Surface segregation of Pd–Cu alloy in various gas atmospheres

Pd–Cu alloys have been investigated as promising candidates for hydrogen separation membranes. Surface segregation influences the long-term performance of these membranes since their catalytic effect is mainly controlled by the surface composition. In the present research, surface segregation of Pd-40 at.% Cu alloy in vacuum and various gas atmospheres (H 2 , CO and CO 2 ) was investigated with both XPS and LEISS probing different depths below the surface. Adsorption of H 2 and CO on the surface has a significant impact and the surface segregation trend can be reversed as compared to segregation in vacuum, however, CO 2 has almost no influence on the segregation behaviour. A thermodynamic model is also presented to explain these phenomena and to understand surface segregation behaviour of binary alloys in various gas atmospheres. The results can be considered as basic guidelines to design novel alloys for hydrogen separation membranes and predict their long-term performance under actual working conditions. • Surface segregation of Pd–Cu was studied in vacuum and various gas atmospheres. • Cu content of the surface region was measured by LEISS and XPS respectively. • Opposite surface segregation behaviours were observed in vacuum and in H 2 or CO. • Effect of gas adsorption on surface segregation were theoretically discussed.