Catalysis

Overview

A catalyst is a substance that increases the rate of a chemical reaction without being consumed. Catalysts work by providing an alternative reaction pathway with a lower activation energy.

How Catalysts Work

Energy Diagram

Energy
  ↑
  |       ∧ Uncatalyzed pathway
  |      / \     (high Ea)
  |     /   \
  |    /  ∧  \  Catalyzed pathway
  |   / /  \  \  (lower Ea)
  |  / /    \  \
  | /_/      \__\___
  |________________→ Reaction Progress

Key Points

• Catalysts lower activation energy ($E_a$)
• Products and reactants remain the same
• $\Delta H$ is unchanged
• Equilibrium position is unchanged
• Both forward and reverse rates increase equally

Types of Catalysts

Homogeneous Catalysis

Catalyst is in the same phase as reactants.

Example: Acid-catalyzed esterification

$$\text{CH}_3\text{COOH} + \text{CH}_3\text{OH} \xrightarrow{\text{H}^+} \text{CH}_3\text{COOCH}_3 + \text{H}_2\text{O}$$

Advantages:

• All molecules accessible
• High selectivity possible

Disadvantages:

• Hard to separate catalyst
• May need neutralization

Heterogeneous Catalysis

Catalyst is in a different phase from reactants.

Example: Haber Process

$$\text{N}_2 + 3\text{H}_2 \xrightarrow{\text{Fe}} 2\text{NH}_3$$

• Iron (Fe) catalyst
• Gases react on solid surface

Advantages:

• Easy separation
• Can be reused
• Continuous processes possible

Disadvantages:

• Only surface is active
• Can be poisoned

Surface Catalysis Steps

1. Adsorption: Reactants bind to catalyst surface
2. Reaction: Bonds break and form on surface
3. Desorption: Products leave the surface

Adsorption Types

Type	Bonding	Strength	Reversibility
Physisorption	Van der Waals	Weak	Reversible
Chemisorption	Chemical bonds	Strong	May be irreversible

Enzyme Catalysis

Biological catalysts (proteins) that are highly specific and efficient.

Characteristics

• Extremely specific (lock and key model)
• Work at mild conditions (body temperature, neutral pH)
• Rate increases of $10^6$ to $10^{12}$ times
• Subject to inhibition

Lock and Key Model

$$\text{E} + \text{S} \rightleftharpoons \text{ES} \rightarrow \text{E} + \text{P}$$

Where E = enzyme, S = substrate, P = products

Michaelis-Menten Kinetics

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

Where:

• $v$ = reaction rate
• $V_{max}$ = maximum rate
• $[\text{S}]$ = substrate concentration
• $K_m$ = Michaelis constant

Enzyme Inhibition

Type	Mechanism	Effect
Competitive	Binds to active site	Increases apparent $K_m$
Non-competitive	Binds elsewhere	Decreases $V_{max}$
Irreversible	Permanent binding	Destroys enzyme

Important Catalytic Processes

Industrial Processes

Process	Reaction	Catalyst
Haber	$\text{N}_2 + 3\text{H}_2 \rightarrow 2\text{NH}_3$	Fe
Contact	$2\text{SO}_2 + \text{O}_2 \rightarrow 2\text{SO}_3$	V₂O₅
Ostwald	$4\text{NH}_3 + 5\text{O}_2 \rightarrow 4\text{NO} + 6\text{H}_2\text{O}$	Pt
Hydrogenation	$\text{C}=\text{C} + \text{H}_2 \rightarrow \text{C}-\text{C}$	Ni, Pd, Pt
Cracking	Large alkanes → smaller	Zeolites

Catalytic Converters

Three-way catalysts in cars:

1. Oxidation: $\text{CO} \rightarrow \text{CO}_2$, $\text{HC} \rightarrow \text{CO}_2 + \text{H}_2\text{O}$
2. Reduction: $\text{NO}_x \rightarrow \text{N}_2 + \text{O}_2$

Catalysts: Pt, Pd, Rh on ceramic support

Catalyst Poisoning

Loss of catalytic activity due to blocking of active sites.

Common Poisons

• Lead (poisons Pt catalysts)
• Sulfur compounds
• Carbon monoxide (some catalysts)
• Heavy metals

Prevention

• Remove poisons from feed
• Regenerate catalyst
• Use poison-resistant catalysts

Autocatalysis

Product of reaction acts as catalyst.

Example: Permanganate reaction

$$\text{MnO}_4^- + \text{C}_2\text{O}_4^{2-} \rightarrow \text{Mn}^{2+} + \text{CO}_2$$

Mn²⁺ product catalyzes the reaction.

Rate Curve

Rate
  ↑
  |      ___________
  |     /
  |    /
  |   / Induction
  |  /  period
  | /
  |/_______________→ Time

Catalyst Properties Summary

Property	Effect
Lowers $E_a$	Increases rate
Not consumed	Regenerated
Doesn't change $\Delta H$	Thermodynamics unchanged
Doesn't shift equilibrium	$K$ unchanged
Speeds both directions	Equilibrium reached faster
High surface area	More active sites
Specificity	Selectivity for desired products

Promoters and Inhibitors

Promoters

Substances that enhance catalyst activity. Example: Al₂O₃ and K₂O in Haber process (with Fe)

Inhibitors

Substances that reduce catalyst activity. Used to control reaction rates or protect catalysts.