Photocatalytic and photoelectrocatalytic degradation and mineralization of small biological compounds amino acids at TiO2 photoanodes

• PC and PEC mineralization of small biological compounds amino acids were compared.
• PEC degradation efficiency was higher than PC for all compounds investigated.
• Frontier electron densities were calculated to predict the initial reaction sites.
• Experimental and theoretical results show PEC and PC reaction mechanism is different.

Quantitative evaluation of photocatalytic (PC) and photoelectrocatalytic (PEC) degradation of small biological compounds like amino acids (AAs) were systematically investigated at illuminated nanoparticulate TiO2 photoanode using a thin-layer photoelectrochemical cell. Three model AAs like phenylalanine (Phe), tyrosine (Tyr) and tryptophan (Trp), were found to be photocatalytically and photoelectrocatalytically degradable. PEC degradation efficiency was much higher than that of PC method for all compounds investigated, and the superiority becomes more obvious at higher concentrations. Organic nitrogen atoms in AAs can be oxidized to either NH3/NH4+ or NO3–, or both, depending the chemical structures of AAs and degradation methods used. HPLC analysis found that, for a given AA except Phe, the hydrophilicity characteristics of intermediates are differed slightly between PC and PEC process. For a given method, similar hydrophilicity characteristics were obtained from all AAs investigated except Phe. Theoretically calculated results showed that initial reaction sites for all single-ringed AAs are likely to occur on the atoms within 6-membered ring structure, while for double-ringed AA, the initial reaction sites are likely to occur at 6-membered ring structure (PC) or at 5-membered ring structure (PEC). Both experimental and theoretical results demonstrated PEC reaction mechanisms differed remarkably from that of PC process.

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