Revealing the Dynamic Evolution and Reactivity of Oxygen Species over Cu-Exchanged Zeolites: Insights from In Situ Multiwavelength Raman Spectroscopy

Copper-exchanged zeolites have attracted broad interest owing to their facile conversion of methane to methanol, which involves oxygen activation, methane oxidation, and methanol extraction. Raman spectroscopy has revealed the presence of multiple oxygen species, such as superoxo, peroxo, and mono-oxo types, during oxygen activation. However, the dynamic evolution and specific functions of these intermediates under operating conditions remain poorly understood. In this study, we applied in situ multiwavelength Raman spectroscopy, employing lasers ranging from visible to deep ultraviolet (UV), to track their formation, interconversion, and reactivity throughout the catalytic cycle. By selecting appropriate excitation wavelengths and combining these with isotopic oxygen experiments under in situ conditions, we successfully identified physically adsorbed O2, μ-(η2:η2) peroxo dicopper, trans-μ-1,2-peroxo, superoxo, and mono-μ-oxo dicopper species. Our findings demonstrate that both the gas environment and temperature critically influence the nature of the copper oxide species formed. The mono-μ-oxo dicopper species on Cu-ZSM-5, generated during oxygen activation at 450 °C, was found to be unstable at reaction temperature (200 °C), accounting for the lower methanol formation activity compared to Cu-mordenite. This study underscores the importance of laser excitation selection and in situ methodology, and further emphasizes how pretreatment conditions and zeolite structure jointly affect the stabilization of active oxygen species and overall catalytic performance.