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| Nanofibrous MnO2 Bird's Nest Superstructure Catalyst |
Inframat Corporation (“IMC”) has developed an unique class of nanofibrous materials that have a “bird’s-nest” superstructure morphology consisting of inter-woven nanofibers using a low cost wet chemical synthesis technique. The advantages of this unique class of nanofibrous materials are explained below.
Morphology: Each “bird’s-nest” is about 10 mm in diameter, consisting of an complex assembly of many individual nanostructured fibers. Combined transmission electron microscopy ("TEM"), x-ray diffraction ("XRD") and porosity studies indicate that this nanofibrous MnO2 has an average fiber diameter of ~ 15 nm, with many different types of molecular and nano-sized pores (see Fig. 1a and b). |
Fig. 1. (a) scanning electron microscopy of nanofibrous MnO2 material showing a bird’s-nest superstructure and (b) high resolution transmission electron microscopy showing transverse view of a nanofiber showing the 402-type lattice fringe |
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| Microstructure: Unique features of the nanofibers are: 4.6Å x 4.6 Å square “lattice tunnels” for diffusion of mobile species (see Fig. 2a and b); large lattice defects formed during the growth of the nanofibers, which we term “nano-tunnels;” and edge dislocation dipoles seen when the fiber is viewed perpendicular to its cylindrical surface, which will create rows of missing or insertion atoms symmetrically arranged in the nanofibers. These features are important for catalytic applications, where such lattice defects perform as active sites for reactions and provide paths for diffusion of mobile species. |
Fig. 2. (a) High resolution transmission electron microscopy showing fiber cross-section view showing the tunnel structures including lattice tunnels and nanotunnels where catalytic reactions can take place, and (b) schematic representation of the lattice view of the fiber cross-section |
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| Control of morphology: The initially synthesized MnO2 suspensions are amorphous nanoparticle agglomerates (Fig. 3a.) Under controlled aging conditions, (time and temperature), these nanoparticle agglomerates in solution gradually transform into nanofibrous entities (Fig. 3b, c, and d). Complete transformation yields an open-weave structure (Fig 3e), with a volume density typically ~20%. A mixed-morphology structure can be developed, where each nanoparticle agglomerate is with covered with nanofibrous material (Fig.3c). |
Fig. 3. Transformation: (a) nanoparticle agglomerates, (b) nucleation embryonic fibers, (c) tranformed fibers, (d) fiber bundles, and (e) fully developed bird’s-nest |
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Porosities: Nanofibrous MnO2 is highly porous. The micro-porosities are about 0.323 cm3/g consisting of two major types, (i) 85% populated at 4.6Å, which is the structure of an octahedral hollandite lattice, (ii) 15% populated at ~10Å. With different metal dopants, the volume fraction and population of these two porosities can be altered. The meso-porosities and macro-porosities are related to the way nanofibers are stacked and randomly weaved into a bird’s-nest superstructure, with pore sizes ranging from a few nm to a few hundred nanometer.
VOCs removal in air: Under a recent EPA project, IMC constructed a prototype VOC destruction device, where the MnO2 bird’s-nest superstructures were assembled into a cartridge form, and installed in a heated tube with air flow control. Performance evaluation using this prototype device demonstrated that 50 of 55 of EPA’s critical VOCs were completely eliminated simultaneously at temperatures range from 200oC to 350oC, at VOC inlet concentrations ranging from 50 ppb levels to 1,000 ppb. Fig. 4. shows a photograph and schematics of the lab-prototypeair filtered constructed for the conversion of toxic VOCs. |
Fig. 4. Photograph (top) shows a lab-prototype VOC fitration unit, schematic (bottom) illustrate filter cartridge constructed using nanofibrous MnO2 bird's-nest superstructure materials. |
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Arsenic removal
in drinking water: IMC is currently working with the US Airforce to
further develop this technology as a means to remove toxic arsenic from drinking
water by combining a nanofibrous MnO2 oxidative process with a granular
ferric hydroxide (“GFH”) adsorptive process.
Taiwan Patent No. 139,071 (Dec 17, 2001)
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