By: Lance Hammell

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CATALYSTS AND ENZYMES

• By Lance Hammell

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What are catalysts?

• Simply put, catalysts are substances which, when
  added to a reaction, increase the rate of
  reaction by providing an alternate reaction
  pathway with a lower activation energy (Ea).
• They do this by promoting proper orientation
  between reacting particles.
• In biochemistry, catalysts are known as enzymes.

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Catalytic Converters

• One common application for catalysts is for
  catalytic converters.
• Catalytic converters are found in automobiles.
• Their role is to reduce to emissions of harmful
  gases (CO, VOCs, NOx) that are the result of the
  combustion of fuel in vehicle engines.

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Specifics of Catalytic Converters

• Most modern cars are equipped with three-way
  catalytic converters. "Three-way" refers to the
  three regulated emissions it helps to reduce --
  carbon monoxide, VOCs and NOx molecules.
• The converter uses two different types of
  catalysts, a reduction catalyst and an
  oxidization catalyst. Both types consist of a
  honeycomb-shaped ceramic structure coated with a
  metal catalyst, usually platinum, rhodium and/or
  palladium.

A Reduction Catalyst B Oxidation Catalyst C
Honeycomb Ceramic Structure
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Step 1 The Reduction Catalyst

• The reduction catalyst is the first stage of the
  catalytic converter.
• It uses platinum and rhodium to help reduce the
  NOx emissions. When an NO or NO2 molecule
  contacts the catalyst, the catalyst rips the
  nitrogen atom out of the molecule and holds on to
  it, freeing the oxygen in the form of O2.
• The nitrogen atoms bond with other nitrogen atoms
  that are also stuck to the catalyst, forming N2.
• The equation for this is as follows
• 2 NO gt N2 O2 or 2 NO2 gt N2 2 O2

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Step 2 The Oxidization Catalyst

• The oxidation catalyst is the second stage of the
  catalytic converter.
• It reduces the unburned hydrocarbons and carbon
  monoxide by burning (oxidizing) them over a
  platinum and palladium catalyst.
• This catalyst aids the reaction of the CO and
  hydrocarbons with the remaining oxygen in the
  exhaust gas.
• The equation for this process is as follows 2
  CO O2 gt 2 CO2
• Once this process is complete, most of the
  harmful substances have been broken down into
  harmless ones such as N2, O2, and CO2.

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Catalysts in Industry

• Of course, reducing vehicle emissions is not the
  only area in which catalysts can prove useful.
  The petrochemical industry also makes great use
  of them in various processes.
• One of these processes, called catalytic
  cracking, is detailed below. Catalytic cracking
  is the name given to the breaking up of large
  hydrocarbon molecules into smaller, more useful
  pieces.

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Catalytic Cracking Part 1

• Hydrocarbons are the result of the fractional
  distillation of gas oil from crude oil
  (petroleum). These fractions are obtained from
  the distillation process as liquids, but are
  re-vaporised before cracking.
• The hydrocarbons are mixed with a very fine
  catalyst powder. These days, the catalysts are
  zeolites (complex alumniosilicates).
• In the past, the catalyst used was aluminum oxide
  and silicon dioxide, however, these are much less
  efficient than the modern zeolite.
• The whole mixture (hydrocarbons and zeolites) is
  blown through a reaction chamber at a temperature
  of about 500 C. The catalyst is recovered
  afterwards, and the cracked mixture is further
  separated by cooling and fractional distillation.

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Catalytic Cracking Part 2

• There isnt any single unique reaction happening
  during this process. The hydrocarbon molecules
  are broken up in a fairly random way to produce
  mixtures of smaller hydrocarbons, some of which
  have carbon double bonds. However, one possible
  reaction might be
• C15H32 ? 2 C2H4 C3H6 C6H18
• zeolite
• This is only one way in which this particular
  molecule might break up. This process is
  important because C2H4 (ethene) and C3H8
  (propene) are important materials for making
  plastics or producing other organic chemicals.
  The octane (C8H18) is one of the molecules found
  in gasoline.

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Enzymes Organic Catalysts

• An enzyme is essentially a catalyst that is found
  in living things.
• Enzymes are proteins.
• They are specific their shape determines which
  substrate they work with.
• Enzymes bind temporarily to one or more of the
  reactants of the reaction they catalyze. In doing
  so, they lower the amount of activation energy
  needed by promoting the proper orientation
  between particles and thus speed up the reaction.
  

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Enzymes Contd

• As we well remember, the function of enzymes and
  catalysts is to speed up a reaction, and they do
  this by promoting proper orientation between
  reacting particles.
• Thus the function of enzymes and how they work
  becomes immediately clear when considered in the
  context of the role of catalysts.
• An example of an enzyme with equation would be
  carbonic anhydrase, which is found in red blood
  cells. During the rxn CO2 H2O ? H2CO3, it
  enables red blood cells to transport CO2 from the
  tissue to the lungs, where it can be expelled.
• Obviously, enzymes are extremely important. By
  allowing rapid processing of CO2 (1,000,000
  molecules of CO2/sec.), they help to ensure that
  we remain alive!
• This is only one example of the importance of
  enzymes there are thousands or even millions of
  these substances at work in all living things.

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Bibliography

• How Catalytic Converters Work. howstuffworks. 19
  Apr. 2005. lthttp//auto.howstuffworks.com/catalyti
  c-converter1.htmgt.
• Clark, Jim. Catalysts in the Petrochemical
  Industry. 19 Apr. 2005.lthttp//chemguide.co.uk/phy
  sical/catalysis/petrochem.htmgt.
• Farabee, M.J. Enzymes Organic Catalysts. 19
  Apr. 2005. lthttp//www.emc.maricopa.edu/faculty/fa
  rabee/8108K/BioBookEnzym.htmlEnzymesgt.

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