Catalyst - Preparation and Characterization

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Catalysis is of importance to today's chemical industry. There are only a small number of chemical processes that are still conducted without the use of a catalyst. Catalysts are use in an array of applications from the production of consumer goods to the protection of the environment. The approach involves a wide number of specific competencies of solid state chemistry, analytical chemistry, physical chemistry, kinetics and rheology. The fundamental aspects in the preparation of heterogeneous catalysts starting from catalyst design up to the catalyst in its final form are briefly reviewed. The literature will also be treating mechanisms of catalyst deactivation. Mechanisms of catalyst deactivation are many; nevertheless, they can be classified into six distinct types: (1) poisoning, (2) fouling, (3) thermal degradation, (4) vapor compound formation accompanied by transport, (5) vapor-solid and/or solid-solid reactions, and (6) attrition/crushing. Finally the characterization of the catalyst is necessary at every stage of development. Product quality in commercial productions is aided with appropriate testing, and the first step in any diagnosis of process problems is to measure changes that have occurred. It is vital that accepted techniques for determining these characteristics be established as an adjunct to research design, preparation, testing, and manufacture.

2. IntroductionDefinition of catalyst is, A substance that increases the rate at which a chemical reaction approaches equilibrium without itself being permanently involved in the reaction . Mainly, the catalyst speeds up the kinetics of the reaction towards thermodynamic completion by introducing a less difficult path for molecules to follow.

The catalyst properties that have to be considered include selectivity, activity, crush strength, abrasion resistance and resistance to fluid flow . As a result the design of a catalyst involves both the active phase and support. Typical forms of industrial catalysts include powder, pellets, rings, spheres, extrudates or granules, as shown in Figure 1. More recent innovations include the monolithic supports in their various forms (see Figure (2)).

2.1 Catalyst ComponentAlthough some catalytic materials are composed of single substances, most catalysts have three types of distinguishable components, as shown in Figure 3 and stated below:1.active components,2.a support or carrier, and3.promoters2.1.1 Active ComponentsActive components are responsible for the principal chemical reaction. Selection of the active component of the active component is the first step in catalyst design. The main type of active component is metals, where approx. 70 per cent of known catalytic reaction involves metallic component . Industrially, metals are used in catalytic reforming, hydro-cracking, ammonia and methanol synthesis. An example of an active component will be copper in the Cu-ZnO-Al2O3 catalyst used in methanol synthesis.

2.1.2 PromotersPromoters are an agent, when added, results in desirable activity, selectivity or stability effects and are designed to assist either the support or the active component. One important example of support promotion is control stability. Support oxides may occur in several different phases, some undesirable. With Al2O3, for example, the preferred phase is γ- Al2O3 . This phase has high surface area, a certain degree of acidity, and forms solid solutions with transition oxides such as CoO. When heated, γ- Al2O3 transforms into α- Al2O3, which has a hexagonal structure and low surface area. The transition begins at about 900oC, a temperature not usually encountered during process conditions but possible during catalyst regeneration. Most often, promoters are added to support in order to inhibit undesirable activity, such as coke formation.

Promotion of the active component may be either structural or electronic. For metals an example can be ammonia synthesis. Ammonia synthesis catalysts...
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