Homogeneous and heterogeneous catalysts differ primarily in their phases relative to the reactants.

Homogeneous catalysts exist in the same phase as the reactants, which means that they are either all in the gas phase or all in the liquid phase. This phase similarity facilitates effective interaction between the catalyst and the reactants, enhancing the speed of the reaction. A common example of a homogeneous catalyst is found in organic chemistry, where metal catalysts such as chromium trioxide are employed for the oxidation of alcohols.

In contrast, heterogeneous catalysts operate in a different phase from the reactants. For instance, a solid catalyst may be utilized in a gas-phase reaction, or a liquid catalyst may be used in a solid-phase reaction. Heterogeneous catalysts function by adsorbing reactants onto their surface, where the reaction occurs. Although this process can be slower compared to that of homogeneous catalysts, heterogeneous catalysts are typically more stable and can be reused multiple times. They are frequently employed in industrial applications, such as in the Haber process for ammonia production, which utilizes an iron catalyst.

In summary, the selection of a catalyst is contingent upon the specific reaction and the desired outcome. Homogeneous catalysts tend to offer greater efficiency but can pose challenges in terms of separation from the reaction mixture. Conversely, heterogeneous catalysts are generally more stable but may require additional time and effort for preparation and implementation.