Catalysts can be categorized as homogeneous or heterogeneous. A homogeneous catalyst is in the same phase as the reactants (for example, gaseous chlorine radicals catalyse the breakdown of ozone in the stratosphere). A heterogeneous catalyst, such as solid vanadium(V) oxide in the reaction between gaseous sulfur dioxide and oxygen, is in a different phase to the reactants. Which type of catalyst might be more difficult to recover after a reaction, and what is a possible approach for recovering it?