Regular perovskite SrTiO3 and monoclinic layered perovskite Nd2Ti2O7 for oxidative coupling of methane: Elucidating the role of chemisorbed oxygen species and lattice oxygen

Abstract

Herein, regular perovskite SrTiO3 and monoclinic layered perovskite Nd2Ti2O7 have been prepared by the sol-gel method and calcined at different temperatures. As the calcination temperature of SrTiO3 increases, the Ti element on their surface is enriched, and the amount of Sr element that provides basic sites is reduced, resulting in a decrease in the number of basic sites on the catalyst surface. For Nd2Ti2O7, its surface is enriched with Nd elements, but the enrichment amount does not change with the increase in calcination temperature, and it contains less surface basic sites less than SrTiO3. Although the enrichment of Ti on the surface of SrTiO3 is conducive to the formation of oxygen vacancies, the oxygen vacancies on the surface of Nd2Ti2O7 are more abundant than those on SrTiO3. The amount of electron-deficient oxygen species on their surfaces is influenced by the presence of basic sites, which make it more favorable to stabilize electrophilic oxygen species. The C2 selective oxygen species are chemisorbed oxygen (O2-, O2δ-, O22-) for SrTiO3 and surface lattice oxygen for Nd2Ti2O7. This is because the [TiO6] octahedra in the crystal structure of SrTiO3 are tightly stacked, whereas the [TiO6] units in the Nd2Ti2O7 structure are loosely stacked, resulting in a weaker Ti-O bond strength in Nd2Ti2O7 compared with SrTiO3. In addition, the lattice oxygen of SrTiO3 has strong basicity, which is beneficial for activating gas-phase oxygen to generate chemisorbed oxygen species and stabilizing them on the catalyst surface, while the nucleophilic lattice oxygen is more likely to cause deep oxidation of methane and coupling products. However, the and basicity of lattice oxygen in Nd2Ti2O7 are weaker than those in SrTiO3, making it more favorable for C2 selectivity.