Deposition-precipitation with urea

The gradual rise in pH required in the true procedure of deposition-precipitation (see Chapter 6) was performed by the addition of urea (CO(NH2)2) for the preparation of gold catalysts. The procedure consists in stirring an aqueous solution containing HAuCLj, urea, and the oxide support in suspension, at 80°C for a given time, then to wash the sample with water and dry it, and finally to apply a thermal treatment. 

Chemical analysis showed that the gold compound that precipitates on Ti02 is close to Au(CO(NH2)2) [41]. This compound results from a reaction between gold anions and urea or products of its decomposition, and forms only when the mixture is heated and reaches pH 3. 

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Figure 1. pH versus time during the deposition-precipitation with urea and gold acetate at 80°C. 

Table 17.3 Gold loading (DP) and average particle size in supported gold catalysts prepared by deposition-precipitation with urea at 80°C followed by calcination at 300°C.

Recent Studies Using Deposition-Precipitation with Urea.332... 

Deposition-Precipitation with Urea (0.14 M of Nickel Nitrate, 0.42 M of Urea, 7.6 g/l" of Silica) from Reference 6... 

RECENT STUDIES USING DEPOSITION-PRECIPITATION WITH UREA... 

Au/oxide Samples Prepared by Deposition-Precipitation with Urea at 80°C with a Nominal Gold Loading of 8 wt %, Followed by Thermal Treatment at 300 C, from References 34 and 37... 

Deposition-precipitation with urea has been also used to prepare Eu doped-YjOj/silica materials (39), which find application not in catalysis, but in the field of phosphor emitting in fluorescent lamps and cathode ray tubes. The method has been applied for depositing yttria (9.7 wt %) doped with Eu (0.30 wt %) from a mixture of nitrates heated at 80°C in the presence of urea and silica for 90 min. After thermal treatment at 900°C, YjOj particles are 12 nm large, coated by a silicate, and after thermal treatment at 1000°C, the a-YjSijO, phase forms. It exhibits interesting luminescent properties, and the authors conclude that this method of preparation is more promising than the sol-gel method. 

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. 

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. 

A modified DP method with urea as a precipitating agent, called the DP urea method, has been developed for Ti02 support [27]. Deposition of gold NPs by this method was appHed to AI2O3 [28], Ce02 [28], and Mn203 [29] supports. Aqueous urea solution produces hydroxyl ions above 60 °C as foUows ... 

Scaling up of the deposition-precipitation procedure does not present any difficulty with urea or sodium nitrite. The preparation of tin oxide on silica catalysts has been scaled up from 11 to a vessel of 2 m3 without any difference in the final catalyst. The injection procedure has been scaled up by recirculating the suspension of the support from a large vessel through a small vessel in which the alkaline solution was injected into the vigorously agitated suspension. Provided the flow of the fluid being injected was kept slow and continuous, excellent results have thus been obtained. 

Extremely good distributions of the active precursor over the internal surface of the support bodies were obtained. The difficulty, however, is the loading that can be achieved. The limited pore volume of preshaped support bodies and the solubility of the precursor to be deposited and of the agent(s) for deposition-precipitation necessitate multiple impregnations in order to achieve loadings characteristic of base metal (compounds). With urea and simple nitrates, however, highly... 

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... 

FIGURE 28 Metal concentration profiles along the length of a monolith for deposition precipitation by use of urea decomposition with nickel nitrate on a cordierite monolith. Metal deposited on a 25-cm-long, 1-cm-diameter, 400 cpsi monolith. Profiles were determined by X-ray fluorescence spectroscopy of finely ground 2.5-cm long sections of the monolith. 

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