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Introduction to catalysis chemistry Introduction to catalysis chemistry

● PreliminariesPreliminaries● DefinitionDefinition● Types of catalysisTypes of catalysis● ExampleExample

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Reaction RateReaction Rate

Function of reactants (or products) Function of reactants (or products) concentrations (or pressure), temperature, concentrations (or pressure), temperature, presence of catalyst (or inhibitor)presence of catalyst (or inhibitor)

– α, β, ... are the orders of the reactantsα, β, ... are the orders of the reactants

– k is the rate constantk is the rate constant

Example: rate of chemical adsorptionExample: rate of chemical adsorption

RRadsads = k P = k P xx

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Activation energyActivation energy

Arrhenius LawArrhenius Law

A: pre-exponential A: pre-exponential factor factor

EEaa: Activation energy: Activation energy

● ObservationsObservations

– Reaction don't start Reaction don't start even if it is even if it is thermodynamically thermodynamically favorable (ΔG < 0) :favorable (ΔG < 0) :

– Reaction rate increase Reaction rate increase with T with T

– This increasing is This increasing is specific for each specific for each reactionreaction

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DefinitionDefinition

What is a catalyst ?What is a catalyst ?

A catalyst modifies the mechanism of A catalyst modifies the mechanism of the reaction the reaction

But never change the equilibrium state But never change the equilibrium state and evolution directionand evolution direction

Catalyst is a chemical species which increase the rate of a reaction without being used up in the process

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Thermodynamic and kinetic Thermodynamic and kinetic

● A catalyst reduces A catalyst reduces

EEaa for both forward for both forward

and back reactionsand back reactions

● More steps in the More steps in the reactionreaction

● Effects of catalyst Effects of catalyst different for each different for each reactionsreactions

● ΔH do not changeΔH do not change

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Catalyst CycleCatalyst Cycle

● Catalyst can be used Catalyst can be used again and again in again and again in theorytheory

● A catalyst has active A catalyst has active sitessites

● But many problems can But many problems can occuroccur

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Catalyst deactivationCatalyst deactivation

PoisoningPoisoning

– impurities which can impurities which can bind strongly to the bind strongly to the active sites on the active sites on the surfacesurface

Sintering of the active Sintering of the active metal particlesmetal particles

Sintering and solid-Sintering and solid-solid phase transitions solid phase transitions of the support of the support (encapsualtion)(encapsualtion)

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Catalyst ParametersCatalyst Parameters

ActivityActivity

– Turnover factor is the number of molecules converted Turnover factor is the number of molecules converted by each catalytic site in a unit of timeby each catalytic site in a unit of time

SelectivitySelectivity

– A selectivity of 99,99% means that the catalyst makes A selectivity of 99,99% means that the catalyst makes one mistake in 10.000 conversionsone mistake in 10.000 conversions

StabilityStability

– The number of reactions performed by each active site The number of reactions performed by each active site before it decays or becomes inactivebefore it decays or becomes inactive

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The three Types of catalysisThe three Types of catalysis

Homogeneous CatalysisHomogeneous Catalysis– The catalyst and reactants are in the same phase, The catalyst and reactants are in the same phase,

usually liquid usually liquid

Heterogeneous catalysisHeterogeneous catalysis– Catalyst and reactants are in different phasesCatalyst and reactants are in different phases

Enzymatic CatalysisEnzymatic Catalysis– Catalyst is an enzyme (macromolecules made of Catalyst is an enzyme (macromolecules made of

amino acids)amino acids)

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Homogeneous CatalysisHomogeneous Catalysis

AdvantagesAdvantages– Good contact with reactants Good contact with reactants

– Greater effective concentration of catalyst than Greater effective concentration of catalyst than with heterogeneous catalystswith heterogeneous catalysts

DisadvantagesDisadvantages– Catalysis needs to be separated after reactionCatalysis needs to be separated after reaction

– Catalyst recovery may be difficult because the Catalyst recovery may be difficult because the temperature for the distillation can destroy the temperature for the distillation can destroy the catalystcatalyst

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Heterogeneous CatalysisHeterogeneous Catalysis

AdvantagesAdvantages– There is little difficulty in separating and recycling There is little difficulty in separating and recycling

the catalystthe catalyst

DisadvantagesDisadvantages– There is a lower effective concentration of There is a lower effective concentration of

catalyst since the reaction occurs only on the catalyst since the reaction occurs only on the exposed active surfaceexposed active surface

– To maximize the surface area catalysts are To maximize the surface area catalysts are spread thinly on a cheap and sturdy supportspread thinly on a cheap and sturdy support

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Enzymatic CatalysisEnzymatic Catalysis We would not survive without We would not survive without

enzymes!enzymes!

Not fully understoodNot fully understood

Extremely efficient and Extremely efficient and highly selective: accept only highly selective: accept only one variety of substratesone variety of substrates

An Enzyme

With a substrate

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Example of simple catalysisExample of simple catalysis

HydrogenationHydrogenationCatalysts used are Ni Pd Pt Ru Rh CoCatalysts used are Ni Pd Pt Ru Rh Counsaturated unsaturated saturated productssaturated products

(Vegetable oils (Vegetable oils margarine) margarine)

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ConclusionConclusion

Catalysis: only a kinetic improvementCatalysis: only a kinetic improvement

Never change the thermodynamic of Never change the thermodynamic of the system the system

Three type of catalysisThree type of catalysis

A majority of industrial processes use A majority of industrial processes use catalysis catalysis