On the long-term stability of Pd-membranes with TiO2 intermediate layers for H2 purification

This work addresses the use of TiO₂-based particles as an intermediate layer for reaching fully dense Pd-membranes by Electroless Pore-Plating for long-time hydrogen separation. Two different intermediate layers formed by raw and Pd-doped TiO₂ particles were considered. The estimated Pd-thickness of the composite membrane was reduced in half when the ceramic particles were doped with Pd nuclei before their incorporation onto the porous support by vacuum-assisted dip-coating. The real thickness of the top Pd-film was even lower (around 3 μm), as evidenced by the cross-section SEM images. However, a certain amount of palladium penetrates in some points of the porous structure of the support up to 50 μm in depth. In this manner, despite saving a noticeable amount of palladium during the membrane fabrication, lower H₂-permeance was found while permeating pure hydrogen from the inner to the outer surface of the membrane at 400 °C (3.55·10^?4 against 4.59·10^?4 mol m^?2 s^?1 Pa^?0.5). Certain concentration-polarization was found in the case of feeding binary H₂–N₂ mixtures for all the conditions, especially in the case of reaching the porous support before the Pd-film during the permeation process. Similarly, the effect of using sweep gas is more significant when applied on the side where the Pd-film is placed. Besides, both membranes showed good mechanical stability for around 200 h, obtaining a complete H₂/N₂ ideal separation factor for the entire set of experiments. At this point, this value decreased up to around 400 for the membrane prepared with raw TiO₂ particles as intermediate layer (TiO₂/Pd). At the same time, complete selectivity was maintained up to 1000 h in case of using doped TiO₂ particles (Pd–TiO₂/Pd). However, a specific decrease in the H₂-permeate flux was found while operating at 450 °C due to a possible alloy between palladium and titanium that is not formed at a lower temperature (400 °C). Therefore, Pd–TiO₂/Pd membranes prepared by Electroless Pore-Plating could be very attractive to be used under stable operation in either independent separators or membrane reactors in which moderate temperatures are required.