Date of Award

2004

Document Type

Thesis

Degree Name

Master of Science in Materials Science and Engineering (MSMSE)

Department

Materials Science

First Advisor

Dr.Kamal Haleel

Second Advisor

Dr.Briyan Merfi

Abstract

Ceria, CeO2, is one of the most important catalytic materials that can play multiple roles owing to its ability to release and uptake oxygen under catalytic reaction conditions with the preservation of it fluorite structure. Textural and thermal stability is a critical issue in determining the promoting and metal supporting functions of CeO2 in its catalytic applications in the three way catalysis. Therefore, significant efforts have been made by industry on finding ways to improve the thermal stability of ceria both by modification of the synthesis of CeO2 and by looking at possible stabilizers.

In the first part of this work, thermally stable ceria aggregates were obtained via a two-step sol gel process of cerium isopropoxide dispersed in isopropanol by ultrasonic radiation. The first step involved a hydrolysis reaction. In the second step, the resultant dispersions were mixed under stirring with neutral or basic water isopropanol solutions. The ceria powders obtained via neutral and basic media were shown to be composed of aggregates of fine particles. However, it was found that the type of media affected both particle texture and morphology. Specific surface areas of 33.1 and 44.2 m2g-1 respectively, were obtained for the neutral and basic materials, which calcined at 650 °C for 3 hours. Moreover, the oxidative nature of ceria was found to help in the removal of the organic impurities while the hydroxylated surface of ceria facilitated the formation of thermally stable agglomerates.

In the second part of this work, ceria supported on silica, ceria/silica, materials of 10 and 20% (w/w) were prepared by calcinations, at 650 °C for 3 h, of the xerogels obtained by the mixing of the corresponding amount of a ceria precursor with freshly prepared sols of spherical silica particles (Stober particles) in their mother liquors. Two different ceria precursors were examined in this investigation. The first was a gel produced by prehydrolysis of cerium (IV)-tetra isopropoxide in isopropanol media, and the second was an aqueous solution of ceric (IV) ammonium nitrate. Different textural and morphological characteristics that developed by calcination, were investigated by TGA, FTIR, XRD, SEM and analyses of N2 adsorption isotherms. The results indicated better ceria dispersion and formation of mesoporous textural composites materials produced by the second precursor, ceric (IV) ammonium nitrate, than the first precursor, prehydrolysed cerium (IV)-tetra isopropoxide.

The results show that properties of composite materials are largely related to the preparation method and the precursor type. Moreover, mixing media affect both nucleation and the growth of ceria particles and their protection against sintering upon calcinations at the test temperature.

Download
COinS