Cocatalyst

A cocatalyst is a substance that works with a catalyst to increase reaction rate, improve selectivity, or enhance efficiency. While the cocatalyst is not the primary site of substrate transformation, it participates in the catalytic cycle and is regenerated over turnover. Cocatalysts are especially important in photocatalysis and electrocatalysis, where they can facilitate charge separation, provide active sites for redox steps, or stabilize reactive intermediates.

Several mechanisms are known. They can act as electron or hole sinks to suppress recombination in semiconductor systems; supply surface sites for hydrogen or oxygen evolution; or mediate key bond-forming steps by stabilizing intermediates or lowering activation barriers. Types include redox mediators, surface cocatalysts such as metal or metal-oxide nanoparticles, and Lewis acid/base additives that tune interfacial chemistry.

In photocatalysis, common cocatalysts include noble metals like platinum for hydrogen evolution and earth-abundant alternatives such as molybdenum sulfide or nickel-based systems. In electrochemical applications such as water splitting and CO2 reduction, cocatalysts improve charge transfer at the electrode–electrolyte interface and steer product selectivity.

Cocatalysts differ from promoters; promoters typically enhance performance without directly entering the catalytic cycle. Practical design emphasizes intimate contact with the main catalyst, suitable electronic interaction, chemical stability under operating conditions, and optimized loading.