Oxidative methane coupling over Mg, Al, Ca, Ba, Pb-promoted SrTiO3 and Sr2TiO4: Influence of surface composition and microstructure

Mg, Al, Ca, Ba, Pb-substituted titanates SrTi(1-x)A(x)O(3) (A= Mg, AI, Ca, Ba, Pb, x = 0.1) and Sr(2)Ti(1-x)A(x)O(4) (A= Mg, Al, x = 0.1) were synthesized using mechanochemical method (sintering at 1100 degrees C in air for 4h) and tested in oxidative coupling of methane (OCM) at 850 and 900 degrees C. The obtained samples were double-phase samples consisting of Mg, Ca, Ba substituted perovskite (SrTiO3) and "layered" perovskite - (Sr2TiO4 or Sr3Ti2O7). In case of Al and Pb, it was shown that these cations most probably did not replace Ti in the perovskite structure and formed the Sr3Al2O6 and SrPbO3 individual phases. The most active samples were Mg- and Al-doped SrTiO3 and Sr2TiO4, which in their mixture with inert quartz particles showed C-2 yield up to 25% and C-2 selectivity around 66%. Microstructure analysis of Mg-substituted titanates revealed that under reaction conditions the "layered" perovskite decomposed releasing (Sr, Mg)O mixed oxide segregated to the surface. The samples characterized by the highest surface content of the (Sr, Mg)0 oxide, which was estimated by IR CO2 adsorption, demonstrated both the highest activity in methane activation and the highest rate of methyl radical generation to the gas phase. The influence of the (Sr, Mg)O segregation on the formation of active oxygen species is discussed. (C) 2014 Elsevier B.V. All rights reserved.