Oxalates, metal, decompositions

There have been many extensive and intensive studies of metal oxalate decompositions. Boldyrev et al. [1015] have suggested that these may be classified according to the three principle reactions... 

The major products from most metal oxalate decompositions can be predicted from thermodynamic data [46,47] (Chapter 2). Interpretation of observations must allow for the possibility that the identifiable sohd phases may not be those initially formed, but arise as stable products of a secondary process. Secondary reactions may involve adsorption-desorption steps at the surfaces of finely-divided, reactive and perhaps non-stoichiometric solids. The composition of product gases may also vary within the mass of reactant, through chemical interactions between primary products. 

The initial or rate limiting step for anion breakdown in metal oxalate decompositions has been identified as either the rupture of the C - C bond [4], or electron transfer at a M - O bond [5], This may be an oversimplification, because different controls may operate for different constituent cations. The decomposition of nickel oxalate is probably promoted by the metallic product [68] (the activity of which may be decreased by deposited carbon, compare with nickel malonate mentioned above [65]). No catalytically-active metal product is formed on breakdown of oxalates of the more electropositive elements. Instead, they yield oxide or carbonate and reactions may include secondary processes [27]. There is, however, evidence that the decompositions of transition metal oxalates may be accompanied by electron transfers. The decomposition of copper(II) oxalate [69] (Cu - Cu - Cu°) was not catalytically promoted by the metal and the acceleratory behaviour was ascribed to progressive melting. Similarly, iron(III) oxalate decomposition [61,70] was accompanied by cation reduction (Fe " - Fe ). In contrast, evidence was obtained that the reaction of MnC204 was accompanied by the intervention of Mn believed to be active in anion breakdown [71]. These observations confirm the participation of electron transfer steps in breakdown of the oxalate ion, but other controls influence the overall behaviour. Dollimore has discussed [72] the literature concerned with oxalate pyrolyses, including possible bond rupture steps involved in the decomposition mechanisms... 

After the loss of the water of hydration, the curve exhibited a horizontal mass level from 250-360°C, which corresponded to the composition for anhydrous metal oxalates. Decomposition of the three oxalates then took place simultaneously, the process being completed al about 500°G The anhydrous metal carbonates were then stable from about SOO-eKTG followed by strontium carbonate, which also began to decompose in this range and was completely decomposed at 1100°C, at which temperature barium carbonate began to decompose. 

The products of decomposition of metal carboxylates vary to some extent with the constituent cation and the final residue is usually either the metal or an oxide, occasionally the carbide and sometimes some elemental carbon deposit. Dollimore et al. [94] have described the use of Ellingham diagrams for the prediction of the composition of the solid products of oxalate decompositions. The complete characterization of residual material can be difficult, however, since the solids may be finely divided, pyrophoric [1010], metallic and amorphous to X-rays. 

Despite the volume of work concerned with metal-catalyzed decomposition of diazo compounds and carbenoid reactions 28>, relatively little work has been reported on the metal-catalyzed decomposition of sulphonyl azides. Some metal-aryl nitrene complexes have recently been isolated 29 31>. Nitro compounds have also been reduced to nitrene metal complexes with transition metal oxalates 32K... 

The tendency for explosive decomposition of heavy metal oxalates is related to the value of the heat of decomposition. Individually entries are ... 

The more stable iron(II) oxalate decomposed [58] between 596 and 638 K to yield FeO, as the initial solid product, which then disproportionated to Fe and Fej04, together with COj and CO (in a 3 2 ratio), a - time data again fitted the Avrami -Erofeev equation (n = 2) with E = 175 kJ mol . The reaction was proposed to proceed by a nucleation and growth process without melting. The behaviour was comparable with the decompositions of other metal oxalates. 

Transition metal oxalates. From studies of the decompositions of six oxalates (Mn, Fe, Co, Ni, Cu and Zn) Kornienko [146] concluded that the initial reaction product is the oxide and that for four of these salts (Fe, Co, Ni and Cu) the oxides are subseqently reduced to the metal by carbon monoxide. The values of the activation energies for these decompositions are comparable (about 176 kJ mol ) and stability is attributed to the polarizing power of the cation. Extensions of this approach to include salts of other cations have been discussed by Dollimore et al. [47] and Kadlec and Danes [55]. 

Thermogravimetric analysis has been widely used to study the thermal decomposition of oxy-salts, such as metal oxalates and metal sulfates. Dollimore, Griffiths and Nicholson have reported TGA data for a wide range of metal oxalates. In an atmosphere of air, these all decompose in three stages, similar to the thermal decomposition of calcium oxalate monohydrate. 

From the curve, it can be seen that the decomposition processes are going on independently of one another. Between 100 and 250CC, the water of hydration is evolved since each ion forms a metal oxalate l-hydrate. According to the curves of individual compounds, the water contents are lost in the... 

From the mass-loss curve, then, the following data are obtained D, mass of dry precipitate at 100°C , mass of water of hydration F, mass of carbon monoxide formed by the decomposition of the anhydrous metal oxalates G, mass of carbon dioxide formed by the decomposition of calcium carbonate and L, the mass of carbon dioxide formed by the decomposition of strontium carbonate. From these data, the amounts of calcium, C, strontium, S, and barium, By can be calculated from... 

Introduction The thermal decompositions of metal oxalates have been studied for more than 130 years. The first paper on the decomposition of PbC204 was published in 1870 [115]. The kinetics of these reactions have been considered in numerous reviews [45, 99, 116-121]. The decompositions of the oxalates of some metals (Ag, Ni, and Pb) have often been used as model reactions. Nevertheless, many features of these processes remained unclear until recently. 

Thermogravimetric curves for solid K2[Pd(C204)2],3H20 and other transition-metal oxalates indicate that the thermal stability of the anhydrous complexes decreases with increase in electron affinity of the central metal ion. AH values were obtained for both dehydration and decomposition. Subsequent studies showed carbon dioxide as the only gaseous product, the decomposition occurring via electron transfer from a 304 ligand to the central palladium. ... 

Reactions of materials in the solid state are strongly influenced by an enormous range of variables, and a complete treatment of this vast subject is beyond the scope of this book or, in fact, any single volume. One factor that becomes apparent immediately when dealing with soHd state reactions is that the rate can generally not be expressed in terms of concentrations. We can illustrate this by means of the following example. The first step in the decomposition of metal oxalates when they are heated normally leads to the loss of carbon monoxide and the formation of a carbonate. In the case of NiC204, the process can be shown as... 

The mechanism of the process is that the polymer reactive centers promote the metal nucleation and aggregation, after which the thermolysis occurs and the metal-containing substance is redistributed. The maximum amount of copper being introduced in PS through a common solvent is about 10%. At the same time, the polymer presence increases the temperature of cadmium trihydrate-oxalate decomposition [97], and the decay products increase the initial temperature of PETF intensive destruction. The copper formate thermal decomposition in the highly dispersed PETF presence allows us to produce a metallopolymeric composition (20-34% of copper) where the NP size distribution is maximal at 4nm, without any chemical interaction between the components. 

The thermal decompositions of weddelite, CaC204.2H20, and other metal oxalates are well known [35-39]. CaC204.2H20 is often used for teaching purposes [40], both as an example and as a standard in thermal analysis. The decomposition of weddelite shows three major mass-loss events (Figure 10). 

Amongst the studies of oxysalt decomposition has been one on oxalates. A number of metal oxalates were subjected to a thermogravimetric run in air and nitrogen up to 1000 °C. The object was to see if there were any differences in behaviour between metals, to determine the end product, to see if the atmosphere made any difference and to check the experimental mass losses against the theoretical ones to confirm the reaction mechanism. The shapes of the curves produced fell into 5 types as shown in Figure 7. 

We will take the example of decomposition of a metallic oxalate into a metal... 

Microwave-assisted route for the synthesis of nanomaterials has gained importance in the field of synthetic technology because of its faster, cleaner and cost effectiveness than the other conventional and wet chemical methods for the preparation of metal oxide nanoparticles. Synthesis of metal oxide nanoparticles, for example, y-Fe Oj, NiO, ZnO, CuO, Co-y-FejOj was carried out by microwave-assisted route through the thermal decomposition of their respective metal oxalate precursors employing polyvinyl alcohol as a fuel (Lagashetty et al., 2007). 

Thermal analysis data of metal hydrazine carboxylates, given in Table 4.2, reveal that the compounds decompose exothermically. The two exotherms observed in all cases are due to the decomposition of hydrazine carboxylate complex. These complexes decompose to the corresponding metal oxalate first, and later to form the carbonate salt. Formation of the oxalate intermediate is confirmed by interrupting the DTA... 

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