1. Chemical Kinetics Catalysts

2. Catalysts
A catalyst is a substance that increases the rate of the reaction but is neither
created nor destroyed in the process. Catalysts can be divided into two
broad categories.
Homogeneous catalysts are those that are in the same phase as the
reacting substances.
Heterogeneous catalysts are in a different phase from the reacting species.
CHEM 3310

3. CF2Cl 2 (aka R-12, CFC-12, Freon 12)
Catalysts
Example: The catalytic destruction of ozone in the stratosphere involves
chlorine as a catalyst. (homogeneous catalysis)
UV radiation breaks off a chlorine atom from a CFC molecule.
Step 1: CF2Cl2 + UV  CF2Cl + Cl 
The chlorine atom attacks an ozone molecule (O3), breaking it apart and destroying the ozone.
Step 2: Cl + O3  ClO + O2
The result is an oxygen molecule (O2) and a chlorine monoxide molecule (ClO).
ClO is attacked by an ozone molecule,
Step 3: ClO + O3  Cl + 2 O2
Chlorine atom is released and an oxygen molecule (O2) is formed.
CHEM 3310

4. Catalysts
Example: The catalytic destruction of ozone in the stratosphere involves
chlorine as a catalyst. (homogeneous catalysis)
Step 1: CF2Cl2 + UV  CF2Cl + Cl 
Step 2: Cl + O3  ClO + O2
Overall reaction:
2 O3 (g)  3 O2(g)
Step 3: ClO + O3  Cl + 2 O2
Note:
The chlorine atoms serve as a catalyst for ozone decomposition because
They are not part of the reaction stoichiometry
They are not consumed by the reaction
They increase the rate of the reaction
One chlorine atom may cycle through this mechanism to destroy 100,000 ozone molecules.
CHEM 3310

5. Powerful oxidizing agent
Catalysts
Example: The reaction between persulphate ions and iodide ions (homogeneous catalysis)
Slow reaction
S2O82- (aq) + 2 I- (aq)  2 SO42- (aq) + I2 (aq)
Powerful oxidizing agent
Easily
oxidized
The reaction needs a collision between two negative ions. Repulsion is going to get seriously in the way of that!
The catalysed reaction avoids that problem completely! Catalysts Cu (II) – Experiment Fe (II) or Fe (III) ions which are added to the same solution.
CHEM 3310

6. Catalysts S2O82- (aq) + 2 Fe2+ (aq)  2 SO42- (aq) + 2 Fe3+ (aq)
Example: The reaction between persulphate ions and iodide ions (homogeneous catalysis)
Fe2+ catalyst:
S2O82- (aq) + 2 Fe2+ (aq)  2 SO42- (aq) + 2 Fe3+ (aq)
2 Fe3+ (aq) + 2 I- (aq)  2 Fe2+ (aq) + I2 (aq)
Cu2+ catalyst:
2 Cu2+ (aq) + 2 I- (aq)  2 Cu+ (aq) + I2 (aq)
S2O82- (aq) + 2 Cu+ (aq)  2 SO42- (aq) + 2 Cu2+ (aq)
Both of these individual stages in the overall reaction involve collision between positive and negative ions. This will be much more likely to be successful than collision between two negative ions in the uncatalyzed reaction.
CHEM 3310

7. Catalysts Hydrogenation of ethene (heterogeneous catalysis) CHEM 3310
The four pictures show the effect of a catalyst on hydrogenation of ethylene (or any C=C compounds).
Metal atoms attract electrons away form the H2 molecules. The bonds break.
The free radicals move along the surface of the metal until they collide with an ethene molecule.
The ethane, CH3CH3 is not as "electron rich" as the ethylene, and breaks away from the catalyst.
CHEM 3310

8. Catalysts Schematic diagram of how catalytic converters in automobiles
Remove NO from exhaust gas. (heterogeneous catalysis)
NO molecule
collides with the Pt catalyst.
NO molecule adsorbs on the Pt surface.
NO molecule dissociates and bonds with the Pt atom. NO
Oxygen atom
Nitrogen atom
Platinum surface (Pt, Pd, Rh) N2 O2
N2 and O2 desorb from the Pt surface, leaving more surface to catalyze further reaction.
Pairs of N atoms react.
Pairs of O atoms react.
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9. Catalysts
The uncatalyzed reaction
must overcome a relatively high
activation barrier.
Catalysts increase the rate of reaction by providing an alterative reaction pathway with lower activation energy.
Catalysis leads to a new reaction pathway that has a relatively lower activation barrier.
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10. Catalyzed reaction speed
Catalysts
Catalyzed reaction
Ea = 25 kJ/mole
Example: Reaction at 293K
Uncatalyzed reaction
Ea = 50 kJ/mole
Catalyzed reaction speed
increases.
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11. OCl- + I-  OI- + Cl- Catalysts Uncatalyzed Reaction:
High Ea ( reaction involves two negative ions)
Requires high energy input
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12. OCl- + I-  OI- + Cl- Catalysts Catalyzed Reaction:
Water acts as a catalyst for this reaction. It is not consumed in the reaction.
It provides an alternate mechanism with a lowering of Ea.
ClOH, OH- and IOH are intermediate species
Forms intermediates
Forms intermediate
Forms the product, Cl-
Forms the product, IO-
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13. OCl- + I-  OI- + Cl- Catalysts Catalyzed Reaction: Ea3 Ea2 Ea1
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14. OCl- + I-  OI- + Cl- Catalysts
Compare the Catalyzed and Uncatalyzed Reaction:
Euncatalyzed
Ea3
Ea2
Ea1
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15. The importance of Chemistry!!
Catalysts
Important in billion dollar industries such as petroleum refining
Research of materials and design process for better catalysts
Selective so that it speeds up one reaction and not all the reactions
Historically, selection of catalysts is by trial and errors Modern days, catalysts are better understood by:
Reaction mechanisms
Molecular structure
Material properties
The importance of Chemistry!!
CHEM 3310