In order to have a representative view of the surface adsorption properties over Pd, Ir, and Rh, the H/MS theoretical chemisorption stoichiometric factors versus the theoretical H/M ratio are depicted in Figure 2d. The adsorption of one hydrogen atom per surface M atom (MS) is reasonably constant (near unity) for H/Pd < 0.54, H/Ir < 0.28, and H/Rh < 0.36, which corresponds to the large particle size domain. However, when H/Pd ≥ 0.44, H/Ir ≥ 0.28, and H/Rh ≥ 0.36 (small particle size domain), the H/MS ratio increases with the H/M ratio to reach a maximum value of 1.17, 3.00, and 2.50 for Pd, Ir, and Rh, respectively. This particular behavior directly originates from the different sites considered for hydrogen adsorption (Equations (3)–(5)), as well as their relative proportion (Table 1).