Cobalt-doped cryptomelane-type manganese oxide (K-OMS-2) microwires have 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 toward 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 250 °C where high sulfur sorption capacities (49.4 g of sulfur/100 g of sorbent) were observed. Here, structure-controlled synthesis was performed using a facile one-step microwave-assisted hydrothermal method (MWHY) associated with a rapid temperature ramping (200 °C/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 the (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 as-synthesized 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 the 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.
Pahalagedara, L. R., King'ondu, C. K., Dharmarathna, S., Pahalagedara, M. N., Meng, Y-T., Kuo, C-H., & Suib, S. L. (2014). Microwave-Assisted Hydrothermal Synthesis of α-MnO2: Lattice Expansion via Rapid Temperature Ramping and Framework Substitution. Journal of Physical Chemistry C, 118(35), 20363-20373. https://doi.org/10.1021/jp505306q