Oxalates, cobalt-nickel, decompositions

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... 

The r-time curves for the decomposition of anhydrous cobalt oxalate (570 to 590 K) were [59] sigmoid, following an initial deceleratory process to a about 0.02. The kinetic behaviour was, however, influenced by the temperature of dehydration. For salt pretreated at 420 K, the exponential acceleratory process extended to flr= 0.5 and was followed by an approximately constant reaction rate to a = 0.92, the slope of which was almost independent of temperature. In contrast, the decomposition of salt previously dehydrated at 470 K was best described by the Prout-Tompkins equation (0.24 < a< 0.97) with 7 = 165 kJ mol . This difference in behaviour was attributed to differences in reactant texture. Decomposition of the highly porous material obtained from low temperature dehydration was believed to proceed outwards from internal pores, and inwards from external surfaces in a region of highly strained lattice. This geometry results in zero-order kinetic behaviour. Dehydration at 470 K, however, yielded non-porous material in which the strain had been relieved and the decomposition behaviour was broadly comparable with that of the nickel salt. Kadlec and Danes [55] also obtained sigmoid ar-time curves which fitted the Avrami-Erofeev equation with n = 2.4 and = 184 kJ mol" . The kinetic behaviour of cobalt oxalate [60] may be influenced by the disposition of the sample in the reaction vessel. 

On decomposition, cobalt oxalate shows a greater tendency than the nickel salt to form oxide. In vacuum, or in an inert atmosphere, reaction proceeds in two steps [61,62] ... 

The thermal decompositions of nickel(II)-cobalt(II) oxalate solid solutions were studied using TG and TM [103], A series of the mixed binary Ni(II)-Co(II) oxalate samples was prepared at 25% (atom) intervals across the system. Physical mixtures were also prepared by mixing the pure end members. The DTG and DTM curves showed that the decomposition proceeds to completion in two overlapping stages. The kinetics of the individual steps were not studied. From the DTG curves, the authors stated that the physical mixtures behaved as individual oxalates, while the coprecipitate decomposed as a single entity. The TM curves showed that the products formed from the physical mixture and the coprecipitate were distinctly different. The magnetic behaviour of the product from the coprecipitate was consistent with the behaviour predicted for a Ni-Co alloy, but the products from the physically mixed oxalate do not show the transition temperature predicted for an alloy. The kinetics of decomposition of iron-nickel mixed oxalates have been studied by Doremieux et al. [104]. 

During the decomposition of Ni-Co mixed oxalates the production of nickel metal accelerates reaction, because the nickel constituent reacts about 60 K lower than cobalt oxalate (Sections 16.4.2. and 16.4.4. above). The significance of the relative yields of metal and oxide in the decompositions of the mixed oxalates Co-Ni Co-Mg and Ni-Mg has been discussed [99],... 

Zhabrova etal. [151] identified the reactions of nickel, cobalt and copper oxalates ( , = 150, 159 and 129 kJ mol respectively) as redox processes in which there is an autocatalytic effect by product metal on the electron transfer step. The decomposition rate was determined by the area of the reactant and results were fitted by the Prout-Tompkins equation. In contrast, the reactions of magnesium, manganese and iron oxalates (f, = 200,167 and 184 kJ mol ) are not autocatalytic and the area... 

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. 

Beside the tartrate, other organic salts—notably the oxalate and the formate—yield the more easily produced pyrophoric metals, i.e. lead, nickel, and cobalt, if the pyrogenic decomposition is performed at low temperature. These organic salts or the oxides of nickel, cobalt, and iron, under hydrogen at about 300X, also furnish the self-flammable metals, but in the case of iron, a pyrophoric ferrous oxide (FeO) may first appear by reduction with hydrogen as well as by reduction under carbon monoxide. 

Table 4.7 gives thermal decomposition data of N2H5M(N2H3COO)3-H2O. The iron complex shows a single step in TG-DTG and two exotherms in DTA, while both cobalt and nickel complexes show two-step decomposition in TG-DTG and two exotherms in DTA. The observed weight loss after the first step corresponds to the formation of metal oxalate hydrazine... 

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