Homogeneous catalysts are in the same state as the reactants. They usually work by forming an intermediate compound with the reactants, shown by the hump on the enthalpy profile. This is also known as the transition state. An example of this would be the removal of Ozone by chlorine and bromine.
Small amounts of chloromethane (\(CH_3Cl\)) and bromomethane (\(CH_3Br\)) reach the stratosphere naturally
Both of these photodissociate to give chlorine and bromine radicals from UV radiation
The radical signs are omitted from the following reactions \[\begin{aligned}
Cl+O_3\Rightarrow ClO+O_2 &&\text{Propagation}\\
ClO+O\Rightarrow Cl+O_2 &&\text{Propagation, $Cl$ effectively acting as a catalyst}\end{aligned}\] There are now two radical reactions competing to remove Ozone (See reaction 4 and 6): \[\begin{aligned}
O+O_3\Rightarrow O_2 + O_2 && \text{Termination}\\
Cl+O_3\Rightarrow ClO+O_2 && \text{Propagation}\\\end{aligned}\] The concentration of \(Cl\) atoms is much lower than the concentration of \(O\) atoms. However, in stratospheric conditions, the reaction is 1500 times faster. Furthermore, the \(Cl\) from reaction 6 is regenerated in reaction 7, giving the overall reaction \[O+O_3\Rightarrow O_2+O_2\] where the \(Cl\) atoms act as a homogeneous catalyst. A single \(Cl\) can remove approxiamtely one million Ozone molecules, so low concentrations can be devastating.
Similarly, bromine can take the place of chlorine. Although the concentrations are again much lower, bromine is 100 times more effective in removing ozone than chlorine.
\[\begin{aligned} X+O_3\Rightarrow XO+O_2\\ XO+O\Rightarrow X+O_2\\ \cline{1-2} O+O_3\Rightarrow O_2+O_2\\\end{aligned}\]
Where \(X\) represents a given radical.
Formed by reaction of oxygen with water in the stratosphere
\[\begin{aligned} HO+O_3\Rightarrow HO_2+O_2\\ HO_2+O\Rightarrow HO+O_2\\\end{aligned}\]
\[\begin{aligned} NO+O_3\Rightarrow NO_2+O_2\\ NO_2 + O \Rightarrow NO+O_2\\\end{aligned}\]
\(NO\) and \(NO_2\) are both unusual radicles
Relatively stable molecules
Can be prepared and collected like ordinary molecular substances