During the preparation of heterogeneous catalyst by impregnation or co precipitation how can I choose the r.p.m of mixer and time of mixing ?
What exactly are you asking? What do you want to control with r.p.m. setting on your mixer?
Suraj Punathil if you are talking about the calculation part then it depends on what you are synthesizing. The reaction kinetics, conc. of reagents being used, which catalyst being synthesized and process employed. If you are asking about the operational part then it depends on the type of instrument you are using.
Bartlomiej Wojciechowski I want to change crystalline properties and increase surface uniformity of my catalyst
Adjusting crystalline properties and increasing surface area are important properties that need to be tailored/optimized for specific applications. So it would be necessary to know the system u r working on and the reaction of interest.
Having said that , you may still adopt certain guidelines as follows.
For co precipitation, uniformity and optimum crystalline size are better obtained by variables such as solid concentration in the slurry , simultaneous mixing at constant pH and use of a buffer . Type of stirring will depend on the instrument being used , but generally for laboratory work a baffled vessel and a limiting rpm that does not cause a vortex is best suited for reproducible results.
In the case of impregnation by incipient wetting , stirring may be less of an issue and gentle rolling in say a round bottomed flask and slow addition of liquid to be impregnated should be OK. Limiting the solution quantity to ~ 90% of pore volume helps
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