Recent Developments in the Electrocatalysis of the O Reduction Reaction - Fuel Cell Catalysis: A Surface Science Approach - page 288

Figure 9.15 Kinetic current density (squares) at 0.8 V for O 2 reduction on supported
Pt monolayers in a 0.1 M HClO 4 solution, and the calculated activation energy barriers for O 2
dissociation (filled circles) and OH formation (open circles) on Pt ML /Au(111), Pt(111), Pt ML /
Pd(111), and Pt ML /Ir(111), as a function of the calculated binding energy of atomic oxygen
(BE O ). The current density data for Pt(111) were obtained from [Markovic et al., 1999] and are
included for comparison. Key: 1, Pt ML /Ru(0001); 2, Pt ML /Ir(111); 3, Pt ML /Rh(111); 4, Pt ML /
Au(111); 5, Pt(111); 6, Pt ML /Pd(111). Surface coverage is 4 ML O 2 in O 2 dissociation and 4 ML
each for O and H in OH formation. (Reproduced with permission from Zhang et al. [2005a].)
adsorbate weakly tend to facilitate the kinetics of bond formation steps in which the
adsorbate is a reactant [Mavrikakis et al., 1998; Wintterlin et al., 2003; Xu et al.,
2004]. Not surprisingly, E a for O 2 dissociation is smallest on Pt ML /Au(111) and lar-
gest on Pt ML /Ir(111). The exact opposite trend is found for E a of oxygen hydrogen-
ation, which is most facile on Pt ML /Ir(111) and most difficult on Pt ML /Au(111). In
accord with the Sabatier principle, the volcano-type dependence of ORR activity on
1 d as seen in Fig. 9.14 is a consequence of two opposing demands on the catalyst con-
nected via a common intermediate. Pt(111) lies close to the intersection of the two E a
trend lines, suggesting that the optimum compromise between the two opposite trends
is well represented by this intersection. The only Pt monolayer in the vicinity of the
intersection of the two trend lines is Pt ML /Pd(111), which immediately raises the
possibility that this bimetallic system is a promising ORR catalyst. Indeed, according
to experiments, Pt ML /Pd(111) performs better than all the other Pt monolayers, and
even shows an approximately 30% increase in current density compared with
Pt(111). Although the exact crossing point may depend on factors including surface
coverage, electric field and potential, and solvent, the binding energy of O appears
to be a robust reactivity descriptor for the ORR on these surfaces. More recently,