Surface Segregation in Pt₃Nb and Pt₃Ti using Density Functional Theory-based Methods

First-principles DFT calculations were used to investigate surface segregation processes in ordered Pt3X (where X=Nb, Ti) alloys. Using pristine Pt (111) surface as a reference, the effect of surface segregation on the adsorption energy of O2 atoms in Pt3X alloys was evaluated. Our results showed that surface segregation due to direct exchange is only feasible for the Pt3Nb alloy (Esegr = - 0.3833 eV) but not for its Ti analogue (Esegr = 0.516 eV). In contrast, for both Pt3X alloys, surface segregation due to antisite migration and leading to the formation of a Pt-skin or overlayer, favouring oxygen atom adsorption, an essential step in ORR, is possible. Interestingly, reverse migration of X atoms from the bulk to replace Pt atoms on the surface is an endothermic process and is thus very unlikely. Analysis of the surface segregation energy for configurations involving a direct exchange of Pt atoms located beyond the third layer in the slab model with Nb atoms at the surface indicates the formation of pristine bulk like Pt (111) surface from Pt3Nb surface is unlikely. The energy of adsorption for the O-atom on pristine Pt (111) surface shows that the presence of minute quantities of dopant Nb atoms in the sub-surface layer could enhance its suitability for ORR. Comparison of O-atom adsorption energy on the various surface segregation models of Pt3X alloys to that of pristine Pt (111) surface shows that the formations of a Pt-skin or overlayer on the Pt3Nb surface due to surface segregation change the O-atom adsorption energy on this surface to 0.34 eV which is just 0.14 eV higher than the optimal value of 0.20 eV. Our results also show that the binding of an oxygen atom to the fcc Pt site in Pt3Ti is lower in energy compared to its binding on a pristine Pt (111) surface. In comparison, the binding of an oxygen atom to the fcc Pt site in Pt3Ti is of the same magnitude as that of the pristine Pt (111) surface.