Surface properties of sol–gel synthesized δ-MnO2 as assessed by N2 sorptometry, electron microscopy, and X-ray photoelectron spectroscopy

Pure δ-MnO2 materials were prepared by calcination at 250–700°C for 3 h of a parent gel obtained by (i) a sol–gel synthesis involving a mild reduction of KMnO4 solution; (ii) drying at room temperature; (iii) calcination to 700 or 900°C; and (iv) a subsequent acid treatment at room temperature. The bulk crystalline structure of the δ-MnO2 materials was evidenced by thermogravimetry, X-ray powder diffractometry (XRD) and high-resolution transmission electron microscopy (HRTEM). Then, surface properties were determined by X-ray photoelectron spectroscopy (XPS), scanning and transmission electron microscopy, and N2 sorptometry. The results revealed that the bulk structure is morphologically organized in the form of needle-like nanofibers. Surfaces thereon exposed were found to assume high specific area (74 m2 g−1) and heat of adsorption (BET-C=130), as well as a mesoporous texture, when initially calcined to 700°C. These surface properties were also found to withstand subsequent calcination at up to 350°C for 3 h.