Probing Terrace and Step Sites on Pt Nanoparticles Using CO and Ethylene

Temperature desorption spectroscopy (TDS) and infrared reflection absorption spectroscopy (IRAS) were used to study CO adsorption on the terrace and step sites of Pt nanoparticles grown in ultrahigh vacuum on SiO2. The percentage of terrace sites obtained by TDS measurements from four average particle sizes (>4, 4.2, 3.3, 2.6, and 2.5 nm) are compared to a simple hard sphere counting model of a truncated cuboctahedron. It is demonstrated experimentally that when the average Pt particle size is reduced from 4.2 to 2.5 nm, the percentage of terrace sites decreases by similar to 50%, consistent with the hard sphere models. Pt particle morphology is further explored by blocking the terrace sites with ethylidyne (derived from ethylene). CO adsorption on the unoccupied sites of Pt nanoparticles precovered with ethylidyne demonstrates a continuous red-shift in IRAS with increasing particle size. Similarities and differences between Pt nanoparticles and high-index single crystals are discussed.