The Design and Preparation of Supported Catalysts
G.J. K. ACRES, A. J. BIRD, J. W. JENKINS AND F. KING
1 Introduction In this Report of catalyst-preparation technology we have placed particular emphasis on catalyst design as opposed to preparation. A properly designed catalyst should have the essential attributes of activity, stability, selectivity, and regenerability. These can berelated to the physical and chemical properties of the catalyst, which in turn can be related to the variable parameters inherent in the method used for the preparation of the catalyst. In the past much of the literature on supported catalysts has not included this information. In part this was due to the lack of techniques for physically and chemically characterizing supported catalysts. Manyadvances have been made in recent years in this area, as described in Chapier 2, so that the design of supported catalysts has become a feasible activity. In addition to a wide range of techniques for the preparation of supported catalysts a substantial number of supports are available for such systems. In this Chapter we highlight the technology of catalyst preparation and the role of the support inits application. In Table 1 are listed the total U.S. sales of catalyst support materials for 1977. The predominence of alumina and zeolites is reflected in the literature on the preparation of supported catalysts and hence in the contents of this Chapter. 2 General Methods of Preparation for Supported Catalyst Systems The principal catalyst-preparation technique involves two stages. First,rendering a metal-salt component into a finely divided form on a support and secondly; conversion of the supported metal salt to a metallic or oxide state. The first stage is known as dispersion and is achieved by impregnation, adsorption from solution, co-precipation, or deposition, while the second stage is variously called calcination or reduction.’ It is brought about by a thermal treatment ineither an inert atmosphere or an active atmosphere of either oxygen or hydrogen. When the active atmosphere is hydrogen the process is known as reduction. Although calcination/reduction can cause major problems in catalyst preparation on a large scale, it is a generalization to say that once the metal species has been bound to the support surface its
B. Delmon, P. Grange, P. A. Jacobs, and G.Poncelet, ed., ‘Preparation of Catalysts 11’, Elsevier, Amsterdam, 1979.
Table 1 US. sales of catalyst supports in 1977*
Catalyst support products Alumina Zeolites Monolith cordierite Diatomaceous earth Activated carbon Silica gel Silica/alumina
U.S. saZes (% wt.)
56.9 22.2 79 . 75 .
3.0 1.3 1.2
Adapted from Chem. Tech., 1979, April
degree ofdispersion and location will be retained during subsequent treatments. This Chapter therefore concentrates on the dispersion stage of catalyst preparation rather than the thermal treatment stage, although where this is known to cause difficulty it is discussed. The primary aim of applying a catalytically active component to a support is to obtain the catalyst in a highly dispersed form and hence in ahighly active form when expressed as a function of the weight of the active component. This feature of supported catalysts is especially important with regard to precious-metal catalysts, because it allows more effective utilization of the metal than can be achieved in bulk-metal systems. However, in the case of base-metal catalysts the use of the support is often primarily aimed at improving thecatalyst stability. This can be achieved by suitable interaction between the active material and the support. For example: unsupported copper oxide is a very active oxidation catalyst but suffers from thermal instability at high temperatures. However, when copper oxide is supported on a high-surface-area alumina, its thermal stability is improved. A wide range of techniques has been employed for...