AS/A-level Chemistry - Catalysts

Do you recognize the role of catalysts in AS/A-level Chemistry?

• A pair of reacting molecules must collide, with a combined energy greater than the activation enthalpy for the reaction in order to make a successful collision.
• They are used in order to overcome the activation energy barrier more easily.
• They speed up the rate of a reaction but can be recovered chemically unchanged at the end.
• The process of speeding up a chemical reaction using a catalyst is called catalysis.
• They do not undergo any permanent chemical change, though sometimes they may be changed physically.
• Only small amounts of a catalyst are needed usually.
• The catalyst does not affect the amount of product formed, only the rate at which it is formed.

Homogenous catalysts: when the reactants and catalyst are in the same physical state
Heterogeneous catalysts: when the reactants and catalyst are in different physical states

• It is important that the catalyst has a large surface area for contact with reactants.
• Solid catalysts are used in finely divided form or as a fine wire mesh.
• Sometimes the catalyst is supported on porous material to increase its surface area and prevent it from crumbling.

• Zeolites are widely used in industry as heterogeneous catalysts
• E.g. in the cracking of petroleum fractions.
• Many of the heterogeneous catalysts used in industrial processes are transition metals or transition metal compounds.

• Catalysts can be poisoned so they no longer function properly.
• Many substances which are poisonous to humans operate by blocking an enzyme-catalysed reaction.
• In heterogeneous catalysis, the ‘poison’ molecules are adsorbed more strongly to the catalyst surface than the reactant molecules.
• The catalyst cannot catalyse a reaction of the poison and so becomes inactive with poison molecules blocking the active sites on its surface.
• Leaded petrol cannot be used in cars fitted with catalytic converter - lead is strongly adsorbed to the surface of the catalyst.
• It is not possible to replace the very costly metals such (platinum and rhodium) in catalytic converters by cheaper metals (such as copper and nickel).
• These metals are vulnerable to poisoning by the trace amounts of sulphur dioxide always present in car exhaust gases.
• Once the catalyst in a converter becomes inactive, it cannot be regenerated.

Catalyst poisoning can be a problem in industrial process?

• In the UK, nearly all the hydrogen for the Haber process is prepared by steam reforming of methane.
• Methane reacts with steam in the presence of a nickel catalyst.
• In the feedstock for the process contains sulfur compounds, these must be removed first to prevent severe catalyst poisoning.
• Sometimes it is possible to clean or regenerate the surface of a catalyst.
• In the catalytic cracking of long-chain hydrocarbons, e.g., carbon is produced and the surface of the zeolite catalyst becomes coated in a layer of soot.
• This blocks the adsorption of reactant molecules and the activity of the catalyst is reduced.
• The catalyst is constantly recycled through a separate container where hot air is blown through the zeolite powder.
• The oxygen in the air converts the carbon to carbon dioxide and cleans the catalyst surface.

That's the end of the topic!

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