Identifying Active Oxygen and Copper Species in Cu/CeO<sub>2</sub> Catalysts Using Raman and UV–Vis Modulation Excitation\nSpectroscopy

Modulation excitation\nspectroscopy (MES) with so-called phase-sensitive\ndetection (PSD) is receiving increasing attention for the identification\nof active species and the elucidation of reaction mechanisms in heterogeneous\ncatalysis. We report the combination of Raman- and UV–vis-MES\nto be a powerful approach to gain new mechanistic insight into oxide-supported\nmetal catalysts, as illustrated for the CO oxidation over low-loaded\nCu/CeO<sub>2</sub> catalysts. Due to the enhanced sensitivity of the\nMES/PSD approach, Raman spectroscopy provides direct evidence for\nthe active participation of surface lattice oxygen of the ceria support\nin the reaction mechanism. While Raman-MES enables exclusive insight\ninto the role of the ceria support, UV–vis-MES is shown to\nbe sensitive to the low copper loading. ME-UV–vis spectra highlight\nthe redox activity of copper species and the electronic interactions\nbetween copper and ceria, revealing major reduction of Cu<sup>2+</sup> to Cu<sup>+</sup> while reduction of Ce<sup>4+</sup> to Ce<sup>3+</sup> is minor under the studied reaction conditions. Based on the mechanistic\nfindings provided by the combined Raman/UV–vis-MES/PSD methodology,\nwe expect this approach to be a valuable tool to gain information\nabout active species and sites of catalysts relevant for renewable\nenergy.