Gold in bulk shines brilliantly and can maintain its brightness forever. This property has led to a common sense that gold is chemically inert. In fact, it was confirmed by surface science and computer calculation study that hydrogen and oxygen molecules do not dissociatively adsorb over the smooth surfaces of gold. In the history of heterogeneous catalysis longer than a century, gold was accordingly regarded as being poorly active as a catalyst. Dr. Masatake Haruta has changed this understanding by his discovery in 1982 that tiny gold nanoparticles(NPs) supported on base transiton metal oxides behave very differently from bulk gold and exhibit surprisingly high catalytic activity for CO oxidation with molecular oxygen(O2) at a temperature as low as -70 degree. Since this finding was commercially applied to odor eaters in rest room in Japan in 1992, the catalysis by gold has attracted growing interests both in academia and industry. The followings are Dr. Haruta's research achievements.
1. Development of several methods for depositing gold as nanoparticles (NPs)
Because gold is weak in bonding with other elements and has lower melting points(1063 degree with respect to 1769 degree for Pt), none had yet succeeded in depositing gold as NPs on base metal oxides. Dr. Haruta developed several techniques to deposit gold NPs with a size from 1 to 10 nm on a variety of materials including carbons and polymers. They are now widely used in the world as standard methods for preparing active gold catalysts.
As wet methods easy to handle, developed are coprecipitation from mixed aqueous solutions of HAuCl4 and the nitrates of the metal components of the supports, deposition-precipitation of gold hydroxide or sulfide on base metal oxides prepared beforehand or commercially available, deposition-reduction from an aqueous solution of HAuCl4 or ethylenediamine Au(III) chloride on carbons and polymers by NaBH4.
His research group has also developed dry methods, solid grinding of the support powder with dimethyl Au(III) acetylacetonate, a solid having a very low vapor pressure at room temperature, and vapor grafting of the gold complex. These dry techniques are useful to deposit gold as clusters smaller than 2nm in diameter on almost every kind of solid materials. .
２．Mechanism for the genesis of catalysis by gold NPs
The catalytic activity of supported gold NPs for CO oxidation is markedly dependent on the kind of the support materials, the contact structure and the size of gold NPs. Dr. Haruta studied the relationships between these factors and the catalytic activity and through kinetic investigation, O2 isotope experiments, in situ FT-IR of surface adsorbates, Electron Spin Resonance analyses for oxygen species he has proposed a "perimeter reaction site" hypothesis that CO adsorbed at the surfaces of gold NPs reacts with oxygen molecule which is activated as O2- at the perimeter interfaces to form CO2.When gold NPs become smaller than 5 nm, the fractions of edges and corners appreciably increase and the perimeter distance is enlarged in proportion to the inverse second power of the diameter of gold NPs. The reaction of the two adsorbates at the perimeter interfaces is considered to be rate determining. The above mechanism comprehensively explains the unique nature of the catalysis by gold and presents useful design principles for metal catalysts in general. .
3. Exploitation of novel catalysis by gold clusters
Gold clusters with diameters smaller than 2 nm(less than 300 atoms) are expected to exhibit novel catalysis owing to the very large fractions (above 50%) of edges and corners and the change in electronic structure. Dr. Haruta reported for the first time in 1998 that over Mg(OH)2 gold clusters composed of 13 atoms (0.8 nm in diameter) having icosahedron structure are extraordinarily active for CO oxidation at -70℃. Recently, more significant results which lead to green chemistry have been found in selective oxidation and hydrogenation. Propylene epoxidation with molecular oxygen alone can be catalyzed by gold clusters deposited on mesoporous titano-silicalite and by water. One pot synthesis of secondary amine from benzyl alcohol and aniline can be realized without using O2 and H2 but in N2 atmosphere, by gold clusters supported on porous coordination polymers.
It has long been thought that gold is particularly inferior to Pd and Pt catalysts in hydrogenation because gold is much less active in the dissociation of H2 molecule. However, gold clusters have been proved, by a surface science study using well-defined model catalysts, to dissociate H2 at temperatures below 200℃and at the perimeter interfaces around gold clusters. This finding indicates that gold can be tuned as a hydrogenation catalyst through the control of the size and the selection of the support materials.
As described in the above, Dr. Masatake Haruta has established a new field of chemistry research, "catalysis by gold" by exploiting many interesting and useful reactions catalyzed by gold NPs supported on a variety of materials. These research achievements are well recognized in and outside Japan and deserve The Chemical Society of Japan Award.