Investigation of Pt particles on gold substrates by IR spectroscopy particle structure and catalytic activity

Abstract

The CO monolayer adsorption and electro-oxidation is investigated on model electrodes consisting of small Pt clusters (2–8 nm size) supported on polycrystalline gold in 0.1 M HClO 4. The study is performed by in situ infrared spectroscopy and cyclic voltammetry. The electrodes were prepared from an aqueous Pt colloid and polycrystalline gold. The size and size distribution of the clusters is determined by TEM images. The cyclic voltammogram of the CO monolayer oxidation on 3 nm Pt particles exhibits three oxidation peaks located at 100 to 500 mV RHE higher potentials compared to a polycrystalline Pt electrode. A pronounced 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 vibrational frequency can also be established by using electrodes prepared from an aged Pt colloid. TEM analysis of this colloid showed particles sizes of 2.2 nm (primary size) and 8.5 nm (secondary size) and aggregates of the larger particles. At low Pt particle coverage, two bands can be distinguished: a band at 2013 cm −1 is attributed to CO adsorbed on the small primary particles while a second band at 2046 cm −1 is assigned to linearly bonded CO adsorbed on the larger particles and on the aggregates. The importance of the lateral dipole field interactions between the adsorbed vibrating molecules is discussed and also the dependence of the vibrational frequency on the coordination of the adsorbate Pt site. Both interactions are too weak to account for the pronounced vibrational shifts in comparison with CO bonded on polycrystalline Pt and therefore the results are tentatively explained by particle-substrate interaction.