Photocatalytic environmental decontamination using 2D-BiOCl/TiO₂ heterostructures: Coupled 4-nitrophenol reduction...

Water contamination by hazardous organic pollutants poses a significant threat to human health. Among these, phenolic compounds, particularly 4-nitrophenol (4-NP), are of special concern due to their high toxicity and carcinogenic, endocrine-disrupting, and genotoxic effects. This study explores the photocatalytic reduction of 4-NP using, for the first time in this reaction, BiOCl-TiO₂ composites prepared in various ratios by ball milling of home-made BiOCl nanosheets and commercial TiO₂. The selective photocatalytic reduction of 4-NP to 4-aminophenol (4-AP), a high-value chemical intermediate, was investigated in aqueous dispersion as an integrated approach that combines pollutant removal with the production of value-added compounds. The influence of the presence of triethanolamine (TEOA), sodium borohydride (NaBH₄) and 4-methoxybenzyl alcohol (4-MBA), used as sacrificial agents, in the reagent system was systematically evaluated. Formation of 4-AP occurred only in the presence of these hole scavengers, confirming their essential role in promoting the reduction pathway. It should be noted, however, that 4-MBA took part in the reaction not only as a trap for the holes, allowing the reduction of 4-NP to 4-AP by the photoproduced electrons, but also by being transformed itself by partial oxidation into the high-value compound 4-methoxybenzaldehyde (4-MBAL). In the integrated system, the optimal performance was achieved with 5%BiOCl-TiO₂ in the presence of both TEOA and 4-MBA, yielding 94% 4-NP conversion and 99% selectivity toward 4-AP after 3 h of irradiation, along with 6% 4-MBA conversion and 50% selectivity toward 4-MBAL.