A hydrogen separator comprising a porous ceramic substrate and a hydrogen-separating layer, such as palladium or palladium alloy, formed on the substrate has been used in order to separate only hydrogen selectively from a hydrogen-containing gas, such as steam-reformed gas. Such a hydrogen separator is used for hydrogen separation at high-temperatures in some cases. Therefore, the hydrogen separator is required to have high gas tightness at high temperatures or during heat cycle in which temperature increase and decrease are repeated.
Conventional hydrogen separators have, as shown in FIG. 2, a structure comprising a porous substrate 12 and a hydrogen-separating layer 13 formed on one surface 15 of the porous substrate 12. Incidentally, the porous substrate 12 has a large number of pores connecting from one surface 15 of the substrate to other surface (not shown), and is constituted by a material such as ceramic or metal. In producing a hydrogen separator 11 having a structure such as shown in FIG. 2, for example, there can be mentioned a method of plating palladium (which becomes a hydrogen-separating layer 13) on one surface 15 of a porous substrate 12 [see, for example, Patent Document 1 (JP-A-3-146122), Patent Document 2 (Japanese Patent No. 3213430) and Patent Document 3 (JP-A-62-273030)].
When palladium layer has been formed directly on a porous ceramic substrate, however, there is a problem that the affinity between the palladium and the porous ceramic substrate is not good. As a result, when the hydrogen separator obtained is exposed to a heat cycle, there may happen peeling of palladium from porous substrate or generation of other defects in palladium layer, and gas tightness of the palladium layer may reduce. Further, when, after the formation of palladium layer on a porous substrate, other metal, such as silver, is formed on the palladium and then heated to form a palladium alloy as a hydrogen-separating layer. There is a problem that the hydrogen-separating layer made of the alloy tends to peel from the porous substrate more easily, because of additional heating for alloying.