In-Situ Reaction Monitoring and Mechanistic Studies by Mass Spectrometry: Heterogeneous Photocatalysis and Initial Contact Reaction.

Elucidating reaction mechanisms in heterogeneous catalysis demands real-time tracking of transient species at the solid-liquid interface. To address limitations in directly probing surface processes with conventional mass spectrometry (MS), this work develops an innovative "in-source reaction" strategy by integrating the reactor with the electrospray ionization (ESI) source. Diverse catalysts (e.g., g-C3N4, TiO2, zeolites, and Pd/C) were robustly immobilized within a glass probe (GP), integrating the microreactor and ionization source. This enables direct, subsecond resolution detection of species desorbing from catalytic surfaces. This strategy has been successfully verified by in-situ tracking of the photocatalytic degradation of Brilliant Green (BG) and other dyes. Over 40 intermediates in BG photocatalytic degradation were detected, with different evolution pathways revealing distinct mechanisms for g-C3N4 under visible light and TiO2 under UV light. Besides photocatalyst, typical solid catalysts were also immobilized on the inner wall of the GP to study the initial contact reactions of a β-O-4 model compound of lignin conversion. The obvious dehydrogenation of surface-adsorbed GGGE on Pd surface was observed by in-situ monitoring, while basic or amphoteric support surface spontaneously induced cyclization of GGGE, even without the input of external energy such as light and heat. This method provides a novel, extremely feasible, and sensitive strategy for tracking surface processes in liquid-solid heterogeneous catalytic reactions and holds promise for broader application to diverse catalytic reaction systems in the future.