Surface Active Species in Amorphous–Crystalline SnO 2 Thin Films: Chemisorbed Oxygen Activity and CO Oxidation Pathways Revealed by In Situ XPS and Mass Spectrometry

This study examines fundamental aspects of CO interaction with SnO 2 surfaces, focusing on (i) the temperature and oxygen partial pressure conditions governing chemisorbed oxygen formation and (ii) the relative participation of chemisorbed versus lattice oxygen species in CO oxidation up to 250 °C. SnO 2 thin films deposited by low-temperature (60 °C) plasma-enhanced atomic layer deposition exhibit a mixed amorphous–crystalline microstructure characterized by nanoscale structural disorder, high defect density, and pronounced oxygen understoichiometry. Using a combination of operando X-ray photoelectron spectroscopy (XPS), mass spectrometry (MS), electrochemical impedance spectroscopy (EIS), and transmission electron microscopy (TEM), we directly correlate surface chemical states with gas-phase reaction products and electronic transport behavior. Operando XPS and MS measurements reveal that under the investigated operando conditions (200 °C), CO oxidation proceeds via chemisorbed oxygen species, while no measurable lattice oxygen participation is detected within the sensitivity limits of the operando XPS measurements. This conclusion is supported by concurrent CO 2 evolution detected by MS and stable impedance signatures indicative of sustained electronic transport. Quantitative analysis further demonstrates dynamic modulation of chemisorbed oxygen populations during repeated CO exposure, establishing their central role in the reaction pathway. At temperatures approaching 250 °C, a critical transition is observed in which chemisorbed oxygen desorbs, accompanied by a collapse in electronic transport and a marked reduction in surface reactivity. These findings clarify the mechanistic role of chemisorbed oxygen in CO oxidation on SnO 2 and highlight how amorphous–crystalline microstructures produced by low-temperature deposition enhance oxygen defect chemistry and surface reactivity.