cocatalysten

A cocatalyst is a substance that works with a primary catalyst to accelerate a chemical reaction without itself acting as the main site of bond making or breaking. The cocatalyst typically facilitates a specific step in the reaction mechanism, such as charge transfer, stabilization of reaction intermediates, or the regeneration of active sites, thereby increasing overall activity and selectivity.

In many catalytic systems, particularly in photocatalysis and electrocatalysis, cocatalysts enhance efficiency by promoting charge separation and directing redox processes. They can act as electron or hole traps, lower reaction overpotentials, and improve the selectivity toward desired products. The cocatalyst often remains separate from the primary catalytic center while delivering crucial support for the reaction.

Common forms and examples include noble metal cocatalysts such as platinum or palladium loaded on semiconductor photocatalysts to drive hydrogen evolution. Other cocatalysts include metal oxides or sulfides such as RuO2, CoP, Ni3S2, and MoS2, which favor either hydrogen evolution or oxygen evolution depending on the system. In overall water splitting, distinct cocatalysts for the hydrogen- and oxygen-evolution half-reactions are frequently applied to a single semiconductor to balance the two processes.

Preparation and evaluation: Cocatalysts are deposited on the primary catalyst by methods such as photodeposition, impregnation, electrodeposition, or chemical vapor deposition. Their performance is evaluated through activity, selectivity, stability, turnover frequency, and the overpotential required for the target reaction.

Relation to catalysis: The cocatalyst is not typically the main site of bond formation or cleavage but enables the main catalyst to operate more efficiently. The concept is widely used in solar fuel generation, CO2 reduction, nitrogen fixation, and related catalytic systems.