XPS and DFT studies of γ-Al2O3 modified by NO

Abstract

The modification of γ-Al2O3 after NO treatment and during the NO + H2 reaction was studied using X-ray photoelectron spectroscopy (XPS) and density functional theory. The XPS N1s spectra contain features with binding energies BE = 399.0 and 403.0 eV; the first is preserved, and the second disappears when NO is removed from the gas phase. The features were identified from experimental and modeled O-KLL regions of possible modification products − NAln oxynitrides (n = 3; 4) of the γ-Al2O3(110) surface and adsorbed states. Calculations show that NAln adsorbs NO, forming normal states with a distance to the surface d = 1.32–1.70 Å and distant (d-NO) states (d = 1.91–2.06 Å) in which the spin-polarized NO molecule is retained due to magnetic NAln and Coulomb interaction. An increase in the similarity of the O-KLL spectra in the sequence of NO/γ-Al2O3(110), NO/NAln and d-NO/NAln at an adsorption heat of ∼ 0.53, 1.33 and 1.26 eV, respectively, suggests that the modification includes the formation of NO/NAl4 corresponding to BE = 399.0 eV and d-NO/NAl4, which corresponds to BE = 403.0 eV and, being chemically unbound, is able to leave the surface in the absence of NO in the reaction medium.