Photocatalytic/ magnetic composite particles have been invented as improved means of exploiting established methods of photocatalysis for removal of chemical and biological pollutants from air and water. The photocatalytic components of the composite particles are formulated for high levels of photocatalytic activity, while the magnetic components make it possible to control the movements of the particles through the application of magnetic fields. The combination of photocatalytic and magnetic properties can be exploited in designing improved air- and water-treatment reactors.
A typical composite particle according to the invention (see Figure 1) includes a magnetic substrate or core particle having a size of the order of 100 μm, a layer to protect the core against chemical attack, and an outer coat of nanosized photocatalyst particles on the protective layer. In an application in which the protective layer is not needed, it can be omitted and the photocatalyst particles coated directly onto the core. Another option is to make the core of a nonmagnetic material and to coat it with nano-sized photocatalyst and magnetic particles.
As used here, "nano-sized" refers to sizes in the range from a few nanometers to about 100 nanometers. The reason for choosing this size range is simply that experience has shown that nano-sized photocatalyst particles are more photocatalytically active than are micron- and larger-sized photocatalyst particles.
The magnetic core (or, optionally, the nano-sized magnetic particles) can be made of any of a variety of suitable magnetic materials — for example, Fe3O4, BaO(Fe2O3)6, SrO(Fe2O3)6, or AlNiCo. The protective layer can be a polymer or a ceramic — for example, poly(tetraethylfluoroethylene) or poly(methyl methacrylate) or silica. The photocatalyst can be TiO2, ZnO, or Fe2O3. Nano-sized photocatalyst particles can be coated onto the larger core particles by various techniques, e.g., a dry coating machine, wherein a mixture of the two types of particles is forced to pass through a narrow clearance under high stress.
A representative reactor for treatment of air by use of photocatalytic/magnetic composite particles (see Figure 2) includes a fluidized bed loaded with the particles. An ultraviolet lamp provides the excitatory photons needed for photocatalysis. An electromagnet coil generates a magnetic field to control the movement of the composite particles.
In operation, polluted air or water enters at the bottom and flows upward through the fluidized bed. The polluted fluid agitates, and mixes with, the composite particles in the bed. Additional agitation is provided by an alternating magnetic field generated by supplying alternating current to the electromagnet coil. The agitation enhances the fluidization, and is almost entirely responsible for fluidization when the flow is not rapid enough to fluidize the particles sufficiently. The agitation promotes the exposure of the photocatalyst particles to ultraviolet light from the lamp and increases the rate of generation of hydroxyl radicals, which react with the pollutants. The exhaust flowing from the top of the reactor consists of purified fluid.
This work was done by Chang-Yu Wu, Yogi Goswami, Charles Garretson, Jean Andino, and David Mazyck of the University of Florida for Johnson Space Center. For further information, contact the Johnson Commercial Technology Office at (281) 483-3809. In accordance with Public Law 96-517, the contractor has elected to retain title to this invention. Inquiries concerning rights for its commercial use should be addressed to: University of Florida, Environmental Engineering Black Hall Gainesville, FL 32611 Refer to MSC-23829, volume and number of this NASA Tech Briefs issue, and the page number.