Structural Characterization and Catalytic Activity of Ce0.65Zr0.25RE0

Article pubs.acs.org/JPCC

Structural Characterization and Catalytic Activity of Ce0.65Zr0.25RE0.1O2−δ Nanocrystalline Powders Synthesized by the Glycine-Nitrate Process D. Hari Prasad, S. Y. Park, H.-I. Ji, H.-R. Kim, J.-W. Son, B.-K. Kim, H.-W. Lee, and J.-H. Lee* High-Temperature Energy Materials Center, Future Convergence Research Division, Korea Institute of Science and Technology, Seoul 136-791, Republic of Korea

ABSTRACT: In this study, Ce0.65Zr0.25RE0.1O2−δ (RE = Tb, Gd, Eu, Sm, Nd, Pr, and La) solid solutions were successfully prepared by the glycine-nitrate process and tested for CO oxidation activity. The X-ray diffraction results confirmed the formation of complete Ce0.65Zr0.25RE0.1O2−δ solid solutions. The Raman spectroscopy measurements suggested the presence of oxygen vacancies due to defective structure formation and further evidenced the formation of solid solution. The high-resolution transmission electron microscopy observations showed the nanocrystalline nature of the solid solutions. From X-ray photoelectron spectroscopy analysis it was revealed that the cerium, terbium, and praseodymium are present in +3 and +4 oxidation states. The UV−vis diffuse reflectance spectroscopy indicated that the Pr3+ ions in the Ce0.65Zr0.25Pr0.1O2−δ system provoked a significant increase in the Ce3+ fraction on the surface. H2 temperature-programmed reduction measurements showed an enhanced surface reduction at much lower temperatures for Ce0.65Zr0.25Pr0.1O2−δ sample compared to others, indicating increased oxygen mobility in these samples, which enable the enhanced oxygen diffusion at lower temperatures. Significantly high CO oxidation activity is exhibited by Ce0.65Zr0.25Pr0.1O2 solid solution.

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reduction efficiency of redox couple Ce4+/Ce3+, and excellent OSC compared to those of pure ceria. Rare earth (RE)-doped CZ oxides showed improvement in OSC, redox property, and thermal resistance compared to CZ itself. Generally, for every two RE+3 ions that replace Ce4+, one oxygen vacancy is needed to balance the charge. These oxygen vacancies will increase the diffusion rate of oxygen, thereby improving the OSC property. Besides, all the RE elements have larger ionic radii than Zr4+ (0.87 Å), which generally provide more effective stabilization of fluorite-type structure compared to undersized ones.17 It has been observed in the literature18 that among the CexZr1−xO2−δ (x = 0.1−0.9) solid solutions investigated those with the highest zirconia content compatible with a cubic symmetry showed the best redox properties and the highest OSC. The phase transition from the tetragonal CexZr1−xO2 to the cubic phase is located close to x = 0.6 even

INTRODUCTION Ceria-based materials displaying high oxygen mobility and oxygen storage capacity (OSC) have been extensively investigated because of their wide application in the field of catalysis and solid oxide fuel cells.1−11 Incorporation of transition metals or other rare earth elements has been studied to improve the thermal stability and to increase the OSC of ceria. Particularly, doped ceria materials have been extensively investigated because of their improved physiochemical properties and decreased oxidation enthalpies compared to pure ceria.12−16 Zirconia proved to be an excellent additive to increase the mobility of bulk oxygen and to prevent the sintering of ceria at high temperatures.3−5 The incorporation of zirconia in ceria forms ceria-zirconia (CZ) solid solutions, which enhances the thermal stability and maintains a high OSC even at high temperatures and thereby replaces pure ceria, whose characteristics were inadequate to sustain the high degree of conversion and the thermal resistance required for catalytic converters. The main features that contribute to the success of the CZ system are high thermal resistance, higher © 2012 American Chemical Society

Received: July 25, 2011 Revised: December 1, 2011 Published: January 5, 2012 3467

dx.doi.org/10.1021/jp207107j | J. Phys. Chem. C 2012, 116, 3467−3476

The Journal of Physical Chemistry C

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if, because of its metastable nature,19−21 an exact location of this boundary is still a matter of debate. Here we have used 65 mol % of Ce in the Ce−Zr−RE system. The homogeneity of a Ce−Zr−RE system critically affects both the redox and textural properties. The presence of metastable phases in the phase diagram of CZ immediately points out the critical importance of the method of synthesis of the mixed oxides and the relevance of the kinetic liability/inertness toward phase separation.2 In the present study, the glycine-nitrate process (GNP) has been employed to synthesize the Ce0.65Zr0.25RE0.1O2−δ (RE = Pr, Tb, Sm, Gd, La, Nd, and Eu) oxides since it is the most suitable method for producing fairly fine, homogeneous, and complex compositional metal oxide powders. Additionally, GNP has many other advantages such as relatively low cost, high energy efficiency, fast heating rates, short reaction times, and high compositional homogeneity.11,22−25 The aim of the present work is to investigate the influence of the RE dopants on the redox properties that affect the catalytic properties of the CZ system. The reducibility of the support material plays an important role in suppression of carbon formation during reforming reactions. To screen the support materials that can be used for internal reforming solid oxide fuel cells (SOFCs) applications, we focused on CO oxidation reaction as this reaction is mostly affected by the enhanced reducibility of the oxide solid solutions. The obtained powders were characterized by using X-ray diffraction (XRD), BET surface area, Raman spectroscopy (RS), transmission electron microscopy (TEM), UV−visible diffuse reflectance spectroscopy (UV−vis DRS), X-ray photoelectron spectroscopy (XPS), H2 temperature-programmed reduction (H2-TPR), and energydispersive X-ray spectroscopy/scanning electron microscopy (EDS/SEM) techniques. Furthermore, the catalytic activity toward CO oxidation has also been evaluated.

Catalyst Characterization. The XRD patterns were obtained by an X-ray generator (Phillips PW 3830, Holland) using Ni-filtered Cu Kα radiation. The intensity data were collected over a 2θ range of 20−80° with a 0.02° step size using a counting time of 1 s per point. The mean crystallite size (DXRD) was measured by applying the Scherrer’s equation, D = (0.9λ)/(β cos θ), where D is the crystallite size, λ is the wavelength of the radiation, β is the corrected peak width at half-maximum intensity, and θ is the peak position. The XRD phases present in the samples were identified using Powder Diffraction File-International Center for Diffraction Data (PDF-ICDD). The cell a parameter was calculated by a standard cubic indexation method with the intensity of the most prominent peaks using the relation a = d(h2 + k2 + l2)1/2 where a is the lattice parameter and d is the interplanar spacing calculated from Bragg equation. Raman spectra were measured using a conventional back scattering geometry with a Raman spectrometer (T 64000, Jobin-Yvon, France) which consists of a triple polychromator and a charged-couple device (CCD) detector. The excitation source was an Ar+ ion laser (λ = 514.23 nm) and the laser power was 10 mW at the sample point. XPS analysis was performed in ultrahigh vacuum using PHI 5800 Versa probe instrument (Physical Electronics, USA). Charging of the samples was minimized by referencing the spectra to the C 1s line at BE 284.6 eV. The BET surface area measurements were made on a Quantachrome instrument (Quadrasorb SI, USA). Prior to the analysis, samples were degassed at 200 °C under vacuum for 3 h to remove any residual moisture and other volatiles. EDS/SEM (EDAX/SEM, XL-30 FEG ESEM, USA) and TEM (Technai G2 F20, The Netherlands) were used for the compositional and microstructural analysis such as size, shape, and morphology of the obtained powders and its agglomerates. Samples for TEM were prepared by dispersing them ultrasonically in ethyl alcohol. After dispersion a droplet was deposited on a copper grid supporting a perforated carbon film and allowed to dry. The specimen was examined under vacuum at room temperature. The UV−vis DRS measurements were performed over the wavelength range 200−800 nm, using a UV−vis NIR spectrometer (Cary 5000, Australia) with integration sphere diffuse reflectance attachment. Samples were diluted in a KBR matrix by pelletization. The reducibility of the catalysts was studied by temperature-programmed reduction. Approximately 0.03 g of the catalyst sample was placed in a quartz reactor. The sample was pretreated at 300 °C under a pure Argon atmosphere at a flow rate of 30 mL min−1 for 30 min. After cooling down to room temperature, the gas atmosphere was switched to 5 vol % H2/Ar, and the reactor was programmatically heated to 950 at 5 °C min−1. The consumption of the hydrogen was monitored by an in situ thermal conductivity detector (TCD) using a BELCAT apparatus. CO Oxidation. The catalytic activity of the samples was evaluated for oxidation of CO at normal atmospheric pressure and temperatures in the range of 25−500 °C in a fixed-bed reactor at a heating ramp of 5 °C min−1. About 100 mg of the sample was placed in a quartz reactor for evaluation. The reactor was heated in an electric furnace equipped with a Ktype thermocouple. The temperature of the catalyst bed was monitored and controlled by using a temperature controller (Model UT 150, Yokogawa, Japan). Prior to the oxidation of CO, the catalyst was heated to 500 °C in a 10% O2/He gas mixture, with a heating ramp of 10 °C min−1, and kept at the final temperature for 1 h. The oxidized sample was then purged

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EXPERIMENTAL SECTION Catalyst Preparation. The Ce0.65Zr0.25RE0.1O2−δ (RE = Pr, Tb, Sm, Gd, La, Nd, and Eu) powders were successfully prepared by GNP. In this method, aqueous precursor solutions containing metal nitrates and glycine were heated on a hot plate until they autoignite, producing metal oxide powder. Here, glycine plays two important roles: first, it complexes the metal cations and thereby prevents selective precipitation, and second, it is oxidized by nitrate anions, thereby serving as a fuel for combustion.25,26 Cerium(III) nitrate hexahydrate (Kanto Chemicals), zirconyl(IV) nitrate hydrate (Acros Organics; 99.5% Zr), and corresponding RE nitrates (Aldrich Chemicals) were used as sources of Ce, Zr, and RE, respectively. Glycine (Junsei Chemicals) was used as a combustion fuel. In typical GNP method, stoichiometric amounts of cerium(III) nitrate hexahydrate, zirconyl(IV) nitrate hydrate, and RE nitrates were taken to obtain Ce0.65Zr0.25RE0.1O2−δ solid solution. These chemical compounds were dissolved in distilled water and 0.55 mol of glycine was added for each mole of nitrate. The mixture was heated to evaporate the water present in the solution. Combustion was carried out in a large volume beaker on a hot plate. The produced powder from spontaneous ignition was collected from the reaction chamber. For comparison, Ce0.75Zr0.25O2−δ powder has also been synthesized as stated above. The obtained powders were ball-milled and calcined at 600 °C for 2 h in air atmosphere. The rates of heating and cooling were maintained at 5 °C min−1. 3468



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toward a lower degree with respect to the undoped sample. This phenomenon could be associated with the expansion of the crystal lattice, which is induced by the larger cation radius of the dopants relative to the Zr4+ (0.87 Å) ions. According to Vegard’s rule,31 a linear increase of the lattice constant is expected because of the linear increase of the cation radius of the dopants [from Tb (1.04 Å) to La (1.16 Å)]. Figure 1b shows the lattice constant of Ce0.65Zr0.25RE0.1O2−δ solid solutions as a function of ionic radius. As shown in Figure 1b, the increase of lattice constant is not so linear with the increase of the cation radius of dopants. This may be due to the factors like the existence of Pr and Tb in mixed valence states and change of Ce3+ concentration when doping with some RE elements.15,32,33 Table 1 shows the surface area, crystallite size,

with helium and cooled to the desired starting temperature. The gas flow rates were controlled by using mass flow controllers (Smart-trak: Sierra Instruments, USA) and the total flow rate was 60 mL min−1 with a CO/O2 reactant feed ratio of 1. The effluent gas mixture was detected by an online Agilent 6890N gas chromatograph equipped with HP PLOT Q and molecular sieve 5 Å capillary columns and a thermal conductivity detector (TCD). N2 was used as a tie balance. The conversion of CO to CO2 was calculated using the following equation:27−29

CO conversion (%) =

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[CO]in − [CO]out × 100 [CO]in

RESULTS AND DISCUSSION The XRD patterns of Ce0.65Zr0.25RE0.1O2−δ samples calcined at 600 °C are presented in Figure 1a. These results reveal the

Table 1. BET Surface Area (S), Crystallite Size from XRD (DXRD), Particle Size from BET (DBET), and Gain of Oxygen Vacancies (%) of Ce0.65Zr0.25RE0.1O2−δ Samples Calcined at 600 °C for 2 h sample Ce0.65Zr0.25RE0.1O2−δ

S (m2 g−1)

DXRD (nm)

DBET (nm)

gain of oxygen vacancies (%)a

Ce0.75Zr0.25O2−δ Ce0.65Zr0.25Tb0.1O2−δ Ce0.65Zr0.25Gd0.1O2−δ Ce0.65Zr0.25Eu0.1O2−δ Ce0.65Zr0.25Sm0.1O2−δ Ce0.65Zr0.25Nd0.1O2−δ Ce0.65Zr0.25Pr0.1O2−δ Ce0.65Zr0.25La0.1O2−δ

52 41 47 40 62 47 45 65

11.6 14.4 11.6 13.5 7.9 12.3 13.4 7.7

16.6 21.1 18.4 21.6 13.9 18.4 19.2 13.3

70.6 36.7 34.4 40.1 43.0 73.9 37.9

a

Gain of oxygen vacancies are calculated with respect to Ce0.75Zr0.25O2−δ from Raman spectra.

and particle size of the samples obtained from XRD and BET analysis. It can be observed from the table that the crystallite size and particle size are between 7 and 14 nm and 13−22 nm, respectively. Since the particle size is slightly greater than the crystallite size, it can be assumed that the samples are slightly agglomerated. The solid solution composition of the samples was confirmed from EDS analysis which is shown in Table 2. Figures 2 and 3 show the selected TEM and HR-TEM images of Ce0.75Zr0.25O2−δ and Ce0.65Zr0.25Pr0.1O2−δ samples, respectively. TEM image of Ce0.75Zr0.25O2−δ sample calcined at 600 °C is shown in Figure 2a. Analysis of these and other images (not shown) reveal that the crystallite size was in the range of 10−12 nm, confirming the result of XRD and its interplanar distance corresponding to 0.31 nm could be indexed to the (111) plane from the HR-TEM image in Figure 2b. TEM image of Ce0.65Zr0.25Pr0.1O2−δ sample in Figure 3a shows the particle size of the sample was around 10−14 nm, which is also well matched to the XRD result and its interplanar distances corresponding to 0.31 and 0.27 nm could be indexed to (111) and (200) planes, respectively, from the HR-TEM image in Figure 3b. Since the mobility of oxygen atoms in the crystal lattice is a critical property not only to the ionic/electronic conductors but also to redox catalysts, a deep insight into structural details of the crystal lattice is desirable. Raman spectroscopy is a good technique, sensitive to both M−O bond arrangement and lattice defects.34 Thus, it acts as a potential tool to obtain additional structural information apart from XRD data.35 The Raman spectra of the Ce0.65Zr0.25RE0.1O2−δ (RE = La, Nd, Sm, Eu, and Gd) samples were depicted in Figure 4a. As shown in

Figure 1. (a) XRD patterns of Ce0.65Zr0.25RE0.1O2−δ samples calcined at 600 °C for 2 h. (b) Lattice constant of the samples as a function of ionic radius of RE element.

formation of solid solutions with typical cubic fluorite structure.25,30 From Figure 1a it can be observed that all the RE-doped samples show a slight shift of the diffraction peaks 3469



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Table 2. Compositional Analysis of Ce0.65Zr0.25RE0.1O2−δ Powder by Using EDS Analysis sample Ce0.65Zr0.25RE0.1O2−δ Ce0.75Zr0.25O2−δ Ce0.65Zr0.25Tb0.1O2−δ

Ce0.65Zr0.25Gd0.1O2−δ

Ce0.65Zr0.25Eu0.1O2−δ

Ce0.65Zr0.25Sm0.1O2−δ

Ce0.65Zr0.25Nd0.1O2−δ

Ce0.65Zr0.25Pr0.1O2−δ

Ce0.65Zr0.25La0.1O2−δ

element

wt %

mol %

Ce Zr Ce Zr Tb Ce Zr Gd Ce Zr Eu Ce Zr Sm Ce Zr Nd Ce Zr Pr Ce Zr La

65.36 14.41 69.38 15.46 15.16 58.77 14.07 10.40 69.61 17.06 13.33 57.50 13.29 11.12 71.01 15.75 13.25 69.60 16.51 13.20 72.41 15.93 11.66

74.70 25.30 65.15 22.29 12.56 65.56 24.11 10.34 64.39 24.24 11.37 65.14 23.12 11.74 65.70 22.39 11.91 63.99 23.32 12.69 66.65 22.52 10.83

Figure 4a, a slight shift in the Raman frequency and absence of peaks related to RE oxides evidence the formation of Ce0.65Zr0.25RE0.1O2−δ solid solutions and thus corroborates the XRD results. Furthermore, the Raman spectra of the samples are very similar with a predominant strong band at ∼470 cm−1 and a less prominent broad band at ∼600 cm−1. The broad band at ∼600 cm−1 corresponds to the longitudinal optical (LO) mode of ceria,36 arising because of the relaxation of symmetry rules which can be linked to the oxygen vacancies in the ceria lattice.37 This is ascribed to a localized substitution defect vibration.21 The broad band at ∼600 cm−1 can be deconvoluted into two peaks with a lower peak at around ∼560 cm−1. In the present study, an excitation source of only 514.23 nm has been used and a broad peak at ∼600 cm−1 observed. The deconvoluted peaks can be easily observed with a higher excitation source of 633 and 785 nm.38 The peak at 570 cm−1 could be assigned to the oxygen vacancies introduced into the ceria to maintain the charge neutrality when Ce4+ ions are replaced with trivalent cations. Besides, the 600 cm−1 band is ascribed to the intrinsic oxygen vacancies because of the presence of Ce3+ in the solid solutions.38 The band at 470 cm−1 can be attributed to the F2g Raman active mode of the fluoritetype lattice. It can be viewed as a symmetric breathing mode of the oxygen atoms around cerium ions (O−Ce−O).39According to the literature, six Raman active modes (A1g + 3Eg + 2B1g) are expected for t-ZrO2 (space group P42/nmc) while for the cubic fluorite structure of ceria (space group Fm3m) only one mode is Raman active.21 It can be observed from the figure that with the increase of ionic radius of RE element in Ce0.65Zr0.25RE0.1O2−δ solid solution, the F2g band has been slightly shifted to lower values. The shift in the F2g mode is attributed to change in M−O vibration frequency after incorporation with the dopants which account for the difference in the ionic radius.33,40,41 As envisaged earlier, the difference in the ionic radius of the dopants enables changes in

Figure 2. (a) TEM and (b) HR-TEM micrographs of Ce0.75Zr0.25O2−δ sample calcined at 600 °C.

the lattice parameter owing to cell contraction or expansion. Hence, vibrations are rapid for contracted lattice and slow down for expanded lattice so that band shifts to higher and lower wave numbers, respectively. Figure 4b shows the Raman spectra of Ce0.65Zr0.25RE0.1O2−δ (RE = Tb, Pr) samples. As can be noted from Figure 4b, the band at ∼465 cm−1 becomes weaker and a broad stronger band at 590 cm−1 has been observed compared to Figure 4a. Pr and Tb doped solid solutions show quite intense absorption in the visible region and the laser line (in the present study, 514.23 nm) cannot penetrate into the deep layers of the sample, which makes the band at 465 cm−1 weaker and the band at 590 cm−1 stronger, which is the combination of both types of oxygen vacancies.38 It is well-known that the RE ions, including Ce, 3470



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Figure 4. Raman spectra of Ce0.65Zr0.25RE0.1O2−δ samples: (a) RE = La, Nd, Sm, Eu, and Gd; (b) RE = Pr, Tb. Inserted figure shows the variations in the intensity ratio of Raman bands at 600 and 465 cm−1.

about 3−4 times because of the generation of vacancies for charge compensation as a result of the presence of Pr4+/Pr3+ and Tb4+/Tb3+ atoms. For Ce0.65Zr0.25RE0.1O2−δ samples, the gain of oxygen vacancies (%) were calculated with respect to Ce0.75Zr0.25 O2−δ sample and the data is presented in Table 1. It can be noted from this table that over 70% of gain of oxygen vacancies were obtained for Pr and Tb doped ceria-zirconia samples and for the rest of the samples it was only 30−45%, which demonstrated that Pr and Tb doped ceria-zirconia samples had higher oxygen vacancies than other doped samples. The UV−vis DRS has been used extensively to study dopedceria materials and transition metal oxides to obtain information on the surface coordination and different oxidation states of the metal ions by measuring d-d, f-d transitions and oxygen−metal ion charge-transfer bands.44,45 Pure ceria normally exhibits three absorption maxima centered at 255, 285, and 340 nm in its DR spectra. The former two bands are ascribed to O2− → Ce3+ and O2− → Ce4+ charge-transfer transitions, respectively, and the latter are assigned to interband transitions.46 The Ce0.65Zr0.25RE0.1O2−δ samples calcined at 600 °C are investigated by DRS technique and the spectra are

Figure 3. (a) TEM and (b) HR-TEM micrographs of Ce0.65Zr0.25Pr0.1O2−δ sample calcined at 600 °C.

easily form a series of nonstoichiometric oxides of the type REOx (1.5 ≤ x ≤ 2). However, neither Pr nor Tb tends to crystallize in the dioxide or sesquioxide phases; they would rather form compounds with Pr6O11 and Tb4O7.42 Thus, it can be expected that Pr or Tb doped samples should increase the oxygen vacancy concentration, which has been observed in the present study. The ratio between the intensities of 600 and 470 cm−1 Raman bands has been related to the concentration of oxygen vacancies in the material, and it is noted that the higher the I600/I470 ratio, the higher the oxygen vacancies.43 The relationship between I600/I470 ratio and RE doping is presented in Figure 4 b (inserted figure). It can be observed from this figure that the incorporation of Pr and Tb into corresponding ceria-zirconia lattice increases the vacancy concentration by 3471



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that the bands corresponding to charge-transfer transitions are shifted to lower wavelength and the interband transition is shifted to higher wavelength. The shift in the interband transition could be due to the presence of Zr4+ ions in the system, which provokes a significant increase in the Ce3+ fraction on the surface, thereby increasing the charge-transfer gap between the O 2p and Ce 4f orbital.47 The shift in the charge-transfer transitions could be due to the result of decreasing particle size of ceria (quantum size effect) when zirconium is incorporated into the ceria lattice.14,44 The DRS spectra of the Ce0.65Zr0.25RE0.1O2−δ (RE = La, Sm, Eu, and Gd) samples were depicted in Figure 5b. From the figure it can be manifested that these samples also showed trend similar to that of Ce0.75Zr0.25O2−δ sample. For Ce0.65Zr0.25Nd0.1O2 sample, additionally two more peaks at 590 and 745 nm are shown because of the presence of Nd in the sample.48 It is quite interesting to see the DRS spectra of the Ce0.65Zr0.25RE0.1O2−δ (RE = Tb, Pr) samples that are shown in Figure 5c. Apart from the three bands that were present in the rest of the samples, one more intense absorption band in the visible region (400− 650 nm) has been observed for Ce0.65Zr0.25Pr0.1O2−δ and Ce0.65Zr0.25Tb0.1O2−δ samples. The presence of this absorption band is associated with Pr3+ and Tb3+ ion transitions in their corresponding samples.33,43 It can be observed from the figure that Ce0.65Zr0.25Pr0.1O2−δ sample showed that the interband transition peak moved to higher wavelength (380 nm) than all other samples. This indicates that the Pr3+ ions in the system provoked a significant increase in the Ce3+ fraction on the surface, thereby increasing the charge-transfer gap between O 2p and Ce 4f orbitals compared to all other samples.33 There is no evidence for the presence of different phases of RE oxides from DRS study in agreement with XRD and Raman measurements. Further, to understand the chemical states of Ce, Pr, and Tb ions in Ce0.65Zr0.25RE0.1O2−δ (RE = Pr, Tb), samples calcined at 600 °C were examined by the XPS technique. Figure 6 depicts the XPS spectra of the samples at Ce 3d core level. The Ce 3d

presented in Figure 5. The spectra, in general, are broad and exhibited poorly resolved peaks. Figure 5a shows the DRS spectrum of Ce0.75Zr0.25O2−δ sample where it can be observed

Figure 6. Ce 3d XPS patterns of Ce0.75Zr0.25O2−δ, Ce0.65Zr0.25Tb0.1O2−δ, and Ce0.65Zr0.25Pr0.1O2−δ samples calcined at 600 °C.

Figure 5. UV−vis DR spectra of (a) Ce 0.75 Zr 0.5 O 2−δ , (b) Ce0.65Zr0.25RE0.1O2−δ (RE = La, Nd, Sm, Eu, and Gd), and (c) Ce0.65Zr0.25RE0.1O2−δ (RE = Pr, Tb) samples calcined at 600 °C.

core level peak of ceria is known to be complicated by the hybridization of the O 2p valence band with the Ce 4f level.49,50 3472



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This includes several final states for the Ce emission, which are seen in the spectra. In the case of two possible cerium oxidation states (+3 and +4) as many as 10 different features could be found in the Ce 3d region. Peaks labeled as “v” correspond to Ce 3d5/2 contributions and those labeled as “u” represent the Ce 3d3/2 contributions. The bands uo and u are the main Ce 3d3/2 lines and the bands vo and v are the main Ce 3d5/2 lines of Ce3+ and Ce4+, respectively. The band labeled as v′ is a satellite to the Ce3+ 3d5/2 main line vo, whereas v″ and v‴ are related to Ce4+ (main line v). Analogous assignments are valid for the corresponding “u” features. The relative intensity of the u‴ feature, which is well separated from the remaining signals, is often used to assess the reduction degree of the Ce ions in the surface region. Table 3 shows the lower binding energy corresponding to surface lattice oxygen of Ce0.65Zr0.25Pr0.1O2−δ Table 3. O 1s Binding Energy of Ce0.65Zr0.25Tb0.1O2−δ and Ce0.65Zr0.25Pr0.1O2−δ Samples sample

O 1s binding energy/eV

Ce0.65Zr0.25Tb0.1O2−δ Ce0.65Zr0.25Pr0.1O2−δ

530.10 530.35

and Ce0.65Zr0.25Tb0.1O2−δ samples. It can be noticed that this binding energy is slightly shifted to higher binding energy for Ce 0.65 Zr 0.25 Pr0.1 O 2−δ sample, indicating its greater Ce3+ component than Ce0.65Zr0.25Tb0.1O2−δ sample.51 Figure 7a illustrates Pr 3d core level XPS spectra of Ce0.65Zr0.25Pr0.25O2−δ sample. Generally, the Pr 3d spectra consist of two sets of spin−orbit multiplets at binding energies of ∼933 and ∼953 eV, which correspond to 3d5/2 and 3d3/2, respectively.52 The 3d3/2 sublevel presents complex features due to the multiplet effect, while the 3d5/2 sublevel consists of two features corresponding to two possible oxidation states (Pr3+ and Pr4+).53 Therefore, we have taken the Pr 3d5/2 region to understand the oxidation states of Pr. The spectra obtained show a pronounced shoulder at 929.7 eV and a maximum at 934.5 eV. According to the literature,43 we assigned the former to Pr3+ and the latter to Pr4+. This indicates that the prepared sample contains both +3 and +4 oxidation states. Figure 7b shows the Tb 4d core level XPS spectra of Ce0.65Zr0.25Tb0.1O2−δ sample. In general, Tb3+ gives a signal below 150 eV, whereas Tb4+ is related to the features above 150 eV (may be around 155 eV).54 The spectra obtained in this study with a pronounced shoulder below 150 eV, a peak at 151.7 eV, and a maximum peak at 154.6 eV along with a small peak at 160.6 eV suggests the presence of more than one oxidation state. This indicates that the surface of the sample contains Tb in both +3 and +4 oxidation states. XPS analysis confirms the presence of both +3 and +4 oxidation states of Tb and Pr in their respective oxide samples, which further corroborates the gain of oxygen vacancies (Raman analysis) for these samples compared to other samples. The reducibility of Ce0.65Zr0.25RE0.1O2−δ samples calcined at 600 °C was studied by using H2-TPR experiments and the corresponding TPR profiles are shown in Figure 8. The Ce0.75Zr0.25O2−δ sample exhibits a very broad reduction peak at 570 °C along with a shoulder peak in a lower temperature region (350−500 °C). It is hard to differentiate these two reduction peaks which resemble a single reduction peak. It is observed that the onset of H2 consumption occurs around 320 °C. According to the literature, the H2-TPR profile of pure high surface area CeO2 shows two well-resolved peaks at around 500

Figure 7. (a) Pr 3d XPS patterns of Ce0.65Zr0.25Pr0.1O2−δ sample and (b) Tb 4d XPS patterns of Ce0.65Zr0.25Tb0.1O2−δ sample calcined at 600 °C.

and 900 °C.55 Reduction of ceria is generally accepted to occur by a stepwise mechanism; first, at lower temperatures, reduction of most outer layers of Ce4+ (surface reduction) occurs and then reduction of inner Ce4+ layers (bulk reduction) at higher temperatures.2,56,57 Incorporation of small quantities of isovalent Zr4+ into CeO2 lattice enhances its redox properties by creating structural defects with size effect and increasing the channel diameter for oxygen migration in the lattice.2 This results in higher mobility of the lattice oxygen, which leads to one major low-temperature peak in the TPR profile of ceriazirconia solid solutions in contrast to ceria with two main peaks.58 The higher mobility of the lattice oxygen causes 3473



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indicating a higher mobility of surface oxygen at lower temperatures. We focused on the CO oxidation reaction, which is most likely to be affected by the enhanced reducibility of the oxide solid solution. The catalytic activities for CO oxidation of Ce0.65Zr0.25RE0.1O2−δ samples calcined at 600 °C are presented in Figure 9. It is evident from Figure 9 that the

Figure 8. H2-TPR profiles of Ce0.65Zr0.25RE0.1O2−δ samples calcined at 600 °C.

reduction not to be limited to the surface but deeply extended into the bulk of ceria-zirconia solid solution. This is in line with the shift of the major reduction peak of pure ceria at 900 to 570 °C for Ce0.75Zr0.25O2−δ sample. For Ce0.65Zr0.25RE0.1O2−δ samples, it can be observed from Figure 8 that the bulk reduction peak was around 550−570 °C. The H2-TPR profiles of Ce0.65Zr0.25RE0.1O2−δ (RE = Gd, Eu, Sm, Nd, and La) samples also showed a shoulder peak in the lower temperature region (350−500 °C) similar to Ce0.75Zr0.25O2−δ sample corresponding to surface reduction of the samples. It is quite interesting to see that the Ce0.65Zr0.25Pr0.1O2−δ sample showed two differentiable peaks instead of a very broad reduction peak which was observed for other Ce0.65Zr0.25RE0.1O2−δ samples. For this sample, the onset of H2 consumption occurred at 180 °C and the surface reduction peak was located around 375 °C which is a much lower temperature than other samples. The peak area of surface reduction is much higher than the peak area of bulk reduction, indicating that reducibility of this sample is much higher and can be reduced at lower temperatures than the other samples. Moreover, addition of Pr induces ordered arrangements of vacancies, thereby creating a pathway for oxygen diffusion. This would promote oxygen ion diffusion from the bulk to surface.59 From the figure it can be observed that the Ce0.65Zr0.25Tb0.1O2−δ sample also showed two reduction peaks corresponding to surface and bulk reduction peaks. It is interesting to see that the surface reduction peak splits into multiple peaks. From the literature it can be observed that the TPR profile of TbO1.75 shows reduction bands at 303, 667, and 720 °C.60 For this sample, the first two peaks at 400 and 490 °C can be attributed to the reduction of both Tb4+ and Ce4+ to Tb3+ and Ce3+, respectively.15 The peak at 560 °C could be attributed to the bulk reduction of the sample. The reducibility of Ce0.65Zr0.25RE0.1O2−δ samples clearly shows that the Pr doped CZO can be easily reduced at lower temperature,

Figure 9. Conversion of CO over Ce0.65Zr0.25RE0.1O2−δ samples calcined at 600 °C as a function of reaction temperature.

Ce0.65Zr0.25Pr0.1O2−δ sample exhibited better activity in terms of total conversion as well as light-off temperature (50% conversion). The Ce0.65Zr0.25Pr0.1O2−δ sample exhibited 100% conversion at 300 °C and light-off temperature of 208 °C, respectively. The light-off temperatures of various catalysts followed the order Pr > Tb > La > Nd > Sm > Gd > Eu > Zr. The oxidation of CO on ceria occurs via Mars-van Krevelen redox type mechanism.61 In the absence of the oxygen feed, CO gets oxidized by consuming lattice oxygen and leaving the oxygen vacancy. In the presence of oxygen feed, the lattice oxygen is replenished.62 During the oxidation process, lattice oxygen, conversely an oxygen vacancy is involved and acts as an active site for the dissociation of gaseous oxygen. Accordingly, in the oxidation process vacancy plays an essential role. The lower reduction temperature and an increased ability to shift between Ce4+/ Ce3+ at much lower temperature and having high oxygen vacancies are the key for the increased oxidation activity of the Ce0.65Zr0.25Pr0.1O2−δ sample. Development of these materials (Ce0.65Zr0.25RE0.1O2−δ (RE= Pr, Tb, and La)) for internal reforming of Solid Oxide Fuel Cells (SOFCs) as anode supports would be the topic of a forthcoming paper.

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CONCLUSIONS Ce0.65Zr0.25RE0.1O2−δ (RE = Tb, Gd, Eu, Sm, Nd, Pr, and La) solid solutions were successfully prepared by the glycine-nitrate process and the structural characteristics and CO oxidation activity of these solid solutions have been systematically investigated. The XRD results confirmed the formation of Ce0.65Zr0.25RE0.1O2−δ solid solutions with a cubic phase of fluorite structure. XPS results revealed the existence of cerium, terbium, and praseodymium in both +3 and +4 chemical valence states. Raman spectroscopy results showed the remarkable increase of oxygen vacancies for 3474



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Ce 0.6 5 Zr 0.2 5 Pr 0 .1 O 2− δ solid solution. From H 2 -TPR, Ce0.65Zr0.25Pr0.1O2−δ sample showed enhanced surface reduction at lower temperatures, indicating a high mobility of oxygen ions in this sample. This can be attributed to the simultaneous presence of enhanced mobile oxygen vacancies, easy surface and bulk reduction, and the cooperative redox couple (+3/+4) of Ce and Pr. The Ce0.65Zr0.25Pr0.1O2−δ solid solution also exhibits superior activity toward CO oxidation compared to other solid solutions. The ability to release a substantial amount of oxygen at relatively low temperatures makes Ce0.65Zr0.25Pr0.1O2−δ solid solution a potential material for oxygen storage/release and catalytic applications.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 82-2958-5532. Fax: 82-2958-5529. E-mail: [email protected].

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ACKNOWLEDGMENTS This work was supported by the Fundamental R&D Program for Core Technology of Materials funded by the Ministry of Knowledge Economy, Republic of Korea, and Institutional Research Program of Korea Institute of Science and Technology (KIST) (2E22132). One of the authors (D.H.P.) acknowledges KIST for the award of a STAR Post-Doc Fellowship.

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Structural Characterization and Catalytic Activity of Ce0.65Zr0.25RE0

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