Metal-structured catalyst carriers

Kolodziej, A., Krajewski, W. and Dubis, A. (2001). Alternative Solution for Strongly Exothermal Catalytic Reactions A New Metal-Structured Catalyst Carrier, Catal. Today, 69, pp. 115-120. 

In many cases there is an interaction between the carrier and the active component of the catalyst so that the character of the active surface will change. For example, the electronic character of the supported catalyst may be influenced by the transfer of electrons across the catalyst-carrier interface. In some cases the carrier itself has a catalytic activity for the primary reaction, an intermediate reaction, or a subsequent reaction, and a dual-function catalyst is thereby obtained. Materials of this type are widely employed in reforming processes. There are other cases where the interaction of the catalyst and support are much more subtle and difficult to label. For example, the crystal size and structure of supported metal catalysts as well as the manner in which the metal is dispersed can be influenced by the nature of the support material. 

Processing aids are used to directly influence the synthesis process function as reaction controllers. Depending on their chemical state they can function as reaction accelerators (the actual catalysts and starters or initiator substances), crosslinkers and/or hardeners, reaction inhibitors or catalyst deactivators, molecular weight controllers, chain splitters or lengtheners. From a chemical standpoint (structure and method of function) the radical builders, mainly peroxides and azo compounds, are treated separately from the catalysts which are mainly metals, metal oxides, salts (redox systems) and organo-metal compounds. The carrier substances, promoters and deactivators are placed in the catalyst class of substances. 

For the majority of industrial catalysts, the sizes of supported metal particles arc less than the mean free path of the electrons analysed. All the metal in the particles is effectively analysed. For highly dispersed systems, XPS surface analysis and bulk X-ray nuorcsccncc analysis therefore give similar results. Comparing information from these two techniques can be used to show a change in the distribution of metals on the surface due, for example, to sintering or to the inclusion of one of the metals into the carrier structure. 

Other powder metal or ceramic parts may have to become filters or catalyst carriers requiring large numbers of penetrating pores (see Section 5.3.2), uniform structure, and high strength. In those cases sintering of preforms is carried out such that no densification occurs and porosity remains unchanged. 

Formates, for example, those of Co [17], Ni [2, 9, 17] and Cu [17]. Decomposition of these compounds yields a very porous active metal structure of crystallites the same is true of Ni and Co oxalates [3, 13]. The decomposition of these salts is, to a large extent, a topochemical reaction, in which the nascent free-metal atoms regroup themselves within a very small region. To obtain these metals in the form of carrier-supported catalysts, one can start with a mixture of salts [3]. 

To maintain catalytic performance during long-term vehicle operation, porosity of the cordierite material is important to fix the catalyst layer on the cell wall of the honeycomb substrates. Figure 13.1.7 shows the washcoat layer, which contains y-alumina composite carrier and precious metal catalyst, on the porous cell wall surface. This photograph shows a 6mil/400cpi standard cell structure catalyst made of 35% porosity cordierite. 

Van Hardeveld and Hartog describe the effect of metal particle size on the properties of a metal on carrier catalyst. They have related the adsorptive and catalytic properties of metal crystals to crystal size and to the structure of the crystal surface. 

Most industrial catalysts are heterogeneous catalysts consisting of solid active components dispersed on the internal surface of an inorganic porous support. The active phases may consist of metals or oxides, and the support (also denoted the carrier) is typically composed of small oxidic structures with a surface area ranging from a few to several hundred m2/g. Catalysts for fixed bed reactors are typically produced as shaped pellets of mm to cm size or as monoliths with mm large gas channels. A catalyst may be useful for its activity referring to the rate at which it causes the reaction to approach chemical equilibrium, and for its selectivity which is a measure of the extent to which it accelerates the reaction to form the desired product when multiple products are possible [1],... 

Catalysts vary both in terms of compositional material and physical structure (18). The catalyst basically consists of the catalyst itself, which is a finely divided metal (14,17,19) a high surface area carrier and a support structure (see Catalysts, supported). Three types of conventional metal catalysts are used for oxidation reactions single- or mixed-metal oxides, noble (precious) metals, or a combination of the two (19). 

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