Catalysis by gold

Despite occasional references in the older literature to the ability of gold to catalyze certain reactions, the metal has until recently had the reputation of being one of the least catalytically useful. The recent discovery that some supported gold catalysts can affect the oxidation of carbon monoxide at or below ambient temperature has, however, focused attention on the metal's ability in this respect. For oxidation of carbon monoxide at low temperature, catalysts comprising small (<5 nm) gold particles supported preferably on an oxide of the first transition series (e.g., TiOp(2), alpha-Fe2O3) an needed. Deposition-precipitation and coprecipitation are better methods than impregnation for this purpose and provide the desired intimacy of contact between metal and support. High activity may well originate at sites at the gold-support interface, with the support making a vital contribution. Stable activity can result by optimizing aging in solution during the preparation, and low calcination temperatures are generally desired. Gold catalysts also have potential for both selective and nonselective oxidation of hydrocarbons, for methanol synthesis by hydrogenation of carbon monoxide or dioxide, for the water-gas shift, and for the reduction of nitric oxide by hydrogen, propene, or carbon monoxide. The hydrogenation of unsaturated hydrocarbons also occurs on highly dispersed gold catalysts. Supported gold chloride is the most active catalyst for the hydrochlorination of ethyne, the activity being correlated with the mesal's high standard electrode potential. The remarkable catalytic behavior shown by gold depends on forming it into very small particles. This is because the massive metal and large particles cannot chemisorb typical reactant molecules to any useful extent; this only occurs when an adequate number of law-coordination surface atoms are present, ideally on particles so small that they lack full metallic character. The long neglect of gold as a catalyst is chiefly due to the failure to appreciate the necessity of creating particles that are sufficiently small and, for oxidations, of selecting a helpful support. Other relevant factors may be the likely high mobility of surface atoms on small particles and the electronegative character of gold, both stemming from the relativistic contraction of the s-electron orbitals. Attention is drawn to the uncertainties concerning reaction mechanisms and to areas for possible future development, such as use of promoters and bimetallic compositions and the extension of concepts already revealed to other reactions and metals (e.g., the platinum group metals). The practical application of gold catalysts in oxidations and other processes can be confidently expected.