The role of carbon deposition from CO dissociation on platinum crystal surfaces during catalytic CO oxidation: Effects on turnover rate, ignition temperature, and vibrational spectra

Experiments were performed to understand the process of ignition during platinum catalyzed CO oxidation using sum frequency generation (SFG) vibrational spectroscopy, Auger electron spectroscopy (AES), gas chromatography (GC), and temperature programmed desorption (TPD). Both CO dissociation and CO oxidation ignition studies on the (100), stepped (557), and (111) surfaces of platinum are presented. Rapid CO dissociation on the Pt(100), Pt(557), and Pt(111) occurred in narrow temperature ranges (+/-10 K) at 500, 548, and 673 K, respectively. The CO ignition temperature at a pressure of 40 Torr of CO and 100 Torr of O-2 is lower on Pt(100) than on Pt(111) and Pt(557). Thus, both CO dissociation and the ignition of CO oxidation are structure sensitive. An in depth study of CO oxidation on Pt(557) was performed on both initially clean and initially carbon covered platinum surfaces to investigate the role of carbon obtained from CO dissociation in CO oxidation. Under excess O-2 and excess CO conditions, a clean platinum surface will remain carbon free below and above ignition. However, a carbon oxide species was formed on initially carbon covered platinum surfaces once oxygen was added at a high temperature (548 K). This carbon oxide species results in a large SFG background signal, allowing us to measure the formation and reactivity of this species during oxidation reactions. The carbon oxide species also formed on initially clean platinum below the ignition temperature when the Pt crystal was exposed to equal partial pressures of CO and O-2. The turnover rates on the carbon oxide covered platinum surfaces were higher than on the initially clean surfaces below ignition. The ignition temperature on the carbon covered surface (648 K) was lower than on the clean platinum (700 K) surface at equal pressures of CO and O-2. All of this evidence indicates the surface carbon oxide species is better at oxidizing CO than platinum under certain pressure and temperature conditions. CO dissociation is an important step during the onset of ignition, when surface carbon oxidation provides a new exothermic reaction channel in addition to the Pt surface catalyzed oxidation of molecular CO.