1. Chemsheets AS006 (Electron arrangement)
14/04/2017
CATALYSIS
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2. 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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3. INTRODUCTION

4. HETEROGENEOUS CATALYSTS
Usually a solid catalyst and gaseous reactants.
Most industrial catalysts are like this (e.g. Haber, Contact).
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6. 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).
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7. HETEROGENEOUS CATALYSTS

8. 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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9. HETEROGENEOUS CATALYSTS
Specificity
Cu catalyst - dehydrogenation
Al2O3 catalyst - dehydration
CH3CH2OH → CH3CHO + H2
CH3CH2OH → CH2=CH2 + H2O
Spacing of sites suitable to bond O-H
Spacing of sites suitable to bond C-O

10. HETEROGENEOUS CATALYSTS
Nature of catalyst
Large surface area.
Spread thinly over ceramic honeycomb.

11. 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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12. HOMOGENEOUS CATALYSTS
Most examples involve reactions in solution with catalyst in solution.
Some gas phase examples.
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13. 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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14. 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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15. HOMOGENEOUS CATALYSTS
e.g. transition metal catalyst
works by metal varying oxidation state
Fe2+ or Fe3+
e.g I- + S2O82- → I SO42-
Slow because both –ve ions so repel.
2 Fe2+ + S2O82- → 2 Fe SO42-
2) 2 Fe I- → 2 Fe2+ + I2
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16. HOMOGENEOUS CATALYSTS+ V V
I2 + 2 e-  2 I-
S2O e-  2 SO42- V
Fe3+ + e-  Fe2+
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17. HOMOGENEOUS CATALYSTS
e.g. transition metal catalyst
works by metal varying oxidation state
Mn2+
e.g. 2 MnO H+ + 5 C2O42- → 2 Mn H2O + 10 CO2
the reaction is catalysed by one of the products (Mn2+) – this is called autocatalysis
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