The Choice of Active Metal

The metal atoms of the surface exposed to the adsorbing medium should be spaced such that the transition-state complex formed has the lowest possible potential energy. It follows, therefore, that reaction activation energies will be reduced and progress under considerably milder reaction conditions than required 

At first glance, there are numerous potential candidates present within the first to third transition metal series, although those which are reduced from the oxide only with difficulty can be excluded. This effectively precludes, Ti, V, Cr, Mn, Zr, Nb, Mo, Tc, Hf, W, and Re. Furthermore, Ag and Au have low hydrogenation activity (electronic factor) and Os can also be excluded on toxicity grounds. In terms of ethyne (C2H2) hydrogenation activity, the following trend is broadly true [7,8]  

Combination of Eqs (2) and (3), shows full alkyne hydrogenation to be a serial process and the ability to terminate at the alkene stage determines the selectivity. This may be quantified from  

Selectivity = Yield (alkene)A ield (alkene + alkane) 

Studies of the hydrogenation of 2-pentyne over various metals  

---