Study of Oxygen Vacancies in Ce0.9Pr0.1O2-δ Solid Solution by in

Nov 29, 2007 - the fluorite structure and at 570 cm-1 ascribed to oxygen vacancies in the solid solution were studied by in situ Raman spectroscopy us...
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J. Phys. Chem. C 2007, 111, 18695-18702

18695

Study of Oxygen Vacancies in Ce0.9Pr0.1O2-δ Solid Solution by in Situ X-ray Diffraction and in Situ Raman Spectroscopy Zhi-Ying Pu, Ji-Qing Lu, Meng-Fei Luo,* and Yun-Long Xie Zhejiang Key Laboratory for ReactiVe Chemistry on Solid Surfaces, Institute of Physical Chemistry, Zhejiang Normal UniVersity, Jinhua 321004, China ReceiVed: July 28, 2007; In Final Form: September 13, 2007

A Ce0.9Pr0.1O2-δ solid solution was prepared by a sol-gel method. Changes in microstructure of the solid solution under different atmospheres (O2, He, and H2) and temperatures were characterized by an in situ X-ray diffraction (XRD) technique. Raman peaks at 460 cm-1 ascribed to the F2g vibration mode of CeO2 in the fluorite structure and at 570 cm-1 ascribed to oxygen vacancies in the solid solution were studied by in situ Raman spectroscopy using 785- and 514-nm excitation laser lines, providing bulk and surface information, respectively. With a 785-nm laser line, the A570/A460 ratio reflecting the oxygen vacancies concentration increased under O2 and He while it first increased and then decreased under H2 with increasing temperature. With a 514-nm excitation laser line, the A570/A460 ratio decreased with increasing temperature under all atmospheres. The growth of the A570/A460 ratio under the 785-nm laser line was due to the positive effects of high temperature and high concentration of oxygen vacancies and the negative effect of reduction of the sample under reducing atmospheres (He and H2), while the decline in the A570/A460 ratio under 514 nm was due to the dominant negative effect of the migration of surface Pr from surface to bulk during the heating process.

1. Introduction As is well-known, CeO2 is widely applied in the three-way catalytic (TWC) converter,1 water gas shift (WGS) reaction,2-4 gas sensors,5 and solid fuel cell,6-8 and it has been extensively investigated for the past few years. Among these functions, oxygen storage is the most important one because CeO2 can release O2 to form nonstoichiometric oxides CeO2-x when it was treated in reducing atmosphere, and the CeO2-x can be oxidized to form CeO2 in rich O2 conditions.9-11 This phenomenon is associated with a fast Ce4+/Ce3+ redox process in the catalyst. However, pure CeO2 has poor thermal stability under high-temperature conditions. It was reported that introduction of other rare earth or transition metal oxides to a CeO2 lattice could improve both the thermal stability and oxygen storage capacity (OSC) of cerium oxide,12 such as Ce-Zr-O13-16 and Ce-Pr-O17,18 mixed oxides. Oxygen vacancies were formed when another element was doped to the CeO2 lattice, due to different ion radii and valences. In accordance with the literature, the formation of oxygen vacancies could enhance the oxygen mobility, reduction/ oxidation capability, and thermal stability. Previous research found that Praseodymium doped to CeO2 was propitious to form oxygen vacancies.17 McBride et al. used Raman spectroscopy to study a series of rare earth doped CeO2 and found a band at 570 cm-1 that was ascribed to the oxygen vacancies.19 In our previous study, the Ce-Pr-O solid solution under different atmospheres was studied by Raman spectroscopy, and it was found that the I570/I460 in He or H2 was higher than that in air (I570 refers to the intensity of the band at 570 cm-1, and I460 refers to the intensity of the band at 460 cm-1).20 Moreover, Rossignol et al.18 studied the Ce0.7Pr0.3O2 oxides by in situ Raman spectroscopy and found that the intensity of the Raman * To whom correspondence may be addressed. Fax: +86-579-82282595. E-mail: [email protected].

band decreased and shifted to lower frequency when the sample was heated to 523 K in hydrogen, and the band could return to its initial position after cooling down to room temperature. They concluded that the shift of the 460-cm-1 band was due to the increase in particle size because of thermal expansion. However, Spanier et al.21 and Popovic´ et al.22 concluded that the red shift of the-460 cm-1 band was due to the removal of nanoeffects such as nonstoichiometry and structure defects during the heating process. However, these investigations were usually carried out at room temperature or low temperature. Since catalytic reaction that involves CeO2 generally occurs at high temperatures (e.g., 473-773 K), it is very important to study the property of oxygen vacancies at reaction temperature range, thus to provide relevant information on the relationship between the oxygen vacancies and the catalytic property. Meanwhile, the influence of different atmospheres on the oxygen vacancies was given much attention too. In situ Raman spectroscopy is a powerful technique because it can provide fundamental molecular-level information about catalyst surface structure and reactive surface intermediates under practical reaction conditions (temperature, atmospheres, etc.).23,24 In this work, in situ Raman spectroscopy was employed to study the surface property of a Ce0.9Pr0.1O2-δ solid solution under different atmospheres and temperatures. By use of various excitation laser lines, information of different surface layers was obtained, which may give new insight on the property of oxygen vacancies in doped CeO2 solid solution. 2. Experimental Section 2.1. Catalyst Preparation. CeO2 and Ce0.9Pr0.1O2-δ mixed oxides were prepared by the citrate sol-gel method, which was described in our previous work.25 In a typical preparation, Pr6O11 (99.9%) was dissolved by the least amount of dense nitric acid

10.1021/jp0759776 CCC: $37.00 © 2007 American Chemical Society Published on Web 11/29/2007

18696 J. Phys. Chem. C, Vol. 111, No. 50, 2007 required. Deionized H2O (150 mL) and an appropriate amount of Ce(NO3)3‚6H2O (99.9%) were added to form a Ce-Pr nitrate solution. Then citric acid with the same molar amount of Ce + Pr was added to the above solution. The mixture was heated at 90 °C under stirring until a viscous gel was obtained. The gel was dried at 120 °C overnight and calcined at 900 °C in air for 4 h with a heating rate of 10 °C min-1. 2.2. Characterization. The gases used in this study were O2 (99.999%), He (99.999%), and H2 (99.999%). The He and H2 were deoxygenized before they were introduced to the system. X-ray diffraction (XRD) patterns were collected on a Philips PW3040/60 automated powder diffractometer equipped with a Philips X’celerator detector using Cu KR radiation (λ ) 0.1542 nm). The working voltage of the instrument was 40 kV, and the working current was 40 mA. The flat samples were collected with a 2θ range from 20 to 130°. The lattice parameters of the samples were determined by the Rietveld method, and the CeO2 particle size was calculated by full curve fitting, using JADE 6.5 software. The in situ XRD were collected in an Anton Paar TCU 750 chamber linked to the XRD diffractometer. Ce0.9Pr0.1 O2-δ sample (100 mg) was ground in a carnelian mortar and placed in the chamber with a thermocouple located rear the sample cell, which allowed us to measure the temperature of the sample. The sample was heated under pure O2 (15 mL min-1) from room temperature to 500 °C with a heating rate of 10 °C min-1. XRD patterns were collected at 25, 100, 200, 300, 400, and 500 °C, after each temperature was held for 30 min. The scanning 2θ range was 20-110°. After that, the sample was cooled down to room temperature (normal cooling) and scanned again, which was denoted as point A. The in situ XRD experiments in He and H2 were measured in the same manner. For these two gases, after the scan at room temperature in the same atmosphere, the previous gas (He or H2) was stopped and O2 was introduced and patterns were collected, denoted as point B. Ultraviolet-visible (UV-vis) diffuse reflectance spectrum was recorded on a JASCO V-500 spectrophotometer equipped with an integrating sphere. Raman measurements were performed on a Renishaw RM1000 with a confocal microprobe Raman system using an excitation wavelength of 785 and 514 nm, a dwell time of 20 s, a number of scans of 2, and a resolution of 1 cm-1. A power of 1% for the 785-nm laser line (HPNIR laser) and a power of 50% for the 514-nm laser line (Ar+ laser) were used. The in situ Raman measurements of the Ce0.9Pr0.1O2-δ sample were performed in a homemade in situ Raman cell linked to the Raman equipment. About 20 mg of the sample was inserted in the cell and heated up to 500 °C under O2 atmosphere with a heating rate of 10 °C min-1. Prior to the measurement, each temperature was held for 30 min. Finally, the sample was cooled down to room temperature and a spectrum was collected again. The in situ Raman spectra in He and H2 were measured in the same manner. For these two gases, after the spectra were collected at room temperature in the same atmosphere, the previous gas (He or H2) was stopped and O2 was introduced and the spectrum was collected again. X-ray photoelectron spectra (XPS) were recorded on a Vg Escalab MK2 spectrometer with an attached sample pretreatment chamber, using Al KR radiation (1486.6 eV). The X-ray source was operated at an accelerating voltage of 10 kV and a power of 200 W. The sample was made into a pellet of 10 mm in diameter and placed in the pretreatment chamber under He atmosphere at 500 °C for 0.5 h and then cooled down to room temperature before being displaced into an ultrahigh vacuum

Pu et al.

Figure 1. XRD patterns of CeO2 and Ce0.9Pr0.1O2-δ samples.

(UHV) chamber at 2 × 10-6 Pa housing the analyzer. Binding energies were calibrated by using the containment carbon (C 1s ) 284.6 eV). 3. Results Figure 1 shows the XRD patterns of the CeO2 and the Ce0.9Pr0.1O2-δ samples. It can be seen that only the diffraction peaks due to cubic CeO2 were observed in the Ce0.9Pr0.1O2-δ mixed oxide, indicating that the solid solution was formed in the Ce0.9Pr0.1O2-δ sample. The lattice parameter of the Ce0.9Pr0.1O2-δ sample was analyzed by the JADE 6.5 software. The lattice parameter of the Ce0.9Pr0.1O2-δ sample was 0.5412 nm, which was almost identical to that of pure CeO2 (0.5410 nm).26,27 This is because the radius of Pr3+ (0.096 nm) is very similar to that of Ce4+ (0.097 nm), which leads to a slight change of the lattice parameter when Ce4+ was partially substituted by Pr3+. The CeO2 particle size was about 39 nm using full curve fitting by the JADE 6.5 software. The in situ XRD patterns of the Ce0.9Pr0.1O2-δ sample at different temperatures under O2, He, and H2 atmospheres were shown in Figure 2. It can be seen from Figure 2 that only diffraction peaks due to cubic CeO2 were observed in the Ce0.9Pr0.1O2-δ mixed oxide in all atmospheres, indicating that the Ce0.9Pr0.1O2-δ solid solution retained the same structure at high temperature. However, the diffraction peaks shifted to lower angle as the temperature increased, which can be ascribed to the increase of lattice parameter. The shift of the diffraction peaks in different atmospheres followed the order of H2 > He > O2, suggesting that the lattice expanded most dramatically in H2. After the sample was cooled down to room temperature and exposed to O2, the diffraction peaks returned to the initial position. Also, results of full curve fitting show that CeO2 particle size did not change much in all the atmospheres during the heating process (about 40 nm) compared to that at room temperature (39 nm). The lattice parameters of the Ce0.9Pr0.1O2-δ sample were calculated via the in situ XRD results. The relationship between lattice parameter and temperature under different atmospheres was shown in Figure 3. In the operating temperature range, the lattice parameter linearly increased in an O2 atmosphere, which could be ascribed to the thermal expansion.18 It reverted completely after the sample was cooled down to room temperature. In He and H2, the lattice parameter was similar to that in O2 below 200 °C, while it increased more significantly in He and H2 atmospheres than that in O2 at higher temperature (>200 °C). Between 200 and 300 °C, an abrupt increase of the lattice parameter could be observed in the H2 atmosphere, so

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Figure 4. UV-visible diffuse reflectance spectra of Ce0.9Pr0.1O2-δ sample.

Figure 2. In situ XRD patterns of Ce0.9Pr0.1O2-δ sample in (a) O2, (b) He, and (c) H2 atmospheres.

Figure 5. Raman spectra of Ce0.9Pr0.1O2-δ sample with the 785- and 514-nm laser lines.

TABLE 1: 4a in He and H2 Atmospheres at Different Temperatures 4aa (10-3)

Figure 3. Relationship between lattice parameter and temperature under different atmospheres. (A) Cool down to 25 °C in same atmosphere; (B) switch to O2 after cooling down to 25 °C.

its extent of expansion was maximal. After the sample was cooled down to room temperature, the lattice parameter reverted partially, and it returned to its initial value when the sample was exposed in O2. Table 1 summarized the change of the CeO2 lattice parameter under He and H2 compared to that under O2. 4a was calculated as LCH2 (or LCHe) - LCO2, where LCH2, LCHe, and LCO2 correspond to the CeO2 lattice parameter under H2, He, and O2 at the same temperature, respectively. Therefore, 4a indicated the cell expansion other than the thermal cause. It could be seen that 4a gradually increased with temperature and returned to the initial value when the sample was cooled down to room temperature and O2 was introduced. Meanwhile, 4a was larger in H2 than in He. The UV-visible diffuse reflectance spectrum of the Ce0.9 Pr0.1O2-δ sample is shown in Figure 4. It can be seen from the figure that the sample shows an absorption band from 200 to 650 nm. The band below 400 nm is due to the O 2p f Ce 4f

temp (°C)

He

H2

25 100 200 300 400 500 cool down to 25 °C cool down to 25 °C with introduction of O2

0 0 0.08 0.94 1.63 1.76 1.83 0

0 0 0.25 2.17 2.18 2.37 2.13 0

a

∆a t LCH2 (or LCHe) - LC02.

transition and the band at 400-650 nm is due to a Pr(III) transition.28 Furthermore, the absorption intensity at 514 nm was stronger than that of 785 nm. The Raman spectra of the Ce0.9Pr0.1O2-δ sample with 785and 514-nm laser lines are shown in Figure 5. From Figure 5, it can be seen that the intensity of the band at 460 cm-1 with the 785-nm laser line is stronger than that of the 514-nm laser line. Its intensity is about 7 times that of the 514-nm excitation laser line. There is a 4-cm-1 shift of the band at 460 cm-1 compared to the bulk value (464 cm-1) because the Ce0.9Pr0.1O2-δ sample employed in the current study is a nanosized material (40 nm), which is consistent with the previous result that a red shift of the band occurred for nanosized CeO2.21 Figure 6 shows the in situ Raman spectra of the Ce0.9Pr0.1O2-δ sample at different temperatures in O2, He, and H2 atmospheres

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Figure 7. Relationship between A570/A460 and temperature with the 785-nm laser line under different atmospheres. (A) Cool down to 25 °C in same atmosphere; (B) switch to O2 after cooling down to 25 °C.

Figure 6. In situ Raman spectra of Ce0.9Pr0.1O2-δ sample under (a) O2, (b) He, and (c) H2 atmospheres with the 785-nm excitation laser line.

with the 785-nm excitation laser line. Two peaks were observed at 460 and 570 cm-1. The Raman band at 460 cm-1 was attributed to the F2g vibration mode of the fluorite structure of pure CeO2.29 Another broad band at about 570 cm-1 in the tail of the main band at 460 cm-1 was clearly visible, which was ascribed to oxygen vacancies according to McBride et al.19 The band at 460 cm-1 hardly changed in intensity, but it shifted to a lower frequency with increasing temperature in all atmospheres. The band at 570 cm-1 did not change significantly with temperature in O2 and He. But it obviously shifted to lower frequency when the temperature was higher than 300 °C in H2. The intensity and the position of these two bands completely reverted when the sample was cooled down to room temperature in the previous atmosphere and then exposed in O2. The peak areas of the band at 460 cm-1 and the band at 570 cm-1 based on the results in Figure 6 were calculated and denoted as A460 and A570, respectively. The A570/A460 value reflects the concentration of oxygen vacancies in the solid solution. The relationship between A570/A460 and temperature under O2, He, and H2 atmospheres with the 785-nm excitation laser line was shown in Figure 7. From the figure, it can be seen that the value of A570/A460 increased under O2 and He, while it first increased then decreased under H2 with increasing temperature. The A570/A460 in He was higher than that in O2 below 200 °C, while it was opposite when the temperature was higher than 200 °C. It completely reverted when the sample was cooled down to room temperature in the previous atmosphere and exposed in O2 (point B). The in situ Raman spectra of the Ce0.9Pr0.1O2-δ sample at different temperatures under O2, He, and H2 atmospheres with

Figure 8. In situ Raman spectra of Ce0.9Pr0.1O2-δ sample under (a) O2, (b) He, and (c) H2 atmospheres with the 514-nm excitation laser line.

the 514-nm excitation laser line are shown in Figure 8. It could be seen that under all atmospheres, besides the peaks at 460 and 570 cm-1, two new peaks were observed at 195 and 1150 cm-1. The weak band at 1150 cm-1 was ascribed to the primary A1g asymmetry of CeO2.29 The trend of the change of the 195 cm-1 band was consistent with the peak at 570 cm-1, so it could also be attributed to the asymmetric vibration caused by the formation of oxygen vacancies.30

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Figure 9. Relationship between A570/A460 and temperature with the 514-nm laser line under different atmospheres. (A) Cool down to 25 °C in same atmosphere; (B) switch to O2 after cooling down to 25 °C.

The Raman spectra of the Ce0.9Pr0.1O2-δ sample at different temperatures in O2 (Figure 8a) show that the peak at 570 cm-1 became broader and its intensity decreased with increasing temperature and it was fully recovered after the sample was cooled down to room temperature. Also, the peak at 460 cm-1 shifted to a lower frequency with increasing temperature, and it returned to its initial position after the sample was cooled down to room temperature. The Raman spectra in He (Figure 8b) were quite similar to those in O2. However, in the He atmosphere, the intensity of the 570-cm-1 band decreased more obviously than that in O2 with increasing temperature, and it was completely recovered when the sample was cooled down to room temperature with introduction of O2. The Raman spectra in H2 (Figure 8c) were very different from that in O2 and He atmospheres: the intensity of the 570-cm-1 band decreased with temperature, while its position did not change below 300 °C. However, its position shifted to lower frequency at about 540 cm-1 when the temperature exceeded 300 °C with little change in its intensity in the temperature range of 300-500 °C. When the sample was cooled down to room temperature and O2 was introduced, the position and intensity of the band due to oxygen vacancies also reverted completely. Figure 9 shows the relationship between A570/A460 and temperature under O2, He, and H2 atmospheres with the 514nm excitation laser line. Different from that with the 785-nm laser line, the value of A570/A460 decreased with increasing temperature in all atmospheres. Moreover, the value was higher in He than in O2, with a slightly higher slope of decline. However, the decline of A570/A460 in H2 was much faster compared to those in O2 and He. A570/A460 can also be reverted completely when the sample was cooled down to room temperature in the previous atmosphere and then exposed in O2. The Ce and Pr 3d XPS spectra of the Ce0.9Pr0.1O2-δ sample treated in He at 25 and 500 °C are shown in Figure 10. Because of the requirement of UHV conditions for the XPS analysis, it is not possible to do in situ experiments because oxygen would be continuously released under high temperature. Therefore, the sample was pretreated in He at 500 °C for 0.5 h and then cooled down to room temperature before measurement, which was to mimic the process for Raman experiments, from room temperature to 500 °C and then to room temperature again (point A). It could be seen that the pretreatment at 500 °C hardly changed

Figure 10. Ce 3d (a) and Pr 3d (b) XPS spectra of the Ce0.9Pr0.1O2-δ sample. (1) at 25 °C; (2) the sample was heated to 500 °C in He and then cooled down to 25 °C.

TABLE 2: XPS Analysis of Pr/Ce Atomic Ratio in the Surface of Ce0.9Pr0.1O2-δ Pretreated at Different Temperatures pretreatment temp (°C) 25 500

Pr/Ce atomic ratio 0.24/0.76 0.34/0.66

any features of the spectra; the two spectra were almost identical. The XPS of Ce 3d (Figure 10a) showed six peaks at 881.1, 887.8, 896.7, 899.4, 906.2, and 915.5 eV. The peaks at 881.1 and 899.4 eV were ascribed to the principle binding energy of Ce 3d5/2 and Ce 3d3/2, respectively.31 The main peaks at 915.5 and 896.7 eV could be assigned to the 3d104f0 initial electronic state corresponding to the Ce4+ ion,31 whereas the bands at 881.1 and 887.8 eV and the bands at 900.0 and 906.2 eV doublets could be ascribed to the final states with a strong mixing of the 3d94f2 and 3d94f1 configurations.32 Note that no peak ascribed to the Ce3+ ion was observed after pretreatment at 500 °C in He, suggesting the Ce4+ ion in the Ce0.9Pr0.1O2-δ solid solution was not reduced after treatment in He at 500 °C. The Pr 3d XPS spectra shown in Figure 10b consist of four peaks at binding energies of 926.9, 932.1, 947.1, and 952.6 eV. In accordance with He et al.,32 the signals at 952.6 and 932.1 eV were assigned to Pr4+ and the peaks at 947.1 and 926.9 eV to Pr3+. Note that the intensities of Pr3+ ion signals after pretreatment at 500 °C were higher than that at 25 °C, indicating higher content of Pr3+ on the surface of the Ce0.9Pr0.1O2-δ sample after the pretreatment. This result implied that the Pr4+ ion in the Ce0.9Pr0.1O2-δ solid solution was partially reduced after the pretreatment process. In combination with the Ce 3d XPS spectra of the Ce0.9Pr0.1O2-δ sample, it could be concluded that the Pr4+ ion was easier to reduce than the Ce4+ ion after being treated at 500 °C in He. This was in accordance with the literature result that the Ce4+ ion was reduced at 570 °C and the Pr4+ ion at 530 °C.30 Table 2 lists the XPS analysis of surface Ce and Pr contents at different temperatures. As seen in Table 2, it was obvious that the content of surface Pr increased when the temperature was cooled down to 25 from 500 °C in He.

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Figure 11. Schematic description for Raman scattering with different excitation laser lines.

4. Discussion 4.1. Change in the Microstructure of the Ce0.9Pr0.1O2-δ Sample during Heating Processes under Different Atmospheres. As shown in Figure 1, the Ce0.9Pr0.1O2-δ sample has the same structure as the pure CeO2 at room temperature. However, its microstructure changes when it is heated under different atmospheres, as shown in Figures 2 and 3. The growth of the CeO2 lattice parameter under O2 with increasing temperature is due to the intrinsic thermal expansion of the cell.18 When the sample was heated in He or H2, the growth of lattice parameter was faster than in O2 (Figure 3). In addition to the thermal expansion of the cell, reduction of the Pr ion under the heating process could also play a role. As oxygen in the sample could be continuously depleted with increasing temperature under He or H2, the Pr4+ ion could be reduced to Pr3+, as evidenced by XPS results (Figure 10). Pr3+ has a larger ionic radius than Pr4+, which results in an increasing lattice parameter. Moreover, as the H2 is a stronger reducing agent than He, the increase is larger in H2 than in He, as confirmed in Figure 3 and Table 1. Therefore, it could be concluded that the heating process causes a change in microstructure of the sample, especially in reducing atmosphere and high temperature. 4.2. Distinguishing the Surface and Bulk Information via Different Excitation Laser Lines. The UV-visible result (Figure 4) of the Ce0.9Pr0.1O2-δ catalyst shows that a higher optical absorption at 514 nm was stronger than that at 785 nm. That is to say, when it was measured by Raman technique, the majority of excitation laser and scattering light were absorbed by the Ce0.9Pr0.1O2-δ sample when the 514-nm excitation laser line was used, while it had almost no absorption with the 785nm excitation laser line. According to Li et al.,33 when the excitation laser and scattering light were strongly absorbed by the sample, only partial light escaped, so the signal was weakened. Then the Raman spectra reflect more surface signal than the bulk of the sample. So the surface region information was obtained when the 514-nm excitation laser line was used. And the 785-nm excitation laser line reflects both the bulk and surface information due to its weak absorption. Furthermore, a comparison between Raman spectra of the Ce0.9Pr0.1O2-δ catalyst using the 785- and the 514-nm laser lines (Figure 5) reveals a much higher intensity for the 785 nm laser line than the 514 nm laser line, which is consistent with the UV-visible result. It further suggests that the majority of scattering light was absorbed by the Ce0.9Pr0.1O2-δ sample when the 514-nm laser line was used and only partial surface information was obtained, while both the bulk and surface information were obtained when the 785-nm laser line was used. A scheme illustrating the difference between the two laser lines is shown in Figure 11. 4.3. Relationship between Pr Content and the A570/A460 Value. In our previous study, the CexPr1-xO2-δ samples with different Pr contents were analyzed by XPS spectra. It was concluded that the content of Pr in the surface region was higher

than that of the nominal composition,20 indicating that Pr richened in the surface region of CexPr1-xO2-δ samples. Furthermore, the I570/I460 ascribed to oxygen vacancies concentration increased with Pr content. In the present work, it was found that the A570/A460 value using the 514-nm laser line was always higher than that using the 785-nm laser line under the same temperature and atmosphere (Figures 7 and 9). This is due to the fact that the Raman signal obtained using the 514-nm laser line provides surface information and the enrichment of Pr on the surface (higher Pr content on the surface than in the bulk) results in a higher A570/ A460 value; however, the signal obtained using the 785-nm laser line provides both the surface and the bulk information. Therefore, the overall Pr content does not change. This is in agreement with our previous study that Pr richened on the surface region of Ce0.9Pr0.1O2-δ sample and higher Pr content on the catalyst surface results in higher A570/A460 value. It suggests that the higher A570/A460 value using the 514-nm laser line than that using the 785-nm laser line is due to Pr enriched on the surface of Ce0.9Pr0.1O2-δ solid solution. 4.4. Influence of Temperature and Atmosphere on the A570/A460 Value. The Raman spectra of the Ce0.9Pr0.1O2-δ sample in different temperatures and atmospheres are shown in Figures 6 and 8. For the spectra using the 785-nm laser line, the band at 460 cm-1 hardly changed in intensity but it shifted to lower frequency with increasing temperature in all atmospheres, while it returned to its initial position after the sample was cooled down to room temperature. Similar results were observed by Spanier et al.21 and Popovic´ et al.,22 and they concluded that the red shift of the 460-cm-1 band was due to the removal of inharmonic effect (nonstoichiometry, inhomogeneous strain, and structure defects) on the nanosized CeO2 under the heating process but not the particle size effect. This is in agreement with the results in the current study, as the CeO2 particle size remains around 40 nm during the heating process. The 570-cm-1 band merely changed in O2 and He, while it shows a clear shift toward lower frequency in H2, indicating a change in microstructure of the sample due to the reduction of Ce4+ or Pr4+ ions by H2, especially at high temperatures beyond 300 °C.30 Under all atmospheres, after the sample was cooled down to room temperature, the A570/A460 value could return to a close level to the initial stage and completely return to the initial stage when the sample was exposed in O2. Similar results were obtained using the 514-nm laser line. However, as clearly shown in Figures 7 and 9, the A570/A460 value increased under the 785-nm laser line under O2 and He while it first increased and then decreased under H2 with increasing temperature. With the 514-nm excitation laser line, the A570/A460 ratio decreased with increasing temperature under all atmospheres. As the A570/A460 ratio reflects the oxygen vacancies concentration in the sample, therefore, the two opposite results under the 785- and 514-nm laser lines are closely related to the change

Study of Oxygen Vacancies in Ce0.9Pr0.1O2-δ in oxygen vacancies during the heating process under different atmospheres. There are several factors that affect the A570/A460 ratio: temperature, oxygen vacancies concentration, Pr content on the surface, and sample microstructure. Generally, high temperatures could enhance the vibration of the metal-oxygen vacancies and thus increase the A570/A460 ratio. Also, high temperature is propitious to form oxygen vacancies due to the release of oxygen from the sample during the heating process.34 Higher oxygen vacancy concentration would certainly increase the A570/A460 ratio, and higher Pr content on the surface will increase the ratio as well, as discussed earlier. The effect of sample microstructure on the A570/A460 value is discussed as follows: Since the Raman signal using the 785-nm laser line provides both surface and bulk information of the sample, so the difference between the surface and the bulk Pr content is normalized and should not be considered. The vibration was enhanced with increasing temperature, and the concentration of oxygen vacancies increased simultaneously. Therefore, the A570/A460 value increased with temperature. Interestingly, as seen in Figure 7, the increase under He is lower than that under O2, which is contradictory to the common understanding that the release of oxygen from the sample will enhance the formation of oxygen vacancies and thus result in a higher A570/A460 ratio. As the in situ XRD results (Figures 2 and 3) indicate the reduction of the sample during the heating process and the XPS results (Figure 10) show the reduction of Pr4+ to Pr3+, therefore, the observation in the A570/A460 (Figure 7) implies that reduction of the sample, especially the reduction of Pr4+ to Pr3+, would decrease the A570/A460 ratio. When the sample was cooled down to room temperature, the temperature effect was eliminated; however, since the sample was still reduced, the A570/A460 (point A) was lower than the initial value. With the introduction of O2, the sample was reoxidized, thus recovering the A570/A460 ratio. This presumption was further confirmed by the finding under H2. As H2 is a stronger reducing agent than He, the change in microstructure of the sample was more pronounced (Figure 3 and Table 1). Therefore, the reduction of the sample (mainly Pr4+ to Pr3+) inhibits the temperature effect and it is even more dominant at temperatures higher than 200 °C, resulting in the decline in the A570/A460 ratio. In fact, a H2 temperatureprogrammed reduction (H2 - TPR) experiment (not shown) on the Ce0.9Pr0.1O2-δ sample shows desorption of H2O at 200 °C, indicating a reduction of the sample occurred at this temperature, which again supported the in situ XRD results. As under He, the A570/A460 ratio was recovered with the introduction of O2 at room temperature because of the recovery of the microstructure of the sample due to the fast redox property of the solid solution. For the Raman spectra obtained using the 514-nm laser line, the situation is more complex because the effect of surface Pr content was involved. As discussed earlier, high surface Pr content leads to high A570/A460 ratio. Compared to the 785-nm laser line, the only difference between these two laser lines is that the spectra obtained using the 785-nm laser line provide both surface and bulk information while those using the 514nm laser line only provide surface information. Therefore, the opposite trend of the A570/A460 ratio under these laser lines is probably due to the change in the surface Pr content. It is unfortunate that in situ XPS experiments could not be carried out to check the surface Pr content during the heating process; however, the surface analysis results (Table 2) show that the content of surface Pr increased when the sample was cooled down to 25 from 500 °C in He, indicating the Pr content in the sample was redistributed during the process. The Raman results

J. Phys. Chem. C, Vol. 111, No. 50, 2007 18701 under He (Figure 9) using the 514-nm laser line suggest that the higher A570/A460 ratio at point B (2.024) than that of the initial state (1.832) is due to a higher surface Pr content, while the ratio hardly changed using the 785-nm laser line (0.182 vs 0.178), which indicates that the change in surface Pr content is not concerned. Therefore, it is reasonable to assume that, during the heating process, the surface Pr species migrated to the bulk, which caused a decline in surface Pr content and consequently a decline in the A570/A460 ratio. When the sample was cooled down, the Pr species in the bulk remigrated to the surface and resulted in an even higher surface Pr concentration than the initial state, which was confirmed by the XPS analysis (Table 2). Furthermore, judging from Figure 9, the effect of Pr redistribution from surface to bulk on the A570/A460 ratio is more pronounced compared to other effects (temperature, oxygen vacancies, and the change in microstructure) because the A570/ A460 ratio increases with increasing temperature under the 785nm laser line (Figure 7), while it decreases under the 514-nm laser line, only due to the influence of surface Pr content. Another observation in this study is that the A570/A460 ratio under He is higher than that under O2 with the 514-nm laser line (Figure 9), while the ratio is lower under He than that under O2 with the 785-nm laser line (Figure 7). It is probably due to the different surface Pr content under various atmospheres, that is, the speed of redistribution of the surface Pr varies under different atmospheres. However, because of the limitation of our experimental conditions, it is unable to detect the surface Pr content under in situ conditions. 5. Conclusions The Ce0.9Pr0.1O2-δ solid solution was prepared by the solgel method and characterized by in situ XRD and Raman techniques. The in situ XRD results reveal the change in microstructure of the sample during the heating process under different atmospheres, among which H2 results in the most dramatic growth in the CeO2 lattice parameter due to its strong reducibility. In situ Raman spectroscopic study with 785- and 514-nm excitation laser lines under different atmospheres gives opposite trends in the A570/A460 ratio, reflecting the concentration of oxygen vacancies. For the Raman spectra obtained using the 785-nm laser line, the growth of the A570/A460 ratio is due to the positive effects of high temperature and oxygen vacancies and the negative effect of the change in microstructure owing to the reduction of the sample under the heating process. For the Raman spectra obtained using the 514-nm laser line, the decline in the A570/A460 ratio is due to the positive effects of high temperature and oxygen vacancies and the negative effects of reduction of the sample and the redistribution of surface Pr content under the heating process. Acknowledgment. This work is financially supported by the Natural Science Foundation of China (Grant 20473075). References and Notes (1) Reddy, B. M.; Bharali, P.; Saikia, P.; Khan, A.; Loridant, S.; Muhler, M.; Gru1nert, W. J. Phys. Chem. C 2007, 111, 1878. (2) Wang, X. Q.; Rodriguez, J. A.; Hanson, J. C.; Gamarra, D.; Martinez-Arias, A.; Fernandez-Garcia, M. J. Phys. Chem. B 2006, 110, 428. (3) Koryabkina, N. A.; Phatak, A. A.; Ruettinger, W. F.; Farrauto, R. J.; Ribeiro, F. H. J. Catal. 2003, 217, 233. (4) Jacobs, G.; Chenu, E.; Patterson, P. M.; Williams, L.; Sparks, D.; Thomas, G.; Davis, B. H. Appl. Catal. A 2004, 258, 203. (5) Jasinski, P.; Suzuki, T.; Anderson, H. U. Sens. Actuators B 2003, 95, 73. (6) Eguchi, K.; Setoguchi, T.; Inoue, T.; Arai, H. Solid State Ionics 1992, 52, 165. (7) Park, S.; Vohs, J. M.; Gorte, R. J. Nature 2000, 404, 265.

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