Microwave-Assisted Hydrothermal Synthesis of α-MnO₂: Lattice Expansion via Rapid Temperature Ramping and Framework Substitution

Abstract

Cobalt doped cryptomelane type manganese oxide (K-OMS-2) microwires had been successfully synthesized and characterized. Their catalytic activity was tested in an oxidation reaction with benzyl alcohol as the substrate and the cobalt doped OMS-2 materials showed 100% selectivity towards benzyl aldehyde with a conversion of 55%. The cobalt doped OMS-2 materials were also investigated as a desulfurization sorbent in a fixed bed reactor at 250ºC where high sulfur sorption capacities (49.4 g sulfur/100 g sorbent) were observed. Here, the structure controlled synthesis was performed using a facile one step microwave assisted hydrothermal method (MWHY) associated with a rapid temperature ramping (200ºC/min). The structural effects induced by the compositional control of transition metal dopants on the cryptomelane (space group I4/m) body centered tetragonal structure were identified with X-ray diffraction (XRD) and transmission electron microscopy (TEM). The XRD and TEM results showed that the systematic variance of the cobalt content was accompanied by a stepwise lattice expansion of (110) plane from 6.70 to 7.43 Å. The XRD, high resolution TEM (HRTEM)/TEM, Raman spectroscopy, Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), and X-ray photoelectron spectroscopy (XPS) data suggested that the assynthesized cobalt doped OMS-2 materials were also crystalline with no segregated metal oxide impurities. The uniform morphology of the metal doped OMS-2 materials was observed by the field-emission scanning electron microscopy (FESEM), whereas energy-dispersive X-ray (EDX) analysis confirmed the successful incorporation of metal dopant into the OMS-2 structure.Inductively coupled plasma atomic emission spectroscopy (ICP-AES) showed a higher degree of doping (Co/Mn = ~0.26) associated with MWHY method over conventional methods. On the other hand, TGA demonstrated that the as-synthesized materials were more thermally unstable than their undoped counterparts. The observed structural and chemical characteristics upon doping with some metal cations were explained by the Jahn-Teller distortion.

Elucidation of Unexpectedly Weak Catalytic Effect of Doping with Cobalt of the Cryptomelane and Birnessite Systems Active in Soot Combustion

“…In the case of cryptomelane the excess of oxygen was confirmed for all samples, whereas the excess of potassium only for cobalt-doped samples. Cobalt presence did not influence the surface status of the other elements (K, Mn, O) in the investigated samples and the obtained results were similar to those already reported elsewhere [24,36].…”

“…As reported by Jinzhu Ma et al [23], cryptomelane can be successfully doped with cobalt by a hydrothermal method, however its consequent activity in ozone decomposition was lower than that of the parent catalyst. Cobalt was introduced to cryptomelane also by microwave synthesis by L. R. Pahalagedara et al [24] and a significant increment of its activity in selective oxidation of benzyl alcohol to benzyl aldehyde was observed. Cobalt-doping of birnessite by addition of CoCl 2 to a boiling solution of KMnO 4 was reported by Hui Yin et al in the context of non-catalytic environmental applications [25].…”

SCILLs as selective catalysts for the oxidation of aromatic alcohols

“…The Raman spectra of OMS-2 and 5wt% Pt/OMS-2,Figure 3(A1, A2), show three main contributions at 183, 576 and 636 cm -1 which are in accordance with literature reports for OMS-2[57,58]. The band at 180 cm -1 corresponds to the deformation mode of the metaloxygen chain of Mn-O-Mn, while the bands at 576 and 636 cm -1 correspond to the stretching modes of the Mn-O lattice.…”

Nanostructured Metal Oxides-Based Electrode in Supercapacitor Applications

“…An inverted temperature gradient takes place during the microwave-assisted process, and a rapid dielectric heating is generated internally within the material due to applied microwave radiation with a commonly used frequency of 0.3-2.45 GHz. Microwave-assisted synthesis has been adopted to prepare metal oxides with highly uniform nanostructures [40][41][42].…”