Sr2TiO4 Layered Perovskites: Effect of Doping and Synthesis Method on Properties and Catalytic Activity in Oxidative Coupling of Methane

Abstract

Layered perovskite-like Sr₂TiO₄ is a promising catalyst for oxidative coupling of methane (OCM)—a potential direct route to produce C₂ (ethane + ethylene) hydrocarbons, especially, ethylene. In this work, Sr_{2 – х}Ln_xTi_{1 – y}Fe_yO₄ (Ln = La, Pr, х = 0, 0.1; y = 0–0.3) layered perovskites with partially substituted positions of Sr (La, Pr) or Ti (Fe), were synthesized by sol–gel method using polymeric precursors and wetness impregnation followed with calcination at 900–1000°C. The samples were characterized by X‑ray diffraction (XRD), field emission scanning electron microscope (FE-SEM), X-ray photoelectron spectroscopy (XPS), adsorption and temperature-programmed hydrogen reduction (H₂-TPR). It was revealed that Sr₂TiO₄ doping with La leads to formation of nanocomposite material containing Sr₂TiO₄, La‑doped SrTiO₃ and La₂O₃ while Sr₂TiO₄, Sr₃Ti₂O₇ and Pr₆O₁₁ are observed in the case of Pr. Fe–doped samples with x = 0.1–0.2 are the single-phase systems. Testing of the obtained doped materials in oxidative coupling of methane has shown the correlation of their catalytic activity with H₂-TPR data and also with the ratio \({\text{O}}_{2}^{{2 - }}\)/O^2– (the surface adsorbed and surface lattice oxygen, correspondingly) and peculiarities of O^2–. The higher СН₄ conversion and C₂ selectivity for the La-doped catalyst is due to appearance of O^2– reduced at medium temperature. The low activity of Pr and Fe doped samples is conditioned by the presence of a large amount of surface adsorbed oxygen \({\text{O}}_{2}^{{2 - }}\) facilitating CO_x formation and decreasing C₂ selectivity.