EFFECT OF ADDED BASIC METAL-OXIDES ON CO2 ADSORPTION ON ALUMINA AT ELEVATED-TEMPERATURES

In order to improve the ability of CO2 adsorption for ceramic membrane with the surface diffusion of CO2, alumina was modified by adding bas ic metal oxides, Due to such additions, CO2 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), alkal ine earth metals (Mg, Ca, Sr, Ba) and alkali metals (Na, K, Rb, Cs). I R observation of CO2 species adsorbed on the surface modified alumina revealed that the added elements reacted with the alumina to form surf ace aluminate layer. The ability of CO2 adsorption was evaluated by us ing a high temperature gas chromatograph: the column was packed with m odified alumina onto which small amounts of a mixture of CO2 and N-2 w ere injected via the helium carrier gas. It was observed that CO2 was adsorbed on the modified alumina even at elevated temperatures above 7 73 K, while N-2 was eluted immediately. The effect on CO2 adsorption v aried from one metal oxide to another. The heat of adsorption, determi ned from the gas chromatogram, increased from 80 to 170 kJ mol(-1) wit h decreasing electronegativity of the cation used for modification. Th e increase in the heat of adsorption among the oxide groups was in the following sequence: rare-earth metal oxides < alkaline-earth metal ox ides < alkali metals oxide. The largest heat (167 kJ mol(-1)) was obta ined for the addition of Rb2O. The observed increase in the heat of ad sorption was discussed in terms of the enhancement in both interaction s between the carbon atom of a CO2 molecule and the oxygen anion of th e added metal oxide and between an oxygen atom of a CO2 molecule and t he cation of the oxide. These materials were considered to be the exce llent candidate materials for surface diffusion membrane. (C) 1998 Els evier Science B.V.