Activity 3 - Heterogeneous Catalysis
The exact mechanism for a lot of heterogeneous reactions is not fully understood as it’s
really difficult to follow the reactions as they progress. However it is possible to have a
general idea of how the reaction works. If we take the example of the use of a nickel
catalyst for the hydrogenation of ethane, it is known that through the reagent gasses
coordinating to the metal the bonds within these gasses weaken making them easier to
break and hence the formation of the new product easier. This is shown schematically in
Figure 5.
Figure 5. Representative reaction pathway for a heterogeneous reaction.
Another very industrially important homogeneously catalysed reaction is the Haber process;
making ammonia from hydrogen and nitrogen using an iron catalyst. Again here the catalyst
speeds up the reaction through the coordination of the reactant gasses weakening the
intramolecular bonds.
Question: Find out some more about the Haber process. How much of the world’s
electricity does it use every year (as a %)? What reaction conditions are used? What are
the products used for?
Question: The contact process and Ziegler-Natta polymerisation are two other synthetically
important reactions. See if you can find out how the catalysts for these reactions were
chosen. Zielger-Natta polymerisation also uses a so-called co-catalyst, can you find out
what this is any why it is needed?
One of the major advantages of heterogeneous catalysts is that they are easily separated
from the products and reactants. This is important both in terms of purification of the
products and the recovery of often very expensive catalyst. This is of particular import in
reactions where, as with many heterogeneous catalysts, the catalysts are rare metals such
as platinum and palladium. One place you can see this in action is in the catalytic
convertors on cars where the catalyst is immobilised whilst the gasses pass through.