Catalysis

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Catalysis
R W Grime Ripon Grammar School
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INTRODUCTION

• Catalyst substance that speeds up reaction
  without being used up.

• They provide an alternative mechanism with lower
  activation energy.

• For an equilibrium, they speed up both reactions
  equally and so do not effect equilibrium position.

• Heterogeneous catalyst is in different phase to
  the reactants.

• Homogeneous catalyst is in the same phase as the
  reactants.

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INTRODUCTION
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HETEROGENEOUS CATALYSTS

• Usually a solid catalyst and gaseous reactants.

• Most industrial catalysts are like this (e.g.
  Haber, Contact).

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HETEROGENEOUS CATALYSTS
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HETEROGENEOUS CATALYSTS
1) Reactants adsorbed onto surface (onto active
sites).

• weakens bonds
• brings molecules closer
• more favourable orientation

2) Reaction takes place.
3) Products are desorbed (leave the surface).
Animation
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HETEROGENEOUS CATALYSTS
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HETEROGENEOUS CATALYSTS
Strength of adsorption
Too strong (e.g. W)

• Reactants cannot move round surface.
• Products cannot desorb.

Too weak (e.g. Ag)

• Reactants not adsorbed.

Ideal (e.g. Ni, Pt)
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HETEROGENEOUS CATALYSTS
Specificity
Cu catalyst - dehydrogenation
Al2O3 catalyst - dehydration
CH3CH2OH ? CH3CHO H2
CH3CH2OH ? CH2CH2 H2O
Spacing of sites suitable to bond O-H
Spacing of sites suitable to bond C-O
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HETEROGENEOUS CATALYSTS
Nature of catalyst

• Large surface area.
• Spread thinly over ceramic honeycomb.

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HETEROGENEOUS CATALYSTS
Poisoning

• Some substances may block active sites (i.e. they
  adsorb and will not come off). Can ruin
  catalyst.
• e.g. S in Haber process
• e.g. Pb in catalytic converters

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HOMOGENEOUS CATALYSTS

• Most examples involve reactions in solution with
  catalyst in solution.
• Some gas phase examples.

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HOMOGENEOUS CATALYSTS

• Catalyst reacts with a reactant to form
  intermediate.
• 2) Intermediate reacts to form product faster
  than original reactant (and regenerates catalyst).

e.g. acid catalyst
X Y ? products

• X H ? HX
• 2) HX Y ? products H

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HOMOGENEOUS CATALYSTS
e.g. acid catalyst in esterification
e.g. CH3COOH(l) CH3OH(l) ? CH3COOCH3(l)
H2O(l)
No H catalyst SLOW step
With H catalyst FASTER
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HOMOGENEOUS CATALYSTS
e.g. transition metal catalyst
works by metal varying oxidation state
Fe2 or Fe3 e.g. 2 I-
S2O82- ? I2 2 SO42-
Slow because both ve ions so repel.

• 2 Fe2 S2O82- ? 2 Fe3 2 SO42-
• 2) 2 Fe3 2 I- ? 2 Fe2 I2

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HOMOGENEOUS CATALYSTS

0
1.51 V
0.54 V
S2O82- 2 e- ? 2 SO42-
I2 2 e- ? 2 I-
0.77 V
Fe3 e- ? Fe2
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HOMOGENEOUS CATALYSTS
e.g. transition metal catalyst
works by metal varying oxidation state
Mn2 e.g. 2 MnO4-
16 H 5 C2O42- ? 2 Mn2 8 H2O 10 CO2
the reaction is catalysed by one of the products
(Mn2) this is called autocatalysis