1.9.4 Catalysts in Industry

• Catalysts can be divided into two types:
  • Homogeneous catalysts
  • Heterogeneous catalysts

• Homogeneous means that the catalyst is in the same phase as the reactants
  • For example, the reactants and the catalysts are all in solution

• Heterogeneous means that the catalyst is in a different phase to the reactants
  • For example, the reactants are gases, but the catalyst used is a solid

• Ammonia production via the Haber Process is a classic example of the use of a heterogeneous catalyst 

N₂ (g) + 3H₂ (g) ⇌ 2NH₃ (g)

• The Haber Process occurs in five stages

1. 1. Hydrogen and nitrogen are obtained from natural gas and air respectively and pumped into the compressor
   2. The gases are compressed to about 200 atmospheres
   3. The pressurised gases, at around 450 ^oC, are pumped into a tank containing beds of solid iron which is a heterogeneous catalyst, where some of the hydrogen and nitrogen react to form ammonia
   4. Unreacted hydrogen, nitrogen and the ammonia product pass into a cooling tank where the ammonia is liquefied and removed 
   5. The unreacted hydrogen and nitrogen are recycled back into the system and start over again

The production of ammonia by the Haber Process

Conditions

Temperature: 450ºC

• A higher temperature favours the reverse reaction as it is endothermic (takes in heat) so a higher yield of reactants would be made
• Lower temperature favours the forward reaction as it is exothermic (releases heat) so a higher yield of products will be made
  • However, at a lower temperature the rate of reaction is very slow
• So 450ºC is a compromise temperature between having a lower yield of products but being made more quickly

Pressure: Approximately 20 MPa

• Lower pressure favours the reverse reaction as the system will try to increase the pressure by creating more molecules (4 molecules of gaseous reactants) so a higher yield of reactants will be made
• Higher pressure favours the forward reaction as it will try to decrease the pressure by creating less molecules (2 molecules of gaseous products) so a higher yield of products will be made
• However, high pressures can be dangerous and very expensive equipment is needed
• So, 20 MPa is a compromise pressure between a lower yield of products being made safely and economically

Catalyst

• A catalyst of iron is used to speed up the reaction
• Without a catalyst, the required temperature and pressure for the Haber Process would be much higher, more dangerous, more expensive and less environmentally friendly

The mode of action of the iron catalyst is as follows:

• Diffusion of the nitrogen and hydrogen gas to the iron surface
• Adsorption of the reactant molecules onto the iron surface by forming bonds between the iron and reactant atoms
• These bonds are so strong that they weaken the covalent bonds between the nitrogen atoms in N₂ and hydrogen atoms in H₂
• But they are weak enough to break when the catalysis has been completed
• The reaction takes place between the adsorbed nitrogen and hydrogen atoms which react with each other on the iron surface to form NH₃
• Desorption occurs when the bonds between the NH₃ and iron surface are weakened and eventually broken
• The formed NH₃ diffuses away from the iron surface

Benefits of catalysts

• Catalysts speed up the rate of reaction, meaning the use of a catalyst may mean lower temperatures and pressures can be used
  • This can save energy costs as there is reduced energy demand for providing high temperatures and less electrical pumping costs for producing the high pressures usually required
  • This can mean fewer CO₂ emissions from burning fossil fuels
• Catalysts can also enable different reactions to be used, with better atom economy and with reduced waste, or fewer undesired products or less use of hazardous solvents and reactants
• Catalysts are often enzymes, generating very specific products, and operating effectively close to room temperature and pressure

The effectiveness of a heterogenous catalyst can be decreased by the presence of impurities in the gas involved in the reaction.

The impurities can: 

• Adsorb onto catalyst surface and occupy active sites
• Prevent bond weakening in the reactants
• 'Take up' the surface area of catalyst by forming strong bonds to surface of catalyst so are less likely to desorb from surface of catalyst