Recent Developments in the Electrocatalysis of the O Reduction Reaction - Fuel Cell Catalysis: A Surface Science Approach - page 293

Figure 9.19 The Pt and Ir L 3 edge in situ XANES spectra for Pt 0.8 Ir 0.2 /Pd/C (20 nmol Pd) in
1 M HClO 4 at 0.47 V. The Pd loading in these nanoparticles has been doubled to enhance the Ir
signal in the specific experiment. (Reproduced with permission from Vukmirovic et al. [2007].)
To verify that intended amounts of Pt and the second metal are indeed deposited on
the substrate, XANES spectra are taken in situ for (Pt 0.8 Ir 0.2 ) ML /Pd/C (20 nmol Pd) in
1 M HClO 4 at 0.47 V. The L 3 edge for Pt and Ir (Fig. 9.19) indicates that the Pt : Ir ratio
is 4.66 : 1, or 82% Pt and 18% Ir, in very good agreement with the 80 : 20 ion concen-
tration used in the Cu-displacement preparation method.
9.3.3 Mechanistic Insights for the ORR
To further substantiate these insights for the ORR mechanism, Wang and co-workers
analyzed the four-electron ORR in acidic media using an intrinsic kinetic equation
with the free energies of activation and adsorption as the kinetic parameters [Wang
JX et al., 2007]. The kinetic model consisted of four essential elementary reactions:
1. dissociative adsorption (DA) and
2. reductive adsorption (RA),
which yield two reaction intermediates, O and OH;
3. reductive transition (RT) O ! OH; and
4. reductive desorption (RD) of OH as H 2 O.
Analytic expressions were obtained for the O and OH adsorption isotherms by solving
the steady state rate equations.