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, Sr2 – хLnxTi1 – yFeyO4 (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 $${\text{O}}_{2}^{{2 - }}$$ /O2– (the surface adsorbed and surface lattice oxygen, correspondingly) and peculiarities of O2–. The higher СН4 conversion and C2 selectivity for the La-doped catalyst is due to appearance of O2– 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 COx formation and decreasing C2 selectivity.