Chromium allotropes

The AIF3 support, characterized by different allotropic phases, has a wide distribution of the pore radii. Incipient wetness impregnation method has allowed the introduetion on the support of chromium phases with amorphous or mierocrystalline structure. The N2 porosimetry has shown that these phases cover the surface of the support, determining a more homogeneous pore radius distribution of the catalytic material. 

Some interesting phenomena have recently come to light which indicate that chemical action may assume a periodic character. The evolution of hydrogen gas, when hydrochloric. acid acts on i one of the allotropic forms of chromium, has recently been studied by W. Ostwald (Zeit. phys. Chem., 35, 33,... 

Of the solids given, ionically bonded sodium chloride is expected to be crystalline, a poor electrical conductor in the solid form, and a good conductor when fused. Diamond, formed of covalently bonded carbon atoms, is a network substance that does not form cubic crystalline patterns, and does not conduct electricity either when solid or fused. None of the allotropic forms of sulfur is expected to conduct electricity. Choice (D), the metal chromium, could possibly form a cubic solid crystalline form, but can be eliminated because it is expected to conduct electricity both when a solid and when fused. The correct choice is (A), because sodium chloride is a crystalline solid that is a poor conductor in the solid state and a good conductor when fused. ... 

R. Bunsen suggested that it would be worth trying to find if allotropic forms of chromium could be produced by electrolyzing green and blue chromic salt soln. Subsequent work, however—by W. R. Whitney, etc.—has shown the hypothesis to be untenable. 8. 0. Cowper-Coles obtained a bright deposit of chromium from a soln. of 25 parts of chromic chloride in 75 parts of water at 88°, with a current of 0-04-0-05 amp. per sq. cm. With a cold soln., gas is evolved at both electrodes, but no metallic deposit is obtained until an excess of hydrochloric acid is added. 

In the patent by Hill, an aUoy of titanium containing 13 wt%vanadium, 11 wt% chromium and 3 wt% aluminum was developed as a hydrogen transport membrane material [12]. In this alloy, the crystal structure of titanium, which is normally hexagonal below 1153 K (880 °C), is stabilized in its high-temperature body centered cubic allotropic form. The body centered cubic crystal lattice is preferred for hydrogen transport. This titanium alloy was found to have hydrogen permeability superior to that of pure palladium in the range 300-450 °C (573-723 K)... 

Isolating the less thermodynamically stable of two interconvertible forms of the same composition (whether it be coordination compounds, allotropes of elements, or any other chemical composition) can be done only if the rate of reaching equilibrium is so slow as to render the conversion of the metastable isomer to the stable one very protracted. This is a form of Ostwald s law of metastable intermediates. Such rates are slow (minutes < ti/2 < years) for the equilibrations of coordinated cobalt(III), chromium(III), a few (spin-paired) d ferrous compounds, such as salts of the ferroin 22a, tris-l,10-phenanthrolineiron(II) cation, and a few octahedral nickel(II) species with strong field ligands (3d, e.g., the tris-l,10-phenanthrolinenickel(II) ion 22b). 

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