Sulfur resistant Pd and Pd alloy membranes by phosphidation

• Pd and Pd-alloy membranes with phosphorus were prepared on a porous hollow fiber α-alumina support.
• The hydrogen permeation membranes were resistant to a gas mixture of 100 ppm H2S in H2.
• X-ray diffraction shows no detectable amounts of sulfur or phosphorus compounds.
• The presence of P suppresses the formation of cracks and pinholes.

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 673 K before and during exposure to a gas mixture of 100 ppm 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 11.2×10−7 mol m−2 s−1 Pa−1 (as-prepared) to 2.3×10−7 mol m−2 s−1 Pa−1, while in the presence of P the H2 permeance varied from 5.2×10−7 mol m−2 s−1 Pa−1 (as-prepared) to 6.5×10−7 mol m−2 s−1 Pa−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 17×10−7 mol m−2 s−1 Pa−1 (as-prepared) to 6.3×10−7 mol m−2 s−1 Pa−1 in contrast to 52×10−7 mol m−2 s−1 Pa−1 (as-prepared) to 5.9×10−7 mol m−2 s−1 Pa−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.

This study shows the effect of phosphorus addition on permselectivity and resistance to sulfur poisoning in supported palladium and palladium-alloy (Pd–Ag and Pd–Cu) membranes. The incorporation of P improves the membranes' recovery from sulfur poisoning after regeneration in hydrogen and produces more homogeneous surfaces in which the formation of cracks and pinholes is suppressed. Phosphorus is most beneficial in Pd–Cu membranes.Figure optionsDownload high-quality image (194 K)Download as PowerPoint slide