INVESTIGATION OF PT PARTICLES ON GOLD SUBSTRATES BY IR SPECTROSCOPY -PARTICLE STRUCTURE AND CATALYTIC ACTIVITY

The CO monolayer adsorption and electro-oxidation is investigated on m odel electrodes consisting of small Pt clusters (2-8 nm size) supporte d on polycrystalline gold in 0.1 M HClO4,. The study is performed by i n situ infrared spectroscopy and cyclic voltammetry. The electrodes we re prepared from an aqueous Pt colloid and polycrystalline gold. The s ize and size distribution of the clusters is determined by TEM images. The cyclic voltammogram of the CO monolayer oxidation on 3 nm Pt part icles exhibits three oxidation peaks located at 100 to 500 mV(RHE) hig her potentials compared to a polycrystalline Pt electrode. A pronounce d influence of the Pt particle coverage on the stretching vibration of linearly bonded CO is also found: while at low Pt coverages the band is located at 2013 cm(-1) an upwards shift to 2060 cm(-1) is observed when increasing the particle coverage. A particle size effect on the v ibrational frequency can also be established by using electrodes prepa red from an aged Pt colloid. TEM analysis of this colloid showed parti cle sizes of 2.2 nm (primary size) and 8.5 nm (secondary size) and agg regates of the larger particles. At low Pt particle coverage, two band s can be distinguished: a band at 2013 cm(-1) is attributed to CO adso rbed on the small primary particles while a second band at 2046 cm(-1) is assigned to linearly bonded CO adsorbed on the larger particles an d on the aggregates. The importance of the lateral dipole held interac tions between the adsorbed vibrating molecules is discussed and also t he dependence of the vibrational frequency on the coordination of the adsorbate Pt site. Both interactions are too weak to account for the p ronounced vibrational shifts in comparison with CO bonded on polycryst alline Pt and therefore the results are tentatively explained by parti cle-substrate interaction. (C) 1998 Elsevier Science B.V.