Structural Complexity in Heterogeneous Catalysis: Cataloging Local Nanostructures

Abstract

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

Site specific and localized structural displacements in open structured multimetallic oxides

“…further highlights the observed structural irregularities in the picometer regime and the huge variety of local structures that can be present in such open structured oxides. 2 However, a direct correlation of the qualitatively described local displacements with the catalytic performance is not possible, due to the involved large parameter space. Prospectively, further theoretical work is needed to convert these local structural descriptors into a detailed understanding of how they impact the performance of a heterogeneous catalyst or of any functional property.…”

“…1 However, detailed defect-activity correlations have not yet been established due to difficulties in capturing, localizing, and quantifying these structural deviations. 2,3 Modern scanning transmission electron microscopes (STEM), equipped with annular detectors, have become powerful tools to visualize the geometric inconsistencies of surfaces and bulk in real space and on the atomic scale. 2,4 In addition, these modern TEMs exhibit more robust columns enabling local structural analyses with 10 pm accuracy.…”

“…2,3 Modern scanning transmission electron microscopes (STEM), equipped with annular detectors, have become powerful tools to visualize the geometric inconsistencies of surfaces and bulk in real space and on the atomic scale. 2,4 In addition, these modern TEMs exhibit more robust columns enabling local structural analyses with 10 pm accuracy. 5,6 Among the huge variety of functional materials, high-performance catalysts, such as multimetallic molybdenum-and vanadium-based oxides, which can crystallize in an open crystal structure, are highly susceptible to geometric and compositional defects, such as different cation occupancies, polyhedral distortions or structural reconnectivities.…”

“…5,6 Among the huge variety of functional materials, high-performance catalysts, such as multimetallic molybdenum-and vanadium-based oxides, which can crystallize in an open crystal structure, are highly susceptible to geometric and compositional defects, such as different cation occupancies, polyhedral distortions or structural reconnectivities. 2,4,[7][8][9] Prominent oxides of this family have an orthorhombic unit cell (space group: Pba2) and crystallize in the so-called M1 structure. The unit cell of the M1 structure is composed of 44 metal atoms that are distributed over 13 crystallographic different metal sites (S).…”

“…31 Detailed beam damage studies and their impact on the structure have been addressed elsewhere for similar M1 samples. 2,23 Structural changes that arise due to the electron beam are expressed by an immediate amorphization that spreads from the surface to the bulk or removal of Te from the channel sites. Our studies suggest that under the applied imaging conditions (Mo,V,Te,Nb)O x can be at least 20 minutes exposed continuously to the electron beam without noticeable morphological changes ( Fig.…”