Aimee Morey

Green decomposition of organic dyes using octahedral molecular sieve manganese oxide catalysts.

The catalytic degradation of organic dye (methylene blue, MB) has been studied using green oxidation methods (tertiary-butyl hydrogen peroxide, TBHP, as the oxidant with several doped mixed-valent and regular manganese oxide catalysts in water) at room and higher temperatures. These catalysts belong to a class of porous manganese oxides known as octahedral molecular sieves (OMS). The most active catalysts were those of Mo(6+)- and V(5+)-doped OMS. Rates of reaction were found to be first-order with respect to the dye. TBHP has been found to enhance the MB decomposition, whereas H(2)O(2) does not. Reactions were studied at pH 3-11. The optimum pH for these reactions was pH 3. Dye-decomposing activity was proportional to the amount of catalyst used, and a significant increase in catalytic activity was observed with increasing temperature. X-ray diffraction (XRD), energy dispersive spectroscopy (EDX), and thermogravimetric analysis (TGA) studies showed that no changes in the catalyst structure occurred after the dye-degradation reaction. The products as analyzed by electrospray ionization mass spectrometry (ESI-MS) showed that MB was successively decomposed through different intermediate species.

Synthesis and characterization of iron-substituted hydroxyapatite via a simple ion-exchange procedure

Hydroxyapatite (HA), the main inorganic component of natural bones, is widely studied as a biomaterial due to its excellent biocompatibility and osteoinductivity. The crystal structure of HA lends itself to a wide variety of substitutions and ion doping, which allows for tailoring of material properties. In this study, iron-doped HA was synthesized via a simple ion-exchange procedure and characterized thoroughly for crystal structure and phase purity using X-ray diffraction, energy-dispersive X-ray spectroscopy, inductively coupled plasma atomic emission spectroscopy, and Fourier transform infrared spectroscopy. Magnetic properties were studied using vibrating sample magnetometer and superconducting quantum interference device analysis. Ion-exchange was attempted using both ferric and ferrous chloride iron solutions, but a substitution was only achieved using ferric chloride solution. The results showed that after iron substitution the powder retained characteristic apatite crystal structure and functional groups, but the iron-doped samples displayed paramagnetic properties, as opposed to the diamagnetism of pure HA. The effect of soaking time on iron content was also examined, and collectively X-ray diffraction and inductively coupled plasma atomic emission spectroscopy results suggested that an increase in soaking time led to an increase in iron content in the sample powder. Iron-substituted HA nanoparticles, a biomaterial with magnetic properties, could be a promising biomaterial to be used in a variety of biomedical fields, including magnetic imaging, drug delivery, or hyperthermia-based cancer treatments.

Enhancement of Catalytic Activities of Octahedral Molecular Sieve Manganese Oxide for Total and Preferential CO Oxidation through Vanadium Ion Framework Substitution

High‐valent vanadium ions were substituted into the synthetic cryptomelane manganese oxide (K‐OMS‐2) framework through a simple and low‐cost reflux method and investigated for total and preferential catalytic oxidation of carbon monoxide. Substitutional doping of V5+ resulted in materials with modified composition, morphology, thermal stability; and textural, redox, and catalytic properties. The catalytic activity increased with V concentration until an optimum amount (≈10 % V incorporated) was reached, beyond that a structural “crash point” was observed, resulting in a material with low crystallinity, nanosphere morphology, and reduced catalytic activity. An increase in O2 concentration in the feed gas resulted in an increase in conversion over 10% V K‐OMS‐2. This most active catalyst was deactivated by moisture only at low temperatures and showed better tolerance than undoped K‐OMS‐2. This catalyst also preferentially oxidized CO to CO2 from 25 °C to 120 °C in large amounts of H2 under dry conditions, without significantly affecting CO conversion. The doped catalyst also showed stable activity and selectivity in long‐run experiments. The mobility and lability of surface oxygen, formation of hydroxyl groups, and enhanced surface redox properties promoted by V doping were strongly correlated with the enhancement of catalytic activities of K‐OMS‐2 nanomaterials.

Nanostructured arrays of semiconducting octahedral molecular sieves by pulsed-laser deposition

Cryptomelane-type manganese oxide (OMS-2) has been widely used to explore the semiconducting and catalytic properties of molecular sieves with mixed-valent frameworks. Selective synthesis of patterned thin films of OMS-2 with hierarchical nanostructures and oriented crystals is challenging owing to difficulties in preserving the mixed valence, porosity and crystalline phase. Here, we report that pulsed-laser ablation of OMS-2 in an oxygen-rich medium produces a three-dimensional nanostructured array of parallel and inclined OMS-2 fibres on bare substrates of (001) single-crystal strontium titanate. Both parallel and inclined OMS-2 fibres elongate along the [001](OMS-2) direction. The parallel fibres interact strongly with the substrate and grow epitaxially along <110>(STO) with lattice misfits of less than 4%, whereas the inclined fibres are oriented with (301) parallel to the substrate surface. The spontaneous orientation of the crystalline OMS-2 domains over the STO surface opens up a new avenue in lattice-engineered synthesis of multilayer materials.

Structural Distortion of Molybdenum-Doped Manganese Oxide Octahedral Molecular Sieves for Enhanced Catalytic Performance.

Due to the excellent catalytic performance of manganese oxide (K-OMS-2) in a wide range of applications, incorporation of various dopants has been commonly applied for K-OMS-2 to acquire additional functionality or activities. However, the understanding of its substitution mechanism with respect to the catalytic performance of doped K-OMS-2 materials remains unclear. Here we present the structural distortion (from tetragonal to monoclinic cell) and morphological evolution in K-OMS-2 materials by doping hexavalent molybdenum. With a Mo-to-Mn ratio of 1:20 (R-1:20) in the preparation, the resultant monoclinic K-OMS-2 shows a small equidimensional particle size (∼15 nm), a high surface area of 213 m(2) g(-1), and greatly improved catalytic activity toward CO oxidation with lower onset temperatures (40 °C) than that of pristine K-OMS-2 (above 130 °C). HR-TEM analyses reveal direct evidence of structural distortion on the cross-section of 2 × 2 tunnels with the absence of 4-fold rotation symmetry expected for a tetragonal cell, which are indexed using a monoclinic cell. Our results suggest that substitution of Mo(6+) for Mn(3+) (rather than Mn(4+)) coupled with the vacancy generation results in a distorted structure and unique morphology. The weakened Mn-O bonds and Mn vacancies associated with the structural distortion may be mainly responsible for the enhanced catalytic activity of monoclinic K-OMS-2 instead of dopant species.

Magnetic study of the Co-MCM-41 catalyst: Before and after reaction

A combined magnetization (both dc and ac techniques), NMR, and EPR study of the magnetic properties has been carried out on a 3 wt. % Co-loaded Co-MCM-41 catalyst, before and after reaction. Before reaction, the Co magnetization can be described by the Curie-Weiss law (50 K ≤ T ≤ 350 K) consistent with the vast majority of the Co existing in the MCM-41 structure as weakly-interacting Co2+ paramagnetic moments with an effective value μp = 4.7(2) μB. In addition, there appears to be a contribution from a small number of Co “clusters” exhibiting a magnetic transition at ≈15 K. After reaction, the Co magnetization is dominated by a magnetic contribution which saturates with a value 58% that for bulk Co, and is attributed to Co metal nanoparticles. In addition, the Co nanoparticles are characterized by a superparamagnetic blocking temperature at 10.5 K. Before reaction, Co-MCM-41 showed two Co EPR signals: (1) a strong signal at g = 1.93 and (2) a weak signal at g = 1.99. After reaction, Co59 zero-field spin-e...

Substrate Control of Anisotropic Resistivity in Heteroepitaxial Nanostructured Arrays of Cryptomelane Manganese Oxide on Strontium Titanate

Resistivity and resistance measurements have been carried out for thin films of cryptomelane-type manganese oxide (OMS-2) grown onto (001), (110), and (111)STO single crystals substrates via pulsed laser deposition. While the symmetries of the (001) and (111)STO substrate surfaces give deposits consisting of multiple nanofiber arrays with isotropic in-plane resistivities, only a single nanofiber array is formed on (110)STO giving highly anisotropic electrical properties with very low resistivity values measured parallel to the fibers and similar to the lowest value ever reported.