High-Pressure CO Dissociation and CO Oxidation Studies on Platinum Single Crystal Surfaces Using Sum Frequency Generation Surface Vibrational Spectroscopy

Abstract

Using sum frequency generation surface vibrational spectroscopy, platinum single crystal surfaces were investigated at high-pressures and high-temperatures under pure CO or CO and O2 environments. In 40 Ton of CO, the molecule dissociates on the (111), (557) and (100) surfaces of platinum single crystals at 673 K, 548 K, and 500 K, respectively, indicating CO dissociation is structure sensitive. The Pt(111) surface must be heated to a temperature where the surface is roughened creating step and kink sites, which are known to dissociate CO. The stepped Pt(557) surface does not need to be heated as high as Pt(111) to dissociate CO since there are step sites already available on the surface. The outer most surface atoms of Pt(100) are mobile compared to the low energy (111) surface and so the surface can roughen at a much lower temperature than observed on Pt(111). Under 40 Ton of CO and 100 Torr of O2, CO oxidation ignition temperature of 620 K, 640 K and 500 K were observed for Pt(111), Pt(557) and Pt(100), respectively, indicating ignition is also structure sensitive. The ignition temperatures for Pt(111) and Pt(557) are similar because the higher concentration of surface atoms on the (111) terraces, common to both surfaces, are more dominant during oxidation than the step sites. Since both CO dissociation and CO oxidation ignition are structure sensitive and follow the same trend of decreasing temperatures for the two processes, it is likely that CO dissociation is important for the onset of ignition.