Nitrogen Adsorption and Surface Texture of High-Area Manganese Oxides Derived from Potassium-Contaminated NH 4 MnO 4

Abstract

Nitrogen sorption isotherms have been determined at 77 K on pre-characterised MnO_x bulk samples obtained by calcination at 423–873 K of K⁺-contaminated NH₄MnO₄. The isotherms have been analysed for estimating the surface area and pore structure of the test samples adopting the α_s-method and standard adsorption data determined on a local reference material (calcination product of the permanganate at 1273 K for 20 h). The most prominent result of the present investigation is that the presence of K⁺ leads to the formation of a channel-structured KMn₈O₁₆ bulk phase at 573 K and the following consequent textural modifications: (i) a considerable increase in the surface area to 205 m²/g, (ii) the construction of a uniform pore system consisting of slit-shaped pores with narrow entrances, and (iii) the evolution of energetically uniform internal surfaces of considerable area (= 92 m²/g). The results also reveal that the test surfaces generally exhibit a low heat towards N₂ adsorption.