Topotactic synthesis of vanadium nitride solid foams
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Vanadium nitride has been synthesized with a surface area of 120 m2 g"1 by temperature programmed nitridation of a foam-like vanadium oxide (35 m2 g"1), precipitated from vanadate solutions. The nitridation reaction was established to be topotactic and pseudomorphous by x-ray powder diffraction and scanning electron microscopy. The crystallographic relationship between the nitride and oxide was {200}//{001}. The effect of precursor geometry on the product size and shape was investigated by employing vanadium oxide solids of different morphologies.
I. INTRODUCTION Transition metal carbides and nitrides have been studied extensively for catalytic1^5 and ceramic6"8 applications. Many of these uses rely upon production of fine powders.9'10 For example, particles of small sizes yield ceramics of greater densification and higher ductility,11-12 while high surface areas provide good contact with a reactive fluid phase in catalysis. The particle morphology of metal carbides or nitrides is difficult to control by direct synthesis, but can be achieved through phase transformation using precursors of desired shapes and dimensions. When external particle geometry and size are retained, transformations are said to be pseudomorphous. If the atomic density is higher in the product, the solid develops voids that give rise to large specific surface areas. The solid-state reactions can also be topotactic. In the latter case, a crystallographic relationship is maintained between precursors and products throughout their bulk,13 with typically one or more crystallographic planes remaining parallel to the original ones. These changes can take place by preserving one-dimensional structural elements, as in the dehydration of gypsum fibers,14 or two-dimensional layers, as in the reduction of MOO3 to MOO2 by hydrogen,15 where the basal plane of the product is parallel to that of the reactant. In general, anisotropic solids tend to conserve one- or two-dimensional elements of structure, whereas layered solids preserve sheets in two dimensions, such as in the dehydration of Mg(OH) 2 , or MoO 3 • 2H 2 O. 16 These reactions are relatively slow and occur at moderate conditions without significant alteration of structure. The structure may be conserved in three dimensions, as in the carburization of Mo 2 N to MoCi_x by a CH4/H2 mixture, where carbon replaces interstitial nitrogen without perturbing the metal atoms in the lattice or causing sintering.13 Since these reactions take 1450 http://journals.cambridge.org
J. Mater. Res., Vol. 8, No. 6, Jun 1993
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place with limited mobility of atoms, the structures change only slightly. Topotaxy, thus, allows synthesis of compounds that are difficult to obtain by equilibrium processes.17 In some cases topotactic and pseudomorphic changes can take place in the same sample. For example, in the nitridation of M0O3 by NH 3 , 18 the Mo atoms undergo rearrangement to form Mo 2 N, with the crystallographic relationship being {100} Mo 2 N//{010} MOO3. In the same samples voids are formed
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