In Situ ATR-IR\nSpectroscopy for the Degradation of\nAcetaminophen on WO<sub>3</sub>–AgCl Photocatalysts

Photocatalysts made of WO<sub>3</sub> coupled with AgCl\nhave demonstrated\nenhanced photocatalytic activity for degrading organic contaminants.\nWhile various studies have focused on the degradation of organic molecules\nand the formation of intermediate species during photocatalysis, the\ncatalytic interface during illumination has received less attention.\nAttenuated total reflection infrared spectroscopy (ATR-IR) is ideal\nfor studying solid–liquid interfaces and has shown potential\nfor studying catalytic reactions. In this study, the application of\nin situ ATR-IR for the degradation of the model contaminant acetaminophen\nusing pure WO<sub>3</sub> and WO<sub>3</sub>–AgCl with and\nwithout a surface-directing agent (citric acid) was explored. ATR-IR\nexperiments in batch and continuous flow modes were conducted to characterize\nthe interface and its adsorbates during illumination. Liquid chromatography–high-resolution\nmass spectrometry (LC-HRMS) was used to further explore the reaction\nintermediates and to support the ATR-IR data. ACT dimers, ACT trimers,\ndimers of ACT and <i>p-</i>benzoquinone, and <i>p-</i>benzoquinone were identified as degradation products. NH<sub>3</sub>-TPD was employed to investigate the surface acidity and its role\nin the photocatalytic process, revealing that the combination of AgCl\nwith WO<sub>3</sub> reduced the number of weak and moderate acidic\nsites, while the addition of citric acid promoted the formation of\nstrong acidic sites in the photocatalyst, which significantly impacted\nthe degradation efficiency. Some limitations of the ATR-IR technique\nwere highlighted; nevertheless, it remains a powerful tool for probing\nsolid–liquid interfaces during photocatalytic degradation.