Oxidative Coupling of Methane with Substituted Strontium Titanate Perovskite

Abstract

Oxidative coupling of methane (OCM) was studied on strontium titanate perovskite oxides whose A site ion (Sr²⁺) was partly substituted by various metal ions. The pulse reaction of methane with oxides gave either C₂ hydrocarbons (C₂) or carbon oxides (CO_x). When Sr²⁺ in SrTiO₃ was substituted by Ca²⁺, its activity and selectivity for producing C₂ decreased whereas when the Sr²⁺ was substituted by Mg²⁺ its activity for C₂ formation was promoted. Substitution of Sr²⁺ by Mg²⁺ (especially, when the substitution was 0.04-0.10), it especially promoted the catalytic activity of OCM while its chemical character (basicity) was unchanged. Substitution by Mg²⁺ reduced the crystal size and strain in the crystallite and promoted the specific surface area. At 4% substitution, the crystallite size was minimum and the catalytic function was maximum. It was concluded that the introduction of Mg²⁺ suppressed the crystal growth while maintaining the perovskite structure provided a larger surface area and that the promotion of catalytic activity was attributed to the increased number of active sites stemming from the larger surface area.