General principles concerning hydrogenation catalysts

Catalysts are essential to most hydrogenation reactions. Very few hydrogenations occur in the absence of some sort of catalyst. Cases in 

Two forms of adsorption are now recognized physical adsorption and chemisorption. It is important to distinguish between them so that the mechanism of any pf icular heterogeneous catalysis can be identified.  

The chemisorption of hydrogen on various metals is commonly studied in attempts to understand catalytic activity as well as the complex phenomenon that occurs in the crystal lattices of metals.  

Catalyst supports or carriers greatly increase the effective surface area of catalysts, and when the cost is significant this may be of great importance. However, the support often performs a more vital role. For instance, in the isomerization of ri-pentane to isopentane, an alumina support containing a small proportion of silica was found to be superior to a support consisting of pure alumina alone. 

Mixed-metal Catalysts. Because an apparently small and inappreciable amount of substance might impair or poison the activity of a catalyst, strenuous efforts must be made to prepare the catalyst as nearly chemically pure as possible. It will usually be found that, in this state, activity is either nil or veiy low. The solution to this dilemma is to recognize more clearly the class of substances to which poisons and promoters belong. Catalyst poisons are usually substances that can react chemically with the catalyst or else, because of their volatility, can condense onto and blot out the active areas of the catalyst. The common poisons are halogens, sulfur, arsenic, and sometimes metals of low melting point such as mercuty, lead, and tin. 

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