cocatalysts

Cocatalysts are substances that participate in a catalytic process alongside a primary catalyst to enhance the efficiency, selectivity, or rate of a reaction. They are distinct from the main catalyst in that they do not constitute the principal site of bond making and breaking, but they interact with the catalytic cycle to facilitate key steps. Often, cocatalysts are regenerated at the end of each turnover and remain available for subsequent cycles.

Cocatalysts function by providing alternative reaction pathways, transferring electrons or protons, stabilizing reactive intermediates, or improving charge separation on a catalyst surface. In photocatalysis and electrocatalysis, cocatalysts frequently serve as dedicated sites for challenging transformations such as hydrogen evolution or oxygen evolution, thereby lowering overpotentials and increasing efficiency. Common examples include noble metal cocatalysts like platinum on semiconductor photocatalysts, as well as earth‑abundant candidates such as nickel, cobalt, or copper compounds.

In polymerization catalysis, cocatalysts are used to activate metal centers and govern polymer growth. A well-known class involves metallocene catalysts paired with activators such as methylaluminoxane (MAO) or other non-coordinating anions, which generate the actual active catalytic species. In homogeneous catalysis more broadly, boron-based and other Lewis acid or salt cocatalysts can serve to modulate the electronic environment of the active site.

Practical considerations for cocatalysts include cost, stability, compatibility with the main catalyst, and potential impacts on purity and recyclability. While cocatalysts can markedly improve activity and selectivity, they can also introduce new deactivation pathways or contamination. Ongoing research seeks robust, inexpensive cocatalysts that extend performance across energy conversion, organic synthesis, and materials processing.