Synthesis of doped ceria with mesoporous flowerlike morphology and its catalytic performance for CO oxidation

Microporous and Mesoporous Materials 120 (2009) 426–431

Contents lists available at ScienceDirect

Microporous and Mesoporous Materials journal homepage: www.elsevier.com/locate/micromeso

Synthesis of doped ceria with mesoporous flowerlike morphology and its catalytic performance for CO oxidation Guoliang Xiao, Shuai Li, Hong Li *, Liquan Chen Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China

a r t i c l e

i n f o

Article history: Received 16 October 2008 Received in revised form 11 December 2008 Accepted 12 December 2008 Available online 25 December 2008 Keywords: Ceria Flowerlike Doping CO oxidation Stability

a b s t r a c t Mesporous flowerlike ceria Ce0.9M0.1O2d (M = Y, La, Zr, Pr and Sn) have been synthesized successfully by a hydrothermal method. The impacts of doping on their physical properties are investigated by means of X-ray diffraction (XRD), scanning electron microscopy (SEM), nitrogen adsorption, Raman and X-ray photoelectron spectroscopy (XPS). The doped materials show relatively high stability against the grain growth at 800 °C under reducing and oxidizing atmosphere. The catalytic activities of all flowerlike ceria materials for CO conversion are quite high (200 < T50 < 320 °C) due to their high specific surface area (>100 m2/g), open mesoporous structure (pore size  3.9 nm) and nano-crystalline nature (grain size < 10 nm). Among flowerlike materials, Pr and Sn doped ceria show enhanced activity while La, Y, Zr doped ceria show decreased activity compared to undoped ceria. Ó 2008 Elsevier Inc. All rights reserved.

1. Introduction Ceria materials have attracted continuous attention because of their wide applications [1–5]. As an important component in catalysts, ceria promotes many redox processes [1–3] due to the properties that Ce4+ can be reversely reduced to Ce3+, high oxygen storage capacity (OSC) and high oxygen ion conductivity [6,7]. The physical and chemical properties of ceria can be tuned by doping. Low-temperature reducibility is enhanced by doping with reducible elements, such as Pr and Sn [8–10]. The oxygen storage capacity (OSC) is increased by doping with tetravalent cations, such as Zr and Hf [11–13]. High ionic conductivities are achieved by doping with trivalent cations, such as La, Sm, Gd and Y [14,15]. The tuning of these properties has strong impact on their catalytic properties [1–15]. In addition, the catalytic activity of ceria can be improved greatly by reducing its scale [7,16,17]. Various nano-scaled ceria based materials have been synthesized [18–22]. Among them, a kind of flowerlike ceria, prepared by a hydrothermal method, is very attractive due to its open mesoporous structure, large specific surface area and nano-crystalline feature. It showed high reactivity as catalysts for ethanol reform and anode reactions in intermediate temperature solid oxide fuel cell (ITSOFC) using hydrocarbon as fuels [23–25]. Catalytic oxidation of CO is not only an important reaction for many applications, such as removing of exhaust gas and fuel cells,

* Corresponding author. Tel.: +86 10 82648067; fax: +86 10 82649046. E-mail address: [email protected] (H. Li). 1387-1811/$ - see front matter Ó 2008 Elsevier Inc. All rights reserved. doi:10.1016/j.micromeso.2008.12.015

but also a fundamental process involving surface oxygen and oxygen vacancy participation [26–29]. This process is also related to catalytic activity of ceria. It is desirable to prepare a ceria catalyst combining the feature of flowerlike microstructure and doping effect. However, it is not clear whether the addition of the doping components in the precursor solution will influence the formation and growth of flowerlike structure during hydrothermal reaction. Here we report that Y, La, Zr, Pr and Sn doped ceria with flowerlike structure can be prepared successfully by the similar hydrothermal method. The impacts of the doping on their physical properties and the catalytic reactivity on CO oxidation reaction have also been investigated. 2. Experimental 2.1. Synthesis of flowerlike materials Hydrate cerium (III) nitrate (AR, 99%), hydrate yttrium nitrate (AR, 99%), hydrate lanthanum nitrate (AR, 99%), hydrate zirconium nitrate (AR, 99%), praseodymium oxide (AR, 99.99%) and tin sulfate (AR, 99%), glucose, acrylamide and ammonia aqueous solution were all purchased from Beijing Chemical Reagents Company. In a typical process, metal salts were dissolved in de-ionized water in advance (praseodymium oxide was dissolved in nitric acid). Then, 10 ml of 0.5 M solution (the molar ratio of dopant cation Mn+ to Ce3+ is 1:9 in all doped samples), 0.01 mol glucose, 0.015 mol acrylamide and 70 ml de-ionized water were mixed in a Teflon autoclave with 100 ml capacity. The pH value of the solution was then adjusted to 10–12 by adding ammonia aqueous

427

G. Xiao et al. / Microporous and Mesoporous Materials 120 (2009) 426–431

solution dropwise. The transparent solution became a sticky gel. After stirring continually for 3 h, the gel turned brown. Then, the autoclave was sealed and kept in an oven for 24 h at 180 °C. The precipitate was collected from the suspension in the autoclave when it was cooled down to room temperature. After washing with de-ionized water and alcohol and drying at 80 °C, the flowerlike basic carbonate of doped ceria was obtained [23]. The flowerlike ceria based material was obtained by calcinating the as-prepared basic carbonate via two steps, firstly being calcinated at 600 °C for 6 h under Ar protection and then at 400 °C for another 4 h in the air [23]. They are named as CeYO, CeLaO, CeZrO, CePrO and CeSnO according to the dopants. 2.2. Characterization The phase and purity of the samples were examined on a Holland X’Pert Pro MPD X-ray diffractometer, equipped with a monochromatized Cu–Ka radiation (k = 1.5405 Å), at a step size of 0.017o. The morphology of the product was observed by a scanning electron microscope (SEM, XL30s-FEG, 10 kV). Gold coating was employed to avoid charge accumulating. The induced coupled plasma (ICP, Thermo Electron Corporation) analysis was adopted to test the chemical composition of the doped ceria. The nitrogen adsorption was measured using a Quantachrome Instruments NOVA2000e from 196 °C. The Raman spectra were recorded on a JY HR800 spectrometer at ambient temperature, with laser source of 532 nm and resolution of 1 cm1. The X-ray photoelectron spectroscopic (XPS) analysis was performed on a PHI Quantera SXM using an Al X-ray source and the binding energies were calibrated with the binding energy of the C 1s (284.8 eV) as reference. The flowerlike ceria materials were calcined at 800 °C for 10 h under reducing atmosphere (15% H2/Ar, 30 ml/min) and oxidative (O2, 30 ml/min) atmosphere, respectively in order to study the microstructure stability of the flowerlike doped ceria. The alternation of morphology, lattice constant and grain size after calcination were investigated by means of SEM and XRD. CO oxidation test was carried out on the sample with amount of 50 mg from 150 °C to 400 °C, under a stream (2% CO, 3% O2 and N2 95%) at a total flow rate of 50 ml/min. The outlet product was detected by a GC (7890a Agilent) equipped with a gas sampling valve, a switching valve, two packed columns of Porapak Q and ShinCarbon ST, and a thermal conductivity detector (TCD).

3. Results and discussion The morphologies of doped and undoped flowerlike ceria can be seen in Fig. 1. They all have a similar appearance of flowerlike particles assembled by wrinkled petals. The size of most particles ranges from 0.5 to 10 lm. The BET specific surface area and the BJH volume of all the samples are summarized in Table 1. The closed surface area and the BJH pore size distribution indicate their similarity in mesoporous structure. The chemical composition of these doped ceria is closed to nominal composition as listed in Table 2. Nearly all samples show the pure face centered cubic phase (Fm–3m (225) space group (JCPDS 43-1002)) except CeSnO as shown in Fig. 2. A very weak peak is found in the pattern of CeSnO which could be assigned to the phase of tin oxide species. This indicates that CeSnO is a mixture of oxides rather than a homogenous solid solution. The lattice constants and the crystalline sizes are evaluated from the XRD patterns as shown in Table 2. The crystalline sizes of all the samples are below 10 nm. Generally, the lattice constant should change accordingly to the Vegard’s law with the radius of doping cations [30,31]. This tendency is seen as the constant changes of CeYO and CeLaO but results of the other doped samples are quite complicated. It is reported that Ce3+ concentration in ceria increases when doping with Zr4+ [32] and Pr and Sn exist in mixed valence state [33,34]. In other words, lattice constant change in CeZrO, CePrO or CeSnO should be a combination of all these factors and is not according to the law mentioned above. The Raman spectra of all samples are shown in Fig. 3A. The ceria normally exhibits three bands at the frequencies around 272, 462 and 600 cm1, corresponding to the doubly degenerated TO mode,

Table 1 The pore parameter of the sample calculated form N2 sorption isotherm. Samples

BET surface area (m2 g1)

BJH pore size (nm)

Pore volumn (cm3 g1)

CeO2 CeLaO CeYO CeZrO CePrO CeSnO

166 155 128 100 140 144

3.9 3.9 3.9 3.8 3.8 3.8

0.15 0.36 0.25 0.17 0.17 0.23

Fig. 1. SEM images of: (a) CeO2, (b) CeLaO, (c) CeYO, (d) CeZrO, (e) CeSnO and (f) CePrO.

428

G. Xiao et al. / Microporous and Mesoporous Materials 120 (2009) 426–431

Table 2 Lattice constant, grain size, doping ratio and reference cation radius of the samples. Samples

Lattice constanta (nm)

Grain sizeb (nm)

Cation ratioc (At.)

Dopant cation (pm)

CeO2 CeLaO CeYO CeZrO CePrO CeSnO

0.5412 ± 0.0002 0.5441 ± 0.0003 0.5411 ± 0.0002 0.5410 ± 0.0002 0.5425 ± 0.0001 0.5414 ± 0.0001

8.0 ± 0.1 7.0 ± 0.1 8.8 ± 0.1 10.1 ± 0.1 6.0 ± 0.1 5.6 ± 0.1

– 10:90 8:92 8:92 – –

Ce4+(97), Ce3+(114) La3+(116) Y3+(102) Zr4+(84) Pr4+(96), Pr3+(113) Sn4+(81), Sn2+(127)

A B c

Calculated by Jade program. Estimated from the (1 1 1) peak of XRD data. ICP results.

Fig. 2. XRD patterns of: (a) CeO2, (b) CeLaO, (c) CeYO, (d) CeZrO, (e) CePrO and (f) CeSnO at a step of 0.017 degree, peak  assigns to the phase of tin oxide species.

Fig. 3. (A) Raman spectra of the flowerlike materials: (a) CeO2, (b) CeLaO, (c) CeYO, (d) CeZrO, (e) CeSnO and (f) CePrO. (B) Raman shift of the sample versus its lattice constant.

the triply degenerate Raman-active mode and the nondegenerate LO mode. The strong band at 462 cm1 is assigned to the F2g Raman active mode of fluorite structure which can be viewed as a symmetric breathing mode of the oxygen atoms around cerium

ions [13,35–37]. This band can be obviously observed in each spectrum and its wave number shifts as while as the dopant cation changes. This shifting also reflects the lattice distortion caused by doping and it consists to the results which we got from XRD, as seen in Fig. 3B. In Fig. 3A, an additional band ranged between 540 cm1 and 600 cm1 is observed in CeYO, CeLaO and CePrO respectively and the relative intensity is more significant in CePrO. It is attributed to oxygen vacancies in the ceria doped by cations with lower valence and the intensity difference of CePrO among them is due to the color change by Pr doping, which enhance optical adsorption [37]. This indicates that oxygen vacancy exist in the Re3+ doped samples. It has been reported that a pseudo cubic phase with a space group of P42/nmc exists in ceria doping with high content of ZrO2 (>20% mole ratio) [13,38]. At 10 mol% doping level, none of the bands originated from the tetragonal phase can be observed in the Raman spectrum of CeZrO. Fig. 4A shows the binding energy of Ce 3d core level of pristine flowerlike CeO2, CeYO, CeLaO and CeZrO. There are ten peaks with the assignment defined in the figure. The signals of Ce4+ are labeled as u, u00 and u000 of 3d3/2 and those of 3d5/2 are labeled as v, v0 0 and v000 . The Ce3+ signal exhibits four peaks of u0, u0 , v0 and v0 . As literature reported [39], the peak v0 and u0 are arising from the Ce 3d5/2 and Ce 3d3/2 ionizations of Ce3+. It can be seen that no significant difference can be seen in XPS spectra among CeO2, CeYO and CeLaO. The relative high intensity of v0 and u0 in CeZrO can be observed. It reveals that the surface ratio of Ce3+ to Ce4+ is enhanced by the incorporation of zirconia. This enhancement is also reported in previous researches [40]. The bind energy signals for Ce3d5/2 and Ce 3d3/2 of CePrO and CeSnO are same as CeO2. As shown in Fig. 4B, four peaks at 929.1, 933.4, 949.6 and 954.5 eV belonging to Pr 3d5/2 and Pr 3d3/2 can be seen. According to literature [36], these signals are assigned to Pr4+ (954.5 and 933.4 eV) and Pr3+ (949.6 and 929.1 eV), respectively. The signals from Sn 3d5/2 and Sn 3d3/2 are shown in Fig. 4C. The signal from Sn 3d5/2 shows an asymmetric peak at 486 eV. It was reported that the shoulder peak at 485.3 eV may be ascribed to Sn2+ species while the peak at 486.3 eV belongs to Sn4+ [41]. The existence of mixed valence state in the samples is also consistent with the changes of lattice constant mentioned above. It is known that the radius of Ce3+ is larger than that of Ce4+ and the conversion between them will cause lattice expansion and shrink. In addition, high migration of ions under high temperature may cause grain growth or sintering. For practical application, whether flowerlike ceria materials can maintain their microstructure at high temperature under different oxygen partial pressures needs investigation. The morphologies of pristine flowerlike CeO2, CeYO, CeLaO and CeZrO after calcination under H2/Ar and O2 atmosphere at 800 °C for 10 h are shown in Fig. 5. It seems that the appearances of CeYO, CeLaO and CeZrO are closed to original morphologies while CeO2 changes relative significantly. The value of BET surface area of CeO2 drops to 17 m2 g1 and 24 m2 g1 after being calcinated in H2/Ar and O2 atmosphere respectively. Its pore size distributions after different treatments are shown in Fig. 6. It is clearly to be seen that lots of pores with diameter below 5 nm disappeared and some larger pores formed after calcination. Additionally, diameter of the formed larger pores arranges above 8 nm in the sample calcinated in O2 and it is above 13 nm in the one calcinated in H2/Ar. This seems that the microstructure of CeO2 is more changeable in the reducing atmosphere. The microstructure change is also indicated further by XRD investigation. The crystalline sizes evaluated from the XRD patterns of the calcinated samples are shown in Table 3. The doped flowerlike ceria restrain from grain growth relatively, especially for CeLaO. Grain sizes of all the samples are larger after calcinating in reduced atmosphere

G. Xiao et al. / Microporous and Mesoporous Materials 120 (2009) 426–431

429

Fig. 4. (A) Ce 3d XPS spectra of the sample (a) CeO2, (b) CeLaO, (c) CeYO and (d) CeZrO. (B) Pr 3d XPS spectrum of the sample (e) CePrO. C: Sn 3d spectrum of the sample (f) CeSnO.

Fig. 5. Morphologies of the flowerlike materials: (a) CeO2, (b) CeLaO, (c) CeYO and (d) CeZrO after different treatments: (1) original, (2) calcinated for 10 h under reducing atmosphere, (3) calcinated for 10 h under oxidizing atmosphere.

than in oxidizing atmosphere. These results indicate that the microstructure of flowerlike ceria materials are not very stable at 800 °C under reducing and oxidizing atmosphere although doping can improve its stability to a certain level. However, they show high stability at intermediate temperature (550 °C) even under hydrothermal condition [23]. The CO conversion curves of pristine flowerlike CeO2, CeYO, CeLaO, CeZrO, CePrO and CeSnO are shown in Fig. 7. As a comparison, commercial pristine CeO2 with surface area of 4 m2 g1 is also included in the test. It can be seen that the flowerlike mesoporous

materials show much higher catalytic activity than normal ceria. In a closed view, light-off temperatures T50 (temperature at 50% CO conversion) of the doped and undoped flowerlike samples illustrate strongly dependence on doping. It was reported that the concentration of releasable lattice oxygen on ceria surface may be the key reactant for oxidation of the adsorbed CO [42–44]. Reversible redox process of Ce4+/Ce3+ in the surface lattice may be an important rate limiting step of this chemical reaction. It is known that the oxygen diffusion and OSC of ceria can be improved by doping with La, Zr [8,36]. However,

430

G. Xiao et al. / Microporous and Mesoporous Materials 120 (2009) 426–431

cious metals or transitional metals on doped flowerlike ceria will be investigated further in future. 4. Conclusions

Fig. 6. Pore size distribution of the flowerlike CeO2: (a) original sample, (b) sample after calcinated in H2/Ar atmosphere for 10 h and, (c) sample after calcinated in O2 atmosphere for 10 h.

Table 3 Grain size (nm) of the sample calcinated under reducing and oxidizing atmospheres, evaluated from the (111) peak of XRD data. Atmosphere

CeO2

CeYO

CeZrO

CeLaO

H2/Ar O2

58.7 ± 0.8 41.9 ± 0.6

39.6 ± 0.7 36.3 ± 0.5

38.6 ± 0.7 23.0 ± 0.4

34.0 ± 0.5 21.5 ± 0.3

Fig. 7. Conversion profile of carbon monoxide versus temperature.

CeYO, CeLaO and CeZrO exhibit negative doping impact on the activity of CO oxidation. This may be ascribed to that the doping cations with stable valence state partially suppressed the conversion of the redox Ce4+/Ce3+ couple and further brought down the dynamic OSC of ceria. Similar negative effect has also been observed in Gd doped ceria previously [45]. On the contrary, CePrO and CeSnO exhibit higher catalytic activity than that of pristine flowerlike CeO2. This improvement is due to the feature of variable valence states of Pr and Sn in ceria [10,34]. It has to be mentioned that the loading of Au and Cu is very effective to promote CO oxidation [46–49]. Direct loading of pre-

In summary, mesoporous flowerlike Y, La, Zr, Pr and Sn doped ceria have been prepared through the hydrothermal method. The flowerlike structure of ceria is maintained after doping at 10 mol% doping level. They all have large surface area (above 100 m2 g1) and similar pore size parameters. Their physical and chemical properties are tuned by different dopants. Grain sizes are increased from less than 10 nm to above 20 nm for all flowerlike materials after calcinating under reducing and oxidizing atmosphere. The hindrance of grain growth is noticed in all the doped flowerlike ceria. The catalytic activity of the flowerlike ceria in CO conversion reaction is improved greatly by the mesoporous structure. It can be further enhanced by doping with Pr and Sn due to variable valence property, but can be decreased by doping with Y, La and Zr. Acknowledgments Financial support from the NSFC key project (50730004) of China is appreciated. References [1] M.A. Centeno, K. Hadjiivanov, T. Venkov, H. Klimev, J.A. Odriozola, J. Mol. Catal. A 252 (2006) 142. [2] N. Laosiripojana, S. Assabumrungrat, Appl. Catal. B 66 (2006) 29. [3] J.A. Rodriguez, P. Liu, J. Hrbek, J. Evans, M. Pérez, Angew. Chem. Int. Ed. 46 (2007) 1329. [4] O. Costa-Nunes, R.J. Gorte, J.M. Vohs, J. Power Sources 141 (2005) 241. [5] B.C.H. Steele, A. Heinzel, Nature 414 (2001) 345. [6] F. Zhang, P. Wang, J. Koberstein, S. Khalid, S.W. Chan, Surf. Sci. 563 (2004) 74. [7] S. Deshpande, S. Patil, S.V. Kuchibhatla, S. Seal, Appl. Phys. Lett. 87 (2005) 133113. [8] B.M. Reddy, P. Bharali, P. Saikia, A. Khan, S. Loridant, M. Muhler, W. Grulnert, J. Phys. Chem. C 111 (2007) 1878. [9] T.Y. Zhang, S.P. Wang, Y. Yu, Y. Su, X.Z. Guo, S.R. Wang, S.M. Zhang, S.H. Wu, Catal. Commun. 9 (2008) 1259. [10] Z.X. Song, W. Liu, H. Nishiguchi, A. Takami, K. Nagaoka, Y. Takita, Appl. Catal. A 329 (2007) 86. [11] Y.C. Hu, J. Am. Ceram. Soc. 89 (2006) 2949. [12] A. Martínez-Arias, M. Fernández-García, V. Ballesteros, L.N. Salamanca, J.C. Conesa, C. Otero, J. Soria, Langmuir 15 (1999) 4796. [13] B.M. Reddy, A. Khan, P. Lakshmanan, J. Phys. Chem. B 109 (2005) 3355. [14] S. Omar, E.D. Wachsman, J.C. Nino, Appl. Phys. Lett. 91 (2007) 144106. [15] M. Mogensena, N.M. Sammes, G.A. Tompsett, Solid State Ionics 129 (2000) 63. [16] S. Patil, S. Seal, Y. Guo, A. Schulte, J. Norwood, Appl. Phys. Lett. 88 (2006) 243110. [17] T. Masui, T. Ozaki, K. Machida, G. Adachi, J. Alloys Compd. 303 (2000) 49. [18] C.W. Sun, H. Li, H.R. Zhang, Z.X. Wang, L.Q. Chen, Nanotechnology 16 (2005) 1454. [19] F. Zhang, Q. Jin, S.W. Chan, J. Appl. Phys. 95 (2004) 4319. [20] D.C. Sayle, X.D. Feng, Y. Ding, Z.L. Wang, T.X.T. Sayle, J. Am. Chem. Soc. 129 (2007) 7924. [21] J. Roggenbuck, H. Schäfer, T. Tsoncheva, C. Minchev, J. Hanss, M. Tiemann, Micropor. Mesopor. Mater. 101 (2007) 335. [22] C.Y. Ni, X.Z. Li, Z.G. Chen, H.H. Li, X.Q. Jia, I. Shah, J.Q. Xiao, Micropor. Mesopor. Mater. 115 (2008) 247. [23] C.W. Sun, J. Sun, G.L. Xiao, H.R. Zhang, X.P. Qiu, H. Li, L.Q. Chen, J. Phys. Chem. B 110 (2006) 13445. [24] C.W. Sun, Z. Xie, C.R. Xia, H. Li, L.Q. Chen, Electrochem. Commun. 8 (2006) 833. [25] L.S. Zhong, J.S. Hu, A.M. Cao, Q. Liu, W.G. Song, L.J. Wan, Chem. Mater. 19 (2007) 1648. [26] C.Y. Shiau, M.W. Ma, C.S. Chuang, Appl. Catal. A 301 (2006) 89. [27] Z.K. Zhao, M.M. Yung, U.S. Ozkan, Catal. Commun. 9 (2008) 1465. [28] F.Y. Wang, S.F. Cheng, B.Z. Wan, Catal. Commun. 9 (2008) 1595. [29] M.S. Chen, Y. Cai, Z. Yan, K.K. Gath, S. Axnanda, D.W. Goodman, Surf. Sci. 601 (2007) 5326. [30] A.R. Denton, N.W. Ashcroft, Phys. Rev. A 43 (1991) 3161. [31] S. Omar, E.D. Wachsman, J.C. Nino, Solid State Ionics 178 (2008) 1890. [32] F. Zhang, C.H. Chen, J.M. Raitano, J.C. Hanson, W.A. Caliebe, S. Khalid, S.W. Chan, J. Appl. Phys. 99 (2006) 084313. [33] A. Boulahouache, G. Kons, H.G. Lintz, P. Schulz, Appl. Catal. A 91 (1992) 115. [34] R. Sasikala, N.M. Gupta, S.K. Kulshreshtha, Catal. Lett. 71 (2001) 69. [35] X.M. Lin, L.P. Li, G.S. Li, W.H. Su, Mater. Chem. Phys. 69 (2001) 236.

G. Xiao et al. / Microporous and Mesoporous Materials 120 (2009) 426–431 [36] Z.Y. Pu, J.Q. Lu, M.F. Luo, Y.L. Xie, J. Phys. Chem. C 111 (2007) 18695. [37] J.R. McBride, K.C. Hass, B.D. Poindexter, W.H. Weber, J. Appl. Phys. 76 (1994) 2435. [38] R. Si, Y.W. Zhang, L.M. Wang, S.J. Li, B.X. Lin, W.S. Chu, Z.Y. Wu, C.H. Yan, J. Phys. Chem. C 111 (2007) 787. [39] C. Larese, M.L. Granados, R. Mariscal, J.L.G. Fierro, P.S. Lambrou, A.M. Efstathiou, Appl. Catal. B 59 (2005) 13. [40] F. Zhang, S.W. Chan, J.E. Spanier, E. Apak, Q. Jin, R.D. Robinson, I.P. Herman, Appl. Phys. Lett. 80 (2002) 127. [41] V. Matolín, M. Cabala, V. Cháb, I. Matolínová, K.C. Prince, M. Škoda, F. Šutara, T. Skálab, K. Veltruská, Surf. Interface Anal. 40 (2008) 225.

431

[42] K. Ramesh, L.W. Chen, F.X. Chen, Y. Liu, Z. Wang, Y.F. Han, Catal. Today 131 (2008) 477. [43] R. Cracium, B. Nentwick, K. Hadjiivanov, H. Knözinger, Appl. Catal. A 243 (2003) 67. [44] L.M. Molina, B. Hammer, Phys. Rev. B. 69 (2004) 155424. [45] U. Hennings, R. Reimert, Appl. Catal. A 325 (2007) 41. [46] C.W. Sun, H. Li, L.Q. Chen, J. Phys. Chem. Solids 68 (2007) 1785. [47] J.A. Rodriguez, S. Ma, P. Liu, J. Hrbek, J. Evans, M. Perez, Science 318 (2007) 1757. [48] Z. Ma, S. Brown, S.H. Overbury, S. Dai, Appl. Catal. A 327 (2007) 226. [49] N.F.P. Ribeiro, M.M.V.M. Souza, M. Schmal, J. Power Sources 179 (2008) 329.