d and f Block Elements
A catalyst is a substance that alters the rate of a reaction by directing the reaction in an alternative pathway that requires lower activation energy. Catalysts play a crucial role to allow chemical processes to change reaction steps and proceed at an economical rate. The ability of transition elements to use the 3d and 4s electrons to form weak bonds to small reactant molecules makes them effective heterogeneous catalysts and homogeneous catalysts as they provide a favorable environment for the reactant molecules to come together with the correct orientation. A heterogeneous catalyst is in a different state of matter than the reactants and it essentially provides surface is for the reaction to occur microscopically. For example, the reactants may be gases and the catalyst a solid.
Homogeneous catalysts are in the same state of matter as the reactants and they form transition complex ions before producing the final compound. Transition metals show variable oxidation states which allows them to be effective homogeneous catalysts in redox reactions. Homogeneous catalysts are of fundamental biological importance as enzymes.
In summary, the catalytic action of transition metals is due to the following reasons:
Variable oxidation state: With the help of variable oxidation state, transition metals form unstable intermediate compounds. These intermediate compounds provide a new path with the lower activation energy for the reaction (Intermediate compound formation theory)
Large Surface area: When transition metals or their compounds are finely divided, they provide a large surface area for adsorption and the adsorbed reactants react faster due to the closer contact.
Transition metals and their compounds effective and important catalysts for industrial and biological applications. The availability of 3d and 4s e- and the ability to change oxidation state are among factors which make transition metals such good catalysts. The catalyst in a different phase from reactants means that a solid transition metal catalyst with reactants in liquid or gas phases. Transition metals can use the electrons from 3d and 4s orbitals on the complex ion (ligand) surface to form weak bonds to the reactants. Once the reaction has occurred on a metallic surface, these bonds can break to release products. An important example is the hydrogenation of alkenes using Ni or Pt catalyst. As a homogeneous catalyst, transition metals ionize in aqueous form. The ion forms an intermediate compound with one or more of the reactants and the intermediate then breaks down to form products.
|Transition element/compound||Catalytic Action|
|TiCl3||Used as Ziegler – Natta catalyst|
|V2O5||Converts SO2 to SO3 in the contact process to produce H2SO4|
|MnO2||Used as a catalyst to break down KClO3 to give O2|
|Fe||Used in Haber Bosch process for producing NH3|
|FeCl3||Production of CCl4 from CS2 and Cl2|
|FeSO4 & H2O2||Fenton’s reagent|
|PdCl2||Wacker process :C2H2 + H2O + PdCl2 → CH3CHO + 2HCl + Pd|
|Pd||For hydrogenation (Phenol → Cyclohexanone)|
|Pt/PtO||Adams catalyst used for reduction|
|Pt||SO2 → SO3 contact process|
|Cu||In the production of (CH3)2SiCl2|
|Cu/V||Oxidation of cyclohexanol|
|CuCl2||Deacon process for making Cl2 from HCl|