Deposition-precipitation with alumina

The success of Haruta s early work lay in his choice of preparation method and support. Gold particles of the necessary small size were first obtained by coprecipitation (COPPT) and later by deposition-precipitation (DP) (see Sections 4.2.2 and 4.2.3) classical impregnation with HAuCLj does not work. The choice of support is also critical transition metal oxides such as ferric oxide and titania work well, whereas the more commonly used supports, such as silica and alumina, do not work well or only less efficiently. This strongly suggests that the support is in some manner involved in the reaction. 

Effecting deposition-precipitation by decreasing the pH level is interesting with metal ions present in the stable state in aqueous solution as anions [35]. With silica no interaction is observed, which has led to the development of the electrochemical reduction procedure. To apply metal ions, such as, molybdenum or vanadium, on alumina, a homogeneous decrease in pH level is interesting. The pH level has been decreased by injection of nitric acid or perchloric acid and electro-chemically. However, the rate of crystallization of the hydrated oxides of vanadium(V) and molybdenum(VI) was observed to be fairly low. To prevent dissolution of the alumina supports the pH cannot be decreased to levels below about 3, at which the crystallization of the hydrated metal oxides does not proceed rapidly. 

Precipitation-deposition can be used to produce catalysts with a variety of supports, not only those that are formed from coprecipitated precursors. It has been employed to prepare nickel deposited on silica, alumina, magnesia, titania, thoria, ceria, zinc oxide and chromium oxide.36 It has also been used to make supported precious metal catalysts. For example, palladium hydroxide was precipitated onto carbon by the addition of lithium hydroxide to a suspension of... 

S. Belochapkine, J. Shaw, D. Wenn, J.R.H. Ross, The synthesis by deposition-precipitation of porous gamma-alumina catalyst supports on glass substrates compatible with microreactor geometries, Catal. Today 110 (2005) 53. 

Klissurski et al. [87] have examined the combustion of acetone, toluene and styrene by zinc-cobalt spinel oxides supported on alumina. Catalysts were prepared by co-precipitation with sodium carbonate from a mixed zinc/cobalt nitrate solution at pH 9. The supported catalyst was prepared by deposition of the precursor on Y-AI2O3 from a suspension in dimethylformamide and water. The supported precursor was dried at 150°C and calcined at 300°C to produce the catalyst. The bulk and supported catalysts both showed the formation of zinc cobaltite spinel structures which were thermally stable. Microreactor studies at 15,000 h- space velocity showed that the components of a mix of acetone, toluene and styrene were destroyed at 225°C, 280°C and 350°C respectively. The VOC concentrations were not specifically expressed but it is assumed that they... 

In this work three Ni-based catalysts were synthesized and their catalytic performance in methane steam reforming was investigated. Catalyst denoted as Ni/Si02 was prepared using precipitation deposition method while catalyst denoted as Ni/SA was prepared via deposition precipitation method using alumina modified with silica. Similarly catalyst denoted as Ni.NPs in the first step Ni nano-particles were synthesized and decorated on AI2O3. 

This method of deposition-precipitation is probably the most used for the preparation of gold catalysts since it readily leads to the formation of small gold particles (2-3 nm). This method was first proposed by Haruta et al. [25, 26]. The pH of the solution containing HAuCh and the oxide support is adjusted by addition of NaOH, often 7-8 for titania or alumina. The suspension is stirred for 1 hour at 70-80°C. The catalyst is then washed with water to eliminate as much chloride and sodium ions as possible, dried between RT and 100°C, then usually calcined in air. 

The deposition-precipitation method using a base has been applied for the preparation of various catalysts. Upon raising the pH of the solution, the precipitation of a hydroxide onto the support is expected. In fact, it was shown in several cases (Table 14.2) that mixed compounds such as phyllosilicates for silica support or hydrotalcite for alumina support formed, involving support dissolution and neoformation of a mixed compound with a layered structure. 

Carbon fibrils can be produced rather easily, e.g., by exposing supported, finely dispersed iron or nickel particles to reducing carbon containing gas flows. To this end, one has to produce first finely dispersed iron or nickel particles on a support material, such as alumina or silica. The desired catalyst can be prepared, e.g., by incipient wetness impregnation of the support material with a suitable metal salt solution or by means of homogeneous deposition-precipitation of the metal ions onto the carrier. 

Deposition precipitation method was found suitable to obtain a homogeneous distribution of nickel on monolith washcoated with alumina. Drying in a microwave oven is helpful in maintaining a homr eneous metal dispersion over the monolith. 

The second most widely used catalyst type is constituted by two phases, the support and the active material, obtained by impregnation, precipitation, or deposition-precipitation. In general, the support is not an active phase, but it serves to increase the area and to disperse the active phase. The active phase can be a metal or oxide which is the active component to interact with the molecules during the chemical reaction. The most used supports are alumina, silica, carbons, and other inert oxides. Often, the material known as support can also be active, so that there are two active phases, with different functionalities, for example, Pt/Y, in which both are active for isomerization processes. The difference is that the Pt concentration is very small, but it plays a preponderant role [1]. 

Membranes with extremely small pores ( < 2.5 nm diameter) can be made by pyrolysis of polymeric precursors or by modification methods listed above. Molecular sieve carbon or silica membranes with pore diameters of 1 nm have been made by controlled pyrolysis of certain thermoset polymers (e.g. Koresh, Jacob and Soffer 1983) or silicone rubbers (Lee and Khang 1986), respectively. There is, however, very little information in the published literature. Molecular sieve dimensions can also be obtained by modifying the pore system of an already formed membrane structure. It has been claimed that zeolitic membranes can be prepared by reaction of alumina membranes with silica and alkali followed by hydrothermal treatment (Suzuki 1987). Very small pores are also obtained by hydrolysis of organometallic silicium compounds in alumina membranes followed by heat treatment (Uhlhom, Keizer and Burggraaf 1989). Finally, oxides or metals can be precipitated or adsorbed from solutions or by gas phase deposition within the pores of an already formed membrane to modify the chemical nature of the membrane or to decrease the effective pore size. In the last case a high concentration of the precipitated material in the pore system is necessary. The above-mentioned methods have been reported very recently (1987-1989) and the results are not yet substantiated very well. 

F-T Catalysts The patent literature is replete with recipes for the production of F-T catalysts, with most formulations being based on iron, cobalt, or ruthenium, typically with the addition of some pro-moter(s). Nickel is sometimes listed as a F-T catalyst, but nickel has too much hydrogenation activity and produces mainly methane. In practice, because of the cost of ruthenium, commercial plants use either cobalt-based or iron-based catalysts. Cobalt is usually deposited on a refractory oxide support, such as alumina, silica, titania, or zirconia. Iron is typically not supported and may be prepared by precipitation. 

Sepiolite and palygorskite have a rather special composition and seem to be related to specific mineral parageneses. They appear to be stably associated with montmorillonite, corrensite, serpentine, chert, sulfates, carbonates and various salts. They are found in deposits typified by processes of chemical precipitation or solution-solid equilibria (Millot, 1964) and are therefore rarely associated in sediments with large quantities of detrital minerals. Their chemical environment of formation is in all evidence impoverished in alumina and divalent iron. Their frequent association with evaporites, carbonates and cherts indicate that they came from solutions with high chlorinity. 

The relationship between aluminous sepiolite and the magnesian form is not at all clear as yet. The question of whether or not aluminous sepiolite can be precipitated directly from solution should be posed since alumina concentration in alkaline solutions (pH 7-9) is assumed to be quite low. For the present, palygorskite should probably be considered as a phase produced through solid-solution equilibria due to its high alumina content. Possibly most sepiolites are produced in this way. Even the inonomineralic deposits in saline lakes or deep sea cores contain sepiolites with high alumina contents (Parry and Reeves, 1968 McLean, et al., 1972). 

This reaction is used, also, to a very small extent in calico-printing. A hyposulphite of alumina is formed in the cold by adding a solution of chloride of aluminium to a sol ution of hyposulphite of soda. The result is a solution of chloride of sodium and hyposulphite of alumina. This Is thickened iu the regular manner, and printed on the fabrio. The piece is then exposed to a temperature of 212° by means of steam, when the salt is decomposed, alumina being deposited on or in the cloth. Tin s alumina so deposited dyes np with madder precisely like the alumina precipitated in the ordinary manner from the acetate. 

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