Hydrocarbon-NO_x interactions at low temperatures-1.Conversion of NO to NO₂ promoted by propane and the formation of HNCO

Reactant conversions and product distributions for reactions between NO (0-350 ppm), propane (425-1000 ppm), and O₂ (2.3-16%) in a laminar flow reactor at temperatures in the range 500°C-800°C have been determined by long-path-length Fourier transform infrared (FTIR) spectroscopy. Addition of NO significantly decreases the temperature at which oxidation of propane commences, but the formation of intermediate products (the olefins, ethylene and propylene, and formaldehyde) occurs over a wider temperature range than does the formation of these products in the absence of added NO. Oxidation of the propane coincides with NO to NO₂ conversion, although significant NO to NO₂ conversion, occurs before substantial oxidation of the hydrocarbon. The NO to NO₂ conversion occurs in stages and increases when the intermediate carbon-containing products begin themselves to oxidise. For experiments in the presence of added NO, CH₄ is not observed as a product, implying that steady-state concentrations of CH₃ radicals are low; rapid conversion of CH₃ to CH₃O₂ followed by reaction of CH₃O₂ with NO is a probable reason for this observation. At high NO and propane concentrations (350 and 1000 ppm, respectively), HNCO was detected as a product of these reactions. Formation of HNCO coincides with NO to NO₂ conversion. These observations provide evidence for hydrocarbon-NO interactions that result in the formation of species containing C-N bonds occurring at lower temperatures than previously reported.