Oxidative coupling of methane on alkali metal-promoted nickel titanate

Nickel titanate, a mixed oxide with ilmenite structure, develops activity for methane oxidative coupling reaction upon promotion with alkaline metals (Li, Na, K). Typically, higher hydrocarbon yields of ca. 13% with ca. 70% selectivity can be obtained with a 9.7% Li/NiTiO3 catalyst and C2 + yields of ca. 8% with ca. 60% selectivity with a 22% Na/NiTiO3 catalyst. Transient experiments involving steps, pulses, and steady-state isotopic switches indicate that while on a 9.7% Li/NiTiO3 catalyst the activity is mainly related to lattice oxygen, there is little lattice oxygen involvement in oxidative coupling over sodium-loaded nickel titanate. Isotopic switch results revealed that carbon dioxide interacts strongly with both lithium- and sodium-promoted surfaces, but the concentration of long-lived adsorbed methane is too low to be differentiated from the interaction of argon with the surface. X-ray diffraction and BET results indicate that increasing the degree of alkali metal loading lowers the surface area and creates different phases after pretreatment at elevated temperatures. X-ray photoelectron spectroscopy revealed that different types of carbon and oxygen species are present on the surface.