Preparation precipitation-deposition

As the metal particle size decreases the filament diameter should also decrease. It has been shown that the surface energy of thirmer filaments is larger and hence the filaments are less stable (11,17-18). Also the proportion of the Ni(l 11) planes, which readily cause carbon formation, is lower in smaller Ni particles (19). Therefore, even though the reasons are diverse, in practice the carbon filament formation ceases with catalysts containing smaller Ni particles. Consequently, well dispersed Ni catalysts prepared by deposition precipitation of Ni (average metal particle size below 2-3 nm) were stable for 50 hours on stream and exhibited no filamentous coke [16]. 

Figure 1. TEM image of a titania supported gold catalyst (1.7wt.% Au) prepared by deposition-precipitation (gold particle size = 5.3+ 0.3 nm, dispersion = 36%). (Reprinted from Reference [84], 2000, with permission from American Chemical Society).

Figure 13. Specific 2-CP (open symbols) and 2,4-DCP (solid symbols) hydrodechlorination rate constant K) as a function of the average Ni particle diameter ( nO for reaction over Ni catalysts prepared via impregnation with nitrate (0,0), deposition-precipitation (A,A) and impregnation with nickel ethanediamine ( , ) r= 423K reaction data refer to aqueous solutions. (Reprinted from Reference [147], 2003, with permission from Royal Society of Chemistry).

The activity of gold catalyst is normally strongly size dependent and the control as well as the narrowest possible distribution of particle size represent the main goal for the production of an active gold catalyst. From a catalytic point of view, several preparation methods have been proposed for obtaining highly dispersed gold catalyst, most of them derived from deposition-precipitation method proposed by Haruta et al. [3]. 

Bezemer, G. L., Radstake, P.B., Koot, V., van Dillen, A. J., Geus, J. W., and de Jong, K. P. 2006. Preparation of Fischer-Tropsch cobalt catalysts supported on carbon nanofibers and silica using homogeneous deposition-precipitation. Journal of Catalysis 237 291-302. 

Andreeva, D., Tabakova, T., Idakiev, V., Christov, P., and Giovanoli, R. 1998. Au/a-Fe203 catalyst for water-gas shift reaction prepared by deposition-precipitation. Appl. Catal. A Gen. 169 9-14. 

In 1989, Gadalla and Sommer (252) reported that a solid-solution NiO/MgO (1 1.35) catalyst prepared by precipitation can inhibit the carbon deposition in the CO2 reforming of methane however, they obtained a low CO2 conversion (66%), a low H2 selectivity (79%), and a low CO selectivity (77%), even at the very low WHSV of 3714 cm3 (g catalyst)-1 h-1 with a CH4/CO2 (1/1, molar) feed gas and the high temperature of 1200 K. Their relatively high CH4 conversion was partly a consequence of homogeneous gas-phase reactions that occurred under their conditions. Indeed, the authors found extensive carbon deposits plugging the reactor upstream and downstream of the reaction zone. 

Catalysts were prepared by deposition-precipitation method over a commercial CeOz. Among the catalysts, Ir/Ce02 exhibited a stable activity for 300 h time-onstream due to the prevention of metal sintering and coke resistance of highly dispersed Ir... 

Au/Ti02 Deposition precipitation onto titania prepared from P-25 109 (from 6) 11.6 4.7 2 8 20... 

Andreeva and coworkers518 improved on the preparation of Au/a-Fe203 by using a deposition-precipitation method. The preparation involved deposition of Au hydroxide with a solution of sodium carbonate at 60 °C and pH = 8.0 on a fresh precipitate of iron hydroxide. The precipitate was aged for 1 hour at 60 °C, filtered, washed, dried under vacuum at 80 °C, and calcined in air at 400 °C for 2 hours. A comparison of properties and activities is provided in Table 122. The iron oxide crystallites were about 10 nm in both samples investigated. 

Au/Ti02 systems prepared by deposition-precipitation method. Therefore, it is probable that a more relevant effect is the presence of some residual alkali on the surface due to the preparation method for these Titania nanotubes. 

For the study of the electrocatalytic reduction of oxygen and oxidation of methanol, our approach to the preparation of catalysts by two-phase protocol " provides a better controllability over size, composition or surface properties in comparison with traditional approaches such as coprecipitation, deposition-precipitation, and impregnation. " The electrocatalytic activities were studied in both acidic and alkaline electrolytes. This chapter summarizes some of these recent results, which have provided us with further information for assessing gold-based alloy catalysts for fuel cell reactions. 

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. 

You X, Chen, F, Zhang J, Anpo M (2005) A novel deposition-precipitation method for preparation of Ag-loaded Ti02. Catal Lett 102 247-250... 

Supported Au catalysts have been extensively studied because of their unique activities for the low temperature oxidation of CO and epoxidation of propylene (1-5). The activity and selectivity of Au catalysts have been found to be very sensitive to the methods of catalyst preparation (i.e., choice of precursors and support materials, impregnation versus precipitation, calcination temperature, and reduction conditions) as well as reaction conditions (temperature, reactant concentration, pressure). (6-8) High CO oxidation activity was observed on Au crystallites with 2-4 nm in diameter supported on oxides prepared from precipitation-deposition. (9) A number of studies have revealed that Au° and Au" play an important role in the low temperature CO oxidation. (3,10) While Au° is essential for the catalyst activity, the Au° alone is not active for the reaction. The mechanism of CO oxidation on supported Au continues to be a subject of extensive interest to the catalysis community. 

Two 1% Au/Ti02 catalysts, designated as HAuC and AuCls were prepared by deposition-precipitation of HAuC (Aldrich) and AuCls (Alfa Asar) onto Degussa-... 

Pd on ceria catalysts prepared by various methods were shown to exhibit varying characteristics depending on the preparation methods.625 Specifically, Pd on ceria samples prepared by the deposition-precipitation method are highly active in comparison with the catalyst prepared by the conventional impregnation method. Cationic palladium species are present in the former samples after reduction with hydrogen at 300°C, suggesting that the active species are produced by strong... 

Carbides and nitrides can be prepared in many ways (chemical vapour deposition, physical vapour deposition, precipitation of salts containing metal, carbon and oxygen followed by reduction and annealing, reaction of a metal or its oxides with a gas or with solid carbon). Carbides and nitrides are often nonstoichiometric with complex phase diagrams.4-9 The compounds sometimes contain multiple phases and impurities, notably oxygen. This can lead to even more complex compounds, like oxycarbides, carbonitrides or oxycarbonitrides. 

Figure 3.2 (a) TEM micrograph of an Au/Ti02 catalyst prepared by deposition precipitation (b) schematic model of the interface [31]. 

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