Heterogeneous catalysis and catalytic converters

Catalysts are able to lower the activation enthalpy, E_a, for a reaction, increasing the number of successful collisions between reactants and hence the reaction rate. The catalyst itself is either not used up or is regenerated at the end of the reaction, and does not change the amount of product in a reaction.

We can illustrate this on a Maxwell-Boltzmann distribution curve. The area under the curve beyond E_a corresponds to the number of molecules in a reaction with sufficient energy to overcome the energy barrier and react when they collide.  If a catalyst is added, the activation enthalpy is lowered –  E_a will move to the left, and a significantly greater number of molecular collisions will be successful.

Heterogeneous catalysis

Heterogeneous catalysts are in a different phase to the reactants e.g. a solid catalyst in a gaseous reaction.

Step 1:  Reactant molecules adsorb onto the surface of the catalyst.  Weak bonds form between the reactant molecule and the catalyst using some of the bonding electrons in the molecule, hence weakening them and making a subsequent reaction easier.

Step 2:  Adsorbed gases are held onto the surface in a favourable orientation for the reaction between them to occur. 

Step 3:  Bonds in the reactant molecules are broken; new bonds are made in the product molecules.

Step 4:  Product molecules desorb from the catalyst surface.

Catalysts are specific to the reactions they catalyse …  for example, ethanol undergoes different reactions depending on the metal catalyst used. The distance between active sites and their similarity with the length of bonds determines the method of adsorption and affects which bonds are weakened.

Catalytic converters

Catalytic converters facilitate the conversion of primary pollutants in exhaust gases to less polluting substances, usually carbon dioxide, nitrogen and water.

The three-way catalytic converter is the most widely used as it is capable of dealing with both nitrogen oxides (NO_x), carbon particulates, unburnt hydrocarbons and carbon monoxide in a series of redox reactions.

• nitrogen oxides are reduced to nitrogen (rhodium / platinum)

2CO_(g) + 2NO_(g) ⇾ N_2(g) + 2CO_2(g)

4CO_(g) + 2NO_2(g) ⇾ 4CO_2(g) + N_2(g)

2H_2(g) + 2NO_(g) ⇾ 2H₂O_(g) + N_2(g)

• carbon particulates, unburnt hydrocarbons and carbon monoxide are oxidised to carbon dioxide and water (palladium / platinum)

C_(s) + O_2(g) ⇾ CO_2(g)

2CO_(g) + O_2(g) ⇾ 2CO_2(g)

C₇H_16(g) + 11O_2(g) ⇾ 7CO_2(g) + 8H₂O_(g)

The efficiency of a catalytic converter depends on its operating temperature (short car journeys are the most polluting as the catalytic converter never gets up to optimum temperature) and on the fuel:air ratio (there needs to be enough oxygen in the exhaust gases for the oxidation reactions). The catalyst is poisoned by the presence of lead (anti-knocking agent) in the petrol which coats the surface preventing the pollutant gases from adsorbing efficiently, hence the move to unleaded petrol in the 1980s.

Practice questions

1. (a)  The exhaust gases of car engines are a major source of carbon monoxide in the atmosphere. Explain why.

  (b)  Give two reasons why carbon monoxide is classes as a polluting gas.

2. Platinum, palladium and rhodium are used in catalytic converters. The metals are held in a very thin layer on a ceramic support. 

(a) Name the type of catalysis that occurs in a catalytic converter.

(b) Describe the stages in the process by which the catalyst converts carbon monoxide and nitrogen oxides into less harmful gases. 

(c) Describe the properties of the ceramic support that contribute to the efficiency of the catalytic converter. 

Answers

1. (a) Incomplete combustion of hydrocarbon fuels / petrol leads to carbon monoxide being present in the exhaust gases.

   (b)  CO is toxic / poisonous and it is a primary pollutant contributing to the formation of photochemical smog.

2. (a) heterogeneous 

(b) Carbon monoxide and nitrogen oxide molecules are adsorbed onto the surface of the catalyst. Weak bonds form between the molecules and the catalyst, causing the bonds in the molecules to weaken. New bonds form in the product molecules (N₂ and CO₂). The products desorb from the catalyst surface. 

(c) The ceramic support has a honeycomb structure which gives it a very large surface area  – this means there is a greater opportunity for reactions to happen.