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

Abstract

This work addresses the use of TiO 2 -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 TiO 2 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 H 2 -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 H 2 –N 2 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 H 2 /N 2 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 TiO 2 particles as intermediate layer (TiO 2 /Pd). At the same time, complete selectivity was maintained up to 1000 h in case of using doped TiO 2 particles (Pd–TiO 2 /Pd). However, a specific decrease in the H 2 -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–TiO 2 /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. • Pd-doped titania highly reduces the Pd-thickness for reaching fully dense membranes. • TiO 2 barriers yield H 2 permeance up to 4.17·10 −4 mol/s m 2 Pa 0.5 and complete α H2/N2 . • Pd-doped titania allows stability for long-time operation up to 1000 h at 400 °C. • Temperature above 450 °C leads to certain PdTi alloy and H 2 permeance decrease.