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Manganese Oxides

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Syntheses of Nanostructures of Cobalt Hydrotalcite Like Compounds and Co3O4 via a
Microwave-Assisted Reflux Method

An efficient microwave-assisted reflux route has been used to synthesize layered nanostructures of cobalt hydrotalcite like compound spheres. Scanning electron microscopy images showed that these structures are self-assemblies of the nanosheets with uniform 5-nm thickness. The layered cobalt hydrotalcite like compound has high surface area and large pore volume, which are 176 m2/g and 1.13 m3/g, respectively. These cobalt hydrotalcite like compounds can transfer into the spinel structure of Co3O4, which keeps a uniform spherical shape. Surface area and pore volume of the Co3O4 are 61 m2/g and 0.5 cm3/g, respectively. Both materials show electrocatalytic activity and stability for the electrochemical reduction of oxygen. High surface area of these catalysts correlates with high rates of electrochemical reduction of oxygen. Therefore, these materials have potential as electrocatalytic materials or as noble metal catalyst supports. This efficient route is also used to synthesize other porous transition metal oxides such as manganese oxide, nickel oxide, and others.

[Journal of Physical Chemistry C (2008), 112(17), 6786-6793]

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Systematic Control of Particle Size in Rapid Open-Vessel Microwave Synthesis of K-OMS-2 Nanofibers

Multigram quantities of manganese oxide (K-OMS-2) nanomaterials in the size range of 4-20 nm and with very high surface areas up to 227 m2/g were produced rapidly via microwave-reflux route with use of mixed aqueous and nonaqueous solvents. The formation process, particle size, crystallite size, crystal structure, and properties of these nanomaterials were characterized by X-ray diffraction, scanning electron microscopy (SEM), transmission electron microscopy (TEM), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), infrared spectroscopy (FT-IR), Raman spectroscopy, and nitrogen adsorption-desorption. The OMS-2 nanofiber diameter was systematically controlled by varying the concentration or type of the cosolvent. Catalytic studies of these K-OMS-2 nanomaterials for oxidation of anisyl alcohol were performed. These nanomaterials show excellent catalytic activity when compared with conventionally prepared bulk OMS-2 catalysts.

[Journal of Physical Chemistry C (2008), 112(17), 6786-6793]

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Flowerlike a-Nickel Hydroxide with Enhanced Electrochemical Activity Synthesized by Microwave-Assisted Hydrothermal Method

Nickel hydroxide has received increased attention especially due to its electrochemical properties and potential applications in rechargeable Ni-base alkaline batteries, e.g., Ni/Cd, Ni/Zn, and Ni/MH. Ni(OH)2 has a hexagonal layered structure with two polymorphs, a- and ß-Ni(OH)2 . a-Ni(OH)2 shows superior electrochemical properties compared to those of the ß-form. Nanosized flowerlike a-nickel hydroxide materials with an interlayer spacing of 7.0 Å have been prepared by a microwave-assisted hydrothermal method. The experimental results from XRD and FT-IR showed that the Ni(OH)2 sample prepared by this method had the typical a-phase. FE-SEM images showed many uniform flowerlike architectures with diameters of 700 nm-1µm which consisted of the aggregated flakes. TEM results showed the flakes were built up from many nanocrystals with 2–3 nm diameters. TGA and TPD were employed to investigate thermal stability and gas evolution during the heating process. a-Nickel hydroxide was transferred to NiO with a cubic crystalline structure after being calcined at 450 °C; the NiO still kept the morphology of a-Ni(OH)2. Cyclic voltammetry was used to determine the electrochemical properties of the Ni(OH)2 electrode in 1 M KOH. a-Ni(OH)2 prepared by MW-HT had the best electrochemical activity for the electrochemical reduction of O2 compared with a-Ni(OH)2 synthesized by conventional HT methods and ß-Ni(OH)2. The effects of nickel sources and precipitators on the phase and morphology of the products were studied. Conventional hydrothermal methods were used to study the role of microwave irradiation. The possible growth mechanism is discussed here. The CV experiments showed that H2O2 can be reduced to OH- on the a-Ni(OH)2 electrode. The Levich equation was used to calculate the number of electrons transferred during the O2 redox reaction.

[Chemistry of Materials 2008, 20(1), 308-316]

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Preparation of Multicomponent Metal Oxides Using Nozzle Spray and Microwaves

Multicomponent metal oxide (MMO) crystallites are prepared by spraying a reactant solution into a receiving solution or into air under microwave radiation at atmospheric pressure. The injection of a nitric acid solution through an ultrasonic nozzle into a receiving solution of metal precursor and the use of microwave radiation are combined to form a novel preparation technique called the nozzle-spray/microwave (NMW) method. The inclusion of an additional step, the in situ mixing of precursor solutions prior to their injection through the ultrasonic nozzle spray, leads to another procedure called the in situ/nozzle-spray/microwave (INM) method. For comparison, MMO materials with the same metal constituents as those prepared by our novel techniques are prepared by conventional hydrothermal (CH) methods. Fresh materials prepared by NMW, INM, and CH methods were heat treated to study the effect of calcination. All materials were characterized before and after calcination using X-ray diffraction, scanning electron microscopy, Brunauer–Emmett–Teller surface area, and inductively coupled plasma elemental analysis. The NMW method produces particles with rodlike morphologies different from those obtained using CH methods. The INM method produces an amorphous material that crystallizes after calcination into small (ca. 200 nm) particles with interesting morphologies. Notably, calcination of materials prepared by both NMW and INM reduces the particle size and increases the surface area. The work presented in this paper paves the way to use NMW and INM to prepare MMOs with unique morphologies.

[Advanced Functional Materials, 2007, 17, 2572-2579]

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An initial study into the use of Microwave remediation of hexachlorobenzene treated soil using selected oxidants and coated graphite rods

This research focused on microwave energy combined with oxidants, potassium hydroxide and potassium persulfate, and, coated and uncoated, graphite rods to remove hexachlorobenzene (HCB) from treated soil samples.

[Journal of Soils and Sediments, 2007, 7(3), 147-152]

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Methods of manufacturing coated articles with uniform coating thickness

A method for coating articles includes contacting a substrate with a mixture comprising a coating composition and a carrier fluid effective to wet at least a portion of the substrate, and removing the carrier fluid by microwave heating for a time and at a temperature effective to produce a coating comprising the coating composition on at least a portion of substrate. The coated articles may be useful in a variety of applications including ion, molecule, and gas separation/filtration; ion-exchanging; semiconductors; catalysis; and as electrodes, among others.

[US PCT Int. Appl., 2007, WO 2007056156, 34pp.]

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Microwave-assisted desulfurization of NOx storage-reduction catalyst

The activity of NOx storage-reduction (NSR) catalysts is greatly reduced by sulfur poisoning, caused by the SO2 present in the exhaust stream. Desorption of sulfur species from poisoned NSR catalysts occurs at temperatures in excess of 600 °C using reducing atmospheres and conventional heating. In this work, microwave (MW) heating has been used to promote desulfurization of poisoned NSR catalysts. The experiments were carried out by heating the catalyst with MW radiation and using hydrogen as the reducing gas. Desorption of H2S at 200 °C was observed. Desorption at even lower temperatures (150 °C) was observed when water was introduced to the system. In the presence of water, sulfur species desorbed as both H2S and SO2. An overall reduction of sulfur species of about 60% was obtained. The use of MW heating proves to be an efficient way to achieve regeneration of poisoned NSR catalysts.

[Applied Catalysis, B: Environmental, 2007, 69(3-4), 235-239]

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Microwave frequency effects on synthesis of cryptomelane-type manganese oxide and catalytic activity of cryptomelane precursor

Cryptomelane-type manganese oxides (OMS-2) were synthesized in the presence of microwave heating at different microwave frequencies and also using variable-frequency heating. The materials were prepared using a two-step hydrothermal procedure. Catalytic activity of the materials was tested for the oxidation of 2-thiophenemethanol. OMS-2 prepared at a high-frequency limit of 5.5 GHz showed the highest conversion (50%) to the 2-thiophenecarboxaldehyde among all of the tested OMS-2 samples. The OMS-2 precursor showed remarkable conversion (89%) in the oxidation reaction. In addition, the OMS-2 materials and the precursor showed differences in oxygen evolution based on the thermal decomposition experiments, as well as differences in porosity.

[Journal of Catalysis, 2006, 239, 290–298]

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Effects of Microwave Processing on Chemical, Physical, and Catalytic Properties of Todorokite-Type Manganese Oxide

A microwave-assisted hydrothermal method of synthesis was used to prepare todorokite. Synthetic todorokite, OMS-1, is a multivalent manganese oxide with a 3×3 tunnel structure. OMS-1 was synthesized from its layered precursor much faster by the microwave method than by conventional heating (8 vs 48 h). In addition, the microwave-synthesized materials reveal some properties superior to those of the conventionally synthesized ones such as better stability, crystallinity, and catalytic activity in the production of phthalic anhydride. Microwave-prepared todorokite also shows novel cubic morphology, which was not found for conventionally made OMS-1 and is catalytically active in the oxidation of benzyl alcohol. Materials synthesized by microwave as well as conventional methods were characterized with XRD, TGA, SEM, TEM, TPD, and BET. Physical, chemical, and catalytic properties of todorokite prepared by the two different methods were compared based on the experimental results.

[Chemistry of Materials, 2004, 16, 4296-4303]

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Oligomerization of methane via microwave heating using Raney nickel catalyst

Raney nickel has been used for the catalytic activation of methane using microwave radiation as a heating source. The effects of irradiation time, power level, and pretreatment of the catalyst have been studied. Compared to previous studies, higher catalytic activity was observed for Raney nickel than for regular nickel powder. The maximum conversion obtained was 24% at 400 W and 10 min of irradiation time. For regular nickel powder that conversion can be achieved only after 700 W of power and more than 20 min of reaction. BET surface area, scanning electron microscopy, X-ray photoelectron spectroscopy, and temperature-programmed desorption and reduction analysis were performed to characterize the catalyst before and after reaction. Oligomers such as ethylene, benzene, and ethane have been prepared selectively under different conditions. Ethylene, with a selectivity of 70%, was the major product at 530 W and 5 min of irradiation time. Deactivation of Raney nickel by fouling and sintering was observed after 500 W and/or 15 min of reaction.

[Journal of Catalysis, 2003, 218, 201-208]

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Catalyst Nature and Frequency Effects on the Oligomerization of Methane via Microwave Heating

Microwave-induced oligomerization of methane was carried out at fixed frequencies of 2.4 and 4.6 GHz and variable frequencies of 2-7 GHz (with a sweep rate of 0.5 s) in the presence of Ni, Fe powder, and activated carbon catalysts. Changes in product distribution, because of changes in frequency, were observed, most probably because of the different transient heating patterns that occurred at different frequencies. Oligomers (C2-8) were obtained, in which C2 compounds were the most abundant. Benzene was produced, with a selectivity of 28% with activated carbon catalyst. Different transverse modes generate different transient heating patterns, consequently changing the dielectric properties of the catalyst (i.e., dielectric loss and dielectric constant).

[Journal of Physical Chemistry B, 2003, 107, 3663-3670]