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.