Hydrogenation by gold catalysts: an unexpected discovery and a current assessment

Gold in the massive state shows little tendency either to chemisorb simple molecules or to act catalytically. The discovery made in 1973 that low loadings of Au on Al2O3 and particularly on SiO2 were active in alkene hydrogenation when prepared by thermal decomposition of adsorbed AuCl4 (-) paved the way for later extensive studies of catalysis by this metal. The activity was attributed to that being Au particles < 4 nm in size, and it soon appeared that similar catalysts performed the selective hydrogenation of 2-butyne and 1,3-butadiene. Later work strongly suggests that Au particles below 2.5 nm in size, and having non-metallic character, are especially adept at H-2 chemisorption and H-2-D-2 equilibration, and that alkene hydrogenation then occurs in an H-rich environment. Larger Au particles have more limited activity, perhaps associated with atoms of low coordination number and an H-poor environment.