(356e) 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,
Ø. 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

spaglieri@tda.com

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.

References

1.   Gade,
S. K.; Keeling, M. K.; Davidson, A. P.; Hatlevik, Ø.; Way, J. D.,
Palladium?ruthenium membranes for hydrogen separation fabricated by electroless
co-deposition. Int. J. Hydrogen Energy 2009, 34,
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2.   (a)
Hatlevik, Ø.; Gade, S. K.; Keeling, M. K.; Thoen, P. M.; Davidson, A. P.; Way,
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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
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(1-2), 35-41.

3.   Ermilova,
M. M.; Orekhova, N. V.; Skakunova, E. V.; Gryaznov, V. M., Changes in the
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37 (4), 637-640.

4.   Coulter,
K. E.; Way, J. D.; Gade, S. K.; Chaudhari, S.; Alptekin, G. O.; DeVoss, S. J.;
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