26.2 homogenous and heterogenous catalysts

Define homogeneous catalyst

Define heterogenous catalyst

What do most examples of homogenous catalysts involve

Redox reactions

Define adsorption

The first stage in heterogeneous catalysis when reactant molecules form bonds with atoms on the catalyst surface.

Explain the stages in adsorption of hydrogen onto nickel

• hydrogen gas diffuses to the surface of the nickel

• Hydrogen is physically adsorbed onto surface: weak forces link H2 molecules to the nickel

• H2 becomes chemically adsorbed onto surface: this causes stronger bonds to form betwee hydrogen and nickel.

• This causes weakening on H-H covalent bond

Explain the haber process in terms of heterogenous catalysis DARDD

1. Diffusion: N2 gas and H2 gas diffuses to the to the surface of the iron

2. Adsorption: reactant molecules are chemically adsorbed onto the surface of the iron. The bonds formed between the reactant molecules and the iron are strong enough to weaken covalent bonds with N-H so the atoms can react with each other and weak enough to break and allow the products to leave the surface.

3. Reaction: adsorbed N and H atoms react on surface of iron to form ammonia

4. Desorption: bonds between the ammonia and the surface of the iron weaken and are eventually broken

5. Diffusion: ammonia ammonia diffuses away from surface

Explain the structure of platinum in the catalytic removal of nitrogen oxides from the exhaust gases of car engines

2NO + CO → N2 + CO2

And

2NO2 + 4CO → N2 + 4CO2

• The ‘honeycomb’ structure in the catalytic converter contained small beads coated with platinum which act as heterogenous catalysts

Explain the catalytic process of to reduce the amount of harmful nitrogen oxides.

• adsorption of nitrogen oxides and carbon monoxide onto the catalyst surface

• weakening of the covalent bonds within the nitrogen oxides and carbon monoxide

• formation of new bonds between:

  • adjacent nitrogen atoms (to form nitrogen molecules)

  • carbon monoxide and oxygen atoms to form carbon dioxide

• desorption of nitrogen molecules and carbon dioxide molecules from the surface of the catalyst.

Explain the homogenous catalysis in the iodine-peroxodisulfate reaction

• very slow reaction in which the peroxydisulfate (S_2,O₈^2- ) ions oxidise the iodide to iodine

S₂O₈^2- (aq) + 2I^- (aq) → 2SO₄^2- (aq) + I₂ (aq)

• Since both the S₂O₈^2- and I^- ions have a negative charge, it will require a lot of energy for the ions to overcome the repulsive forces and collide with each other

• Therefore, Fe³⁺ (aq) ions are used as a homogeneous catalyst

• The catalysis involves two redox reactions:

  • First, Fe³⁺ ions are reduced to Fe²⁺ by  I^-

2Fe³⁺ (aq) + 2I^- (aq) → 2Fe²⁺ (aq) + I₂ (aq)

• Then, Fe²⁺ is oxidised back to Fe³⁺ by S₂O₈^2-

2Fe²⁺ (aq) + S₂O₈^2- (aq) → 2Fe³⁺ (aq) + 2SO₄^2-  (aq)

Draw the reaction pathway for a two-stage catalysed reaction

Nitrogen oxides and acid rain: explain how NOx can act as a catalyst

• As fossil fuels contain sulfur, burning the fuels will release sulfur dioxide which oxidises in air to sulfur trioxide

• dilute H₂SO₄ is formed by reaction with water, causing acid rain

• Nitrogen oxides can act as catalysts in the formation of acid rain by catalysing the oxidation of SO₂ to SO₃

NO₂ (g) + SO₂ (g) → SO₃ (g) + NO (g)

• The formed NO gets oxidised to regenerate NO₂

NO (g) + ½ O₂ (g) → NO₂ (g)

• The regenerated NO₂ molecule can again oxidise another SO₂ molecule to SO₃ which will react with rainwater to form H₂SO₄ and so on