Topotactic transformation

In acid media (pH 2) magnetite crystals ca. 10 nm across transform topotactically to maghemite via an adsorption reaction which traps mobile electrons from the bulk material and reduces interfacial Fe the Fe ions that form are selectively leached into solution (Jolivet Tronc, 1988). Electron delocalization also induces ferrihydrite in contact with small magnetite particles to transform into a spinel layer (Belleville etal., 1992). 

X-Ray and electron diffraction measurements have been most usually used to characterize the phases present in any reactant mixture, and provide a means of identification of solid reactants, intermediates and products. In addition to such qualitative analyses, the method can also be used quantitatively, with suitable systems, to determine the amounts of particular solids present [111], changes in lattice parameters during reaction, topotactical relationships between reactants and products, the presence of finely divided or strained material, crystallographic transformations, etc. 

Shao-Hom Y., Hackney S. A., Comilsen B. C., Structural characterization of heat-treated electrolytic manganese dioxide and topotactic transformation of discharge products in the Li-Mn02 cells, J. Electrochem. Society, (1997) 144, 3147-3153. 

Decomposition of akaganeite starts at 150 °C and complete conversion to hematite is achieved at ca. 500 °C. This is not a topotactic transformation it involves a complete breakdown of the bcc anion packing of akaganeite followed by reconstruction of the hep anion array of hematite. Initially, the product is in the form of elongated, porous... 

Figure 3.22. Dynamic electron diffraction (ED) image of the topotactic transformation of the VHPO precursor to active VPO catalyst in N2 (a) (010) VHPO at room temperature (b) physical mixture of VHPO and VPO at 425 °C (c) final VPO in the (010) active plane and (d) VPO microcrystals (V) and cracks (arrowed) on the precursor surface.

Figure 3.23. Model for topotactic transformation of the precnrsor to the active catalyst (a) face-sharing VOe octahedra and H-phosphate tetrahedra (H atoms are shaded) (b) removal of bonded water (resulting in VO5 which rotate, and transfer of H-atoms from H-phosphates) and (c) final reconnected VPO after removal of water.

SEM micrographs (Figure 13.1(a,b) illustrate the different morphology of Mo2N-A and Mo2N-B powders. The higher Sg in the platelets is due to the development of microporosity during the topotactic transformation.7... 

High-surface-area MgO is usually prepared by decomposition of brucite, Mg(OH)2, at 520-550 K in vacuo. The hexagonal platelets of brucite are topotactically transformed into linear aggregates of cubes preferentially... 

The TPR method has now been applied to TlN O [52], VN [23], and NbC [53]. Although the TPR method produces high surface area materials, the pore structure of these is usually not controllable. Often, the pores are in the micropore regime (< 3 nm). However, a number of the solid state transformations that lead to carbides and nitrides are topotactic and exhibit pseudomorphism (retention of external particle size and shape) This provides a possible means of engineering the pore structure by preparing oxide precursors with the desired external morphology. 

Two types of transformations can be very broadly distinguished. The first is the formation of a solid solution, in which solute atoms are inserted into vacancies (lattice sites or interstitial sites) or substitute for a solvent atom on a particular sublattice. Many types of synthetic processes can result in this type of transformation, including ion-exchange reactions, intercalation reactions, alloy solidification processes, and the high-temperature ceramic method. Of these, ion exchange, intercalation, and other so-called soft chemical (chimie douce) reactions produce no stmctural changes except, perhaps, an expansion or contraction of the lattice to accommodate the new species. They are said to be under topotactic, or topochemical, control. 

Control of fhe precursor phase is an imporfanf aspecf in fhe formafion of an active catalyst. The topotactic transformation to the pyrophosphate phase leaves many of fhe feafures of fhe precursor unchanged. Controlling the morphology and preferentially exposing the desired crystal faces have the potential to increase the activity of fhe catalyst or its selectivity to MA. 

Vanadyl pyrophosphate is widely considered to play an important catalytic role in the oxidation of -butane to MA, specifically the (100) face (Figure 18b), which is retained from the topotactic transformation (6,43,84—86) of the catalyst precursor phase (Figure 18a). Furthermore, this active phase has been reported to be an efficient catalyst for the oxyfimctionalization of light paraffins (a) for the oxidation of ethane to acetic acid (3,87), (b) for the oxidation and ammoxidation of propane to acrylic acid (88) and acrylonitrile (89,90), respectively, and (c) for the oxidation of n-pentane to maleic and phthalic anhydrides (90-102). 

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