Understanding Electrocatalytic Activity Enhancement of Bimetallic Particles to Ethanol Electro-oxidation: (1) Water Adsorption and Decomposition on Pt(n)M (n=2,3 and 9; M=Pt, Ru, Sn).

The fundamental assumption of the bi-functional mechanism for PtSn alloy to catalyze ethanol electro-oxidation reaction (EER) is that Sn facilitates water dissociation and EER occurs over Pt site of the PtSn alloy. To clarify this assumption and achieve a good understanding about the EER, H(2)O adsorption and dissociation over bimetallic clusters PtM (M=Pt, Sn, Ru, Rh, Pd, Cu and Re) are systematically investigated in the present work. To discuss a variety of effects, Pt(n)M (n=2, and 3; M=Pt, Sn and Ru), one-layer Pt(6)M (M=Pt, Sn and Ru), and two-layer (Pt(6)M)Pt(3) (M=Pt, Sn, Ru, Rh, Pd, Cu and Re) clusters are used to model the PtM bimetallic catalysts. Water exhibits atop adsorption on Pt and Ru sites of the optimized clusters Pt(n)M (n=2, and 3; M=Pt and Ru), yet bridge adsorption on Sn sites of Pt(2)Sn as well as distorted tetrahedral Pt(3)Sn. However, in the cases of one-layer Pt(6)M and two-layer Pt(9)M cluster models water preferentially binds to all of investigated central atom M of surface layer in atop configuration with the dipole moment of water almost parallel to the cluster surface. Water adsorption on the Sn site of Pt(n)Sn (n=2 and 3) is weaker than those on the Pt site of Pt(n) (n=3 and 4) and the Ru site of Pt(n)Ru (n=2 and 3), while water adsorptions on the central Sn atom of Pt(6)Sn and Pt(9)Sn are enhanced so significantly that they are even stronger than those on the central Pt and Ru atoms of PtnM (n=6 and 9; M=Pt and Ru). For all of the three cluster models, energy barrier (E(a)) for the dissociation of adsorbed water over Sn is lower than over Ru and Pt atoms (e.g., E(a): 0.78 vs 0.96 and 1.07 eV for Pt(9)M), which also remains as external electric fields were added. It is interesting to note that the dissociation energy on Sn site is also the lowest (E(diss): 0.44 vs 0.61 and 0.67eV). The results show that from both kinetic and thermodynamic viewpoints Sn is more active to water decomposition than pure Pt and the PtRu alloy, which well supports the assumption of the bi-functional mechanism that Sn site accelerates the dissociation of H(2)O. The extended investigation for water behavior on the (Pt(6)M)Pt(3) (M=Pt, Sn, Ru, Rh, Pd, Cu and Re) clusters indicate that the kinetic activity for water dissociation increases in the sequence of Cu < Pd < Rh < Pt < Ru < Sn < Re.