Regular perovskite SrTiO 3 and monoclinic layered perovskite Nd 2 Ti 2 O 7 for oxidative coupling of methane: Elucidating the role of chemisorbed oxygen species and lattice oxygen

Herein, regular perovskite SrTiO 3 and monoclinic layered perovskite Nd 2 Ti 2 O 7 have been prepared by the sol-gel method and calcined at different temperatures. As the calcination temperature of SrTiO 3 increases, the 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 Nd 2 Ti 2 O 7 , its surface is enriched with Nd elements, but the enrichment amount does not change with the increase in calcination temperature, and contains less surface basic sites less than SrTiO 3 . Although the enrichment of Ti on the surface of SrTiO 3 conducive to the formation of oxygen vacancies, the oxygen vacancies on the surface of Nd 2 Ti 2 O 7 are more abundant than those on SrTiO 3 . 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 selective oxygen species are chemisorbed oxygen (O 2 - , O 2 delta- , O 2 2- ) for SrTiO 3 and surface lattice oxygen for Nd 2 Ti 2 O 7 . This is because the [TiO 6 ] octahedra in the crystal structure of SrTiO 3 are tightly stacked, whereas the [TiO 6 ] units in the Nd 2 Ti 2 O 7 structure are loosely stacked, resulting in a weaker Ti -O bond strength in Nd 2 Ti 2 O compared with SrTiO 3 . In addition, the lattice oxygen of SrTiO 3 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 Nd 2 Ti 2 O 7 are weaker than those in SrTiO 3 , making it more favorable for C 2 selectivity.