The Effect of the Particle Size on the Kinetics of CO Electrooxidation on High Surface Area Pt Catalysts

Abstract

Using high-resolution transmission electron microscopy (TEM), infrared reflection−absorption spectroscopy (IRAS), and electrochemical (EC) measurements, platinum nanoparticles ranging in size from 1 to 30 nm are characterized and their catalytic activity for CO electrooxidation is evaluated. TEM analysis reveals that Pt crystallites are not perfect cubooctahedrons, and that large particles have “rougher” surfaces than small particles, which have some fairly smooth (111) facets. The importance of “defect” sites for the catalytic properties of nanoparticles is probed in IRAS experiments by monitoring how the vibrational frequencies of atop CO (ν_CO) as well as the concomitant development of dissolved CO₂ are affected by the number of defects on the Pt nanoparticles. It is found that defects play a significant role in CO “clustering” on nanoparticles, causing CO to decrease/increase in local coverage, which yields to anomalous redshift/blueshift ν_CO frequency deviations from the normal Stark-tuning behavior. The observed deviations are accompanied by CO₂ production, which increases by increasing the number of defects on the nanoparticles, that is, 1 ≤ 2 < 5 ≪ 30 nm. We suggest that the catalytic activity for CO adlayer oxidation is predominantly influenced by the ability of the surface to dissociate water and to form OH_ad on defect sites rather than by CO energetics. These results are complemented by chronoamperometric and rotating disk electrode (RDE) data. In contrast to CO stripping experiments, we found that in the backsweep of CO bulk oxidation, the activity increases with decreasing particle size, that is, with increasing oxophilicity of the particles.