In previous project periods, the most stable CeO₂(111) and its interaction with small molecules from the gas phase was examined in great detail. In this project period, the goal was to understand the influence of the surface termination on the catalytic behavior. The CeO₂(100) and CeO₂(110) terminations are accessible by hydrothermal synthesis and can be modeled in theory. For the catalytic experiments the samples were loaded with gold and the gold structures were also studied theoretically. First, we took a closer look at the oxygen activation, which is particularly interesting for reactions where lattice oxygen is needed, e.g. the water-gas shift reaction or the CO oxidation. Regarding the oxygen activation we can observe a strongly surface facet dependent behavior. Overall, we find facilitated peroxide formation at the CeO₂(100) surface leading to distinct Raman signals. This is in line with the DFT calculations that predict the formation of a surface oxygen vacancy followed by peroxide formation as a net exothermic process at the CeO₂(100) surface, but not at the more stable facets, namely, CeO₂(110) and CeO₂(111). As one of the surface peroxide features, we observe a Raman band at 354 cm⁻¹, the symmetry of which had not been resolved yet in the literature. In combination with DFT calculations we readily can assign this band to a deformation vibration of surface peroxides (see Figure 1). Furthermore, DFT calculations show that the interaction of gold with ceria depends on the surface facets and that gold lowers the defect formation energy, which is in line with our experimental results obtained during CO oxidation. Moreover, we detect an adsorbate band at 1647cm⁻¹ for the rods, which contain both the CeO₂(110) and CeO₂(111) facet. Using the combination of theory and experiment this band can be assigned to carbonate species, which are formed at lower temperatures (60 ◦C); remarkably, and these carbonate species may block active sites, because of the strong adsorption energies on CeO₂(100) and CeO₂(110), and are therefore of relevance for catalysis. To elucidate the mechanism of the water gas shift reaction we performed isotopic exchange experiments. In this context a strong red-shift of the F_2g band, which is an indicator for lattice oxygen, is observed. This leads to the question how complete the exchange of lattice oxygen is. To answer this question, the shifts of the F_2g band for a complete substitution of ¹⁶O by ¹⁸O were calculated using DFT calculations (for spectra of Ce¹⁸O₂ and Ce¹⁶O₂ see Figure 2), indicating a red-shift of 23 cm^-1. Interestingly during the water-gas shift reaction a band at 1079 cm⁻¹ is detected for the rods, which has not been discussed in literature. Using DFT this band can assigned to superoxides adsorbed on a CeO₂(110) facet, which occurs at significantly lower wavenumber than on CeO₂(111) or CeO₂(100) facets. Furthermore, the exclusive occurrence of the 1079 cm⁻¹ band under reaction conditions is in line with our DFT calculations, which show that superoxide adsorption is facilitated on a reduced surface. Thus, based on our experimental and theoretical findings, we can attribute the Raman band at 1079 cm⁻¹ to superoxide adsorbed on a reduced CeO₂(110) surface. Summarizing, our results show the potential of a combined operando Raman and DFT approach to characterize e.g. adsorbates at the low index surfaces of ceria, which is of importance for a detailed understanding of the activity behavior of ceria-based catalysts.