Many catalysts are not so effective on their own, but work in conjunction with promoters. The role of the promoter varies.
Chemical promoters improve the effectiveness of the catalyst often by altering the electron distribution at the surface.
Structural promoters improve the mechanical properties and help prevent sintering.
The activity of a catalyst is greatly influenced by the surface structure. Promoters simply enhance this catalytic activity. However, after nearly 100 years of study in the area, the precise way by which they do this is still a subject of debate.
In homogeneous catalysis promoters must intervene in the catalytic cycle. Again their exact role is often unclear.