Effect of Platinum Deposits on TiO2 on the Anoxic Photocatalytic Degradation Pathways of Alkylamines in Water: Dealkylation and N-Alkylation

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Abstract
Most photocatalytic degradation (PCD) reactions of aquatic pollutants require the presence of dissolved oxygen and hence do not occur in anoxic suspensions. We investigated the PCD reactions of alkylamines in anoxic water using TiO2 deposited with Pt nanoparticles. Unlike typical PCD reactions, the absence of dissolved oxygen increases the PCD rates of alkylamines on Pt/TiO2 and generates products that are different from those formed on pure TiO2. In particular, N-alkylated amines (e.g., (CH3)3N produced from (CH3)2NH) as well as dealkylated amines are generated in a deaerated Pt/TiO2 suspension. This anoxic N-alkylation pathway is enabled only in the presence of Pt deposits on TiO2 and is applicable only to neutral alkylamines and not to alkylammonium cations. The Pt surface appears to interact with the lone-pair electron on the N atom and catalyze the anoxic degradation of alkylamines mainly through a radical mechanism. Methyl radicals generated on Pt participate in the N-methylation reaction. The presence of intermediate methyl radicals on the Pt surface was verified by the detection of CH4 and CH3CH3 gases evolved during the PCD of (CH3)3N in an anoxic Pt/TiO2 suspension, whereas no such products were observed in a pure TiO2 suspension. The anoxic PCD of N-methylethylamine on Pt/TiO2 also produces both N-ethylated and N-methylated amines as byproducts, which indicates that both methyl and ethyl radicals are generated during the anoxic degradation process. From a practical point of view, the present finding that undesirable alkylated amines can be produced on Pt/TiO2 in anoxic conditions indicates that caution is necessary when applying Pt/TiO2 photocatalyst to the treatment of water that contains amines.