Mechanistic Insights into Nitroarene Hydrogenation Dynamics on Pt(111) via In Situ Tip-Enhanced Raman Spectroscopy

Mechanistic insights into the molecular-level dynamics of nitroarene hydrogenation on Pt remain limited, largely because most prior studies rely on ex situ, ensemble-averaged measurements, or simulations considered in isolation. Here, we address this gap and demonstrate a novel methodology combining in situ tip-enhanced Raman spectroscopy (TERS) with density functional theory (DFT) modeling to track, at a well-defined single plasmonic junction, the hydrogenation of chloronitrothiophenol (CNTP) on atomically flat Pt(111). In situ TERS captures the dynamic transformation of CNTP → chloroaminothiophenol (CATP) under ambient H₂ exposure with a characteristic time scale of ∼6 s. Complementary DFT modeling maps the reaction energetics, revealing novel mechanistic insights: CNTP desorption is rapid initially (barrier 0.61 eV) but slows down once the Pt(111) surface is at about half-coverage; molecular bending on the half-covered Pt(111) surface is barrierless and exergonic; the first hydrogen addition to CNTP is facile (barrier 0.26 eV), while the second hydrogen addition is kinetically most demanding (barrier 0.83 eV), yielding a time scale of seconds that matches experimental results and identifies the rate-determining step. These findings advance molecular-level understanding of nitroarene hydrogenation on Pt(111) and demonstrate in situ TERS integrated with first-principles DFT modeling as a powerful platform for operando mechanistic studies of heterogeneous catalytic processes at the nanoscale.