Gold Catalysis: How Nanoparticles Revived a "Dead" Metal

For most of chemical history gold was assumed to be catalytically dead, a consequence of its inertness as a noble metal. That changed in 1987 when Masatake Haruta discovered, partly by serendipity, that gold deposited as nanometer-scale particles on a metal-oxide support (his original was Au/Fe2O3) catalyzes the oxidation of carbon monoxide at strikingly low temperatures — active even well below room temperature, far colder than any prior CO-oxidation catalyst. The finding launched gold catalysis as a research field and has since reached practical use in CO-removing safety masks. The central lesson is that catalytic activity is a size effect. Gold becomes active only when particles shrink below roughly 5 nm, with activity rising sharply toward 2 nm and smaller; the active sites are the low-coordination corner and edge atoms abundant on tiny particles but absent on bulk surfaces. The oxide support matters too — common choices are TiO2, Fe2O3, and Al2O3 — and for the water-gas shift reaction the rate scales steeply with shrinking particle size. Key reactions catalyzed by supported gold include low-temperature CO oxidation, the water-gas shift (a route to hydrogen), selective hydrogenation (gold can reduce a carbonyl while sparing an adjacent C=C double bond), and electrochemical CO2 reduction, where gold is one of the few catalysts that selectively makes carbon monoxide — a feedstock for fuel synthesis — rather than just splitting water. A separate field of homogeneous gold catalysis exploits gold's affinity for activating alkynes and alkenes in solution. See Gold Nanoparticles and Localized Surface Plasmon Resonance and Platinum: The Rarer-Than-Gold Metal That Cleans Your Car's Exhaust.