Tunable synchrotron vacuum-ultraviolet photoionization mass spectrometry resolves reactive gas-phase species during ethanol oxidation over SnO2.

Resolving low-abundance and short-lived species in heterogeneous gas-solid reactions remains difficult because electron-ionization mass spectrometry fragments extensively and chromatography may lose reactive intermediates, limiting mechanistic understanding of ethanol oxidation on SnO2, a reaction central to SnO2-based ethanol sensing. Here, tunable synchrotron vacuum-ultraviolet photoionization mass spectrometry characterized gas-phase species during ethanol oxidation over SnO2 nanofibers at 300-500 °C. Photon-energy-dependent mass spectra and photoionization-efficiency curves supported the assignment of a broad set of stable products and reactive species, including methyl, methoxy, ethenol, and ketene. Signals were converted to relative mole fractions using photoionization cross sections and mass discrimination factors. At 300 °C, SnO2 enhanced acetaldehyde and water fractions by 23.4- and 8.6-fold relative to the control, whereas at 500 °C most carbon-containing intermediates were depleted by 95% and deep-oxidation products dominated. A SnO2 chemiresistor showed an ethanol response increasing from 5.8 (300 °C) to 22.1 (500 °C), consistent with the shift toward deep oxidation-a shift that returns more electrons per molecule via consumption of ionosorbed oxygen. The study demonstrates the value of tunable SVUV-PIMS for reaction-network-resolved characterization of functional oxide-gas interfaces.