Which of the following is an example of a heterogeneous catalyst?

• A. Acid catalyst in solution
• B. Enzyme in solution
• C. A dissolved metal complex in solution
• D. Pt solid in hydrogenation

Answer: (D)

The main idea here is that heterogeneous catalysis involves a catalyst in a different phase than the reactants. A solid platinum catalyst provides a surface for the reaction, while the reacting species (the hydrogen and the substrate) are in a different phase (gas or liquid). Reactions occur at the solid–gas or solid–liquid interface, and the catalyst can be separated from the products and reused.

The other scenarios are all homogeneous catalysts because they operate in the same phase as the reactants, typically dissolved in solution. An acid catalyst in solution participates in the reaction entirely within the liquid phase. An enzyme in solution also acts while dissolved in the same medium. A dissolved metal complex in solution is another soluble, single-phase catalyst.

So a Pt solid used in hydrogenation is a classic example of a heterogeneous catalyst.