Andrew A. Herzing

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.

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.

Electron Vortex Beams with High Quanta of Orbital Angular Momentum

Diffraction holograms are used to create electron vortex beams that should enable higher-resolution imaging. Electron beams with helical wavefronts carrying orbital angular momentum are expected to provide new capabilities for electron microscopy and other applications. We used nanofabricated diffraction holograms in an electron microscope to produce multiple electron vortex beams with well-defined topological charge. Beams carrying quantized amounts of orbital angular momentum (up to 100ℏ) per electron were observed. We describe how the electrons can exhibit such orbital motion in free space in the absence of any confining potential or external field, and discuss how these beams can be applied to improved electron microscopy of magnetic and biological specimens.

Nanoscale zero-valent iron (nZVI): Aspects of the core-shell structure and reactions with inorganic species in water

Aspects of the core-shell model of nanoscale zero-valent iron (nZVI) and their environmental implications were examined in this work. The structure and elemental distribution of nZVI were characterized by X-ray energy-dispersive spectroscopy (XEDS) with nanometer-scale spatial resolution in an aberration-corrected scanning transmission electron microscope (STEM). The analysis provides unequivocal evidence of a layered structure of nZVI consisting of a metallic iron core encapsulated by a thin amorphous oxide shell. Three aqueous environmental contaminants, namely Hg(II), Zn(II) and hydrogen sulfide, were studied to probe the reactive properties and the surface chemistry of nZVI. High-resolution X-ray photoelectron spectroscopy (HR-XPS) analysis of the reacted particles indicated that Hg(II) was sequestrated via chemical reduction to elemental mercury. On the other hand, Zn(II) removal was achieved via sorption to the iron oxide shell followed by zinc hydroxide precipitation. Hydrogen sulfide was immobilized on the nZVI surface as disulfide (S(2)(2-)) and monosulfide (S(2-)) species. Their relative abundance in the final products suggests that the retention of hydrogen sulfide occurs via reactions with the oxide shell to form iron sulfide (FeS) and subsequent conversion to iron disulfide (FeS(2)). The results presented herein highlight the multiple reactive pathways permissible with nZVI owing to its two functional constituents. The core-shell structure imparts nZVI with manifold functional properties previously unexamined and grants the material with potentially new applications.

Selective Oxidation of Glycerol by Highly Active Bimetallic Catalysts at Ambient Temperature under Base-Free Conditions†

Au–Pt alloy nanoparticles deposited on Mg(OH)2 (see STEM-HAADF image) show high activity in the selective oxidation of polyols using molecular oxygen as oxidant at mild and base-free conditions.

Direct Synthesis of H2O2 from H2 and O2 over Gold, Palladium, and Gold–Palladium Catalysts Supported on Acid‐Pretreated TiO2

Palladium-ringed gold: The acid-pretreated Au–Pd catalysts supported on TiO2 have a well-defined gold-rich core (blue) and palladium-rich shell (green). This type of core and shell enhances the catalytic activity of the catalyst for the direct synthesis of H2O2 from H2 and O2.

Determination of the Oxide Layer Thickness in Core−Shell Zerovalent Iron Nanoparticles

Zerovalent iron (nZVI) nanoparticles have long been used in the electronic and chemical industries due to their magnetic and catalytic properties. Increasingly, applications of nZVI have also been reported in environmental engineering because of their ability to degrade a wide variety of toxic pollutants in soil and water. It is generally assumed that nZVI has a core-shell morphology with zerovalent iron as the core and iron oxide/hydroxide in the shell. This study presents a detailed characterization of the nZVI shell thickness using three independent methods. High-resolution transmission electron microscopy analysis provides direct evidence of the core-shell structure and indicates that the shell thickness of fresh nZVI was predominantly in the range of 2-4 nm. The shell thickness was also determined from high-resolution X-ray photoelectron spectroscopy (HR-XPS) analysis through comparison of the relative integrated intensities of metallic and oxidized iron with a geometric correction applied to account for the curved overlayer. The XPS analysis yielded an average shell thickness in the range of 2.3-2.8 nm. Finally, complete oxidation reaction of the nZVI particles by Cu(II) was used as an indication of the zerovalent iron content of the particles, and these observations further correlate the chemical reactivity of the particles and their shell thicknesses. The three methods yielded remarkably similar results, providing a reliable determination of the shell thickness, which fills an essential gap in our knowledge about the nZVI structure. The methods presented in this work can also be applied to the study of the aging process of nZVI and may also prove useful for the measurement and characterization of other metallic nanoparticles.

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.

Oxidation of glycerol using gold-palladium alloy-supported nanocrystals

The use of bio-renewable resources for the generation of materials and chemicals continues to attract significant research attention. Glycerol, a by-product from biodiesel manufacture, is a highly functionalised renewable raw material, and in this paper the oxidation of glycerol in the presence of base using supported gold, palladium and gold-palladium alloys is described and discussed. Two supports, TiO(2) and carbon, and two preparation methods, wet impregnation and sol-immobilisation, are compared and contrasted. For the monometallic catalysts prepared by impregnation similar activities are observed for Au and Pd, but the carbon-supported monometallic catalysts are more active than those on TiO(2). Glycerate is the major product and lesser amounts of tartronate, glycolate, oxalate and formate are observed, suggesting a sequential oxidation pathway. Combining the gold and palladium as supported alloy nanocrystals leads to a significant enhancement in catalyst activity and the TiO(2)-supported catalysts are significantly more active for the impregnated catalysts. The use of a sol-immobilisation preparation method as compared to impregnation leads to the highest activity alloy catalysts and the origins of these activity trends are discussed.