Size dependence of the CO monolayer oxidation on nanosized Pt particles supported on gold

The CO monolayer electro-oxidation was investigated on model electrodes consisting of small Pt particles (2-5 nm size) and large particles (8-20 nm size) supported on polycrystalline gold in 0.1 M HClO4. The study was performed by cyclic voltammetry and the recording of CO oxidation current transients. The electrodes were prepared from aqueous Pt colloids and polycrystalline gold. The size and size distribution of the clusters was determined by transmission electron microscope (TEM) images. Whereas it is likely that the small particles (2-5 nm size) are single-crystalline the TEM images reveal that the larger particles (8-20 nm) are polycrystalline consisting of aggregates of smaller particles. The cyclic voltammogram of the CO monolayer oxidation on 3 nm Pt particles exhibited three oxidation peaks located at more positive potentials compared with a polycrystalline Pt electrode. The observed difference was clear especially for low particle coverages. On the contrary, the larger particles showed CO monolayer oxidation potentials, which resemble the behavior of polycrystalline Pt. These varying properties of the small particles of different sizes were evident in the CO oxidation transients. The time-dependent oxidation on the 3-nm particles differs from the classical Langmuir-Hinshelwood mechanism and can be better described by an Eley-Rideal mechanism. This is explained either with the absence of oxygen adsorption or by the absence of transportation processes on the electrode surface. The larger particles exhibit bulk behavior in this respect. Finally, the deviations from bulk behavior are discussed in terms of size and structure effects. (C) 2000 Elsevier Science Ltd. All rights reserved.