Density Functional Theory Study of OH and CO Adsorption on the Pt2Ru3 Surface

It is well established that the catalytic properties of metal can be markedly changed by alloying with a second metal in polymer electrolyte fuel cells [1]. Pt-Ru alloys are known to substantially improve the catalytic performance in the electrochemical oxidation of CO from CO contaminated hydrogen fuels. The presence of small amounts of CO that are carried over in the H₂ feed from the reformer act to poison Pt surface sites. The addition of Ru to Pt helps to prevent CO poisoning of the Pt surface sites that carry out H₂ oxidation. Ru promotes the catalytic oxidation of CO to form CO₂ and may also weaken the Pt-CO interaction. Most practical anode fuel cell catalysts consist of Pt nanoparticles alloyed by a second components are Ru, Mo and Sn. Among the binary systems the Pt-Ru combination has proved to be most successful at enhancing electro-catalytic activity [2]. The CO itself is usually assumed to follow a so-called bifunctional mechanism, originally suggested by Watanbe and Motoo [3]. For H₂ oxidation in the presence of CO, Watanabe and Motoo have proposed a combined mechanism, in which the ligand and bifunctional effects may coexist [2,4]. The improvement of the catalytic performance requires an understanding of the reaction mechanism at the atomistic level. Multi-scale simulation study of the electro-oxidation of CO at the Pt-Ru alloy interface is very important to provide new insights into the reaction mechanism.

All calculations were performed using density functional theory (DFT) under the generalized gradient approximation with the Perdew-Burke-Ernzerhof (PBE) exchange-correlation functional and Monte Carlo (MC) simulation method. The computational method used in the present study is implemented in the DMol³ [5]. MC simulation has been performed to know the stability of Pt-Ru alloy catalysts during CO oxidation processes.

    Pt-Ru surface is modeled as a five layer slabs with Pt:Ru (2:3) ratio. Initially, we have checked the surface stability of Pt-Ru surface with different positions of Pt and Ru remaining same mixing ratio. We found that fcc based Pt-Ru alloy shows higher stability than the hcp based alloy and we also confirmed that the surface coverage with Pt atoms indicates stable combination. We also notice that 2^nd layer positioned with Ru atoms shows higher stability than the other combinations. As a whole the tendency indicates, top surface contains Pt sufficiency and 2nd layer contains Ru sufficiency make the structure stable. Model of Pt-Ru surface where CO are located on the atop, bridge and hollow sites were also examined. We found that the well mixing of Pt by Ru leads to a weaker bond of CO on the surfaces. Many studies assumed that Pt-metal alloy shows more reactivity than pure Pt for CO oxidation but very few studies regarding atomistic level. Coverage change of Pt on the surface of alloy catalysts is further studied by MC simulations. Based on the DFT calculations we correlate the binding energy of alloy catalysts with different Pt coverage. From our MC simulation study we successfully modeled the temperature effect on Pt-Ru alloy surface change as well as CO poisoning influence to Pt coverage of this bimetallic surface.

References

1. V. Ponce, G.C. Bond, Catalysis by Metals and Alloys; Studies in Surface Science and Catalysis 95; Elsevier: Amsterdam, 1995.
2. M.Watanabe, M. Uchida, S. Motoo, J. Electroanal. Chem.229, 395 (1987)
3. M.Watanabe, S. Motoo, J. Electroanal. Chem. 60,275 (1975) 
4. M. Watanabe, M. Shibata, S. Motoo, J. Electroanal. Chem. 206, 197 (1986)
5. B. Delley,  J. Chem. Phys. 92, 508 (1990)