how many groups can catalyst be broadly divided ...
Answers
Lecture 5
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:
i. Metallic conductors ( e.g Fe, Pt, Ag, etc.)
ii. Semiconductors (e.g. NiO, ZnO,etc.)
iii. Insulators (e.g. Al2O3, SiO2,MgO 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.
Answer:
Catalysts can be classified into two types: homogeneous and heterogeneous. Homogeneous catalysts are those which exist in the same phase (gas or liquid ) as the reactants, while heterogeneous catalysts are not in the same phase as the reactants.
Explanation: