Gas permeation redox effect on binary lanthanum cobalt silica membranes with enhanced silicate formation

This work investigates the characterisation and performance of binary lanthanum cobalt doped silica membranes. The prepared membranes resulted in temperature dependent gas transport, a characteristic of molecular sieving membranes. Under reduction and oxidation cycles at 500 degrees C, the maximum steady state permeance of the membranes reached 1.5 x 10(-7) mot m(-2) s(-1) Pa-1 at 500 degrees C. It was found that the membranes contained both cobalt oxide and lanthanum silicate phases embedded in the silica matrix. The permeance of all tested gases under redox cycling decreased during the 35011 permeation test, though the reduction was slightly more pronounced for larger gas molecules (H-2, CO2 and N-2). As a result, He/CO2 permselectivity increased from similar to 80 at the initial tests to 196 during the redox cycling test, an improvement of 145%. This indicates that the pore sizes >= 2.89 angstrom (i.e. kinetic diameter of H-2) preferentially collapsed/closed instead of the smaller pores available for the permeation of the smaller gas He (d(k)=2.6 angstrom). These results are contrary to other silica and metal doped silica membranes which undergo thermal densification and closure of the smaller pore sizes. Therefore, the formation of lanthanum silicates conferred superior structural stability in the silica matrix. (C) 2015 Elsevier B.V. All rights reserved.