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Abstract: Thin PdAg alloy membranes with 20–25% Ag are being developed for hydrogen separation technology. Despite many investigations on such membranes as well as representative experimental and theoretical model systems, unresolved issues remain concerning the effect of the alloy surface structure and composition on adsorption and vice versa. Therefore, the interaction between hydrogen, carbon monoxide or oxygen with the surface of a PdAg model alloy was studied using periodic self-consistent density functional theory (DFT-GGA) calculations. In particular, the adsorption structure, coverage dependence and possible adsorption-induced segregation phenomena were addressed using Pd3Ag(111) model surfaces with varying degrees of surface segregation. In agreement with previous experimental and theoretical investigations, we predict Ag surface termination to be energetically favorable in vacuum. The segregation of Ag is then reversed upon adsorption of H, CO or O. For these adsorbates, the binding is strongest on Pd three-fold hollow sites, and hence complete Pd termination is favored at high coverage of H or CO, while 25% Ag may remain under oxygen because of the lower O-saturation coverage. CO adsorption provides a somewhat stronger driving force for Pd segregation when compared to H, and this may have implications with respect to permeation properties of PdAg alloy surfaces. Our predictions for high coverage are particularly relevant in underlining the importance of segregation phenomena to the hydrogen transport properties of thin PdAg alloy membranes. [Copyright &y& Elsevier]

Abstract: The performances and long-term stability in the 350°–450°C temperature range of a Pd–Ag alloy thin film membrane are reported. The membrane was prepared by electroless plating deposition (EPD) on the inner walls of a tubular alumina asymmetric support using a novel seeding procedure based on Pd-only complexes. A defect-free film with thickness around 12 microns is obtained. Scanning electron microscopy (SEM) images evidence the presence of aggregates of small crystallites of few hundred nanometers. The membrane shows a stable behaviour for at least 260h of time-on-stream in pure H2 and permeability tests indicate a high hydrogen flux with an activation energy of about 14kJ/mol. The permeability follows Sievert''s law with an exponent of 0.5, indicating a bulk diffusion of hydrogen as the rate-controlling step. There is an initial increase in the transmembrane H2 flux during the time-on-stream tests, due to hydrogen absorption in the Pd–Ag alloy which increases the permeation rate. The presence of two mechanisms of transport, within Pd–Ag alloy crystallites and in the intergrain boundary region, is discussed. The permeability behaviour using H2:N2 mixtures as the feed is also shown. The presence of N2 induces a large decrease in the H2 permeation rate, greater than that expected by dilution effect. The membrane is stable in the 350°–400°C temperature range with a high separation factor between H2 and N2, but at 450°C shows an initial fast decrease of the selectivity with time-on-stream up to reach a nearly constant value. The effect is interpreted in terms of formation of NH x species which inhibit hydrogen diffusion in the intergrain boundary region, as suggested also from literature data. [Copyright &y& Elsevier]