1 Lecture 7. Molecular kinetic. Characterization of Catalysts. Catalytic Reactions A Y C A T L S I S Prepared by PhD Halina Falfushynska
Dec 30, 2015
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Lecture 7. Molecular kinetic. Characterization of Catalysts. Catalytic
Reactions
A YC
A
T L S IS
Prepared by PhD Halina Falfushynska
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What is a Catalyst?
Catalyst is a substance that increases the rate
of the reaction at which a chemical system
approaches equilibrium , without being
substantially consumed in the process.
Catalyst affects only the rate of the
reaction,i.e.Kinetics.
It changes neither the thermodynamics of
the reaction nor the equilibrium composition.
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Reaction path for conversion of A + B into ABReaction path for conversion of A + B into AB
Kinetic Vs. Thermodynamic
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Activation Energy : The energy required to overcome the reaction barrier. Usually given a symbol Ea or ∆G≠
The Activation Energy (Ea) determines how fast a reaction occurs, the higher Activation barrier, the slower the reaction rate. The lower the Activation barrier, the faster the reaction
Activated Complex Theory
Heterogeneous Catalysis• Consider the hydrogenation of ethylene:
C2H4(g) + H2(g) C2H6(g), H = -136 kJ/mol.– The reaction is slow in the absence of a catalyst.– In the presence of a metal catalyst (Ni, Pt or Pd) the reaction occurs
quickly at room temperature.– First the ethylene and hydrogen molecules are adsorbed onto active
sites on the metal surface.– The H-H bond breaks and the H atoms migrate about the metal
surface.
CatalysisCatalysis
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Catalyst Preparation
(1) Unsupported Catalyst Usually very active catalyst that do not require high surface area
e.g., Iron catalyst for ammonia production (Haber process)
(2) Supported Catalyst requires a high surface area support to disperse the primary catalystthe support may also act as a co-catalyst (bi-functional)or secondary catalyst for the reaction (promoter)
Enzymes• Enzymes are biological catalysts.• Most enzymes are protein molecules with large molecular
masses (10,000 to 106 amu).
• Enzymes have very specific shapes.• Most enzymes catalyze very specific reactions.• Substrates undergo reaction at the active site of an enzyme.• A substrate locks into an enzyme and a fast reaction occurs.• The products then move away from the enzyme.
CatalysisCatalysis
Enzymes• Only substrates that fit into the enzyme lock can be
involved in the reaction.• If a molecule binds tightly to an enzyme so that another
substrate cannot displace it, then the active site is blocked and the catalyst is inhibited (enzyme inhibitors).
• The number of events (turnover number) catalyzed is large for enzymes (103 - 107 per second).
CatalysisCatalysis
Enzymes
CatalysisCatalysis
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SEkMentenMichaelisRate
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Enzymes function best or are most active in specific conditions known as optimum
conditions.
Optimum Condition
Enzymes applicationEnzymes for detergents and personal care
Enzyme applications in the food industry
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Medical and Clinical Application• Amylase and proteases and lypases used in
drugs to cure digestive disorders
• Papain (papaya proteinase I)ingredient in some toothpastes or mints
as teeth-whitener;
enzymatic debriding preparations, notably Accuzyme• Papacarie, a gel used for chemomechanical dental caries
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