Catalysis: Homogeneous and Heterogeneous

Real-Life Connection: From Life to Industry

Every breath you take, every heartbeat, every industrial process that makes modern life possible - all depend on catalysts!

In your body:

Carbonic anhydrase helps transport CO₂ (speeds reaction by 10⁷ times!)
Catalase decomposes H₂O₂ (prevents cell damage)
DNA polymerase copies genetic material (life itself!)
3,000+ different enzymes catalyze biochemical reactions

In industry:

Haber process (ammonia for fertilizers) - feeds 40% of world population
Catalytic converters (clean car exhaust) - reduce pollution by 90%
Petroleum cracking (gasoline production) - powers transportation
Margarine production (hydrogenation) - food industry

Without catalysts:

Life would be impossible (metabolism too slow)
Industrial processes uneconomical (too slow, too hot)
Environment more polluted (harmful emissions)

Understanding catalysis is crucial for chemistry, biology, and industry!

What is a Catalyst?

Definition

A catalyst is a substance that:

Increases the rate of a chemical reaction
Is not consumed in the overall reaction
Provides alternative pathway with lower activation energy
Is regenerated at the end of the reaction

$$\boxed{\text{Catalyst: Speeds up reaction without being permanently changed}}$$

What Catalysts Do NOT Do

Common misconceptions:

❌ Do NOT change equilibrium position (K unchanged) ❌ Do NOT change thermodynamics (ΔG, ΔH unchanged) ❌ Do NOT make unfavorable reactions favorable ❌ Do NOT change product distribution at equilibrium

✓ DO help reach equilibrium faster ✓ DO speed up both forward AND reverse reactions equally ✓ DO make economically unfeasible processes feasible

How Catalysts Work

Energy profile comparison:

Without Catalyst:          With Catalyst:
Energy                     Energy
  |                          |
  |     ╱╲                   |  ╱╲
  |    ╱  ╲                  | ╱  ╲╱╲
  |   ╱    ╲                 |╱      ╲
  |__╱      ╲___             |        ╲___
  |________________          |________________
     Progress                   Progress

Ea(uncat) = 100 kJ/mol      Ea(cat) = 50 kJ/mol

Key mechanism:

Catalyst provides alternative pathway
Lower activation energy (Ea)
More molecules have E ≥ Ea
Rate increases exponentially

$$\boxed{k_{cat} = Ae^{-E_{a,cat}/RT} > k_{uncat} = Ae^{-E_{a,uncat}/RT}}$$

Since Ea,cat < Ea,uncat → kcat » kuncat

Types of Catalysis

1. Homogeneous Catalysis

2. Heterogeneous Catalysis

3. Enzyme Catalysis (Biochemical)

4. Autocatalysis

Let’s explore each in detail.

Homogeneous Catalysis

Definition

Homogeneous catalysis: Catalyst and reactants are in the same phase (usually all gases or all in solution).

$$\boxed{\text{Catalyst phase} = \text{Reactant phase}}$$

Characteristics

Same phase as reactants
Molecular distribution uniform
Better contact between catalyst and reactants
Temperature control easier
Often acid-base catalysis in solutions
Difficult to separate catalyst from products

Mechanism

General pathway:

Step 1: Reactant + Catalyst → Intermediate (fast) Step 2: Intermediate → Product + Catalyst (fast)

Overall: Reactant → Product (Catalyst regenerated)

Examples

Example 1: Lead Chamber Process (H₂SO₄ Production)

Reaction:

$$2SO_2(g) + O_2(g) \rightarrow 2SO_3(g)$$

Catalyst: NO(g) - same phase as reactants

Mechanism:

$$2NO(g) + O_2(g) \rightarrow 2NO_2(g)$$ $$NO_2(g) + SO_2(g) \rightarrow SO_3(g) + NO(g)$$

Overall: 2SO₂ + O₂ → 2SO₃ (NO regenerated)

Example 2: Acid Catalysis of Ester Hydrolysis

Reaction:

$$CH_3COOC_2H_5 + H_2O \rightarrow CH_3COOH + C_2H_5OH$$

Catalyst: H⁺ (acid) - in same aqueous phase

Mechanism: H⁺ protonates carbonyl, making it more electrophilic and susceptible to nucleophilic attack by water.

Example 3: Oxidation of Oxalic Acid by KMnO₄

Reaction:

$$5(COOH)_2 + 2KMnO_4 + 3H_2SO_4 \rightarrow K_2SO_4 + 2MnSO_4 + 10CO_2 + 8H_2O$$

Catalyst: Mn²⁺ (produced in reaction) - autocatalysis

Initially slow, speeds up as Mn²⁺ is produced!

Example 4: Wilkinson’s Catalyst

Reaction: Hydrogenation of alkenes

$$RCH=CH_2 + H_2 \xrightarrow{[RhCl(PPh_3)_3]} RCH_2CH_3$$

Catalyst: Wilkinson’s catalyst [RhCl(PPh₃)₃] - dissolved in organic solvent (same phase)

Use: Organic synthesis, pharmaceutical industry

Advantages of Homogeneous Catalysis

All active sites available (molecular dispersion)
High selectivity (can be tuned chemically)
Mild conditions often sufficient
Well-understood mechanisms

Disadvantages

Difficult to separate catalyst from products
Catalyst loss in each batch
Product contamination possible
Limited thermal stability

Heterogeneous Catalysis

Definition

Heterogeneous catalysis: Catalyst and reactants are in different phases.

$$\boxed{\text{Catalyst phase} \neq \text{Reactant phase}}$$

Most common: Solid catalyst + Gas/liquid reactants

Characteristics

Different phase from reactants (usually solid)
Surface phenomenon - reaction occurs on catalyst surface
Easy separation (just filter out solid catalyst)
Active sites on surface
Surface area crucial - more area, faster reaction
Reusable catalyst

Mechanism: Three-Step Process

Step 1: Adsorption

Reactants bind to catalyst surface
Physical or chemical bonding
Concentration of reactants on surface

Step 2: Reaction on Surface

Bonds weakened/broken
New bonds formed
Lower activation energy pathway

Step 3: Desorption

Products leave catalyst surface
Catalyst surface regenerated
Ready for next cycle

         Adsorption    Reaction    Desorption
            ↓            ↓            ↓
Surface: [ ] → [A-B] → [A B] → [A B] → [ ] + Products
        Empty  Adsorbed Reacting Adsorbed Empty
                                Products

Types of Adsorption

Physical Adsorption (Physisorption)

Weak van der Waals forces
Reversible
Low heat of adsorption (< 40 kJ/mol)
Multi-layer adsorption possible
Non-specific

Chemical Adsorption (Chemisorption)

Strong chemical bonds form
Irreversible or slow reversal
High heat of adsorption (> 40 kJ/mol)
Monolayer adsorption only
Specific to certain sites
Important for catalysis

Factors Affecting Heterogeneous Catalysis

1. Surface Area

$$\boxed{\text{Activity} \propto \text{Surface Area}}$$

Methods to increase surface area:

Porous materials (zeolites, activated carbon)
Nanoparticles (higher surface-to-volume ratio)
Support materials (Al₂O₃, SiO₂) to disperse catalyst

Example: 1 cm³ cube has 6 cm² surface

Divide into 1 mm cubes: 600 cm² surface (100× increase!)
Nanoscale: millions of times more surface area

2. Temperature

Optimum temperature exists
Too low: slow reaction
Too high: desorption too fast, catalyst deactivation

3. Catalyst Poisoning

Catalyst poisoning: Unwanted substances block active sites

Examples:

CO poisons platinum catalysts
Sulfur compounds poison many metal catalysts
Lead poisons catalytic converters (why unleaded gasoline needed)

Effect: Drastically reduces catalyst activity

4. Promoters

Promoter: Substance that enhances catalyst activity (though not catalytic itself)

Example: In Haber process

Fe is catalyst
Mo (molybdenum) is promoter
Mo increases Fe activity significantly

Examples of Heterogeneous Catalysis

Example 1: Haber Process (Ammonia Synthesis)

Reaction:

$$N_2(g) + 3H_2(g) \rightleftharpoons 2NH_3(g)$$

Catalyst: Fe (iron) - solid catalyst, gaseous reactants

Promoter: Mo, Al₂O₃, K₂O

Conditions:

Temperature: 450-500°C
Pressure: 200-300 atm
Iron catalyst with promoters

Mechanism:

N₂ and H₂ adsorb on Fe surface
N≡N and H-H bonds weaken
N and H atoms react on surface
NH₃ desorbs

Importance: Produces fertilizers feeding 40% of world population!

Example 2: Contact Process (H₂SO₄ Production)

Reaction:

$$2SO_2(g) + O_2(g) \rightleftharpoons 2SO_3(g)$$

Catalyst: V₂O₅ (vanadium pentoxide) - solid catalyst

Conditions:

Temperature: 450°C
Pressure: ~1-2 atm
V₂O₅ catalyst

Mechanism:

$$V_2O_5 \rightarrow V_2O_4 + \frac{1}{2}O_2$$

(provides oxygen)

$$V_2O_4 + SO_2 \rightarrow SO_3 + V_2O_3$$ $$V_2O_3 + \frac{1}{2}O_2 \rightarrow V_2O_4$$

V₂O₅ regenerated!

Example 3: Catalytic Converters (Automobile Exhaust)

Reactions:

$$2CO + 2NO \xrightarrow{Pt, Pd, Rh} 2CO_2 + N_2$$ $$\text{Hydrocarbons} + O_2 \xrightarrow{Pt, Pd} CO_2 + H_2O$$

Catalyst: Platinum, Palladium, Rhodium on ceramic honeycomb

Function:

Converts harmful gases (CO, NO, hydrocarbons)
Into less harmful gases (CO₂, N₂, H₂O)

Structure: Honeycomb provides huge surface area

Poisoning: Lead poisons catalyst (reason for unleaded petrol)

Example 4: Hydrogenation of Oils (Margarine Production)

Reaction:

$$\text{Vegetable oil (liquid)} + H_2(g) \xrightarrow{Ni(s)} \text{Margarine (solid)}$$

Catalyst: Ni (nickel) - solid catalyst, finely divided

Mechanism:

C=C double bonds in oil adsorb on Ni surface
H₂ dissociates on Ni surface
H atoms add to C=C (hydrogenation)
Saturated fat desorbs

Result: Liquid oils become semi-solid (spreadable)

Example 5: Ostwald Process (Nitric Acid Production)

Reaction:

$$4NH_3(g) + 5O_2(g) \xrightarrow{Pt(s)} 4NO(g) + 6H_2O(g)$$

Catalyst: Pt (platinum) - solid catalyst

Conditions: 800-900°C, 5-9 atm

Use: Industrial production of HNO₃

Example 6: Zeolite Catalysis (Petroleum Cracking)

Reaction:

$$\text{Long-chain hydrocarbons} \xrightarrow{Zeolite} \text{Shorter hydrocarbons (gasoline)}$$

Catalyst: Zeolites (aluminosilicates with porous structure)

Mechanism:

Shape-selective catalysis
Pores allow only certain molecules
Acid sites in zeolite break C-C bonds

Applications:

Petroleum refining
Gasoline production
Isomerization

Advantages of Heterogeneous Catalysis

Easy separation from products (just filter)
Reusable (cost-effective)
High thermal stability
Less corrosive than acids/bases
Continuous operation possible (industrial reactors)

Disadvantages

Only surface active (bulk of solid inactive)
Catalyst poisoning issues
Less selective than homogeneous catalysts
Diffusion limitations
Deactivation over time (sintering, coking)

Enzyme Catalysis

What are Enzymes?

Enzymes are biological catalysts:

Proteins (complex 3D structures)
Highly specific (one enzyme, one reaction)
Extremely efficient (increase rates by 10⁶ to 10¹⁷!)
Work at mild conditions (body temperature, neutral pH)

$$\boxed{\text{Enzyme: Nature's perfect catalyst}}$$

Characteristics

Substrate specificity - catalyze only specific reactions
High efficiency - small amounts needed
pH sensitive - work at specific pH
Temperature sensitive - denature at high temp (>50°C typically)
Can be regulated - activity controlled by body

Lock-and-Key Model

Early model: Enzyme (lock) + Substrate (key)

Lock-and-Key Model

Enzyme has a rigid active site (the lock)
Substrate has a complementary shape (the key)
Only the correct substrate fits perfectly
Forms enzyme-substrate complex

Only specific substrate fits in enzyme active site

Induced Fit Model

Modern model: Enzyme shape changes when substrate binds

Induced Fit Model

Enzyme active site is flexible
When substrate approaches, enzyme changes shape to fit
Better explains enzyme-substrate interactions
More accurate than lock-and-key model

More accurate - explains enzyme flexibility

Mechanism

Step 1: Substrate binding

$$E + S \rightleftharpoons ES$$

(enzyme-substrate complex)

Step 2: Catalysis

$$ES \rightarrow EP$$

(enzyme-product complex)

Step 3: Product release

$$EP \rightarrow E + P$$

(enzyme regenerated)

Overall: E + S → E + P (enzyme unchanged)

Examples of Enzymes

1. Carbonic Anhydrase

Reaction:

$$CO_2 + H_2O \rightleftharpoons H_2CO_3$$

Function:

Helps transport CO₂ in blood
Speeds reaction by 10⁷ times!
Essential for respiration

Without enzyme: Reaction too slow for life

2. Catalase

Reaction:

$$2H_2O_2 \rightarrow 2H_2O + O_2$$

Function:

Decomposes H₂O₂ (toxic to cells)
One of fastest enzymes (10⁶ molecules/second!)
Found in liver, blood cells

Demonstration: Add liver to H₂O₂ → vigorous bubbling (O₂ evolution)

3. Invertase

Reaction:

$$\text{Sucrose} + H_2O \rightarrow \text{Glucose} + \text{Fructose}$$

Function:

Breaks down table sugar
Used in confectionery industry
Present in yeast, intestines

4. Urease

Reaction:

$$NH_2CONH_2 + H_2O \rightarrow 2NH_3 + CO_2$$

Function:

Hydrolyzes urea
First enzyme crystallized (1926)
Used in clinical diagnostics

5. DNA Polymerase

Function:

Copies DNA during cell division
Life depends on it!
Extremely specific (error rate < 1 in 10⁹)

Factors Affecting Enzyme Activity

1. Temperature

Activity
  |      ╱╲
  |     ╱  ╲___
  |    ╱      ╲___
  |___╱___________╲___
     10  37  50    Temp(°C)
        optimal

Optimum: Usually 37°C (body temp)
Too low: Slow reaction
Too high: Enzyme denatures (structure destroyed)

2. pH

Activity
  |      ╱╲
  |     ╱  ╲
  |    ╱    ╲
  |___╱______╲___
     2   7   10  pH
        optimal

Each enzyme has optimum pH
Pepsin (stomach): pH 2
Trypsin (intestine): pH 8
Extreme pH denatures enzyme

3. Substrate Concentration

Rate
  |         _______
  |       /
  |     /
  |   /
  |__/____________
     [Substrate]

Initially: Rate ∝ [S]
At high [S]: Saturation (all active sites occupied)
Maximum rate (Vmax) reached

Michaelis-Menten equation:

$$v = \frac{V_{max}[S]}{K_M + [S]}$$

4. Inhibitors

Competitive inhibition:

Inhibitor resembles substrate
Competes for active site
Can be overcome by increasing [S]

Non-competitive inhibition:

Inhibitor binds elsewhere
Changes enzyme shape
Cannot be overcome by increasing [S]

Cofactors and Coenzymes

Cofactor: Non-protein component needed for enzyme activity

Metal ions (Zn²⁺, Mg²⁺, Fe²⁺)

Coenzyme: Organic cofactor

NAD⁺, FAD, Coenzyme A
Often vitamins or vitamin derivatives

Apoenzyme (protein alone) + Cofactor = Holoenzyme (active)

Autocatalysis

Definition

Autocatalysis: Product of reaction acts as catalyst for the same reaction.

$$A \rightarrow B$$

where B catalyzes A → B

Characteristics

Initially slow (no catalyst present)
Speeds up as product forms
S-shaped kinetic curve
Self-accelerating reaction

[Product]
  |           ___/
  |        __/
  |      /
  |    /
  |___/__________
         Time

Example: Permanganate Oxidation

Reaction:

$$5(COOH)_2 + 2KMnO_4 + 3H_2SO_4 \rightarrow \text{Products}$$

Autocatalyst: Mn²⁺ (produced in reaction)

Observation:

First drop of KMnO₄: Decolorizes slowly
Subsequent drops: Decolorize faster and faster

Explanation: Mn²⁺ catalyzes the reaction, and is produced by the reaction!

Memory Tricks

“HORSE” for Homogeneous Catalysis

Homogeneously mixed (same phase) Often in solution Regenerates (catalyst recovered) Same phase as reactants Examples: Acid catalysis, NO in lead chamber

“HEAT” for Heterogeneous Catalysis

Huge surface area important Easy to separate Adsorption on surface Three steps: Adsorb-React-Desorb

“SELF” for Enzyme Specificity

Specific to substrate (lock and key) Efficient (high rate increase) Life depends on them Fragile (pH, temp sensitive)

“CAT” Properties

Catalyst accelerates reaction Activation energy lowered Thermodynamics unchanged (ΔH, ΔG, K same)

Common JEE Mistakes

Mistake 1: Catalyst Changes Equilibrium

Wrong: “Adding catalyst increases product yield”

Correct: Catalyst does not change equilibrium position!

Increases rate to reach equilibrium faster
K unchanged
Both forward and reverse rates increase equally

Mistake 2: Catalyst Changes ΔH

Wrong: “Exothermic reaction becomes less exothermic with catalyst”

Correct: ΔH completely unchanged by catalyst

Ea lowered for both forward and reverse
Energy of reactants and products unchanged

Mistake 3: Confusing Homogeneous and Heterogeneous

Wrong: “Fe catalyst in Haber process is homogeneous”

Correct: Fe is solid, N₂ and H₂ are gases

Different phases → heterogeneous!

Mistake 4: Enzyme Temperature

Wrong: “Higher temperature always increases enzyme activity”

Correct: Above optimum, enzyme denatures

Activity decreases
Irreversible damage to protein structure

Practice Problems

Level 1: JEE Main Foundation

Problem 1: Identify the type of catalysis: (a) Fe in Haber process (b) H⁺ in ester hydrolysis (c) NO in lead chamber process

Solution: (a) Heterogeneous - Fe (solid), N₂ and H₂ (gas) (b) Homogeneous - H⁺ (aqueous), ester and water (aqueous) (c) Homogeneous - NO (gas), SO₂ and O₂ (gas)

Level 2: JEE Main/Advanced

Problem 2: In presence of catalyst, Ea decreases from 80 kJ/mol to 40 kJ/mol. By what factor does rate increase at 300 K? (R = 8.314 J mol⁻¹ K⁻¹)

Solution:

$$\frac{k_{cat}}{k_{uncat}} = e^{-(E_{a,cat} - E_{a,uncat})/RT}$$ $$= e^{-(40000 - 80000)/(8.314 \times 300)}$$ $$= e^{40000/2494.2} = e^{16.04}$$ $$= 9.2 \times 10^6$$

Rate increases by factor of ~10⁷!

Problem 3: For a reaction at 300 K:

Without catalyst: k = 10⁻⁶ s⁻¹, Ea = 100 kJ/mol
With catalyst: Ea = 60 kJ/mol

Calculate: (a) Rate constant with catalyst (b) Factor of increase in rate

Solution:

(a)

$$\frac{k_{cat}}{k_{uncat}} = e^{-(E_{a,cat} - E_{a,uncat})/RT}$$ $$\frac{k_{cat}}{10^{-6}} = e^{40000/(8.314 \times 300)} = e^{16.04} = 9.2 \times 10^6$$ $$k_{cat} = 9.2 \text{ s}^{-1}$$

(b) Factor = 9.2 × 10⁶

Problem 4: Which statement is correct? (a) Catalyst increases equilibrium constant (b) Catalyst decreases activation energy (c) Catalyst increases ΔH of reaction (d) Catalyst is consumed in reaction

Answer: (b)

Explanation: (a) Wrong - K unchanged (b) Correct - Ea decreased (c) Wrong - ΔH unchanged (d) Wrong - Catalyst regenerated

Level 3: JEE Advanced

Problem 5: A reaction proceeds through the mechanism:

Without catalyst:

$$A + B \xrightarrow{slow} AB$$

(Ea = 120 kJ/mol)

With catalyst C:

$$A + C \xrightarrow{fast} AC$$

(Ea = 40 kJ/mol)

$$AC + B \xrightarrow{slow} AB + C$$

(Ea = 60 kJ/mol)

Calculate: (a) Overall Ea with catalyst (b) Factor of rate increase at 300 K

Solution:

(a) With catalyst, RDS (rate-determining step) is second step Ea(catalyzed) = 60 kJ/mol (Ea of RDS)

(Note: First step has Ea = 40 kJ/mol but it’s fast; second step is slow with Ea = 60 kJ/mol, so that determines overall rate)

(b)

$$\frac{k_{cat}}{k_{uncat}} = e^{-(60000 - 120000)/(8.314 \times 300)}$$ $$= e^{60000/2494.2} = e^{24.06}$$ $$= 3.0 \times 10^{10}$$

Rate increases by factor of ~10¹⁰!

Comparison Table

Aspect	Homogeneous	Heterogeneous	Enzyme
Phase	Same as reactants	Different from reactants	Usually aqueous
Contact	Molecular	Surface only	Active site
Separation	Difficult	Easy (filter)	Difficult
Selectivity	Good	Moderate	Extremely high
Conditions	Mild to harsh	Often harsh	Mild only
Examples	Acid catalysis, NO	Fe, Pt, Ni, V₂O₅	Enzymes
Cost	Can be high	Moderate	Can be high
Reusability	Difficult	Easy	Difficult

Industrial Importance

Economic Impact

80% of chemical processes use catalysts!

Without catalysts:

Haber process would need 1000°C instead of 450°C (uneconomical)
Cars would emit 10× more pollutants
Many drugs couldn’t be synthesized

Catalyst industry: Billions of dollars annually

Green Chemistry

Catalysts enable:

Lower temperatures → Less energy
Better selectivity → Less waste
Atom economy → Efficient use of materials

12 Principles of Green Chemistry - catalysis features prominently!

Connection to Other Topics

Link to Arrhenius Equation

Catalysts lower Ea, increasing k exponentially
See: Arrhenius Equation

Link to Rate Law

Catalysts appear in rate law for elementary steps
Change mechanism, may change apparent order
See: Rate Law

Link to Equilibrium

Catalysts don’t change K (equilibrium constant)
Help reach equilibrium faster
See: Chemical Equilibrium

Link to Factors Affecting Rate

Catalysts are one of the main factors
Surface area matters for heterogeneous catalysis
See: Factors Affecting Rate

JEE Previous Year Questions

JEE Main 2021: Which is an example of heterogeneous catalysis? (a) Hydrolysis of sugar by H⁺ (b) Decomposition of H₂O₂ by I⁻ (c) Haber process (d) Lead chamber process

Answer: (c) Haber process

Explanation:

Fe catalyst (solid) with N₂, H₂ (gases) = different phases
(a), (b), (d) all homogeneous (same phase)

JEE Advanced 2019: An enzyme catalyzed reaction has Vmax = 10 μmol/min and KM = 2 mM. What is the rate when [S] = 2 mM?

Solution:

Michaelis-Menten equation:

$$v = \frac{V_{max}[S]}{K_M + [S]}$$ $$v = \frac{10 \times 2}{2 + 2} = \frac{20}{4} = 5 \text{ μmol/min}$$

Answer: 5 μmol/min (half of Vmax when [S] = KM)

Quick Revision Points

Catalyst speeds reaction without being consumed
Lowers Ea (activation energy)
No change in ΔH, ΔG, or K
Homogeneous: Same phase as reactants
Heterogeneous: Different phase (usually solid)
Mechanism: Adsorption → Reaction → Desorption (heterogeneous)
Enzymes: Biological catalysts, highly specific
Surface area crucial for heterogeneous catalysis
Catalyst poisoning blocks active sites
Promoters enhance catalyst activity

Summary

Catalysis is fundamental to:

Life (enzyme-catalyzed metabolism)
Industry (80% of chemical processes)
Environment (catalytic converters, green chemistry)

Three types:

Homogeneous - same phase, good selectivity
Heterogeneous - different phase, easy separation
Enzyme - nature’s catalysts, extremely specific

Key principle: Catalysts provide alternative pathway with lower Ea, dramatically increasing reaction rates without affecting thermodynamics.

Understanding catalysis is essential for:

JEE success (frequent topic)
Industrial chemistry
Biochemistry and medicine
Environmental protection

Previous Topic: Arrhenius Equation

Related Topics:

Factors Affecting Rate
Chemical Equilibrium
Coordination Compounds (catalyst complexes)