Nitrates, transition-metal, decompositions

The decomposition rates of several glycol nitrates and glyceryl trinitrate are greatly enhanced by the presence of transition metal oxides or chelates. 

Decomposition of a mixture of nitrates is often a convenient method for the preparation of mixed transition metal oxides. The success of the method depends upon the fact that typically, the decomposition of the nitrates results in an intimate oxide mixture that readily reacts at higher temperatures to form a homogeneous, complex oxide. Oftentimes nitrate mixtures can be formed as precnrsors by dissolving a stoichiometric mixture of the oxides or carbonates in nitric acid and evaporating to dryness. This method is one of the options used to prepare the superconducting YBa2Cn30y and isotypes. 

The gas phase reaction proceeds very much as described for nickel carbonyl, but the product does not contain the nitrite group (10). A smoke is formed immediately the gases come into contact, but the analysis and infrared spectrum of the solid formed show it to be the oxide-nitrate Fe0(N03). It seems likely that initial reaction involves the NO2 radical, and an iron nitrite such as Fe(N02)3 may be produced initially. The oxidation-reduction properties of the ferric and nitrite ions may render them incompatible Fe0(N03) would then be left as a decomposition product. So little is known about transition metal nitrites that this must remain conjecture at present, but it may be relevant to recall that it has not yet been possible to isolate pure samples of Fe(N03)3, A1(N03)3, or Cr(N03)3. 

Several hydrated transition metal nitrates (Co, Cu, Cr, Zn and Ni) undergo aqueous fusion [62] prior to hydrolysis which yields basic salts. Vratny and Gugliotta [63] reported sigmoid -time curves for the decomposition of Pb(N03)2 between 523 and 708 K, and concluded that the cation was not further oxidized. Margulis et al. [64] identified both dissociation and autoxidation steps during the decomposition of lead nitrate. The first step involved melting of the eutectic formed from the reactant and the initial product, 2Pb0.Pb(N03)2. 

The pulse radiolysis study also provides evidence that the route of nitrate production from ONOO is not via radical recombination within the solvent cage. Heterolytic decomposition to give an NOrlike species requires only —12 kcal/mol and is readily catalyzed by transition metals (Scheme 3, upper pathway) (Beckman etal., 1992). Later we will examine how ONOO" might be generating hydroxylating and nitrating species in a metal independent manner. 

The thermal decomposition of ammonium salts is an important field of study. Kinetic parameters for the decomposition of NH4NO3 have been determined, it being found that the addition of small amounts of NH4CI reduces both the thermal stability and the initial decomposition temperature of the nitrate. Confirmation of a previously proposed mechanism for the chloride-transition-metal-ion synergistically catalysed thermal decomposition of NH4NO3 has come from visible absorption spectroscopy. Ammine, nitrato-, and chloro-complexes of Cu and Ni° and chloro-complexes of Co i were detected in fused NH4NO3. 

Another solution-based method for material synthesis is based on the freezedrying of frozen aqueous solutions of components, followed by thermal decomposition of freeze-dried precursors [10]. The nitrates of a number of cations that are readily available and easily soluble in water are also used in this method. However, freezing of the nitrate solutions of transition metals and rare earth elements or their soluble complex compounds is accompanied by their vitrification [12]. Freeze-... 

However, if transition metal exchanged zeolites are active materials in NH3-SCR, N2O emission can be also observed. N2O emission constitutes one of the main drawbacks of this system. For instance, Wilken et al. [112] reports a maximum N2O production at 200 °C over Cu-Beta zeolite. Mechanism of N2O emission is proposed to proceed through the decomposition of ammonium nitrates (reaction 19.28) ... 

Transition metal nitrate hydrates are industrially favored precursors for the preparation of supported metal (oxide) catalysts because of their high solubility and facile nitrate removal. The final phase and particle size depend on the experimental conditions, as reported for both supported and unsupported metal nitrates [1-3]. Several authors report that decreasing the water partial pressure during the decomposition of unsupported nickel nitrate hexahydrate, via vacuum or a high gas flow, increases the final NiO surface area [3, 4], The low water partial pressure results in dehydration of the nickel nitrate hydrate to anhydrous nickel nitrate followed by decomposition to NiO. Decomposition at higher particle pressures, however, occitrred through the formation of intermediate nickel hydroxynitrates prior to decomposition to NiO. Thus, NiO obtained via intermediate nickel hydroxynitrate species showed a poorer siuface area (1 m /g) compared to NiO obtained via anhydrous nickel nitrate species (10 mVg) [4]. 

The Ca(N03)2 decomposition is catalyzed by metal oxides. Comparative studies [61] of the relative activities of 16 selected metal oxides in promoting nitrate ion breakdown in Ca(N03)2 led to the conclusion that all of the reactions were accompanied by melting. Most of the mixtures containing oxides of metals of the first transition period underwent reaction at temperatures below or close to the melting point of Ca(N03)2, 836 K, and well below the decomposition temperature, 949 K. Cations with only partially filled d shells were the most active, Mn and Fe, followed by Cr " and Co ". It was concluded that these reactions were promoted by the electron transfer steps ... 

The superconducting oxides La gSr 2Cu04 and I YCujOy were prepared by decomposition of mixed metal nitrates. Thermogravimetric analysis and electron spin resonance measurements indicated the presence of Cu(I) and Cu(III) in the Lai.8Sr.2 u 4 phase. The compound I YC C prepared from the nitrates and subjected to an oxygen anneal at 425°C gave a sharp superconducting transition at 92 K. The phase was stoichiometric but readily decomposed when kept in contact with moist air. 

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