Fujishima, X. Zhang and D. A. Tryk, TiO2 photocatalysis andrelated surface phenomena, Surf. Sci. Rep., 2008, 63(12), 515–582.
10 Q. Y. Zhang, Y. Li, E. A. Ackerman, M. Gajdardziska-Josifovskaand H. L. Li, Visible light responsive iodine-doped TiO2 forphotocatalytic reduction of CO2 to fuels, Appl. Catal., A, 2011,400(1–2), 195–202.
11 I. H. Tseng, J. C. S. Wu and H. Y. Chou, Effects of sol–gelprocedures on the photocatalysis of Cu/TiO2 in CO2 photoreduc-tion, J. Catal., 2004, 221(2), 432–440.
12 W. Y. Su, Y. F. Zhang, Z. H. Li, L. Wu, X. X. Wang, J. Q. Li andX. Z. Fu, Multivalency iodine doped TiO2: Preparation, characteriza-tion, theoretical studies, and visible-light photocatalysis, Langmuir,2008, 24(7), 3422–3428.
13 R. Asahi, T. Morikawa, T. Ohwaki, K. Aoki and Y. Taga, Visible-light photocatalysis in nitrogen-doped titanium oxides, Science,2001, 293(5528), 269–271.
14 C. J. Wang, R. Thompson, J. Baltrus and C. Matranga, VisibleLight Photoreduction of CO2 using CdSe/Pt/TiO2 HeterostructuredCatalysts, J. Phys. Chem. Lett., 2010, 1(1), 48–53.
15 Y. Kohno, H. Hayashi, S. Takenaka, T. Tanaka, T. Funabiki andS. Yoshida, Photo-enhanced reduction of carbon dioxide withhydrogen over Rh/TiO2, J. Photochem. Photobiol., A, 1999,126(1–3), 117–123.
16 K. Koci, K. Zatloukalova, L. Obalova, S. Krejcikova, Z. Lacny,L. Capek, A. Hospodkova and O. Solcova, Wavelength Effect onPhotocatalytic Reduction of CO2 by Ag/TiO2 Catalyst, Chin. J.Catal., 2011, 32(5), 812–815.
17 C. Y. Zhao, A. Kroll, H. L. Zhao, Q. Y. Zhang and Y. Li,Ultrasonic spray pyrolysis synthesis of Ag/TiO2 nanocompositephotocatalysts for simultaneous H2 production and CO2
reduction, Int. J. Hydrogen Energy, 2012, 37(13), 9967–9976.18 J. C. S. Wu, Photocatalytic Reduction of Greenhouse Gas CO2 to
Fuel, Catal. Surv. Asia, 2009, 13(1), 30–40.19 L. J. Liu, C. Y. Zhao and Y. Li, Spontaneous Dissociation of CO2
to CO on Defective Surface of Cu(I)/TiO2�x Nanoparticles atRoom Temperature, J. Phys. Chem. C, 2012, 116(14), 7904–7912.
20 Y. H. Xu and Z. X. Zeng, The preparation, characterization, andphotocatalytic activities of Ce-TiO2/SiO2, J. Mol. Catal. A: Chem.,2008, 279(1), 77–81.
21 B. M. Reddy, A. Khan, P. Lakshmanan, M. Aouine, S. Loridant andJ. C. Volta, Structural characterization of nanosized CeO2–SiO2,CeO2–TiO2, and CeO2–ZrO2 catalysts by XRD, Raman, and HREMtechniques, J. Phys. Chem. B, 2005, 109(8), 3355–3363.
22 K. Ogura, M. Kawano, J. Yano and Y. Sakata, Visible-Light-Assisted Decomposition of H2O and Photomethanation of CO2
over CeO2–TiO2 Catalyst, J. Photochem. Photobiol., A, 1992, 66(1),91–97.
23 J. M. Xie, D. L. Jiang, M. Chen, D. Li, J. J. Zhu, X. M. Lu andC. H. Yan, Preparation and characterization of monodisperseCe-doped TiO2 microspheres with visible light photocatalyticactivity, Colloids Surf., A, 2010, 372(1–3), 107–114.
24 G. S. Li, D. Q. Zhang and J. C. Yu, Thermally stable orderedmesoporous CeO2/TiO2 visible-light photocatalysts, Phys. Chem.Chem. Phys., 2009, 11(19), 3775–3782.
25 X. J. Sun, H. Liu, J. H. Dong, J. Z. Wei and Y. Zhang, Preparationand Characterization of Ce/N-Codoped TiO2 Particles for Productionof H2 by Photocatalytic Splitting Water Under Visible Light, Catal.Lett., 2010, 135(3–4), 219–225.
26 G. Magesh, B. Viswanathan, R. P. Viswanath andT. K. Varadarajan, Photocatalytic behavior of CeO2–TiO2 systemfor the degradation of methylene blue, Indian J. Chem., Sect. A:Inorg., Bio-Inorg., Phys., Theor. Anal. Chem., 2009, 48(4), 480–488.
27 F. Galindo, R. Gomez and M. Aguilar, Photodegradation ofthe herbicide 2,4-dichlorophenoxyacetic acid on nanocrystalline
TiO2-CeO2 sol–gel catalysts, J. Mol. Catal. A: Chem., 2008,281(1–2), 119–125.
28 N. Sasirekha, S. J. S. Basha and K. Shanthi, Photocatalyticperformance of Ru doped anatase mounted on silica for reductionof carbon dioxide, Appl. Catal., B, 2006, 62(1–2), 169–180.
29 B. Srinivas, B. Shubhamangala, K. Lalitha, P. A. K. Reddy,V. D. Kumari, M. Subrahmanyam and B. R. Dema, PhotocatalyticReduction of CO2 over Cu-TiO2/Molecular Sieve 5A Composite,Photochem. Photobiol., 2011, 87(5), 995–1001.
30 S. C. Zhang, D. Jiang, T. Tang, J. H. Li, Y. Xu, W. L. Shen, J. Xuand F. Deng, TiO2/SBA-15 photocatalysts synthesized through thesurface acidolysis of Ti(OnBu)4 on carboxyl-modified SBA-15,Catal. Today, 2010, 158(3–4), 329–335.
31 C. C. Yang, J. Vernimmen, V. Meynen, P. Cool and G. Mul,Mechanistic study of hydrocarbon formation in photocatalyticCO2 reduction over Ti-SBA-15, J. Catal., 2011, 284(1), 1–8.
32 H. C. Yang, H. Y. Lin, Y. S. Chien, J. C. S. Wu and H. H. Wu,Mesoporous TiO2/SBA-15, and Cu/TiO2/SBA-15 CompositePhotocatalysts for Photoreduction of CO2 to Methanol, Catal.Lett., 2009, 131(3–4), 381–387.
33 L. Q. Jing, X. J. Sun, B. F. Xin, B. Q. Wang, W. M. Cai andH. G. Fu, The preparation and characterization of La doped TiO2
nanoparticles and their photocatalytic activity, J. Solid StateChem., 2004, 177(10), 3375–3382.
34 D. Y. Zhao, J. L. Feng, Q. S. Huo, N. Melosh, G. H. Fredrickson,B. F. Chmelka and G. D. Stucky, Triblock copolymer syntheses ofmesoporous silica with periodic 50 to 300 angstrom pores, Science,1998, 279(5350), 548–552.
35 L. J. Liu, H. L. Zhao, J. M. Andino and Y. Li, Photocatalytic CO2
reduction with H2O on TiO2 nanocrystals: Comparison withanatase, rutile, brookite polymorphs and exploration of surfacechemistry, ACS Catal., 2012, 2, 1817–1828.
36 Q. Y. Zhang, T. T. Gao, J. M. Andino and Y. Li, Copper and IodineCo-modified TiO2 Nanoparticles for Improved Activity of CO2 Photo-reduction with Water Vapor, Appl. Catal., B, 2012, 123–124, 257–264.
37 T. A. Kandiel, A. Feldhoff, L. Robben, R. Dillert andD. W. Bahnemann, Tailored Titanium Dioxide Nanomaterials:Anatase Nanoparticles and Brookite Nanorods as Highly ActivePhotocatalysts, Chem. Mater., 2010, 22(6), 2050–2060.
38 A. Di Paola, G. Cufalo, M. Addamo,M. B. Ellardita, R. Campostrini,M. Ischia, R. Ceccato and L. Palmisano, Photocatalytic activity ofnanocrystalline TiO2 (brookite, rutile and brookite-based) powdersprepared by thermohydrolysis of TiCl4 in aqueous chloride solutions,Colloids and Surf., A, 2008, 317(1–3), 366–376.
39 J. Fang, X. Z. Bi, D. J. Si, Z. Q. Jiang and W. X. Huang,Spectroscopic studies of interfacial structures of CeO2–TiO2 mixedoxides, Appl. Surf. Sci., 2007, 253(22), 8952–8961.
40 T. Kidchob, L. Malfatti, D. Marongiu, S. Enzo and P. Innocenzi,An alternative sol–gel route for the preparation of thin films inCeO2–TiO2 binary system, Thin Solid Films, 2010, 518(6),1653–1657.
41 Z. L. Liu, B. Guo, L. Hong and H. X. Jiang, Preparation andcharacterization of cerium oxide doped TiO2 nanoparticles,J. Phys. Chem. Solids, 2005, 66(1), 161–167.
42 X. L. Nie, S. P. Zhuo, G. Maeng and K. Sohlberg, Doping of TiO2
polymorphs for altered optical and photocatalytic properties,Int. J. Photoenergy, 2009, 2009, 294042.
43 J. L. Blin and B. L. Su, Tailoring pore size of ordered mesoporoussilicas using one or two organic auxiliaries as expanders, Langmuir,2002, 18(13), 5303–5308.
44 Z. B. Wu, F. Dong, W. R. Zhao and S. Guo, Visible light inducedelectron transfer process over nitrogen doped TiO2 nanocrystalsprepared by oxidation of titanium nitride, J. Hazard. Mater., 2008,157(1), 57–63.
45 A. Henglein, Small-Particle Research – Physicochemical propertiesof extremely small colloidal metal and semiconductor particles,Chem. Rev., 1989, 89(8), 1861–1873.
46 D. R. Sahu, L. Y. Hong, S. C. Wang and J. L. Huang, Synthesis,analysis and characterization of ordered mesoporous TiO2/SBA-15matrix: Effect of calcination temperature,MicroporousMesoporousMater., 2009, 117(3), 640–649.
47 J. Yang, J. Zhang, L. W. Zhu, S. Y. Chen, Y. M. Zhang, Y. Tang,Y. L. Zhu and Y. W. Li, Synthesis of nano titania particlesembedded in mesoporous SBA-15: Characterization and photo-catalytic activity, J. Hazard. Mater., 2006, 137(2), 952–958.
2568 Catal. Sci. Technol., 2012, 2, 2558–2568 This journal is c The Royal Society of Chemistry 2012
48 H. G. Zhu, Z. W. Pan, B. Chen, B. Lee, S. M. Mahurin,S. H. Overbury and S. Dai, Synthesis of ordered mixed titaniaand silica mesostructured monoliths for gold catalysts, J. Phys.Chem. B, 2004, 108(52), 20038–20044.
49 J. P. Holgado, R. Alvarez and G. Munuera, Study of CeO2 XPSspectra by factor analysis: reduction of CeO2, Appl. Surf. Sci.,2000, 161(3–4), 301–315.
50 I. H. Tseng, W. C. Chang and J. C. S. Wu, Photoreduction of CO2
using sol–gel derived titania and titania-supported copper catalysts,Appl. Catal., B, 2002, 37(1), 37–48.
51 W. N. Wang, W. J. An, B. Ramalingam, S. Mukherjee,D. M. Niedzwiedzki, S. Gangopadhyay and P. Biswas, Size andStructure Matter: Enhanced CO2 Photoreduction Efficiency bySize-Resolved Ultrafine Pt Nanoparticles on TiO2 Single Crystals,J. Am. Chem. Soc., 2012, 134, 11276–11281.
52 D. C. Hurum, A. G. Agrios, S. E. Crist, K. A. Gray, T. Rajh andM. C. Thurnauer, Probing reaction mechanisms in mixed phase TiO2
by EPR, J. Electron Spectrosc. Relat. Phenom., 2006, 150(2–3), 155–163.53 D. C. Hurum, A. G. Agrios, K. A. Gray, T. Rajh and
M. C. Thurnauer, Explaining the enhanced photocatalytic activityof Degussa P25 mixed-phase TiO2 using EPR, J. Phys. Chem. B,2003, 107(19), 4545–4549.
54 A. Di Paola, M. Bellardita, R. Ceccato, L. Palmisano andF. Parrino, Highly Active Photocatalytic TiO2 Powders Obtained
by Thermohydrolysis of TiCl4 in Water, J. Phys. Chem. C, 2009,113(34), 15166–15174.
55 T. Ozawa, M. Iwasaki, H. Tada, T. Akita, K. Tanaka and S. Ito,Low-temperature synthesis of anatase-brookite composite nano-crystals: the junction effect on photocatalytic activity, J. ColloidInterface Sci., 2005, 281(2), 510–513.
56 G. H. Tian, H. G. Fu, L. Q. Jing, B. F. Xin and K. Pan,Preparation and characterization of stable biphase TiO2 photo-catalyst with high crystallinity, large surface area, and enhancedphotoactivity, J. Phys. Chem. C, 2008, 112(8), 3083–3089.
57 J. Yang, L. W. Zhu, J. Zhang, Y. M. Zhang and Y. Tang,Synthesis of nanosized TiO2/SiO2 catalysts by the ultrasonicmicroemulsion method and their photocatalytic activity, React.Kinet. Catal. Lett., 2007, 91(1), 21–28.
58 S. Choi, J. H. Drese and C. W. Jones, Adsorbent Materials forCarbon Dioxide Capture from Large Anthropogenic PointSources, ChemSusChem, 2009, 2(9), 796–854.
59 Y. Z. Li and S. J. Kim, Synthesis and characterization of nanotitania particles embedded in mesoporous silica with both high photo-catalytic activity and adsorption capability, J. Phys. Chem. B, 2005,109(25), 12309–12315.
60 Q. A. Wang, J. Z. Luo, Z. Y. Zhong and A. Borgna, CO2 captureby solid adsorbents and their applications: current status and newtrends, Energy Environ. Sci., 2011, 4(1), 42–55.