Deposition-Precipitation with NaOH

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. 

Several remarks can be made regarding this preparation method  

Cold Complex Interaction with Oxide Supports 

The decrease in gold uptake as pH increases is explained by an equilibrium between the adsorbed species and those in solution, which shifts toward the solution side as the pH is increased. 

If one reconsiders the case of pH lower than the PZC of Ti02 (conditions of anion adsorption), electrostatic adsorption of the gold anions is possible. However, the increasing gold uptake when pH increases is not consistent. The presence of neutral AuCl3(H20) between pH 2 and 5 deduced from thermodynamic data, which may interact with the support, could explain that the gold uptake increases as pH increases and becomes closer to the PZC. Literature data in geochemistry reports the same type of observation, that is 

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Zanella, R., Delannoy, L. and Louis, C. (2005). Mechanism of deposition of gold precursors onto Ti02 during the preparation by cation adsorption and deposition-precipitation with NaOH and urea. Appl. Catal. A-Gen. 291(1-2), 62-72. 

Supported Metal Samples Prepared by Deposition-Precipitation with NaOH... 

This method has been also applied to the development of supported gold catalysts. In the case of the so-called deposition-precipitation with NaOH, the procedure of preparation has been, in fact, completely modified and rather corresponds to preparations at a fixed pH in excess of solution, in other words, to a procedure of ion adsorption. In the case of deposition-precipitation with urea, there is precipitation of a gold compound, not gradual as in classical deposition-precipitation, but sudden and fast at low pH, followed by a period of maturation during the increase of pH, which induces a redispersion of the supported phase. 

Au/TiOj catalysts were prepared at various pH by deposition-precipitation using NaOH as a base (31, 32). Typical procedure of preparation was as follows After adding the support to an aqueous solution of HAUCI4, the pH of the suspension was raised to a fixed value by adding sodium hydroxide or carbonate, after which it was heated at 70 or 80°C with stirring for 1 h. After thorough washing with water to remove as much of the sodium and chlorine as possible, the solid... 

In the other two studies, selenosulphate was used. In one, a formamide complex of Hg, made by dissolving HgO in formamide, was used [154]. The solution was made ca. 0.5 M in NaOH, and a trace of polyvinyl pyroUidone was added. The deposition was carried out at room temperature. The polyvinyl pyrollidone slowed the deposition somewhat and apparently improved fihn uniformity and adherence as well as slightly increased terminal thickness (500 mn). It was noted that films were not obtained with the usual complexants, such as ammonia, triethanolamine, and cyanide. It is not mentioned in which way these complexants were unsuitable ammonia and triethanolamine might be too weak, resulting in immediate precipitation in solution. Also, addition of ammonia to some mercuric salts tends to lead to precipitation of insoluble products. Cyanide, however, is a very strong com-plexant and would be expected to control such bulk precipitation better than formamide. Iodide, a strong complex for Hg (and successfully used to deposit HgS, as described earlier), resulted in film deposition but with poor reproducibility. 

Vapor-phase epoxidation of propylene using H2 and O2 was carried out over gold catalysts supported on mesoporous ordered (MCM-41) and disordered titanosilicates prepared hydrothermally or by modified sol-gel method. Gold nanoparticles were homogeneously dispersed on the titanosilicate supports by deposition-precipitation (DP) method. The catalysts and support materials were characterized by XRD, UV-Vis, surface area measurements (N2 adsorption) and TEM. NaOH was found to be the best precipitant to prepare Au catalysts with optimum propylene oxide yields and H2 efficiency. The extent of catalysts washing during preparation was found to affect the activity of the catalyst. The activity and hydrogen efficiency was found to depend on the type of mesoporous support used. 

Deposition-precipitation. The samples were synthesized accordingly to the method reported in literature [2, 6]. Aqueous solutions of the metal (10 M) were prepared by dissolving HaPtCle.hHaO (Acros) or (NH4)2PtCl4 (Acros) in distilled water. An amount of Ti02, chosen to obtain a desired metal loading of 3 wt%, was dispersed in these solutions. The pH of the titania dispersion was then adjusted to 10 with 0.1 M NaOH. The deposition reactions were carried out by stirring the solution for 2 h or 24 h at room temperature or 60°C (exact conditions are specified in the text). The products were washed 4 times with distilled water to remove chloride and sodium ions and then dried in a vacuum oven at 60°C for 16 h. The precursors were then heated at 300°C for 4 h under a flow of Hz in He (10/90). 

According to Chariot [11], PtJ CU should be very easily hydrolyzed and the hydroxide of Pt is almost not soluble in NaOH solution. Preparations of Pt/Ti02 catalysts by the deposition-precipitation method with a tetrachloroplatinate (II) containing salt were thus investigated. 

FIGURE 14.8 Deposition-precipitation of gold on titania with NaOH for various pH (a) An loading (b) gold particle size from Reference 32. 

For example, to understand the mechanism of development of morphology of the synthesis product in the hydrothermal method, the t) pe of alkati source employed and its reactivity with the fly ash, can influence the input and output streams of each as depicted in Fig. 4.4. In fact, alkali activation of the fly ash particles, with NaOH, causes etching of the outer surface of its particles and increases its surface roughness because of the large scale dissolution of Si" " and Al " in the alkali solution. This can also be attributed to the precipitation reaction products, their nucleation and subsequent crystallization, as fine crystals of zeolite P, mostly seen as surface deposits in the activated fly ash residue particles (refer Fig. 4.4a) [12]. The increase in the rate of dissolution can be directly correlated with the increase in alkali concentration and/or temperature, which can finally result in the rapid nucleation and crystallization of big spherical crystals of Sodahte, which has been shown as projecting out of the surface as seen in Fig. 4.4a. 

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