A catalyst is a substance that changes the rate of a chemical reaction without itself being consumed in the overall change. It works by providing an alternative reaction path of lower activation energy. A catalyst does not alter the position of equilibrium or the value of $\Delta G$; it only helps the system reach equilibrium faster by speeding both the forward and the backward reaction equally. Substances that increase the activity of a catalyst are promoters (e.g. molybdenum in the Haber process), while those that destroy its activity are poisons (e.g. arsenic on the platinum catalyst in contact process).
Homogeneous catalysis. Here the catalyst is in the same phase as the reactants. Examples: oxidation of $\text{SO}_2$ to $\text{SO}_3$ by $\text{NO}$ in the lead-chamber process (all gases); hydrolysis of an ester catalysed by mineral acid (all in solution); inversion of cane sugar catalysed by $\text{H}^+$. In heterogeneous catalysis the catalyst is in a different phase from the reactants, usually a solid catalysing gaseous or liquid reactants. Examples: the Haber synthesis of $\text{NH}_3$ over finely divided iron; the contact process oxidation of $\text{SO}_2$ over $\text{V}_2\text{O}_5$; hydrogenation of vegetable oils over nickel; Ostwald oxidation of $\text{NH}_3$ over platinum gauze.
Adsorption theory of heterogeneous catalysis. The action of a solid catalyst is explained in five steps: (i) diffusion of reactant molecules to the catalyst surface; (ii) adsorption of reactant molecules on the surface; (iii) formation of an intermediate on the surface, weakening the bonds within the reactants; (iv) reaction of the adsorbed species to give the product; (v) desorption of the product, freeing the surface for fresh reactant. Adsorption increases the local concentration of reactants on the surface and lowers the activation energy, so the reaction speeds up. Old intermediate-compound ideas are special cases of this surface picture.
Features of solid catalysts. Two properties matter most. Activity is the ability to speed up a reaction; it depends on the strength of chemisorption — the gases must be adsorbed strongly enough to react but not so strongly that products cannot leave. For hydrogenation the catalytic activity falls roughly in the order $\text{Pt} > \text{Pd} > \text{Ni} > \text{Fe}$. Selectivity is the ability to steer a reaction towards a particular product. The same reactants can give different products on different catalysts; for example, carbon monoxide and hydrogen give methane over nickel, methanol over a $\text{Cu}/\text{ZnO}/\text{Cr}_2\text{O}_3$ catalyst, and a mixture of hydrocarbons over cobalt.
Shape-selective catalysis by zeolites. Zeolites are microporous aluminosilicates with a honeycomb of cavities and channels of molecular dimensions. They catalyse reactions selectively according to the size and shape of the reactant and product molecules, which must fit into the pores; this is called shape-selective catalysis. The well-known zeolite ZSM-5 converts alcohols directly into petrol (gasoline) by dehydrating them to a mixture of hydrocarbons. Zeolites are also used as cracking and isomerisation catalysts in the petroleum industry.
Enzyme catalysis. Enzymes are complex protein molecules produced by living cells that catalyse biochemical reactions; they are nature's catalysts. The reactant is the substrate. A small region of the enzyme, the active site, binds the substrate (lock-and-key fit), forms an enzyme-substrate complex, converts it to product and is then released. Characteristic features: enzymes are highly efficient (one molecule can transform millions of substrate molecules per minute), highly specific (each enzyme acts on a particular substrate, e.g. urease only on urea), work best at an optimum temperature (around $298$–$310\ \text{K}$) and an optimum pH ($5$–$7$), and their activity is increased by coenzymes/activators and decreased by inhibitors and poisons. Examples: maltase converts maltose to glucose; zymase converts glucose to ethanol; pepsin hydrolyses proteins.