Tailored Macroporous ZrO2-Al2O3 Mixed Oxides by Template-Assisted Method: Novel Materials for Catalytic Applications

J. Ortiz-Landeros, M. E. Contreras-García and H. Pfeiffer

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AZojomo (ISSN 1833-122X) Volume 4 July 2008

Topics Covered

Abstract
Introduction
Experimental Procedure
   Synthesis of Submicrometric Polystyrene Latex Particles
   Preparation of the Zirconia-alumina Gels
   Preparation of the Macroporous Structures
   Characterization
Results and Discussion
   Submicrometric Polystyrene Latex Particles
   Preparation of the Macroporous Structures
   Characterization of Structures of Mixed Oxides
Conclusions
Acknowledgements
References
Contact Details

Abstract

Macroporous ZrO₂ - Al₂O₃ mixed oxides containing different amounts of Al₂O₃ were prepared via colloidal processing using negatively charged polystyrene spheres as template and ultrafine particles of zirconia-alumina oxi-hydroxide gels as the building blocks which were prepared by sol-gel method from inorganic precursors.The final three dimensional porous arrays with controllable pore size in the submicrometer range could be obtained by calcination of the organic template. These materials combine the chemical properties of mixed oxides with a very open structure of interconnected and uniform pores that can provide diffusional efficiency in potential catalysis and gas separation aplications. The influence of the suspension conditions and the characteristics of the ordered porous microstuctures derived from this synthesis pathway were discussed. The physicochemical characterization of the samples was carried out by N₂ physisorption (SBET), XRD, FT-IR, SEM, wet chemical analysis and Zeta potential measurements.

Keywords
Mixed Oxides; ZrO₂- Al₂O₃; Macroporous Materials, Colloidal Processing.

Introduction

ZrO₂–Al₂O₃ mixed oxides have been the subject of several studies into the catalytic materials field. They are used as catalysts and/or supports because the mixed system presents higher catalytic behaviour than pure ZrO₂ or Al₂O₃ and other classical supports like SiO₂ [1-5].

These sorts of materials are commonly synthesized by soft chemistry methods which include coprecipitation and sol-gel techniques [2-5]. Both these, are good methods to avoid phase segregation and then, they offer the possibility to prepare ultrahomogeneus structures at the molecular level [4, 6-8].

On the other hand, the catalytic supports require a combination of good thermal stability, chemical and textural properties. The control of textural characteristics in a catalytic support like pore size, shape and conectivity improve selectivity and efficiency to the active phase increasing the performance of catalytic systems.

Nowadays, porous inorganic oxides with tailored porosity and pore sizes ranging from a few hundred micrometers to a few nanometers can be achieved by many methods, which include the use of polymer spheres array as templates. A number of related papers that have been published [9-12], and several of the strategies follow the colloidal processing as synthesis pathway [13-16].

The aim of this study was to prepare ZrO₂-Al₂O₃ mixed oxides with a well-ordereded and interconnected macroporous structure on a submicrometer scale with potential applications as catalysts and catalysis supports. Mixed oxides containing different amounts of Al₂O₃ were prepared by sol-gel method, and the resulting porous architectures were fabricated via colloidal processing using submicrometric polystyrene latex spheres (PS) as the sacrificial template.

Experimental Procedure

Synthesis of Submicrometric Polystyrene Latex Particles

Negatively charged sulfated-stabilized polystyrene (PS) latex spheres were synthesized by emulsifier-free emulsion polymerization using ammonium persulphate as initiator and sodium bicarbonate as buffer. Based on literature [17-20], the reaction was carried out for 14 h at 70ºC with mechanical stirring at 350 rpm. The system was purged with nitrogen to eliminate oxigen inhibitor effects before the process was initiated. Once the polymerization was completed, the reaction mixture was cooled and filtered to remove any aggregates. Then, colloidal PS particles were centrifuged to remove the residual monomer, initiator and water-soluble oligomers. The average diameter of the spheres was estimated by image analysis using Sigma Scan Pro 5 software based in SEM micrographs.

Preparation of the Zirconia-alumina Gels

Sol precursors were prepared from disolution of reagent grade aluminium nitrate [Al(NO₃)₃•9H₂O] and zirconium oxychloride [ZrOCl₂•8H₂O] at room temperature under vigorous stirring. The salts were mixed in stoichiometric proportions so that the resulting mixed oxide powders in the final oxidized state contained 0, 10, 25, and 50 wt% of alumina; labeled as Z, ZA10, ZA25, and ZA50, respectively. The gelation of sol precursors was carried out by dropping ammonium hydroxide solution until a pH equal to 9 was reached. The gels obtained were washed several times with distilled water to remove chloride and nitatre residual ions.

Preparation of the Macroporous Structures

According to the synthesis route, in a first step, aqueous suspensions with the template polymers and the other with nanosized gel particles were prepared separately. In the two cases, these suspensions were ultrasonically treated for 5 min and further a vigorous stirring treatment was conducted for 1 h, to ensure their good dispersion. After adjusting the pH to different levels, of both suspensions, they were mixed together maintaining the volume ratio of PS:gel particles of 70:30.

The different pH values were selected based on zeta potential meassurements in order to generate template-gel particles compact assemblies via two different mechanisms i.e., by the hetero-coagulation phenomenon and by settling and evaporation of a template-nanoparticle stabilized suspension.

In a second experimental step, the mixed suspensions were heated at 75ºC in an oven until the total water was removed. Finally, macroporous inorganic oxides were obtained by calcination at 550ºC in air for 4 h of polymer/ceramic assemblies to remove the PS template, which resulted in the macroporous structures. The calcination rate was 1ºC/min. Experimental procedure described above was made for the different mixed oxide compositions. Samples named as Z, ZA10, ZA25 and ZA50 were processed according to the Z-potential conditions described on line b of Figure 2. On the other hand, the samples processed at heterocoagulation conditions were named with an aditional H, i.e. ZH, ZA10H, etc.

Characterization

The characterization of the samples was carried out by X-ray diffraction; the patterns were recorded in the range 20º ≤ 2θ ≤ 80º in a Bruker D8 Advance difractometer, using Cu Kα radiation. The morphology of porous structure was observed by Scanning electron microscopy in a SEM model Stereoscan 440. The measurements of zeta potential were performed in a Zeta meter 3.0+ instrument. Reagent grade hydrochloric acid and ammonium hydroxide were used to adjust the pH suspension, and NaCl was used to adjust the ionic strength when measuring Zeta potential. The products were also analyzed by FT-IR spectroscopy with samples prepared as KBr pellets, wet chemical analysis and N₂ physisorption (S_BET) measurements.

Results and Discussion

Submicrometric Polystyrene Latex Particles

Monodisperse PS is required to form ordered close-packed arrays, which are directly related with the morphology of final porous structure. Therefore, template diameters should not vary by more than 8 % [11]. A diameter particle size variation of 3 % was estimated by image analysis, i. e. polystyrene spheres could be synthesized with a narrow size distribution and average diameter of 685 ± 20 nm.
The above-mentioned without discart that certain amount of smaller particles was removed with supernatan by centrifugation separation process. Figure 1, shows SEM micrograph of the synthesized latex spheres with unimodal size distribution leading to close packing.

Figure 1. SEM image of monodispersed PS.

Preparation of the Macroporous Structures

The higher and negative charge values in the range of pH used. For the mixed hydroxide hydrogels suspensions, the isoelectric point is higher as the alumina content is increased. Higher values of higher and negative charge values in the range of pH used. For the mixed hydroxide hydrogels suspensions, the isoelectric point is higher as the alumina content is increased. Higher values of negative Z-potential are obtined for pure zirconia hydrogels at pH = 11 and they decrease as the alumina hydrogels content is increased. An inverse behaviour is shown at pH = 3 with higher positive Z-potential values.

An analysis of the Z-potential versus pH curves, for all the obtained suspensions shows the pH values at which each suspension is electrokinetically stable. These data are reported in Table 1.

Table 1. Zeta potential values of different particles in suspension.

Figure 2 shows two regions corresponding to two suspension conditions, one is indicated in the acidic region as a and a’. The other, in basic region, corresponding to pH condition with the higher Z-potential, named b.

Figure 2. Zeta potential values of the PS and mixed oxide gels in aqueous suspensions.

Characterization of Structures of Mixed Oxides

Region a and a’ is the suspension conditions for heterocoagulation process, since polymer and gels present opposite surface charges and can form core-shell structures; the b, is the suspension conditionwhich leads to settling and evaporation i.e., the method reported by Wang et al. [15]. Such methodology is based in the liquid fase evaporation of a stable suspension formed by the gel particles and the template. The slow evaporation rate promotes the gel formation in the interstitials of PS array forming a hybrid structure. Subsequent calcination of this hybrid leads to the template elimination generating the designed porous structure.

Table 2 shows the chemical analysis results for the Al and Zr content. The formuled Al₂O₃/ZrO₂ ratio on the prepared samples is very close to Al₂O₃/ZrO₂ ratio on the obtained mixed oxides.

Table 2. Wet chemical analysis results for the various samples.

The X-ray diffraction profiles are shown in Figure 3. It can be seen that there are some wide peaks corresponding to tetragonal (t) and monoclinic (m) zirconia (Figure 3 (a)). The diffraction patterns (b), (c) and (d) of samples AZ10, AZ25 and AZ50 show only broad peaks around 30° corresponding to the signal of the main peak of the tetragonal zirconia showing amorphous phase with incipient crystallization.

Figure 3. XRD patterns of samples calcined at 550ºC for 4h: (a) Z; (b) ZA10; (c) ZA25 and (d) ZA50.

Figure 4 shows FT-IR spectra of samples heat-treated to 550ºC. All samples exhibit bands at 3430 and 1630 cm^-1 assigned to the bending vibration and stretching vibration of the O-H bond in absorbed and coordinated water, showing that there are residual structural OH.

Figure 4. FT–IR spectra: (a) Z; (b) ZA10; (c) ZA25 and (d) ZA50

Bands at 494 and 744, 576, 416 cm^-1 in sample Z are characteristics of tetragonal and monoclinic phases of zirconia, respectively [21]. In the sample ZA10, bands at 416, 494 and 744 cm^-1 were not observed in agreement with XRD data, which did not show crystallized zirconia.

In samples ZA10, ZA25 and ZA50 bands due to alumina are present at 1400, 1034 and 563 cm^-1 and the band at 576 cm^-1 in sample Z characteristic of (m) zirconia shifted to 600 cm^-1 in ZA10 and ZA25. However, it is necessary to mention that those samples, corresponding to mixed oxides, show a trend. As the concentration of alumina increased, the intensity of these bands decreased until they formed a shoulder and a broad band located at 1040 and 530 cm^-1 in sample ZA50, respectively. The broad band can be explained by the relative contribution of two bands, one at 494 cm^-1 characteristic of (t) ZrO₂ and the other at 563 cm^-1 corresponding to alumina.

According to literature [6], a band related to the existence of Zr-O-Al bonds has been reported in the range of 2100 cm^-1. However, in the present work the presence of this band in the alumina containing spectra was not clear and it was also found on the pure system ZrO₂, which indicates that this vibration is not exclusive of the Zr-O-Al system.

SEM images of ceramic materials for samples ZA10, ZA25 and ZA50, Figures 5 to 9, show an interconnected and open macroporosity with average pore size diameter of 420 nm, corresponding to approximately 60% of sphere template diameter. Once stablished the above mentioned; by varying the PS spherical particles diameter, it is possible to systematically control the pore size in the final porous structure. In this way, macroporous catalysts and supports can be designed to provide optimal flow and improved efficiences in catalysis processes.

Figure 5. Macroporous open structure of sample ZA10.

Figure 6. Macroporous open structure of sample ZA25.

Figure 7. Macroporous open structure of sample ZA25.

Figure 8. Macroporous structure with average pore size diameter of 420 nm for sample ZA50.

Figure 9. Macroporous structure with average pore size diameter of 420 nm for sample ZA50.

Figure 10. Macroporous closed structure of sample ZA50H.

SEM photos for the different samples show that there is a closely related morphology with the alumina content. Well defined open structures and interconected porosity were obtained when alumina content was increased. Attemps made to synthesize macroporous structures of pure zirconia composition were unsuccessfull. The structures obtained for Z sample were powders without any macroporosity. In this case, the macrostructure might have collapsed because ZrO₂ is not stabilized by Al₂O₃, so the monoclinic phase of zirconia is formed.

The samples processed in heterocoagulation conditions a-a’ (Figure 2), did not present interconnected porous structures, showing only closed porosity, as it can be seen in the micrograph Figure 10. So those conditions were not further studied.

The specific surface area of the samples, measured by the single point BET method, are given in Table 3. As expected, an increase is observed with the alumina content. This increase is related to the textural promotional effect of alumina on the zirconia.

Table 3. Surface area and crystalline phases present in various samples.

Conclusions

This work demostrates that high surface area macroporous zirconia-alumina mixed oxides can be synthesized by sol-gel method, using polystyrene spheres as sacrifical template.

Such structured materials present the advantage of associating the zirconia-alumina system great versatility in terms of composition with the benefits of an open macroporous framework.

Through this approach, it can be produce porous structures with well defined and controllable interconnected porosity that provides textural characteristics, which are helpful for advanced catalytic materials.

Acknowledgements

The authors thank to CONACYT and ECOES program which J.Ortiz-Landeros is fellow. Furthermore, authors thank to Leticia Baños for technical help with XRD analyses.

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Contact Details

This paper will be also published in “Advances in Technology of Materials and Materials Processing Journal, 9[2] (2007) 119-124”.