Microwave-assisted Heterogeneous Photocatalytic Oxidation

Photocatalytic oxidation makes use of light of appropriate frequency to activate a catalyst. In the presence of water on the surface of the catalyst, rates of some photocatalytic reactions are slowed. Microwaves are known for an ability to couple with water. Herein a photocatalytic reactor system was integrated with a waveguide allowing for concurrent application of microwaves and photocatalysis. Data from initial experiments are reported on the ability of this system to oxidize ethylene gas in an air stream at two levels of temperature and water vapor. INTRODUCTION Photocatalysis is the use of photons from UV-rich light to generate electronhole pairs near the surface of a metal oxide material. In our laboratory, photocatalytic reactions are studied in feed streams that contain high molar flow rates of molecular oxygen that, when adsorbed to the surface of the material, act as electron acceptors. If the remaining holes can reach the surface of the material before recombining with an electron, the holes become available to react with adsorbed water and other adsorbed inorganic and organic molecules. It is known that the presence of water vapor can deleteriously alter the kinetics of some photocatalytic reactions that occur in the gas-phase. While some reactions require certain amounts of water as a stoichiometric ingredient, many other compounds suffer reduced degradation kinetics in humid conditions. It has been hypothesized that water competes for reaction sites on the surface of the semiconducting photocatalysts and therefore depresses the rates of these reactions as the concentration of water vapor is increased [1]. Since microwaves preferentially heat molecules that have a high dielectric, one might expect microwaves to couple directly with water molecules adsorbed at the semiconductor interface. It has been demonstrated that concurrent application of UV light and microwaves has a beneficial effect on organic reactions [2] and photodegradation of pollutants and drugs [3]. Herein, the application of microwaves (MW) and photocatalytic oxidation (PCO) is examined to determine if concurrent MW-PCO enhances conversion (or removal) of a test gas – ethylene – above that achieved if PCO is employed alone. EXPERIMENTAL Apparatus Schematics are provided of the entire MW-PCO reactor system (Fig. 1), and in particular, the PCO reactor (Fig. 2). The reactor system is a closed-cycle system consisting of a microwave generator (2.45 GHz) containing a variablepower magnetron, a rectangular waveguide, a sampling port and the PCO reactor. In order to measure the temperature and the microwave power, optical probes are inserted into the photocatalytic reactor. Also, cooling air is supplied in order to control the temperature. The catalyst is located in the middle of the reactor. A fluorescent light source or bulb is inserted along the axial centerline of the reactor.