Sunlight assisted photocatalytic mineralization of nitrophenol isomers over W⁶⁺ impregnated ZnO

The synthesis, characterization and sunlight photocatalytic activity of W⁶⁺ impregnated ZnO for the degradation of nitrophenol isomers (2-nitrophenol, 3-nitrophenol, and 4-nitrophenol) is reported. Compared to pure ZnO, the impregnated catalysts exhibited a relatively improved spectral response in the visible region. The significant decrease in luminescence intensity for W⁶⁺ impregnated catalysts, relative to pure ZnO, established the effective trapping of charge carriers by surface bonded W⁶⁺ states. The characterization of the impregnated catalysts by FESEM, XRD and XPS revealed the presence of impregnated W⁶⁺ entities at the surface without affecting the morphology of ZnO. The synthesized catalysts exhibited superior activity for the mineralization of three nitrophenol isomers under sunlight exposure. Based on HPLC analysis, ~99% of 2- and 4-nitrophenol was degraded in a short span of 120min while a degradation of >80% was observed in 3-nitrophenol in the same period. Time scale TOC measurements ratified the mineralization capability of impregnated catalysts. The release of NO₂^-, NO₃^- ions in the solution, pH measurements and non-existence of hydroxylated aromatic intermediates revealed that superoxide anion radicals (O₂^-) are the major contributors in the degradation process. Besides the traces of coupling products, major intermediates identified by GC-MS analysis were either aliphatic dihydroxy alcohols, carboxylic acids or other oxygen containing species depicting that the degradation process proceeds through ring opening mechanism. The W⁶⁺ impregnated catalyst also furnished high activity in the visible portion of sunlight. Suitable kinetic models were applied to estimate the rate constants (k) of various processes involved in the degradation process. The stability of the photocatalyst against photocorrosion and the possible decrease in the activity of the catalyst in the successive use was also monitored.