Standard Nitrogen Adsorption Data for Manganese Oxide and like (C BET = 40–60) Surfaces

Abstract

Calcination to 1170 K of materials obtained by mild and intensive reduction of KMnO₄ and HMnO₄ solutions was found to produce manganese oxides assuming the crystalline bulk structure of α-Mn₂O₃ Nitrogen adsorption isotherms at 77 K revealed that they also assume non-porous surfaces of low specific area (4 ± 1 m²/g) with C_BET equal to 40–60; t- and α_s-curves derived therefrom were found to coincide with those determined reportedly on non-porous surfaces of 11 < C_BET < ∞, irrespective of the surface chemical similarity. The experimental and reported standard adsorption data were used to analyze a type-IV nitrogen adsorption isotherm determined at 77 K on a porous manganese oxide (δ-MnO₂) which was obtained by sol–gel processing of a KMnO₄ solution, and found to similarly reveal a mesoporous surface of S_BET = 73 m²/g and C_BET = 125. The close agreement between the surface area values (S_t and S_α) derived for the test material (δ-MnO₂) from the t- and α_s-plots thus obtained and its C_BET value (73 m²/g) was the basis on which reference materials with 11 < C_BET < ∞ were found to be appropriate. Since most of the appropriate reference materials were dissimilar chemically to the test material, the results of the present investigation may imply that the surface chemical similarity demanded by the α_s-method is better manifested in terms of the surface activity (heat of adsorption monitored by the C_BET value) than the chemical composition.