In homogeneous catalysis, the catalyst is in the same phase (usually liquid) as the reactants. This allows for easier mixing and interaction at the molecular level. Examples include the use of acid or base catalysts in solution. On the other hand, heterogeneous catalysis involves the catalyst being in a different phase (usually solid) than the reactants. This type is commonly seen in processes like hydrogenation where gases react on the surface of a solid metal catalyst.
