A Kinetic Analysis of the CO Electro-Oxidation Reaction on Pt Bimetallics: Understanding the Interplay of Bifunctional and Electronic Effects

Abstract

The CO electro-oxidation reaction has been one of the most thoroughly studied reaction mechanisms in electrocatalysis, mostly due to its role as a model reaction for the oxidation of larger organics like methanol and ethanol, each of which have important renewable energy applications in direct alcohol fuel cells. Bimetallics of Pt-M (where M is an oxophilic modifier such as Ru or Sn) have been shown extensively to have improved CO tolerance compared to pure Pt. The reason for this enhancement is often attributed to some combination of bifunctional (oxophilic metal provides OH to remove poisoning CO on Pt) and electronic (oxophilic metal weakens the CO binding strength on the Pt sites) effects. However, it has been difficult to determine the extent to which these different effects are operative in a given system. In this work, we perform a kinetic analysis of the CO electro-oxidation reaction in order to elucidate the roles of bifunctional and electronic effects. We look specifically at the classic Pt-Ru bimetallic system, and determine that the rates of bifunctional coupling are generally lower than the rates of coupling on Pt sites electronically modified by Ru. This indicates that, at least for the Pt-Ru system, the electronic effects that weaken CO binding on Pt are more operative than the bifunctional effect. This implies that that the prospects for bifunctional gains in general may be more limited than once thought, and that a more rational design parameter for bimetallic materials may be the electronic effects between the two components.