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Cocatalyst refers to a secondary catalyst that is used in conjunction with a primary catalyst to enhance or alter the outcome of a chemical reaction. Unlike a single catalyst that alone drives a transformation, a cocatalyst typically works in tandem, either by activating the primary catalyst, stabilizing reaction intermediates, or providing an additional reaction pathway. The term is widely used in heterogeneous catalysis, photocatalysis, and homogeneous catalysis, and is a key concept in designing efficient catalytic processes.

Historically cocatalysts emerged in the mid‑20th century when chemists discovered that adding trace amounts of metals or acids could drastically improve catalytic performance. One early example involved palladium‑based cross‑coupling reactions, where a copper cocatalyst was found to accelerate the coupling of aryl halides with alkenes. In photocatalytic systems, cocatalysts such as platinum or non‑metal dopants are often deposited on semiconductor surfaces to capture photoexcited electrons and facilitate charge separation.

Cocatalysts are chosen based on their ability to influence selectivity, turnover frequency, and stability. Common classes include noble metals (platinum, gold, palladium), transition metal oxides, and simple proton donors or bases. In industrial settings, cocatalysts enable processes such as hydrogenation, dehydrogenation, and selective oxidation to operate at lower temperatures or with higher yields. They also play a vital role in renewable energy technologies, where cocatalysts improve water‑splitting efficiency and the conversion of biomass into fuels.

In summary, cocatalysts are auxiliary catalysts that work synergistically with primary catalysts to achieve desired reaction outcomes. Their judicious selection and optimization remain central to advancing catalytic science and technology.