Recent Developments in the Electrocatalysis of the O Reduction Reaction - Fuel Cell Catalysis: A Surface Science Approach - page 287

Figure 9.14 Kinetic current density (squares) at 0.8 V for O 2 reduction on the Pt monolayer
deposited on various metal single-crystal surfaces in a 0.1 M HClO 4 solution, and calculated
binding energies (circles) of atomic oxygen (BE O ), as a function of calculated d-band center
(relative to the Fermi level, 1 d 2 1 F ) of the respective surfaces. The 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). (Reproduced with permission from Zhang et al. [2005a].)

unduly hindering either. To identify the fundamental reasons behind the volcano-type
behavior shown in Fig. 9.14, DFT calculations were performed to study the following
elementary steps:

O 2 ! O þ O (9 : 3)

O þ H ! OH (9 : 4)

These two steps were chosen because they are the most activated versions of O - O
bond scission and O - H bond formation, respectively. There is evidence that the reac-
tivity of other hydrogenated forms of oxygen follows a similar trend to that of atomic
oxygen [Shao et al., 2007a].
The calculated activation energy barrier E a for the elementary steps (9.3) and (9.4)
are plotted against O binding energy in Fig. 9.15. The binding energy of O is empha-
sized because atomic oxygen is the common intermediate for (9.3) and (9.4), and inter-
acts more strongly than any other ORR intermediate with the surface, thus making O
adsorption more sensitive to surface properties. Previous studies have shown that sur-
faces that bind an adsorbate strongly tend to enhance the kinetics of bond scission
steps in which the adsorbate is a product. On the other hand, surfaces that bind an