Seeding method for deposit of thin selective membrane layers
A process of producing transition metal-based membranes or other layers on a porous support is provided. The layers are suitable for hydrogen separation, oxygen separation, or protective or decorative purpose sand are produced by pretreating the porous support by coating with a solution of a transition metal salt, drying the seeded support, reducing the transition metal salt to transition metal metal, and electroless plating with a complex of a transition metal (palladium, silver or other) and optionally other metals. The membranes can be tubular with a transition metal layer of 1-10 μm on its outside.
1. A process of producing layers of one or more transition metals on a porous support, comprising:
(a) pre-treating the porous support by film-coating with a solution of a transition metal chloride, nitrate, sulphate or selenate salt at a coating rate between 5 and 100 mm/s,
(b) drying the pre-treated support at a temperature of between 40 and 100° C.,
(c) reducing the transition metal salt to the corresponding transition metal, and
(d) electroless plating the pre-treated support with one or more transition metals.
2. The process according to claim 1 , wherein the transition metal salt is a transition metal chloride or nitrate of a metal of the platinum group (Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt).
3. The process according to claim 2 , wherein the transition metal salt is a nickel, palladium or platinum chloride.
4. The process according to claim 1 , wherein the solution is an aqueous solution.
5. The process according to claim 4 , wherein the reducing is performed at a temperature of between 475 and 625° C.
6. The process according to claim 1 , wherein the reducing is performed with a hydrogen-containing gas at a temperature of between 400 and 700° C.
7. The process according to claim 1 , wherein the support is tubular and the amount of transition metal in the pre-treated support ranges from 5 to 12 mg per 100 mm tube length.
8. The process according to claim 1 , wherein the support is tubular and the coating rate is between 15 and 75 mm tube length per sec.
9. The process according to claim 1 , wherein the transition metal penetrates into the support for no more than 5 μm.
10. The process according to claim 1 , wherein the solution comprises between 113-188 mM of the transition metal.
11. The process according to claim 1 , wherein the one or more transition metals used for plating are selected from palladium, nickel and silver.
12. The process according to claim 11 , wherein the one or more transition metals comprise at least 40 wt. % of palladium.
13. A tubular membrane based on a transition metal on a porous tubular support, said membrane being characterised by a first transition metal layer comprising at least 65 wt. % of silver, said layer having a thickness of between 1 and 5 μm at one side of the support, with seeds of a second transition metal selected from the group consisting of Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, said seeds being present in an amount of 10-30 mg per dm 2 of support and having a depth of between 0.5 and 5 μm into the support, and by having an oxygen to nitrogen selectivity of at least 10.
14. A process of separating oxygen from a gas mixture, comprising subjecting the gas mixture to a tubular membrane according to claim 13 .