Operando ATR-FTIR elucidation of surface-mediated photocatalytic pathways on metal-free nanomaterials

This work presents the first systematic in-situ investigation of surface adsorption and photocatalytic reaction pathways on metal-free nanomaterials, namely graphitic carbon nitride and boron carbon nitride (g-C₃N₄ and BCN). Although g-C₃N₄- and BCN-based photocatalysts are widely studied, mechanistic understanding of surface-bound intermediates during photocatalysis is still often inferred rather than directly tracked in real time. Here, in situ attenuated total reflectance Fourier-transform infrared (ATR-FTIR) spectroscopy is used to directly monitor the adsorption and subsequent light-driven transformation of three representative organics: methanol, phenol, and methylene blue (MB) at the catalyst/water interface. Methanol adsorption and its time-dependent spectral evolution are consistent with formation of surface methoxy species followed by stepwise oxidation toward carbonyl/formate-type intermediates and eventually CO₂. The spectral evolution of phenol is consistent with its interaction via hydrogen bonding and π-π interactions, followed by hydroxylation, quinone formation, and ring-opening pathways. MB adsorbs strongly through electrostatic and π-π interactions and undergoes N-demethylation, chromophore disruption, and sequential oxidation. Across all probes, BCN shows stronger adsorbate-induced spectral perturbation and more pronounced intermediate evolution than g-C₃N₄, consistent with boron-induced modification of surface polarity/acid-base character and charge-transfer behavior that promotes interfacial transformation. The results demonstrate the value of operando ATR-FTIR for resolving surface-controlled photocatalytic reaction sequences and diagnosing intermediate accumulation and interfacial OH/water dynamics, which are directly relevant to activity and stability considerations under more complex treatment conditions.