The Design and Preparation of Supported Catalysts

BY 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 be related 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. Many advances have been made in recent years in this area, as described in Chapter 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 in its 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 in either 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 degree of dispersion 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 a highly 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 the catalyst 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 the incorporation of a catalytically active species onto a support material. A summary of the most widely used techniques is given below as an introduction to later Sections in this Chapter, which describe the more important chemical and physical factors involved in the dispersion of metal salts onto supports and their influence on the activity, selectivity, and durability of the catalyst system.

Impregnation. – Impregnation as a means of supported catalyst preparation is achieved by filling the pores of a support with a solution of the metal salt from which the solvent is subsequently evaporated. The catalyst is prepared either by spraying the support with a solution of the metal compound or by adding the support material to a solution of a suitable metal salt, such that the required weight of the active component is incorporated into the support without the use of excess of solution. This is then followed by drying and subsequent decomposition of the salt at an elevated temperature, either by thermal decomposition or reduction. With this method of preparation it is essential to have an understanding of both chemical and physical properties of the support and the chemistry of the impregnating solution in order to control the physical properties of the finished catalyst. Comment on these factors is reserved for discussion in a later Section of this Chapter. When used for the preparation of mixed metal catalysts, care has to be taken to confirm that a component in an impregnating solution of metal salts is not selectively adsorbed, resulting in an unexpectedly different and undesirable concentration of metals in a mixed-metal catalyst. This technique has been widely used for the preparation of small amounts of catalyst for basic studies,

Adsorption from Solution. – Adsorption is defined as the selective removal of metal salts or metal ion species from their solution by a process of either physisorption or chemical bonding with active sites on the support. Depending upon the strength of adsorption of the adsorbing species, the concentration of the active material through the catalyst particle may be varied and controlled. This technique is widely used in the preparation of industrial catalysts as it permits a greater degree of control over the dispersion and distribution of the active species on the support. In some systems, however, the weight of the active component that can be incorporated into the support is limited. Although multiple adsorption is often possible it is not recommended when close control of physical parameters is required.

Co-precipitation. – The preparation of supported catalysts by the co-precipitation of metal ions with the support ions usually produces an intimate mixing of catalysts and support. An example of this technique is the co-precipitation of metal ions with aluminium ions to produce a precipitated alumina gel containing the metal hydroxide. This precipitate when calcined produces a refractory support with active component dispersed throughout the bulk as well as at the surface. However, in the preparation of multi-component catalysts, it is possible under improper conditions to obtain a heterogeneous product because of the different solubility products of the constituents. Care should be taken therefore to avoid this undesirable situation by appropriate forethought.

Deposition. – Deposition, as used in preparing supported catalysts, is the laying down or placing of the active components on the exterior surface of a support. One means by which this may be achieved is the preparation of catalysts by sputtering, which involves condensing the metal vapour onto an agitated finely dispersed support. However, as this process is performed under a high vacuum, the technique is probably only useful for the preparation of 'model' catalysts. Alternatively, the process may be performed in the liquid phase by the deposition of a metal sol onto a suspended support.

Chemical Vapour Deposition (CVD). – Another example of deposition is the vapour plating of the support with a volatile inorganic or organometallic compound. The process requires only a moderate vacuum and is currently one of the methods under research in industry as a means of preparing catalysts with a purely surface deposition.

Also included in this preparation category is the addition of a precipitating agent for the metal ion to a suspension of the support in an impregnating solution. A layer of precipitated metal ion adheres to the support material, which can be thermally decomposed as before.

In the case of vapour-phase processes for metal deposition on the support, only limited control of dispersion and distribution of the metal crystallites is possible. In the case of liquid-phase systems, they do not provide as wide a range of catalysts as is possible with techniques based on adsorption from solution. However, the technique does provide a means of preparing well characterized surface-impregnated supports.

3 Catalyst Design Parameters

For catalyst design purposes it is first necessary to translate the catalyst performance parameters into a physical picture of catalyst structure. As we shall see, different performance parameters can give rise to different structural features and so a compromise is generally required. For example it is commonly found in industrial applications that initial catalyst activity may be sacrificed in favour of improved catalyst stability, since a lower activity and a prolonged operating catalyst life is in general preferable to a higher initial activity that decays rapidly. First, we should therefore discuss some of the relationships between the catalyst performance parameters and physical structure.

Activity. – In general activity arises from maximizing both the dispersion and availability of the active catalytic material. Ideally, from an activity viewpoint, the catalyst material should be highly dispersed and concentrated on the external surface of the support. Already, however, there is an inherent conflict as high concentrations of active material become progressively more difficult to disperse.

Stability. – By stability we refer to the loss in activity with time. This is due to one or several of four main causes; fouling of the active surface with involatile reaction by-products, sintering or crystal growth of the active material, poisoning of the active surface by feed impurities, and blockage of the support pore structure.

Sintering during catalyst use is usually not a problem if catalysts are properly designed for their end use, although it is perhaps an important problem during catalyst preparation, activation, and reduction if the impregnated metal is not bound to the support surface. It also becomes an important factor under the more severe conditions imposed during catalyst regeneration.

Fouling of the active surface by reaction by-products is a real problem, which typically can be partially met by selective poisoning of the active ingredient. In a general sense the use of bimetallic supported catalysts would also commonly fall into this category, since selective poisoning implies a close control over the ratio of poison to active material. In this case a severe constraint is imposed upon catalyst design in that both active and moderating components should ideally be in a constant ratio throughout the catalyst support, that is to say, the placement of both should be the same.

Poisoning of the catalyst by impurities introduced with the reactants can often be minimized by placing the active material deep within the catalyst support structure, and since most catalyst supports are also good absorbents, poisons frequently can be selectively removed by such absorption before reaching the active surface. An example would be the removal of traces of lead and phosphorous from a car exhaust by the surface of the catalyst support. A catalyst design modification of this same technique would be the deposition of a poison-resistant catalyst component close to the surface and a poison-sensitive component deep within the support. This technique can be taken even further; an inert material can be used as a poison trap close to the support's external surface. In this way each catalyst support particle can be viewed as coming complete with its own catalyst guard bed. Once again for poison resistance the location of the active component becomes a critical factor in proper catalyst design.

Finally, blockage of the support-pore structure is critically dependent upon the pore-size distribution of the support. Normally a correct balance of large and small pores is required; the former to aid reactant transport and the latter to provide the large surface necessary for the optimal dispersion of the active components. Whereas one might intuitively expect that small pores would block more readily, an important exception has only recently been recognized in the case of ZSM 5 type zeolites. In these the structure is small enough to prevent the formation of the high molecular weight involatile by-products that normally are the pore blocking agents, and yet is still large enough to allow for the transport of reactants and products to and away from the catalytically active sites.

Selectivity. – Catalyst selectivity can change due either to physical or chemical reasons. For sequential reactions diffusivity and mass transport through the pore structure can lead to apparent loss in selectivity in the formation of intermediate products. Location of active ingredients and pore-size distributions are therefore again of importance. Changes in selectivity can also arise from changes in intrinsic chemical activity of the active component. Typically this can be affected by use of multicomponent catalysts in which case, as we saw earlier for stability improvement, the location of the difference components ideally should be the same. A specific example of this type of selectivity arises in the case of multifunctional catalysts in which a hydro-genation function is combined with an acid function. Since the latter is typically provided by the support and the former by the impregnated material, a uniform impregnation is required.

Regenerability. – Regenerability refers to the reactivation of a catalyst, which typically will involve an air calcination followed in some cases by a redispersion of the active components. From the catalyst design viewpoint this will generally imply enhanced thermal-hydrothermal stability of the support itself, combined with stability of the active components under the high temperature oxidizing environments required for the oxidation of the deactivating carbonaceous deposits. It is now generally recognized that many metals sinter more readily under oxidizing conditions and in extreme cases may even dissolve in the underlying support and become effectively removed from the reaction system. A further complication arises with multicomponent catalysts in which the combination ratio is all important, since such combinations frequently are destroyed under oxidizing conditions.

Summarizing this Section, the activity, stability, and selectivity are determined by the correct dispersion and location of the active ingredients. Dispersion, location, and regenerability are each in their turn determined by the interaction of the active components with the support surface and with each other during preparation, activation, use, and regeneration. It is the purpose of this Report to examine in greater detail the extent to which our knowledge of these matters has progressed in the past few years. It is our thesis that substantial progress has been made and although much remains to be done, the present status is such as to justify our claim that we can now talk of catalyst design rather than catalyst preparation.

4 The Control of Metal Dispersion and Location during Catalyst Preparation

It is not our purpose in this Section to give an extended list of catalyst-preparation recipes. Neither is it our intent to give an exhaustive and complete review of all the published papers on catalysis in recent years that may have some aspect of catalysis preparation. The former has been amply covered in recent reviews and the proceedings of two recent symposia are devoted to this subject. The latter would be a virtual impossibility. Rather we intend to try and identify those factors that contribute most to the preparation of viable catalysts and the recent papers that exemplify these requirements and contribute to what we feel have been some of the major significant advances in this field.

Techniques used for Characterization. – As is true in other fields of scientific endeavour, much of this advance has been due to the introduction and wider use of new analytical techniques for catalyst characterization. These are discussed elsewhere in this Volume, but since these form an integral part of much of the work to be discussed, some recapitulation is appropriate. This recent period has been particularly fortunate in the introduction and dissemination of these newer techniques, and this has done much to put a firmer foundation to catalyst preparation and augers well for the immediate future.