THE ROLE OF LATTICE AND GASEOUS OXYGEN ON THE OXIDATIVE COUPLING OF METHANE STUDIED BY A PULSE REACTION TECHNIQUE

A comparative study of the role of lattice and gaseous oxygen in theoxidative coupling of methane over Na~ /Cao and Ca_xSr_(1-x)TiO_3 perovskiteoxide catalysts was performed by using a pulse reaction technique with CH_4,O_2 and mixtures of CH_4and O_2.It was found that there is an oxygen species onthe Surface of Cao which is active only for the total oxidation of methane at thereaction temperature.These oxygen species.once reacted with methane andconsumed,can be regenerated by the adsorption of oxygen molecules from thegas phase or by the migration of the lattice oxysen from the bulk onto thesurface.In contrast,no such oxygen species and no reaction can be detected bymethane pulsed over Ca_xSr_(1-x)TiO_3perovskite oxide catalysts.Na~ can heavilysuppress the non-selective oxygen species on the surface,and When 5mol%Na~ was incorporated onto the surface of Cao,the sample showed no reactivityto methane.When oxygen exists in the gas phase,or the pulse containsmethane and oxygen,the reaction over(Cao is mai

THE ROLE OF LATTICE AND GASEOUS OXYGEN ON THE OXIDATIVE COUPLING OF METHANE STUDIED BY A PULSE REACTION TECHNIQUE*

A comparative study of the role of lattice and gaseous oxygen in the oxidative coupling of methane over Na /CaO and Cax,Sr(1-x)TiO3 perovskite oxide catalysts was performed by using a pulse reaction technique with CH4, , O2and mixtures of CH4and O2. It was found that there is an oxygen species on the surface of CaO which is active only for the total oxidation of methane at the reaction temperature. These oxygen species, once reacted with methane and consumed, can be regenerated by the adsorption of oxygen molecules from the gas phase or by the migration of the lattice oxygen from the bulk onto the surface. In contrast, no such oxygen species and no reaction can be detected by methane pulsed over CaxSro(1-x)TiO3 perovskite oxide catalysts. Na can heavily suppress the non-selective oxygen species on the surface, and when 5 mol% Na was incorporated onto the surface of CaO, the sample showed no reactivity to methane. When oxygen exists in the gas phase, or the pulse contains methane and oxygen, the reaction over CaO is mainly non-selective. The samples of Na / CaO with a Na content higher than 5 mol% and CaxSr(1-x)TiO3 perovskite oxide catalysts show activity and high selectivities to ethane and ethylene. XPS characterization of the fresh and used Ca0.1 Sr0.9TiO3 perovskite oxide catalysts shows a peak developing at 531. 1 eVafter reaction, which suggests that oxygen species active for the oxidative coupling of methane are produced during reaction and not present on the fresh catalyst surface of the Ca0.1Sr0.9TiO3 perovskite oxide catalysts at a reaction temperature of 923 K. These results give evidence that gaseous oxygen plays key roles in the oxidative coupling of methane over either Na / CaO and CaxSr(1-x)TiO3 perovskite catalysts.