Heterogeneous Catalysts

Heterogeneous catalysts are those that are in a different phase from one or more of the reactants. Commonly solid metals and metal oxides are used to catalyze the reactions of gaseous or liquid reactants. Solid catalysts often serve as a surface on which reactions can occur, where one or more reactants will adsorb (bind to the surface) to the solid.

A common example of heterogeneous catalysis is the hydrogenation reaction of simple alkenes. The conversion of ethene (C₂H₄) to ethane (C₂H₆) can be performed with hydrogen gas in the presence of a metal catalyst such as palladium (Figure 1.1 “Conversion of Ethene to Ethane with Hydrogen and a Metal Catalyst”).

C₂H₄(g) + H₂(g) → C₂H₆(g)

Figure 1.1 Conversion of Ethene to Ethane with Hydrogen and a Metal Catalyst

Heterogeneous catalysis mechanisms of reaction for ethene with hydrogen on a catalytic metal surface

Ethene and hydrogen adsorb onto the metal surface, where the H₂ breaks into two individual hydrogen atoms bonded to the metal surface. A reaction occurs between adjacent ethene and hydrogen atoms on the metal surface, first to generate a C₂H₅ intermediate, then to generate ethane, C₂H₆, which desorbs from the surface.