Identifying Active Oxygen and Copper Species in Cu/CeO₂ Catalysts Using Raman and UV–Vis Modulation Excitation Spectroscopy

Modulation excitation spectroscopy (MES) with so-called phase-sensitive detection (PSD) is receiving increasing attention for the identification of active species and the elucidation of reaction mechanisms in heterogeneous catalysis. We report the combination of Raman- and UV–vis-MES to be a powerful approach to gain new mechanistic insight into oxide-supported metal catalysts, as illustrated for the CO oxidation over low-loaded Cu/CeO₂ catalysts. Due to the enhanced sensitivity of the MES/PSD approach, Raman spectroscopy provides direct evidence for the active participation of surface lattice oxygen of the ceria support in the reaction mechanism. While Raman-MES enables exclusive insight into the role of the ceria support, UV–vis-MES is shown to be sensitive to the low copper loading. ME-UV–vis spectra highlight the redox activity of copper species and the electronic interactions between copper and ceria, revealing major reduction of Cu²⁺ to Cu⁺ while reduction of Ce⁴⁺ to Ce³⁺ is minor under the studied reaction conditions. Based on the mechanistic findings provided by the combined Raman/UV–vis-MES/PSD methodology, we expect this approach to be a valuable tool to gain information about active species and sites of catalysts relevant for renewable energy.