Christopher J. Kiely

Identification of Active Gold Nanoclusters on Iron Oxide Supports for CO Oxidation

Gold nanocrystals absorbed on metal oxides have exceptional properties in oxidation catalysis, including the oxidation of carbon monoxide at ambient temperatures, but the identification of the active catalytic gold species among the many present on real catalysts is challenging. We have used aberration-corrected scanning transmission electron microscopy to analyze several iron oxide–supported catalyst samples, ranging from those with little or no activity to others with high activities. High catalytic activity for carbon monoxide oxidation is correlated with the presence of bilayer clusters that are ∼0.5 nanometer in diameter and contain only ∼10 gold atoms. The activity of these bilayer clusters is consistent with that demonstrated previously with the use of model catalyst systems.

Tunable gold catalysts for selective hydrocarbon oxidation under mild conditions

Oxidation is an important method for the synthesis of chemical intermediates in the manufacture of high-tonnage commodities, high-value fine chemicals, agrochemicals and pharmaceuticals: but oxidations are often inefficient. The introduction of catalytic systems using oxygen from air is preferred for ‘green’ processing. Gold catalysis is now showing potential in selective redox processes, particularly for alcohol oxidation and the direct synthesis of hydrogen peroxide. However, a major challenge that persists is the synthesis of an epoxide by the direct electrophilic addition of oxygen to an alkene. Although ethene is epoxidized efficiently using molecular oxygen with silver catalysts in a large-scale industrial process, this is unique because higher alkenes can only be effectively epoxidized using hydrogen peroxide, hydroperoxides or stoichiometric oxygen donors. Here we show that nanocrystalline gold catalysts can provide tunable active catalysts for the oxidation of alkenes using air, with exceptionally high selectivity to partial oxidation products (∼98%) and significant conversions. Our finding significantly extends the discovery by Haruta that nanocrystalline gold can epoxidize alkenes when hydrogen is used to activate the molecular oxygen; in our case, no sacrificial reductant is needed. We anticipate that our finding will initiate attempts to understand more fully the mechanism of oxygen activation at gold surfaces, which might lead to commercial exploitation of the high redox activity of gold nanocrystals.

Spontaneous ordering of bimodal ensembles of nanoscopic gold clusters

The controlled fabrication of very small structures at scales beyond the current limits of lithographic techniques is a technological goal of great practical and fundamental interest. Important progress has been made over the past few years in the preparation of ordered ensembles of metal and semiconductor nanocrystals. For example, monodisperse fractions of thiol-stabilized gold nanoparticles have been crystallized into two- and three-dimensional superlattices. Metal particles stabilized by quaternary ammonium salts can also self-assemble into superlattice structures,. Gold particle preparations with quite broad (polydisperse) size distributions also show some tendency to form ordered structures by a process involving spontaneous size segregation,. Here we report that alkanethiol-derivatized gold nanocrystals of different, well defined sizes organize themselves spontaneously into complex, ordered two-dimensional arrays that are structurally related to both colloidal crystals and alloys between metals of different atomic radii. We observe three types of organization: first, different-sized particles intimately mixed, forming an ordered bimodal array (Fig. 1); second, size-segregated regions, each containing hexagonal-close-packed monodisperse particles (Fig. 2); and third, a structure in which particles of several different sizes occupy random positions in a pseudo-hexagonal lattice (Fig. 3).

Solvent-Free Oxidation of Primary Carbon-Hydrogen Bonds in Toluene Using Au-Pd Alloy Nanoparticles

A gold- and palladium-based catalyst can be used to oxidize toluene and form a commercially useful ester. Selective oxidation of primary carbon-hydrogen bonds with oxygen is of crucial importance for the sustainable exploitation of available feedstocks. To date, heterogeneous catalysts have either shown low activity and/or selectivity or have required activated oxygen donors. We report here that supported gold-palladium (Au-Pd) nanoparticles on carbon or TiO2 are active for the oxidation of the primary carbon-hydrogen bonds in toluene and related molecules, giving high selectivities to benzyl benzoate under mild solvent-free conditions. Differences between the catalytic activity of the Au-Pd nanoparticles on carbon and TiO2 supports are rationalized in terms of the particle/support wetting behavior and the availability of exposed corner/edge sites.

Some recent advances in nanostructure preparation from gold and silver particles: a short topical review

Recent developments in nanostructure self-assembly from gold and silver particles are reviewed. A brief historical background of the field is given, followed by a selection of topics which are of particular current interest. An overview of the preparation of thiol-stabilised gold and silver nanoparticles and their spontaneous self-organisation into well-ordered superlattices is presented. Distance-dependent metal insulator transitions in ensembles of nanoparticles are discussed, along with a previously unpublished measurement of optical properties of dithiol-linked thin films of gold nanoparticles. Recent approaches to more complex nano-architectures are reviewed, including the use of various templates and of DNA base pair recognition. Some aspects of nanoscopic surface chemistry of gold particles including the evolution of molecular recognition sites are reviewed. Current and potential future applications are discussed.

Switching Off Hydrogen Peroxide Hydrogenation in the Direct Synthesis Process

Hydrogen peroxide (H2O2) is an important disinfectant and bleach and is currently manufactured from an indirect process involving sequential hydrogenation/oxidation of anthaquinones. However, a direct process in which H2 and O2 are reacted would be preferable. Unfortunately, catalysts for the direct synthesis of H2O2 are also effective for its subsequent decomposition, and this has limited their development. We show that acid pretreatment of a carbon support for gold-palladium alloy catalysts switches off the decomposition of H2O2. This treatment decreases the size of the alloy nanoparticles, and these smaller nanoparticles presumably decorate and inhibit the sites for the decomposition reaction. Hence, when used in the direct synthesis of H2O2, the acid-pretreated catalysts give high yields of H2O2 with hydrogen selectivities greater than 95%.

Direct synthesis of hydrogen peroxide from H2 and O2 using TiO2-supported Au-Pd catalysts

Abstract The direct synthesis of H 2 O 2 at low temperature (2 °C) from H 2 and O 2 using TiO 2 -supported Au, Pd, and Au–Pd catalysts is discussed. The Au–Pd catalysts performed significantly better than the pure Pd/TiO 2 and Au/TiO 2 materials. Au–Pd particles were found with a core–shell structure, with Pd concentrated on the surface. The highest yields of H 2 O 2 were observed with uncalcined catalysts, but these were particularly unstable, losing both metals during use. In contrast, samples calcined at 400 °C were stable and could be reused several times without loss of performance. These catalysts exhibited low activity for CO oxidation at 25 °C; conversely, catalysts effective for low-temperature CO oxidation were inactive for H 2 oxidation to H 2 O 2 . This anticorrelation is explored in terms of the mechanism by which the catalysts function and the design of catalysts for the selective oxidation of one of these substrates in the presence of the other.

Facile removal of stabilizer-ligands from supported gold nanoparticles

Metal nanoparticles that comprise a few hundred to several thousand atoms have many applications in areas such as photonics, sensing, medicine and catalysis. Colloidal methods have proven particularly suitable for producing small nanoparticles with controlled morphologies and excellent catalytic properties. Ligands are necessary to stabilize nanoparticles during synthesis, but once the particles have been deposited on a substrate the presence of the ligands is detrimental for catalytic activity. Previous methods for ligand removal have typically involved thermal and oxidative treatments, which can affect the size or morphology of the particles, in turn altering their catalytic activity. Here, we report a procedure to effectively remove the ligands without affecting particle morphology, which enhances the surface exposure of the nanoparticles and their catalytic activity over a range of reactions. This may lead to developments of nanoparticles prepared by colloidal methods for applications in fields such as environmental protection and energy production.

Oxidation of glycerol using supported Pt, Pd and Au catalysts

The oxidation of aqueous solutions of glycerol is described and discussed for Pd, Pt and Au nanoparticles supported on graphite and activated carbon. The oxidation in a batch reactor at 60 °C and 1 bar pressure using air as oxidant was initially investigated. Under these conditions, supported Pd and Pt catalysts give some selectivity to glyceric acid, but the main reaction products are considered to be non-desired C1 by-products, e.g. CO2, HCHO and HCOOH. In addition, under these conditions, supported Au catalysts were totally inactive. Using an autoclave with pure oxygen at 3 bar pressure gave a significant improvement in reactivity and, for Pt and Au catalysts, the formation of C1 by-products was eliminated when NaOH was added. In particular, it was noted that, in the absence of NaOH, the Au/C catalyst was inactive. For 1 wt.% Au/graphite or activated carbon, 100% selectivity to glyceric acid at high conversion was readily achieved. The role of the base is discussed and it is proposed that the base aids the initial dehydrogenation via H-abstraction of one of the primary OH groups of glycerol and, in this way, the rate limiting step in the oxidation process is overcome.

Direct synthesis of hydrogen peroxide from H2 and O2 using Pd and Au catalysts

The direct synthesis of hydrogen peroxide from H2 and O2 using a range of supported metal catalysts is described and discussed. A detailed study of the factors influencing the formation and decomposition of hydrogen peroxide is presented for a Pd/sulfonated carbon catalyst in a methanol/water solvent. The use of low temperatures (1–2 °C) and short reaction (residence) time are identified as the key factors that favour high selectivity to hydrogen peroxide. Decomposition of hydrogen peroxide, mainly via further hydrogenation, prevents the formation of high concentrations of hydrogen peroxide. Combustion of hydrogen to water is a competing reaction that becomes significant at higher temperatures, but this can be partially inhibited by the addition of HBr. A second set of supported Pd and Au catalysts are evaluated for the direct synthesis of hydrogen peroxide using supercritical CO2 as a solvent. The use of supercritical CO2 is shown to be beneficial when compared with hydrogen peroxide formation at a temperature just below the critical temperature for CO2. However, at the critical temperature of CO2 (31.1 °C), the decomposition of hydrogen peroxide is rapid and only low rates of hydrogen peroxide formation are observed. At low temperature (2 °C) supported Au catalysts are shown to be very selective for the synthesis of hydrogen peroxide. The rate of hydrogen peroxide synthesis is enhanced markedly when Pd is present with Au and a detailed scanning transmission electron microscopy study shows that the 2–9 nm metal nanoparticles present in this supported catalyst are a Au∶Pd alloy.