Selective Photocatalytic Oxidation of NH3 to N2 on Platinized TiO2 in Water

Selective photocatalytic oxidation of NH3 to N2 is proposed as a new treatment method for controlling the levels of ammonia in water. The photocatalytic oxidation of ammonia on naked and metallized TiO2 in water saturated with air, nitrogen, or N2O gas was investigated. While the slow photocatalytic oxidation of NH3 to NO2-/NO3- is the only pathway for decomposition of NH3 on naked TiO2 and Au/TiO2, a new pathway, that of selective oxidation of ammonia to dinitrogen, opens up on Pt/TiO2. The formation of dinitrogen from the oxidation of 15NH3 was confirmed by mass spectrometric detection of 15N2. The photocatalytic conversion of NH3 to N2 greatly increases when the Pt/TiO2 suspension is saturated with N2O gas, whereas N2O itself shows little reactivity with naked TiO2 and Au/TiO2. Over 80% of the total nitrogen available in ammonia (0.1 mM) is converted into N2 within 40 min illumination of the N2O-saturated Pt/TiO2 suspension. The ability of N2O to accept the conduction band electrons of Pt/TiO2 was verified by photoelectrochemical measurements. N2O reductively decomposes to generate OH radicals on Pt/TiO2; the rate of ammonia degradation in the N2O-saturated Pt/TiO2 suspension significantly decreases in the presence of excess tert-butyl alcohol, an OH radical scavenger. The presence of Pt deposits on the TiO2 particles changes the photocatalytic pathway of ammonia conversion by both enhancing OH radical production from N2O and stabilizing intermediate NHx (x=0,1,2) species to facilitate their recombination into N2.