Iron oxalates, decompositions

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... 

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... 

The decomposition of ferrocenyl amines in oxalic acid solution gives iron oxalate 356>. 

Grinding of iron oxalate with NH4-Y zeolite should result in the Fe(ll) and, if oxydation by oxygen occurs, in the Fe(lll) form of Y zeolite and ammonium oxalate. Since CO is formed as decomposition product of the latter compound, secondary reactions between this strong reducing agent and iron species may also occur. 

Nevertheless, the iron oxalate/NH4 Y system heat-treated at high temperatures might be considered for catalytic applications since the decomposition products (metallic iron, iron carbide and magnetite) may be highly dispersed and. hence, may exert catalytic activity. Studies are in progress to prove this suggestion. 

Iron oxalate hydrazine, the synthesis of which is discussed in Chapter 3, is used to prepare iron oxide. The decomposition/combustion of FeC204 (N2H4)2 in air at —250 °C yields the oxide Fe203 by the following reaction ... 

The oxide (prepared at 300°C) bums in air above 200°C, while the finely divided oxide prepared by reduction may be pyrophoric at ambient temperature [1]. That prepared by thermal decomposition under vacuum of iron(II) oxalate is also pyrophoric [2],... 

Thermal decomposition of mixtures of iron(II) and iron(III) oxalates, giving 7-Fc203, has been studied by XRD, Mossbauer and FTIR spectroscopy. " A standard addition kinetic method for the simultaneous determination of Fe and Fe is based on the vastly different rates of complex formation of Fe and Fe " with gallic acid " " this study is complemented by kinetics of formation of gallate (3,4,5-trihydroxybenzoate) complex. " ... 

Music, S. Gessner, M. Wolf, R.H.H. (1979b) Sorption of trace amounts of gallium(III) on iron(III) oxide. Radiodiim. Acta 26 51-53 Music, S. Gotic, M. Popovic, S. (1994) Formation of y-Fe203 by thermal decomposition of a mixture of Fe(II)- and Fe(III)-oxalate salts. 

Iron(II) oxide may be prepared by thermal decomposition of iron(ll) oxalate ... 

Thermal decomposition—Thermal decomposition methods may be used to prepare metal oxide fumes. An aerosol of a precursor to the metal oxide (i.e., a substance that is readily decomposed, thermally, to yield the oxide) is first generated and then is heated by passing it through a heated tube to decompose it to the oxide. Metal formates, oxalates, and the like, which readily yield the oxides and do not produce objectionable side products, are commonly used precursors. In this program, fumes of iron oxide, vanadium oxide, and copper oxide were generated using this method. 

Chemically pure Fe can be prepared by reduction of pure iron oxide (which is obtained by thermal decomposition of iron(II) oxalate, carbonate, or nitrate) with H2, by electrodeposition from aqueous solutions of Fe salts, or by thermal decomposition of Fe(CO)5. 

Curve C is characterized by the rapid onset of an acceleratory reaction and no induction period is found. Any initial small evolution of gas will probably be obscured by onset of the main reaction. Such behaviour is found in the decompositions of iron(II) and iron(III) oxalates [91]. 

Iron(n) oxalate dihydrate loses water above 420 K to yield the anhydrous salt [53] in an inert atmosphere, but in the presence of oxygen FejOj may be formed [54]. At about 590 K the anhydrous oxalate decomposed slowly to yield magnetite and iron but no wustite was detected [53]. Kadlec and Danes [55] showed that the ar-time curves are sigmoidal and Zr, = 167 kJ mol. During decomposition of the mixed (Fe-Mg) oxalates, magnesium may stabilize the wustite product against either oxidation or reduction [56]. 

The decomposition of iron(II) oxalate in hydrogen is an autocatalytic reaction which fits the Avrami-Erofeev equation with n = 2 (0.02 < nr < 0.25) and = 3 (0.3... 

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. 

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]. 

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. 

Iron(II) oxide is a black, insoluble solid with an NaCl lattice above its Curie temperature (200 K) the lattice suffers defects because it is always deficient in Fe (see Section 27.2). Below 200 K, FeO undergoes a phase change and becomes antiferromagnetic. It can be made in vacuo by thermal decomposition of iron(II) oxalate but the product must be cooled rapidly to prevent disproportionation (equation 21.67). 

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