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CatalystsCatalysts
Learning ObjectivesLearning Objectives Catalysts can be classified as either heterogeneous Catalysts can be classified as either heterogeneous
or homogeneous.or homogeneous.
Heterogeneous catalysts work by the adsorption of Heterogeneous catalysts work by the adsorption of reactant molecules.reactant molecules.
The surface activity of a catalyst can be reduced by The surface activity of a catalyst can be reduced by poisoning.poisoning.
Impurities in the reactants result in the industrial Impurities in the reactants result in the industrial catalysts having to be regenerated or renewed.catalysts having to be regenerated or renewed.
Catalytic converters are fitted to cars to catalyse the Catalytic converters are fitted to cars to catalyse the conversion of poisonous carbon monoxide and conversion of poisonous carbon monoxide and oxides of nitrogen to carbon dioxide and nitrogen.oxides of nitrogen to carbon dioxide and nitrogen.
Definition of CatalystsDefinition of Catalysts
Speed up the rate of a chemical reactionSpeed up the rate of a chemical reaction
Are not chemically changed or used up Are not chemically changed or used up in the reactionin the reaction
May be May be specificspecific for a particular reaction for a particular reaction
Types of catalysts
HomoHomogeneous catalysts –geneous catalysts –reactants and catalyst in same statereactants and catalyst in same state
HeteroHeterogeneous catalysts – geneous catalysts – reactants and catalysts in different reactants and catalysts in different statesstates
How do heterogeneous catalysts work?How do heterogeneous catalysts work?
Catalyst is usually solidCatalyst is usually solid
Reactants are Reactants are adsorbedadsorbed onto surface onto surface
Adsorb – Adsorb – formation of weak bondsformation of weak bonds
CatalystCatalyst
CatalystCatalyst
Reactant AReactant A
CatalystCatalyst
Adsorption occurs at Adsorption occurs at active siteactive site
CatalystCatalyst
CatalystCatalyst
Reactant BReactant B
CatalystCatalyst
CatalystCatalyst
CatalystCatalyst
CatalystCatalyst
CatalystCatalyst
ProductsProducts
H H C CH H
H H
E.g Hydrogenating alkenes (turning oils into fats)
H H
Bonds within reactant molecules are Bonds within reactant molecules are weakened and breakweakened and break
H H C CH H
H HH H
New bonds form between the reactantsNew bonds form between the reactants
H H C CH H
H HH H
H
H
H
C
C
H
H
•New bonds form between the reactant molecules
H HH
The product(s) leave the catalyst The product(s) leave the catalyst surfacesurface
H H C CH H
HHH H
CatalystsCatalystsN
UM
BE
R O
F M
OL
EC
UE
S W
ITH
A P
AR
TIC
UL
AR
EN
ER
GY
MOLECULAR ENERGY Ea
NUMBER OF MOLECULES WITH SUFFICIENT ENERGY TO OVERCOME THE ENERGY BARRIER
CatalystsCatalysts Catalysts lower the activation energy for a Catalysts lower the activation energy for a
reactionreaction
Ea
EXTRA NUMBER OF MOLECULES WITH SUFFICIENT ENERGY TO OVERCOME THE ENERGY BARRIER
NU
MB
ER
OF
MO
LE
CU
ES
WIT
H A
PA
RT
ICU
LA
R E
NE
RG
Y
MOLECULAR ENERGY
EXAMPLES OF CATALYSTS
• MetalsMetals Ni, Pt Ni, Pt hydrogenation reactionshydrogenation reactions FeFe Haber ProcessHaber Process Rh, PdRh, Pd catalytic converterscatalytic converters
• Format is FINELY DIVIDED Format is FINELY DIVIDED
• This increases the surface area and provides more This increases the surface area and provides more active sitesactive sites
Catalyst poisoningCatalyst poisoning
In heterogeneous catalysis Catalyst In heterogeneous catalysis Catalyst poisoned by a chemical binding to surfacepoisoned by a chemical binding to surface
Poison bonds irreversibly with catalystPoison bonds irreversibly with catalyst
Sulphur and lead are two examples of Sulphur and lead are two examples of catalyst poisonscatalyst poisons
PoisoningPoisoning
CatalystCatalyst
PoisoningPoisoning
CatalystCatalyst
PoisoningPoisoning
Active site taken up by poisonActive site taken up by poison
Effectiveness of catalyst reduced.Effectiveness of catalyst reduced.
CatalystCatalyst
Cleaning a catalystCleaning a catalyst
Catalysts can sometimes be Catalysts can sometimes be regeneratedregenerated by blowing hot air over by blowing hot air over catalyst – oxidises chemicals on surfacecatalyst – oxidises chemicals on surface
Used to remove carbon (soot) after Used to remove carbon (soot) after catalytic crackingcatalytic cracking
Catalytic ConvertersCatalytic Converters
Found in car exhaust systemsFound in car exhaust systems Reduce levels of pollutant gasesReduce levels of pollutant gases Pollutants react in exhaust to form less Pollutants react in exhaust to form less
harmful gases.harmful gases.