(701b) The Effect of H2s on the Long-Term Stability of Pd-Cu Membranes and the Characteristics of H2s Poisoning of Electroless Deposited Pd

Hydrogen selective Pd membranes can improve the efficiency and cost effectiveness of the coal gasification process by recovering high purity H₂ from the syngas. Also, the high pressure CO₂ in the retentate reduces the cost of carbon capture required for carbon sequestration. However, small quantities of H₂S present in the gas stream poison the Pd membrane and reduce the hydrogen permeance by either adsorbing on the surface or reduce the selectivity of the membrane by forming a sulfide scale. Pd/Cu alloys have been shown to be more tolerant to H₂S than pure Pd. Therefore, the objective of this study was to examine the interactions of H₂S with Pd that led to membrane poisoning and to test the long-term stability of Pd/Cu membranes in the presence of H₂S with the purpose of designing sulfur tolerant membranes.

Coupons of 316L PSS (0.5 micron media grade) were plated with Pd by the electroless deposition method. The Pd layers were approximately 12 μm thick. Prior to plating, the coupons were oxidized for 12 hours at 800ºC to form an intermetallic diffusion barrier between the PSS and the Pd layer. The plated samples were exposed to either pure H₂ or a mixture of 50 ppm H₂S/H₂ for 24 hours at temperatures of 350, 400, 450 and 500ºC. All samples were analyzed with XPS, XRD, SEM and EDS. A Pd/Cu membrane was fabricated by the electroless plating of sequential layers (12.7 μm Pd + 3.8 μm Cu) on a porous Inconel tubular support (0.5? diameter, 0.1 micron media grade) which was graded with Al₂O₃ and plated with a Pd/Ag intermetallic diffusion barrier before the dense layer was deposited. The membrane was exposed to a mixture of 50 ppm H₂S/H₂ for 120 hours at 450ºC and the permeance was recovered by exposing the membrane to pure H₂. The concentration of H₂S in the retentate and the H₂ flux were monitored throughout the entire experiment.

At temperatures of 400, 450 and 500ºC, the exposure of the Pd coupons to the 50 ppm H₂S/H₂ mixture resulted in sulfur adsorbing on the Pd surface and forming sulfide bonds with Pd. High resolution XPS studies of the pure Pd coupons confirmed the presence of both Pd ? Pd and Pd ? S chemistries, indicating the formation of surface sulfides. At 350ºC, a Pd₄S scale (bulk sulfide) formed on top of the Pd layer. In addition, XPS quantitative analysis of the samples exposed to the 50 ppm H₂S/H₂ mixture showed a decreasing S/Pd atomic ratio and an increasing Pd-Pd/Pd-S ratio with increasing temperature, showing that increasing temperature decreased the amount of sulfur adsorbed on the Pd surface, due to an increase in the rate of H₂S desorption and surface sulfides becoming less stable. The presence of the bulk sulfide, Pd₄S, was verified by XRD and EDS on the coupon exposed to the H₂S mixture at 350ºC. The morphology of the deposited Pd changed from a granular structure to a coral-like structure upon the Pd₄S scale formation. At 400, 450 and 500ºC, pores formed in the grain boundaries of the Pd deposit after H₂S adsorption. Both the morphology change and the pore formation could cause leaks in a membrane.

The Pd/Cu membrane was characterized in H₂ and He for over 600 hours between the temperatures of 250 - 450ºC. The ideal H₂/He separation factor after the long period of testing was 220. Upon exposure to 50 ppm H₂S/H₂ at 450ºC, the permeance of the membrane had an instantaneous drop to 21% of the original permeance at that temperature. The drop was due to H₂S forming surface sulfides on the membrane and decreasing the effective area for H₂ adsorption. The permeance remained stable for 120 hours in the H₂S mixture. The recovery rate of the permeance in pure H₂ at 450ºC was 0.017 m³/(m²*bar^0.5*h)*h or 0.11%/h. At the end of the recovery period which lasted for 600 hours, the permeance stabilized at 64% of the value of the permeance before the initial H₂S poisoning. The inability to completely recover the permeance showed that part of the sulfide poisoning was irreversible at 450ºC. The activation energy of hydrogen permeance was 15.0 kJ/mol before the H₂S testing and slightly decreased to 13.8 kJ/mol after the recovery. Because of the drop in permeance, the ideal H₂/He separation factor decreased to 170 after the H₂S testing. However, the He leak also decreased from 1.24 sccm to 1.01 sccm over a period of 700 hours at 450ºC, showing that exposure to H₂S increased the leak stability of the membrane. The decrease in the He leak could have been caused by sulfur segregating to the grain boundaries of the deposits. In contrast to the implications of the results of the pure Pd coupon study, the Pd/Cu alloy showed leak stability in the presence of H₂S. In accordance with the XPS studies, the tolerance to sulfur should increase with increasing temperature due to less surface sulfides formation.