Catalysis: homogeneous and heterogeneous

A catalyst increases a reaction's rate without being permanently consumed and without shifting the reaction's equilibrium position — it changes how fast equilibrium is reached, not where the equilibrium lies, since both the forward and reverse reaction rates are accelerated by exactly the amount needed to keep the equilibrium constant unchanged. The mechanism behind this rate increase, in every case, is the same: a catalyst provides an alternative reaction pathway with a lower activation energy than the uncatalysed route. Recalling the Arrhenius equation, a lower activation energy means a substantially larger fraction of molecular collisions have enough energy to react, which is exactly why even a small reduction in activation energy can produce a dramatic increase in rate.

Catalysts are classified by whether they share a phase with the reactants. In homogeneous catalysis, the catalyst is dissolved or dispersed in the same phase as the reactants — typically the same liquid solution — and interacts with reactant molecules through ordinary solution-phase collisions and complex formation. Enzymes, the biological catalysts that accelerate nearly every reaction inside living cells, are a prominent example: an enzyme dissolved alongside its substrate in the cell's aqueous interior binds the substrate at an active site shaped to stabilise the reaction's transition state, dramatically lowering the effective activation energy for that specific reaction, often by many orders of magnitude in rate.

In heterogeneous catalysis, the catalyst exists in a different phase from the reactants, most commonly a solid catalyst acting on gas- or liquid-phase reactants. The catalytic action in this case typically proceeds through adsorption: reactant molecules first adsorb onto the solid catalyst's surface (governed by the same Langmuir-type equilibrium that describes gas-solid adsorption generally), where the surface's local electronic and geometric structure weakens particular bonds in the adsorbed molecule or brings reacting species into favourable proximity and orientation, lowering the activation energy for the surface-bound reaction compared with the same reaction occurring between free-flying gas molecules. After reacting on the surface, the products desorb, freeing the site for further catalytic cycles. Industrial heterogeneous catalysis — solid catalysts in car exhaust converters, in ammonia synthesis, and in petroleum refining — exploits exactly this surface-mediated pathway, and because reaction occurs specifically at the catalyst's surface, maximising surface area (fine powders, porous supports) is a standard and direct way to increase a heterogeneous catalyst's overall effectiveness.