Heterogeneous photocatalysis is the general term that describes the technical approach. Mills, A., Le Hunte, S., “An Overview of Semiconductor Photocatalysis,” J. PhotoChem. & PhotoBio. A: Chemistry 108 (1997) 1-35; and Hoffman, M. R., Martin, S. T., Choi, W., Bahnemann, D. W., “Environmental Applications of Semiconductor Photocatalysis,” Chem Rev 1995, 95, 69-96. Of particular importance is the formation of OH., the hydroxyl radical. The hydroxyl radical is an extremely potent oxidizing agent (redox potential of +2.8 eV vs. SHE (Standard Hydrogen Electrode)) that is capable of oxidizing almost all organic compounds. By comparison, the redox potentials for the more conventional oxidants chlorine and ozone are +1.36 and +2.07 eV, respectively. Hydroxyl radicals also kill and breakdown microorganisms and endotoxins.
Contaminants in fluid streams, such as organic compounds, nitrogen and sulfur oxides, acid gasses, dissolved inorganic solids, and microorganisms are converted by the oxidizing and reducing potential of the activated semiconductor. The conversion may take the form of, but is not limited to, the oxidation of organic compounds, degradation of microorganisms, or reduction of dissolved ionic species. The products of the conversion are ideally less harmful or more easily removed from the fluid stream than the parent compounds.
Semiconductor photocatalysts that have been demonstrated for the destruction of organic contaminants in fluid media include but are not limited to: TiO2, ZrO2, ZnO, CaTiO3, SnO2, MoO3, Fe2O3, and WO3. TiO2 is the most widely investigated because it is chemically stable, has a suitable bandgap structure for UV/Visible photoactivation, and is relatively inexpensive.
TiO2 exists as three principal crystalline forms: rutile, brookite, and anatase. The rutile form of TiO2 is widely used as a pigment and can be found in almost anything white, e.g., paint, paper, textiles, inks, plastics and cosmetics. Anatase, the low temperature form, is the most photoactive form. One method for making a brookite-containing TiO2 photocatalyst is described in U.S. Pat. No. 6,337,301, the disclosure of which is incorporated by reference in its entirety.
The inclusion of co-catalysts (e.g., platinum, palladium, silver and/or their oxides and sulfides) with titanium dioxide can increase the photocatalytic activity. A variety of methods improve the quantum efficiency of TiO2 by adding various metals to increase the minority carrier diffusion length, Augustynski, J.; Hinden, J. Stalder, C.; J. Electrochem. Soc. 1977, 124, 1063, or achieve efficient charge separation to increase carrier lifetimes. Vogel, R., Hoyer, P., Weller, H., “Quantum-Sized PbS, CdS, Ag2S, Sb2S3 and Bi2S3 Particles as Sensitizers for Various Nanoporous Wide-Bandgap Semiconductors,” J. Phys. Chem. 1994, 98, 3181-3188.
The inventors have found that the performance of a photocatalytic fluid purification device is primarily a function of the activity of the semiconductor photocatalyst, the specific surface area of the irradiated semiconductor surface, the mass transfer characteristics of the device, the pressure drop across the device, and light distribution and absolute light intensity at the semiconductor surfaces. All of these factors should be considered concurrently in the design of the photoreactor. To take advantage of the activity of the semiconductor surface, conditions are preferably controlled such that the rate of transport of contaminants from the fluid bulk to the semiconductor surface is not a limiting factor. Because the rate of contaminant conversion increases with increased surface concentration of the contaminant, the concentration gradient between the fluid bulk and the fluid at the semiconductor surface should be preferably minimized. The mass transfer characteristics of the device increase with the velocity of the fluid through the device and with closer proximity of the bulk fluid to the semiconductor surfaces. Preferred mass transfer characteristics are achieved by altering pore diameter, packing diameter, or otherwise physically ensuring small characteristic lengths. Forcing the fluid to flow in a tortuous path past the semiconductor surfaces also inherently improves mass transfer.
According to the invention, the absolute conversion rate is a function of the surface concentration of the organic compound, the light intensity, and the irradiated surface area. The inventors believe that the reaction order in light intensity is <1, and that it is half order based on theoretical consideration of the accepted reaction mechanism and observed experimentally. Gerischer, H., Electrochimica Acta 38, 9 (1993) and Turchi, C. S., Ollis, D. F., Journal of Catalysis 119, 483 (1989). Based on the inventors' understanding that the reaction order for light intensity is less than unity, it is desirable to spread the light from a given source over as much surface area as possible. Therefore, according to the principles of the invention, an efficient device is one that provides intimate contact between the fluid and the semiconductor surfaces while spreading activating light over as large a surface area as is practical. Note that if the reaction order were, in fact, unity in light intensity, increasing the specific irradiated surface area, for a given light source, would not improve performance.
Since until recently known semiconductor surfaces have been opaque to UV light; the spreading of light has typically been done by using geometry or waveguides. U.S. Pat. No. 5,516,492, to Dong, uses a plurality of curved plates while U.S. Pat. No. 6,063,343, to Say, uses a series of close packed plates orientated normal to the lamp axis to provide high specific irradiated surface area. U.S. Pat. No. 6,285,816, to Anderson, uses a waveguide to distribute the light. The previously mentioned methods rely on geometry or optics to distribute light to a semiconductor film and do not rely on a semitransparent substrate/semiconductor film pairing for radiation distribution.
Much of the early research on semiconductor photocatalysis concerned methods using opaque titanium dioxide (TiO2) slurries or TiO2 wash coatings onto or inside a glass tube and the photodegradation of organic compounds and their intermediates in water. These methods of using TiO2 have limitations for commercial applications. For example, TiO2 slurry has the serious limitation of the removal of the TiO2 particles from the purified water. While wash coating TiO2 onto glass avoids the removal limitations of the slurry approach, it has its own problems in that insufficient surface area is presented for effective destruction of organics within a reasonable time period. Additionally, the wash coat is not firmly attached to the glass resulting in a loss of TiO2 to the water stream and a concomitant reduction in photocatalytic activity.
Kraeutler and Bard made a photocatalytic reactor with a water slurry of suspended TiO2 powder, in the anatase crystalline form, and studied the decomposition of saturated carboxylic acid. J. ACS 100 (1978) 5985-5992. Other researchers used UV-illuminated slurries of TiO2 to study the photocatalyzed degradation of organic pollutants in water.
Mathews created a thin film reactor by wash coating TiO2, (Degussa P25™), particles to the inside of a 7 millimeter long borosilicate glass tube wound into a 65-turn spiral. He monitored the destruction of salicylic acid, phenol, 2-chlorophenol, 4-chlorophenol, benzoic acid, 2-naphthol, naphthalene, and florescin in water. J. Physical Chemistry 91 (1987) 3328-3333. U.S. Pat. No. 5,766,455, to Berman, wash coated an opaque coating onto a glass fiber mesh. Neither of these devices uses a transparent substrate/semiconductor pairing.
U.S. Pat. No. 4,892,712, to Robertson et al., discloses the attachment by the sol-gel process of anatase TiO2 to a fiberglass mesh substrate. This mesh was wrapped around a light source contained within a quartz glass cylinder and emitting UV radiation in a wavelength range of 340 to 350 nanometers (nm). Unlike the present invention, Robertson's mesh is not rigid, lacks permanent bonding of the semiconductor to the mesh, and does not specify the transparency of the substrate semiconductor pairing.
Professor I. R. Bellobono prepared photocatalytic membranes immobilizing 23% of Titanium Dioxide (Degussa P-25). Controlled amounts of appropriate monomers and polymers, containing the semiconductor to be immobilized and photoinitiated by a proprietary photocatalytic system was photografted onto a non-woven polyester tissue. The final porosity of the photosynthesized membrane was regulated at 2.5-4.0 microns. He trade named this membrane “Photoperm”™. “Effective Membrane Processes. New Perspectives,” R. Paterson, ed., BHR, Mech. Eng. Publ., London (1993), 257-274. The process was patented in Italy in 1995, Italian Pat. No. IT1252586. Unlike the present invention, Bellobono's apparatus is not inert, not durable, and would display excess pressure drop due to low fluid permeability.
Cittenden, et al. discloses a method and apparatus for destroying organic compounds in fluids. See The 1995 American Society of Mechanical Engineers (ASME) International Solar Energy Conference, Maui, Hi., USA. An opaque coating of TiO2 was attached by wash coating to a 35×60-mesh silica gel substrate. The substrate was placed within a plastic tube that allowed the penetration of UV light. Organic pollutants in a water stream passed axially through the tube. Unlike the present invention with a semitransparent semiconductor coating, Cittenden's invention is not durable, the photocatalytic coating is not semitransparent, and has very limited fluid permeability.
Another method to make ceramic titanium membranes uses a refined sol-gel process. J. Membrane Science 30 (1988) 243-258, and U.S. Pat. No. 5,006,248, to Anderson. These membranes are porous and transparent to UV illumination. They are made from a titanium alkoxide and then fired to form the anatase crystalline structure. Unlike the present invention, Anderson's invention has very limited fluid permeability.
Thus, while attempts have been made in the prior art to enhance quantum yields by increasing semiconductor specific irradiated surface area and improving UV light penetration, serious limitations remain to the commercial development of an efficient, durable, photocatalytic purification apparatus with acceptable pressure drop characteristics and methods for its use.