Nitrates, decomposition

Fig. 5. HREM of enclosed silver particles in CNTs. The metallic particles were obtained by electron irradiation-induced decomposition of introduced silver nitrate. Note that the gases produced by the nitrate decomposition have eroded the innermost layer of the tube.

The comparison with the TPD data showed that the presence of hydrogen did not affect significantly the temperature threshold for nitrate decomposition, but leads to a different product distribution. As suggested by Cant and Patterson [42], the observed product distribution can be ascribed to the occurrence of the following reactions ... 

It is, however, worth of note that over the ternary Pt—Ba/y-Al203 sample, the complete decomposition of stored NO, is achieved at lower temperatures if compared to the binary sample. This indicates that Pt promotes the rate of nitrate decomposition [45], The... 

The TPD experiments were also performed after NO, adsorption at different temperatures, namely 300 and 400°C [44], The temperature threshold for nitrate decomposition was always observed close to the adsorption temperature, thus indicating that the adsorption temperature rules the thermal stability of the NOx adsorbed species. 

The TPD data showed that decomposition of adsorbed nitrates occurs at temperatures very close to that of adsorption. The process was not yet completed at temperatures as high as 600°C. This was in line with literature indications [11], claiming that NO, spillover processes from Ba to Pt (not possible on the physical mixture) could affect the nitrate decomposition process. 

Dining evaporation of an alkaline aqueous solution of the nitrate, decomposition led to gas evolution and a pressure explosion occurred. This was attributed to the use of recovered alkali containing high levels of lead and iron, which were found to catalyse the thermal decomposition of the nitrate. Precautions to prevent recurrence are detailed. 

Recently, it has been reported that a novel calcination procedure relying on nitric oxide gas in lieu of air also results in smaller cobalt crystallites over silica supports.15 17 The idea is to use a less oxidative gas to prevent rapid decomposition of the nitrate precursor during thermal nitrate decomposition, which has been observed when 02 is present.17 As a result, the mobility of the precursor on the oxide carrier surface is hindered, resulting in a smaller average Co oxide cluster... 

Based on this scheme, different proposals have been advanced to explain the O release, that is, the nitrate decomposition. It has been suggested that the release is provoked by the heat generated upon the reducing switch (thermal release) 1113], by the decrease in the gas-phase oxygen concentration that destabilizes the stored nitrates [114], by spillover and reduction of NO2 at the reduced Pt sites or by the establishment of a net reducing environment which decreases the stability of nitrates [114—120]. 

To darify better the role of Pt in the reduction mechanism, a physical mixture of the binary Pt/Y-Al2O3 (1 100 w/w) and Ba/Y-Al2O3 (20 100 w/w) samples was prepared and tested [117]. Also in this case NO was stored at 350 °C upon adsorption of NO in the presence of excess O2 [102], then the stability/reactivity of stored nitrates was investigated by means of TPD and H2 TPSR. The TPD data showed that decomposition of adsorbed nitrates occurs at temperatures very dose to that of adsorption the presence of H2 (TPSR run) does not decrease significantly the temperature threshold for nitrate decomposition (which is still observed near 350 °C), but instead provokes the reduction of the evolved NO to NH3 and N2. Hence the previously invoked Pt-catalyzed route is active only when Pt and Ba are dispersed over the same particle of the support. 

Spent acid from nitroglycerine manufacture is not used to prepare nitrating mixture since it contains too much organic matter with which it is undesirable to contaminate the nitration. Decomposition of these substances during nitration could make the process difficult to control and might lead to accidents. 

Ammonium nitrate prepared from ammonia obtained by the dry distillation of coal should not be used as component of any explosive material because of the ammonium thiocyanate and pyridine present in it (the latter as nitrate). When the ammonia liquor from dry distillation of coal was the sole source af ammonia and ammonium nitrate, decomposition of mixtures containing ammonium nitrate with TNT (amatols), was brought about at the melting point TNT reacted with ammonium thiocyanate or with pyridine nitrate and evolved gaseous products. Minute traces of these impurities were sufficient to cause abundant gas evolution to develop during the fusion, pouring, and cooling of amatol. 

F.H. Pollard, R.M.H. Wyatt, H.S.B. Marshall Retarding (Inhibiting) and Sensitizing Effects of Nitrogen Dioxide in Alkyl Nitrate Decompositions, Nature 165 (1950) 564-565. 

Thanks to Dr. Elizabeth Mayfield of the Mathematics Department of Hood College, Frederick, MD, who kindly calculated the partial pressures for nitrate decomposition in Chapter 5. 

Although N203 has been reported as a product of nitrate decomposition, it is unstable with respect to dissociation into N02 and NO above 400 K it could therefore exist in equilibrium mixtures only near ambient temperatures. 

Since the nitrite of a given metal is generally much less stable than the nitrate, the former can appear only as an unstable intermediate in the decomposition of the nitrate. This is particularly true for covalent nitrates. In the case of ionic nitrates, however, both salts may be more or less equally unstable over some temperature range, so that the decomposition reactions can become quite complex. This is particularly so since the salts may be oxidized or reduced by the gaseous decomposition products. For example, N02 produced by the decomposition of the nitrate may oxidize the nitrite ion also formed back to the nitrate. Since the experimental arrangement usually determines the gas-phase composition, reports by different authors frequently conflict. In such cases the common features have been emphasized in this chapter. A consequence of the complexity of most nitrate decompositions is that kinetic studies have usually been restricted to identifying the reactions. Even when rate constants and activation energies are reported it is frequently not clear with which particular reactions they are identified. 

All the partial pressures calculated from the equilibrium constants assume unit activity for the condensed-phase components. This assumption is good when they are solid. Above the melting points of the salts, however, continued decomposition of the salt will result in a solution containing dissolved oxide and the partial pressures will depend on the melt composition, and will therefore change as the decomposition proceeds. Because of the form of Kt, the partial pressure calculation will be worst for small oxide concentrations. An examination of the various tables shows that 02 and NO are the major products of nitrate decomposition, the concentration of N02 being rather minor. This results from the fact that the equilibrium 2 N02 = 2 NO + 02 lies to the right for low pressures. 

The second stage is the decomposition of the nitrate. However, this occurs in the range 320 to 450°C and seems to proceed directly to PbO, a result which is inconsistent with other studies of nitrate decomposition (see below). 

The kinetics of the nitrate decompositions of Nd, Dy, and Yb were found to be first order with activation energies ranging from 23 to 46 kJ higher than those of the corresponding nitrites.98... 

The mechanisms of n- and iso-propyl nitrate decomposition flames appear to be the same as that of the ethyl nitrate flame, the main attack on the nitrate ester being by nitrogen dioxide and nitrous acid [136]. 

In fused sodium nitrate-potassium nitrate, decomposition of chlorate and bromate is thought to occur through acid-base reactions fundamentally similar to some which have been proposed for aqueous solutions . The acid used was dichromate and the processes appear to involve the net reaction... 

We used HZSM-5 zeolites (Si02/AI203 = 40 - 120) and H[Ga]ZSM-5 zeolites (Si02/AI203 = 60) prepared by decationization of the Na-form with a 1 N aqueous solution of HCI. The preliminary activation of all catalyst samples was carried out at temperatures of 820 - 1120 K either in an Ar flow or in an air flow for 6 h. For comparison, we prepared the HZSM-5 zeolite modified by 2 wt % Cu (wet impregnation with a 1 M nitrate solution with further nitrate decomposition at 820 K in air) and tested this sample in the fluorobenzene oxidation by nitrous oxide. 

C03O4 nitrate decomposition at various temperatures 0.001-0.2 mol dm KNO3 ... 

Despite its brilliant results, it seems unlikely that the Solutia process can become a major source of phenol. Nitrous oxide availability is quite limited and its production on-purpose (by the conventional ammonium nitrate decomposition, which enables nitrous oxide of high purity to be produced for medical anesthetic applications, or even by selective oxidation of ammonia) would result too expensive. Therefore, the only reasonable scenario to exploit the Solutia process is its implementation close to adipic acid plants, where nitrous oxide is co-produced by the nitric oxidation of cyclohexanol-cyclohexanone mixtures and where it could be used to produce phenol instead of being disposed of However, the stoichiometry of the process is such that a relatively small phenol plant would require a world-scale adipic acid plant for its nitrous oxide supply. In fact, a pilot plant has been operated using this technology, but its commercialization has been postponed. 

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