Formation of gas-phase hydroxyl radicals over metal oxide catalysts

Abstract

The formation of surface-generated gas-phase OH- radicals was investigated during the catalytic oxidation of CH4 and the oxygen-assisted decomposition of H20. The OH- radicals were detected by laser-induced fluorescence spectroscopy. The ability of different types of metal oxides to produce gas-phase OH- radicals during the reaction of 0, with H 2 0 was examined. Surface-generated gas-phase hydroxyl radicals were formed at temperatures between 983 and 1250 K over 7 mg La 2 0 3 during the reactions of CH4+02, CH 4 +N2O and H 2 0+0 2' The activation energy for the production of hydroxyl radicals during these three reactions was approximately 40 kcal/mol. It was determined that the OH- radicals were actually formed from the reaction 1/2H 2 0+1/40 2 @ OH-. Evidence for the reaction being at equilibrium includes (i) the activation energy for hydroxyl radical production was equal to the heat of formation of OH- radicals from H,O with 0,; (ii) the orders of OH- radical production were those expected from the law of mass action; and (iii) the absolute concentration of hydroxyl radicals was the same as that calculated from thermodynamics. The production of OH, radicals was investigated over other rare earth oxides catalysts. The strongly basic rare earth oxides, including La203, and Nd 203) produced hydroxyl radicals at equilibrium with 7 mtorr each of H2 0 and 0 2 and 7 mg catalyst at 1220 K. These catalysts also produced significant concentrations of methyl radicals during methane oxidative coupling. The weakly basic rare earth oxides, Yb 203 and CeO2), did not produce any detectable hydroxyl radicals or any significant concentration of methyl radicals during methane oxidative coupling. The production of OH- radicals became kinetically controlled at higher pressures or when the amount of catalyst was decreased. The OH- radical production became zero order for both H,O and 0, and had an activation energy of 10 kcal/mol at pressures over 200 mtorr for a 1: I water-to-oxygen ratio. When the amount of La 203 catalyst was less than 2 mg, the activation energy decreased from 45 kcal/i-mol to 31 kcal/mol as the total pressure was increased froi- n 50 mtorr to 120 mtorr. At pressures below 50 mtorr the orders were 0.47 and 1.02 for 0, and H,O, respectively.