Compositional Decoupling of Bulk and Surface in Open-Structured Complex Mixed Oxides

The local composition of heterogeneous catalysts can deviate from average values obtained by integral techniques. However, due to the difficulties in capturing nontranslational parts, they are often not determined. We have examined local alterations in the metal cation distribution of open-structured orthorhombic (Mo,V)­O x by scanning transmission electron microscopy coupled to electron energy loss spectroscopy (STEM-EELS). A HyperSpy-based routine was adapted and implemented that allows for automated EELS analysis. This analysis revealed compositional variations that reflect differences among the bulk, termination layers, and different surface facets. For instance, the lateral surface is preferentially enriched in molybdenum compared to the bulk, while V-rich surfaces can also be detected for the basal plane. These compositional inhomogeneities can modulate the surface reactivity on the atomic scale. The knowledge of local compositional gradients is of general importance for any material as they alter its local chemistry and, thus, its physicochemical properties. Furthermore, such knowledge allows establishing novel composition–activity correlations, which could provide a deeper understanding of the origin of the function of materials.