Waveguide

Details Waveguide Waveguide

1999-04-13

2001-09-04

Font, Frank G. (Department: 2877)

Optical waveguides

Having particular optical characteristic modifying chemical...

C385S128000, C385S129000, C385S130000, C385S144000

Reexamination Certificate

active

06285816

ABSTRACT:

CROSS-REFERENCE TO RELATED APPLICATIONS
Not Applicable
BACKGROUND OF THE INVENTION
Optical waveguides are used in industry and science in a variety of processes and purposes and have been proposed for use in supporting and illuminating photocatalysts. Mariangeli, R. E. and D. F. Ollis, AIChE J. 23 (4): 1000 (1977). In this configuration they could be used in remediating effluent waste streams (e.g., water or air) by photocatalytic oxidation or photooxidation. Titanium dioxide (TiO
2
) and other transition metal oxides are well-known as effective photocatalysts that can oxidize organic compounds to CO
2
and H
2
O in the presence of UV light (at wavelengths of about 380 nm or less for TiO
2
) and a suitable electron acceptor, such as O
2
. Metal oxide-mediated photocatalytic processes can occur at ambient temperatures. The coated waveguides can also remove inorganic ions from solution and can find utility in processes for converting ionic species into neutral species, such as metals.
Several shortcomings in metal oxide-based photocatalytic processes have been identified. For instance, (a) the ratio of illuminated catalyst surface area to reactor volume is often low, (b) the photocatalyst must be fixed in the reactor to separate from the reactant, and (c) the photocatalyst uses the activating ultraviolet radiation inefficiently. For example, UV light distribution throughout a typical TiO
2
packed-bed reactor design is hindered by the high UV absorptivity of TiO
2
and by losses due to reflection and scattering. Attempts to overcome these limitations have generally not succeeded.
For example, TiO
2
-coated optical fibers have been used for photocatalytic oxidation of organic compounds in water. In such systems, UV light is propagated through an optical fiber substrate to photoactivate the TiO
2
coating. The UV light is not completely absorbed in a single coated region of the fiber. TiO
2
-coated optical fibers as developed thus far are not an adequate solution to the identified shortcomings in that it has only been possible to propagate UV light for about 10-15 cm. Peill, N. J., and Hoffmann, M. R., “Mathematical Model of a Photocatalytic Fiber-Optic Cable Reactor for Heterogeneous Photocatalysis,”
Environ. Sci. Technol.,
32:398-404 (1998); Peill, N. J., and Hoffmann, M. R., “Development and Optimization of a TiO
2
-Coated Fiber Optic Cable Reactor: Photocatalytic Degradation of 4-Chlorophenol,”
Environ. Sci. Technol.,
29:2974-81 (1995).
Peill and Hoffmann determined that at each reflection at the fiber/TiO
2
interface a portion of the UV light was refracted out of the fiber and absorbed by the TiO
2
coating. Successive reflections quickly diminished the UV light intensity in the fiber. According to Peill and Hoffmann, the UV light propagated through the optical fibers in a frustrated total reflection (FTR) mode which is expected when light is incident from an optically rarer medium (i.e., silica) to an optically denser medium (i.e., TiO
2
). Peill and Hoffmann point out that “[t]he refractive index of TiO
2
is higher than that of fused-silica glass . . . . For this reason, it is impossible that total refection occurs at the interface . . . the light flux is divided: one part of it is reflected and the other part leaves the fiber.” Peill and Hoffmann, (1998), supra.
In another approach, U.S. Pat. Nos. 5,194,161 and 4,997,576 disclose processes for oxidizing organic compounds in an oil film floating on water. These patents describe coating photocatalytic metal dioxides, including TiO
2
, onto water-floatable waveguiding materials such as silica beads. UV light trapped in the coated bead is scattered onto the photocatalytic material where it is completely absorbed so as to create a photon flux for photocatalytic oxidation of the oil by oxygen. Although the patents envision using the coated materials to oxidize organic compounds, the patents describe preparing the coated materials so as to rapidly absorb as much trapped UV light as possible from the substrate. This approach is contrary to the articulated desire to improve the efficiency with which UV light is used in photocatalytic systems.
The art is still in need of a photocatalytic system that efficiently propagates UV light in a coated photocatalytic waveguide. Such a waveguide would allow for the controlled interaction of light energy with a large photocatalyst surface area, thus, enhancing the efficiency of heterogeneous photocatalytic processes, including but not limited to photooxidation or solar energy conversion. Such waveguides could also be the basis for novel optical chemical and biochemical sensors.
BRIEF SUMMARY OF THE INVENTION
In one aspect, the present invention is summarized in that a waveguide that propagates light in an attenuated total reflection (ATR) mode comprises a transparent internal reflection element (IRE) and a porous, particulate transition metal oxide coating on one or more surfaces of the IRE, the coating being provided so that its boundaries are parallel to the IRE surface or surfaces. The particles and pores of the coating are small relative to the wavelength of the propagating light (i.e., at most less than {fraction (1/10)} size of the wavelength of the light). The pores can be mesoporous, microporous or nanoporous, depending upon the wavelength of the light to be propagated. A constant distance is maintained between the IRE/film interface and the film/external medium interface.
In another aspect, the invention is summarized in that a method for forming the waveguide of the invention includes the steps of applying a particulate metal oxide coating to a transparent IRE as described herein.
It is an object of the invention to provide an apparatus comprising a waveguide or a plurality of waveguides that propagates light in an attenuated total reflection mode and distributes the light to a photocatalytic coating on a surface of the waveguide.
Another object of the invention is to provide a photocatalyst for efficient photocatalysis.
It is a feature of the invention that a waveguide according to the invention includes a particulate coating on an IRE, where the coating has a refractive index higher than that of the transparent substrate on which the coating is coated.
It is an advantage of the invention that the waveguide propagates light in an ATR mode.
Another advantage of the metal oxide-coated waveguide of the invention is its ability to efficiently capture diffuse light (such as sunlight or light from a fluorescent lamp) for photocatalysis, for biological or chemical sensing, or for increasing the efficiency of a photovoltaic cell. Diffuse light sources are typically disfavored photoilluminators, since the intensity of light from a diffuse light source diminishes with distance from the source. Using the waveguides of the invention, especially in an apparatus comprising a plurality of waveguides, the light intensity gradient is less severe than in conventional photoreactors that are activated by direct illumination.
Yet another advantage of the present invention is that the waveguides yield the benefit of an even distribution of activated catalyst throughout the reactor volume. Furthermore, the activated photocatalyst in a waveguide reactor is evenly distributed throughout a comparatively larger reactor volume. These attributes could facilitate the economical scale-up of photocatalytic reactors to the dimensions required for commercial remediation applications. Photocatalytic reactors that incorporate the waveguides of the invention reduce the cost of UV light generation because fewer light sources are required. This particularly important for materials such as TiO
2
that require UV light for activation.
Other objects, advantages, and features of the present invention will become apparent after examination of the specification and claims.


REFERENCES:
patent: 3801352 (1974-04-01), Furuuchi et al.
patent: 4597913 (1986-07-01), Kimoto et al.
patent: 4604248 (1986-08-01), Dehm
patent: 4987158 (1991-01-01), Eckberg
patent: 4997576 (1991-03-01), Heller et al.
patent: 5028568 (1991

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