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

We have studied the effects of CO surface coverage on the diffusion rates of CO adsorbed on commercial Pt-black in sulfuric acid media by using ¹³C electrochemical nuclear magnetic resonance (EC-NMR) spectroscopy in the temperature range 253-293 K. The temperature range chosen for these measurements was such that the electrolyte is in a liquid-like and liquid environment. For CO coverage between θ = 1.0 and 0.36, the CO diffusion coefficients (D_CO) follow a typical Arrhenius behavior and both the activation energies (E_d) as well as the pre-exponential factors (D_CO⁰) show CO coverage dependence. For partially CO covered samples, E_d decreases linearly with increasing CO coverage, indicating that the repulsive CO-CO interactions exert a stronger influence on the coverage dependence of the activation energy than does the nature of the CO adlayer structure. On the other hand, D_CO⁰ shows an exponential decrease with increasing CO coverage, consistent with the free site hopping model [Gomer, R. Rep. Prog. Phys. 1990, 53, 917] as the major mechanism for surface diffusion of CO at partial coverages, unlike the situation found with a fully CO covered surface [Kobayashi et al., J. Am. Chem. Soc, 2005, 127, 14164]. Overall, these results are of interest since they improve our understanding of the surface dynamics of molecules at electrochemical interfaces, and may help facilitate better control of fuel cell reactions in which the presence of surface CO plays a crucial role in controlling electrocatalytic reaction rates.