279616 Long-Term Thermal Stability and Chemical Tolerance of Supported Thin Film Pd-Au-Ru Membranes
Long-term thermal stability and chemical tolerance of supported thin film Pd–Au–Ru membranes
S.N. Paglieri*,1, Ř. Hatlevik2, H. Abuelhawa2, and J.D. Way2
1TDA Research, Inc., Wheat Ridge, Colorado, USA
2Department of Chemical Engineering, Colorado School of Mines, Golden, Colorado, USA
Long-term thermal stability and chemical tolerance are essential for hydrogen membranes and membrane reactors. A 3.5 µm thick Pd–11Au–5Ru (wt.%) membrane was tested for >3000 h at 400-500°C in pure hydrogen and helium, and for 120 h in simulated water-gas shift (WGS: 50% H2, 19% H2O, 1% CO and 30% CO2) mixtures at 400 and 450°C. The Pd–Au–Ru alloy membranes were thermally stable at 400°C and tolerant of WGS gases, but were attacked by WGS gases containing 20 ppm H2S.
Pd-11Au-5Ru (3.5 µm thick) and Pd–10Au–5Ru (5.3 µm thick) membranes were deposited onto zirconia (ZrO2)/porous stainless steel supports using electroless plating techniques. Simultaneous co-deposition of Pd–Ru was followed by displacement plating (a variant of electroless plating) of Au.1 This sequence was repeated until a dense film was obtained. Preparation details and the testing apparatus have been described previously.2 SEM/EDX, XRD, AA and DSC were used to characterize morphology, microstructure, chemical composition, and thermal stability.
The initial H2/He pure gas permeation ratio for the Pd–11Au–5Ru membrane was ~20,000 at 400°C and 1.4 MPa pressure differential across the membrane. A minimal increase in helium leak rate through the membrane occurred at 400°C for >1000 h as shown in Figure 1, which indicated that the membrane was very stable, however, hydrogen flux was still increasing even after 1400 h. Subsequent annealing at 450°C for 300 h appeared to result in further alloying and higher flux, especially after exposure to simulated WGS gases for >120 h. At 450 and 500°C, the helium leak rate increased linearly over time, but it more than doubled when the temperature was increased to 500°C. A hydrogen permeance of 4.8×10-3 mol m-2 s-1 Pa-0.5 was measured at 500°C.
Exposing the Pd–11Au–5Ru and Pd–10Au–5Ru membranes to WGS conditions for >120 hours at either 400 or 450°C resulted in minimal increase in helium leak rate through the membrane (Figure 2). However, loss of feed CO2 flow or exposure to excess CO for only several hours caused a permanent increase in the impurity leak rate. Next, the Pd–10Au–5Ru membrane was annealed at 400°C and exposed to WGS gases containing 20 ppm H2S as shown in Figure 3, which resulted in an immediate 60% loss in hydrogen flux and a rapidly increasing concentration of impurities (CO, CO2) in the hydrogen permeate.
Pd–Au–Ru membranes were thermally stable at 400 and 450°C in pure hydrogen, and exposure to WGS conditions resulted in a minimal increase in the helium leak rate at both 400 and 450°C. Exposure to WGS gases apparently increased the hydrogen permeability by accelerating atomic rearrangement of the film (without increasing the impurity leak rate), as was previously observed for thicker Pd–Ru alloy membranes.3 Exposure to WGS gases demonstrated the carbon tolerance, however, addition of 20 ppm H2S to the WGS mixture rapidly degraded both flux and purity. Therefore, it appears that Pd–Au–Ru alloys may be less H2S tolerant than binary Pd–Au or ternary Pd–Au–Pt alloys due to accelerated rearrangement in the presence of H2S.2b, 4 Potential mechanisms affecting pore growth rate in the membrane film include alloying, recrystallization, grain boundary diffusion and coalescence of microvoids. Development of alloys with greater durability is essential for the long-term operation of membranes and membrane reactors. The results for these Pd–Au–Ru alloy membranes will be compared to the performance of other binary (Pd–Au, Pd–Ru) and ternary (Pd–Au–Pt) alloy membranes and presented at the meeting.
1. Gade, S. K.; Keeling, M. K.; Davidson, A. P.; Hatlevik, Ř.; Way, J. D., Palladium–ruthenium membranes for hydrogen separation fabricated by electroless co-deposition. Int. J. Hydrogen Energy 2009, 34, 6484-6491.
2. (a) Hatlevik, Ř.; Gade, S. K.; Keeling, M. K.; Thoen, P. M.; Davidson, A. P.; Way, J. D., Palladium and palladium alloy membranes for hydrogen separation and production: History, fabrication strategies, and current performance. Sep. Purif. Technol. 2010, 73 (1), 59-64; (b) Gade, S. K.; DeVoss, S. J.; Coulter, K. E.; Paglieri, S. N.; Alptekin, G. O.; Way, J. D., Palladium-gold membranes in mixed gas streams with hydrogen sulfide: Effect of alloy content and fabrication technique. J. Membr. Sci. 2011, 378 (1-2), 35-41.
3. Ermilova, M. M.; Orekhova, N. V.; Skakunova, E. V.; Gryaznov, V. M., Changes in the catalytic activity and hydrogen permeability of a palladium–ruthenium alloy membrane catalyst under the influence of reagents. Bull. Acad. Sci. USSR 1988, 37 (4), 637-640.
4. Coulter, K. E.; Way, J. D.; Gade, S. K.; Chaudhari, S.; Alptekin, G. O.; DeVoss, S. J.; Paglieri, S. N.; Pledger, B., Sulfur tolerant PdAu and PdAuPt alloy hydrogen separation membranes. J. Membr. Sci. 2012, 405–406, 11-19.