The reactivity of gold and platinum metals in their cluster phase

Platinum surfaces are known to be among the most active ones for the oxidat ion of carbon monoxide, whereas gold surfaces are completely inert for this reaction. The question remains: Do small clusters of these two metals main tain these distinct differences? To answer this question, we have employed temperature-programmed reaction (TPR) and Fourier transform infrared (FTIR) spectroscopy to investigate the reactivity of small platinum and gold clus ters consisting of up to 20 atoms. These clusters are generated in the gas phase by a laser evaporation source and, after mass selection, are deposite d with low kinetic energy onto thin MgO films. The oxidation of CO is studi ed for the octamer and the icosamer under UHV conditions. Surprisingly, all investigated cluster sizes are catalytically active. Pt-20 shows the highe st reactivity and oxidizes almost 6 CO molecules, whereas Pt-8, Au-20, and Au-8 oxidize just one CO molecule under identical experimental conditions.