Amorphous hydroxides

As also mentioned in Sect. 2.1.2, Varin et al. [27] showed that a long-term air exposure of nanocrystalUne MgH for a few months led to a massive formation of crystalline Mg(OH)j not only on the surface but also in the bulk by conversion of the entire MgH particles into Mg(OH)2. Apparently, the initial amorphous hydroxide layer Mg(OH) grows and transforms into a crystalline-phase Mg(OH)2 (the reaction of (2.3)). 

The surface of these nuclei (and therefore d2) is a measure of the activation energy of the above reaction. We can therefore understand the common observation that only the amorphous hydroxide undergoes a spontaneous change whereas the crystalline modifications may be kept under water for a long time without being converted into ZnO. However, these considerations apply only to two-phase reactions. Oxides and hydroxides may undergo one-phase transformations as well. The above speculations must therefore be used with care. 

The thermal dehydration of Cr(0H)3,3H20 to the amorphous hydroxide has been studied.108 X-Ray diffraction data recorded for aqueous solutions of [Cr(H20)6]Cl3 show that the [Cr(H20)6]3+ ions pass relatively undisturbed from the solid into solution.109 A deuterium magnetic resonance study of [Cr(H20)6]3+ has confirmed110 the linear trend of the spin densities at the hydrogen atoms in [M(H20)6]3 + (M = Ti, V, or Cr) ions predicted earlier. 

In the case of passive layers the outer hydroxide film is always present. It is hydrated to a large extent, and thus does not correspond to crystalline Ni(OH)2. The incorporation of water leads to an amorphous structure of the outer part of the passive layer with the observed characteristic dense package of small grains. All these detailed investigations lead to the structure of the passive layer on Ni with a an inner crystalline oxide with a antiparallel orientation of its (111) planes relative to the (11 l)-oriented Ni surface with a tilt of 3° and an outer amorphous hydroxide part. 

Extensive characterisation studies have been performed with various techniques (XAFS, XPS, 31P CP/MAS-NMR, IR and Raman spectroscopies)13 to characterise Aul after deposition, and its decomposition and conversion into metal particles upon thermal treatment. The transformation of hydroxide supports into oxides was also followed. The amorphous hydroxides contain a larger number of surface hydroxyl groups and surface defects than do the corresponding oxides, and they interact more efficiently with Aul, which dissociates on the surface, releasing the... 

Amorphous hydroxide Al(0H)3.nH20 Amorphous aluminum hydroxide... 

Thermal decomposition (dehydration) of air-dried X-ray amorphous CPH proceeds in different ways. Similar to the decomposition of X-ray amorphous hydroxides, it occurs gradually at a wide temperature range, while the total mass loss at isothermal dehydration gradually increases at elevated temperatures. 

Reactions that take place under hydrothermal conditions in CaO -SiOj - HjO system are complicative the number of silicates synthesized in this system approaches 20 [26]. The state of calcium hydroxide has a substantial effect on final products. If amorphous hydroxide is used, the reaction runs faster, but at the same time the probability of metastable phases formation increases. Detailed investigations that were carried out by the authors [26] allowed to state the preferable regions of the synthesis of various calcium compounds depending on Ca0/Si02 ratios and hydrothermal treatment temperature (450-850°C). 

Iron oxide sediments are formed in the absence of or at low concentrations of active forms of silicic acid, carbonic acid, and sulfur in the waters. According to the data of investigations of the composition of recent ferric iron sediments, they consist mainly of amorphous hydroxides of Fe " " (Strakhov, 1947, 1960 Ostroumov, 1955 Volkov and Tikhomirova, 1966 ... 

Sevast yanov, 1968) in some cases finely dispersed magnetite has been found (Strakhov, 1960 Stashchuk et al., 1964 Harrison and Peterson, 1965), and very rarely, in highly reducing conditions—for instance in the sediments of swamps and peat bogs—amorphous hydroxides of Fe " " are encountered (Astapova et al., 1969 Kovalev and Generalova, 1969 Stashchuk, 1969). 

Ce species formed upon reduction of Ce02 are known to be reoxidized quickly by exposure to even small O2 pressures [127] this easy Ce oxidation was observed even for the (highly amorphous) hydroxides which appear in the first stages of preparation via precipitation [128], It is thus striking that nanocrystalline... 

In aqueous media oxides can spontaneously turn into hydroxides or oxohydroxides and vice versa. Let us explore the hydration-dehydration reactions for Al(III). Example 1 shows that corundum is the most stable oxide. Likewise hydrargillite has a more negative negative G° (and AGf) than the amorphous hydroxide and thus the former from is stable (not a big surprise), and diaspore is more stable than boehmite. In order to find which of the three forms (corundum, hydrargillite and diaspore) is thermodynamically stable, let us calculate AG" for the following reactions. 

PZC/IEP of Boehmite Obtained by Aging of Amorphous Hydroxide Electrolyte T Method Instrument pH, Reference... 

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