Modelling of surface segregation for palladium alloys in vacuum and gas environments

Abstract

Surface segregation of a series of forty Palladium-based binary alloys has been investigated using a thermodynamic model based on an atom exchange approach. Their surface segregation behaviour, both in vacuum and in gas environments, were comprehensively estimated. The calculated results are in good agreement with the available experimental and computational data reported in literatures. Effects of mixing enthalpy, temperature, crystal orientation on the surface, elastic strain energy, adsorption and absorption of gases like H 2 , O 2 , CO have been discussed in detail. These results can be considered as basic guidelines to design novel Pd alloys for hydrogen separation membranes, sensors or catalysts. The model itself also offers a convenient and accurate routine to predict the surface segregation of other than Pd-based binary alloys in different gas atmospheres.

Surface Segregation of Ternary Alloys: Effect of the Interaction between Solute Elements

“…For binary alloys, the reduction of the surface energy is the most important driving force of surface segregation, which indicates that the element with lower surface energy usually tends to segregate to the surface, although the elastic strain energy and chemical bonding energy are also of influence 36. For example, in the Pd‐Ag and Pd‐Cu binary alloys, the segregation of Ag and Cu in vacuum have been proven by experiments, while Pd segregation in the Pd‐Mo binary alloys has also been predicted due to the much higher surface energy of Mo 23,37,38. However, as mentioned above, the interaction between two solute elements makes the situation much more complicated in ternary alloys.…”

“…Interfaces 2020, 7,1901784 For example, in the Pd-Ag and Pd-Cu binary alloys, the segregation of Ag and Cu in vacuum have been proven by experiments, while Pd segregation in the Pd-Mo binary alloys has also been predicted due to the much higher surface energy of Mo. [23,37,38] However, as mentioned above, the interaction between two solute elements makes the situation much more complicated in ternary alloys. In general, the interaction, either attractive or repulsive, between two kinds of solute elements results in different segregation behaviors in ternary alloys: co-segregation, sitecompetition, and blocking.…”

“…The effect of hydrogen adsorption on the surface and absorption in the bulk on the segregation enthalpy can be included as23 where θ is the adsorption coverage of H atoms on the surface (the molar ratio between hydrogen atoms adsorbed on the surface and the total surface metal atoms), ε i is the adsorption energy of the corresponding solute element i . In the present work, a hydrogen atmosphere is considered, adsorption of other gas molecules can also be included by adapting the adsorption energy ε i and θ. x H is the concentration of absorbed H atoms in the bulk (the molar ratio between hydrogen atoms absorbed in the bulk and the total metal atoms) and is the formation enthalpy of the corresponding metal hydride.…”

“…The effect of hydrogen adsorption on the surface and absorption in the bulk on the segregation enthalpy can be included as [23] γ σ γ σ ω…”

“…Most research addressed surface segregation of binary alloys, and has already provided good results for Pd‐based binary alloys 22,23. Pioneering work on surface segregation of ternary alloys has been performed by Guttmann based on statistical thermodynamics of adsorption isotherms 24.…”