Coverage Dependence of CO Surface Diffusion on Pt Nanoparticles - an EC-NMR Study
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Coverage Dependence of CO Surface Diffusion on Pt Nanoparticles - an EC-NMR Study Andrzej Wieckowski, Takeshi Kobayashi, Panakkattu K Babu, Jong Ho Chung, and Eric Oldfield Chemistry, University of Illinois at Urbana-Champaign, 600 S. Mathews Ave., Urbana, IL, 61801 ABSTRACT 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 (Ed) and pre-exponential factor (Dco) show CO coverage dependence. Ed increases from 6.0 to 8.4 kcal/mol and DCO varies from 1.1 × 10-8 to 3.7 × 10-6 cm2/s when the coverage is increased from θ = 0.36 to θ = 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.
INTRODUCTION Study of the diffusion of small molecules adsorbed on catalyst surfaces is of fundamental importance since it can lead to interesting correlations between surface structure and reactivity of electrode surfaces.1,2 CO diffusion on Pt surfaces has been extensively investigated under ultrahigh vacuum or gas phase conditions.2-4 Both diffusion coefficients (DCO) as well as activation energies (Ed) for diffusion are of importance in the context of CO hydrogenation in fuel synthesis 4 and CO oxidation in heterogeneous catalysis.5 Due to problems associated with the presence of the electrolyte, the direct experimental determination of DCO in an electrochemical environment is a difficult task. However, incorporating a selective spin inversion pulse sequence 6 into the technique of electrochemical NMR (EC-NMR),7-9 we managed to determine the diffusion constants of CO on Pt in a liquid electrochemical environment, together with the activation energy for diffusion. Our initial study was carried out on a nanoparticle Pt electrode saturated with CO (at CO coverage, θ, close to 1) where we proposed that the surface diffusion was due to CO exchange
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