Transition chromium nitrides

Chromium nitrides have been prepared by several routes heating of chromium metal in N2, reaction of chromium borides with NH3, and heating of CrCly in gaseous NH3. The two stable nitrides have the composition Cr2N and CrN see Nitrides Transition Metal Solid-state Chemistry). At very high temperatures, both decompose into the constituent elements (CrN, > 1425 °C CryN, >700 °C). CrN is very stable chemically, while CryN dissolves in dilute acid with liberation OfH2. 

Hedegaard ED, Schau-Magnussen M, Bendix J (2011) [Cr(N)(acac)2] a simple chromium nitride complex and its reactivity towards late transition metals. Inorg Chem Commun 14 719-721... 

Further concentration-independent diffusion coefficients were measured for vanadium nitrides [46], hafnium nitrides [88], and chromium carbides, 5TiNi c [89] and 5ZrNi c [90]. Although the metal dilfusivity in transition metal nitrides has not yet been investigated, the activation energy of that process is much higher than for nitrogen diffusion and can be estimated to be of the order of about 8 eV. Tables 3a and 3b summarize some nonmetal diffusivity data of transition metal carbides and nitrides. 

C. Constantin, M. B. Haider, D. Ingram and A. R. Smith Metal/semiconductor Phase Transition in Chromium Nitride(OOl) Grown by Rf-plasma-assisted Molecular-beam Epitaxy ,/Ipp/. Phys. Lett., 85, 6371-6373 (2004)... 

In retrospect, it is ironic to it that when I met Ernst Schumacher in 1969 (he was then Professor at the University of Bern in Switzerland) we did not talk about the experiments he did at Zurich in the same building where I was at that time. Instead, his interest focussed on our work on borazine transition metal compounds and we discussed in some detail whether it would be possible to incorporate metal atoms like chromium or molybdenum between the layers of hexagonal boron nitride (BN) in a similar way as it can be done with graphite. In the course of these discussions I did not mention that, after I had moved to Zurich, we had begun to investigate the reactivity of nickelocene towards both nucleophilic and electrophilic substrates. The reason was that we were still at the beginning, and while we had been able to prepare a series of monocyclopentadienyl nickel complexes from Ni(C5H5)2 and Lewis bases, our attempts to obtain alkyl- or acyl-substituted nickelocenes by the Friedel-Crafts reaction failed. 

Most borides are chemically inert in bulk form, which has led to industrial applications as engineering materials, principally at high temperature. The transition metal borides display a considerable resistance to oxidation in air. A few examples of applications are given here. Titanium and zirconium diborides, alone or in admixture with chromium diboride, can endure temperatures of 1500 to 1700 K without extensive attack. In this case, a surface layer of the parent oxides is formed at a relatively low temperature, which prevents further oxidation up to temperatures where the volatility of boron oxide becomes appreciable. In other cases the oxidation is retarded by the formation of some other type of protective layer, for instance, a chromium borate. This behavior is favorable and in contrast to that of the refractory carbides and nitrides, which form gaseous products (carbon oxides and nitrogen) in air at high temperatures. Boron carbide is less resistant to oxidation than the metallic borides. 

The catalyst preparation procedure starting with the adsorption of a metal hydroxide followed by its reduction in a stream of H2 at 600 C and a further activation step in acetonitrile at 1,000°C was expanded to all transition metals of the first row. Catalysts for O2 reduction were obtained only with Cr, Fe, and Co. Cr203/C, Fe/C, and Co/C were detected after the reduction step in H2, while Co, Fe3C, and a chromium carbide-nitride (Cr6.2 C3 5 N0.3/C) were detected in the catalyst. The nominal loading of all metals was 10 wt%. Catalytic activity decreased as Cr > Fe > Co. Tests in fuel cells indicated that the Cr-based catalyst was not stable, while Fe and Co-based catalysts were stable (see Section 4 for details). 

The metals of the nine early-transition elements, i.e., titanium, zirconium, and hafiiium of Group IV, vanadium, niobium, and tantalum of Group V, and chromium, molybdenum, and tungsten of Group VI, fit the criteria of size and site availability, and form interstitial nitrides.1 1... 

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