Catalyst science Essay

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Properties of Al-Ce mixed oxides prepared mechanochemically
Květa Jirátováa, Alla Spojakinab, Jan Klempaa, Jana Balabánováa, Joro Tyulievb, Ludek Kaluzhaa, Radostina Palchevab, Radek Fajgara

aInstitute of Chemical Process Fundamentals of the ASCR, v.v.i., Prague, Rozvojová 135, 165 02, Czech Republic bInstitute of Catalysis, Bulgarian Academy of Sciences, Sofia, acad. G.Bonchev str., bl.11, 1113, Bulgaria

Abstract
High surface alumina and its mixtures with different quantities of CeO2 (1- 18 wt.%) were synthesized by simple mechano-chemical method from aluminum nitrate, cerium nitrate and ammonia bicarbonate. Properties of the prepared samples were investigated by SEM, TG-DTA, IR and Raman spectroscopy, N2 adsorption at -195 °C, TPR, NH3-TPD, CO2-TPD determination of isoelectric point and XPS. Aluminum ammonium carbonate hydroxide NH4Al(OH)2CO3,was found to be main product in the dried Al precipitate, while cerium carbonate hydroxide CeCO3OH was prevailing in the dried Ce precipitate. After calcination at 500 °C, amorpous structure of alumina and cubic fluorite crystallites of ceria with preferred orientation along (1 1 1) direction were detected by X-ray diffraction pattern. XPS spectrum confirmed the existence of Ce4+ oxidation states in cerium oxide nanoparticles. Surface acidity of the mixed oxides prepared mechanochemically is lower than that observed at oxides prepared by other methods because of presence of adsorbed ammonia on the surface. Gradual increase of isolectric point with increasing CeO2 amount in the mixed Al2O3-CeO2 and IR spectroscopy results is in accord with the findings. Addition of CeO2 to alumina slightly decreased surface area and mesopore volume of resulting mixed oxides.

Introduction
Ceria containing catalysts play an essential role in heterogeneous catalytic processes. Pure ceria has been subject of numerous investigations because of its wide applications in catalysis [1,2]. However, ceria shows poor thermal stability and low specific surface and therefore, many studies have been made to improve its properties by combination with other oxides like TiO2, ZrO2, Al2O3 and other [3]. Alumina-ceria are substantial component of the three ways catalysts [1], due to the ceria ability to function as the buffer of oxygen [7] and enhance the oxygen storage capacity of the catalysts [1,6], what is important in eliminating of pollution in automobile exhausts [4]. Ceria in these catalysts also functions as structural promoting component, increasing alumina stability towards thermal sintering [5]. In addition, ceria is effective in supported precious metals as activity promoter.
It is known that the activity of catalysts is influenced by the method of their preparation. For example, properties of alumina support strongly depend on a preparation method [20]. The neutralizing agents, usually added to acidic Al salts like aluminum nitrate, are sodium hydroxide [21], sodium carbonate[22], sodium bicarbonate [23], ammonia [24], etc. Several important studies have also been reported on the use of urea as a precipitating agent by many workers [25-30].
Ceria properties are affected by preparation technique as well. Several methods have been adopted for the preparation of CeO2, e.g.azeotropic distillation [31], coprecipitation [32], glycine–nitrate combustion [33,34], glycothermal decomposition [35], hydrothermal [36-38], pyrolysis [39], reverse micelles [40], sol–gel [41,42], sonochemical [43,44], solvothermal [45] and simple precipitation method [46-49, 55]. Among these methods, ammonia precipitation method is widely used in laboratories because of its low preparation cost and simple process.
Relation of preparation methods and properties of both pure alumina and ceria has been widely investigated, but properties of the mixed oxides containing cerium were not studied so frequently. CeO2 catalysts containing compounds with identical valence (B, Al, Ga, or In) have been often synthesized by
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