Improved pore connectivity by the reduction of cobalt oxide silica membranes

Abstract

This work investigated the permeation of binary gas mixtures in non-reducing (He/CO 2 ) and reducing (H 2 /Ar) conditions at temperatures ranging from 200 to 500 ºC. A common performance aspect under both non-reducing and reducing conditions was that the He and H 2 purity in the permeate stream was independent of temperature for the tested binary gas mixtures, except at very high He/CO 2 or H 2 /Ar concentrations (≥90/10) in the retentate stream. Under non-reducing conditions, the transport of gases was consistent with molecular sieving properties of silica derived membranes, and He permeance was constant irrespective of the He/CO 2 binary concentration tested.An anomalous H 2 transport was observed under reduced conditions, as unexpectedly the H 2 permeance was higher for gas mixtures instead of single gas. Further tests showed that H 2 permeance increased 170% as the gas mixture changed from single H 2 gas to H 2 /Ar gas mixtures.This was attributed to the experimental procedure, as the membranes were partially reduced each day and tested for gas permeation from pure H 2 to lower H 2 concentration in gas mixtures. Under these partial reducing conditions, H 2 slowly reacts with the surface of the dense Co 3 O 4 particle, thus 2 forming a porous CoO region. The increase in H 2 permeance was therefore attributed to improved pore connectivity between the silica structure and the porous CoO region.

Estimation of Pore Size Distribution of Amorphous Silica-Based Membrane by the Activation Energies of Gas Permeation

“…The permeate side was open to the atmosphere, to maintain pressure at 1 atm. More details about the mixed gas permeation could be found in Ji et al [61]. The CO 2 single gas permeation proved to be too slow to be measured, and was considered below the measurable region of the bubble flow meter.…”

“…After the H 2 single gas permeation test, H 2 and Ar mixture separation was measured for feed compositions from 90%, 70%, 50%, and 30% down to 20% H 2 . The H 2 feed fraction was reduced step-wise after each day of testing [61].…”

“…However, it is interesting to observe that the H 2 permeance was higher for gas mixtures than for the pure gas. Miller et al [69] and Ji et al [61] reported that cobalt oxide embedded in silica undergoes reduction after exposure to H 2 , resulting in pore size enlargement. As the single H 2 gas permeation was tested at the beginning in this series of experiments, the permeance was the lowest as the membrane was just exposed to H 2 for a short time and possibly in an oxidized state.…”

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

“…By embedding metal oxides into silica films, the pore tailoring from the reducing 3 step has been mainly attributed to loss of oxygen from the metal oxide, thus opening a molecular gap and generally increasing the flux for the larger gas molecules. Very recently, Ji and co-workers [14] reported an anomalous 170% increase in H 2 permeance from H 2 /Ar gas mixture to single H 2 gas using cobalt oxide membranes. This was attributed to the shrinking core model, causing the larger metal oxide particles to crackle as they break down in a gas and solid reaction, and leading to the formation of smaller particles with inter-particle porous structures accessible for gas transport [15,16].…”

“…4) suggests that the reduction step has caused a structural rearrangement of the Fe/Co=10/90 silica matrix. Ji and co-workers [14] attributed the increased permeance on the reduction cycle to the crackling core effect of metal oxide particles, associated with H 2 reaction with solid particles and leading to formation of smaller particles with porous structures [15,16]. However, the results observed for a silica membrane prepared with binary metal (FeCo) are transient in this work and they differ from the consistent switching redox effect for single metal (Co) oxide silica membranes reported by Miller et al [4].…”