The Aqueous Chemistry of Chromium(III) above 100 °C: Hydrothermal Synthesis of Chromium Spinels

Can. J. Chem. 53, 12 (1975). The possibility of forming spinel-type ferrites MFez04 from M(0H)z or MO and a-FeO(0H) (goethite) or a-Fez03 (hematite) under hydrothermal conditions up to 350" has been investigated experimentally for M = Mg, Ca, Mn, Co, Ni, Cu, and Zn. Formation ofMFez04 was observed for M = Mn (jacobsite), Co, Ni (trevorite), and Zn (franklinite), and was in general favored by high temperatures, high pH, and the presence of NaNO3 or, especially, NaCl in solution. a-FeO (0H) is more reactive than a-FezO3, but generally converts to the latter at 220-260°, independently of pH, in neutral or alkaline media. Interpretation of the results is complicated by the ionexchanging properties and varying degrees of crystallinity of M(OH)z, and by the tendency of M(OH)2 to decompose to MO or to be oxidized at the higher temperatures. Nevertheless, the [Tradult par le journal]

Section: Apparatr~smentioning

confidence: 99%

“…require strict maintenance and control of a hydrogen pressure, since, at the temperatures and pHvalues of interest, the Fe(OH), produced in the formal reaction would decompose to magnetite and hydrogen (2), the net reaction being…”

Section: Introductionmentioning

confidence: 99%

Hydrothermal Formation of Ferrite Spinels

Gainsford¹,

Sisley²,

Swaddle³

et al. 1975

“…or NH4C1 (up to 0. The optimal conditions for magnetite preparation were found to be [FeU1NTA] = 0.10 m, 0.20 5 [NaOH] 5 0.40 m, under nitrogen at 497 K for 70 h or more; these give shining, well-formed, jet-black octahedra, 10-100 pm in diameter. These crystals can easily be freed from the surface of the liner, in 70% yield.…”

Section: Resultsmentioning

confidence: 99%

“…Preparative methods involving solid reactants, e.g., the oxidation of elemental iron by steam containing hydrogen (I), tend to give a sintered product, are usually slow, and leave room for doubt concerning the homogeneity and purity of the product. The latter problems persist in heterogeneous hydrothermal syntheses of Fe304 from metallic iron and hematite (a-Fe,03) (2, 3) or from solid iron(I1) hydroxide by the Schikorr reaction (4,5).…”

mentioning

confidence: 99%

Hydrothermal preparation of magnetite from iron chelates

Booy¹,

Swaddle²

1978

“…Weighed amounts of P-Ni(OH)2 were placed in capped 35 cm3 PTFE liners with (usually) 20 cm3 NaOH solution, and sealed into titanium/0.2% palladium alloy autoclaves (35) leaving -25 crn3 gas space. These operations were carried out in a nitrogen-filled glovebox, when oxygen was to be excluded; measurements using a Rexnord oxygen meter showed the O2 content of solutions handled in this way to be less than 0.1 pg/g (cf.…”

Section: Kit~etics Of the Hydrorl~erninl Renctiot~mentioning

confidence: 99%

Hydrothermal reaction kinetics. The decomposition of nickel(II) hydroxide

Swaddle¹,

Wong²

1978

The fractional extent α of decomposition of ripened Ni(OH)2 into green NiO in alkaline hydrothermal conditions under nitrogen is given by the rate law α1/3 = kc[OH−]t, where kc = 2.4 × 10−5 kg mol−1 s−1 at 561 K. The Arrhenius activation energy is 220 + 25 kJ mol−1. The reaction is accelerated moderately by O2 (giving black NiO1+δ) and strongly by added NiO. A reaction mechanism involving dissolution of Ni(OH)2 as Ni(OH)3− (aq) and rate-determining, surface-controlled reprecipitation of this as NiO is proposed.