Finally, we discuss about the reason why H adsorption energy on Pt17(X)/graphite is smaller than that on a large Pt NP. The variation in the H adsorption energy is attributed to three factors. The first factor is the H adsorption structure. The bridge positions in sites i and iv results in the large stabilization of Hads than the ontop positions. The second is the low coordination-number site in Pt17(X)/graphite. The pair of Hads on the identical Pt atom (I = 11) at the site v results in the small adsorption energy. The last is variety in the electronic property among the Pt atoms of Pt17(X)/graphite. In the case of site iii (I = 9, 11), both Hads are bounded to Pt in the ontop manner. In addition, in this structure, both Pt atoms are negatively charged (Table ). Accordingly, both Hads on site iii are negatively charged (Table ). Although the higher electronegativity of Pt compared with H (2.28 for Pt cf. 2.20 for H, respectively) could promote the electron transfer from H to Pt, a negatively charged Pt atom is less likely to facilitate electron transfer from H to Pt, causing the less stabilization. Accordingly, the adsorption of Hads is weak at site iii, and thereby HER progression occurs easier than at other reaction sites. This is in contrast to site ii where both Hads are on the ontop position in a similar manner to the case of site iii. In this case, the pair of Pt atoms at site ii are positively charged, resulting in the strong adsorption of H via the electron transfer from H to Pt.