Sulfur resistant Pd and Pd alloy membranes by phosphidation

Novel ultra-thin Pd and Pd-alloy (Pd-Ag, Pd-Cu) membranes, with a phosphorus component, were prepared on a porous hollow fiber α-alumina support. The aim was to study the effect of the presence of a P additive on the behavior of the membranes towards sulfur poisoning. To incorporate P in the structure, the membranes were exposed to PH₃ as a gaseous phosphorous source. The performance and sulfur resistance of the membranes with and without P was evaluated by measuring the hydrogen permeance and the H₂/N₂ selectivity at 673K before and during exposure to a gas mixture of 100ppm H₂S in H₂, and also after regeneration in H₂. The membranes were characterized by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM), both before exposure to H₂S and after exposure/regeneration. XRD shows no detectable amounts of sulfur or phosphorus compounds in any of the exposed or P-treated membranes respectively. XPS indicates that S was located on the surface of the H₂S/H₂ treated Pd and Pd-Ag membranes. No S was detected on the surface of the Pd-Cu membranes indicating that no irreversible S compounds were formed in the latter. P was also detected on the surface of the P-treated membranes. The presence of phosphorus lowered the hydrogen permeance and H₂/N₂ selectivity; however it drastically improved the Pd-alloy membranes' capacity of recovery in H₂ after H₂S poisoning. The Pd-Cu membrane recovered only part of its H₂ permeance after regeneration, going from 11.2×10^-7molm^-2s^-1Pa^-1 (as-prepared) to 2.3×10^-7molm^-2s^-1Pa^-1, while in the presence of P the H₂ permeance varied from 5.2×10^-7molm^-2s^-1Pa^-1 (as-prepared) to 6.5×10^-7molm^-2s^-1Pa^-1 and its H₂/N₂ selectivity increased from 30,000 to 40,000. Similarly, the Pd-Ag membrane recovered better in the presence of P going from 17×10^-7molm^-2s^-1Pa^-1 (as-prepared) to 6.3×10^-7molm^-2s^-1Pa^-1 in contrast to 52×10^-7molm^-2s^-1Pa^-1 (as-prepared) to 5.9×10^-7molm^-2s^-1Pa^-1 in the absence of P. SEM images show that in the absence of P, more cracks and pinholes are visible on the surface of the H₂S/H₂ exposed membranes. The presence of P produces more homogeneous surfaces and less segregation, conferring a structural integrity to the membrane by suppressing the formation of cracks and pinholes.