| Abstract
| - The photocatalytic degradation of a series of (CH3)nNH4-n+(0 ≤ n ≤ 4) was systematically studied in the UV-illuminated TiO2 aqueous suspensions at pH ranges of3−11. By investigating the pH-dependent kinetics andanalyzing intermediates and products, we elucidated themechanistic pathways and the role of OH radicals in thephotocatalytic oxidation. The deprotonated neutral speciesmore rapidly degraded than their protonated counterpartsfor these homologous compounds because the OHradicals favorably reacted with the lone-pair electron onthe nitrogen atom. Therefore, the photocatalytic degradationwas highly enhanced at alkaline solutions for all substancesexcept (CH3)4N+. The H-atom abstraction (from (CH3)4N+)by OH radicals initiated successive demethylation processesto generate tri-, di-, and monomethylammonium/amine asan intermediate and NH3/NH4+ as a final product. On theother hand, the OH-addition to the N-atom with the lone-pair electron led to NO2-/NO3- whose production was highlyfavored at alkaline conditions. The photocatalyticdegradation rates of (CH3)4N+ were comparable at bothacidic and alkaline conditions, which could not be explainedby a simple electrostatic surface charge model. Byusing OH-scavenging tert-butyl alcohol as a diagnosticprobe into the mechanism, it is suggested that thephotocatalytic oxidation of (CH3)4N+ at acidic conditionsproceeds through free OH radicals in the solution bulk, noton the surface of TiO2.
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