Surface chemistry of 2-propanol and O₂ mixtures on SnO₂(110) studied with ambient-pressure x-ray photoelectron spectroscopy

Tin dioxide (SnO₂) has various applications due to its unique surface and electronic properties. These properties are strongly influenced by Sn oxidation states and associated defect chemistries. Recently, the oxidation of volatile organic compounds (VOCs) into less harmful molecules has been demonstrated using SnO₂ catalysts. A common VOC, 2-propanol (isopropyl alcohol, IPA), has been used as a model compound to better understand SnO₂ reaction kinetics. We have used ambient-pressure x-ray photoelectron spectroscopy (AP-XPS) to characterize the surface chemistry of IPA and O₂ mixtures on stoichiometric, unreconstructed SnO₂(110)-(1 × 1) surfaces. AP-XPS experiments were performed for IPA pressures ≤3 mbar, various IPA/O₂ ratios, and several reaction temperatures. These measurements allowed us to determine the chemical states of adsorbed species on SnO₂(110)-(1 × 1) under numerous experimental conditions. We found that both the IPA/O₂ ratio and sample temperature strongly influence reaction chemistries. AP-XPS valence-band spectra indicate that the surface was partially reduced from Sn⁴⁺ to Sn²⁺ during reactions with IPA. In situ mass spectrometry and gas-phase AP-XPS results indicate that the main reaction product was acetone under these conditions. For O₂ and IPA mixtures, the reaction kinetics substantially increased and the surface remained solely Sn⁴⁺. We believe that O₂ replenished surface oxygen vacancies and that SnO₂ bridging and in-plane oxygen are likely the active oxygen species. Moreover, addition of O₂ to the reaction results in a reduction in formation of acetone and an increase in formation of CO₂ and H₂O. Based on these studies, we have developed a reaction model that describes the catalytic oxidation of IPA on stoichiometric SnO₂(110)-(1 × 1) surfaces.