Abstract text (incl. figure legends and references)

The heterogeneous catalytic oxidation of CO is an important reaction which impacts many technical applications such as fuel cells (preferential oxidation of CO instead of H₂), air purification, exhaust-gas emission treatment, CO sensors and CO₂ lasers.^[1] Therefore, research focuses on the development and investigation of active and selective catalysts, preferentially containing abundant metals. Copper is a promising and well-investigated candidate, which, upon immobilization on CeO₂, forms redox-active Cu²⁺/Cu⁺ couples. CeO₂ itself has a high oxygen mobility and the ability to form Ce⁴⁺/Ce³⁺ couples, two properties which can be even improved by incorporation of titanium.

Three Cu doped CeO₂-TiO₂ catalysts and the pure support were prepared by a sol-gel method adding 0 (support), 0.06 (Cat1), 0.26 (Cat2), or 0.66 wt% Cu (Cat3) with Cat2 showing the best catalytic performance. To elucidate structure-activity relationships, the materials have been characterized by XRD, Raman and IR spectroscopy, STEM-EELS, NAP-XPS, and operando EPR. STEM-EELS was thereby applied to spatially resolve potential differences in the oxidation state of the containing elements.

A probe corrected JEOL JEM-ARM200F (Schottky emitter) with a Gatan Enfinium ER EELS were used for electron microscopic investigation of the catalysts. A rather high beam current was applied to keep the acquisition time low. The FWHM of the zero-loss peak (ZLP) was about 1.2 eV and the lack of de-scan was compensated by the DualEELS mode. The spectra were recalibrated at each pixel by the position of ZLP.

Cat2 (0.26 wt% Cu) shows the best catalytic performance. The relative intensity of the Raman band at 463 cm^-1 and the appearance of separate CeO₂ entities observed by STEM increase with the Cu content. However, Cu is not visible in the STEM HAADF and BF images due to its small Z contrast and high dispersion. This high dispersion could be proven by EPR and DRIFTS. STEM-EELS enabled the determination of the oxidation state of Ce and its distribution with respect to TiO₂ (Fig. 1 and 2). It revealed that Ce is present either as Ce³⁺ in well-dispersed form or as Ce⁴⁺ in separate CeO² crystallites. It could be shown that Ce³⁺ and Ti⁴⁺ form a Ce-Ti oxide solid solution.

Figure 1. STEM ADF image of Cat2 (left) with the corresponding Ce M edge electron energy loss (EEL) spectra of the highlighted areas (right). The shape and intensity of the edge signal points to Ce³⁺ in Area 1 and Ce⁴⁺ in Area 2.^[2] Reprinted with permission from ref [1]. Copyright © 2021, American Chemical Society.

Figure 2. STEM ADF image of Cat2 (left) with an overlaid false color elemental map. On the right, the distribution of two different cerium species (Ce-M (a) and Ce-M (b)) and titanium is shown independently as well as in a single-color elemental map. Reprinted with permission from ref [1]. Copyright © 2021, American Chemical Society.

Using STEM-EELS with this catalyst system, it was possible to benefit from this additional spectroscopic method by providing information on the Ce oxidation state distribution which is not possible with EDXS. Although EPR and XPS provide already evidence for Ce³⁺, its exact localization is only possible with STEM-EELS.

[1] J. Mosrati, A.M. Abdel-Mageed, T.H. Vuong, R. Grauke, S. Bartling, N. Rockstroh, H. Atia, U. Armbruster, S. Wohlrab, J. Rabeah, A. Brückner, ACS Catal. 2021, 11, 10933-10949.

[2] Y. Zhang, S. Bals, G. Van Tendeloo, Part. Part. Syst. Charact. 2019, 36, 1800287.