3.5.4 Transition Metals - Catalysis
Transition metal catalysts
Catalysts are substances that increase the rate of a reaction and that can be recovered unchanged chemically at the end of the reaction.
Transition metals and their salts have proven to be very effective catalysts for many reactions. This is normally attributed to the availability of the 'd' orbitals and the variable oxidation states exhibited by transition metals. Below are a few examples.
|Transition metal catalyst||Process|
|Nickel||Hydrogenation of alkenes|
|Iron||Haber process for ammonia production|
|Vanadium(V) oxide||Contact process for sulfuric acid production|
|Molybdenum(VI) oxide||Oxidation of methanol to methanal|
|Platinum/Rhodium||Nitric acid production|
|Palladium||Exhaust gas conversion|
|Titanium compounds||Zeigler Natta addition polymerisation|
|Manganese(IV) oxide||Hydrogen peroxide decomposition|
Example: The industrial manufacture of methanol
On industrial scale methanol is prepared from a mixture of carbon monoxide and hydrogen. The gaseous mixture is subjected to 200 atmospheres and then passed over heated catalyst mixture of ZnO and Cr2O3 kept at 400º C to 450º C. This reaction results the formation of methanol vapours which are then condensed to liquid state.
CO + 2H2 CH3-OH
Heterogeneous and homogeneous catalysis
The state of the catalyst may be the same as that of the reactants, in which case it is termed homogeneous, or it may be in a different state, heterogeneous.
The latter is the most common form of catalysis, as it is more controllable industrially.