Spatial Distribution of the Vanadium Atomic Species in MoVTeO and MoVTeNbO Oxide Catalysts as Revealed by High‐Angle Annular Dark‐Field Scanning Transmission Electron Microscopy

Abstract

High‐angle annular dark‐field scanning transmission electron microscopy (STEM–HAADF) was used to characterize a light‐alkane mild‐oxidation catalyst corresponding to two different MoVTe(Nb)O M1 phases. For that purpose a semiautomatic integration routine was developed, which allows the analysis of several hundred atomic columns with a normalization procedure to quantitatively determine the vanadium content of mixed (V, Mo) crystallographic sites with a low statistical variability. The obtained results were compared with data previously obtained on other M1 phases with different compositions. The comparison showed that the V distribution over the sites was not strongly influenced by the presence or absence of niobium, the method of synthesis used, and the nature of the Te or Sb cations in the hexagonal channels, but depended mainly on the total V content of the phases. Extrapolation to low V content enabled a discussion on the possible location of the vanadium active sites.

Site specific and localized structural displacements in open structured multimetallic oxides

“…12 An unequal Te occupancy in the hexagonal channels of orthorhombic (Mo,V,Te)O x has been previously reported. 9,20 Our finding of Te disorder in the hexagonal channels of various synthesized (Mo,V,Te,Nb)O x with different composition shows that these structural irregularities are a general feature in Te-containing molybdenum/vanadium mixed oxides. Although the origin of the observed structural displacement remains unclear, it seems reasonable that the occupancy of Te in the hexagonal channels is influenced by the nature of neighboring metal sites.…”

“…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).…”

“…17,18 They have been the subject of numerous studies that focused on the understanding of their catalytic and structural properties also including operando/ in situ investigations. 7,[9][10][11][12][13]16,[19][20][21][22][23][24][25][26][27][28][29] A crystallographic model is shown in Fig. 1A.…”

Multi-slice frozen phonon simulations of high-angle annular dark field scanning transmission electron microscopy images of the structurally and compositionally complex Mo–V–Nb–Te oxide catalyst

“…In other words, the detailed ordering of the Mo and V along the columns for an identical composition leads to changes in ADF image intensity equivalent to an uncertainty in the composition of ± 3.3 atomic % V. In the worst cases, the uncertainty in the V concentration is equivalent to ± 2 atoms (i.e., ± 6.6% V). Given these results, it seems unlikely that quantitative HAADF STEM of M1 will be able to determine the V content in an atomic column to the precision previously reported [ 21 – 23 ].

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“…Multi-slice image simulations will be required to make quantitative comparisons between HAADF STEM images and atomic column compositions. Recently, Epicier et al [ 21 ] reported on the spatial distribution of V using a combination of HAADF STEM and multi-slice image simulations. They found that their images followed a power-law relationship between mass thickness and image intensity.…”