In Situ ATR-IR Spectroscopy for the Degradation of Acetaminophen on WO₃–AgCl Photocatalysts

Photocatalysts made of WO3 coupled with AgCl have demonstrated enhanced photocatalytic activity for degrading organic contaminants. While various studies have focused on the degradation of organic molecules and the formation of intermediate species during photocatalysis, the catalytic interface during illumination has received less attention. Attenuated total reflection infrared spectroscopy (ATR-IR) is ideal for studying solid–liquid interfaces and has shown potential for studying catalytic reactions. In this study, the application of in situ ATR-IR for the degradation of the model contaminant acetaminophen using pure WO3 and WO3–AgCl with and without a surface-directing agent (citric acid) was explored. ATR-IR experiments in batch and continuous flow modes were conducted to characterize the interface and its adsorbates during illumination. Liquid chromatography–high-resolution mass spectrometry (LC-HRMS) was used to further explore the reaction intermediates and to support the ATR-IR data. ACT dimers, ACT trimers, dimers of ACT and p-benzoquinone, and p-benzoquinone were identified as degradation products. NH3-TPD was employed to investigate the surface acidity and its role in the photocatalytic process, revealing that the combination of AgCl with WO3 reduced the number of weak and moderate acidic sites, while the addition of citric acid promoted the formation of strong acidic sites in the photocatalyst, which significantly impacted the degradation efficiency. Some limitations of the ATR-IR technique were highlighted; nevertheless, it remains a powerful tool for probing solid–liquid interfaces during photocatalytic degradation.