Catalysis Compared: Homogeneous, Heterogeneous and Enzymatic

What all catalysis has in common

A catalyst provides an alternative reaction pathway with a lower activation energy. It is not consumed overall, though it is chemically involved during the cycle.

It also cannot make a thermodynamically unfavourable reaction occur. It only accelerates one that is already favourable, which is a second common misconception.

The three types compared

Heterogeneous catalysis in more detail

The sequence is adsorption of reactants onto the surface, reaction there, then desorption of products.

Adsorption weakens bonds within the reactant and holds molecules in favourable orientations, which is why the surface route has a lower barrier. But adsorption must be neither too weak nor too strong — too weak and the reactant does not stick, too strong and the product does not leave.

That intermediate optimum produces a volcano-shaped plot of activity against binding strength, with the best catalysts in the middle. Explaining why the best catalyst is one of intermediate binding strength is a standard higher-order question.

Practical features

• Surface area determines the number of active sites, so catalysts are finely divided or dispersed on a support.
• Promoters enhance activity without being catalysts themselves.
• Poisons bind irreversibly to active sites and destroy activity, so feedstocks must be purified.
• Shape selectivity arises where pores admit only molecules of a certain size, as in zeolites.

Homogeneous catalysis

Because the catalyst is molecular and in solution, its ligands can be varied systematically to tune selectivity — including enantioselectivity, which heterogeneous catalysts achieve only with difficulty.

The mechanism can also be studied directly by spectroscopy, so homogeneous catalytic cycles are generally much better understood than surface mechanisms. The offsetting problem is separation, and much industrial effort goes into anchoring homogeneous catalysts to supports to combine the advantages of both.

Enzymes

Enzymes achieve rate enhancements far beyond synthetic catalysts, at ambient temperature and pH, with selectivity that distinguishes between enantiomers.

The active site binds the substrate in a specific orientation, and the enzyme is generally understood to bind the transition state more tightly than the substrate — which is precisely what lowering the activation barrier means. That framing explains enzyme catalysis better than the older lock-and-key picture, and it is the one worth giving.

Frequently asked questions

Why can a catalyst not shift equilibrium?

Because it lowers the forward and reverse barriers equally, so both rates rise by the same factor and their ratio is unchanged.

Why is intermediate binding strength optimal in surface catalysis?

Because weak binding means reactants do not adsorb, while strong binding means products do not desorb. Activity peaks in between.

Why are homogeneous catalysts hard to use industrially?

Because separating them from the product is difficult and expensive, whereas a solid catalyst is simply filtered off.

What makes enzymes so selective?

The active site's precise three-dimensional structure, which binds only substrates of matching shape and charge distribution and stabilises the transition state specifically.