Homogeneous Catalysts

The CFC story

The depletion of the ozone layer has been linked to the release of chlorofluorocarbons (CFCs) into the atmosphere. These molecules were widely used as propellants for aerosols, as refrigerants, as cleaning solvents and as blowing agents for making expanded plastics.

The CFCs are very good for these uses, partly because the molecules are so inert. It is this very inertness, however, that causes the ozone problem. The molecules can persist in the atmosphere for a very long time, up to a hundred years, without being broken down as most molecules are. Eventually they will move to the upper atmosphere. Here they are no longer inert. The high energy ultra violet radiation from the sun breaks them down, releasing highly reactive chlorine atoms.

CF3Cl + hn

CF3 + Cl

It is these chlorine atoms that catalyse the breakdown of ozone causing the 'hole' on the ozone layer over Antarctica.

Catalysing ozone removal

Step 1

In the stratosphere, CFCs are broken down by sunlight releasing very reactive chlorine atoms.

 

Step 2

The chlorine atoms react with ozone to produce chlorine oxide and dioxygen molecules.

 

Cl + O3

ClO + O2

 

 

Step 3

The chlorine oxide molecule is also very reactive and can react with oxygen atoms to regenerate the chlorine atom.

 

ClO + O

Cl + O2

 

Step 4

The chlorine atoms are involved in a catalytic cycle. It is estimated that one chlorine atom can remove about a million ozone molecules.

 

   
 
     

Free radicals

Chlorine and bromine atoms are also known as free radicals. Other free radicals in the stratosphere can also remove ozone. If we show a free radical by the general symbol X then the mechanism for the catalytic destruction of ozone is as follows

X + O3

XO + O2

 

 

 

XO + O

X + O2