The formation of the intermetallic diffusion barrier layer by the controlled in-situ oxidation method for Pd and Pd/alloy porous stainless steel composite membranes was investigated. SEM and EDS results showed the existence of an oxide layer as the intermetallic diffusion barrier for oxidation temperatures higher than 600°C. At oxidation temperatures lower than 600°C, there might still be an oxide layer at the membrane-substrate interface although it was too thin to be detected by SEM and EDS. The alloy formation study showed that annealing at 500°C under helium atmosphere did not produce alloys with uniform compositions either for Pd/Ag or Pd/Cu membranes. However, annealing at 600°C gave a uniform Pd/Cu-porous stainless steel (PSS) composite membrane, with no detectable presence of elements from the PSS substrate, further demonstrating the oxide layer as an effective intermetallic diffusion barrier.

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Composite palladium and Pd-based membranes represent an appealing technology option to increase the CO conversion and the H 2 recovery in the waterÀgas shift (WGS) reactor as a result of the continuous removal of hydrogen over the course of the reaction. Even though many studies have been performed in this area, their outcome typically represents a proof-ofconcept involving reactors with small membrane area. The present study therefore addresses the scaling up of the process to obtain high hydrogen production rates through the use of large surface area, ∼0.02 m 2 , composite Pd membranes. Two thin, δ < 10 μm, defect-free composite membranes were prepared by the electroless plating method on porous stainless steel tubular supports and tested under pure gases and waterÀgas shift reaction conditions. Syngas similar to the actual gasifier reacting mixture (40% H 2 , 42.2% CO, and 17.8% CO 2 and steam to carbon ratio varying between 2.5 and 3.5) was fed to the WGS catalytic membrane reactor (WGS-CMR) with a total flow rate up to 1.5 Nm 3 h À1 , 20 bar maximum pressure, and temperatures ranging 420À440°C. In the presence of crushed catalyst, CO conversions higher than the equilibrium conversions were obtained within the entire gas hour space velocity (GHSV) range considered in the present study, for pressures between 7 and 20 bar. At a relatively low feed flow rate, GHSV = 1130 h À1 , a maximum CO conversion of 98.1% was achieved, with a hydrogen recovery of 81.5% at 440°C. On the other hand, at the highest GHSVs, the system appeared to be limited by the activity of the ferrochrome catalyst. At 20 bar of absolute pressure in the retentate side, 440°C, and GHSV = 5650 h À1 , the hydrogen production rate was found to be 5.6 Nm 3 day À1 . Remarkably high hydrogen purity, in excess of 99.97% and 99.2% for the two membranes, respectively, was achieved also in experiments performed with a retentate pressure of 20 bar.

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in Wiley InterScience (www.interscience.wiley.com).The leak (of He gas) in composite Pd membranes prepared by the electroless deposition method was studied. The leak was distributed over the entire surface of membranes, and was due to the formation of pinholes, 0.1-2 lm in dia., located at the boundaries of Pd clusters and Pd crystallites. The rate at which the leak developed in H 2 atmosphere was determined at 450, 500 and 5508C, and an apparent activation energy for the leak growth was calculated to be equal to 238 kJ/mol, which was close to the activation energy of Pd self-diffusion coefficient of 266 kJ/mol. From experimental data at 450, 500 and 5508C, the leak rate increase at 4008C was extrapolated. In order to corroborate the calculated rate at 4008C, a 4 lm thick composite Pd membrane, M-53, was studied for over 2,000 h in H 2 atmosphere. M-53 showed a H 2 permeance of 42 m 3 /m 2 h bar 0.5 , and an outstanding selectivity (H 2 /He) higher than 22,000 over 2,200 h, which was in agreement with the calculated leak rate increase at 4008C.