Original Carbon-Free Electroless Plating Bath for Sequential Deposition of PdAg Composite Membranes

Metal membranes are of interest for hydrogen separation as they have a wide range of operational temperatures, are capable of high selectivities, and are scalable [1].   Palladium (Pd) and its alloys have been studied extensively due to their ability to dissociate hydrogen and transport the atoms via a solution-diffusion mechanism.  At low temperatures (below 300 °C), the hydrogen absorbed in Pd can form a hydride phase with a larger lattice constant than the Pd, which leads to irreparable damage in the film [2].  An alloy of particular significance is palladium-silver (Pd-Ag) as it has the ability to operate at temperatures less than 300°C and still maintain high selectivity [3].

Electroless co-deposition of Pd-Ag films can be difficult to control to obtain reproducible compositions and microstructures [4, 5].  Therefore, an alternative approach to fabricate high flux, thin-film composite Pd-Ag alloy membranes is the deposition of thin, sequential layers of Pd and Ag with subsequent annealing.  Synthesis of Ag via electroless plating is traditionally done with a bath stabilizer, the most common being ethylenediaminetetraacetic acid (EDTA) [6-8]. EDTA has been shown to remain in the membrane and cause contamination during mixed gas testing [9, 10].  

Two original plating baths have been formulated for Ag deposition.  The first bath reduces the amount of EDTA used from the common amount of 40 g/L to 20 g/L.  The second bath eliminates the use of EDTA.  Proof-of-concept membrane fabrication consisted of a single Ag layer between Pd layers with both of the metals contain the same amount of EDTA in their respective plating baths.  Three Pd₉₀Ag₁₀(in wt%) membranes have been made with each metal’s electroless bath containing 0, 20, and 40 g/L EDTA.

Preliminary pure-gas permeation has shown promising results.  The hydrogen permeability of the Pd₉₀Ag₁₀ alloy membranes is up to 30% higher compared with a pure Pd film at 400 °C.  Current work is twofold: water gas shift (WGS) testing on the membranes and increasing the Ag content.  The WGS testing will be interesting to see if the films made with EDTA degrade more quickly than those without EDTA.  This speculation is due to some literature evidence that the carbon contamination remaining in the film from the EDTA reacts with carbon dioxide via the reverse Boudouard reaction to form pin holes in the membrane [11].  An Ag content of ~27 wt% yields the maximum H₂permeabilities for Pd-Ag alloy [12].  Increasing the Ag content will be done with multiple, thin Ag layers in between Pd layers.  We expect that multiple, thinner Ag layers will also facilitate annealing.

References

1.         Bose, A.C., Inorganic Membranes for Energy And Environmental Applications. 2008: Springer.

2.         Lewis, F.A., The Palladium-Hydrogen System. Platinum Met. Rev. 26(1):(1982) p. 20-27.

3.         Athayde, A.L., R.W. Baker, and P. Nguyen, Metal composite membranes for hydrogen separation. Journal of Membrane Science. 94(1):(1994) p. 299-311.

4.         Chen, H.-I., C.-Y. Chu, and T.-C. Huang, Characterization of PdAg/Al2O3 composite membrane by electroless co-deposition. Thin Solid Films. 460(1-2):(2004) p. 62-71.

5.         Shu, J., et al., Simultaneous deposition of Pd and Ag on porous stainless steel by electroless plating. Journal of Membrane Science. 77(2-3):(1993) p. 181-195.

6.         Kikuchi, E. and S. Uemiya, Preparation of supported thin palladium-silver alloy membranes and their characteristics for hydrogen separation. Gas Separation & Purification. 5(4):(1991) p. 261-266.

7.         Bhandari, R. and Y.H. Ma, Pd-Ag membrane synthesis: The electroless and electro-plating conditions and their effect on the deposits morphology. Journal of Membrane Science. 334(1-2):(2009) p. 50-63.

8.         Witjens, L.C., et al., Improving the control of the electroless plating synthesis of Pd/Ag membranes for hydrogen separation using Rutherford backscattering. Journal of Membrane Science. 254(1-2):(2005) p. 241-248.

9.         Collins, J.P., et al., Catalytic Dehydrogenation of Propane in Hydrogen Permselective Membrane Reactors. Industrial & Engineering Chemistry Research. 35(12):(1996) p. 4398-4405.

10.       Volpe, M., et al., Optimized bath for electroless deposition of palladium on amorphous alumina membranes. Surface and Coatings Technology. 200(20-21):(2006) p. 5800-5806.

11.       Kulprathipanja, A., et al., Effects of Water Gas Shift Gases on Pd-Cu Alloy Membrane Surface Morphology and Separation Properties. Industrial & Engineering Chemistry Research. 43(15):(2004) p. 4188-4198.

12.       Hunter, J.B., Silver-palladium film for separation and purification of hydrogen. 1956, U.S. Patent 2,773,561.