(288b) Long Term Thermal Stability of Pd and Pd-Alloy Composite Membranes

Long Term Thermal
Stability of Pd and Pd-alloy Composite Membranes  

Hani Abuelhawa¹,
Oyvind Hatlevik¹, Stephen N. Paglieri², Aadesh
Harale³, and J. Douglas Way¹

¹Department
of Chemical & Biological Engineering, Colorado School of Mines,

Golden, CO 80401, USA

²TDA
Research Inc., Wheat Ridge, CO 80033, USA

³ Saudi
Arabian Oil Company, Dhahran, 31311, KSA

The integration of a Pd-based
membrane within a catalytic reforming reactor to produce hydrogen has received
much more attention during the last few decades [1].  In addition to
being permselective to hydrogen, Pd-based membranes are capable of maximizing
the conversion of thermodynamically-limited reactions, such as the steam
reforming of methane, beyond the equilibrium conversion based on the feed composition,
by simply withdrawing the hydrogen from the reaction mixture as it forms. 
However, there are still some issues that prevent this technology from being
fully commercialized, such as the long term thermal stability and the chemical
tolerance of these membranes in real-world mixed gas environments.

Guazzone, et al. [2] has
investigated the long term thermal stability of electrolessly plated pure Pd
membranes by identifying the mechanism that leads to the leak evolution in
hydrogen atmosphere.  They concluded the leak evolution is a
thermally-activated process induced by the self-diffusion of Pd at temperatures
above 400°C.  In this work, we will investigate the thermal stability of Pd and
Pd-alloy membranes such as PdRu, PdAu, and/or PdAg.  Previous literature [1]
has investigated the performance and properties of Pd-alloy membranes, however,
the long term thermal stability of these alloys did not receive so much
attention.

In this paper, long term high
temperature stability data for a 5.1 micron thick sequentially plated PdAu10wt%
membrane prepared by the electroless plating method on a porous-zirconia/porous-stainless-steel
support will be discussed.  The membrane has been tested for approximately 1000
hours and showed superior thermal stability at 500°C and below, in comparison to
similar pure Pd membranes. The membrane also maintained an ideal H₂/N₂
pure gas flux ratio of at least 1000 at 100 psig for more than 1000 hours of
testing.  As shown in Figure 1, nitrogen leak did not grow significantly in the
temperature range (450-500°C) during several hundred hours of testing.   Figure
2 compares the performance of PdAu membrane to a pure Pd of similar thickness
at 500°C. Addition of Au to Pd enhanced the thermal stability of the membrane
by reducing the leak rate by at least one order of magnitude. Additionally, by comparing
the leak rate of this PdAu membrane with the results of Guazzone, et al. [2]
for a pure Pd membrane of comparable thickness, the leak growth rate in the
PdAu membrane is also lower by at least one order of magnitude.  The
calculation of the activation energy of the leak growth rate in Pd-alloy
membranes and comparing it to the Pd self-diffusion activation energy could
give some insights in revealing the leak growth mechanism in these membranes.

References

[1] Ø. Hatlevik et al., Separation and Purification
Technology 73 (2010) 59?64

[2] F. Guazzone et al., AIChE Journal, (2008) Vol. 54, No.
2, 487-494.

Figure (1): The high temperature thermal stability
data for the 5.1 micron PdAu10wt% membrane deposited on zirconia/SS porous
support.

Figure (2): A comparison of the thermal stability
data of a PdAu10wt% and a pure Pd control at 500 °C, 100 psig.  Both Membranes
are of comparable thickness.