Catalyst characterization. After synthesis, the materials were generally present as polycrystalline solids, with the exception of the "amorphous" vanadium-phosphorus oxide, referred to here as a-VPP, which contains an amorphous component and poorly crystalline hemihydrate VOHPO4•½H2O. As far as possible, the observed diffraction patterns were analyzed and interpreted using whole powder pattern fitting according to the Rietveld method. The corresponding crystal structure and peak shape models used are listed in Table . A detailed analysis of the XRD data and a discussion of possible structural changes observed after catalysis are provided in the SI (see text and Tables S6-S11, Fig. ). The specific surface area of the fresh catalysts varies between 0.70 and 60 m 2 /g. The mostly macroporous materials have total pore volumes below 0.5 cm 3 /g. Structural transformations and crystallization processes that take place during the activation procedure are accompanied by an increase or decrease in the specific surface area and the pore volume (Fig. ), and changes in the surface composition are thus to be expected. The changes differ depending on the reaction carried out, especially with regard to the pore volume (Fig. ). Even the phase-pure materials, for which no phase transformations are observed under reaction conditions, exhibit considerable fluctuations in their interfacial properties. The connection between the surface composition and the chemical potential of the environment becomes clear through a detailed analysis of the XPS data. The surface contents of the RAEs vanadium or manganese on fresh catalyst samples, denoted L M,$" OPQ and measured by XPS at ultra-high-vacuum (UHV) conditions, are displayed in the abscissa of the plots in Fig. . The values lie in the range 0.01 to 0.30, in units of atomic fraction. The perovskite SmMnO3 and the vanadium oxide V2O5 present the highest RAE surface contents, whereas the mixedmetal oxides MoVOx, MoVTeNbOx, and VWPOx are the materials with lowest values. The comparison of RAE surface content with activity towards the analyzed oxidation reactions does not suggest a direct link.