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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
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
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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HETEROGENEOUS CATALYSTS
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
CH3CH2OH → CH3CHO + H2
Spacing of sites suitable to bond O-H
Al2O3 catalyst - dehydration
CH3CH2OH → CH2=CH2 + H2O
Spacing of sites suitable to bond C-O
HETEROGENEOUS CATALYSTS
Nature of catalyst
• Large surface area.
• Spread thinly over ceramic
honeycomb.
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
1) 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
1) 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)
O
R
H+
No
catalyst
SLOW step
C
OH
O
H
R
+
H
O
O
OH
+
-H
R
C
With H+ catalyst
FASTER
OH
R
C
OH
O
H
R © www.chemsheets.co.uk
R
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O
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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.
1) 2 Fe2+ + S2O82- → 2 Fe3+ + 2 SO422) 2 Fe3+ + 2 I- → 2 Fe2+ + I2
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HOMOGENEOUS CATALYSTS
+
0
+ 0.54 V
+ 1.51 V
I 2 + 2 e-  2 I -
S2O82- + 2 e-  2 SO42-
+ 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
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