Effect of added basic metal oxides on CO2 adsorption on alumina at elevated temperatures Original Research Article

In order to improve the ability of CO2 adsorption for ceramic membrane with the surface diffusion of CO2, alumina was modified by adding basic metal oxides. Due to such additions, CO2 was effectively separated from an N2-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 CO2 species adsorbed on the surface modified alumina revealed that the added elements reacted with the alumina to form surface aluminate layer. The ability of CO2 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 CO2 and N2 were injected via the helium carrier gas. It was observed that CO2 was adsorbed on the modified alumina even at elevated temperatures above 773 K, while N2 was eluted immediately. The effect on CO2 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 Rb2O. The observed increase in the heat of adsorption was discussed in terms of the enhancement in both interactions between the carbon atom of a CO2 molecule and the oxygen anion of the added metal oxide and between an oxygen atom of a CO2 molecule and the cation of the oxide.These materials were considered to be the excellent candidate materials for surface diffusion membrane.