Catalysts in Action (College Board AP Chemistry)

• Catalysts are classified into two categories depending on whether they are in the same phase as the reactants or not
  • Homogeneous
  • Heterogeneous
• Heterogeneous catalysts are in a different phase to the reaction
  • For example, the use of platinum or palladium metals (solid) in organic reactions such as hydrogenation reactions
• Homogeneous catalysts exist in the same phase as the reactants
  • Homogeneous catalysts most often catalyze gaseous and aqueous reactions
  • For example, aqueous zinc chloride, ZnCl₂, is used to catalyze the organic reaction between hydrogen chloride and alcohol

Enzyme Catalysis

• Enzymes are examples of biological catalysts that can speed up the rate of biological reactions in living organisms
  • Most enzymes are large protein molecules with molecular weights ranging from about 10,000 to about 1 million amu
  • Enzymes are very selective in the reactions they catalyze
  • Some are absolutely specific, operating for only one substance in only one reaction
• Enzyme molecules possess an active site
  • This is part of the molecule with a shape that allows a specific reactant molecule (substrate) to bind
  • The binding between the substrate and active site involves dipole-dipole attractions, hydrogen bonds, and dispersion forces
• Two models currently exist to explain how an enzyme and its substrate interact
  • Model 1:
    • The substrate molecule fits into the active site on the enzyme molecule, somewhat in the way a key fits into a lock
    • This results in the formation of an enzyme-substrate complex as a reaction intermediate
    • On binding to the enzyme, the substrate may have bonds weakened or new bonds formed that help yield the products
  • Model 2:
    • This is often called the induced fit model
    • This suggests that the active site of an enzyme changes its shape to fit its substrate

Enzyme Catalysis Models

Diagram A shows the lock and key model relationship between the enzyme’s active site and the reactant molecules (substrate) while diagram B shows the enzyme’s active site changing shape to allow the substrates to bind. 

• Some heterogeneous catalysts can speed up the reaction rate through the formation of weak covalent bonds with the reactant molecules
• One example is the decomposition of N₂O on gold, the solid catalyst

N₂O (g) → N₂ (g) + ½ O₂ (g)

• In the catalyzed decomposition, N₂O is chemically adsorbed on the surface of the solid gold
  • A weak covalent bond is formed between the oxygen atom of an N₂O molecule and a gold atom on the surface
  • This weakens the bond joining nitrogen to oxygen, making it easier for the N₂O molecule to break apart
  • The process can be represented as follows:

Decomposition of N₂O with Gold Catalyst

Diagram showing the catalytic action of gold in the decomposition of dinitrogen oxide, N₂O. A weak covalent bond represented by the broken lines is formed between oxygen and the solid gold catalyst

• Another example of catalysis involving covalent bond formation between a catalyst and the reactants is acid-base catalysis
  • In such reactions, the catalyst donates protons to the reactant molecules (usually a base), a process known as protonation
  • This leads to the formation of an intermediate which is more reactive than the original reactant
  • The conversion of esters into alcohol and water in the presence of a hydrochloric acid catalyst is a good example of such reactions

Acid Catalysed Hydrolysis of Esters

• 

Reaction mechanism showing acid-catalyzed hydrolysis of esters. The H₃O⁺ acid catalyst pronates the ester, forming a protonated intermediate. This intermediate is then easily hydrolysed

• In industry, many reactions are catalyzed by the surfaces of solids
  • These reactions often involve gaseous reactants being adsorbed on the surface of a solid catalyst
  • Adsorption refers to the collection of one substance on the surface of another substance

Examples of surface catalysis

• Some of the more important examples of surface catalysis include:
  • The Contact Process - a multistep reaction for making sulfuric acid

2SO₂ (g) + O₂ (g)  2SO₃ (g)

• The Haber Process - for making ammonia

N₂ (g) + 3H₂ (g)  2NH₃ (g)

• Catalytic converters - for the removal of toxic / poisonous gases from car exhausts

2CO (g) + 2NO (g)  2CO₂ + N₂

• Hydrogenation - for the hardening of vegetable oils

C₂H₂ (g) + H₂ (g)  C₂H₄ (g)

How surface catalysis works

• The mode of action of a heterogeneous catalyst consists of 2 main steps:
• Adsorption of the reactants on the catalyst surface
  • The reactants diffuse to the surface of the catalyst
  • The reactant is physically adsorbed onto the surface by weak forces
  • The reactant is chemically adsorbed onto the surface by covalent bonds
  • This causes bond weakening between the atoms of the reactants
• Desorption of the products
  • The bonds between the products and catalyst weaken so much that the products break away from the surface

Surface catalysis in hydrogenation

• Hydrogenation is an important industrial process that is used to convert Unsaturated fats, occurring as oils, to Saturated fats
• Hydrogenation involves the use of a heterogeneous catalyst
• The hydrogenation process involves the addition of hydrogen across the carbon=carbon double bond / C=C which converts them into carbon-carbon single bonds / C-C
• A simple example of hydrogenation involves ethylene and hydrogen gas:

C₂H₂ (g) + H₂ (g) → C₂H₄ (g)

• Without a catalyst, this reaction is very slow
  • However, when the reaction is catalyzed by a metal such as nickel, palladium or platinum, the rate increases dramatically
  • The role of the catalyst is to allow the formation of metal–hydrogen interactions that weaken the H‒H bond, making the reaction easier

How hydrogenation happens

• Using the ethylene and hydrogen example, hydrogen and ethylene adsorb onto the catalyst surface, where the reaction occurs
• This causes weakening of the hydrogen bond / H-H, ultimately breaking the bond
• This leaves two H atoms loosely bonded to the metal surface but relatively free to move
• When a hydrogen atom encounters an adsorbed ethylene molecule, it forms a single covalent bond with one of the carbon atoms
  • Effectively, the C‒C pi bond is destroyed
• This leaves an ethyl group, C₂H₅, bonded to the surface via a metal-to-carbon sigma bond
  • This sigma bond is relatively weak
  • So, when the other carbon atom also encounters a hydrogen atom, a bond is readily formed
  • This forms an ethane molecule, C₂H₆, which is released from the metal surface

Surface Catalytic Hydrogenation of Ethylene

Diagram showing the reaction mechanism between ethylene and hydrogen on the surface of platinum metal catalyst 

• In general, it is important to note that for solid catalysts, an increase in surface area makes them more effective