Sols as Precursors to Transitional Aluminas and These Aluminas as Host Supports for Ceo2 and ZrO2 Micro Domains

Abstract

Abstract The possible relationship of alumina sols to the final three dimensional structure of calcined aluminas prepared by commercial procedures has been referred to only briefly in the literature. We have established that the pore sizes and pore volumes of transitional aluminas prepared by sol precipitation using ammonium hydroxide are strongly dependent on the size of the alumina sol precursor. Preparation of alumina from gelation of a 2 nm diameter alumina sol produces a relatively dense phase with an average pore diameter of 5.5 nm. Aluminas made by gelation of a 20 nm average diameter size alumina sol have three-fold the pore volume of the alumina produced by gelation of the 2 nm alumina sol. In the case of the 20 nm size sol the average pore size is 7 nm in diameter which is very close to that of aluminas produced by conventional aqueous procedures. The high temperature stability of the alumina formed from sols argues that very little amorphous alumina is formed as a separate phase. Ceria sols from 1 to 20 nm size were co-precipitated with the 20 nm alumina sol such that the CeO2 component is the “guest phase” within the three-dimensional alumina phase acting as the “host phase” of CeO2 micro domains. By varying the ceria content, highly dispersed CeO2 with rod-shaped morphology could be prepared within the relatively stable alumina host structure. It was established that these rod-shaped CeO2 structures would transform at increasing temperatures to cylinder morphologies which were isolated from each other within the alumina host structure. Studies with a CeO2 sol size of 12 nm size revealed that the micro domains formed within the alumina were very similar to the starting sol size. However, in the case of a CeO2 sol of 1 nm size agglomeration to produce larger domains than the starting sol only occur at CeO2 contents of 5% or greater. The high temperature stability and characterization results for Ce-Al and temperature stability and characterization results for Ce-Al and Ce-Zr-Al systems will be described.