Alumina stabilisation

The long terra effects of steam and hydrogen on the properties of this catalyst have been studied by Williams et al [20]. The catalyst contained only 23% 7 A1 O, and may be considered almost as alumina stabilised nickel. The... 

As the transition alumina stabilisation is more effective when the added cation is larger or more charged, it is recommanded that silicon, lanthanum and zirconium are used. 

The washcoat provides a high, stable surface area for dispersion of the precious metals [2,3]. The bulk of the washcoat consists of alumina, stabilised by the addition of small amounts of baria or lanthana. Thermal stabihsation is essential, since temperatures in the catalyst bed can rise to over 1000°C. The choice of alumina precursors (boehmite or gibbsite) and the preparation/stabihsation procedures are vital to the stability of the washcoat [2,3]. 

Wash with cone H2SO4, then Na2C03 soln, dry with anhydrous Na2C03, and finally pass through a 50cm column of activated alumina before distn. Alternatively, wash with 10% ferrous sulfate soln to remove peroxides, then H2O, dry with CaS04, and dist in vac. Add 0.2% of catechol to stabilise it. VERY TOXIC. 

Chlorinated hydrocarbons can be purchased without stabiliser, or the stabiliser can be removed by adsorption onto alumina, or by extraction with water, followed by drying. IJnstabilised di- or trichloromethane will slowly decompose, producing HC1, which corrodes stainless steel. The rate of decomposition may be accelerated by the presence of other solvents. 

Ethers contain additives to stabilise them against peroxide formation. For instance, tetrahydrofuran is commonly stabilised by the addition of small amounts of hydroquinone. This absorbs uv radiation strongly and so interferes with uv absorbance detection. It can be removed by distilling the solvent from KOH pellets. If you use inhibitor-free tetrahydrofuran, it should be stored in a dark bottle and flushed with nitrogen after each use. Any peroxides that form should be periodically removed by adsorption onto alumina. 

The activation energy represents the ease of ion hopping, as already indicated above and shown in Fig. 2.5. It is related directly to the crystal structure and in particular, to the openness of the conduction pathways. Most ionic solids have densely packed crystal structures with narrow bottlenecks and without obvious well-defined conduction pathways. Consequently, the activation energies for ion hopping are large, usually 1 eV ( 96 kJ mole ) or greater and conductivity values are low. In solid electrolytes, by contrast, open conduction pathways exist and activation energies may be much lower, as low as 0.03 eV in Agl, 0.15 eV in /S-alumina and 0.90 eV in yttria-stabilised zirconia. 

In magnesia-stabilised / "-alumina, the formula is Na, (Mg2 Al5 (08 and X is essentially fixed at 0.175. An alternative way of representing it is in terms of a derivation from the stoichiometry NaAlnOi, with the formula Nai+2Mg2Aln jOi7. Various values of 2 are quoted in the literature, between 0.5 and 0.8. The value 0.747 which is in this range coincides with that obtained using the alternative, spinel-like formula given above, with x = 0.175. 

Lately, a number of papers have dealt with microwave-assisted reactions on palladium-doped A1203. Villemin reported on Stifle, Suzuki, Heck and Trost—Tsuji reactions where potassium fluoride on alumina was used as the base26. The reactions were carried out without solvent or stabilising phosphine ligands in single-mode reactors. The Stifle reactions were noteworthy as the toxic organotin residue remained adsorbed on the solid support, thus allowing a simplified work-up procedure for the otherwise unpleasant, and toxic, stannous by-products. Both the Stifle and the Suzuki reactions could be performed under air. Furthermore, it was noted that with experiments where the... 

Hydration stabilisation. Due to the polarisability of the water molecules, even in a deionised aqueous solution, stabilisation may occur. Positively charged alumina particles, for example, bind preferentially to the negative oxygen of the water molecule. As a result, a double layer is formed, similar to ionic electrostatic repulsion. In ionic solutions, the hydration repulsive force occurs simultaneously with the electrostatic force, while the proportion of the two forces depends on the ionic concentration [22],... 

Stable suspensions are necessary to obtain homogeneous casts by colloidal filtration. Different kinds of stabilisation mechanisms for alumina suspensions are used in literature. Pure electrostatic stabilisation can be obtained using HNO3, while electrosteric stabilisation is obtained using polyelectrolytes like PolyMethylAcrylicAcid (PMAA) and PolyAcrylicAcid (PAA). In... 

Hidber et al. [9] described the colloidal processing of wet-milled a-alumina suspensions by centrifugal casting from a 80 wt-% suspension at pH 4.3. The mean particle diameter of the a-alumina powder was 0.3 pm and the BET surface area 8.59 m /g. Nitric acid or ammonia was used for electrostatic stabilisation. Very dense ceramics could be obtained with relative densities up to 99.9%. 

In two articles, J. Cesarano HI et al. [23, 24] described the stability of aqueous a-alumina suspensions stabilised with PMAA and PAA polyelectrolytes. The powders used were AKP-20 with a mean particle diameter of 0.52 pm and a surface area of 4.5 m2/g and AKP-30 as described above. The electrolytes used were the Na-salt of PMAA with an average molecular weight of 15000 g/mol and PAA of various molecular weights (1800, 5000 and 50 000 g/mol). The structural formulas of the polymers are shown in Figure 5. 

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