Precipitation-deposition

Like incipient wetness impregnation, this method provides supported metal oxides with high loadings. The support material is suspended in a precursor solution, and then precipitation of the supported metal oxide is induced such that the metal oxide nanoparticles (NPs) are nucleated and grown on internal and external surfaces of the support. 

This method allows obtaining surface with homogeneous active and highly dispersed phases. It is a hybrid method known as deposition-precipitation, when carriers may be added to the precipitant solution. The precipitating precursor is deposited on this support in suspension. This preparation depends on various parameters, such as, pH of the solution and mainly of the external surface of the support. The main problems are the onset of nucleation and crystal growth over the surface, thus nucleation and formation of small particles over the surface. 

Step 1—Starting from nitrate, it forms Ni ions and the silica provides hydroxyls  

The silica ions SiO attract Ni ions, which are adsorbed and in turn react with the hydroxyls, forming the precipitate  

In addition to this precipitate, there is the possibility of forming silicates, releasing the hydroxyl again 

In this process, it enhances the interaction between the ion and the support, resulting better metal dispersion and higher competition between the rate growth and nucle-ation rate on the surface, as shown in Fig. 7.10. 

As coprecipitation methods are not easy to control and reproduce, and impregnation techniques can not always be made to yield the desired active-phase distribution, loading and/or dispersion, it is worthwhile considering alternative methods. One of these is deposition-precipitation, which will be discussed in this section. 

Traditionally, the objective of precipitation is to obtain a well-dispersed metal hydroxide (or carbonate) phase on a support through its precipitation from an aqueous metal solution onto a support powder by adding a base. The support powder is suspended in the metal salt solution. Upon addition of, say, NaOH, the pH increases strongly and metal hydroxide species are generated. When their concentration exceeds the (super)solubility limit, metal hydroxide particles can 

To establish a very uniform distribution of small active particles over a support, and to get around the mixing problem, the procedure of deposition-precipitation was developed. For the method to work well it is essential, as for precipitation in general, that the support facilitates the nucleation of an active precursor (cf. below). In the classical embodiment of the method, a metal salt solution containing urea is well mixed with the support powder at room temperature. Then, the temperature is raised to 70-90°C, where the urea slowly hydrolyses according to 

Apart from raising the pH level, precipitation can be induced in a variety of other ways. Anionic species, for example, can be deposited on the surface of suspended carriers by decreasing the pH level. This procedure has been used for vanadium(V) and Mo(VI). Oxidation at a pH level where the ions of the lower valency are soluble and the oxidized species insoluble, can also be utilized to precipitate from a homogeneous solution. Iron(n/m) and Mn(III/IV) are cases in point. Oxidation can be 

After the deposition-precipitation step has been completed, the solid is filtered, washed and dried. Before activation and use, it has to be shaped (e.g. pelleted), and, in most cases, calcined. 

This basic method has numerous variations, such as pH, temperatures of preparation and washing, for instance at room temperature instead of higher temperature,25,63,64 and use of other bases, such as ammonia.65,66 Some of the parameters have been systematically investigated in a study 

The method works well with supports having an PZC greater than five, such as magnesia, titania (usually Degussa P25, 70% anatase and 30% rutile), alumina, zirconia and ceria,10,37,65 but it is not suitable for silica (PZC 2), silica-alumina (PZC 1), tungsta (PZC l),10 or for supports such as activated carbon68 (see Section 4.5.4). For zeolites, surprisingly, it does seem to work (see Section 4.5.1). 

An alternative explanation combining interpretations found in two separate studies on Au/Ti0237 67 could be that when pH is above but close to 6, which is the PZC of titania, there is surface complex formation  

This method called DP, which consists of fixing the pH of the HAuCU solution by addition of a base, obviously does not correspond to that of the principle of deposition-precipitation, which consists of a gradual increase of the pH to avoid precipitation in solution (Section 4.1.1). In the present method, there is no Au(OH)3 precipitation, but only grafting of gold complexes. 

This method is convenient and is used for producing commercial gold sup -ported catalysts [7, 28]. It is also the one that can be applied to the widest range of different support materials [32]. 

In the procedure described by Haruta, after the pH of an aqueous solution of HAuCH is adjusted with NaOH, to a fixed point in the range of 6-10, a metal oxide support, in the form of a powder, bead, honeycomb, or thin film, is immersed in the solution. The partially hydrolysed species [Au(OH) Cl4 ]- (n = 1-3) then react with the surface of the support. Ageing for about 1 h results in the deposition of Au(OH)g exclusively on the surface of the metal oxide support, if the concentration and temperature are properly chosen [18,28,31]. 

The dispersion and size of the particles and therefore their catalytic activity, depend critically upon the pH used (typically 6-10) and the amount of gold in the solution [8,9,31-33,35-41]. 

Haruta emphasises that the influence of the pH on the particle size of Au is remarkable. Above pH 6, the main species of Au in solution are transformed from AuClJ to [Au(OH) Cl4 ] (rr = 1-3), and the mean particle diameters of Au in the calcined catalysts become smaller than 4nm [31]. 

Bond and co-workers [43] showed that pH 9 was the optimum value to be reached during deposition precipitation for high activity of Au/Ti02 catalysts used in CO oxidation. At this pH, the main species in solution were anionic Au complexes, from which most of the chlorine had been removed by hydrolysis. At lower pH, the Au complexes contained more chloride, Au particles were larger, and activities were lower [43]. 

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A weigh tank containing chlorosulfonic acid needed to be cleaned to remove salt deposits. The salt deposits precipitated from the material and occasionally plugged the downstream control valve. Since the material was water reactive, heptane was chosen to clean the vessel. Chemists had not anticipated the material would be reactive with heptane. While cleaning the vessel the pressure... 

Figure 3.2 Schematic representation of deposition-precipitation (DP) method.

Successful applications of the oxygen-modified CNFs are reported on immobilization of metal complexes ]95], incorporation of small Rh particles [96], supported Pt and Ru CNFs by adsorption and homogeneous deposition precipitation ]97, 98], Co CNFs for Fischer-Tropsch synthesis ]99], and Pt CNFs for PEM fuel cells [100]. 

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).

Gas-Phase Grafting Liquid-Phase Grafting Deposition-Precipitation... 

The approach comprises deposition-precipitation (DP) of Au(OH)3 onto the hydroxide surfaces of metal oxide supports from an alkaline solution of HAUCI4 [26] and grafting of organo gold complexes such as dimethyl gold (Ill)acetylacetonate (hereafter denoted as Au acac complex) [27] and Au(PPh3)(N03) [28] either in gas and liquid phase are advantageous in that a variety of metal oxides commercially available in the forms of powder, sphere, honeycomb can be used as supports. 

Figure 6. Postulated role of magnesium citrate in the deposition precipitation of gold hydroxide on Mg(OH)2.

One solution-based approach that works for gold catalysts, in that it produces highly active catalysts, is the deposition-precipitation (DP) method [8]. The DP method entails adjusting the pH, temperature, and gold concentration of an HAUCI4 solution to form a gold hydroxide species which is then deposited onto the support material [8]. This catalyst precursor is washed, dried, and annealed to form small (<5nm) catalyst particles [9]. The DP method has a number of limitations for example, DP cannot produce Au particles with diameters less than 5 nm on support materials with low-isoelectric points (lEPs) like SiOz and WO3 [5,10,11]. 

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]. 

U and Th concentrations in secondary deposits precipitated from solution generally reflect relative abundances in the hydrosphere. Uranium is co-precipitated with CaCOs in subaerial environments on exsolution of CO2 (or evaporation), while the immediate daughter products are essentially absent. This represents extreme chemical fractionation of parent and daughter isotopes within the hydrosphere. 

Volcano-sedimentary ore deposits are syngenetic deposits precipitated from sea water enriched in metals by submarine volcanic activity. Deposits of this type are also called submarine exhalative-sedimentary deposits. Stratabound lead-zinc-copper deposits associated with marine sedimentary volcanic sequences belong to this category. Important examples are Kuroko deposit in Japan, Mt. Isa in Australia, Sullivan deposit in British Columbia, Canada, Rammelsberg in Germany and Rampura-Agucha in Rajasthan, India. 

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. 

Input data for the most detailed soil model include parameters describing atmospheric deposition, precipitation, evapotranspiration, litterfall, foliar uptake, root uptake, weathering, adsorption and complexation of Pb, Cd, Cu, Zn, Ni, Cr and Hg. The input data mentioned above vary as a function of location (receptor area) and receptor (the combination of land and soil type) as shown in Table 6. 

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... 

Fu et al. 02 reported enhanced OSC with Au addition to ceria based on the CO pulsing technique. They were also unable to discern the active state of Au, as both metallic and ionic species were present in XPS measurements after calcination at 400 °C. In a later report, the authors403 removed metallic Au particles by extraction with a 2%NaCN solution and observed that the lightoff curves remained unchanged, with activation energies being virtually identical for the leached materials relative to the materials prior to leaching, 47.8 kJ/mol for deposition-precipitation method... 

Au/TiOz Deposition precipitation onto anatase titania 60 80 90 88... 

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