Photocatalytic oxidation of aqueous ammonia by Ag2O/TiO2 (P25): New insights into selectivity and contributions of different oxidative species

Oxidation of ammonia (NH4+/NH3) from wastewater is a challenge due to its high stability and solubility in aqueous environments. A simple Ag2O/TiO2 (P25) catalyst was synthesized and used for photocatalytic oxidation of aqueous NH4+/NH3. Under UV illumination, the oxidation rate of NH4+/NH3 by 1% Ag2O/P25 is increased with increasing pHs, with a rate constant of 0.383 h(-1) at pH 11.8. (OH)-O-./h(+) are important oxidative species that dominate the oxidation of NH4+/NH3. The respective contributions of Pathway I (the h(+) mediated process) and Pathway II (the e(-) mediated process) are 66.2% and 33.8% to the oxidation of NH4+/NH3. Oxidation selectivity toward N-2 is more favorable under high concentrations of NH4+/NH3 and low concentrations of Ag2O/P25. The improved separation of e(-)-h(+) pairs on Ag2O/P25 facilitates generation of (OH)-O-. and h(+), thus triggering the high conversion yield of NH4+/NH3. XPS analysis further suggests that Ag(I), instead of the photogenerated Ag(0), plays the predominant role in photocatalytic oxidation of NH4+/NH3. This work enriches our understanding on the photooxidation mechanism of NH4+/NH3 by Ag2O/P25.