Although gold is the subject of one of the most ancient themes of investigation in science, its renaissance now leads to an exponentially increasing number of publications, especially in the context of emerging nanoscience and nanotechnology with nanoparticles and self-assembled monolayers (SAMs). We will limit the present review to gold nanoparticles (AuNPs), also called gold colloids. AuNPs are the most stable metal nanoparticles, and they present fascinating aspects such as their assembly of multiple types involving materials science, the behavior of the individual particles, size-related electronic, magnetic and optical properties (quantum size effect), and their applications to catalysis and biology. Their promises are in these fields as well as in the bottom-up approach of nanotechnology, and they will be key materials and building block in the 21st century. Whereas the extraction of gold started in the 5th millennium B.C. near Varna (Bulgaria) and reached 10 tons per year in Egypt around 1200-1300 B.C. when the marvelous statue of Touthankamon was constructed, it is probable that “soluble” gold appeared around the 5th or 4th century B.C. in Egypt and China. In antiquity, materials were used in an ecological sense for both aesthetic and curative purposes. Colloidal gold was used to make ruby glass 293 Chem. Rev. 2004, 104, 293−346

Gold nanoparticles: assembly, supramolecular chemistry, quantum-size-related properties, and applications toward biology, catalysis, and nanotechnology.

Crystalline metal-organic frameworks (MOFs) are formed by reticular synthesis, which creates strong bonds between inorganic and organic units. Careful selection of MOF constituents can yield crystals of ultrahigh porosity and high thermal and chemical stability. These characteristics allow the interior of MOFs to be chemically altered for use in gas separation, gas storage, and catalysis, among other applications. The precision commonly exercised in their chemical modification and the ability to expand their metrics without changing the underlying topology have not been achieved with other solids. MOFs whose chemical composition and shape of building units can be multiply varied within a particular structure already exist and may lead to materials that offer a synergistic combination of properties.

The Chemistry and Applications of Metal-Organic Frameworks

This critical review shows the basis of photocatalytic water splitting and experimental points, and surveys heterogeneous photocatalyst materials for water splitting into H2 and O2, and H2 or O2 evolution from an aqueous solution containing a sacrificial reagent Many oxides consisting of metal cations with d0 and d10 configurations, metal (oxy)sulfide and metal (oxy)nitride photocatalysts have been reported, especially during the latest decade The fruitful photocatalyst library gives important information on factors affecting photocatalytic performances and design of new materials Photocatalytic water splitting and H2 evolution using abundant compounds as electron donors are expected to contribute to construction of a clean and simple system for solar hydrogen production, and a solution of global energy and environmental issues in the future (361 references)

Heterogeneous photocatalyst materials for water splitting

The surface science of titanium dioxide

The interest in nanoscale materials stems from the fact that new properties are acquired at this length scale and, equally important, that these properties * To whom correspondence should be addressed. Phone, 404-8940292; fax, 404-894-0294; e-mail, mostafa.el-sayed@ chemistry.gatech.edu. † Case Western Reserve UniversitysMillis 2258. ‡ Phone, 216-368-5918; fax, 216-368-3006; e-mail, burda@case.edu. § Georgia Institute of Technology. 1025 Chem. Rev. 2005, 105, 1025−1102

Chemistry and properties of nanocrystals of different shapes.

Gold nanoparticles were deposited on TiO2 nano-rods by a deposition precipitation method. This Au/TiO2 nano-rods catalyst is active for CO oxidation even at temperatures less than 273K and is as active as the catalyst of Au supported on conventional TiO2 particles. Transmission electron microscopy (TEM) showed that the TiO2 nano-rods were composed of a single anatase crystal and had a pillar shape with an axis along the direction surrounded by flat four [101] planes. High angle annular dark field scanning TEM (HAADF-STEM) also revealed that a Au platelet with a thickness of 0.5-1 nm was formed at the interface between TiO2 nano-rods. The Au platelet was not observed when conventional round particles of TiO2 were used as a support.

TEM and HAADF-STEM study of a Au catalyst supported on a TiO2 nano-rod.

We prepared a series of Nb2O5 polymorphs by the sol-gel method and the sol-gel hydrothermal method. Gold nanoparticles supported on Nb2O5 were prepared by solid grinding method. Au on pseudohexagonal (TT-phase) Nb2O5 containing an amorphous phase exhibited higher catalytic activity for CO oxidation than did other Au/Nb2O5 polymorphs. For the electrochemical properties for cathode materials of Li-ion batteries, Nb2O5 (TT) exhibited high capacity of 208 mA h g-1, and the deposition of Au onto Nb2O5 (TT) improved the capacity to 232 mA h g-1. Capacity and CO oxidation activity were related; the higher the catalytic activity Au/Nb2O5 shows, the higher is the discharging capacity. From the characterization results, Nb2O5 (TT) contains oxygen vacancies (Nb4+ sites), and the oxygen vacancies of Nb2O5 (TT) would play an important role for CO oxidation and the performance for Li-ion batteries.

Gold Nanoparticles Supported on Nb2O5 for Low-Temperature CO Oxidation and as Cathode Materials for Li-ion Batteries

Catalysis by Gold Nanoparticles

Publisher Summary Propylene oxide (PO) can be formed by a single step either in the liquid phase or in the gas phase through the reaction of propylene with hydrogen–oxygen mixtures. In a semibatch reactor using methanol or butanol as a solvent, Pd/TS-1 catalysts promoted by Pt selectively produce PO with a space time yield (STY) above 120 gPO kgcat–1 h–1, which is comparable to that of industrial ethylene oxide production. In the gas phase, gold nanoparticles deposited on anatase TiO2 exhibit selectivities to PO higher than 90%. The high STY is obtained when the support for gold nanoparticles is changed from TiO2 to microporous crystalline TS-1 or mesoporous Ti-SiO2. In both the liquid- and gas-phase epoxidation, H2O2 is formed by the catalysis of noble metals and transforms at isolated tetrahedrally coordinated Ti cation sites into Ti-OOH species, which produces PO by the reaction with propylene.

Epoxidation of Propylene with Oxygen-Hydrogen Mixtures

Ag-based catalysts, especially Ag/Al2O3, show high NH3 oxidation activity in NH3-selective catalytic oxidation (NH3–SCO). However, the low N2 selectivity limits their further application. To improve N2 selectivity, herein, a series of Ag/Al2O3-X catalysts (X = calcination temperature) are designed by simple calcination. Only Ag nanoparticles (Ag NPs) were observed on the Ag/Al2O3-400 catalyst, while highly dispersed Ag species (Ag HDs) were dominated on the Ag/Al2O3-800 catalyst. NH3–SCO results showed that Ag NPs/Al2O3 forms much N2O by-products, whereas Ag HDs/Al2O3 achieves >99% N2 selectivity over the Ag/Al2O3-800 catalyst at <200 °C. More importantly, various intermediates (NO3 ads, NH2 ads, N═N–M, and N═N–O–M) and their internal transformations were detected, and the reaction pathways for the formation of N2O and N2 are evidenced by in situ NH3-Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). This study not only provides a convenient and effective approach to obtain excellent N2 selectivity in NH3–SCO but also affords a systematic insight into the reaction pathways over Ag/Al2O3 catalysts. 

Masatake Haruta

Papers

TEM and HAADF-STEM study of a Au catalyst supported on a TiO2 nano-rod.

Gold Nanoparticles Supported on Nb2O5 for Low-Temperature CO Oxidation and as Cathode Materials for Li-ion Batteries

Catalysis by Gold Nanoparticles

Epoxidation of Propylene with Oxygen-Hydrogen Mixtures

Understanding the Distinct Effects of Ag Nanoparticles and Highly Dispersed Ag Species on N2 Selectivity in NH3–SCO Reaction