Structural Complexity in Heterogeneous Catalysis: Cataloging Local Nanostructures

We present an analytical route toward a detailed and quantitative description of individual defects in heterogeneous catalysts. The investigation is based on a high resolution scanning transmission electron microscopy (STEM) study using complex (Mo,V)­O_x mixed oxide as an example. Tiling the structural regions simplifies the identification of local modifications in the microstructure. Up to 19 different structures were observed that can be listed and classified into different structural motifs, intergrowth, channels, interstitial regions, and inclinations. The observed defects are expressed by the rearrangement of the {(Mo)­Mo₅O₂₇} building blocks, exhibit different sizes, penetrate the bulk, and can form decoupled surface regions that partially cover the crystallographic bulk. The evaluation of 31 crystals yields an average defect concentration of 3.3% and indicates the absence of identical particles. We have, for example, observed 54 of these rearranged structures close to the surface of one (Mo,V)­O_x particle (100 × 50 nm²). A detailed analysis of the atomic arrangement at the surface of this particle suggests a surface composition of (Mo₆₁₀V₂₃₀M₇₀)­O_x (M = Mo and/or V). The resulting catalog of motifs reproduces individual fragments of the real structure of a catalyst and can reveal detailed defect–activity correlations that will contribute to a better understanding of heterogeneous catalysis.