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Lecture 7. Molecular kinetic. Characterization of Catalysts. Catalytic

Reactions

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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

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Catalyst lowers the activation energy for both forward and reverse reactions.

Activation Energy

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CatalysisCatalysis

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Effect of a catalyst

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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

CatalysisCatalysis

Hydrogenation of ethylene

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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)

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Supported Catalyst

Nickel clusters

SiO2

Highly dispersed metal on metal oxide

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Molecules in Zeolite Cages and Frameworks

+ p-xylene

ZSM-5

Y-zeolite

Paraffins

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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Enzyme activityTemperature and pH affect the activity of an enzyme

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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Medical and Clinical Application• Thrombin obtained from beef plasma used to stop

bleeding during operation

• Enzymes are used as surface disinfectants – Trypsin used in cleaning

• Glucose oxidase, Peroxidases used in determining the Glucose in blood

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