OPQ and L M,9BC OPQ in Fig. . This is in particular the case for the phosphate-based materials and MoVOx (displayed as diamonds and grey circles, respectively, in Fig. ). The analysis of spent catalysts (Fig. ) shows a similar RAE surface contents compared to the activated samples (Fig. ), indicating that the changes in surface occurring during the reaction are reversible. The changes in RAE surface contents under reaction conditions are accompanied by changes in RAE oxidation states, as shown in Fig. and Tables .