Gas permeation redox effect of binary iron oxide/cobalt oxide silica membranes

Abstract

Abstract This work investigates the redox cycling effect on the physicochemical characteristics and gas permeation behaviour of binary metal oxide (iron/cobalt) silica membranes prepared by sol–gel method from tetraethyl orthosilicate, cobalt and iron nitrates, peroxide and water. A Fe/Co ratio of 10/90 conferred more stable silica microstructure under redox cycling, contrary to Fe/Co ratios ⩾25/75 which led to structural densification after the first redox cycle. The gas permeance redox effect of Fe/Co = 10/90 silica membranes resulted in a switchable permeance increase and decrease as the membrane was reduced and oxidised, respectively. However, the extent of the switchable changes were not constant and tended to decrease at each cycle, as evidenced by the He/CO 2 permselectivity shift from a higher value of 130 (first oxidation cycle) down to 80 (third oxidation cycle). It was found that a shift in temperature corresponding to the exothermic oxidation peak, attributed to the progressive changes in the formation of Fe x Co 1−x O y mixed oxide embedded in the silica matrix at each redox cycle. This was further supported by the sequential disappearance of the infrared absorbance peak at 565 cm −1 upon redox cycling, thus demonstrating structural re-arrangement of the membrane. The permselectivity of He and H 2 over CO 2 sequentially decreased at a higher ratio for after the oxidation cycle, as a result of the silica matrix densification at high temperatures (400 °C).