Philip Landon

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.

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.

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.

Direct synthesis of hydrogen peroxide from H2 and O2 using Au-Pd/Fe2O3 catalysts

The direct synthesis of hydrogen peroxide from H2 and O2 using a range of Au, Pd and Au–Pd metal nanoparticles supported on iron oxide is described and discussed, and in particular the microstructure of the catalysts are investigated using a detailed electron microscopy study. Iron oxide was selected as a support because Au/Fe2O3 catalysts are known to be very active for low temperature CO oxidation. Hydrogen peroxide synthesis was investigated at low temperatures (2 °C) and short reaction (residence) time, and the addition of Pd to the Au catalyst was found to increase the rate of hydrogen peroxide synthesis as well as the concentration of hydrogen peroxide formed. Indeed the rates of hydrogen peroxide synthesis are higher for the Au–Pd alloy catalysts as compared to the Au or Pd only catalysts. These catalyst materials were also investigated for CO oxidation at 25 °C and all were found to be almost inactive. In contrast, Au-based catalysts that are very effective for low temperature CO oxidation were found to be totally inactive for H2 oxidation to H2O2. This suggests an inverse correlation between catalysts that are active for either CO or H2 activation. The microstructure of the Au–Pd/Fe2O3 catalysts was studied using scanning transmission electron microscopy and the metal alloy nanoparticles were found to have a core–shell morphology with Pd concentrated on the catalyst surface.

Direct formation of hydrogen peroxide from H2/O2 using a gold catalyst.

Supported Au catalysts are very selective for the direct formation of hydrogen peroxide from H2/O2 mixtures at 2 degrees C; the rate of H2O2 synthesis is markedly increased if Au-Pd alloy nanoparticles are generated by the addition of Pd.

New approaches to designing selective oxidation catalysts: Au/C a versatile catalyst

Selective oxidation is of key importance in the synthesis of chemical intermediates. For many years a number of oxides and supported metal catalysts have been used. The key questions involved in the design of selective oxidation catalysts are discussed in the initial part of this paper. One of the most exciting recent developments in the field of selective oxidation has been the discovery that supported gold catalysts are active. The second part of the paper discusses Au/C catalysts, which are shown to be particularly versatile for oxidation reactions. Four examples of selective oxidation are described using molecular oxygen as oxidant: (a) selective oxidation of glycerol to glycerate in the presence of base; (b) the oxidation of cyclohexane to cyclohexanol and cyclohexanane in the presence of a radical initiator; (c) the oxidation of hydrogen to hydrogen peroxide, and (d) the oxidation of benzyl alcohol to benzaldehyde under solvent free conditions. In contrast, the Au/C catalysts are not active for oxidation of carbon monoxide at ambient temperature. These examples demonstrate that there exists a rich potential for Au/C as a selective oxidation catalyst and that research efforts should now be focussed on selective oxidation using supported gold catalysts.

Selective oxidation of CO in the presence of H2, H2O and CO2 utilising Au/α-Fe2O3 catalysts for use in fuel cells

Au/Fe2O3 catalysts prepared using co-precipitation are described and discussed for the preferential oxidation of CO in the presence of H2, H2O and CO2. A catalyst prepared using a two-stage calcination procedure (400 °C followed by 550 °C) achieves target conversion and selectivity (>99.5% CO conversion and >50% selectivity, based on O2, for the competing conversion of H2 with O2 at 80–100 °C) for the competitive oxidation of dilute CO in the presence of moist excess H2 and CO2. The effect of the preparation method of the uncalcined precursor is described and the effects of calcination on the catalyst activity in the absence of H2, CO2 and H2O is initially explored. The catalysts are characterised in detail using electron microscopy (TEM), X-ray photoelectron spectroscopy and Mossbauer spectroscopy. For the target conversion to be achieved, it is necessary that the activity for the reverse water gas shift activity (CO2 + H2 → CO + H2O) of the catalyst is suppressed, since under the fuel cell conditions this reaction reforms CO at high CO conversions due to the presence of excess CO2 and H2. It is proposed that the two stage calcination procedure removes active sites for the water gas shift reaction whilst retaining active sites for preferential CO oxidation.

Selective oxidation of CO in the presence of H2, H2O and CO2via gold for use in fuel cells

An Au/Fe2O3 catalyst prepared using a two-stage calcination procedure achieves target conversion and selectivity for the competitive oxidation of dilute CO in the presence of moist excess H2 and CO2.

Decreased methane formation from the hydrogenation of carbon monoxide using zeolite/cobalt–manganese oxide composite catalysts

A composite catalyst comprising a physical mixture of a zeolite and a cobalt/manganese oxide Fischer-Tropsch catalyst decreases the formation of methane in the hydrogenation of carbon monoxide without significantly affecting conversion.