In this work we synthesize and apply a unique class of supported Pd membranes and their use for in gas separation and reaction/separation applications. Pd based membranes have been researched for more several decades because of their potential for high temperature separation of hydrogen from gas mixtures. The main challenges facing the development of Pd membrane separation devices and membrane reactors are cost and durability. Cost demands the synthesis of ultrathin membranes while durability demands that membranes maintain high flux and permselectivity during prolonged use. Often these two challenges are conflicting: Membranes with thickness less than a few microns are prone to degradation during high temperature operation, especially transient in nature.

Many attempts have been made by researchers over the years to overcome the above-said challenges using a host of different synthesis strategies. Among the most studied has been the coating of porous ceramic and metallic supports with a thin layer of Pd or Pd alloys by electroless plating. In our research we have developed a new class of submicron encapsulated Pd hollow fiber membranes which involve the growth of Pd within the pores of a mesoporous support such as g-Al₂O₃. Some unique advantages of these membranes over conventional Pd impregnated membranes are increased separation power (product of flux and permselectivity), reduced thickness (or cost) and improved durability. In this study we report the results of a systematic study in which the Pd encapsulated membranes are tested in terms of separation and separation and reaction. The test reaction system is the catalytic decomposition of NH₃ to produce an ultra pure hydrogen stream. Ammonia decomposition is an appealing reaction system for on-demand generation of high purity H₂ for use in fuel cells and other applications (Choudhary et al., 2002; Zhang et al,. 2006; Collins et al., 1993). Ammonia can be stored or transported as a liquid at room temperature and 8 atm. The importance of membrane reactor system in coal gasification power plants to reduce the production of NO_x emissions by catalytic decomposition of NH₃ in the gas feed to the turbine is also reported (Collins and Way, 1994). The decomposition of NH₃ is a mildly endothermic reaction

2NH₃ ↔ N₂ + 3 H₂ ; DH = 11kcal/mol

which is limited by equilibrium, especially at low temperatures.

The encapsulated Pd membranes are supported within the pores of a 2-4 µm thick g-Al₂O₃ layer on the surface of a a-Al₂O₃ hollow fiber. Pre-plating of Pd is carried out on this g-Al₂O₃ layer by conventional sensitization and activation techniques. Electroless plating is carried out at a temperature around 50-60 C to achieve a rate of deposition of the order of 3-4µm/h. The rate of deposition and the thickness deposited on the hollow fiber is monitored online by a quartz crystal microbalance. The plating is stopped when the thickness is ~ 0.5µm. After a drying step at 120 C a layer of ~ 2µm thick g-Al₂O₃ is coated by film coating over the submicron level Pd film and fired at 500 C. The resulting composite hollow fiber membrane has a Pd film encapsulated between two g-Al₂O₃ layers. Additional electroless plating is carried out to grow Pd inside the pores of g-Al₂O₃. Capillary forces (~100 atm) enable the plating solution to fill the pores of the g-Al₂O₃ top layer. Plating time is kept very short in the order of induction time for nucleation. Diffusion time of Pd complex into the surface of the Pd film from the surface through the nanometer sized g-Al₂O₃ layer pores is of the order of 0.01s whereas the induction time for nucleation is in the order of minutes (~20 minutes). This ensures the continuous growth of Pd from the base film. The resulting “Type-I” nanowire membrane consists of a submicron thick Pd film embedded beneath a g-Al₂O₃ layer and the pores of the g-Al₂O₃ layer is filled with Pd.

The “Type-I” membranes are compared to two other membrane types. The second type termed “Type-II” is synthesized without a base film of Pd. This is accomplished by slip casting a layer of g-Al₂O₃ immediately after the Pd seed pre-plating step. Subsequent electroless plating allows the Pd “nanowires” to grow from the encapsulated Pd nuclei. This membrane consists only of nanowires growing from the Pd nuclei and a continuous film of Pd is often absent in this case. If we continue the plating step beyond the thickness of the g-Al₂O₃ over layer, we will obtain a Pd film anchored in the g-Al₂O₃ matrix using nanowires and is designated as “Type-III”. Permeation characteristics of all the three types of nanowire membranes will be presented.

A single tube catalytic packed bed membrane reactor is used to evaluate the capabilities of the three types of encapsulated Pd membranes to extract and purify hydrogen during the catalytic decomposition of NH₃ reaction. A supported Pt/Al₂O₃ catalyst was used. The single Pd composite hollow fiber membrane with sealed end is positioned within a packed bed of catalyst particles. Reaction is conducted at varying conditions of temperatures (350-600 C), pressures (2-5 atm) and space velocities. At steady state the permeate side of the membrane is under 1 atm of H₂ and high purity H₂ is withdrawn through the lumen of the membrane. Axial temperatures in the reactor are measured by a multipoint thermocouple placed inside the packed bed. The effects of temperature, pressure and space time on the conversion of NH₃ and its comparison with conventional packed bed reactor will be presented.

References

1) Zhang, J., Liu, D., He, M., Xu, H., and Li, W. (2006). Experimental and simulation studies on concentration polarization in H₂ enrichment by highly permeable and selective Pd membranes. J. of Memb. Sci., 274, 83-91.

2) Collins, J. P., and Way J. D. (1994). Catalytic decomposition of ammonia in a membrane reactor. J. of Memb. Sci., 96, 259-274.

3) Collins, J. P., Way J. D., and Kraisuwansarn, N. (1993). A mathematical model of a catalytic membrane reactor for the decomposition of NH₃. J. of Memb. Sci., 77, 265-282.

4) Choudhary, T. V. and Goodman, D. W. (2002). CO-free fuel processing for fuel cell applications. Catalysis Today 77, 65-78.

5) Choudhary, T. V. Sivadinarayana, C., and Goodman, D. W. (2001). Catalytic ammonia decomposition: CO_x -free hydrogen production for fuel cell applications. Catalysis Letters 72, 3-4,197-201.