Activators and Deactivators of Catalysts

Efficiency of catalysts is affected by the presence of some compounds. Even small amounts of alien admixtures, especially with noble metal catalysts, can increase or decrease the rate of hydrogenation and, in some cases, even inhibit the hydrogenation completely. Moreover some compounds can even influence the selectivity of a catalyst. 

On the other hand, some compounds slow down the uptake of hydrogen and may even stop it at a certain stage of hydrogenation. Addition of lead acetate to palladium on calcium carbonate makes the catalyst suitable for selective hydrogenation of triple to double bonds (Lindlar catalyst) [36] (Procedure 9, p. 206). 

Many nucleophiles act as inhibitors of platinum, palladium and rhodium catalysts. The strongest are mercaptans, sulfides, cyanide and iodide weaker are ammonia, azides, acetates and alkalis [26]. 

Acidity or alkalinity of the medium plays a very important role. Hydrogenation of aromatic rings over platinum catalysts requires acid medium. Best results are obtained when acetic acid is used as the solvent. Addition of 

On the other hand, addition of tertiary amines accelerated hydrogenation of some compounds over Raney nickel [6I. In hydrogenation of halogen derivatives over palladium [62] or Raney nickel [63] the presence of at least one equivalent of sodium or potassium hydroxide was found necessary. 

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