Practice of Kinetics (Comprehensive Chemical Kinetics, Volume 1)

2 THE SOLID-GAS INTERFACE 22 1(b) Impregnated catalystsImpregnation can be carried out on an inert porous support. This procedureoften serves to greatly increase the effective surface area and provide mechanicaland chemical stability. If the support is not porous, it is still possible to coat thesurface, although the adhesion of the catalyst may be less satisfactory. The generalprocedure is to immerse the support in a solution of thecatalyst component, removethe excess solution after a suitable impregnation time, dry and ignite. Nitrates areagain preferred, because of their solubility and ease of decomposition. The otherpremises mentioned under (a) also apply. If necessary, the support can be evacuated,prior to immersion, to remove air which might otherwise prevent a uniformdistribution of active material.(c) Precipitated and gelled catalystsThis method can be applied to the preparation of single catalysts or, by coprecipitation,to multi-component catalysts. Hydrous oxides, sulphides, carbonatesand phosphates can be used. After precipitation, the catalyst must be washedfree of impurities which might have an adverse effect on the final catalytic properties.The presence of these impurities can be minimised by the use of dilute solutions.An alternative is the mixing of a metal salt with ammonia or an ammonium salt.Any ammonium nitrate remaining in the precipitate is readily removed by washingbecause of its high water solubility, and the last traces can be eliminated by calcination.Where two or more components are co-precipitated, special steps must be takento ensure homogeneity of the final catalyst. This can be achieved by adding asolution of both components to an excess of the precipitating agent, rather thanthe other way round. The physical properties of precipitated catalysts will oftendepend on the conditions of precipitation (eq. concentration of solutions, orderand rate of mixing, temperature of precipitation, washing, drying and calcination)all of which must be carefully studied.Gel formation is a special case of precipitation and is particularly suitable forcatalysts whose major components are hydrous oxides such as silica or alumina.The production of silica-alumina catalysts by this technique has been studied ingreat detail because of their commercial application^^'^. Crystalline aluminosilicatesor zeolites more commonly known as ‘molecular sieves’ have recentlybeen the subject of a great deal of research. Preparative details are still mainly inthe patent literatureis0.(d) Skeletal catalystsA special type of catalyst which is typified by “Raney Nickel”’s’ is preparedby leaching out one component from a binary alloy leaving a skeletal structure ofthe desired catalyst. Raney Nickel itself is made by leaching out aluminium froman aluminium-nickel alloy with sodium hydroxide. Cobalt and iron catalysts havealso been prepared in this manner.References pp. 270-278