Washcoat calcination

Figure 1 XRD pattern of the different washcoats calcined at 500 C (left) and 1000 "C (right). The following crystal phases are identified PdO ( ), y-A Os (A), 0-AI2O3 ( ) and a-AbOj ( ).

The hydroxide deposited onto the surface from an aqueous sol before calcination is called a hydrogeh the corresponding gel from a sol using alcohol as a solvent is called alcogel. Due to a smaller surface tension of alcohols compared to that of water, the washcoat layer from an alcogel is less easy to crack and often is superior to that from a hydrogel [60]. 

Figure 1 shows the block scheme of washcoating. A dry monolith is dipped in an Al-sol. Afterwards, it is drained or blown with air to remove the remaining sol. After drying and calcination at appropriate temperatures, the alumina washcoating is completed. 

An example is washcoating of platinum and silica using a sol containing both a silica and a platinum precursor [58,59]. After calcination a catalyst containing platinum on silica-washcoated monolith is obtained. Since the platinum precursor is homogeneously mixed with silica, encapsulation of platinum may occur, which reduces the effectiveness of platinum as compared to that of a two-step preparation. In this respect, the method may be more suitable for less expensive active phases. 

Zirconium oxides are the preferred supports for the precious metal component rhodium. The cerium oxide and/or the zirconium oxide are added to the washcoat either as preformed oxides or as oxide precursors, such as their respective carbonates or nitrates - the oxides are then formed in situ during washcoat drying and calcination. 

Various methods exist for the application of the washcoat to the support. With ceramic monolithic supports, the washcoat is typically applied as an aqueous slurry by a dipping process, followed by drying and calcination [29]. 

Typically, the microstructured plates are coated prior to their assembly. After cleaning and possible thermal treatment of the bare plates, inlet and outlet parts of the structures are protected, for example, with a thin polymer film. The suspension is deposited on the microchannel plates, and any excess suspension is wiped off. Then, the washcoat is dried at room temperature, whereby it shrinks. After cleaning of the top parts of the microchannel fins, the washcoats are calcined at temperatures of 500-600 °C. After calcination, a catalytically active compound can be introduced by impregnation. 

For the preparation of the three-way catalysts several procedures were used, which are summarized in Table 1. The reference catalyst samples were prepared by coating monolithic ceramic substrates with a cell density of 400 cpsi and a wall thickness of 6.5 mil. After drying and calcination of the coated monoliths, they were impregnated Avith the precious metals, precious metal loadings and precious metal ratios of choice. This method will be referred to as preparation method A. The novel catalyst technologies were prepared by placing the precious metals directly onto the washcoat. To do so, several methods were used. Preparation method C was used to apply the precious metals on all the washcoat components. The methods B and D were used to apply the precious metals selectively on one or more of the washcoat components. Details of the catalysts are given in Table 2. 

Cordierite monoliths were coated with an alumina washcoat and stabilised at 550°C. Some of the samples were then immersed in either an aqueous solution of cerous or cobalt nitrate, dried and calcined in air at 550°C at which the metal nitrates decomposed into their oxides [11]. The samples were weighed and the procedure was repeated until 40 mg of the metal oxide had been deposited onto the alumina washcoated monolith samples. Pt and Pd were applied by direct impregnation using aqueous solutions of HaPtCla and PdCb followed by diying and calcination in air at 550°C [8]. The Pt and Pd loadings (2.0 and 1.09 mg, respectively) of the catalysts were equal on molar basis. The nominal composition of the eight catalysts prepared are listed in Table 1. 

For the preparation of washcoated monoliths the suspensions of sol-aluminium hydroxide with pseudoboehmite structure have been used. This sol formed during the reaction between the hydroxide and nitric acid serves both as a binder and a source of y-A Oa in the final product after calcination. Salts of additives were introduced into sol. The influence of the following parameters on the formation of thermostable washcoated layer have been studied concentration of anhydrous alumina in the sol amount of added HNO3 dipping time number of dippings drying and calcination duration. 

In addition, there is pressure to reduce the cost of the catalyst. Washcoat aluminas have desired surface areas and porosities and are thermally stable. They are best produced by calcination of particular precursors, and aluminium isopropoxide or boehmite have been suggested to be useful materials to calcine [42]. Both of these precursors are not cheap, and less expensive raw materials would be desired,... 

This is shown in Fig 3b, where the Rh was supported on zirconia first and the resulting powder was incorporated into 7-AI2O3 washcoat on a monolithic body. The activity of this catalyst remains virtually intact after calcination in air at 1100°C for one hour [6]. 

Study of the formation and decomposition of aluminium sulphates has shown that low cost gibbsite can readily be converted to thermally stable washcoat alumina through the intermediate formation of the sulphate. Optimal reaction between hydroxidic aluminium starting materials and sulphuric acid occurred in the presence of water. A protective sulphate layer was formed on the surface of gibbsite on reaction with concentrated sulphuric acid which limited conversion. Higher conversion could be achieved by reaction with diluted acid. Conversion of the resultant aluminium sulphate to alumina was essentially complete on calcination at about 1000°C for 4 hours. 

The thermal stability of aluminas in the presence and absence of dopants was then examined. Samples were calcined at 1000 C for 2 h and at 1200°C for 4 h, the latter treatment being a standard washcoat test. The results, summarised in Table 1, show that thermally stable alumina can be produced from the sulphate in the presence of additives. At least in the presence of baria, the formation of Ba0.Al203 and BaO.bAlOs was detected by X-ray diffraction [6]. 

Comparable tests were carried out using doped washcoat alumina. A final surface area of 30.7 m g" after calcination for 4 h at 1200°C was observed. 

The monolithic samples were prepared by coating cordierite honeycombs (cell density 62 cells/cm wall thickness 20 pm) with an aqueous sluny of the desired washcoat oxide. After drying and subsequent calcination at 530°C for 2 h in air, the washcoated supports were impregnated with an aqueous solution of the platinum salt, dried and activated in hydrogen for 2h at 530°C. The total platinum loading was fixed at 50 g/ft3 Pt. 

Magnesia (MgO) - Magnesia is also a possible support material having a good thermal stability that can be used as washcoat material as well. Hashimoto et al. have described a preparation by vapour oxidation that can lead to a large surface area and ultra-fine single crystal MgO. It is very resistant to thermal treatment and has a surface area above 72 m g after calcination at 1500°C. ... 

The precipitate was separated from the excess liquid by centrifugation and washed once with acetone. The precipitate was dried at 120 °C for 10 h. The dried powder was consecutively calcined at 1000 °C and then at 1200 °C for 4 h each. Samples were taken out after each calcination. Finally a small fraction of the powder was aged at 1400 °C in 15% steam for 10 h. The main fraction of the powder was ball-milled and washcoated onto cordierite monoliths (Coming 400 cpsi) by dip-coating technique. The washcoated catalysts were then calcined at 1000 °C for 4 h. 

Palladium and platinum was added to the washcoat, corresponding to 2.5 wt-%. This was carried out with a conventional incipient wetness technique, using platinum and palladium nitrates. After addition of the precious metals the catalysts was once again calcined at 1000 °C for 4 h. 

Table 4. The BET surface area after consecutive calcinations to 1000 °C, 1200 °C (for 4 h each) and 1400 °C (for 10 h in 15 % steam) and crystal phases after calcination to 1200 °C for the three washcoat materials.

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