CATALYSTS

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CATALYSTS

• A catalyst alters the rate of a chemical reaction
  without itself undergoing any permanent change
• A catalyst does not appear in the overall
  equation for the reaction
• They are not chemically changed but can sometimes
  be physically changed
• Only small amounts are needed
• They do not affect amount of product formed only
  rate of the reaction
• Often specific for a particular reaction e.g
  enzymes

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TYPES OF CATALYSTS
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HOW DO HETEROGENEOUS CATALYSTS WORK?
Reactants form bonds with atoms on the surface of
the catalyst (absorbed onto the surface)
As a result bonds in the reactants are weakened
and break
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New bonds form between the reactants held close
together on catalyst surface
This in turn weakens bonds between product and
catalyst and product leaves
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Examples of uses of catalysts

• Car exhaust systems
• Zeolites are used in industry as catalysts e.g
  in the cracking of petroleum fuels
• Transition metals are good catalysts for industry
  e.g iron in the Haber process

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CATALYTIC CONVERTERS

• Catalytic converters remove harmful gases from
  car exhausts.
• It consists of a honeycomb of ceramic with metals
  such as platinum,palladium and rhodium coated on
  the honeycomb
• It removes up to 90 of the harmful gases

Catalytic converter
CO Nox C8H18
CO2 N2 H2O
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EQUATIONS FOR REACTIONS IN THE CATALYTIC CONVERTER
2
2
2
25
8
12 1/2
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Catalyst poisoning
Catalysts can be poisoned. In heterogeneous
catalysis the poison molecules are absorbed
more strongly to the catalyst surface than the
reactant molecules, the catalyst becomes inactive
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EXAMPLES OF POISONING OF CATALYSTS
Leaded petrol cannot be used in cars fitted with
a catalytic converter since lead strongly absorbs
onto the surface of the catalyst
Cannot use copper or nickel in a catalytic
converter on a car instead of the expensive
platinum or Rhodium. REASON - Any SO2 present in
the exhaust fumes (trace amounts ) would poison
the catalyst Once the catalytic converter has
become inactive it cannot be regenerated
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REGENERATION OF CATALYST

• This is possible sometimes.
• Catalytic cracking of long-chain hydrocarbons
  produces carbon
• This can coat the zeolite catalyst, making it
  become inactive .
• The catalyst is recycled through a container
  where hot air is blown over it.
• Oxygen in the air converts the carbon to carbon
  dioxide and the catalyst is regenerated.