Atomic-level imaging of Mo-V-O complex oxide phase intergrowth, grain boundaries, and defects using HAADF-STEM

Abstract

In this work, we structurally characterize defects, grain boundaries, and intergrowth phases observed in various Mo-V-O materials using aberration-corrected high-angle annular dark-field (HAADF) imaging within a scanning transmission electron microscope (STEM). Atomic-level imaging of these preparations clearly shows domains of the orthorhombic M1-type phase intergrown with the trigonal phase. Idealized models based on HAADF imaging indicate that atomic-scale registry at the domain boundaries can be seamless with several possible trigonal and M1-type unit cell orientation relationships. The alignment of two trigonal domains separated by an M1-type domain or vice versa can be predicted by identifying the number of rows/columns of parallel symmetry operators. Intergrowths of the M1 catalyst with the M2 phase or with the Mo 5 O 14 -type phase have not been observed. The resolution enhancements provided by aberration-correction have provided new insights to the understanding of phase equilibria of complex Mo-V-O materials. This study exemplifies the utility of STEM for the characterization of local structure at crystalline phase boundaries.propane | oxidation | catalyst | acrylonitrile | bronze S elective catalytic oxidation of light hydrocarbons is of tremendous commercial importance for the production of a variety of key industrial organic chemicals and intermediates. Existing processes, valued in the billions of US dollars per annum, are predominantly based on oxidation of olefin feeds; however, there are significant economic and environmental benefits to replacement by more energy-and carbon-efficient paraffin-fed processes. Pursuit of active and selective catalysts for use in such replacement processes is a currently very active area of research. Mixed-metal oxides in the system Mo-V-Nb-Te-O, when prepared under mildly reducing conditions, may consist of one or more of several network "bronze" structures with mixed valences for Mo and V (1-4). Materials from this system show promise as catalysts for selective oxidation of ethane to ethylene, propane to acrylic acid, and, in the presence of cofed ammonia, propane ammoxidation to acrylonitrile. One phase in particular, commonly designated as "M1," has been found to be essential as a catalyst component for selective paraffin oxidation, and under some conditions seems to be promoted by coexistence with another phase designated as "M2" (1-18). Our interest in chemical and structural inhomogeneities observed in this system is driven by the apparent need for composite M1/M2 phase coexistence to realize the most active, selective, and stable catalysts.The M1 phase has an orthorhombic unit cell with a structure comprised of a network of interconnected pentagonal fMo 6 O 21 g rings joined together by linking octahedra, resulting in the formation of nanoscale hexagonal and heptagonal channels (4,14,(19)(20)(21). The M2 phase is belongs to the hexagonal tungsten bronze family, but exhibits a small orthorhombic distortion (4). In its pure form, the M2 phase is efficient in ...

Catalytic Consequences of a Revised Distribution of Key Elements at the Active Centers of the M1 Phase of the MoVNbTeOx System

“…We have enlarged an HAADF-STEM image of M1 in [001] view, taken from the work of Pyrz et al [30], as shown in Fig. 11.…”

Aberration-corrected scanning transmission electron microscopy for atomic-resolution studies of functional oxides

“…The application of such quantitative approaches to STEM images has led in recent years to a flurry of work in which atom positions are now routinely measured with picometre precision and in which we have moved beyond simple qualitative matching of atomic structures to images into a new paradigm where structures are now determined quantitatively from STEM images. 17,[76][77][78][79][80][81][82][83][84][85][86][87] Probably, the first published atomic structure quantification of a functional oxide using HAADF-STEM was carried out by Pyrz et al on Mo-V-Nb-Te-O catalyst materials. 76 The issue at stake was that, while these materials are promising catalysts for selective oxidation in organic processes, it is difficult to have perfect confidence in Rietveld refinements of the structure from diffraction data because of the sheer number of possible adjustable parameters in such a complex structure.…”

“…Further work by this group examined other structures in the doped Mo-V-O system and then used the STEM results to challenge previous assumptions about structures determined using diffraction techniques alone. 79 While STEM is a very valuable addition to the arsenal of techniques for studying complex bulk crystal structures, it really comes into its own when studying structures that are only present at the nanoscale. One such situation is 'incommensurate' structures occurring close to a phase boundary, such as that between the rhombohedral ferroelectric and orthorhombic antiferroelectric phases in Zr-rich Pb(Zr,Ti)O 3 .…”

Building Block Synthesis of Crystalline Mo–V-based Oxides: Selective Oxidation Catalysts

“…The structure of the trigonal Mo _ V _ oxide was further confirmed by analyzing the powder XRD pattern using the Rietveld analysis and HAADF-STEM imaging 13), 14) .…”