Catalysis

• The distinction between homogeneous and heterogeneous catalysis with industrial examples
• How promoters and poisons modify catalyst activity, with specific examples (Al₂O₃/K₂O for Fe; CO and As₂O₃ as poisons)
• The lock-and-key model of enzyme catalysis and how pH/temperature affect enzyme activity
• How zeolites provide shape-selective catalysis (ZSM-5 in petroleum cracking)
• How to read and interpret activation energy diagrams showing the catalysed vs uncatalysed reaction path

What is a Catalyst?

A catalyst is a substance that alters the rate of a chemical reaction without being consumed in the reaction and can be recovered chemically unchanged at the end.

Key features of a catalyst:

• Changes reaction rate (usually increases it; rarely decreases — negative catalyst)
• Is not consumed — present in the same form and amount before and after the reaction
• Does not alter the equilibrium position (both forward and reverse rates are increased equally)
• Lowers the activation energy (Eₐ) by providing an alternate reaction pathway
• Is required in small amounts (except when it is also a reactant in a non-catalytic sense)

Catalytic action — energetics: Without catalyst: Eₐ (uncatalysed) = high barrier With catalyst: Eₐ (catalysed) = lower barrier (alternate pathway)

ΔG of the reaction (thermodynamics) is unchanged — the catalyst cannot make an energetically unfavourable reaction spontaneous. It only speeds up a reaction that is already thermodynamically feasible.

Homogeneous Catalysis

In homogeneous catalysis, the catalyst and reactants are in the same phase (gas or liquid).

Examples:

Mechanism of homogeneous catalysis — Intermediate compound theory:

1. Catalyst reacts with reactant to form an intermediate
2. Intermediate reacts with another reactant to give the product + regenerated catalyst

Example: Lead Chamber process

• 2NO + O₂ → 2NO₂ (NO is oxidised)
• NO₂ + SO₂ → NO + SO₃ (SO₂ is oxidised; NO regenerated)
• Net: SO₂ + ½O₂ → SO₃

Heterogeneous Catalysis

In heterogeneous catalysis, the catalyst and reactants are in different phases. The catalyst is almost always a solid; reactants are gases or liquids.

Examples:

Steps in heterogeneous catalysis (adsorption theory):

1. Diffusion of reactant molecules to the catalyst surface
2. Adsorption of reactants on active sites (chemisorption — bonds form with surface)
3. Reaction — adsorbed molecules react (activation energy is lowered by weakening bonds on adsorption)
4. Desorption of products from the surface
5. Diffusion of products away from the surface

The surface provides active sites — atoms or groups of atoms with special catalytic activity (often defects, edges, corners of the crystal).

Promoters

Promoters (activators) are substances that enhance the activity of a catalyst. They are not catalysts themselves.

Al₂O₃ is a structural promoter (maintains high surface area); K₂O is an electronic promoter (changes electron density).

Catalyst Poisons

Catalyst poisons are substances that reduce or destroy the activity of a catalyst. They bind to active sites and block them (often irreversibly).

Selectivity (related to poisoning): Some poisons selectively poison one type of active site, making the catalyst selective for a specific product — this is called selective poisoning and can be used intentionally.

Enzyme Catalysis

Enzymes are biological macromolecules (proteins) that act as catalysts in living systems. They are the most efficient and specific catalysts known.

Characteristics of enzyme catalysis:

1. Highly specific — each enzyme catalyses only one reaction (one substrate)
2. High efficiency — turnover number can be 10⁶ to 10⁷ reactions per second per enzyme molecule
3. Optimum temperature — activity peaks at ~37°C for human enzymes; denatured above ~40–50°C
4. Optimum pH — each enzyme has a specific pH optimum (pepsin: pH 2; trypsin: pH 8; most enzymes: pH ~7)
5. Activated by cofactors — some enzymes require metal ions (Mg²⁺, Zn²⁺) or coenzymes (vitamins)
6. Inhibited by inhibitors — heavy metal ions (Hg²⁺, Pb²⁺) inhibit enzymes (like catalyst poisons)

Lock-and-Key Model (Fischer, 1894):

• The enzyme has a specific active site (the "lock") with a precise geometric shape
• Only a substrate with the complementary shape (the "key") can bind
• Enzyme-substrate complex (ES complex) forms → reaction occurs → products released → enzyme regenerated

Induced Fit Model (Koshland, 1958) — more accurate: the active site is flexible and changes shape to fit the substrate upon binding (like a glove conforming to a hand).

Examples of enzyme catalysis:

Zeolites and Shape-Selective Catalysis

Zeolites are hydrated aluminosilicates with a microporous three-dimensional crystal structure. General formula: Mₓ/ₙ[(AlO₂)ₓ(SiO₂)ᵧ]·wH₂O

Structure: SiO₄ and AlO₄ tetrahedra linked in 3D network with uniform-sized pores and channels (0.3–1.0 nm diameter).

Shape-selective catalysis: The zeolite pores act as molecular sieves — only molecules of the right size and shape can enter the pores, react at active sites, and exit.

Three types of shape selectivity:

1. Reactant selectivity — only reactants small enough to enter pores react
2. Product selectivity — only products small enough to exit pores are formed
3. Transition state selectivity — only certain transition states fit within the cage

ZSM-5 (Zeolite Socony Mobil-5):

• Widely used in petroleum cracking to convert heavy hydrocarbons to petrol-range molecules
• Converts methanol to gasoline (MTG process)
• Produces linear (unbranched) hydrocarbons preferentially due to pore geometry
• Used in isomerisation of xylenes

Other zeolite applications:

• Water softening (ion exchange — Ca²⁺/Mg²⁺ → Na⁺)
• Drying gases (adsorption of water)
• Separation of straight-chain from branched alkanes

Activation Energy Diagram — Catalysed vs Uncatalysed

A catalyst lowers activation energy (Eₐ) by providing an alternate reaction pathway. The energy profile diagram shows:

Energy
  |        ‡ (uncatalysed, high Ea)
  |       /\
  |      /  \
  |     /    \
  |    /    ‡ (catalysed, lower Ea)
  |   /    /\ \
  |  /    /  \ \
  | /    /    \ \
  |/  Reactants  \ Products
  |________________\______ Reaction coordinate

Key points on the diagram:

• Reactants and Products energy levels are SAME with or without catalyst (ΔH unchanged)
• Activation energy Eₐ (catalysed) < Eₐ (uncatalysed)
• The catalysed path may show an intermediate (shallow trough on the way)
• The position of equilibrium (Keq) is unchanged — both forward and reverse Eₐ are lowered by equal amounts

Mathematically (Arrhenius equation): k = A·e^(−Eₐ/RT) Lower Eₐ → larger e^(−Eₐ/RT) → larger k (rate constant) → faster rate.

Comparison: Types of Catalysis

Catalytic Poisoning vs Catalyst Deactivation

Poisoning (chemical): Irreversible blocking of active sites by chemisorption of foreign molecules (CO poisons Fe, As₂O₃ poisons Pt).

Sintering (physical deactivation): At high temperature, catalyst particles fuse → surface area decreases → fewer active sites. Promoters (like Al₂O₃) prevent sintering.

Leaching: Solid catalyst dissolves in reaction medium (relevant for liquid-phase reactions).

This is why promoters (not just catalysts) are essential for industrial processes — Al₂O₃ maintains Fe surface area (anti-sintering), K₂O increases electronic activity.

Example 1: Activation Energy and Rate — Catalyst Effect

Problem: An uncatalysed reaction has Eₐ = 80 kJ mol⁻¹. A catalyst reduces Eₐ
to 50 kJ mol⁻¹. By what factor does the rate increase at T = 300 K?
(R = 8.314 J mol⁻¹ K⁻¹; use ln(rate ratio) = ΔEₐ/RT)

Step 1: ΔEₐ = 80 − 50 = 30 kJ mol⁻¹ = 30,000 J mol⁻¹

Step 2: From Arrhenius equation:
        k₂/k₁ = e^(ΔEₐ/RT) = e^(30000 / (8.314 × 300))

Step 3: Calculate exponent:
        30000 / (8.314 × 300) = 30000 / 2494.2 = 12.03

Step 4: k₂/k₁ = e^12.03 ≈ 1.67 × 10⁵

Answer: The rate increases by a factor of approximately 1.67 × 10⁵ (about 167,000 times faster).

This shows why even a modest reduction in Eₐ (30 kJ mol⁻¹) produces 
an enormous rate increase — the exponential nature of the Arrhenius equation.

Example 2: Enzyme Catalysis — Identifying the Correct Concept

Problem: Which of the following statements about enzyme catalysis is CORRECT?
(A) Enzymes are consumed during the reaction
(B) Enzymes lower the ΔG of the reaction
(C) A single enzyme can catalyse many different reactions with equal efficiency
(D) Enzymes have an optimum pH and temperature beyond which activity decreases

Step 1: Evaluate each option:

(A) Enzymes are consumed — INCORRECT
    Enzymes are catalysts; they are regenerated after each catalytic cycle.
    They are consumed only if denatured (which is not normal catalytic action).

(B) Enzymes lower ΔG — INCORRECT
    Catalysts lower Eₐ (activation energy), NOT ΔG (free energy change).
    ΔG is a thermodynamic quantity and depends only on reactants and products,
    not on the pathway.

(C) One enzyme catalyses many reactions — INCORRECT
    Enzyme catalysis is HIGHLY SPECIFIC. Each enzyme has an active site
    complementary to ONE substrate (or a narrow class). Lock-and-key specificity.

(D) Optimum pH and temperature — CORRECT
    Each enzyme works best at a specific pH and temperature.
    Beyond the optimum: denaturation of protein structure → loss of active site shape
    → reduced activity. Human enzymes: optimum ~37°C, ~pH 7.

Answer: (D) is correct.
Key: Remember Eₐ ↓ (not ΔG ↓), and enzymes are highly specific.

1. What is the difference between a catalyst poison and a promoter? Give one example of each for the Haber process catalyst.
2. Explain the lock-and-key model of enzyme catalysis. Why does enzyme activity decrease above the optimum temperature?
3. In the Contact process, V₂O₅ is the catalyst and SO₂ + O₂ are reactants (gas phase). Identify this as homogeneous or heterogeneous catalysis and justify.
4. Draw and label the activation energy diagram for a catalysed vs uncatalysed exothermic reaction. Mark Eₐ(catalysed), Eₐ(uncatalysed), and ΔH.
5. (Stretch) ZSM-5 zeolite is used to crack long-chain hydrocarbons selectively to produce petrol-range molecules. Explain how the pore geometry of ZSM-5 controls (a) which reactants can enter, (b) which products can exit, and (c) why this makes ZSM-5 more selective than a conventional metal catalyst. What would happen if the pore size were doubled?

• NCERT Class 12 Chemistry, Chapter 5 — Surface Chemistry, Section 5.3 (Catalysis)
• Catalysis gives 1–2 JEE/NEET questions per year; activation energy diagrams, enzyme lock-and-key, and industrial catalyst examples are most tested
• Common MCQ formats: identify incorrect statement about catalysts, identify promoter vs catalyst, interpret energy diagram, explain zeolite selectivity

Study strategy: Draw the activation energy diagram twice from scratch — label all six elements (reactants, products, TS uncatalysed, TS catalysed, Eₐ uncatalysed, Eₐ catalysed). For enzyme questions, focus on what changes with temperature (structure) vs what changes with pH (ionisation of active site). Memorise the 5-row table of industrial catalysts (Haber, Contact, Ostwald, Sabatier, Lead Chamber) with catalysts and promoters.

Practice MCQs on homogeneous vs heterogeneous catalysis, enzyme properties, zeolites, and activation energy diagrams in the Surface Chemistry — Catalysis topic bank. Clear Easy before attempting Medium.

Confused about why a catalyst doesn't change Keq even though it speeds up the reaction? Ask the Drishti AI tutor to explain using the Arrhenius equation and the relationship between Eₐ(forward), Eₐ(reverse), and ΔH.

Complete all 5 Quick Check questions and mark them in your Drishti progress tracker. Aim for 4/5 before moving to the next chapter.

• Read: Chapter 6 — General Principles of Isolation of Elements — thermodynamic and electrochemical principles of metallurgy
• Revise: Full Surface Chemistry — attempt a 20-question mixed MCQ set (Adsorption + Colloids + Catalysis)
• Practice: Surface Chemistry PYQs from JEE Mains 2018–2024