Nickel oxalate, decomposition

Kadlec and Rosmusova [1153] believe that both Ni and Co oxalates initially yield product oxide and that the proportion of metal increases with a. Since nickel oxalate decomposes at temperatures 60 K lower than those for CoC204, even a small proportion of Ni2+ markedly increases the rate of decomposition of cobalt oxalate. The effect was attributed to the catalytic properties of the preferentially formed Ni metal. The a—time curves were generally sigmoid and showed only slight deviations in shape with changes in the Ni Co ratio. In the decomposition of a mechanical... 

Likewise, the direct synthesis of [M(PF3)4] (M = Ni, Pd, Pt) complexes has been achieved from the appropriate metal powder (method F), or alternatively under very mild conditions from highly reactive forms of the metal (e.g., Ni) generated either from the decomposition of nickel oxalate or nickel tetracarbonyl or activated by sulfide (method G). 

Non-isothermal kinetic studies [69] of the decomposition of samples of nickel oxalate dihydrate doped with Li and Cr showed no regular pattern of behaviour in the values of the Arrhenius parameters reported for the dehydration. There was evidence that lithium promoted the subsequent decomposition step, but no description of the role of the additive was given. 

Jacobs and Tariq Kureishy [48] identified three stages in the decomposition of nickel oxalate a surface reaction, a period of slow development of nuclei and finally the growth of these nuclei at constant rate during which the interface advances through the bulk of the reactant crystallites. 

Carboxylate decompositions are generally irreversible and it is noteworthy that compensation behaviour, a pattern of behaviour frequently associated with reversible processes, is not a feature of the literature concerned with these reactants. More usually the values for carboxylate decompositions are accepted as approximating to a constant value. For example, seven values of reported for the decomposition of nickel oxalate (134, 135, 136, 150, 150, 168 and 177 kJ mol ) were (with one exception) all within 151 17 kJ mol, which is typical of the reproducibility found in this field. Greater variation may arise when the decomposition is influenced by other reactions, such as prior dehydration [112]. 

Using DTA measurements, Macklen [148] compared the temperatures of onset of decomposition of manganese, iron, cobalt and nickel oxalates for reactions in nitrogen and in air. The reactivities of these salts follows the sequence of ease of cation oxidation [152] (IvP — and from this it was concluded that the first step for reaction in air is cation oxidation followed by rapid breakdown of the oxalate anion. 

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... 

More detailed experiments with copper and nickel oxalates (184) confirmed the extensive decomposition under radiation, although the doses required to reach 80 to 90% metal appeared to be somewhat larger (perhaps by a factor of 2 to 3) than in the previous work (183). This could easily arise from differences in the techniques or in the materials. A preliminary irradiation insufficient to cause appreciable decomposition by itself (ca. 4 x 10 2 ev/gm) reduced the induction period and increased the rate of a subsequent thermal decomposition. There is considerable work on the effects of irradiation upon subsequent thermal decomposition of various solids, although not with catalyst preparation in mind (132h, 185-188). In the case of CaO, irradiation of the parent material, Ca(OH)2, did not affect the area of the oxide, although irradiation of the latter (with copper X-rays) decreased the specific surface for samples calcined (before irradiation) between 400 and 900°. 

At higher gas pressures, a buoyancy correction must be made to allow for the volume occupied by the sample this increases with pressure. The mass of gas displaced is given by MPV/RT, where P and V are pressure and volume of gas of molar mass M, and/ is the gas constant. During reactant decompositions, the volume of the residual sample changes for example, the nickel metal product formed on heating nickel oxalate dihydrate occupies only about 10% the volume of the reactant from which it was formed. Other corrections include the contributions from any gas flow in and around the balance mechanism and the effects of convection that increase with pressure. Some specific issues are discussed in the following text ... 

A further possibility of preparing mixed-salt catalysts is provided by decomposition of complex salts of the type A [BXm],181 where A is a non-noble metal cation and B is a noble metal present in a complex anion an example is the oxalate complex Li2[Cu(Ox)2]. In all cases, highly active catalysts result they are usually more efficient than other catalysts containing the metal in question, and maximum activity is found with a molar ratio of, e.g., nickel oxalate magnesium oxalate = 1 1. 

Dominey DA, Morley H, Young DA (1965) Kinetics of decomposition of nickel oxalate. Trans Faraday Soc 61 1246-1255... 

In both these equations ao is the fractional decomposition which accompanies the initial formation of the nuclei. [The analysis of the kinetics of the thermal decomposition of nickel oxalate is complicated by an initial deceleratory process like that depicted in Fig. 1. Here ao is the decomposition corresponding to the formation of growth nuclei in the bulk in addition to that occurring in the initial process, a, (Fig. 1).] Comparison with experiment showed that Eq. (31) was to be preferred. 

The combination of Eqs. (31) and (28) certainly provided an excellent fit to the nickel oxalate data, but the model used is by no means unique. Dominey et al. ) showed that the a(t) curves during the acceleratory period could also be fitted by a cube law n = 3) succeeded by a square law n = 2). This analysis accounts beautifully for the fractional decomposition curves but leaves us with the problem of Dominey et ah propose that represents the time taken for the nickel atoms formed during the initial surface reaction to diffuse over the surface to face edges and there form growth nuclei. This model is thus the ultimate in slow growth since it involves the formation of growth nuclei by a purely physical process which is not accompanied by chemical decomposition. Thus w = 0, a = ao = in this model,... 

The nickel complex decomposes in three steps ((5.13)-(5.15)). In the first step water is lost endothermically at 124 °C. Thereafter, the anhydrous complex further decomposes to give nickel oxalate monohydrazine. This is observed as an endotherm with a peak temperature of 280 °C. The final step corresponds to the decomposition of nickel oxalate monohydrazine to nickel oxide (NiO). This decomposition is exothermic at 342 °C (Figure 5.7a) ... 

Various active nickel catalysts obtained not via reduction of nickel oxide with hydrogen have been described in the literature. Among these are the catalysts obtained by the decomposition of nickel carbonyl 10 by thermal decomposition of nickel formate or oxalate 11 by treating Ni-Si alloy or, more commonly, Ni-Al alloy with caustic alkali (or with heated water or steam) (Raney Ni) 12 by reducing nickel salts with a more electropositive metal,13 particularly by zinc dust followed by activation with an alkali or acid (Urushibara Ni) 14-16 and by reducing nickel salts with sodium boro-hydride (Ni boride catalyst)17-19 or other reducing agents.20-24... 

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