Surface Segregation in Pt Based Binary Alloys and its Effect on Oxygen Reduction

The surface segregation of platinum based binary alloys, especially PtNi alloys, and its effect on catalysis is well studied [30, 116, 121, 122, 125, 126]. In 1985, through the use of LEED, Gaultier et. al. first found that surface enrichment of platinum atoms in the topmost layer relative to the bulk Pt concentration was present in PtNi alloys with a range of compositions [116, 126]. The PtNi9{111} alloy was found to exhibit surface segregation of platinum so that

the top most atomic layer contained 30% (3 times the bulk concentration) platinum [121]. A 1:1 PtNi{111} alloy was found to exhibit 88 atomic % platinum [125] in the first layer and the Pt3Ni{111} alloy exhibited a pure platinum (99% +/- 1%) top-most layer [126]. In all cases a

compositional oscillation was observed with surface depth: the second layer being deprived of platinum relative to the bulk and (for PtNi and Pt3Ni) the third layer being platinum rich. This

oscillatory behaviour was found to dampen with depth until the bulk alloy atomic composition was reached. This platinum enrichment, along with oscillatory behaviour, was also observed the {100} PtNi alloy surfaces. The {110} alloys exhibited an inverse behaviour in the surface composition, strong Ni enrichment. The surface composition profiles of the base pane PtNi alloys obtained by Gauthier and Baudoing are shown in the figure below. Studies have shown that other platinum alloys, such as Pt3Fe{111} and Pt3Co{111} [30], also exhibit a pure

platinum outermost layer with a platinum-depleted second layer.

Figure 34. Summary of the results obtained by LEED surface crystallographic studies on platinum-nickel fcc random substitutional alloys. (Gauthier and Baudoing 1990) [116]

It has been shown that by depositing Ni on Pt{111} at 300K, a Ni-Pt-Pt{111} monolayer structure forms [127]. Carrying out the deposition procedure at 600K results in the formation of a Pt-Ni-Pt{111} sandwich structure [128], with nickel only being present in the second atomic layer. In this case the nickel has moved from top to second layer and illustrates the driving force of the above alloys to form a Pt enriched top layer followed by a Ni enriched second layer. Theoretical and experimental studies have shown that exposure of these types of structures to hydrogen or oxygen can influence surface composition. Hydrogen exposure favours the Pt-3d-Pt{111} sandwich structure whereas oxygen favours the 3d-Pt-Pt{111} surface [112, 129, 130].

Surface hydrogen, OH or oxide can also be formed electrochemically and therefore the surface segregation profile could change with potential. The surface composition of the top-layer-Pt- enriched Pt3Ni{111} system has been analysed during CV. The surface alloy structure and

segregation profile was found to be stable between 0.05-1V (vs RHE), with only a contraction of the surface Pt layer upon adsorption of oxygenated species at high potentials [122].

In the last 15 years, interest in the effect of surface segregation in electrocatalysis has increased, as PtM alloys (where for example M=Fe, Co or Ni) have increased activity when compared to pure platinum for the oxygen reduction reaction [30]. Markovic et. al. first found that Pt3Ni

and Pt3Co alloy catalysts had increased activity for oxygen reduction when they were prepared

in such a way as to form a pure platinum top-layer, which he called a “Pt-skin” [131, 132]. Catalysts with a surface layer composition matching the bulk alloy composition were prepared by argon ion etching of a bulk alloy and were found to have higher activity than polycrystalline platinum, but not as high as the Pt-skin surfaces. The Pt-skin catalysts were prepared by annealing the etched alloy. This was the first time that an electrocatalyst’s high activity for the ORR could be associated with a segregation effect. [131]

The high activity of the Pt-skin surface was associated with weaker Pt-OHads interaction on the

electronically modified monoatomic layer of Pt atoms [132] according to the ligand effect. Pt- OHads was assumed to be a site blocking species (θad) which lowered the reaction kinetics

according to the equation;

𝑗 = 𝑛𝐹𝐾𝑐

(1 − 𝜃

)

_{exp (−}

𝑅𝑇

) exp (−

CV of the Pt-skin catalysts showed that at high potentials their coverage of OH was lower than over pure platinum[132]. Also it showed that the potential onset of OH formation was higher for the Pt-skin surfaces. This is illustrated in the voltammetry below (figure 35) by the blue arrow.

Figure 35. Cyclic voltammetry of annealed Pt3Ni, ‘Pt-skin’, surface vs. polycrystalline Pt at 293 K. The blue arrow highlights the shift in OH adsorption. Adapted from [132]

A later study by Markovic et. al. on Pt-skin Pt3Ni single crystal electrodes found that the weaker

interaction with OH for these surfaces is attributed to a downshifted d-band centre relative to the same single crystal pure platinum electrodes [122]. Other studies have found that many alloys of platinum exhibit this skin structure and that the d-band centre is tuneable by the choice of alloying metal [30]. Figure 36, below, shows the electronic effect of alloying platinum with metals such as Ni, Co, Fe, V and Ti and also the activity enhancements possible for the ORR. A negative shift in d-band centre compared to polycrystalline platinum of approximately 0.3eV corresponded in this study to the top of the volcano curve of activity. It is unclear how close to the top of the volcano curve current state of the art catalysts are. Studies on alloys of platinum with early transition metals such as Y or Sc have shown even higher ORR activity than platinum-late transition metal alloys [123, 133]. These results indicate that the optimum d-band centre shift is closer to 0.2eV [133]. It is also unclear how much activity increase is possible through focussing solely on the d-band centre property of alloy catalysts.

The surface segregation effects described so far have been found not only to occur in UHV- prepared extended alloy surfaces, but also in alloy nanoparticles. As such, the synthesis of stable, active Pt-skin nano-structures which have low Pt usage (and hence low cost) for the oxygen reduction reaction has been the focus of recent work.

Figure 36. Relationship between experimentally measured specific activity for the ORR on Pt3M surfaces in 0.1 M HClO4 at 333K versus the d-band centre position for the Pt-skin surfaces. [30]