In terms of porous substrates, the conventional ceramic
and metallic substrates, both in planar or tubular geometries,
are a common option.
The metallic substrates, e.g. porous stainless steel (PSS), have high mechanical and chemical
stability and are commercially available. However, disadvantages
such as intermetallic diffusion at temperatures from
275 C to 350 C, large pore size and non-uniform pore size
distribution still represent a problem.
Commonly, the selection of this type of substrates requires the deposition of an
interlayer, which represents additional costs.
The low surface area to volume ratio of tubular PSS substrates limits
the possibility of fabricating a compact module.
Ceramic supports are a good choice due to their high chemical, thermal
and mechanical stability, together with the small pore size of
0.1e10 mm.
However, the fabrication of palladium membranes
supported on ceramic substrates involves several steps,
leading to considerable production cost.
The possibility of fabricating a module is compromised by the low surface area
to volume ratio of the support configurations, such as flat
sheet and tubular supports.