HYDROXYL RADICAL FORMATION DURING THE REACTION OF OXYGEN WITH METHANEOR WATER OVER BASIC LANTHANIDE OXIDE CATALYSTS

Methane and molecular oxygen react over La2O3 and Nd2O3 in the tempera ture range 1100-1350 K to form OH. radicals which emanate into the gas phase, where they are detected using laser-induced fluorescence spect roscopy. The less basic oxides CeO2 and Yb2O3 form no detectable OH. r adicals. When CH4 is replaced by a comparable amount of H2O, the conce ntration of OH. radicals increases, although the apparent E(a) remains constant at ca. 40 kcal/mol. It is concluded that the surface-catalyz ed equilibrium reaction 1/2H2O + 1/4O2 reversible OH. is responsible f or the formation of hydroxyl radicals with both CH4 and H2O as reagent s. Consistent with the law of mass action, variation in the OH. radica l 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 wi th 57 mTorr of O2 and 3 mTorr of H2O is 1 x 10(11) molecules/cm3, whic h is well within experimental uncertainty of the thermodynamic equilib rium value of 4.5 x 10(11) molecules/cm3. The rotational temperature o f the OH. radicals is essentially the same as the temperature of the c atalyst. Hydroxyl radicals are believed to be formed by the abstractio n of hydrogen atoms from H2O, probably at surface peroxide ions. The r eaction is analogous to the formation of CH3. radicals from CH4 on the se same active oxides.