Catalysts are essential in chemistry because they lower activation energy and speed up reactions without being consumed. However, in both laboratory and industrial settings, catalysts can lose their effectiveness. This phenomenon—called catalyst poisoning—is a critical idea in IB Chemistry Topic 6 (Chemical Kinetics) and Topic 19 (HL). Understanding catalyst poisoning helps students appreciate how real-world chemical processes work, why efficiency drops over time, and how industries prevent catalyst deterioration.
What Is Catalyst Poisoning?
Catalyst poisoning is the process by which a catalyst becomes less effective or inactive because foreign substances bind to its active sites, preventing reactants from adsorbing.
When a substance (the “poison”) adheres strongly to the catalyst’s surface:
- Reactants can no longer attach
- Reaction rate decreases
- Catalyst activity drops
- Efficiency and product yield decline
Poisoning is especially problematic for solid heterogeneous catalysts used in industrial reactions.
Why Catalysts Have Active Sites
Catalysts work by providing active sites, which are specific points on their surface where reactants adsorb and react. If these sites are blocked:
- The catalyst cannot function
- Reaction pathways shut down
- Activation energy effectively increases
Poisoning prevents reactants from reaching these active sites.
Types of Catalyst Poisoning
IB Chemistry focuses mainly on two kinds:
1. Reversible Poisoning
The poison binds loosely and can be removed by:
- Heating
- Washing
- Changing reaction conditions
Example:
- Carbon monoxide (CO) weakly poisons some metal catalysts but can be removed under certain conditions.
Reversible poisoning is less harmful because the catalyst can often be regenerated.
2. Irreversible Poisoning
The poison binds strongly and permanently to the catalyst surface.
It cannot be removed through simple treatment.
Examples:
- Lead damaging catalytic converters
- Sulfur poisoning in industrial catalysts
- Heavy metals binding to enzyme active sites
Irreversible poisoning requires replacing or fully regenerating the catalyst.
Common Catalyst Poisons
Catalyst poisons depend on the reaction and catalyst type:
For metal catalysts (industry):
- Sulfur compounds
- Lead compounds
- Phosphorus
- Chlorine and chlorinated hydrocarbons
- Carbon monoxide (for some catalysts)
For enzyme catalysts (biology):
- Heavy metals (Hg²⁺, Pb²⁺)
- Cyanide
- Certain drugs or inhibitors
These poisons bind strongly to catalytic metals or functional groups.
Industrial Examples of Catalyst Poisoning
Catalyst poisoning poses major problems in manufacturing, fuel processing, and environmental control.
1. Haber Process (ammonia production)
The iron catalyst is poisoned by:
- Sulfur compounds
- Carbon monoxide
- Carbon dioxide
Reactants must be purified before entering the reactor.
2. Catalytic Converters (cars)
Catalysts made of platinum, palladium, and rhodium convert pollutants but are poisoned by:
- Lead compounds
- Sulfur
- Phosphorus from engine oil additives
This is why unleaded petrol became mandatory.
3. Hydrogenation Reactions
Nickel catalysts used in hydrogenation are poisoned by sulfur impurities found in oils and fats.
How Industries Prevent Catalyst Poisoning
To protect expensive catalysts, industries use:
1. Purification of reactants
Removing sulfur, lead, or other contaminants beforehand.
2. Protective filters and traps
Prevent poisons from reaching catalysts.
3. Controlled reaction conditions
Avoiding temperatures or pressures that increase poisoning.
4. Regeneration cycles
Burning off impurities or reactivating catalysts.
Why Catalyst Poisoning Matters in IB Chemistry
Understanding catalyst poisoning helps explain:
- Why catalyst efficiency changes over time
- Why industrial plants spend money on reactant purification
- Why reaction rates drop unexpectedly
- Why green chemistry emphasizes cleaner feedstocks
- How enzyme activity is inhibited in biological systems
It also reinforces ideas about adsorption, reaction pathways, and activation energy.
Common IB Misunderstandings
“Catalysts are never consumed.”
Catalysts aren’t consumed chemically, but they can become inactive due to poisoning.
“Catalyst poisoning breaks the catalyst apart.”
Poisoning blocks active sites; it does not destroy the catalyst structure.
“Only gases poison catalysts.”
Liquids and solids can also act as poisons.
“Poisoning always reversible.”
Many poisons bind permanently and cannot be removed.
FAQs
Why do poisons bind so strongly?
Because they often form strong chemical bonds with the metal surface or active site.
Can poisoned catalysts be reused?
Only if poisoning is reversible; otherwise, they must be replaced.
How expensive is catalyst poisoning?
Extremely costly—industrial catalysts (e.g., platinum) are very valuable.
Conclusion
Catalyst poisoning occurs when foreign substances block the active sites of a catalyst, reducing its effectiveness. It affects industrial reactions, environmental technologies, and even biological enzymes. Understanding catalyst poisoning helps IB Chemistry students see why purity, reaction conditions, and catalyst maintenance are essential for efficient and sustainable chemical processes.
