Catalysis
- A catalyst is a substance that alters the rate of chemical reaction
without itself being permanently chemically changed. It must take part
in the reaction and it must change chemically, albeit on a temporary
basis. A catalyst provides a different 'pathway' or mechanism that
makes the bond breaking processes (or other electronic changes in the
reactants) occur more readily. Catalysis also involves the formation of
intermediates, not just a matter of an 'activated complex' or
'transition state'.
- A catalyst works by providing an alternative reaction pathway of
lower activation energy Ea, compared to the uncatalyzed reaction.
- Catalysts do play a part in the reaction mechanism.
- Catalysts work in either of two main ways, heterogeneously or
homogeneously.
1. Heterogeneous catalysts
- The
catalyst and reactants are in different phases (usually solid
catalyst and liquid/gaseous reactant)
- The reaction occurs on the catalyst surface which may be the
transition metal or one of its compounds. The reactants must be
adsorbed onto the catalyst surface at the 'active sites'.
- This can be physical adsorbed or 'weakly' chemically bonded
(chemisorption/adsorption) to the catalyst surface. Either way, it has
the effect of concentrating the reactants close to each other and
weakening the original intra-molecular bonds within the reactant
molecules and so allows a greater chance of 'fruitful' collision.
- The diagram above illustrates a typical heterogeneous catalysis e.g.
hydrogenation of alkenes with hydrogen and a nickel catalyst.
- In catalytic converters the very expensive metals Pt, Rh and Pd are
used. Cu and Ni are cheaper alternatives but they are vulnerable to
catalytic poisoning of the active sites by traces of sulphur dioxide in
the exhaust gases. Once poisoned, the catalyst in a converter cannot
be regenerated and its a new costly converter!
- The purpose of a catalytic converter is to convert harmful NO and
CO gases to less harmful N2 and CO2
2. Homogeneous Catalysts
- The
catalyst and reactants are in the same phase (usually a solution),
and so the catalyzed reaction can happen throughout the bulk of the
reaction medium.
- Catalysis can be due to temporary changes in the oxidation state and
ligand(s) of a transition metal ion and results in a 'catalytic cycle'. In
other words, the reaction occurs via some intermediate species and the
original catalyst is reformed.