This invention relates to photocatalysis and, more particularly, to the control of photoreactors that catalyze the oxidation of organic compounds.
Photocatalysis is a process that involves illuminating a wide bandgap semiconductor with light having an energy greater than the semiconductor bandgap and then exposing the surface of the semiconductor material to organic compounds. In many cases, the result is oxidation of the organic compound to harmless components, e.g., H.sub.2 O and CO.sub.2. Many types of semiconducting compounds and mixtures thereof have been found to have varying degrees of efficiency. For example, TiO.sub.2, SnO.sub.2, ZNO, WO.sub.3, and SrTiO.sub.3 have been found to be effective photocatalysts. TiO.sub.2 has been found to be the most effective in terms of photoactivity, photostability and cost, but the following discussion is not limited to TiO.sub.2.
As used herein, the use of the term "photocatalyst" means any compound in which irradiation of the compound with electromagnetic radiation of visible or ultraviolet wavelength will result in conduction band electrons and valence band holes that can undergo redox reactions at the catalyst surface with species such as water or inorganic and organic compounds. The illuminating light has an energy higher than the band gap of the semiconductor (e.g., generally in the ultraviolet (UV), i.e., &lt;385 nm for anatase TiO.sub.2).
The major photocatalysis application consists of treatment of aqueous or gaseous waste streams to eliminate toxic components. Photocatalysis has been carried out in the form of fine particles dispersed in the aqueous solutions and immobilized films exposed to aqueous or gaseous streams. The present invention is directed to immobilized film photoreactors.
In immobilized film systems, semiconductor films have been immobilized onto UV transparent or glass substrates of various geometries. For instance, films have been immobilized on the inside surface of an annulus of UV transparent support where the light source is from within or without the annulus and glass or UV transparent tubing wound around the light source in a spiraling manner. The following discussion is not limited to a particular geometry except as expressly set out in the discussion.
The physical processes associated with photocatalysis are generally understood. Typically, electron-hole pairs are created within the semiconductor and are transported to the interface of the semiconductor and treatment stream. Both oxidation and reduction reactions occur at the interface by transfer of electrons from the semiconductor to an oxidant and holes to a reductant. Organic molecules that are oxidized to less harmful constituents are hereinafter referred to as reductants. The oxidant, or electron acceptor, can be a variety of substances, including hydrogen peroxide, but is typically free oxygen (O.sub.2) in most systems. In oxidation reactions of organic materials to less harmful constituents, the hole in an electron-hole pair can be transferred to surface hydroxyls or directly to adsorbed organics. Surface hydroxyl radicals can further attack adsorbed or non-adsorbed organic species at or near the surface of the semiconductor. As used herein, the term "oxidant" refers to a reactant that is reduced by electron transfer from the semiconductor. "Reductant" refers to a reactant that is oxidized by hole transfer from the semiconductor.
The most efficient use of absorbed light occurs when the rates of oxidation and reduction are equal. Under these conditions, surface charging is minimized and recombination processes within the semiconductor are minimized. Under most conditions, the reduction reaction, or transfer of an electron to free oxygen, is the slowest occurring process. As a result, a negative charge caused by excess electrons accumulates within the semiconductor leading to increased electron-hole recombination rates and less efficient hole transfer. In oxidation reactions, the notion of photonic efficiency relates to the ratio of oxidation events to the incident number of photons on the semiconductor surface. In terms of effectively utilizing incident photons, photonic efficiency is maximized when surface charging is minimized. Therefore, removal of the excess photogenerated charge of the nonproductive majority carrier (electrons, in the case of an n-type conducting photocatalyst applied to oxidation reactions) maximizes the photonic efficiency.
Some of the difficulties present in photocatalytic systems include the unpredictability of the reaction rates under varying conditions of light intensity, reactant concentrations and reactant types. Different reactants adsorb to the surface of the photocatalyst to different extents thereby influencing reaction rates. For instance, a reactant that adsorbs less well to the catalyst surface may have a lower reaction rate. At high concentrations of organic materials, the kinetics or rate of reaction may be increasingly independent of the organic concentration. Under these circumstances, the reaction rate is primarily controlled by reduction reactions. Oxidation-reduction rates may be increased by increasing the mass transport rate of reactants to the catalyst surface or the amount of reactant adsorbed on the surface of the catalyst. In flow reactors with immobilized films, increasing the flow rate or concentration can increase mass transport rates. Increased concentrations in the feed stream can also increase the amount of reactant adsorbed on the catalyst surface if the reactant follows langmuirian behavior. Note that, hereinafter, the feed rate is defined as the molar flow rate, and is the product of the concentration and volumetric flow rate. Therefore, the feed rate can be adjusted either by adjusting concentration, flow rates, or both. Likewise, under otherwise fixed operating parameters, increasing the light intensity will increase the reaction kinetics, but at an increasingly slower rate. Additionally, the concentration at which the reaction rate is independent of the organic concentration will vary depending on the light intensity.
It has been shown by K. Vinodgopai et al., "Electrochemically Assisted Photocatalysis. TiO.sub.2 Particulate Film Electrodes for Photocatalytic Degradation of 4-Chlorophenol," 97 J. Phys. Chem., pp. 9040-9044 (1993) that photooxidation rates can be enhanced by applying an anodic bias to an n-type semiconductor in an electrochemical cell configuration. In particular, Vinodgoapi shows that photocatalytic degradation occurs at a faster rate with the applied potential maintained at +0.6 V while little degradation occurs when the potential is maintained around -0.6 V. There is no suggestion on how to usefully apply this result to a photocatalytic process.
Accordingly, it is an object of the present invention to control the voltage bias applied to the photocatalytic film to minimize the recombination rates of electron-hole pairs by enhancing charge separation.
Another object of the present invention is to monitor and adjust process parameters to minimize excess charge buildup on the surface of the photocatalyst.
One other object of the present invention is to adapt computer process control to monitor and adjust process parameters to minimize excess charge buildup.
Additional objects, advantages and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.