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Heterogeneous Catalysts (A2 Only)

Chemistry: AQA A Level, topic 4.2.8

Heterogeneous Catalysis

A heterogeneous catalyst is a catalyst which is in a different phase to the reactants. For example, the catalyst is a solid, but the reactants are in aqueous solution.

Heterogeneous catalysis

Heterogeneous catalysis

  • Heterogeneous catalysts catalyse the reaction on their surface.
  • The reactants attach to the metal in places called active sites.
  • The term for the attachment to the metal active site is adsorption (i.e. the reactants adsorb to the metal surface).
  • If you increase the surface area of the metal, you increase the rate of reaction because more reactants can adsorb.
Efficient catalysis

Efficient catalysis

  • Catalysts are expensive, so you want to get the most out of your money.
  • We commonly coat a support medium in the metal so that we don’t need to buy much metal.
  • We can also powder the metal to increase its surface area.
Catalyst poisoning

Catalyst poisoning

  • Unfortunately, substances other than reactants can adsorb metal surfaces.
  • Sometimes, these molecules will adsorb irreversibly, and so slowly coat the surface in impurities.
  • This stops reactants from absorbing. It is named catalyst poisoning.
  • Catalyst poisoning slows down reactions by reducing the number of available active sites.
  • If a catalyst is heavily poisoned, it might need replacing, which costs money.

The Contact Process

The Contact Process is an example of a reaction that uses a heterogeneous catalyst. It is used to produce sulfuric acid.

The reaction

The reaction

  • The Contact Process converts sulfur dioxide to sulfur trioxide. Adding this to water produces sulfuric acid.
  • The equation is:
  • 2SO2(g) + O2(g) ⇌ 2SO3(g)
  • This is then converted to sulfuric acid:
  • SO3(g) + H2O(l) → H2SO4(aq)
The catalyst

The catalyst

  • This reaction uses a V2O5(s) catalyst.
  • It is done at 450oC and 1 or 2atm pressure.

The Haber Process

The Haber Process is a reaction used to produce ammonia from hydrogen gas and nitrogen gas. It uses a heterogeneous catalyst.

The reaction

The reaction

  • The equation for the reaction is:
  • N2(g) + 3H2(g) ⇌ 2NH3(g)
  • This reaction is done in a sealed vessel at 200atm pressure and 450oC. It has a very low yield so we recycle unreacted gas and run the process again.
The catalyst

The catalyst

  • We use a finely divided iron catalyst to make this reaction happen faster.
  • This allows us to lower the temperature, and get a higher yield (the forward reaction is exothermic).
  • The iron catalyst is powdered, to increase its surface area.
Poisoning

Poisoning

  • The iron catalyst is readily poisoned by the presence of sulfur.
  • Iron sulfide (FeS) can form if sulfur is present.
  • The hydrogen gas is often produced from methane. It will contain some impurities, of which sulfur is one.
  • The Haber Process catalyst is slowly poisoned.

Test yourself on Heterogeneous Catalysts (A2 Only)

  • What is a negative of catalysts?

  • If we increase the surface area, the rate of reaction increases. Why is this?

  • What is the catalyst used in the Contact Process?

  • What's the end goal of the Contact Process?

  • What is the catalyst used in the Haber process to increase the rate?

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