Catalysts

Syllabus Statements• C.4.1 Compare the modes of action of homogeneous

and heterogeneous catalysts.

• C.4.2 Outline the advantages and disadvantages of homogeneous and heterogeneous catalysts.

• C.4.3 Discuss the factors in choosing a catalyst for a process. Factors could include: selectivity (produce only the desired product) efficiency ability to work under mild/severe conditions environmental impact problems caused by catalysts becoming poisoned by impurities.

First a little reminder of what you should already know . . .

• Catalyst = substance that changes the rate of a reaction without being used up.

• They provide an alternative mechanism - usually with lower activation energy.

• For an equilibrium, they change both reactions equally and so do not affect equilibrium position.

Or in terms of a Maxwell – Boltzmann distribution

• A few points:• The definition I have given for catalyst

allows the possibility of “negative catalysts" or inhibitors.

• The overall energy change, ΔH, is not affected.

• Make sure you label both axes on any graphs you draw. The IB mark scheme for these questions will reduce your mark if you forget!

There are 2 ways that catalysts may work:

• Homogeneous catalysts• The catalyst is in the same phase (or

state) as the reactants.• The catalyst reacts to form an

intermediate, and is reformed later in the reaction.

• E.g.• S2O8

2- + 2I- 2SO42- + I2

• Is catalysed by the addition of Fe2+ ions

• S2O82- + 2Fe2+ 2SO4

2- + 2Fe3+

• 2I- + 2Fe3+ 2Fe2+ + I2

Catalyst used

Catalyst reformed

• Note that transition metal ions often make good catalysts. Why?

• More about this later!

• Heterogeneous catalysts• The catalyst and reactant are in different

phases.

• Typically a gas or liquid is adsorbed onto the surface of a solid catalyst.

Absorption means that the one substance has fully entered or permeated into the other. Adsorption is a surface phenomenon wherein one substance sticks to the surface of the other

Some examples of heterogeneous catalysis

• The Contact Process:

• 2SO2 + O2 2SO3

• All reactants and products in gas phase• Catalyst is V2O5 in solid phase

• The Haber Process

• N2 + 3H2 2NH3

• All reactants and products in gas phase• Catalyst . . .• Finely divided iron catalyst• Fe in the solid phase

• The hydrogenation of alkenes

• C2H4 + H2 C2H6

• All reactants and products in gas phase• Catalyst Ni in solid phase

• Notice that you don’t need to draw any intermediates.

How do catalysts work?

• Transition metals make very useful catalysts because they can gain or lose electrons easily.

• They have multiple oxidation states.• This means they can form a number of

stable intermediates.• This kind of catalysis is most often

encountered in homogeneous catalysis.

• E.g.

H2O2 (aq) + 2H+ (aq) + 2Fe2+(aq) 2H2O(l) + 2Fe3+

(aq)

2Fe3+ (aq) + 2I- (aq) I2 (s) + 2Fe2+

(aq)

Both steps of the reaction have low activation energies.

So both steps occur quickly

Catalyst used

Catalyst reformed

• This kind of reaction involving transition metal complexes is also sometimes seen in heterogeneous reactions

• E.g. The Contact process

• SO2 (g) + V2O5 (s) SO3 (g) + V2O4 (s)

• V2O4 (s) + ½ O2 (g) V2O5 (s)

• Make sure you can work out the overall reactions if you are given two steps like this!

• Heterogeneous catalysts may also work through active sites.

• Consider the hydrogenation of an alkene in the presence of a nickel catalyst:

• The electrons in the H-H bond are attracted to the Nickel surface

• Hence the H-H bond is weakened

• The Pi bond in the C=C bond is also attracted to the Ni surface

• Hence the C=C bond is also weakened.• Weaker bonds means that the reaction is

faster.• The product formed is not strongly

attracted to the Ni and so does not remain attached.

• Good catalysts such as nickel, rhodium and platinum must adsorb the reactants, but not adsorb the products.

HETEROGENEOUS CATALYSTS

HETEROGENEOUS CATALYSTS

1) Reactants adsorbed onto surface (onto active sites).• weakens bonds• brings molecules closer• more favourable orientation

2) Reaction takes place.

3) Products are desorbed (leave the surface).

A quick note about enzymes:

• Enzymes are biological catalysts (they are globular protein molecules)

• Enzymes aren’t easy to place in either of the catalyst types we have mentioned.

• They are usually in the same phase as the reactants,• But the reactants bind to the surface of the enzyme in a

similar way to heterogeneous catalysis.• Enzymes are very effective, compared to inorganic

catalysts, so they can be added in small amounts.• Enzymes are specific to particular reactants (“lock and

key” mechanism)

“Lock and key” mechanism

• Enzymes function best at a specific temperature (often body temperature)

• Enzymes function best at a specific pH. (They contain ionisable groups like COOH and NH2 which are affected by pH)

• Their effectiveness is limited by concentration – once all their active sites are full, rate can’t increase.

• They can be rendered ineffective (“poisoned”) by the presence of heavy metals

Advantages and disadvantages of different types of catalyst

• Homogeneous:• Advantages:

– All the catalyst comes into contact with the reactants – so it has high activity

– High selectivity as only certain reactions can happen

• Disadvantages:– Difficult to separate products and catalyst

• Heterogeneous catalysts• Advantages:

– Easy to separate from products by filtration– Long catalytic life

• Disadvantages:– Only effective at the surface – so most of the

catalyst is not used– Any coating on the surface reduces activity

Factors in Selecting a Catalyst• Selectivity:• We only want to make the required

product (no by-products).• Enzymes are very efficient at this.• Some inorganic catalysts can lead to a

variety of products.• Zeolites are an example of a selective

inorganic catalyst.

Zeolites are the aluminosilicate members of the family of microporous solids known as

molecular sieves

An example mineral formula is: Na2Al2Si3O10·2H2O

• Zeolites are lattice structures with open spaces which act as pores or channels

• The pores are tiny, and of a regular size.• Zeolites are selective catalysts as only

molecules with a specific geometry can enter the pores, and only product of a certain size can leave the pores.

• Effectiveness:• Effectiveness is a measure of how quickly

a catalyst gives the desired product.• Enzymes are generally the most effective• Zeolites are also good as they have a

massive surface area.

• Range of effective conditions:

• Enzymes have the disadvantage of only working within a narrow range of pH and temperature

• Inorganic catalysts work well in a much wider range of conditions.

• Environmental Impact:• Catalysts are in huge demand (80% of all

chemical industry uses a catalyst)• In particular rare metals such as Pt, Rh and Pa

are in massive demand for catalytic converters.• Mining these metals has a huge environmental

impact.• However careful we are (and however much we

try to recycle) some metal is lost to the environment.

• Many rare metals can show toxic effects (Pt based drugs are used to treat cancer)

• Scientists are also looking for alternatives to acid catalysts.

• Catalyst Poisoning:

• Catalysts can be “poisoned” by impurities.• We have already discussed this for enzymes

(pH, heavy metals)• The efficiency of a catalyst decreases with time

as its surface becomes coated with impurities.• For example the alumina catalyst used in

cracking gets coated with coke (carbon).• This can be removed by burning it off.

• C.4.1 Compare the modes of action of homogeneous and heterogeneous catalysts.

• C.4.2 Outline the advantages and disadvantages of homogeneous and heterogeneous catalysts.

• C.4.3 Discuss the factors in choosing a catalyst for a process. Factors could include: selectivity (produce only the desired product) efficiency ability to work under mild/severe conditions environmental impact problems caused by catalysts becoming poisoned by impurities.

Remember . . .

• The nasty IB people like you to be able to give examples.