Role of ReOx Species in Ni-Re/Al2O3 Catalyst for Amination of Monoethanolamine

The role of ReOx species in Ni-Re/Al2O3 catalyst for amination of monoethanolamine (MEA) was investigated using a combination of experimental (Raman spectroscopy, in situ X-ray photoelectron spectroscopy, transmission electron microscopy, X-ray diffraction, scanning transmission electron microscopy-energy-dispersive X-ray spectroscopy line scan, and extended X-ray absorption fine structure) and theoretical (density functional theory (DFT)) methods. Characterization and simulation results indicate that rhenium oxide specie prefers to atomically disperse on the surface of NiO in calcined Ni-Re/Al2O3 catalyst. After reduction, the surface of metallic Ni is decorated by ReOx with multiple valence states; meanwhile, a small amount of Ni-Re alloy was detected by extended X-ray absorption fine structure (EXAFS). DFT results indicate that different-valent ReOx species in Ni-Re/Al2O3 catalyst have different functions during reaction. The high-valent ReOx (x >= 3) distributed on the Ni particle surface play an important role in decreasing the surface energy of Ni particle, thus stabilizing Ni particles against sintering and increasing the lifetime of Ni-Re/Al2O3 catalyst during amination reaction. The synergistic effect between Ni and low-valent ReOx (x < 3) sites facilitates the abstraction of hydrogen in MEA and improves the activity of Ni-Re/Al2O3 catalyst for MEA amination.