The subject matter described herein is generally concerned with air purification technology that employs new and improved photocatalyst materials especially efficient in decomposing volatile organic compounds that may be present in an air stream, with methods for manufacturing such photocatalyst materials, and with methods and apparatus for using such materials to clean and purify air streams. A specific application for the photocatalyst materials of this invention is to treat an air stream drawn from a “clean room” environment to remove and/or decompose organic contaminants before recycling the treated air stream back to the “clean room.”
It is known in the art of air purification to prepare photocatalytic materials based on metals or metal oxides having semiconductor-like characteristics for removing contaminants from air. For example, the metals titanium (Ti), zirconium (Zr), tin (Sn), zinc (Zn), and similar metals, and the oxides of these metals, e.g., TiO2, ZrO2, SnO2, and ZnO2, are known to demonstrate the semiconductor characteristic of exhibiting at least two possible energy levels which may be referred to as a valence band state and a conduction band state. Thus, when excited or activated by light energy of a suitable wavelength, these materials respond by exciting electrons into the conduction band and leaving electron “holes” in the valence band. Depending on the photocatalytic material, solar light, fluorescent light, ultraviolet light, or other forms of light irradiation may be used effectively to activate the photocatalytic material.
The field known as heterogeneous photocatalysis has attracted considerable attention in recent years. A comprehensive overview of heterogeneous photocatalysis using titanium dioxide (TiO2) appears in a 1995 Chemical Reviews article by A. L. Linsebigler et al. entitled “Photocatalysis on TiO2 Surfaces: Principles, Mechanisms, and Selected Results,” which article is incorporated herein by reference. This article discusses the potential application of TiO2-based photocatalysts to destroying organic compounds found in polluted air and wastewaters.
The above-cited article provides the following useful summary of how photocatalysis operates to decompose a contaminant such as an organic compound: “In a heterogeneous photocatalysis system, photo-induced molecular transformations or reactions take place at the surface of a catalyst . . . . The initial excitation of the system is followed by subsequent electron transfer and/or energy transfer. It is the subsequent deexcitation processes (via electron transfer or energy transfer) that leads to chemical reactions in the heterogeneous photocatalysis process . . . . Photocatalysis processes involve the initial absorption of photons by a molecule or the substrate to produce highly reactive electronically excited states. The efficiency of the photoinduced chemistry is controlled by the system's light absorption characteristics.”
When the photocatalytic material is activated by light energy, the electrons generated tend to form “super oxide” anions (typically represented as O2−) and the holes form hydroxide (OH−) radicals, with the result that the activated photocatalyst has an unusually strong oxidation capability. Specifically, the O2− and OH− radicals are capable of rapidly and effectively oxidizing an organic contaminant contacted at the surface of the photocatalyst thereby converting the contaminant into water (H2O) and carbon dioxide (CO2) or, sometimes, to small amounts of relatively harmless mineral acids, for example HCl. Thus, volatile organic contaminants in an air stream can be effectively decomposed and removed from the environment by contacting the air stream with the light-activated photocatalyst.
The phenomenon known as “band-gap photoexcitation” is described in more technical terms in the previously cited Chemical Reviews article as follows:
“Unlike metals which have a continuum of electronic states, semiconductors possess a void energy region where no energy levels are available to promote recombination of an electron and hole produced by photoactivation in the solid. The void region which extends from the top of the filled valence band to the bottom of the vacant conduction band is called the band gap. Once excitation occurs across the band gap there is a sufficient lifetime, in the nanosecond regime, for the created electron-hole pair to undergo charge transfer to adsorbed species on the semiconductor surface from solution or gas phase contact. If the semiconductor remains intact and the charge transfer to the adsorbed species is continuous and exothermic the process is termed heterogeneous photocatalysis.”
The Chemical Reviews article then further explains how one of the deexcitation pathways for the electrons and holes is a photoinduced chemical reaction at the semiconductor surface:
“The photoinduced electron transfer to adsorbed organic or inorganic species or to the solvent results from migration of electrons and holes to the semiconductor surface. The electron transfer process is more efficient if the species are preadsorbed on the surface. While at the surface the semiconductor can donate electrons to reduce an electron acceptor (usually oxygen in an aerated solution) (pathway C); in turn, a hole can migrate to the surface where an electron from a donor species can combine with the surface hole oxidizing the donor species (pathway D).”
As also taught in the art, however, the electrons and holes generated by exciting a photocatalyst with light energy have a tendency to quickly recombine thereby neutralizing the powerful oxidation capacity of the activated photocatalyst. Thus, the Chemical Reviews article further teaches that: “In competition with charge transfer to adsorbed species is electron and hole recombination.” Photocatalytic efficiency is therefore generally improved if the photocatalyst can be prepared so as to remain in an excited state (with “free” electrons and holes) for a longer period of time to preserve the high oxidation capability of the material.
An increase in the oxidation capability of a photocatalyst can therefore be achieved by decreasing the rate at which recombination of electrons and holes occurs. This effect can sometimes be achieved by doping a photocatalyst with a relatively small amount of a suitable dopant or dopants, for example with a noble metal. For example, a suitable dopant molecule in a photocatalyst can act as an electron trap side to, at least temporarily, bind a free electron and thereby slow its tendency to recombine with a hole. Similarly, a suitable dopant molecule in a photocatalyst can act as a hole trap site to, at least temporarily, block a hole and thereby slow its tendency to recombine with an electron.
Another means of improving the photocatalytic efficiency of a particular photocatalyst is by increasing the band-gap energy of the photocatalyst. This effect can also sometimes be achieved by doping a photocatalyst with a relatively small amount of a suitable dopant or dopants.
Thus, it is known in the art to improve the photocatalytic efficiency of a metal oxide photocatalyst by doping it with a noble metal. For example, a metal oxide photocatalyst based on Ti, Zr, Sn, Zn, or a similar metal, typically made from a metal alkoxide precursor, can be advantageously doped with a noble metal such as platinum (Pt), gold (Au) or silver (Ag) to provide trap sites in the photocatalyst. Metal alkoxides suitable as precursors in forming these photocatalysts typically have the general chemical formula [M-O-alkyl] in which M is a suitable metal, O is oxygen, and the alkyl group is selected from methyl, ethyl, and similar alkyl groups typically having 6 or fewer carbon atoms.
In a typical application of this preparation process, a noble metal precursor, such as a solution or dispersion of a noble metal in a suitable solvent or dispersant, is prepared. The metal oxide photocatalyst is then doped with the noble metal by any of several familiar methods such as a dipping method, a deposition method, a co-precipitation method, an impregnation method, or a Sol-gel method. In the typical dipping method, the metal oxide photocatalyst is briefly immersed in a bath of the noble metal precursor and then withdrawn. Particles of the noble metal become deposited on and/or within the photocatalyst as the solvent or dispersant evaporates and/or drips off the photocatalyst. The dipping procedure may have to be repeated multiple times to achieve effective doping of the photocatalyst. By comparison with the dipping method, however, the Sol-gel doping method tends to be a very complicated and lengthy (slow) process.
This prior art technique, however, suffers from numerous limitations, drawbacks and disadvantages. One important drawback of the prior art approach is the high costs of the materials. Nobel metals such as Pt, Au, and Ag are also precious metals, which are very expensive. Even the metal alkoxide precursors are relatively expensive industrial commodities. A second disadvantage of the prior art technique is that it is relatively complicated—involving preparation of two precursors—and relatively lengthy.
Another important limitation of the above-described conventional method, however, is that it is very difficult, if not impossible, to reliably prepare nano-sized metal oxide particles for use as the photocatalyst using the metal alkoxide precursor technique. It has been found that the particle size of the photocatalyst is highly inversely correlated with photocatalytic efficiency. Specifically, it has been found that smaller photocatalytic particle size is associated with greater efficiency in decomposing contaminants, presumably because smaller particle size correlates with greater surface area and therefore with a larger active contact area between the photocatalyst and an air stream being treated.
More recent developments in photocatalyst technology have used TiO2 doped with various dopants to prepare useful photocatalyst materials. U.S. Pat. No. 6,627,173, which is incorporated herein by reference, for example, teaches a process for preparing doped, pyrogenically prepared titanium dioxide, doped with zinc oxide, platinum oxide, magnesium oxide or aluminum oxide for use as a photocatalyst or UV absorber. In this patent, the titanium dioxide is doped by injecting an aerosol of the oxide into the production stream. This process, however, requires relatively complex production equipment, does not insure a thorough, homogeneous doping of the TiO2, and does not produce optimally-sized photocatalyst particles.
U.S. Pat. No. 6,365,007, which is also incorporated herein by reference, teaches preparation of a photocatalyst consisting of TiO2 doped with at least one lanthanide metal oxide. The photocatalyst is prepared by forming a titanium-containing gel followed by the steps of drying the gel and subjecting it to calcinations. This process, however, employs relatively uncommon and expensive “inner-transition” lanthanide series elements and relatively high processing temperatures in both the drying and calcination steps.
These and other problems with and limitations of the prior art approaches in this field are addressed in whole or at least in part by the products, methods and apparatus of this invention.