XPS and EPR study of high and low surface area CoO–MgO solid solutions: surface composition and Co2+ ion dispersion

Abstract

Solid solutions CoO–MgO (MCo) of low and high surface area (LSA and HSA) have been studied by XPS and EPR analysis to evaluate the surface composition and the dispersion of cobalt ions in the systems. Different preparation routes were adopted: wet impregnation (I_w), dry impregnation (I_d) and coprecipitation (C). LSA solid solutions (A_s= 10–20 m² g^–1) were obtained after treatment in air at 1173 K (1173 LSA) or at 1473 K (1473 LSA), HSA materials (A_s= 200–300 m² g^–1) after treatment under vacuum at 1173 (1173 HSA). XPS intensity ratios, I_Co/I_Mgvs. x_Co/x_Mg molar fraction showed no appreciable differences between the bulk and surface compositions of LSA and HSA solid solutions from I_d and C precursors, calcined at 1173 and at 1473 K. MCo 1173 LSA I_w samples had evident cobalt enrichment, due to Co₃O₄ segregation, whereas I_w HSA did not. NO₂ or O₂ treatments led to the formation of Co^III. The Co²⁺ bulk dispersion in LSA and HSA was studied by EPR.