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 electrochemical nuclear magnetic resonance spectroscopy (EC-NMR) in the temperature range 253 - 293 K. For CO coverage from θ = 1.0 to 0.36, the diffusion coefficients follow Arrhenius behavior and both activation energy {E_d) and pre-exponential factor (D_CO) show CO coverage dependence. E_d increases from 6.0 to 8.4 kcal/mol and D _CO varies from 1.1 × 10^-8 to 3.7 × 10 ^-6 cm²/s when the coverage is increased from θ = 0.36 to 9 = 1.0. On the Pt catalyst surface at partial CO coverage, our data strongly support the free site hopping model of adsorbed CO as the major surface diffusion mechanism, unlike the situation found with a fully CO covered surface where CO exchange between different surface sites is believed to be the major diffusion mechanism. Our results also indicate that the contributions of lateral repulsive interactions exert a stronger influence on the diffusive motion than does the nature of the surface structure. When the diffusion coefficient was estimated from CO stripping measurements by using an electrochemical modeling protocol, the estimated 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 of molecules at interfaces, and may help facilitate better control of fuel cell reactions where the presence of surface CO plays a crucial role in controlling the reaction rates.