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 infrar ed emission spectroscopy. The selective production of syngas (CO + H-2 ) was observed by the partial oxidation reaction of small alkanes (eth ane, propane, butane) and ethylene at higher surface temperatures (abo ve 1000 K). The infrared spectra of the product CO desorbed from the P t 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 spectr al analyses showed that the desorbed CO molecules were vibrationally e xcited (vibrational temperature, T-V = ca, 2500 K) but rotationally ve ry cool (rotational temperature, T-R = 380 K). The low T-R value was d ue not to the rotational relaxation by the gas phase collision, but th e product CO formed by the reaction between C-ad and O-ad was desorbed without being ad thermally accommodated to the surface. The vibration al and rotational states of the product CO2 formed by the ethylene oxi dation at the higher O-2/C2H4 ratio were also studied in comparison wi th those of CO2 formed by the catalytic oxidation of CO, and implicati ons of these results for the dynamics of the oxidation reactions are d iscussed.