(701c) Characterization of Sulfur Resistance of Pd-Au Hydrogen Separation Membranes Prepared by Galvanic Displacement | AIChE

(701c) Characterization of Sulfur Resistance of Pd-Au Hydrogen Separation Membranes Prepared by Galvanic Displacement

Authors 

Chen, C. - Presenter, Worcester Polytechnic Institute


Sulfur containing impurities are known to cause severe and irreversible poisoning of Pd membranes during the hydrogen separation application (Especially in the coal gasification) resulting in the reduction of hydrogen permeability, selectivity and structure deterioration of Pd membranes. Pd/Au alloys are found to show high sulfur resistance in comparison to pure Pd and other Pd based alloys (i.e., Pd/Ag, Pd/Cu, etc.) in addition to its high hydrogen permeability. As a result, Pd/Au alloy has recently regained great interests for membrane separation process. The preparation of Pd/Au alloys can be carried out by the galvanic displacement method, which does not require external sources of current, reducing agent and complexing agent (such as cyanide) and provides simplified plating equipment and environmentally preferable procedure. The main objectives of this study were to investigate the effect of temperature on sulfur poisoning of pure Pd and Pd/Au alloy morphologically. In addition, the hydrogen permeation characteristics of the Pd/Au membrane in the presence of H2S and the permeance recovery after the sulfur poisoning at different temperatures were investigated.

Pure Pd coupons were prepared by the electroless plating on media-grade 0.1 micron porous 316L stainless steel (PSS) coupons (1 cm x 1 cm), and Pd/Au alloy coupons were prepared by the Au displacement plating in 3mM NaAu(Cl)4?2H2O solutions on the Pd plated PSS coupons (6-9 micron thick Pd) followed by annealing at 550ºC for 24 hours in H2. The prepared pure Pd and Pd/Au alloy coupons were then exposed to a 54.8 ppm (±1 ppm) H2S/H2 mixture for 24 hours at the temperature from 350ºC to 500ºC with the interval of 50ºC.

The SEM, EDX, and XRD analyses indentified the pure Pd phase on the Pd coupons and the Pd/Au alloy phase with the surface Au content of about 10wt% on the Pd/Au alloy coupons before the poisoning experiments, and showed negligible sulfur content on both pure Pd and Pd/Au alloy coupons. After the poisoning, SEM micrographs showed the ?net-like? surface structure with numerous pinholes on pure Pd coupons that were poisoned at the temperature from 350ºC to 450ºC, while fewer pinholes were observed on the ones that were poisoned at 500ºC. The EDX and XRD analyses indentified the Pd4S phase formation with about 4 wt% S on the surface that was poisoned from 350ºC ? 450ºC, and no Pd4S phase and sulfur were detected on the ones poisoned at 500ºC. For the Pd/Au alloy coupons, no surface morphology changes were observed on the SEM micrographs and no new phase and sulfur were detected on the surfaces that were poisoned at entire temperature range studied (i.e. 350ºC - 500ºC). The results indicated that Pd/Au alloy did show higher resistance to sulfur adsorption and sulfide formation.

A Pd/Au membrane was prepared by electroless plating Pd on the porous Inconel supports (0.1 micron media grade) until gas-tight followed by the Au displacement in 3mM NaAu(Cl)4?2H2O solutions. The membrane was annealed at the temperature from 250ºC up to 550ºC in H2 and He for over 2000 hours and re-plated with Pd and Au to dense prior to the sulfur poisoning test. The final thickness of the membrane was estimated gravimetrically as 18 micron with approximately 10wt% Au. The membrane was characterized for its hydrogen permeation characteristics in a 54.8ppm (±1ppm) H2S/H2 mixture for 4 hours and for the permeance recovery characteristics in pure H2 after poisoning at the temperatures from 350ºC to 500ºC.

Prior to the sulfur poisoning, the membrane showed a steady state hydrogen permeance of 1.2 m3/(m2*h*atm0.5) at 250ºC and increased with increasing temperature reaching 6.5 m3/(m2*h*atm0.5) at 500ºC with a stable H2/He separation factor of 100. The activation energy for the H2 permeation was 13.1 kJ/mol. During the exposure to a 54.8 ppm (±1 ppm) H2S/H2 mixture, the permeance declined instantaneously to a steady state value in the entire temperature range tested (i.e. 350ºC, 400ºC, 450ºC, and 500ºC). The percentage of permeance decline decreased as the temperature was increased, form 88% at 350ºC, 85% at 400ºC, 74% at 450ºC, to 60% at 500ºC. After the poisoning, permeance was recovered to a steady state value in pure H2 at each poisoning temperature before the fully recovery in H2 at 500ºC. The recovered permeance was 43% of the original value at 350ºC, 65% at 400ºC, 74% at 450ºC, and 91% at 500ºC, and the recovery time required to reach the steady state value was 100 hours at 350ºC, 66 hours at 400ºC, and 25 - 30 hours at 450ºC and 500ºC. The results suggested that the higher desorption rate of the H2S molecules at higher temperatures resulted in less sulfur adsorption on the membranes surface thereby less poisoning effect of the membrane at higher temperatures. Finally, the He leak showed no significant increase after the sulfur exposure indicating that the adsorbed sulfur molecules might segregate to the grain boundaries and defects in the Pd/Au films to block the passage-way of the non-adsorption gas, such as He.

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