PHOTOOXIDATION OF AQUEOUS AMMONIA WITH TITANIA-BASED HETEROGENEOUS CATALYSTS

The photoassisted oxidation of dilute aqueous ammonia (0-100 muM) to NO2- and NO3- was investigated over heterogeneous catalysts of TiO2 doped with iron oxide in quartz vessels and with radiation from mercury lamps. Effects on the reaction rate of varying concentration, radiation intensity, amount of catalyst, and pH were studied. The rate of ammonia consumption increased with radiation intensity and was independent of the amount of catalyst over the range of 0.1-1.0 g suspended in 50 ml of solution. In acidic solutions (pH 1-6), the simultaneous action of catalyst and radiation was needed, while in strongly basic solutions (pH 12) only illumination was required. Moreover, the nature and yields of products were dependent on pH: in acidic solutions NO3- was the main product and in basic solutions NO2- was the primary one. Control experiments showed that the reaction rate increased with pH in the absence of catalyst, and the catalyst was active only when irradiated. The reaction was first-order in NH3. The chemical reaction and the mechanism of the photooxidation of NH3 are discussed.