Solid   catalysts

Catalyst  components

A solid catalyst consists of mainly three components :

1. Catalytic agent
2. Support /carrier
3. Promoters and Inhibitors

Catalytic agent:
These are the catalytically active component in the catalyst. These components generate the active sites that participate in the chemical reaction. Activity of any catalyst is proportional to the concentration of these active sites. Though concentration of the active sites depends on the amount of catalytically active component, however, it is not always directly proportional. Availability of active sites depends mainly on the dispersion of catalytic agent. The dispersion is defined as ratio of total number of exposed atoms/molecules of catalytic agent available for reaction to total number of atoms/molecules of catalytic agent present in the catalyst sample.

Catalytic agents may be broadly divided in the following categories:

1. Metallic conductors  ( e.g Fe, Pt, Ag, etc.)
2. Semiconductors  (e.g. NiO, ZnO, etc.)      
3. Insulators (e.g. Al₂O₃, SiO₂, etc.)

Metallic conductors: The metals that have strong electronic interaction with the adsorbates are included in this category. The metals are used in various catalytic reactions such as methanol synthesis, oxidation, hydrogenation and dehydrogenation processes.

Examples of metal catalysts:

Cu for water gas shift reaction and methanol synthesis; Ag for oxidation of ethylene to ethylene oxide, Au for oxidation of methanol to formaldehyde; Fe for ammonia synthesis; Pd and Pt for hydrogenation of olefins, dienes, aniline or nitriles as well as dehydrogenation of alkanes, alcohols, cyclohexanes, cyclohexanols etc.

Semiconductors:

The oxides and sulfides of transition metals that have catalytic activity are included in this category. Similar to conducting metals, they are also capable of electronic interaction with adsorbed species and catalyze the same type of reactions. Usually the lower valence band electrons participate in bonding. The upper conduction band separated by band gap energy is empty unless electrons are promoted by heat or radiation. Semiconductor characteristics may be intrinsic or induced by addition of foreign ion, creating cationic or anionic vacancies. Common transition oxides and sulfides such as CuO, AgO, NiO CoO, Fe₂O₃, MnO, Cr₂O₃, FeS, V₂O₅ show conductivity. These materials participate in catalytic reactions and reaction occurs through acceptation or donation of electrons between the reactant material and catalysts. Few applications of semiconductor catalysts are: CuO for oxidation of nitric oxides, NiO for dehydrogenation of alkanes, MnO₂ for oxidation of alcohols, and V₂O₅ for oxidation of hydrocarbons.

Insulators: Catalytic functions of insulators are different from that of conductor and semi conductor materials. Insulators have large values of band gap energy and very low concentration of impurity levels. The electrons remain localized in valence bonds and redox type reactions involving electronic interaction as observed for metal or semiconductor catalysts does not occur. However, insulators have sites that generate protons, thereby, promote carbonium ion based reactions such as cracking, isomerization or polymerization. Al₂O₃, SiO₂, SiO₂-Al₂O₃, zeolites, MgO, CaO, MgAl₂O₄, SiO-MgO are few examples of the insulators used as catalysts.