Theoretical Advances in Electro catalysis over Supported Nanoparticles: the Oxidation of CO and Methanol over PtRu
M. Neurock, S. Desai, and J-S. Filhol
University of Virginia, US
Keywords: PEM Fuel Cell
Theory has rapidly advanced to the stage where it can provide reliable information for kinetic processes carried out over well-defined surfaces in the gas phase. It has therefore been used to a growing extent to probe catalytic reactions over extended surfaces and cluster models, which mimic catalytic sites on supported nanoparticles. There have been relatively few applications, however, electro catalytic systems due to the complexity that arises from the presence of solution, electric fields and applied potentials. Herein we will discuss the development and application of ab initio methods to treat aqueous electro catalytic reactions on bimetallic nanoparticles and their application to fuel cell systems. More specifically we will focus on the electro catalytic oxidation of CO and methanol over PtRu surfaces.
The catalytic performance of both the direct methanol fuel cell and the reformate fuel cell can be significantly inhibited by the formation of CO which acts to poison active Pt surface sites. The addition of ruthenium is known to enhance CO tolerance. Ruthenium is thought to either weaken the binding of CO on the surface or activate water to form surface hydroxyl intermediates, which can in turn oxidize CO to CO2 via the bifunctional mechanism shown below:
H2O* -> OH* + H+ + e-
CO* + OH* -> CO2 + H+ + e- .
We have used first-principle density functional theoretical calculations and ab initio molecular dynamics herein to model the oxidation of CO over different PtRu alloys. We explicitly examine both of the reactions above at various model conditions including: 1) in the vapor phase, 2) in solution, and 3) in the presence of an applied potential in order to understand each of their effects. We describe a set of extensive calculations on Pt, Ru and PtRu alloys for these systems. We find that Pt and Ru play unique roles in this chemistry, which cooperatively work together in order to facilitate the bifunctional mechanism.
NSTI Nanotech 2003 Conference Technical Program Abstract