Coverage dependence of CO surface diffusion on Pt nanoparticles: An EC-NMR study

We have studied the effects of CO coverage on surface diffusion rates of CO adsorbed on nanoparticle Pt catalysts in sulfuric acid media by using 13C EC-NMR at 253 - 293 K. For CO coverage from 1.0 to 0.36, the diffusion coefficients follow Arrhenius behavior and both activation energy (Ed) and pre-exponential factor (DCO) show CO coverage dependence. When the coverage is increased from 0.36 to 1.0, Ed increases from 6.0 to 8.4 kcal/mol and DCO from 1.1 × 10-8 to 3.7 × 10-6 cm2/s. At partial CO coverage, our data support the free site hopping model of adsorbed CO as the major surface diffusion mechanism. When the diffusion coefficient was estimated from CO stripping measurements by using an electrochemical modeling protocol, the diffusion coefficients were a few orders of magnitude larger than those obtained from the EC-NMR experiments. Overall these results are important for improving our understanding of electrochemical surface dynamics that play a crucial role in controlling fuel cell reaction rates.