Structural–chemical disorder of manganese dioxides: II. Influence on textural properties

Abstract

Relationships between structural parameters of MnO 2 and their surface properties at the solid–gas interface were investigated. The studied series ranged from ramsdellite to pyrolusite and encompassed disordered γ-MnO 2 samples. The structural model used takes into account structural defects: Pr (rate of pyrolusite intergrowth in the ramsdellite network) and Tw (rate of microtwinning). Analysis of the N 2 adsorption isotherm evidenced positive correlations between specific surface area and Tw for γ-MnO 2 only and between the energetic constant C and (1-Pr). No microporosity is evidenced. Water adsorption isotherms evidenced the dependence of the H 2O monolayer volume on Tw and showed a positive correlation between the cross-section area of water molecules adsorbed in the first monolayer and Pr, ranging from 13.5 Å 2 for Pr=1 to 6.3 Å 2 for Pr=0.2 (12 sites/nm 2). Energetic heterogeneity is quantified from Ar and N 2 low-pressure adsorption isotherms with the DIS procedure and correlated with H 2O adsorption. High-energy adsorption domains are quantified and assigned to the different crystal faces: (110) faces with a common 1×1 octahedra layer of pyrolusite and ramsdellite and the (001) face of ramsdellite with 2×2 octahedra on which channels and plateaus are differentiated. The specific surface area ratio of ramsdellite high-energy sites to total ramsdellite content is shown to depend on Tw. The dependence on microtwinning of low cross-sectional area of N 2 and much lower cross-sectional of residual H 2O molecules leads us to assume that their adsorption sites on grain boundaries are represented by the twin planes between the structured nanocrystals generated by oxygen evolution during MD synthesis.