The use of intrinsic materials properties as descriptors for selective oxidation is also problematic because they are reaction-independent, i.e., they neglect any effect resulting from the interaction of the reaction mixture with the material. The chemical potential under reaction is, however, known to influence the reactivity in alkane oxidation. We have also verified whether the surface RAE oxidation state measured under UHV on the fresh catalyst samples and under NAP alkane-rich conditions (denoted c M,$" OPQ , and c M,9:;9<# OPQ , respectively, in Fig. , Table ) correlates with the reactivity. The VPP and a-VPP materials, which are highly selective towards MAN in n-butane oxidation (Fig. ), present the highest increase in oxidation state under NAP conditions with respect to UHV (compare black and blue bars in Fig. ). In spite of these observations, no general correlation between the surface RAE oxidation states and activity or selectivity in alkane oxidation is apparent.