Photocatalytic oxidation of aqueous ammonia over platinized microwave-assisted titanate nanotubes

Batch experiments were conducted to study the photocatalytic oxidation of aqueous ammonia (NH_3/NH_4^+) over platinized titanate nanotubes (platinized TNTs). The reaction kinetics of NH_3/NH_4^+ oxidation is considerably enhanced by the platinization of TNTs. The observed rate constant (k_(obs)) for NH_3/NH_4^+ oxidation over platinized TNTs was 1.69 × 10^(−2) min^(−1), while the unamended TNTs gave a kobs = 1.23 × 10^(−3) min^(−1). The selectivity of nitrogen gas (N_2) from degraded NH_3/NH_4^+ increased with platinum loading to 87.8% after 1 h photocatalysis. In contrast, NH_3/NH_4^+ to nitrate (NO_3^−) conversion was low due to a large NH_3/NH_4^+ to nitrite (NO)_2^−) energy barrier. High-resolution transmission electron microscopy shows that platinum deposits are formed primarily on the internal surface of the hollow structure of TNTs instead of being deposited on the external surface of TNTs. Based on the analysis of X-ray photoelectron spectroscopy, a mechanism is proposed in which N_2 is produced from the NH_3/NH_4^+ adsorbed on platinum deposits while NO_2^−/NO_3^− is formed from the NH_3/NH_4^+ adsorbed on titanate sites