Kinetics and mechanism of the oxidative coupling of methane over 1 percent strontium/lanthanum oxide

The kinetics and mechanism of the oxidative coupling of methane on 1% Sr/La{dollar}rmsb2Osb3{dollar} were investigated in a flow reactor coupled with photoionization mass spectrometer. The important radical intermediate produced in the methane coupling process, methyl radicals (CH{dollar}sb3{dollar} (g)), were quantitatively measured along the catalyst bed using photoionization mass spectrometry (PIMS) and laser photolysis technique. The determination of CH{dollar}sb3{dollar} (g) production rate law, stoichiometric conversion of CH{dollar}sb3{dollar} (g) to {dollar}rm Csb2Hsb6{dollar} (g) and of {dollar}rm Csb2Hsb6{dollar} (g) to {dollar}rm Csb2Hsb4{dollar} (g), and conversion efficiency of CH{dollar}sb4{dollar} (g) to CH{dollar}sb3{dollar} (g) on this metal oxide catalyst advances knowledge on mechanism of the catalytic conversion of methane to higher hydrocarbons, a potential process to provide an alternative for the future fuels.; Photoionization mass spectrometry technique was developed to study gas-phase kinetic studies of polyatomic free radicals. This technique has high selectivity for the detection of gas phase polyatomic radicals, which normally has low ionization energy, via selecting appropriate ionizing energy. Applying this technique to study the oxidative coupling of methane can provide not only quantitative measurement of radical concentrations but also direct studying the kinetics of the gas phase/surface reactions of radical intermediates (CH{dollar}sb3{dollar} and {dollar}rm Csb2Hsb5{dollar}) under the methane coupling conditions. The gas phase as well as heterogeneous radical reactions have been speculated to be one of important factors governing the selectivity of the higher hydrocarbon productions.; CH{dollar}sb3{dollar} production rates depend first order on {dollar}rmlbrack CHsb4rbrack{dollar} and shows, (1) zero dependence of {dollar}rmlbrack Osb2rbrack{dollar} when {dollar}rmlbrack Osb2rbrack{dollar} is large (e.g. {dollar}>{dollar}10% of total carrier gas density), and (2) half order dependence of {dollar}rmlbrack Osb2rbrack{dollar} when {dollar}rmlbrack Osb2rbrack{dollar} is small. A mechanism, which assumes gas phase methane reacts with active surface sites generated by dissociatively adsorption of gas phase oxygen on catalyst surface, can best describe the observed CH3 production. Gas phase recombination of CH{dollar}sb3{dollar}(g) can account for observed ethane production. At temperature less than 1100 K, gas phase pyrolysis of ethane cannot account for ethylene production. However, ethylene formation can be accounted for by heterogeneous reaction of ethane. The increase of catalyst activity will decrease the conversion efficiency of CH{dollar}sb4{dollar} to CH{dollar}sb3{dollar}(g). Consequently, high selectivity of C{dollar}sb2{dollar} production can only be achieved at low activity of catalyst surface.