Active and Inactive Sites in COad Electro-Oxidation

COad monolayer (ML) electrooxidation on Pt nanoparticles has a long history as a prototype electrochemical reaction. Furthermore, irreversibly adsorbed COad is a catalyst poison in PEFCs. Numerous studies have revealed strong effects of particle size and morphology on the electrocatalytic activity of COad electro-oxidation (Arenz et al., 2005 Cherstiouk et al., 2003 Friedrich et al., 2000 Maillard et al., 2004 Mayrhofer et al., 2005 Solla-Gulldn et al., 2006). 

Controversy has evolved, in particular, around the question as to whether COad surface mobility could be a limiting process for the overall activity (Kobayashi et al., 2005 Koper et al., 2002 Lebedeva et al., 2002). Widely different values of COad surface mobilities have been determined, with relatively high values found on flat surfaces (Feibelman et al., 2001) and considerably smaller values found on surfaces of small nanoparticles (Ansermet, 1985 Becerra et al., 1993). Maillard et al. (2004) concluded that significant restrictions in COad surface diffusivity arise on the smallest Pt nanoparticles. 

A heterogeneous surface model for COad electro-oxidation on Pt nanoparticles should incorporate the heterogeneous surface morphology and the limited mobility of COad- The minimalist s modeling approach is to use a simple two-state model with a fraction 0 tot 1 of electrocatalytically active sites as exclusive sites, at which OHad can be formed by water splitting. A fraction (1 - tot) of inactive sites merely serves as a reservoir of COad- Finite surface mobility of adsorbed COad is included, defining the time for COad on inactive sites to reach the active sites (Andreaus et al., 2006 Maillard et al., 2004). 

COad removal from active sites (nucleation process), and it accounts for finite surface diffusion of CO ad  

The general solution of the model can be obtained using kinetic Monte Carlo (kMC) simulations. This stochastic method has been successfully applied in the field of heterogeneous catalysis on nanosized catalyst particles (Zhdanov and Kasemo, 2000,2003). It describes the temporal evolution of the system as a Markovian random walk through configuration space. This approach reflects the probabilistic nature of many-particle effects on the catalyst surface. Since these simulations permit atomistic 

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