Catalytic activity of CeO2–ZrO2 mixed oxide catalysts prepared via sol–gel technique: CO oxidation
Introduction
Ceria and Ce-containing mixed oxides have attracted much attention as oxidation catalysts because of their unique redox properties and high oxygen storage capacity. It has been reported that ceria has potential uses for the removal of post-combustion pollutants and of high strength organics from wastewater (catalytic wet oxidation) [1], [2]. However, pure CeO2 is seldom used since it is known to have poor thermal stability [3], [4], [5], [6]. A loss in surface area also occurs due to the changes in pore structure and in crystallite growth. Doping with other rare earth or transition metal oxides [2], [7], [8] may improve the stability of the surface area of ceria at high temperature. However, the improvement is found to be insignificant.
Recently, there is much interest in a new generation of mixed oxide containing CeO2 and ZrO2 [9], [10], [11], [12], [13], [14], [15], [16], [17], [18], [19]. It has been reported that the addition of ZrO2 to ceria leads to improvements in ceria’s oxygen storage capacity, redox property, thermal resistance and better catalytic activity at lower temperature [19], [20], [21]. This was found to be due to the partial substitution of Ce4+ with Zr4+ in the lattice of ceria, which results in a solid solution formation [19], [22]. It was suggested that the role of zirconia is to control the structure or the sites of ceria crystallisate [21].
Many preparation methods have been applied for the preparation of CeO2–ZrO2 solid solution for catalytic applications. These include the high-temperature firing or high-energy milling of a mixture of the oxides, conventional precipitation, and sol–gel techniques [19], [20]. Among these methods, sol–gel was found to be very beneficial since it yields products with high purity, homogeneity, well-controlled properties, and low temperature processing. The properties of the final products were found to be dependent on the temperature and the hydrolysis catalysts [23], [24], [25].
In this paper, the CO oxidation on CeO2–ZrO2 mixed oxides prepared via sol–gel technique using urea as a hydrolysis catalyst was studied. The effects of preparation conditions of mixed oxides and the mole ratio of CeO2 and ZrO2 upon the CO oxidation activity were investigated.
Section snippets
Synthesis
CeO2–ZrO2 mixed oxide catalysts were prepared by using the sol–gel technique. Cerium nitrate (Ce(NO3)3·6H2O (99.0%), Fluka), and zirconium oxychloride (ZrOCl2·8H2O (99.0%), Fluka) were used as sources of Ce and Zr, respectively. The starting solution was prepared by mixing 0.1 M of metal salt solutions with 0.4 M of urea solution at a 2 to 1 volumetric ratio. The ratio between each metal salt was altered depending on the desired solid solution concentration: Ce1−xZrxO2,where x=0, 0.25, 0.50, 0.75
BET, SEM and TEM characterization
The approach we used for the preparation of a mixed oxide solid solution of ceria–zirconia was a sol–gel technique using urea as the hydrolysis catalyst. The results showed that the mixed oxide catalysts prepared by this method are highly uniform. As shown in Fig. 1, pure CeO2 particles are mainly of long thin needle shaped morphology. The aggregation of the primary long thin needle shaped particles was found in the case of zirconia-doped ceria particles. It was noticed that there was no effect
Conclusion
It can be concluded that the highly uniform nano-size solid solution particles of ceria–zirconia can be prepared via sol–gel technique using urea as the hydrolysis catalyst. The effect of reflux time on the characteristics of the mixed oxides was not prominent. The stabilization of surface areas of the catalysts can be achieved by the addition of zirconium. However, it was found that the effect of zirconium was almost demolished under high temperature treatment (900°C).
The CO oxidation activity
Acknowledgements
The authors would like to thank the Petroleum and Petrochemical College and Thailand Research Fund (grant number PDF/38/2541) for their financial support.
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