Manganese Oxide Octahedral Molecular Sieves (OMS-2) Multiple Framework Substitutions: A New Route to OMS-2 Particle Size and Morphology Control

Abstract

Self-assembled multidoped cryptomelane hollow microspheres with ultrafi ne particles in the size range of 4-6 nm, and with a very high surface area of 380 m 2 g − 1 have been synthesized. The particle size, morphology, and the surface area of these materials are readily controlled via multiple framework substitutions. The X-ray diffraction and transmission electron microscopy (TEM) results indicate that the as-synthesized multidoped OMS-2 materials are pristine and crystalline, with no segregated metal oxide impurities. These results are corroborated by infrared (IR) and Raman spectroscopy data, which show no segregated amorphous and/or crystalline metal impurities. The fi eld-emission scanning electron microscopy (FESEM) studies confi rm the homogeneous morphology consisting of microspheres that are hollow and constructed by the self-assembly of pseudo-fl akes, whereas energy-dispersive X-ray (EDX) analyses imply that all four metal cations are incorporated into the OMS-2 structure. On the other hand, thermogravimetric analyses (TGA) and differential scanning calorimetry (DSC) demonstrate that the as-synthesized multidoped OMS-2 hollow microspheres are more thermally unstable than their single-doped and undoped counterparts. However, the in-situ XRD studies show that the cryptomelane phase of the multidoped OMS-2 hollow microspheres is stable up to about 450 ° C in air. The catalytic activity of these microspheres towards the oxidation of diphenylmethanol is excellent compared to that of undoped OMS-2 materials.

Enhanced catalytic degradation of ciprofloxacin over Ce-doped OMS-2 microspheres

“…As shown in Fig. S2, the FTIR spectrum of OMS-2 showed four characteristic bands around 720 cm −1 , 590 cm −1 , 530 cm −1 , and 470 cm −1 with comparable relative intensities that can be assigned to Mn-O lattice vibration modes in [MnO 6 ] octahedra [14,19]. Upon doping of Ce into OMS-2 structure, the bands at 530 cm −1 and 470 cm −1 shifted to 537 cm −1 and 481 cm −1 , respectively, for Ce(0.06)-OMS-2, whereas the ones at 720 cm −1 and 590 cm −1 did not change in peak position.…”

“…To improve the catalytic activity, OMS-2 was usually doped by foreign metal ions, such as Fe 3+ , Co 2+ , Cu 2+ , V 5+ and Mo 6+ [9,10]. The metal dopants can be loaded on the surface, incorporated into the framework, or entered into the tunnel of OMS-2 by different preparation processes, such as ball milling, impregnation, reflux, and hydrothermal methods [11,12]. Inclusion of foreign metals in the framework or tunnel structure could lead to increase in defects inside the catalyst surface thereby creating more number of active sites [13,14].…”

“…The metal dopants can be loaded on the surface, incorporated into the framework, or entered into the tunnel of OMS-2 by different preparation processes, such as ball milling, impregnation, reflux, and hydrothermal methods [11,12]. Inclusion of foreign metals in the framework or tunnel structure could lead to increase in defects inside the catalyst surface thereby creating more number of active sites [13,14]. Cerium (Ce) with a redox cycle between the +3 and +4 states can be manipulated to create efficient catalysts.…”

Structural and chemical disorder of cryptomelane promoted by alkali doping: Influence on catalytic properties

“…Furthermore, the retention of alkali metals on cryptomelane is also governed by the crystal radii and the coordination of the dopant cations. The cations should allow a coordination number of eight and have a crystal radius of almost the same size as K + (1.39 Å) [38].…”

Formation of Platinum (Pt) Nanocluster Coatings on K-OMS-2 Manganese Oxide Membranes by Reactive Spray Deposition Technique (RSDT) for Extended Stability during CO Oxidation

“…Manganese oxide K-OMS-2 is a porous mixed-valent metal oxide with applications in catalysis [1][2][3], environmental remediation [4], sorption processes [5], and microbial fuel cells [6]. The rationale for the synthesis of this octahedral molecular sieve (OMS) is based on its low cost, processability, stability, and excellent catalytic activity in different redox reactions [7].…”