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Adsorption and decompositions of ethanol over Pt_nM clusters (n=6 and 9; M=Pt, Sn and Ru) have been systematically investigated to get the insights into the ethanol electro-oxidation reaction (EER) mechanisms. Although trans-ethanol is more stable than the cis- one, as a result of the adsorptions in all of the investigated bimetallic PtM the latter turns out to be a more favorable adsorption as previously reported for Pt. The adsorption via the hydroxyl is much stronger than that via the α-hydrogen. As shown below for ethanol adsorption over cluster Pt₆Sn, the adsorption energies are -1.40, -1.36 and -0.74eV for the two hydroxyl adsorptions and α-hydrogen, respectively.

           (Eads: -1.40eV)                   (Eads:  -1.36eV)                       (Eads: -0.74eV)

Figure 1. Three major adsorptions of ethanol on Pt₆Sn: via hydroxyls of cis-ethanol (left) and trans-ethanol (middle), and α-hydrogen (right)

The comparison of the adsorptions on the three clusters Pt₆M (M=Pt, Sn, and Ru) indicates that the adsorptions of both hydroxyl and α-hydrogen on the central Sn and Ru of Pt₆M are much stronger than those on the central Pt of Pt₇. The decomposition behaviors for the two hydroxyl adsorptions are rather similar to each other, for an example, the energy barriers to the adsorbed CH₃CH₂O* and H* on Pt₇ are 0.87 and 0.89 eV, respectively. It is interesting to note that on the clusters Pt₇ and Pt₆Ru the decomposition resulting from the α-hydrogen adsorption has lower energy barrier than those of the hydroxyl adsorptions (0.23 eV vs 0.87/0.89eV for Pt₇; 0.06 eV vs 0.21/ 0.36eV), while on the cluster Pt₆Sn oppositely the α-hydrogen decomposition has higher barrier than the hydroxyl dissociation (1.05 vs 0.57/0.59eV). The results indicate whether ethanol decomposition proceeds initially via O-H or α-C-H bond cleavage may depend on the type of catalysts.

      (adsorption via α-hydrogen )     (decomposition TS)                   (decomposition product)

Figure 2. Dissociative adsorptions for hydroxyl and α-H of ethanol over Pt-based bimetallic: A concerted pathway  

 The adsorption via α-hydrogen considerably extends C-H bond by 0.1~0.2Å, and hydroxyl also has significant adsorption with the Pt-H(O) distance of approximately 2.4 Å. Therefore, the transition state in Figure 2 may be characterized as the TS for the co-decomposition of hydroxyl and α-hydrogen, which decompose in a concerted pathway. According to Figure 3, the favorable decomposition pathway ( via α-hydrogen) on Pt₇ has a lower activation energy by 0.17eV than the favorable pathway (via hydroxyl) on Pt₇Sn, and thermodynamically the former is also favorable than the latter ( dissociation energy: 0.35 vs -1.25eV). Thus, Pt is more active to ethanol oxidation reaction than Sn. Together with the investigation of H₂O adsorption on Pt_nM, the present results support the proposed bifunctional mechanism for PtSn alloy to catalyze ethanol electro-oxidation reaction that the dissociative adsorption of ethanol occurs on platinum sites and Sn primarily promotes adsorption and dissociation of water to form OH oxidizing intermediates from ethanol.

Figure 3.  Potential energy profile for the decompositions of ethanol on Pt₆M (M=Pt, Sn and Ru)