HYDROXYL RADICAL FORMATION DURING THE REACTION OF OXYGEN WITH METHANE OR WATER OVER BASIC LANTHANIDE OXIDE CATALYSTS
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Methane and molecular oxygen react over La2O3 and Nd2O3 in the temperature range 11001350 K to form OH radicals which emanate into the gas phase, where they are detected using laser-induced fluorescence spectroscopy. The less basic oxides CeO2 and Yb2O3 form no detectable OH radicals. When CH4 is replaced by a comparable amount of H2O, the concentration of OH radicals increases, although the apparent Ea remains constant at ca. 40 kcal/mol. It is concluded that the surface-catalyzed equilibrium reaction 1/2H2O + 1/4O2OH is responsible for the formation of hydroxyl radicals with both CH4 and H2O as reagents. Consistent with the law of mass action, variation in the OH radical concentration is 0.51 order with respect to H2O and 0.26 order with respect to O2. The measured concentration of OH radicals at 1208 K with 57 mTorr of O2 and 3 mTorr of H2O is 1 1011 molecules/cm3, which is well within experimental uncertainty of the thermodynamic equilibrium value of 4.5 1011 molecules/cm3. The rotational temperature of the OH radicals is essentially the same as the temperature of the catalyst. Hydroxyl radicals are believed to be formed by the abstraction of hydrogen atoms from H2O, probably at surface peroxide ions. The reaction is analogous to the formation of CH3 radicals from CH4 on these same active oxides. 1993, American Chemical Society. All rights reserved.