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Catalysis is broadly classified into homogeneous and heterogeneous systems based on the phase of the catalyst relative to the reactants. In homogeneous catalysis, the catalyst and reactants exist in the same phase, usually liquid. This allows for uniform mixing, intimate contact, and precise control over reaction conditions, resulting in high selectivity and efficiency. Common examples include transition metal complexes in organic synthesis and acid/base catalysts in esterification or hydrolysis reactions. Homogeneous catalysts are particularly valuable in fine chemical and pharmaceutical industries, where product specificity and stereoselectivity are crucial. However, challenges such as catalyst recovery and separation often limit their large-scale application.

In contrast, heterogeneous catalysis involves catalysts in a different phase, typically solid catalysts interacting with gaseous or liquid reactants. These catalysts, including metals, metal oxides, zeolites, and supported catalysts, offer advantages like ease of separation, reusability, and durability, making them suitable for industrial-scale processes. The catalytic mechanism generally involves adsorption of reactants on the active sites, surface reactions, and desorption of products. Heterogeneous catalysts are widely used in petroleum refining, hydrogenation, oxidation, and environmental remediation. Advances in nanocatalysis, surface engineering, and computational modeling have enhanced the activity, selectivity, and stability of both homogeneous and heterogeneous catalysts, enabling more efficient and sustainable chemical processes.