Catalyst Tolerance to Sulfur

In general, the concentration of sulfur and its compounds would likely need to be reduced to the part per billion level to protect unalloyed or palladium-silver-based membranes. Removal of sulfur to tolerable levels from feeds originating from natural gas is much more straightforward relative to removal of sulfur and other impurities from coal according to Riesenfeld and Kohl [73]. 

McKinley studied the effects of poisoning by H2S on membrane materials of pure palladium, and alloys of palladium with silver, nickel, copper and gold [74]. Only the Pd-Au alloys showed reasonable resistance to poisoning by hydrogen sulfide according to McKinley [74]. An alloy of 60 wt% Pd and 40 wt% Au retained 80% of its initial (unpoisoned) permeability upon exposure to 4 ppm H2S and retained 40% of its permeability during exposure to 20 ppm H2S at 623 K (350 °C). In contrast, pure palladium was poisoned to 30% of its initial, unpoisoned value after 6 days exposure to only 4 ppm H2S. An alloy of 73 wt% Pd and 27 wt% Ag was completely poisoned within hours after exposure to only 4 ppm 

H2S [74]. An alloy of 60 wt% Pd and 40 wt% Cu fell to about 5% of its initial, unpoisoned permeability after a few days according to McKinley [74]. 

The Pd-Au and Pd-Cu alloys retained their metallic luster upon exposure to hydrogen sulfide and recovered their activity after removal of H2S [74], implying that both alloys were resistant to formation of bulk sulfides. The unalloyed Pd and Pd-Ag membranes were much changed in appearance, according to McKinley, indicative of the formation of bulk sulfides. X-ray diffraction analysis at Eltron Research Inc. shows that unalloyed Pd forms the bulk sulfide, Pd4S, and that Pd-Ag alloys form bulk palladium-silver sulfides, leading to failure of membranes exposed to under 20 ppmv H2S in the range 593-713 K (320-440 °C) [75]. The Pd 60-Cu 40 (wt%) alloys resist formation of bulk sulfides, but were not completely satisfactory for use with H2S [75], in accord with the work of McKinley [74]. 

Morreale et al. find that 80 wt% Pd-20 wt% Cu alloys exhibit higher resistance to H2S (at very high partial pressures of hydrogen) relative to the 60 wt% Pd-40 wt% Cu composition [77]. However, the 80-20 composition has a lower initial (unpoisoned) permeability relative to the 60—40 composition. The 80-20 Pd-Cu composition has the face centered cubic crystal structure as do unalloyed elemental palladium and copper [77]. 

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