Effect of temperature: The precipitation temperature is a decisive factor in controlling precipitate properties such as primary crystallite size, surface area and the phase formed. Till date it is very difficult to predict the exact nature and extent of effect of the precipitation temperature on the properties and is generally determined experimentally. Nucleation rates are extremely sensitive to temperature. In general, most precipitation process es are carried out above room temperature, often close to 373 K for obvious reason that the precipitation is more rapid. A higher temperature may result in an increase in crystallite size, though this depends on the kinetics of different elementary processes. Sometimes, no effect of temperature or even lowering of size of the crystallites is observed as in the case of ZnO system. Temperature also affects the phase formation. During preparation of Ni/SiO2 catalysts, at high temperatures nickel hydro-silicate is obtained while at lower temperatures, the main precipitate is nickel hydroxide.
When use of high temperature is detrimental, the rotary evaporator is often used to remove the solvent from slurry solution. Rotary evaporator is a vacuum evaporator in which pressure is lowered above the slurry so that boiling point of the solvent is reduced and it can be removed without using excessive heating. In the evaporator a rotating evaporating flask is connected to vapor duct to draw off the vapor and thereby, reduce the pressure within evaporator system. Sample solution is gently heated in bath, usually water bath, to enhance the solvent removal. The separated solvent vapor can be condensed back using a condenser and collected in a separate flask.
Effect of Additives: Additives are substance which are not a necessary ingredients of a precipitation reactions. The properties of the precipitates can strongly be influenced by additives. The most widely used additives are organic molecules which are added to the precipitate in order to control the pore structure. Such organic molecules can later be removed from the precipitate in the calcination step.
A very promising route for the preparation of the high surface area oxides is the use of surfactants as additives. Removal of the surfactant by calcinations steps leaves a well defined pore network. The pore diameter can be adjusted in the range of 2-10 nm. These all are treated as trade secrets and details are not available in the public domain.
Preparation of dual oxides catalysts by coprecipitation
Mixed oxide support and catalyst can be prepared by coprecipitation method. As discussed earlier, for coprecipitation, the solubility of the two components should be in similar range for simultaneous precipitation resulting in homogeneous product. Otherwise the precipitation will be sequential resulting in non-homogeneous product.
Two examples are discussed below.
1. SiO2-Al2O3
SiO2-Al2O3 is used in catalytic cracking process and is also used as support for active metals in various applications. Preparation of dual oxides by coprecipitation is similar to precipitation of single oxide. At pH 6 (at 500C) the precipitation of both silica and alumina sols begins and gelation takes places.
2. NiO-Al2O3
NiO-Al2O3 is used for hydrogenation and methanation reactions. Although this catalyst can be produced by other route, coprecipitation method of preparation is also done to increase the intimate interaction between active metal and support. The sodium bicarbonate can be used as precipitating agent for formation of nickel aluminium hydroxyl carbonate with good homogeneity of final product.
Book References :
• K.P. de Jong., Synthesis of solid catalysts , Wiley –VCH, 2009
• J.T. Richardson, Principle of catalysts development, Plenum Press, 1989
• G. Ertl, H. Knozinger & J. Weitkamp, Handbook of Heterogeneous Catalysis Vol 1, Wiley – VCH, 1997
• R. J. Farrauto & C. H. Bartholomew, Fundamentals of Industrial Catalytic Processes, Blackie Academic & Professional, 1997
• S. P. S. Andrew, Chemical Engineering Science 36 (1981) 1431-1445