Surface Area Dependency: Since the reaction only occurs at the surface, increasing the available surface area directly increases the rate of reaction. This is why industrial catalysts are often used as fine powders or thin coatings rather than large blocks.
Support Mediums: To maximize surface area while minimizing the cost of expensive metals, catalysts are often spread over an inert, porous support medium. A common example is a ceramic honeycomb structure, which provides a massive surface area in a compact volume.
Cost-Effectiveness: Using a support medium allows a very small amount of precious metal (like Rhodium) to be highly effective, as almost every atom of the metal is exposed on the surface and available for catalysis.
| Feature | Heterogeneous Catalysis | Homogeneous Catalysis |
|---|---|---|
| Phase | Different from reactants | Same as reactants |
| Reaction Site | Surface active sites only | Throughout the entire mixture |
| Separation | Easy (filtration/decanting) | Difficult (requires distillation) |
| Industrial Use | Preferred for continuous flow | Often limited to batch processes |
Adsorption vs. Absorption: Always use the term adsorption (with a 'd') when discussing surface catalysis. Absorption implies the substance enters the bulk of the material, whereas catalysis is strictly a surface phenomenon.
Mechanism Steps: When asked to describe the mechanism, always include all three steps: adsorption, reaction (bond weakening), and desorption. Omitting the desorption step is a common way to lose marks.
Oxidation State Changes: If the catalyst is a transition metal oxide, look for how the metal's oxidation state changes during the intermediate steps. Even if it returns to its original state at the end, the temporary change is often the key to the catalytic pathway.
