3.2.5.6 Catalysts

Explain why TM are good catalysts

1.) They have multiple stable oxidation states - They have multiple vacant d orbitals with similar energies, allowing them to transition between different charges.

2.) They readily forming dative bonds with ligands - Their vacant d orbitals can form coordinate bonds with ligands, facilitating electron transfer.

-This allows them to accept or donate electrons during a reaction, significantly lowering activation energies and increasing reaction rates.

Two types of catalysts

homogeneous and heterogeneous

Heterogeneous Catalysts

A heterogeneous catalyst is in a different phase from the reactants and the reaction occurs at active sites on the surface.

Mechanism of Heterogeneous Catalysts

1.) Adsorption: Reactants are adsorbed onto the catalyst's surface.

2.) Bond Weakening: Bonds within the reactant molecules weaken, facilitating the reaction.

3.) Favourable Orientation: Reactants are held in an optimal orientation for the reaction.

4.) Desorption: Products are released, freeing up active sites.

How to increase efficiency of catalysts

To increase efficiency, catalysts are often spread over a support medium, maximizing surface area and reducing costs.

What happens in contact process

V2​O5​ reduces to V2​O4​ and is then regenerated, facilitating continuous conversion of SO2 to SO3​

The Contact Process:

Catalyst
Reaction
Mechanism

1.) Catalyst: Vanadium(V) oxide (V2O5)

2.) Reaction:
2SO2​ + O2 ​→ 2SO3​

3.) Mechanism:
-Step 1: SO2​ + V2​O5​→ SO3​ + V2​O4​

-Step 2: V2​O4 ​+ O2​→ V2​O5

Explain what happens when u increase the surface area to volume ration to Ea

Increasing the surface area to volume ratio increases the number of exposed active sites which leads to a lower activation energy.

Explain the importance of variable oxidation states in catalysis

1.) Transfer Electrons:

-It can accept and donate electrons, which is vital in redox and multi-electron reactions.

2.) Stabilize Intermediates:

-It provides temporary electron storage, lowering activation energies and speeding up the reaction.

3.) Cycle Efficiently:

-Its ability to reversibly change oxidation states allows the catalyst to regenerate and continue catalyzing reactions.

What is haber process

A reversible reaction between nitrogen and hydrogen to form ammonia

What heterogeneous catalyst is used in the Haber process?

iron

What is catalyst poisoning

-Impurities in reactants can poison a heterogeneous catalyst by blocking its active sites.

-This reduces the catalyst's efficiency and increases replacement costs.

What are the consequences of catalyst poisoning

-Reduces available catalyst surface area.

-Lowers reaction rate.

-Increases operating costs due to less product output for a given catalyst mass and energy input.

-Can necessitate costly catalyst replacement.

For example, sulfur poisons the iron catalyst in the Haber process - the hydrogen from natural gas contains sulfur impurities that adsorb onto the iron catalyst surface as iron sulfide.

Homogeneous Catalysts

-A homogeneous catalyst is in the same phase as the reactants, usually in solution.

-These catalysts work by forming an intermediate species that reacts to form the products and regenerates the catalyst.

Intermediate Formation

The catalyst reacts with one reactant to form an intermediate.

Product Formation

The intermediate then reacts with another reactant to form the product and regenerates the catalyst.

Catalysis by Fe2+ Ions:

Reaction
Mechanism

Reaction: 2I− + S2​O8^2− ​→ I2​+ 2SO4^2− -This reaction is slow due to the repulsion between the negatively charged ions.

-Adding Fe2+ ions accelerates the reaction as follows:The mechanism involves:

1. Fe²⁺ ions are oxidised to Fe³⁺ by S₂O₈^2-:

S₂O₈^2-_(aq) + 2Fe²⁺_(aq) ➔ 2Fe³⁺_(aq) + 2SO₄^2-_(aq)

1. The Fe³⁺ ions then oxidise I^- to I₂, regenerating Fe²⁺:

2Fe³⁺_(aq) + 2I^-_(aq) ➔ I_2(aq) + 2Fe²⁺_(aq)

Fe2+ and Fe3+ facilitate electron transfer between I− and S2​O8^2−​, speeding up the reaction.

Autocatalysis

Autocatalysis occurs when a product of the reaction acts as a catalyst, speeding up the reaction as the product accumulates.

Mn2+ Autocatalysis

Reaction
Mechanism
Explaination

The reaction between permanganate ions (MnO₄^-) and oxalate ions (C₂O₄^2-) is an autocatalytic reaction. Autocatalysis occurs when a product of the reaction acts as a catalyst, speeding up the reaction as the product accumulates. In this case, Mn²⁺ is the autocatalytic product that accelerates the reaction.

The overall reaction is:

5C₂O₄^2-_(aq) + 2MnO₄^-_(aq) + 16H⁺_(aq) ➔ 10CO_2(g) + 2Mn²⁺_(aq) + 8H₂O_(l)

• The mechanism showing Mn²⁺ autocatalysis is:

  1. Mn²⁺ catalyses the reaction by first reacting with MnO₄^- to form Mn³⁺ ions:

• MnO₄^-_(aq) + 4Mn²⁺_(aq) + 8H⁺_(aq) ➔ 5Mn³⁺_(aq) + 4H₂O_(l)

• 1. The Mn³⁺ ions then react with C₂O₄^2- to regenerate Mn²⁺:

• 2Mn³⁺_(aq) + C₂O₄^2-_(aq) ➔ 2Mn²⁺_(aq) + 2CO_2(g)