Electron spin resonance spectroscopy of surface species formed upon adsorption of nitrogen oxides and oxygen on high-surface-area Nio–MgO and CoO–MgO solid solutions

Abstract

Solid solutions of high-surface-area (h.s.a.) NiO–MgO (Ni content 0.1–5 atoms per 100 Mg atoms) were activated by heating under vacuum at 1173 K for 5 h before exposure to O₂, NO₂, NO or N₂O at temperatures in the range 298–815 K. With all gases, depending on the temperature of adsorption, two species were observed by electron spin resonance spectroscopy. Both are axial signals of nickel species on the surface of MgO, as shown by broadening experiments with O₂. The e.s.r. analysis, the chemical conditions under which the species are formed and their thermal stability allow the assignment of the two species to (i) a coordinatively unsaturated Ni³⁺(Ni_s³⁺) and (ii) Ni³⁺ ions in a distorted octahedral surface complex (Ni_s³⁺⋯L). Adsorption of NO at 178–298 K produces a (Ni⋯NO)²⁺ adduct, NO^2–₂ and Mg²⁺⋯NO.

Solid solutions of h.s.a. CoO–MgO (Co content 0.1–5 atoms per 100 Mg atoms) activated as for NiO–MgO were exposed to NO₂ or NO. The labile cobalt nitrosyl adduct formed is tentatively assigned to Co²⁺⋯(NO)₂; NO^2–₂ species and Mg²⁺⋯NO are also formed.

Some total and reversible adsorptions (removed by evacuation at the temperature of adsorption) were determined volumetrically. Reflectance spectra were taken under the same conditions for the formation of the paramagnetic species. Surface processes by which the various species were formed are discussed.