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Chemistry LibreTexts

Heterogeneous catalysis

Mechanism

According to Surface adsoprtion theory heterogeneous catalysis has five stages:

Stage 1: Diffusion of Reactant(s) to the Surface: The rate at which reactants will diffuse to the surface will be influenced by their bulk concentration and by the thickness of the boundary layer.

Stage 2: Adsorption of reactants:Bonds are formed as the reactant(s) are adsorbed onto the surface of the catalyst. The ability for an atom or molecule to stick to the surface is known, brilliantly, as the Sticking Co-efficient. This is just the ratio or percentage of molecules that end up sticking on the surface.

Stage 3: Reaction: Bonds form between the atoms and molecules on the surface

Stage 4: Desorption of products: Bonds are broken as the product(s) desorb from the surface.

Stage 5: Diffusion of Product(s) away from the Surface: The products are then desorbed from the surface of the catalyst. 

Example \(\PageIndex{1}\): Contact Process

The contact process is the current method of producing sulfuric acid in the high concentrations needed for industrial processes. Platinum used to be the catalyst for this reaction; however, as it is susceptible to reacting with arsenic impurities in the sulfur feedstock, vanadium(V) oxide (\(V_2O_5\)) is now preferred

In the contact process vanadium [V] oxide (V2O2) is solid whereas the reactants SO2 and O2 are gaseous.

\[2SO_2 (g) + O_2 (g)\overset{V_2O_5(s)}\rightleftharpoons 2SO_3 (g)\]

More detailed method:

\[2V_2O_5(s) + 2SO_2(g)\rightleftharpoons 2SO_3(g) + 2V_2O_4(s)\]

\[2V_2O_4(s) + O_2 (g) \rightleftharpoons 2V_2O_5 (s)\]

Therefore:

\[2SO_2(g) + O_2(g)\rightleftharpoons 2SO_3(g)\]

Example \(\PageIndex{2}\): Haber–Bosch Reaction

The original Haber–Bosch reaction chambers used osmium as the catalyst, less expensive iron-based catalyst, which is still used today

\[N_2 (g) + 3H_2 (g) \overset{Fe(s)}\rightleftharpoons 2NH_3(g)\]

The reaction mechanism, involving the heterogeneous catalyst, is believed to involve the following steps:

  1. N2 (g) → N2 (adsorbed)
  2. N2 (adsorbed) → 2 N (adsorbed)
  3. H2 (g) → H2 (adsorbed)
  4. H2 (adsorbed) → 2 H (adsorbed)
  5. N (adsorbed) + 3 H(adsorbed)→ NH3 (adsorbed)
  6. NH3 (adsorbed) → NH3 (g)