INFRARED-EMISSION STUDY OF VIBRATIONALLY EXCITED MOLECULES FORMED BY CATALYTIC-OXIDATION OF HYDROCARBONS

Vibrationally excited molecules produced by molecular-beam hydrocarbon oxidation reactions on a Pt surface have been studied by using infrared emission spectroscopy. The selective production of syngas (CO + H-2) was observed by the partial oxidation reaction of small alkanes (ethane, propane, butane) and ethylene at higher surface temperatures (above 1000 K). The infrared spectra of the product CO desorbed from the Pt surface were measured during the partial oxidation of ethylene (O-2/C2H4 = 3/1) on Pt at the surface temperature of 1500 K, and the spectral analyses showed that the desorbed CO molecules were vibrationally excited (vibrational temperature, T-V = ca, 2500 K) but rotationally very cool (rotational temperature, T-R = 380 K). The low T-R value was due not to the rotational relaxation by the gas phase collision, but the product CO formed by the reaction between C-ad and O-ad was desorbed without being ad thermally accommodated to the surface. The vibrational and rotational states of the product CO2 formed by the ethylene oxidation at the higher O-2/C2H4 ratio were also studied in comparison with those of CO2 formed by the catalytic oxidation of CO, and implications of these results for the dynamics of the oxidation reactions are discussed.