Effect of added basic metal oxides on CO 2 adsorption on alumina at elevated temperatures

Abstract

In order to improve the ability of CO 2 adsorption for ceramic membrane with the surface diffusion of CO 2, alumina was modified by adding basic metal oxides. Due to such additions, CO 2 was effectively separated from an N 2-containing mixture gas at elevated temperatures. Modifiers examined were the oxides of rare earth metals (La, Ce, Nd, Pr), alkaline earth metals (Mg, Ca, Sr, Ba) and alkali metals (Na, K, Rb, Cs). IR observation of CO 2 species adsorbed on the surface modified alumina revealed that the added elements reacted with the alumina to form surface aluminate layer. The ability of CO 2 adsorption was evaluated by using a high temperature gas chromatograph: the column was packed with modified alumina onto which small amounts of a mixture of CO 2 and N 2 were injected via the helium carrier gas. It was observed that CO 2 was adsorbed on the modified alumina even at elevated temperatures above 773 K, while N 2 was eluted immediately. The effect on CO 2 adsorption varied from one metal oxide to another. The heat of adsorption, determined from the gas chromatogram, increased from 80 to 170 kJ mol −1 with decreasing electronegativity of the cation used for modification. The increase in the heat of adsorption among the oxide groups was in the following sequence: rare-earth metal oxides < alkaline-earth metal oxides < alkali metals oxide. The largest heat (167 kJ mol −1) was obtained for the addition of Rb 2O. The observed increase in the heat of adsorption was discussed in terms of the enhancement in both interactions between the carbon atom of a CO 2 molecule and the oxygen anion of the added metal oxide and between an oxygen atom of a CO 2 molecule and the cation of the oxide.These materials were considered to be the excellent candidate materials for surface diffusion membrane.