Mixed hydroxides

Decompositions of crystalline mixed hydroxides to mixed oxides often occur at temperatures lower than those required to produce the same phases through the direct interaction of metal oxides. This route thus offers an attractive approach for the preparation of catalysts of high area and activity [1147]. Detailed kinetic investigations comparable with those for the dehydroxylations of a number of pure hydroxides (Sect. 2.1) are not, however, available. 

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PROBLEMS Show the mechanism for the reaction that takes place when you mix hydroxide (HO ) with each of the following compounds (remember that you need to look for the most acidic proton in each case). 

The possible strategies are coprecipitation to prepare mixed hydroxides or carbonates [5], cosputtering of gold and the metal components of the supports by Ar containing O2 to prepare mixed oxides [23], and amorphous alloying to prepare metallic mixed precursors [24]. These... 

Thermally activated mixed metal hydroxides, made from naturally occurring minerals, especially hydrotalcites, may contain small or trace amounts of metal impurities besides the magnesium and aluminum components, which are particularly useful for activation [946]. Mixed hydroxides of bivalent and trivalent metals with a three-dimensional spaced-lattice structure of the garnet type (Ca3Al2[OH]i2) have been described [275,1279]. 

Equilibrium constants for formation of iron(III) complexes of several oxoanions, of phosphorus, arsenic, sulfur, and selenium, have been reported. The kinetics and mechanism of complex formation in the iron(III)-phosphate system in the presence of a large excess of iron(III) involve the formation of a tetranuclear complex, proposed to be [Fc4(P04)(0H)2(H20)i6]. The high stability of iron(III)-phosphate complexes has prompted suggestions that iron-containing mixed hydroxide or hydroxy-carbonate formulations be tested for treatment of hyperphosphatemia. " ... 

It is also possible, if the proper conditions are set, to dissolve selectively the rare earth hydroxides which are more basic than thorium hydroxide, see the right hand column of Figure 9. In such a case, the mixed hydroxide water slurry is brought to a pH of 3.4 by a slow and careful addition of hydrochloric acid. The undissolved thorium hydroxide is then separated from the solution by filtration. 

To some extent, the aqueous metal silicate complexes constitute an entire class of placeholders. There is ample evidence that silica forms strong complexes with several metals, but the stoichiometry of such complexes has not yet been established. Different speciation schemes have been proposed to interpret experimental data, for example, 1 2 complexes, chelates, or mixed hydroxide silicate complexes. Despite these ambiguities we decided to include several metal silicate complexes in the data base as guidelines for modellers. Should such complexes turn out to be of crucial importance in particular systems, additional experimental studies would be called for. 

Ternary complexes must be considered in models of environmental systems. However, there is no chance to explore experimentally the huge number of potentially forming ternary complexes. Modellers must first estimate which ternary complexes might be important for a specific system, and only then should experimental investigations be started. Hummel Bemer (2002) have shown, for example, that mixed hydroxide carbonate complexes of U(IV), Np(IV), and Pu(IV) are of particular interest in carbonate-rich groundwaters. 

There are probably several mineral phases, particularly for the highly alkaline systems, that remain to be discovered. Mixed hydroxides may control solubility. Calcium zincate (CaZn2(OH)6), for example, is thermodynamically more stable than Zn(OH)2 above pH 11.5 and may be important in cementitious systems. Another group of minerals is that of the hydrotalcite-like minerals, the layered double hydroxides (LDH, M2+2M3+l/yXy (OH)6 where X is an anion). Cobalt, Ni and Zn can form such minerals (Johnson Glasser 2003) under neutral to alkaline conditions. For the majority of species, however, solubility-limiting phases do not appear to control dissolved concentrations. 

Cd,Zn)0 films were deposited on glass at 45°C from solutions of Cd and Zn chlorides to which ammonia and then H2O2 were added (the purpose of the H2O2 was not given) they were then annealed at a final temperature of 500°C [6]. (No description of the as-deposited films was given they were presumably mixed hydroxides.)... 

Mixed hydroxide was prepared by a method previously described [9]. A mixture of 10% metal nitrate aqueous solution with a Zn Cr ratio of 0.5 was prepared. This solution was heated to 70 °C, while a 5% ammonia (Merck) aqueous solution was added drop wise under constant stirring, with the pH maintained at approximately 7.0. The mixture was digested for another 2 hours at 80 °C in order to complete the precipitation. The precipitate was filtered, washed and dried in air at 110 °C for 12 hours. Chemical analysis yielded Zn 21.4%, Cr 34.1%, O 41.9% and H 2.6%. 

Thermal treatment of layered double hydroxide (LDH) and mixed hydroxide (MH)... 

The mixed hydroxide (MH), however, gave no significant PXRD reflections. The identified formula for this compound was Zni.oCr2(OH)8 0 (normalized for Cr = 2), with a Zn Cr ratio of 0.5, exactly as expected, showing the complete precipitation of the metal cations. 

The mixed hydroxide precursor (MH) was treated at 500, 900 and 1200 °C. Again a progressive increase in crystallinity was observed with the increase of the temperature. FT-IR spectra showed a band around 3400 cm 1 for the sample treated at 500 °C, which was absent in samples treated at 900 and 1200 °C. The explanation follows that given for the LDH system. As observed for the spinel oxide prepared by the solid-reaction method the treatment with mineral acid did not affect the PXRD pattern. 

There are several ways of treating the mixed hydroxide cake. The cake is either dissolved in HC1 or HNOs and thorium is removed. The original Rohden process involved a partial dissolution of the mixed hydroxide in HC1 at 70—80° C (pH 3.5—4). The crude thorium hydroxide was filtered off and the filtrate contained the rare earths practically free from thorium and phosphate. 

The possibility of dissolving the mixed hydroxide in HNOs and obtaining direct extraction of thorium (and uranium) from the nitrate solution has been studied [155,156], but does not seem to be too promising, possibly due to the partial oxidation of tripositive cerium to the tetrapositive state. Kraitzer [157] was able to separate thorium from the mixed hydroxide cake by extracting the cake with sodium carbonate buffer at pH 9.5—10. Thorium was found to form a soluble carbonate complex and a recovery of better than 99% of thorium was claimed after only four extractions. 

Mixed Hydroxides Thorium Cerium Group Lanthanides... 

Narayanan, S. and Krishna, K. (1998). Palladium mixed hydroxide catalysts for phenol hydrogenation. Stud. Surf. Sci. Catal. 113, 359. 

Copper is a more "noble metal than iron and is found both native, i.e. in metallic form, and as mixed hydroxide/carbonate (malachite, azurite) or combined with iron and sulphur as copper pyrites. 

Anionic clays are natural or synthetic lamellar mixed hydroxides with interlayer spaces containing exchangeable anions [10, 104]. The generic terms, layered double hydroxides (LDHs) or hydrotalcites are widely used, the latter because exten-... 

Basic acetate-chloroform test Upon dissolving beryllium hydroxide (reaction 1) in glacial acetic acid and evaporating to dryness with a little water, basic berryllium acetate, BeO. 3Be(CH3COO)2, is produced, which dissolves readily upon extraction with chloroform. This forms the basis of a method for separating beryllium from aluminium, since basic aluminium acetate is insoluble in chloroform. The mixed hydroxides are treated as detailed above. 

Separation. Beryllium is precipitated in Group IIIA. It is ultimately associated with aluminium in solution as tetrahydroxoaluminate and tetrahydroxoberyllate respectively. Upon diluting and boiling, only the Be(OH)2 is precipitated. Alternatively, the quinalizarin test, 7, may be applied to the solution or the basic acetate-chloroform test, 6, to the mixed hydroxides. Beryllium may also be detected in the presence of aluminium by the acetylacetone test, 9 this is specific for Be. 

Precipitated catalysts are generally prepared by rapid mixing of concentrated solutions of metal salts. The product precipitates in a finely divided form with high surface areas. Mixed hydroxides or carbonates are normally prepared by precipitation. Typically, the following reaction is used ... 

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