One of the key elements to the success of Pd-Ag membrane based reactor for the H2 production is the synthesis of thin and highly selective membranes using the electroless plating method. This work describes the effect of electroless plating conditions on the obtained Pd and Ag deposits properties (morphology, compactness, phase structure, compositional homogeneity and adhesion) important from synthesis of thin and H2 selective membrane viewpoint. Both sequential and co-deposition deposition methods were investigated. The conventional Pd and Ag plating conditions (NH3+EDTA based bath) produced dendritic and non-uniform sequential (multi layer) deposits, not suitable for synthesizing the thin and H2 selective Pd-Ag membranes. Ag under the conventional plating conditions deposited at high overpotential resulting in the dendritic and non-uniform sequential deposits. The modified Ag plating conditions eliminated Ag deposition at high overpotential and the sequential deposits obtained were non-dendritic and uniform. Thin (< 10 µm thick) and H2 selective Pd-Ag membranes were successfully synthesized using the modified Ag plating conditions. The membranes were then successfully annealed at 550 oC. After the annealing step, the membranes showed activation energy for the H2 permeation (4.3-11.5 kJ/mole) lower than that of the pure Pd membrane (12-16.4 kJ/mole) meaning that the Pd-Ag membranes were more effective for the H2 separation at lower temperatures than the pure Pd membrane. A Pd-Ag (20 wt%) membrane showed H2 permeance higher by a factor of 2.47 at 250 oC than the pure Pd foil. The Pd-Ag membranes also showed decline in the H2/He selectivity on exposure to the annealing and H2 permeation (300-500 oC) study conditions. The Pd-Ag co-deposits obtained (using NH3+EDTA bath) were dendritic, inhomogeneous with poor substrate adhesion, therefore not suitable for the membrane synthesis. The co-deposits were bi-metallic and required the annealing step to form the Pd-Ag alloy. There existed a large difference in the deposition potentials (600 to 650 mV) of Pd and Ag. The Ag deposition was severely controlled by its mass transfer in the solution resulting in the dendritic and inhomogeneous deposits. Among the different complexing agents investigated, KCl showed the least difference between the Pd and Ag deposition potentials. The co-deposits obtained using the KCl bath were non-dendritic, homogeneous and were Pd-Ag alloy therefore required no annealing step. Finally, the multi step plating, annealing and polishing approach was used to avoid the decline in the selectivity of the sequentially prepared Pd-Ag membranes. The membranes prepared by the plating, annealing and polishing approach showed very high selectivity (H2/He) and no decline in the selectivity was observed between 300-450 oC for the total exposure time > 550 h (> 200 h at 450 oC).

ABSTRACT Recent global interests in developing hydrogen economy generate substantial research and development for hydrogen production worldwide. Pd membranes are especially suited for high temperature hydrogen separation and membrane reactor applications. Alloying Pd with Ag not only suppresses hydrogen embrittlement, but also increases the permeability of the alloy membrane. The main objective of this work was to carry out fundamental studies to understand the properties of the porous stainless steel (PSS), morphologies of Pd and Ag deposits on PSS, and the structural changes of the membrane layer upon heat treatment. Both coating and diffusion and co-plating techniques were employed in the study. The Pd-Ag membranes that had sandwiched Ag layers suffered from very low selectivity due to the voids formed because of high diffusion rate of Ag. Alloy membranes with high selectivity can be prepared by applying intermediate annealing after each Ag deposition. On the other hand, the homogeneity of the alloy depended very much on the thickness of the deposited layers and annealing temperature and time. A stable co-plating bath was developed to co-plate Pd and Ag simultaneously. Pd-Ag membranes were prepared from co-plating bath using ultrasound to accelerate the plating rate.

Thesis (M.S.)--Worcester Polytechnic Institute. / Keywords: Hydrogen Permeation; Pd-Ag Membranes; Electroless Plating. Includes bibliographical references (p. 114-118).

Composite thin Pd/Ag alloy membranes with long-term thermal and chemical stabilities have potential applications for H2 separation via catalytic membrane reactors and may be one of the key determinants to achieve the 21st century's global hydrogen economy. This work provides a detailed microstructure characterization study and a better understanding of the fundamental principles involved in the synthesis of a novel Pd/Ag intermetallic diffusion barrier formed by the bi-metal multi-layer (BMML) deposition technique. The BMML deposition technique formed an extremely effective Pd/Ag intermetallic diffusion barrier and significantly improved the thermal and long-term stability of the composite Pd and Pd/alloy membranes over a temperature range of 500-600oC. In addition, high temperature annealing studies over a temperature range of 500-800oC in H2 atmosphere led a thorough understanding of the surface interactions and the phase changes between the Pd and Ag metals and the porous metal support elements (Fe, Cr and Ni) and it was shown by the SEI, EDX and X-ray phase analyses that the Ag/Fe and Ag/Ni binary systems exerted complete immiscibility compared to the completely miscible solid solutions of Pd/Fe and Pd/Ni phases. A novel characterization method of in-situ time-resolved high temperature X-ray diffraction (HTXRD) analysis was used to elucidate the mechanistic details of the isothermal nucleation and growth kinetics of the Pd/Ag alloy phase over a temperature range of 500-600oC in H2. The nucleation of the Pd/Ag alloy phase was instantaneous where the growth mechanism was through diffusion-controlled one-dimensional thickening of the Pd/Ag alloy layer. The Pd/Ag alloy phase growth was strongly dependent upon the deposition morphology of the as-synthesized Pd and Ag layers due to the presence of the heterogeneous nucleation sites. Based on the empirical rate constants derived from the solid-state reaction models, the estimated activation energies for the Pd/Ag alloy phase transformation were 236.5 and 185.6 kJ/mol and in good agreement with the literature values of 183-239.5 kJ/mol. The successful utilization of surface modification techniques and modified plating conditions led to the synthesis of several dense Pd/Ag layers, which were as thin as 5-15 µm with a bulk Ag content in the 10-40 wt% range. The long-term testing of the composite Pd/Ag membranes (5-15 µm) at 500oC showed stable hydrogen permeances as high as 30 to 54 m3/m2-h-atm0.5 with H2/He selectivities ranging from 200 to 14000. Furthermore, the atomic absorption flame analysis was used for the first time to elucidate the effects of temperature, initial metal ion concentration, initial hydrazine concentration and bath agitation on the electroless plating rates of Pd and Ag. The electroless plating of both Pd and Ag were strongly affected by the external mass transfer in the absence of bath agitation. The external mass transfer limitations for both Pd and Ag deposition have been overcome at or above an agitation rate of 400 rpm, resulting in a maximum conversion of the plating reaction and dramatically shortened plating times with the added advantage of uniform deposition morphology as evidenced by the SEI micrographs. Finally, the agitation rate of 400 rpm was successfully employed for the synthesis of composite Pd and Pd/Ag membranes. The H2 permeance for a 4.7 µm thick pure-Pd membrane at 400oC was as high as 61 m3/m2-h-atm0.5 with H2/He selectivity over 310 after a total testing period of 690 hours.