Gold can be deposited as hemispherical nanoparticles on base metal oxides by coprecipitation, deposition-precipitation, adsorption of organo-gold complexes and so forth. Such Au nanoparticles are usually epitaxially contacted with specific crystal planes of the support metal oxides, for example, Au(111)/anatase TiO₂(112). Gold catalysts are superior to Pd and Pt catalysts in many reactions, including low-temperature CO oxidation, water gas shift reaction, and propylene epoxidation. The catalytic behavior dramatically changes at a critical size: 2 nm in diameter and 2 atoms in thickness, where metallic nature is lost. Gold clusters smaller than this can also exhibit interesting reactivities depending on the size and structure.