Enhanced catalytic degradation of AO7 in the CeO2–H2O2 system with Fe3+ doping

Applied Catalysis B: Environmental 101 (2010) 160–168

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Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb

Enhanced catalytic degradation of AO7 in the CeO2 –H2 O2 system with Fe3+ doping Wandong Cai, Feng Chen ∗ , Xingxing Shen, Lijing Chen, Jinlong Zhang Key Laboratory for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, China

a r t i c l e

i n f o

Article history: Received 27 July 2010 Received in revised form 26 September 2010 Accepted 28 September 2010 Available online 7 October 2010 Keywords: CeO2 Fe3+ doping Ce3+ Peroxide species Degradation

a b s t r a c t Degradation of Acid Orange 7 (AO7) was employed to evaluate the catalytic activity of the Fe3+ doped CeO2 in the presence of H2 O2 . Fe3+ doping in the ceria improved the degradation of AO7 in the dark as well as under the visible light irradiation and showed the highest catalytic activity at Fe/Ce = 1/100 (FC100). XPS, Raman and EPR studies showed that Fe3+ doping in the CeO2 greatly affected the concentration of Ce3+ . FC100 was found to have the highest Ce3+ concentration. Raman and EPR studies confirmed the presence of surface peroxide-like species, which was derived from the complex of the surface Ce3+ and H2 O2 . DMPO spin trapping EPR studies indicated that peroxide-like species played a key role during the dark-reaction, while • OH radical induced the degradation of AO7 under visible irradiation. The amounts of both active species were remarkably increased in the FC100/H2 O2 system than those in the CeO2 /H2 O2 system. © 2010 Elsevier B.V. All rights reserved.

1. Introduction Cerium oxide (ceria, CeO2 ) is an important rare earth metal oxide which has a wide range of applications in various fields including automotive three-way catalysts, water-gas shift reactions, solid oxide fuel cells, UV-blocking filters, oxygen sensors and protection of biological tissues [1–7]. Ceria has attracted intense interest because of its unique structure and properties: oxygen vacancy defects can be rapidly formed and eliminated on the surface of ceria, giving it the redox cycle of Ce4+ /Ce3+ and the high oxygen storage capacity (OSC) [8]. Ceria-based materials (ceria doped with other ions) always exhibit better catalytic performance as compared to ceria. Doping ceria with trivalent ions (Pr3+ and Tb3+ ) was reported to lower the activation energy of oxygen release [9], while doping ceria with smaller homovalent Zr4+ could obviously enhance the thermal stability and OSC by decreasing the Ce4+ /Ce3+ reduction energy, preserving oxygen defects and retarding OSC degradation at high temperatures [10,11]. Fe3+ doped CeO2 has also received attention because of scientific interest in understanding the effect of doping with undersized lower valence ions to the practical applications of CeO2 [12]. Consequently, Fe3+ doped CeO2 has been synthesized by various techniques, including hydrothermal, co-precipitation and template assisted methods [12–15].

∗ Corresponding author. Tel.: +86 21 6425 2062; fax: +86 21 6425 2062. E-mail addresses: [email protected] (W. Cai), [email protected] (F. Chen), [email protected] (X. Shen), [email protected] (L. Chen), [email protected] (J. Zhang). 0926-3373/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2010.09.031

Recently, Fenton-like reaction catalyzed by cerium salt in the presence of H2 O2 has been reported [16]. Specifically, it was proposed that the reaction of cerium and H2 O2 underwent the cycle of Ce4+ /Ce3+ with the generation of superoxide and hydroxyl radical. It is well-known that the catalytic activity of CeO2 also relies closely on the redox cycle of Ce4+ /Ce3+ [1,2]. There have been many reports focused on promoting the redox cycle of the cerium ion, which enhances the catalytic oxidative activity of CeO2 [10,11,17]. Conversely, the catalytic oxidative activity of the CeO2 would be greatly reduced if the redox cycle of the cerium ion in CeO2 was blocked, e.g., the catalytic activity of CeO2 towards the superoxide species was inhibited in the presence of H2 O2 with high concentration (100 mM) [18]. Raman results in our previous work [19] indicated that peroxide-like species (O2 2− ) appeared on the surface of CeO2 after H2 O2 treatment. The as-formed peroxo-like species has a brown color and a relative chemical stability in the absence of the organic contaminants. Further studies showed that the degradation of organics depended strictly on their adsorption on the surface of CeO2 , which suggested a localized surface reaction more than a superoxide/hydroxyl radical-attack pathway for the degradation of organics in the CeO2 –H2 O2 system [19]. It has been reported that impurity metallic doping into the CeO2 can affect the catalytic oxidation activity of CeO2 towards the molecular O2 at high temperature [2,11]. However, the effect of the impurity metallic doping into the CeO2 towards the catalytic oxidation reaction with the common oxidants such as H2 O2 at the ambient temperature has not been concerned yet. Therefore, Fe3+ was selected in this work to be doped into the CeO2 lattice to adjust the concentration of the oxygen vacancies, which might improve the catalytic activity of CeO2 towards the organics degra-

W. Cai et al. / Applied Catalysis B: Environmental 101 (2010) 160–168 Table 1 The theoretical and measured concentration of Fe3+ in Fe3+ doped CeO2 catalysts. Fe% (w/w), theoretical value

Fe% (w/w), measured value

0 1/200 1/100 1/20 1/10 1/1

0.0 0.16 0.32 1.6 3.2 24.6

0.0 0.13 0.25 1.6 3.1 22.0

dation with H2 O2 . Fe3+ doped CeO2 with different doping amounts was prepared by hydrothermal method. The catalytic activity was evaluated by degradation of Acid Orange 7 (AO7). Raman, XPS and EPR spectra were employed to investigate the role of Fe3+ on the catalytic degradation reaction of AO7 catalyzed by Fe3+ doped CeO2 in the presence of H2 O2 . 2. Experimental section 2.1. Materials Ce(NO3 )3 ·6H2 O, Fe(NO3 )3 ·9H2 O, NaOH and H2 O2 (30%) were obtained from Sinopharm Chemical Reagent. AO7 was purchased from Acros. The radical spin trap 5,5-dimethyl-1-pyrroline N-oxide (DMPO) (purity >97%) was obtained from Tokyo Chemical Industry. All of the chemicals were of analytic grade and were used without further purification. Double distilled water was used throughout the experiments. 2.2. Preparation of the catalysts A hydrothermal procedure was employed to synthesize cerium oxide and iron doped cerium oxides [14,17]. 7.81 g Ce(NO3 )3 ·6H2 O and desired amount of Fe(NO3 )3 ·9H2 O (molar ratio of Fe/Ce = 1/200, 1/100, 1/20, 1/10, 1/1) were dissolved in 60 mL water, and stirred at 30 ◦ C for 1 h. An aqueous NaOH solution (5.0 M) was added drop wise to the mixture under continuous stirring until pH = 13.0. After 1 h stirring, the purple precipitates were transferred to a Teflonlined stainless steel autoclave and held at 180 ◦ C for 24 h. After cooling to room temperature, the precipitates were filtered and washed several times with water and then dried under infrared lamps over night. Finally, the catalysts were obtained after ground and calcined under ambient air at 500 ◦ C for 4 h. The as-prepared Fe3+ doped CeO2 composites were labeled as FC200 (Fe/Ce = 1/200), FC100 (Fe/Ce = 1/100), FC20 (Fe/Ce = 1/20), FC10 (Fe/Ce = 1/10), FC1 (Fe/Ce = 1/1). Similarly, pure CeO2 and Fe2 O3 were prepared following the same procedure as control. The practical doping amounts of Fe3+ in CeO2 were detected by ICP-AES (Varian 710-ES). Table 1 shows the practical doping concentration of Fe3+ in the CeO2 , which is slightly less than the corresponding nominal doping concentration. 2.3. Characterization X-ray diffraction (XRD) analysis of the catalysts was carried out with a Rigaku D/Max 2550 VB/PC apparatus using Cu K␣ radiation ( = 0.15406 nm) and a graphite monochromator at room temperature, operated at 40 kV and 100 mA. Diffraction patterns were recorded in the angular range of 20–80◦ . The BET specific surface area (SBET ) and BJH pore size distribution were determined by nitrogen adsorption at 77.3 K (Micromeritics ASAP 2020). Samples were degassed at 473 K for 5 h prior to the measurement. The morphology of the samples was observed by high-resolution transmission electron microscopy (HRTEM, JEOL JEM 2100F), and the electron beam accelerating voltage was 200 kV. Raman measurements

CeO2 -Fe2O3

(111)

CeO2 FC200

Intensity (a.u.)

CeO2 FC200 FC100 FC20 FC10 FC1

Fe/Ce in molar ratio

161

FC100 FC20 FC10 FC1 (110)

20

30

40

-Fe2O3 50

60

70

80

2 (degree) Fig. 1. XRD patterns of CeO2 , Fe2 O3 and Fe3+ doped CeO2 catalysts.

were performed at room temperature using a Via+ Reflex Raman spectrometer with the excitation light of 514 nm. The instrument employed for X-ray photoelectron spectroscopy (XPS) studies was a Perkin-Elmer PHI 5000C ESCA system with Mg K␣ radiation (photon energy 1253.6 eV) operated at 300 W, and calibrated internally by carbon deposit C (1s) binding energy (BE) at 284.6 eV. Electron paramagnetic resonance (EPR) measurements for the catalyst powders were recorded at room temperature on a Bruker EMX8/2.7 by accumulating three scans with the microwave frequency of 9.85 GHz and power of 6.35 mW. Both untreated and H2 O2 pretreated catalysts for measuring were placed into a glass capillary in pasty form. DMPO trapping measurements were detected at room temperature by adding DMPO (50 mM) into the reaction suspension and recorded by one scan. 2.4. Measurements of catalytic activity The catalytic activity was evaluated by measuring the decompositions of the AO7 (35 mg/L) aqueous solution with the addition of H2 O2 under visible irradiation or in dark. In a typical visible light irradiation degradation experiment, a total of 0.025 g of catalyst powder was added to 50 mL of the above AO7 solution in a quartz tube with vigorous agitation. The suspensions were stirred in the dark for 1 h to ensure the establishment of an adsorption/desorption equilibrium, then H2 O2 solution was added to reach a concentration of 1.0 mM. After 2 min stirring, the concentration of AO7 was recorded and set as C0 . A 1000-W tungsten halogen lamp equipped with an UV cut-off filter ( > 420 nm) was used as a visible light source. The lamp was cooled with flowing water in a quartz cylindrical jacket around the lamp, and ambient temperature was maintained during the vis-degradation reaction. At the given time intervals, samples were taken from the mixture and immediately centrifuged. The supernatant AO7 solution was withdrawn and analyzed by recording variations of the absorbance at 484 nm with a UV–vis spectrophotometer (Varian Cary 100). Dark-degradation experiment was carried out following the same procedure as the vis-degradation in the absence of visible light irradiation. 3. Results and discussion 3.1. Lattice structure and the morphology of the Fe3+ doped CeO2 catalysts The XRD patterns of the different catalysts are shown in Fig. 1. The pure CeO2 displays a typical XRD pattern of cubic CeO2 (fluorite

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Table 2 Physical parameters of the CeO2 , Fe2 O3 and Fe3+ doped CeO2 catalysts. Sample

2(CeO2 )/2(Fe2 O3 ) (◦ )

CeO2 FC200 FC100 FC20 FC10 FC1 Fe2 O3

28.56/– 28.56/– 28.58/– 28.62/– 28.74/– 28.86/35.64 –/35.62

a b c

ds a (CeO2 ) (nm) 10.6 10.9 11.4 12.4 13.4 9.9 –

˚ ab (A) 5.409 5.409 5.405 5.398 5.376 5.354

SBET c (m2 /g) 67 75 75 119 121 71 11

Pore volumec (cm3 /g)

Pore diameterc (nm)

0.11 0.13 0.12 0.26 0.31 0.19 0.04

4.9 5.5 4.7 5.8 8.1 85.6 31.6

ds , average crystallite size of CeO2 (1 1 1) according to the Scherrer equation. a, lattice parameter of CeO2 . SBET (BET specific surface area), pore volume and pore diameter were calculated from the desorption isotherm.

structure, JCPDS 34-0349), while the XRD pattern of Fe2 O3 is consistent with that of hematite, ␣-Fe2 O3 (hexagonal, JCPDS 33-0664). The pattern of FC1 sample demonstrates the presence of both ␣Fe2 O3 and CeO2 crystals, while only the characteristic diffraction angles of cubic CeO2 are observed in the patterns of other composites (FC200, FC100, FC20, and FC10). A slight shift of the (1 1 1) peak of the CeO2 is observed for doped CeO2 in Fig. 1. Fe3+ ions were doped into the lattice of ceria to form a solid solution even at a Fe/Ce ratio of 1/10. ␣-Fe2 O3 phase is observed in FC1 as the doping amount of Fe3+ exceeded a critical value [15]. The lattice structure of CeO2 is very flexible (a large fraction of the positions available for cations are unoccupied) and permits significant Ce4+ ion substitution by various metallic cations of different sizes. For instance, it is well-known that CeO2 can hold up to ca. 80–90% of metal cations as Zr4+ , and still keeps the fluorite structure with distortions [1,15]. As for Fe3+ , the XRD signals ascribed to ␣-Fe2 O3 start to appear only when the Fe% in Fe2 O3 /CeO2 composites is beyond 30% [12].

Physical parameters of the catalysts are calculated from the XRD patterns and listed in Table 2. The average crystallite size is estimated using the Scherrer equation base on the (1 1 1) reflection of CeO2 and the (1 1 0) reflection of ␣-Fe2 O3 , respectively. Lattice parameter (a) is calculated by the following lattice parameter formula Eq. (1). a=d



h2 + k2 + l2

(1)

where a refers to the CeO2 FCC (face-centered-cubic) lattice parameter, and h, k, l are the crystalline face indexes while d is the interplanar spacing. The diffraction angle (2) of CeO2 (1 1 1) is shifted to higher values as the doping amount of Fe3+ increased in the solid solution, which means the lattice parameter decreased with the increasing doping amount of Fe3+ in the ceria. Since the size of Fe3+ cation (0.064 nm) is smaller than that of Ce4+ cation (0.101 nm), it is feasible for Fe3+ cation to occupy either network

Fig. 2. TEM images of (a) CeO2 , (b) FC100, (c) FC10, and (d) SAED pattern of CeO2 .

W. Cai et al. / Applied Catalysis B: Environmental 101 (2010) 160–168

Adsorption rate

120

FC200 CeO2

FC1

80

0.2 40

0.5

1

5

10

50

0.6

d b

0.4

c

0.2 0.0

0 0

f e a

Fe2O3 0.0

g

0.8

2

Adsorption rate

0.4

FC10

BET Area (m /g)

FC100 FC20

A

1.0

160

BET Area

C/C0

0.6

163

0

100

2

4

Fig. 3. BET specific surface area of various catalysts and their adsorption towards AO7.

3.2. Catalytic activities of the Fe3+ doped CeO2 catalysts towards the H2 O2 for the degradation of AO7 It is well-known that Fe2+ /Fe3+ ions in aqueous solution catalyze the breakdown of hydrogen peroxide along with the generation of hydroxyl radical, which leads to the oxidation of organics. Recently, it was further extended to the heterogeneous ␣-Fe2 O3 /H2 O2 system for the degradation of organics such as AO7 [22]. Consequently, the catalytic activity of Fe3+ doped CeO2 catalysts with H2 O2 was determined by the degradation of AO7 aqueous solution. 3.2.1. Adsorption capacity It was suggested that the degradation of organics in ceria/H2 O2 system was tightly related to the adsorption of organics on the surface of CeO2 [19,23]. Hence, the adsorption of AO7 on the various kinds of catalysts is presented in Fig. 3. The adsorption of AO7 on ␣-Fe2 O3 is very weak as compared to CeO2 . Besides the larger surface area of CeO2 than that of Fe2 O3 , the stronger adsorption of AO7 should be due to a chelation interaction between the electron-rich groups (sulfonate group, SO3 − ) of AO7 and empty f orbital of cerium ion [23]. The adsorption capacity of Fe3+ doped CeO2 towards AO7 is variable according to different doping amounts of Fe3+ and has an optimal Fe/Ce ratio of 1/100. Considering that the FC10 displays the maximum specific surface area among the catalysts, it seems that surface states other than the specific surface area of CeO2 should be responsible for the maximum adsorption capacity of FC100. It was reported that the impurity metallic doping into ceria enhanced the concentration of oxygen vacancies [2,12]; hence, the increased AO7 adsorption might be attributed to the increase of oxygen vacancies

10

0.6

B

0.3

Degradation rate 2

Degradation rate

Degradation rate per m

FC100 0.4

0.2 CeO2 FC200

FC20

0.2

0.1 FC10

FC1 Fe2O3

0.0

2

sites or interstitial sites in the fluorite lattice [1,15,20]. As a result, lattice shrink of CeO2 occurred. Meanwhile, the crystallite size of the cubic CeO2 grows with the increasing doping concentration of Fe3+ . Moreover, BET specific surface area increases nearly one-fold from the pure CeO2 to FC10. It seems that the Fe3+ doping in the ceria stabilizes the solid solutions against thermal sintering (also see the pore volume). Similar effects of some other trivalent cations such as Al3+ , La3+ , Nd3+ , and Y3+ doping in the ceria have ever been reported [2]. It is suggested that the presence of trivalent cations on the surface of CeO2 inhibits the sintering rate limiting step, that is, the surface diffusion of cerium defects [15,21]. TEM images in Fig. 2a–c show that the particle sizes of the CeO2 and Fe3+ -doped CeO2 are about 10 nm, which are close to the values calculated from XRD results. The selected area electron diffraction (SAED) pattern (Fig. 2d) of prepared CeO2 displays typical polycrystalline rings.

8

Degradation rate per m

Ce4+

6

Time (h)

Fe/Ce (mol%)

0.0 0

0.5

1

5

10

50

100

Fe/Ce (mol%) Fig. 4. (A) Degradation of AO7 aqueous solution, (B) degradation rate of AO7 after 2 h, and degradation rate per m2 of AO7 after 2 h with various catalyst/H2 O2 systems in the dark, (a) CeO2 ; (b) FC200; (c) FC100; (d) FC20; (e) FC10; (f) FC1; and (g) ␣-Fe2 O3 .

from the Fe3+ doping, which promoted the chelation interaction between the sulfonate group and cerium atom [23]. 3.2.2. Dark reaction The heterogeneous Fenton activity of ␣-Fe2 O3 as shown in Fig. 4A is relatively low and confirmed again in this work. As compared with the ␣-Fe2 O3 , the CeO2 exhibited a significant higher catalytic activity for H2 O2 to degrade the AO7. Fe3+ doping obviously changed the reactivity of CeO2 . FC100 presented the highest catalytic activity, either in degradation rate or in degradation rate per m2 (Fig. 4B). The enhanced catalytic activity of CeO2 was not because of tiny Fe2 O3 clusters on the surface of composites. As we know, individual Fe2 O3 clusters did not form in low Fe/Ce ratios; and even they were formed in FC1 and pure ␣-Fe2 O3 , the activities of the corresponding catalysts were particularly low. As a result, chemical factors other than special surface area play an important role in promoting the activity of Fe3+ doped CeO2 . 3.2.3. Visible reaction Fig. 5 presents the degradation of AO7 with CeO2 , ␣-Fe2 O3 and Fe3+ doped CeO2 in the presence of H2 O2 under visible irradiation. The doping of Fe3+ favors the catalytic activity of CeO2 at low level but retards the degradation of AO7 at high level with optimal Fe/Ce ratio of 1/100, similarly as in the dark reaction. The above results confirm that the catalytic activity of the catalyst towards the degradation of AO7 with H2 O2 , either in the dark or under visible irradiation, depends on its adsorption capacity towards AO7 rather than its specific surface area.

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A

1.0 0.8

Ce4d

f

0.6

e a 0.4

b d c

0.2

Ce MINN C(1s)

Ce3d

Fe2p

O(1s)

CeO2

Intensity (a.u.)

C/C0

A

h g

FC200 FC100 FC20 FC10 OKLL-Mg

0.0 0

20

40

60

80

100

0

120

200

Time (min)

v

0.8 FC100 CeO2

600

800

1000

Ce4+: v, v'', v''', u, u'',u''' Ce3+: v0, v', u0, u'

Actual data Fitted Line

B

B

Degradation rate

400

Binding Energy (eV)

v'''

V: 3d5/2

v''

FC20

U: 3d3/2

u

FC200

u'''

u''

FC10 0.4

880

0.0 0

0.5

1

5

u0

v'

v0

10

890

u'

900

910

920

Binding Energy (eV)

Fe/Ce (mol%) 10

C

3.3.1. XPS spectroscopy measurement XPS spectra of the CeO2 and Fe3+ doped CeO2 are presented in Fig. 6A. It shows that no obvious signals of Fe appeared (Fe2 O3 2p3/2 : 711 eV), even on the surface of FC10. This result is in good agreement with the XRD outcome. Because of the flexible lattice structure of CeO2 , Ce4+ can easily be substituted by large amount of metallic cations such as Fe3+ with a slight lattice distortion, which results in the formation of solid solutions [1,12,15,24]. Ce3d fine XPS spectra of the CeO2 and Fe3+ doped CeO2 show that Fe3+ doping results in some changes in the chemical state of cerium ions. The deconvolution of the Ce3d XPS peaks and the corresponding meaning of each contribution were according to the literature method established by Burroughs et al. [25], in which vn and un refer to 3d5/2 and 3d3/2 spin–orbit component of cerium ion, respectively. The Ce3d fine XPS spectrum can thus be decon-

3+

The degradation of AO7 suggests that some active oxidative species are formed in the CeO2 –H2 O2 system. Considering the degradation reaction kinetics depends closely on the adsorption of AO7, the key step of the whole degradation process should be related to the surface of CeO2 . Hence, several observations towards the surface sites of CeO2 and Fe3+ doped CeO2 were carried out.

Ce

3.3. Surface sites of the catalysts and their interaction with H2 O2

3+

4+

/ (Ce + Ce ) (%)

Fig. 5. (A) Degradation of AO7 aqueous solution with various catalyst/H2 O2 systems under visible light irradiation, and (B) the corresponding degradation rate of AO7 after 20 min, (a) CeO2 ; (b) FC200; (c) FC100; (d) FC20; (e) FC10; (f) FC1; (g) ␣-Fe2 O3 ; and (h) without catalyst.

8

CeO2 6.75%

FC200 7.11%

FC100 8.13%

FC20 7.53% FC10 5.58%

6

4

2

0 0

0.5

1

5

10

Fe/Ce (mol%) Fig. 6. (A) Survey XPS spectra for various catalysts, (B) fine XPS spectrum of Ce3d for FC100, and (C) the relative concentration of Ce3+ calculated by XPS spectra.

voluted [26,27] and each deconvoluted peaks refer to Ce3+ {two pairs of doublets: (v0 , u0 ), (v , u )} [28,29] and Ce4+ {three pairs of spin–orbit doublets: (v, u), (v , u ), (v , u )} [29]. The XPS spectrum of Ce3d for FC100 is shown in Fig. 6B with its corresponding deconvoluted peaks. The peaks at v0 , v , u0 , u (880.4, 885.5, 898.8, 903.7 ± 0.7 eV) represent the presence of Ce3+ , while characteristic peaks of Ce4+ present at v, v , v , u, u , u (882.7, 888.96, 898.2, 901.3, 907, 916.7 ± 0.7 eV). The semiquantitation of Ce3+ in the catalysts can be obtained by calculating the relative integrated areas under the

W. Cai et al. / Applied Catalysis B: Environmental 101 (2010) 160–168

165

Scheme 1. Different kinds of oxygen vacancies in the Fe3+ doped CeO2 and their corresponding surface peroxide-like species in the catalytic oxidation of organics.

The evolution of the Ce3+ concentration corresponds well with the trend of the catalytic activity and the adsorption capability of catalysts, which suggests that the catalytic reaction of the CeO2 –H2 O2 system is mostly centered on the surface Ce3+ sites.

curve of each deconvoluted peaks, as shown in Eq. (2) [30]: [Ce3+ ] =

Av0 + Av + Au0 + Au Av0 + Av + Au0 + Au + Av + Av + Av + Au + Au + Au (2)

Ai is the integrated area of peak “i”. The calculated relative concentrations of Ce3+ are illustrated in Fig. 6C. The calculated [Ce3+ ] values of about 7% in the catalysts are in good agreement with the reported XPS analysis in literatures [31,32]. The relative concentration of Ce3+ is initially increased with the increase of Fe3+ doping amount, and decreased at higher Fe3+ doping amount, which presents a maximum Ce3+ concentration in FC100 catalyst at Fe3+ /Ce4+ ratio of 1/100. Two mechanisms have been suggested about the concentration of oxygen vacancies for the impurity metallic doping in the CeO2 , the vacancy compensation [9] and interstitial compensation mechanism [1,15]. The increased concentration of oxygen vacancies should be ascribed to the vacancy compensation mechanism [9]. As shown in Scheme 1, besides the intrinsic oxygen vacancy in the CeO2 , there are two additional kinds of oxygen vacancies: mode 1, substitution of one Ce4+ cation by one Fe3+ cation gives rise to the formation of one oxygen vacancy with the adjacent Ce4+ reduced to Ce3+ ; mode 2, one oxygen vacancy is created to balance the charge when two adjacent Ce4+ cations were substituted by two Fe3+ cations. Conversely, in dopant interstitial compensation mechanism [1,15], Fe3+ cation presented in the interstitial site neutralizes the negative charges caused by Fe3+ substitution, and thus eliminates the appearance of oxygen vacancy. At small Fe3+ doping amount, the vacancy compensation mechanism [9] is the dominant factor to charge the concentration of oxygen vacancy; therefore, the relative concentration of Ce3+ is increased with the increase of impurity doping concentration. Conversely, the interstitial compensation mechanism [1,15] plays a more important role at higher Fe3+ doping amount, which reduces the concentration of Ce3+ .

3.3.2. Raman spectroscopy measurement The Raman spectra for various catalysts are shown in Fig. 7. The pure CeO2 displays a strong band at 463 cm−1 , which has been assigned to the F2g vibration mode of the O atoms around each Ce4+ cation [33,34]. The weak band at 1172 cm−1 is attributed to second-order phonon mode of fluorite structure [35]. The band at 593 cm−1 is known to be associated to the oxygen vacancies and has been widely observed in CeO2−x [36,37]. However, the Fe3+ doping into the CeO2 increased the lattice distortion of the CeO2 crystallite, and hence interfered with the vibrations of CeO2 . As a result, the Raman band intensities of the Fe3+ doped CeO2 decreased significantly with the increase of the Fe3+ doping amount. Therefore, an alternative approach to estimate the concentration of oxygen vacancies (593 cm−1 ) is adopted by comparing the Raman band intensity at 593 cm−1 (I593 ) to that at 463 cm−1 (I463 ) [12]. Fig. 7B shows the evolution of the I593 /I463 according to the different Fe3+ doping amounts in the catalysts. The relative concentration of bulk oxygen vacancies, that is the value of I593 /I463 , increased as the Fe3+ doping amount increased. The result of XPS, in which Fe3+ doped CeO2 has a maximum surface oxygen vacancies concentration for FC100, is however different with that observed in Raman spectra, which should be due to the particularity of surface structure (XPS) to the bulk (Raman). Surface states of the catalyst play an important role in the catalytic oxidation of AO7 in the presence of H2 O2 ; hence, the surface information of the Fe3+ doped CeO2 was further observed via H2 O2 treatment in Raman spectroscopy. H2 O2 pretreatment has not obviously decreased the Raman band at 593 cm−1 (Fig. 8A), which suggests that the oxygen vacancies does not disappear in the presence of H2 O2 . In other words, adsorbed H2 O2 on the sur-

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A

A

CeO2

-1

463 cm

-1

593 cm

-1

1172 cm

x 20

400

600

800

1000

-1

593 cm

x 20

600

800

1000

1200

-1

-1

Raman shift (cm )

Raman shift (cm ) 0.10

0.015

B

B FC100

0.08

FC10 0.010

0.06

I830 / I463

I593 / I463

-1

1172 cm

-1

830 cm

400

1200

H2O2 treated

CeO2 FC200 FC100 FC20 FC10

Intensity (a.u.)

Intensity (a.u.)

FC200 FC100 FC20 FC10

-1

463 cm

FC20

0.04

FC200 FC20

CeO2

FC10

0.005

0.02

CeO2

FC200

FC100

0.5

1

0.000

0.00 0

5

10

Fe/Ce (mol%) Fig. 7. (A) Raman spectra of various catalysts, and (B) the corresponding I593 /I463 ratios.

face of CeO2 does not oxidize the surface Ce3+ to the Ce4+ . A new band appeared at 830 cm−1 in the presence of H2 O2 . The band at 830 cm−1 can be assigned to the stretching vibration of a peroxide-like (O2 2− ) species [19,36–41], which resulted from the surface cerium complex with the H2 O2 { Ce(O2 )+ } [42]. According to our previous work [19], the interfacial peroxide-like species { Ce(O2 )+ } plays an important role in the degradation of organics in the pure CeO2 –H2 O2 system. The as-formed peroxide-like species is relatively chemical stable without organic contaminants, and can bear the pre-treatment for Raman measurement or be held in the desiccator for several days. Fig. 8B presents the evolution of the I830 /I463 according to the different Fe3+ doping amounts in the catalysts. FC100 presents the maximum I830 /I463 value, which indicates the highest amount of surface peroxide-like species is formed on the surface of FC100 among all the catalysts in the presence of the H2 O2 treatment. 3.3.3. EPR spectra measurement EPR spectra of CeO2 and Fe3+ doped CeO2 are presented in Fig. 9. The signal of g = 2.0 should be ascribed to ferric ions in the Fe3+ doped CeO2 , which is significantly enhanced with the increase of Fe3+ doping amount [13]. The signal of g = 1.96 should be assigned to Ce3+ [13,43], which can be strengthened obviously with a trace amount of Fe3+ doped into ceria. It is confirmed that a low doping amount of Fe3+ into ceria increases the concentration of Ce3+ . Fig. 9B shows the EPR spectra of catalysts in the presence of H2 O2 . The signal of g = 2.0 was obviously increased CeO2 after H2 O2 treatment. A weak signal of g = 2.0 is observed in the inset of Fig. 9A. It was reported that the interaction between surface Ce3+ of CeO2

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Fe/Ce (mol%) Fig. 8. (A) Raman spectra of H2 O2 pretreated CeO2 and Fe3+ doped CeO2 , and (B) the corresponding I830 /I463 ratios. *H2 O2 treatment procedure: catalyst powder was added into H2 O2 solution (40 mM) and stirred for 1.0 h, washed with distilled water thoroughly, and then dried at 80 ◦ C for 2 h.

and molecular O2 from air formed Ce–O2 − species and presented signals at g = 2.0 in the EPR spectra [43]. The stronger peroxide-like species (Ce–O2 − ) signals in Fig. 9B originated from the interaction between surface Ce3+ ions and H2 O2 . Further, a gradual g-value shift was observed for the Fe3+ doped CeO2 in FC200 (g1 = 2.0, g2 = 2.05) and FC100 (g1 = 2.0, g2 = 2.10). The g-value shift here should be ascribed to the Fe3+ doping, which changes the electron coupling of the surface peroxide-like species. As for FC20 and FC10, no significant EPR signals for the surface peroxide-like species can be observed in Fig. 9B. 3.3.4. DMPO spin trapping EPR spectra measurement DMPO spin trapping EPR spectra measurement was carried out to study the possible active oxygen species in the reactions (Fig. 10). The sextet marked with “” for control experiment (CeO2 and DMPO only) should be ascribed to the formation of DMPO–CCR (CCR, carbon-centered radical) [44], which was stable and kept unchanged even after 30 min at ambient or several weeks at −15 ◦ C. The interaction between DMPO and some transitional metal ions such as manganese ion and cerium ion resulted in a metal ion-DMPO complex, which induced molecular rearrangement to produce a EPR active species as shown in Fig. 10 (curve a) [45]. In the presence of H2 O2 , a typical signal of the DMPO–• OH adducts (1:2:2:1 quartet) gradually appeared in the dark while that of the DMPO–CCR adducts slowly disappeared (Fig. 10A). There are two possible pathways for the generation of the DMPO–• OH adducts, attack of active • OH radical to the DMPO or surface reaction of DMPO with the surface peroxide-like species. Considering

W. Cai et al. / Applied Catalysis B: Environmental 101 (2010) 160–168

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Fig. 10. DMPO spin trapping EPR spectra of the reaction. (A) Dark-reaction: (a) control (FC100 and DMPO), (b) 1 min, (c) 3 min, (d) 10 min, (e) 30 min. (B) Visiblereaction: (a) control (FC100 and DMPO), (b) 1 min, (c) 3 min, (d) 10 min. () DMPO–CCR; (*) DMPO–• OH.

the peroxide-like species at the surface of catalyst is relative chemical stable without organic contaminants, and the degradation of AO7 depends greatly on its adsorption at the surface of the catalyst, it is mostly that the reaction was carried out at the surface of CeO2 following an intermolecular rearrangement mechanism with DMPO (and AO7). Under the visible irradiation, the DMPO–• OH adducts appear more quickly and present a stronger intensity in the EPR spectra. Accordingly, catalysts exhibit a better catalytic activity under the visible irradiation than that in the dark (Fig. 5). The enhanced catalytic activity and DMPO–• OH signals should be due to the LMCT (ligand-to-metal charge transfer) from the excited colored adsorbate (AO7 here) under visible irradiation, which injects an electron to the surface peroxide-like species and results in the generation of hydroxyl radicals (• OH). The generated • OH radical then attacks and destroys the organic substances such as AO7. Comparing the EPR spectra of FC100 with that of CeO2 (Fig. 11), it can be seen that the EPR intensity of the DMPO–CCR slightly decreases, while that of DMPO–• OH increases obviously, either in dark or under visible irradiation. It indicates that the amounts of either surface peroxide species or the • OH radical were increased in the FC100/H2 O2 system than that in the CeO2 /H2 O2 system, which confirms that the reactivity of the CeO2 can be enhanced with a desired amount of Fe3+ doping.

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4. Conclusions A series of Fe3+ doped CeO2 were prepared with a hydrothermal procedure, which showed typical cubic CeO2 structure from Fe/Ce = 1/200 to 1/10 with little fluorite structure distortion. XPS, Raman and EPR studies showed that Fe3+ doping in the CeO2

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affected greatly the concentration of Ce3+ . Lower doping amount of Fe3+ improved the concentration of Ce3+ as well as enhanced the adsorption capacity of AO7, which resulted in better catalytic performances. Further Fe3+ doping decreased the concentration of surface Ce3+ , which reduced the catalytic activity of the Fe3+ doped CeO2 . FC100 of Fe/Ce = 1/100 showed the maximum catalytic activity for the degradation of AO7 with H2 O2 . Raman and EPR studies confirmed that the amounts of both active species, the surface peroxide species in dark and the • OH radical under the visible irradiation, were increased in the FC100/H2 O2 system than that in the CeO2 /H2 O2 system. Acknowledgments This work was supported by the National Science Foundation of China (20777015) and the Fundamental Research Funds for the Central Universities. References [1] A. Trovarelli, Catalysis by Ceria and Related Materials, Imperial College Press, 2002. [2] J. Kaˇspar, P. Fornasiero, M. Graziani, Catal. Today 50 (1999) 285. [3] C.H. Kim, L.T. Thompson, J. Catal. 230 (2005) 66. [4] S. Park, J.M. Vohs, R.J. Gorte, Nature 404 (2000) 265. [5] Y.W. Zhang, R. Si, C.S. Liao, C.H. Yan, J. Phys. Chem. B 107 (2003) 10159. [6] P. Jasinski, T. Suzuki, H.U. Anderson, Sens. Actuators B: Chem. 95 (2003) 73. [7] J.P. Chen, S. Patil, S. Seal, J.F. McGinnis, Nat. Nanotechnol. 1 (2006) 142. [8] C.T. Campbell, C.H.F. Peden, Science 309 (2005) 713. [9] A. Trovarelli, Comments Inorg. Chem. 20 (1999) 263. [10] G. Balducci, J. Kaˇspar, P. Fornasiero, M. Graziani, J. Phys. Chem. B 102 (1998) 557. [11] E. Mamontov, T. Egami, R. Brezny, M. Koranne, S. Tyagi, J. Phys. Chem. B 104 (2000) 11110. [12] H.Z. Bao, X. Chen, J. Fang, Z.Q. Jiang, W.X. Huang, Catal. Lett. 125 (2008) 160. [13] C.H. Liang, Z.Q. Ma, H.Y. Lin, L. Ding, J.S. Qiu, W. Frandsen, D.S. Su, J. Mater. Chem. 19 (2009) 1417. [14] G.S. Li, R.L. Smith, H. Inomata, J. Am. Chem. Soc. 123 (2001) 11091.

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