Promoters and Selective Poisons

We now encounter a semantic problem of considerable size. It has been recognised for a very long time that the activity of metal catalysts can be helped by the presence of quite small amounts of substances that of themselves have no or little activity. This concept first achieved prominence in the development of iron catalysts for ammonia catalysts, and of iron and cobalt catalysts for Fischer-Tropsch synthesis, and the term promoter was applied to these substances. They were of two kinds (i) structural promoters such as alumina, which acted as grain stabilisers and prevented metal particle sintering and (ii) electronic promoters such as potassium that entered the metallic phase and actually enhanced its activity. In these cases the metal is the major component, so that the catalyst is a promoted metal rather than a supported metal. 

Electron-rich metals Cu, Ag, Au, Hg, Sn, Pb, Ge (a) Improved selectivity in the hydrogenation of multiple bonds (b) Suppression of hydrogenolysis in alkane transformations (c) Suppression of carbon deposition (d) Alteration of hydrogenolysis selectivity  

Oxides of electropositive elements Li+,Na+,K+,Cs+,Ca2+, Mg2+,La3+ (a) Suppression of carbon deposition (b) Control of chain growth in Eischer-Tropsch synthesis 286,327,328,417 

Mid-Transition Series elements, Re, MoOx, VOx, TiOx, (a) Improved yields of oxygenates in Eischer-  

---