A catalyst speeds up a chemical reaction without being permanently consumed by it — the same catalyst can, in principle, keep facilitating the same reaction indefinitely, since it comes out the other side chemically unchanged. Surface chemistry explains how that's possible: much of catalysis, especially the industrially important heterogeneous kind, works by providing a surface where reacting molecules can bind, weaken their internal bonds, and react more easily than they would in open space, with the catalyst's own atoms staying put on that surface rather than being used up in the reaction itself.
The surface is where the actual chemistry happens
Heterogeneous catalysis, where the catalyst is in a different physical phase from the reactants — a solid catalyst speeding up a reaction between gases, for instance — works because reactant molecules can temporarily bind to specific active sites on the catalyst's surface, a process called adsorption. Binding to the surface can weaken or distort the reactant molecules' internal bonds in ways that make the desired reaction proceed with a much lower activation energy than it would need in the open gas or liquid phase, letting the reaction happen faster, or at a lower temperature, than it otherwise could. Once the reaction has occurred, the resulting product molecules detach from the surface, freeing up that same site to bind and process another set of reactant molecules — the catalyst's surface atoms participate in facilitating the reaction over and over, without being chemically transformed or consumed by any single instance of it.
More surface means more active sites, which is why catalysts are often powdered
Because the reaction happens specifically at the catalyst's surface, and only atoms actually exposed on that surface can serve as active sites, a catalyst's effectiveness scales directly with how much surface area it presents relative to its total mass — a solid block of catalytic material has comparatively little surface area exposed relative to its bulk, since most of its atoms are buried inside where reactant molecules can never reach them. This is exactly why industrial and laboratory catalysts are so often used as fine powders, porous pellets, or materials deliberately engineered with a highly porous internal structure: maximising surface area exposed to reactants maximises the number of available active sites doing the actual catalytic work, letting a comparatively small mass of catalyst material process a very large quantity of reactant over time.
What we're still unsure about
The general mechanism of heterogeneous catalysis — surface binding, weakened bonds, and lowered activation energy — is well established physical chemistry, supported by extensive experimental and spectroscopic evidence. What remains a genuinely active area of research is designing new catalysts with greater selectivity and efficiency for specific industrially or environmentally important reactions, since predicting exactly how a candidate material's surface structure will behave with a given set of reactants, at the atomic level, is still difficult to do reliably from first principles alone — much of catalyst development still combines theoretical prediction with substantial empirical testing, rather than being a fully predictable design process.
This sits inside Surface Chemistry & Catalysis, one of seven topics in Physical Chemistry, one of six domains in Chemistry, one of seventeen subjects the app can quiz you on.