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47286-GB5
Density Functional Theory and Car-Parrinello Molecular Dynamics Simulations of Ethanol Electro-oxidation Reaction on Bi- and Trimetallic Nanoparticles

Yixuan Wang, Albany State University

            To gain a good understanding about the mechanism for PtSn alloy to catalyze ethanol electro-oxidation reaction, adsorption and decompositions of H₂O and ethanol over Pt_nM clusters have been systematically investigated. CPMD simulations for the relevant system are in progress.

1.  H₂O adsorption and decomposition over Pt_nM clusters (n=2, 3 and 9; M=Pt, Sn, Ru)

            The decompositions of adsorbed water over the clusters Pt₃, Pt₂Sn and Pt₂Ru, were found to have two pathways: atop and bridge decomposition. For the Pt₃, atop decomposition has lower energy barrier than the bridge one; while for the adsorptions over Sn and Ru sites of Pt₂M the energy barriers for bridge decompositions are lower than atop ones. The kinetic favorable pathways for water decomposition over Pt₂M in Figure 1a, shows that decompositions of H₂O over Sn and Ru sites are kinetically more favorable than Pt site. The adsorption of water on the Sn site of Pt₃Sn is quite weak with O-Sn distance of 2.84 Å and only -0.20 eV of adsorption energy, and failed to obtain the decomposition transition state for the Sn site adsorption.    

Figure 1.  Potential energy profiles for the adsorbed water decomposition over Pt₂M (M=Pt, Sn and Ru) (a, left) and Pt₉M (b, right).       

            To eliminate the edge effect, a two layer cluster, Pt₇Pt₃ has been used to model Pt-M alloy surface,¹ and the central atom on the surface has been replaced by alloyed atoms. To provide structural information and accurate barrier, the transition states are directly searched for rather than estimated by the UBI-QEP formula as in Ref. 1. The binding energy of H₂O on Pt₁₀ was predicted to be -0.26eV in this work, which is rather close to the DFT calculation of water on Pt(111) surface, -0.29eV.²  It shows that the central Sn has a quite stronger adsorption to H₂O (E_ad, -0.80eV), but similar to Pt₃Sn water only weakly binds to Sn as Sn locates on the edge. According to Figure 1b, it is interesting to find that thermodynamically and kinetically the decomposition of the adsorbed water on Sn site is more favorable than the decomposition on Pt and Ru sites. This result supports the assumption of the bi-functional mechanism, where Sn site facilitates the dissociation of H₂O.         

2.  Adsorption and Dissociation of Ethanol over Pt_nM clusters

            Two major adsorptions of ethanol via hydroxyl and methylene/methyl groups, and subsequent decompositions are investigated (shown in Figure 2). The former is more stable than the latter over Pt_n (n=3,4 and 9) by 0.3-0.4eV, and we fail to obtain the methylene adsorption over Pt_nSn clusters.

                             Figure 2. Two major adsorptions of ethanol on Pt₁₀

  Figure 3. The cleavage mechanisms of ethanol through OH and CH₂ adsorptions over Pt₃, Pt₂Sn and Pt₄.

According to Figure 3, in spite of stronger adsorption the energy barrier for the cleavage of the adsorbed OH is significantly higher than that for the homolytic decomposition of the adsorbed CH₂, and thermodynamically is also less favorable. Although the two possible mechanisms via either O or CH₂ adsorption have been proposed, the current investigation shows that the decomposition resulting from the adsorption of CH₂ is more likely to occur preferentially. Relative to individuals, the C-H bond cleavage even exhibits negative energy barriers. Since ethanol adsorption on Sn site of Pt_nSn through CH₂ is unstable, the corresponding cleavage seems unlikely. Thus, the results support the assumed bi-functional mechanism from another viewpoint that ethanol decomposition occurs over Pt site.

3. Impact on my career and on the students

            In the “Computational Chemistry” (Fall, 2007) course, 4 hours were spent on molecular modeling and simulations of fuel cell systems. The projects of two chemistry seniors, Jessica Holiday and Natalie Redmon, are “Quantum chemistry studies on the ethanol oxidation reaction over metal clusters”. They also got opportunity to use the high performance Linux cluster provided by the NCSA at the University of Illinois. Supported by the HBCU-UP program, they worked on the project for two semesters, and presented their research entitled “Quantum Chemistry Studies on the ethanol oxidation over metal clusters” on the research symposium of HBCU-UP research assistant program of Albany State University, the fall of 2007 and the spring of 2008. They were well trained on the G03 software, data collection, problem-solving skill, and paper writing skill. Both students are now in the graduate school of Florida A&M University.

 References:

1. Ishikawa, Y. et al.; Surface Science  513(2002), 98-110.
2. Meng, S. et al.; Physical Rev. B 69, 195404(2004).

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