Pt alloy monolayer

Figure 6.21. Schematics of currently pursued Pt-based electrocatalyst concepts for the ORR. (A) Pt bulk alloys (B) Pt alloy monolayer catalyst concepts (C) Pt skin catalyst concept (D) De-alloyed Pt core-shell catalyst concept.

The Pt alloy monolayer nanoparticle catalysts (e.g., Pt-Re layer on Pd cores) showed a clearly improved specific (Pt surface normalized) ORR activity their Pt mass-based electrocatalytic activity, however, exceeded that of pure Pt catalysts by an impressive factor of 18 x— 20 x. Their noble metal (Pt, Re, and Pd) mass-based activity improvement was still about a factor 4x. The Tafel slope in the 800-950 mV/RHE range suggested that the surface accumulation of Pt-OH species is delayed on the Pt monolayer catalyst. The enormous increase in Pt mass-based activity is obviously due to the small amount of Pt metal inside the Pt monolayer. 

Pt alloy monolayer catalysts exhibited even more active ORR behavior compared to Pt monolayer catalysts. To understand this phenomenon computational DFT studies were carried out. The hypothesis to be tested was that, for instance, Ru metal atoms in the Pt—Ru monolayer are OH-covered and could inhibit the adsorption of additional OH on neighboring surface sites (adsorbate-adsorbate repulsion effect). A very similar hypothesis was put forward about three years earlier by Paulus et al. [105] who postulated that Co surface atoms might exhibit a so-called common-ion effect, that is, they could repel like species from neighboring sites. A combined computational-experimental study finally confirmed this hypothesis [123] If oxophilic atoms such as Ru or Os were incorporated into the Pt monolayer catalysts, the formation of adjacent surface OH was delayed, if not inhibited. Oxo-phobic atoms, such as Au, displayed the opposite effect, would not inhibit Pt—OH formation, and were found to be detrimental to the overall ORR activity. 

---