Pt-skin surfaces

For many years, the oxygen reduction reaction (ORR) on Pt and Pt-based alloys has been studied extensively by experimental " and computational methods, and it has been shown that the formation of a Pt-skin layer is accompanied with a Pt-depleted layer underneath for many Pt-3d alloys. Notably, several studies of O adsorption on Pt-skin surfaces have revealed that the binding strength is weaker than that on the pure Pt(lll) surface and this may facilitate the removal of adsorbed O, therefore in-... 

Equation (1) describes the chemisorption of O2 on a surface site A of a metal (Ma) in an acid medium, where coupled to a proton and an electron transfer leads to the formation of an adsorbed end-on complex HOO-Ma. The unstable intermediate subsequently dissociates into two adsorbed species, one adsorbing on A sites, 0-Ma and the OH species adsorbing on B sites, HO-Mb (Eq. 2). In the rest of the electroreduction steps, represented by Eq. (3), adsorbed O and OH are reduced to H2O and the water molecules are eventually desorbed from the metal surface. Actually, Eqs. (1)-(3) can also be used to interpret the ORR activity for Pt-skin surfaces. The electronic sfructures of surface Pt atoms are not identical due to the existence of 3d metal in the sublayers. Ma and Mb can be looked as two Pt surface sites with different activities for reactions (l)-(3). Ma site possess better performance for the formation of the OOH complex, and Mb site may enhance the dissociation of OOH. The overall ORR is thus facilitated by the skin sfructures. 

In contrast to sputtered surfaces, UHV-annealed PtjM surfaces are rather stable in electrochemical experiments, implying that Pt-skin surfaces are structurally and compositionally identical in both UHV and electrochemical environments. Although the composition of the top most layer is the same (pure Pt) for both structures [21], there are three key differences between the Pt-skin and Pt-skeleton surfaces (1) electronic properties are different for the two surfaces i.e., for each PtjM alloy the corresponding Pt-skin surface has a larger d-band center shift from the Fermi level than the sputtered surface (2) Pt-skeleton is morphologically different than the Pt-skin i.e., the Pt surface atoms have a lower average coordination number and (3) the composition of the 3d element in the second layer of the skeleton surfaces... 

Difference in ORR Activity Catalyzed by Pt, Pt-Skeleton, and Pt-Skin Surfaces 145... 

Wang C, Chi M, Li D, Strmcnik D, Vliet D, Wang D, Komanicky V, Chang KC, Paulikas AP, Tripkovic D, Pearson J, More KL, Markovic NM, Stamenkovic VR (2011) Design and synthesis of bimetallic electrocatalyst with multilayered Pt-skin surfaces. J Am Chem Soc 133 14396-14403... 

Most recently, R alloy catalysts that have the so-called structure of a R skin, i.e., with a pure R topmost atomic layer on the surface of the alloys, were reported to be the most aetive catalysts towards the ORR [14—17]. Stamenkovic et al. [14] studied the surface properties of R-M (M = Co, Ni, Fe) polycrystalline alloys prepared by sputtering, aimealing, and leaching, respectively. They found two kinds of surface structure on the alloys, depending on the preparation procedure. The merely sputtered alloys could form R-skeleton outermost layers due to the dissolution of transition metal atoms in acid electrolyte, whereas the aimealed alloys had a R-skin topmost layer containing only R. The eatalytie activity towards the ORR on these two surfaces was much higher than that on a pure polycrystalline R surface, and the Pt-skin surface displayed the highest activity. In particular, the RsNi alloy with the (111) face was 10 times more active than the... 

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