Influence of Alumina, Silica, and Titania Supports on the Structure and

Jun 22, 2007 - ... Chemistry DiVision, Indian Institute of Chemical Technology, Hyderabad 500 007, ... Brunauer-Emmett-Teller (BET) surface area techn...
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10478

J. Phys. Chem. C 2007, 111, 10478-10483

Influence of Alumina, Silica, and Titania Supports on the Structure and CO Oxidation Activity of CexZr1-xO2 Nanocomposite Oxides Benjaram M. Reddy,*,† Pandian Lakshmanan,† Pankaj Bharali,† Pranjal Saikia,† Gode Thrimurthulu,† Martin Muhler,‡ and Wolfgang Gru1 nert‡ Inorganic and Physical Chemistry DiVision, Indian Institute of Chemical Technology, Hyderabad 500 007, India, and Lehrstuhl fu¨r Technische Chemie, Ruhr-UniVersita¨t Bochum, D-44780 Bochum, Germany ReceiVed: February 22, 2007; In Final Form: May 10, 2007

Nanocomposite oxides of CexZr1-xO2 dispersed over alumina (Al2O3), silica (SiO2), and titania (TiO2) have been synthesized by a deposition coprecipitation method. The physicochemical characterization was carried out by X-ray diffraction (XRD), Raman spectroscopy, high-resolution electron microscopy (HREM), and Brunauer-Emmett-Teller (BET) surface area techniques. The catalytic efficiency toward CO oxidation was investigated under normal atmospheric pressure. Oxygen storage capacity (OSC) of the dispersed nano-oxides was also evaluated and correlated with the structural characterization and CO oxidation activity results. XRD measurements reveal the presence of dispersed cubic Ce0.75Zr0.25O2 nano-oxide phase over all the supports and the absence of unwanted inert compounds such as Ce9.33(SiO4)6O2, ZrSiO4, ZrTiO4, CeAlO3, and Ce-Ti oxides. Raman spectroscopy studies suggest formation of oxygen vacancies, lattice defects, and oxygen ions displacement from the ideal ceria cubic lattice positions. The HREM results indicate well-dispersed cubic Ce-Zr composite oxides of the size ∼5 nm over the surface of various supports. The CO oxidation activity results and the OSC measurements reveal the same order of efficiency: that Al2O3-supported CexZr1-xO2 shows better performance, followed by TiO2- and SiO2- supported systems. The OSC properties of the supported CexZr1-xO2 oxides show a strong influence on the CO oxidation activity.

1. Introduction Nanocomposite metal oxides have been widely investigated in various branches of chemistry, physics, and materials science in recent years.1 Ceria (CeO2) is one of the technologically most important metal oxides since its oxygen vacancy defects can be rapidly generated and eliminated, facilitating it as a high oxygen storage capacity (OSC) material. In the last two decades it has been established that partial incorporation of Zr4+ into CeO2 lattice forms CexZr1-xO2 composite oxides, which show enhanced structural/textural features and redox properties with improved thermal stability at elevated temperatures.2,3 Hence, CexZr1-xO2 composite oxides gradually replaced the pure CeO2 as the OSC promoters in three-way catalysts (TWCs) to reduce the emissions of noxious pollutants such as CO, NOx, and hydrocarbons from automobile exhausts. Besides TWC applications, the CexZr1-xO2 composite oxides are also effective catalysts for selective synthesis of dimethylcarbonate from methanol and CO2, direct conversion of methane to synthesis gas, isosynthesis, dehydration, and CO oxidation.4-8 These composite oxides have also been profitably employed as catalyst supports for noble metals and transition metal oxides and for designing ternary solid solutions by doping small amounts of other transition or rare earth metals to the parent CexZr1-xO2 oxides. These catalysts are applied to different industrially important reactions including steam reforming of ethanol, methane combustion, preferential oxidation of CO, and low-temperature water-gas shift due to their high OSC and coke resistance nature.9-14 * Corresponding author. E-mail: [email protected]; mreddyb@ yahoo.com † Indian Institute of Chemical Technology. ‡ Ruhr-Universita ¨ t Bochum.

The catalytic performance of CexZr1-xO2 composite oxides is strongly influenced by the crystal structure, balance between structural defects and Ce content, the degree of reducibility, and the mobility of oxygen in the bulk.15,16 Despite all these implications, inadequate textural/structural stability and mechanical strength and fall of specific surface area at high temperatures are some of the problems encountered in the case of unsupported ceria-zirconia solid solutions as many applications require high temperatures.17-19 Stabilization of the CexZr1-xO2 nanocomposites on an inert support to form stable and active catalysts could represent a suitable way to overcome these drawbacks. It is a known fact that in the absence of diffusion constraints, the rate of a catalytic reaction is proportional to the surface area of the active phase. For a particular mass of material, to maximize this characteristic behavior it is necessary to make the particles as small as possible, which eventually leads to high dispersion of the active phase. The essential requirements of a better support are nonreactivity with the dispersed phase and high specific surface area. Therefore, selection of a suitable support is an important factor in the design of active catalysts for various reactions. Alumina, silica, and titania are the bestknown supports employed in various catalyst formulations. To the best of our knowledge, only limited reports have been published on Al2O3-supported CexZr1-xO2 composite oxides.17,20,21 Very recently, we have shown that SiO2 and TiO2 exhibit a strong influence on the thermal stability and redox behavior of CexZr1-xO2 nanocomposite oxides.22,23 The CexZr1-xO2/SiO2 combination catalyst was also found to display excellent catalytic activity for dehydration of 4-methylpentan2-ol.24 Furthermore, vanadium oxide-impregnated CexZr1-xO2/ SiO2 nanocomposites also exhibited high and stable activity

10.1021/jp071485h CCC: $37.00 © 2007 American Chemical Society Published on Web 06/22/2007

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toward oxidative dehydrogenation of ethylbenzene to styrene.25 The present investigation was undertaken against the above background in view of the commercial significance of these materials. In this study the influence of various supports, namely, Al2O3, SiO2, and TiO2, on the thermal stability, OSC, and CO oxidation activity of CexZr1-xO2 has been investigated systematically. The supported CexZr1-xO2 nanocomposites have been synthesized by a deposition coprecipitation method and characterized by X-ray diffraction (XRD), Raman spectroscopy, high-resolution electron microscopy (HREM), and BrunauerEmmett-Teller (BET) surface area techniques and subjected to evaluation for CO oxidation activity and OSC properties. 2. Experimental Section Catalyst Preparation. Al2O3-, SiO2-, and TiO2-supported CexZr1-xO2 (support oxide:CeO2:ZrO2 ) 2:1:1 in moles) were prepared by a deposition coprecipitation method with dilute aqueous NH3 solution. The precursors used were (NH4)2Ce(NO3)6 (Loba Chemie, GR grade), Zr(NO3)4‚5H2O (Fluka, AR grade), γ-Al2O3 powder (Harshaw, SA 127 m2 g-1), colloidal SiO2 (Ludox, 40 wt %, Aldrich, AR grade), and TiO2 anatase powder (EU consortium, SA 49 m2g-1). In a typical procedure to prepare the composite oxides, requisite quantities of Ce and Zr precursors were dissolved separately in deionized water and mixed together. The supporting oxides, that is, γ-Al2O3, colloidal SiO2, or TiO2 anatase powders, were first dispersed separately in about 3000 mL of deionized water and stirred for 2 h before the addition of Ce-Zr mixture solutions to it. Then the whole mixture solution was diluted to 4000 mL with deionized water and stirred for another 1 h. Aqueous NH3 solution was added dropwise to the mixture solution until pH ∼ 8.5, under vigorous stirring. The obtained precipitates were filtered off, washed with deionized water, oven-dried at 383 K for 12 h, and subsequently calcined at 773 K for 5 h in air atmosphere. Hereafter the composite oxides will be referred as CZ/A, CZ/S, and CZ/T (where C, Z, A, S, and T stand for Ce, Zr, Al, Si, and Ti, respectively). Catalyst Characterization. The X-ray powder diffraction (XRD) patterns were acquired with a Siemens D-5005 diffractometer by use of Ni-filtered Cu KR radiation. Crystalline phases were identified by matching the experimental patterns with the PDF-ICDD. The mean crystallite size (DXRD) was measured by applying the Scherrer equation. The cell a parameter was calculated by a standard cubic indexation method with the intensity of the base peak (111). The Raman spectra were obtained at ambient and moisture-free conditions with a Dilor XY spectrometer equipped with a charge-coupled device (CCD) detector. The emission line at 514.5 nm from an Ar+ ion laser (Spectra Physics) was focused on the sample under the microscope, the analyzed spot being ∼1 µm. The BET surface area measurements were made on a Gemini 2360 instrument. Prior to analysis, samples were degassed at 393 K under vacuum for 12 h to remove any residual moisture and other volatiles. High-resolution electron microscopy investigations were performed on a Philips CM200 electron microscope with 0.23 nm point-to-point resolution. The samples were supported on holey carbon grids by dropping ethanol suspensions containing uniformly dispersed oxide powders. CO Oxidation. The catalytic activity of the synthesized nanocomposite oxides was evaluated for oxidation of CO at normal atmospheric pressure and temperatures in the range of 300-773 K in a fixed-bed microreactor at a heating ramp of 5 K min-1. About 100 mg of catalyst sample (250-355 µm sieve fraction) diluted with quartz particles of the same sieve

Figure 1. Powder XRD patterns of CZ/A, CZ/S, and CZ/T samples. (*) Ce0.75Zr0.25O2; (Τ) TiO2 anatase.

fraction was placed in a quartz reactor for evaluation. Temperature was measured directly at the catalyst bed, by use of a thermocouple placed in the hollow shaft of the reactor. The following gases and gas mixtures were used (supplied by Air Liquide): argon (>99.999% purity), 9.98% CO in argon (CO purity, >99.997%; argon purity, >99.99%), and 10.2% O2 in argon (oxygen purity, >99.995%). The total flow rates maintained by three mass flow controllers were in the range of 5060 N mL min-1 (milliliters normalized to 273.15 K and 1 atm). The CO and CO2 gas concentrations were measured with an Uras 14 infrared analyzer module, and the O2 concentration was measured with a Magnos 16 analyzer (Hartmann & Braun). Prior to oxidation of CO, the catalyst was heated to 773 K in 10.2% O2/Ar gas mixture, with a heating ramp of 10 K min-1, and kept at the final temperature for 1 h. The oxidized sample was then purged in argon and cooled to the desired starting temperature. The CO/O2 reactant feed ratio was 1, and partial pressures of CO and O2 were in the range of 10 mbar. OSC Measurements. The OSC of the catalysts was determined by thermogravimetry (TG) under cyclic thermal treatments. This technique of OSC evaluation is essentially similar to that reported earlier in the literature.26,27 Oxygen release characteristic was used to evaluate the OSC of the catalysts in the temperature range 573-1073 K. The weight change during the cyclic heat treatments was monitored under flowing argon or dry air. For this purpose, a commercial Netzsch TG-DTA analyzer (Luxx, STA, 409 PC, Germany) was employed. The cyclic thermal treatments consisted of heating the sample from room temperature to 1073 K, cooling to 423 K, and again heating to 1073 K without any interruption. All heating and cooling rates were 5 K min-1. The weight loss of sample during the second heating cycle was used to measure the oxygen release property. 3. Results and Discussion The XRD profiles of all the samples calcined at 773 K are shown in Figure 1. The lines are broad due to smaller crystallite size. The XRD analysis revealed the presence of ceria-zirconia solid solutions with the composition Ce0.75Zr0.25O2 (PDF-ICDD 28-0271) in all cases. Despite apparent simplicity of the CexZr1-xO2 system, both phase diagram and structural properties of CexZr1-xO2 composite oxides are still a matter of investigation. Owing to the relatively large difference (13%) between the cation radii of Ce4+ (0.097 nm) and Zr4+ (0.084 nm), a limited mutual solubility is expected and a cubic ceria-rich Ce0.75Zr0.25O2 phase readily forms due to high thermodynamic

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TABLE 1: BET Surface Area, Average Crystallite Size, and a Cell Parameter Measurements of Cubic CexZr1-xO2 in CZ/A, CZ/S, and CZ/T Samples sample

surface area (m2g-1)

crystallite sizea (nm)

cell parameter (Å)

CZ/A CZ/S CZ/T

146 172 105

4.0 3.1 3.7

5.29 5.35 5.28

a

From XRD measurements.

stability.28 The XRD peaks due to TiO2 anatase in CZ/T sample are clearly noticed, whereas no diffraction patterns corresponding to Al2O3 and SiO2 in CZ/A and CZ/S samples, respectively, are observed. This indicates the amorphous nature of these supports. The nonappearance of characteristic diffraction lines between 20° and 30° and a sharp peak at 31° clearly indicate the absence of Ce9.33(SiO4)6O2, a possible inert compound between the oxides of ceria and silica.29 Similarly, intense peaks at 2θ value of 20° and 27° pertaining to ZrSiO4 are also absent.30 In the case of Al2O3-supported samples, the formation of CeAlO3 is also not observed. XRD patterns of CZ/T sample also reveal no peaks corresponding to ZrTiO4 or any other possible compound between ceria and titania such as Ce2TiO5, Ce2Ti2O7, or Ce4Ti9O24.31 It could be inferred from XRD results that the deposition coprecipitation adopted in the present study may be a good method to obtain highly dispersed CexZr1-xO2 oxides over the supports without the formation of inert compounds. The N2 BET surface areas of various samples and the average crystallite size (DXRD) of Ce0.75Zr0.25O2 on Al2O3, SiO2, and TiO2 supports are presented in Table 1. The synthetic methodology of the CexZr1-xO2 composite oxides applied and the nature of supporting oxides employed strongly influence the physicochemical characteristics of the resulting materials. As can be noted from Table 1, colloidal silica shows the highest surface area among the three samples, followed by Al2O3 and TiO2 counterparts. The measured average crystallite sizes obtained by employing the Scherrer equation reveal that they are in the nano range with similar sizes irrespective of the support employed. However, it is worth mentioning here that the particle size estimate of Ce-Zr oxides is subject to uncertainties due to compositional nonuniformity. As reported earlier, the facile formation of thermodynamically more stable phases between ceria and zirconia retards crystallite growth.32 Among the three supports, the colloidal SiO2 acts as the best carrier to stabilize the smaller Ce-Zr oxides over its surface and inhibits the coagulation or sintering of the particles. The most intense line (111) of the Ce0.75Zr0.25O2 pattern is used to calculate the cell parameter values with the help of the cubic indexation method.33 The values obtained (Table 1) imply that Zr4+, having a smaller ionic radius, would result in a reduction of the lattice parameter as compared to pure ceria (5.4 Å) when Ce4+ ions are substituted by Zr4+ ions in all cases. The Raman spectra of CexZr1-xO2 composite oxides on various supports are shown in Figure 2. Raman spectroscopy is widely used to investigate the oxygen lattice vibration in fluorite-type oxides.34 The strong band observed at 466 cm-1 could be attributed to the F2g Raman-active mode of the fluoritetype lattice, which can be viewed as a symmetric breathing mode of the oxygen atoms around cerium ions.35 Noticeably, the Raman band in the case of CZ/S sample is broader compared to others. The intensity of the Raman band depends on several factors including grain size and morphology.36 The broadness in the case of CZ/S sample may be related to the presence of SiO2, as the broadness reflects the phonon density of states, which is a characteristic of Raman scattering of amorphous

Figure 2. Raman spectra of CZ/A, CZ/S, and CZ/T samples. The bands at 623, 466, 310, and 270 cm-1 are due to ceria-zirconia solid solutions and the bands at 638, 514, 397, and 196 cm-1 are pertaining to TiO2 anatase.

materials.36,37 In the case of CZ/T sample, CeO2 features (466 and 310 cm-1) are dominated by TiO2 anatase bands (196, 397, 514, and 638 cm-1). The absence of inert compound formation such as Ce9.33(SiO4)6O2 (strong bands at 446 and 522 cm-1), and ZrTiO4 (bands at 280, 338, and 412 cm-1) is clearly evident, and the formation of rutile phase was also not observed.29 Although, the anatase to rutile phase transformation is expected in impurity-free samples beyond 773 K, the additives such as silica and ceria are known to inhibit such transformation by stabilizing the anatase phase.38 Sintering of samples under hightemperature calcination leads to the formation of oxygen vacancies, which perturb the local M-O bond symmetry, leading to the relaxation of symmetry selection rules.37 The presence of a weak and less prominent broad band at ∼620 cm-1 could be attributed to a nondegenerate Ramaninactive longitudinal optical (LO) mode of ceria, which arises due to relaxation of symmetry rules as stated above.35,37 In particular, the substitution of zirconium into the ceria lattice gives rise to oxygen vacancies, which are responsible for the emergence of this band.39 The appearance of a very weak band at ∼310 cm-1 could be related to displacement of the oxygen atoms from their ideal fluorite lattice positions.40 In order to substantiate the observations made from XRD and Raman measurements, TEM-HREM analyses were undertaken and the representative micrographs are shown in Figures 3 and 4. As can be noted from these figures, the crystallite size of CexZr1-xO2 composites on various supports are observed to be in the range of 5-8 nm. Figure 3a shows the TEM global picture of the CZ/A sample along with the corresponding selected area diffraction pattern (SAED). A closer inspection of this image and other images (not shown) reveals the existence of smaller crystals (∼5 nm) dispersed over an amorphous matrix with different lighter contrasts. The slight broadening of the rings in the electron diffraction pattern accounts for the presence of such small randomly oriented mixed oxide particles. As presented in Figure 3b, the HREM image of CZ/A sample reveals different spots, which account for the existence of periodic contrasts in the original experimental micrograph, which correspond to different sets of atomic planes of the crystalline structure. The geometrical arrangement of these reflections is directly related to the structural aspects of the analyzed crystals for which digital diffraction patterns are very useful for phase recognition.41 The obtained DDPs at two different nearby locations of the sample within 15 nm range

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Figure 5. Conversion of CO over CZ/A, CZ/S, and CZ/T samples as a function of reaction temperature.

Figure 3. (a) TEM and (b) HREM images of CZ/A sample.

Figure 4. (a) TEM image of CZ/S and (b) HREM image of CZ/T sample.

reveals the presence of d-spacings of 3.1 and 2.6 Å, which correspond to (111) and (002) planes of a cubic Ce-Zr oxide

phase with the fluorite structure.42 Figure 4a shows a TEM image of the CZ/S sample. Well-dispersed, very small, and randomly oriented Ce-Zr oxide particles over the surface of the amorphous SiO2 are mainly observed from both the image and SAED pattern (inset). A careful inspection of the image reveals the presence of nanodomains (crystallites size below 5 nm) with nonuniformity in the structure. Such features are absent in the case of CZ/A and CZ/T samples, indicating a relatively disordered crystal structure of CexZr1-xO2 dispersed on SiO2. The presence of Ce-Zr oxide of size ∼5 nm could be detected in the HREM image of the CZ/T sample (Figure 4b). The d-spacing values of 4.8 and 3.6 Å measured on the big rounded particle in the figure can be respectively assigned to (002) and (101) family planes of anatase TiO2, and the geometry of the whole set of spots in the inset corresponds to the [010] zone axis of this phase. The existence of a small CexZr1-xO2 particle supported on top of this TiO2 crystal has been noted as shown by black arrows in the image. These images reveal that the cubic fluorite-type crystal structure of Ce-Zr oxide is present in all the samples. These conclusions are postulated from the observation of the experimental images and the analysis of the digital and selected area electron diffraction patterns. The catalytic activity of CexZr1-xO2 stabilized on three different supports was tested for CO oxidation. The conversion of CO as a function of reaction temperature is presented in Figure 5. The activity measurements were performed at normal atmospheric pressure and the conversion of CO was evaluated as per the procedure described in an earlier report.43 The CO oxidation starts after the temperature reaches ∼500 K in all cases and subsequently increases rapidly. The CZ/A catalyst exhibited relatively better performance compared to that of titania- and silica-supported samples. The temperature at which 50% conversion occurred (light-off temperature) was found to be 657, 667, and 770 K for CZ/A, CZ/T, and CZ/S samples, respectively, giving rise to the catalytic efficiency order of CZ/A > CZ/T > CZ/S. In all cases there is no appreciable difference in the composition of Ce-Zr oxides over the supports except their specific surface areas. Interestingly, the reactivity order does not correlate with the specific surface areas of the samples. From the results it appears that the catalytic performance of CexZr1-xO2 is certainly influenced by the nature of the support. It is worth mentioning here that pure supports are not active for CO oxidation activity. The Al2O3 and SiO2 supports were found to show no CO oxidation activity and the TiO2 some negligible activity under the experimental conditions employed in the present study. Therefore, the catalytic performance is solely due

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Figure 6. Plot of weight loss (%) versus temperature between 573 and 1073 K, measured in flowing air for CZ/A, CZ/S, and CZ/T samples. A to B, first heating cycle; B to C, cooling; C to B, second heating cycle.

to CexZr1-xO2, which is influenced by the nature of the support. Ceria crystallizes in a cubic fluorite structure, where each cerium cation is coordinated by eight equivalent nearest-neighbor oxygen anions at the corners of a cube. One of the most important desired aspects of Zr(IV) insertion into the Ce core structure is the formation of defects in the ceria-zirconia lattice that induce a distortion of the oxygen sublattice. If the ceria-zirconia solid solution contains much less zirconium, it cannot induce sufficient stress, and the oxygen mobility within the bulk will not increase substantially. On the other hand, if the zirconium content is too high, it will reduce the quantity of the redox element Ce. Therefore, a definite balance between structural defects and the composition of CexZr1-xO2 are essential requirements, and a balance between these two parameters appears to be a key factor to determine the CO oxidation activity of the CexZr1-xO2 composite oxides.15 During the oxidation of CO, the rapid transformation of adsorbed CO into CO2 strongly depends on the availability of lattice oxygen or OSC, as the participation of lattice oxygen and the degree of reducibility significantly affect the performance of the ceria-based catalysts.15 In order to understand this aspect, the potential oxygen storage-release capacity (OSC) of various samples was examined. The OSC property was tested by oxygen release characteristics of the synthesized nanocomposite oxide powders under dry air atmosphere in the temperature range of 573-1073 K. The change of weight of the sample was monitored by a thermogravimetric (TG) method under cyclic heat treatments in flowing air. The heat treatment temperature maximum (1073 K) corresponds to the moderate thermal condition of practical automotive exhaust gases. Figure 6 shows typical TG curves of various samples, which were subjected to consecutive cycles of heating and cooling. In all the cases, the first heat treatment (A to B in the figure) induced a large decrease of weight, corresponding to the release of both water molecules (from surface) and oxygen (from ceria-zirconia solid solutions). The recovery of weight was seen in a cooling back stage (B to C in the figure). A small decrease of weight in the second heating cycle (C to B in the figure) corresponds to the potential oxygen release capacity of the powders in ordinary air atmosphere. The obtained weight loss and the corresponding total OSC values are presented in Table 2. As can be noted from Table 2, the CZ/A sample exhibits highest total OSC, which is

TABLE 2: Percentage Weight Loss and Corresponding Total Oxygen Storage Capacity (OSC) of CZ/A, CZ/S, and CZ/T Samples in Flowing Air Environment catalyst

weight loss (%)

total OSC (µmol of O2/g of CexZr1-xO2)

CZ/A CZ/S CZ/T

1.32 0.65 1.01

697 286 483

followed by CZ/T and CZ/S. Very interestingly, these results corroborate well with the catalytic activity order of CO oxidation, where CZ/A showed better activity than that of CZ/ T, followed by CZ/S catalyst. It appears that the OSC of the dispersed CexZr1-xO2 strongly influences the catalytic activity of these materials. The total OSC of CexZr1-xO2 solid solutions and the factors that affect the OSC, such as surface area, particle size, method of preparation, and nature of supporting oxide, are still a matter of investigation and not yet completely understood. It appears from the present investigation that the total OSC is not merely dependent on the specific surface area of a particular supported ceria-zirconia sample. Therefore, it can be concluded that the bulk oxygen diffusion rate must be sufficiently large and the observed OSC is controlled by thermodynamic equilibrium of the redox reaction.15 XRD and Raman analyses of these samples subjected to a high calcination temperature revealed that CZ/S sample exhibits different thermal behavior compared to that of CZ/A and CZ/T samples.22,23 These studies further gave an impression that there is a severe lack of long-range order in the crystal structure in the case of CZ/S samples. This is evidenced by the observation of very small crystallites as noted from HREM measurements. The presence of nanodomains with nonuniformity in the crystal structure of very small CexZr1-xO2 nanocomposite oxides could be responsible for the poor OSC of CZ/S sample, leading to inferior performance in the CO conversion. Thus, the combined use of XRD, Raman, and HREM techniques provided interesting information regarding the influence of support on the OSC and CO oxidation activity of highly dispersed CexZr1-xO2 nano-oxides. 4. Conclusions (i) Ceria-zirconia nanocomposite oxides dispersed over Al2O3, SiO2, and TiO2 supports possessing high specific surface

CexZr1-xO2 Nanocomposite Oxides area and good thermal stability were synthesized by a deposition coprecipitation method. As revealed by XRD measurements, this method leads to highly dispersed cubic Ce0.75Zr0.25O2 nanooxides over all the supports and avoids the formation of unwanted inert compounds such as Ce9.33(SiO4)6O2, ZrSiO4, Ce-Ti oxides, and CeAlO3. (ii) Raman spectroscopy studies suggest formation of oxygen vacancies, lattice defects, and oxygen ions displacement from the ideal ceria cubic lattice positions in various samples, which are important from the catalytic point of view. (iii) The TEM-HREM studies reveal the nanocrystalline nature of the dispersed CexZr1-xO2 solid solutions (∼5 nm) over various supports investigated. (iv) The catalytic activity of the synthesized nanocomposite oxides for CO oxidation was found to follow the order CZ/A > CZ/T > CZ/S. This reactivity order resembled the order of total OSC values determined by a thermogravimetric method, which gives an impression that OSC is one of the key factors for the catalytic performance of ceria-based nanocomposite oxides. Acknowledgment. We thank DST (New Delhi) and DAAD (Germany) for financial support under a bilateral collaboration program (DST-DAAD-PPP-2005). Thanks are due to Dr. S. Loridant, IRCELYON, France, and Dr. C. L. Cartes, CSICUNSE-Sevilla, Spain, for providing Raman and electron microscopic results, respectively. P.L., P.B., P.S., and G.T. thank the Council of Scientific and Industrial Research (CSIR), New Delhi, for the research fellowships. References and Notes (1) Fernandez-Garcia, M.; Martinez-Arias, A.; Hanson, J. C.; Rodriguez, J. A. Chem. ReV. 2004, 104, 4063. (2) Bernal, S., Kaspar, J., Trovarelli, A., Eds. Recent Progress in Catalysis by Ceria and Related Compounds. Catal. Today 1999, 50, 173. (3) Fornasiero, P.; Balducci, G.; Di Monte, R.; Kaspar, J.; Sergo, V.; Gubitosa, G.; Ferrero, A.; Graziani, M. J. Catal. 1996, 164, 173. (4) Tomishige, K.; Kunimori, K. Appl. Catal., A 2002, 237, 103. (5) Otsuka, K.; Wang, Y.; Nakamura, M. Appl. Catal., A 1999, 183, 317. (6) Li, Y.; He, D.; Zhu, Q.; Zhang, X.; Xu, B. J. Catal. 2004, 221, 584. (7) Solinas, V.; Rombi, E.; Ferino, I.; Cutrufello, M. G.; Colon, G.; Navio, J. A. J. Mol. Catal. A: Chem. 2003, 205, 629. (8) Boaro, M.; de Leitenburg, C.; Dolcetti, G.; Trovarelli, A. J. Catal. 2000, 193, 338. (9) Aneggi, E.; de Leitenburg, C.; Dolcetti, G.; Trovarelli, A. Catal. Today 2006, 114, 40. (10) Thomas, C.; Gorce, O.; Fontaine, C.; Krafft, J. M.; Villain, F.; Mariadassou, G. D. Appl. Catal., B 2006, 63, 201. (11) Tibiletti, D.; Fonseca, A. A.; Burch, R.; Chen, Y.; Fisher, J. M.; Goguet, A.; Hardacre, C.; Hu, P.; Thompsett, D. J. Phys. Chem. B 2005, 109, 22553. (12) Liotta, L. F.; Di Carlo, G.; Pantaleo, G.; Deganello, G. Catal. Commun. 2005, 6, 329.

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