Free Radical Formation in the Catalytic Oxidation of NH₃ over a Polycrystalline Pt Surface at High Temperatures

The catalytic oxidation of NH₃ over a polycrystalline Pt surface in the temperature range 800–1400 K has been investigated with special emphasis on radical production through the use of laser-induced fluorescence detection. Among all the free radicals and transient species probed (NH, NH₂, OH, HNO, and NO₂) only the diatomic species NH and OH were detected. NH production was diminished drastically by O₂ and yielded a desorption energy of 66 kcal/mol. The activation energy for OH desorption was found to depend strongly on the O₂/NH₃ ratio. Its desorption energy varied from 26 kcal/mol at O₂/NH₃ = 100 to a peak value of 42 kcal/mol near the stoichiometric ratios for N₂ and NO production (O₂/NH₃ = 1) and then down to the value of 26 kcal/mol at O₂/NH₃ = 0.03. This interesting variation in OH desorption energy is attributed to site competition between O and N atoms previously noted for NO and N₂ production. An atomic/radical mechanism of NH₃ oxidation on Pt was proposed to account for the formation of these radical products.