Stability of Pt-Ru Alloy for Anode Catalyst in PEFC Fuel Cell: A Density Functional Theory Study

Pt-based material are commonly used as electrocatalysts for low temperature fuel cells. The Pt metal surface is easily poisoned by trace amount of CO, especially in fuel cells [1]. CO is also present in H₂produced by stream reforming of methanol, ethanol and hydrocarbon fuels, it is also a problem in hydrogen fuel cells [2]. Among the most studied catalysts, the supported platinum based particles have been exhibited a notable efficiency for limiting CO poisoning. Most practical anode fuel cell catalysts consist of platinum nanoparticles alloyed by a second components are Ru, Mo and Sn, which have been studied in many recent surface electrochemistry studies. Among the binary systems the Pt-Ru combination has proved to be most successful at enhancing electro-catalytic activity [3].

The carbon CO itself is usually assumed to follow a so-called bifunctional mechanism, originally suggested by Watanbe and Motoo. [4]. In this mechanism, Ru sites act as adsorption centers for the oxygen-containing surface species reacting with the CO to produce CO₂. Attempts have been made to develop catalysts that either enhance the oxidation of adsorbed CO or simply tolerate CO better, or both, by alloying Pt with other metals. The improvement of the catalytic performance requires an understanding of the reaction mechanism at the atomistic level. There are many studies have been performed for CO adsorption and oxidation on the Pt and Pt-based alloys  in order to design highly CO tolerant bimetallic catalysts but there is few studies regarding atomistic scale and dynamical phenomena of the CO adsorption and oxidation mechanism. A combined molecular dynamics (MD)/density-functional theoretical (DFT) study of the electro-oxidation of CO at the Pt-Ru alloy interface is very important to provide insights into the reaction mechanism.

All calculations were performed using DFT under the generalized gradient approximation with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional. A double numerical basis set augmented with polarization functions were used in the optimization. The computational method used in the present study is implemented in the DMol3 [5]. First-principles molecular dynamics is performed to know the dynamical properties of CO/Pt-Ru interface during the CO oxidation process.

To simulate the mentioned first we determined the most stable initial state on each surface. The Pt(111)-(CO+O) and Pt-Ru (111)-(CO+O) system where O adsorbs on the hollow site and CO on the atop site with the carbon head closer to the surface. From previous findings it is assumed that Pt-metal alloy shows   more reactivity than pure Pt for CO oxidation but very few studies regarding atomistic level and dynamical characteristics of this oxidation process.  From our combined MD and DFT study we expect we will be able to examine electronic and geometric change of Pt-metal and its role of CO oxidation process. In our MD calculation we would be able to know temperature effect on Pt-Ru alloy surface as well as CO oxidation mechanism on this bimetallic surface. We also want to investigate how the Pt-Ru alloy structure change by CO and H₂O adsorption using MD.

Reference:

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3. M.Watanabe, M. Uchida, S. Motoo, J. Electroanal. Chem.229, 395 (1987)
4. M.Watanabe, S. Motoo, J. Electroanal. Chem. 60, 275  (1975)
5. B. Delley,  J. Chem. Phys. 92,508  (1990)