Harold H. Kung

Silicon nanoparticles–graphene paper composites for Li ion battery anodes

Composites of Si nanoparticles highly dispersed between graphene sheets, and supported by a 3-D network of graphite formed by reconstituting regions of graphene stacks exhibit high Li ion storage capacities and cycling stability. An electrode was prepared with a storage capacity >2200 mA h g(-1) after 50 cycles and >1500 mA h g(-1) after 200 cycles that decreased by <0.5% per cycle.

Coking- and Sintering-Resistant Palladium Catalysts Achieved Through Atomic Layer Deposition

A Useful Cover-Up Many industrial catalysts that consist of metal nanoparticles adsorbed on metal oxide supports undergo deactivation after prolonged use. Organic reactants can decompose and cover the metal with carbon (“coking”), and other processes can push the size distribution to fewer but larger particles that have less overall surface area available for reaction (“sintering”). Lu et al. (p. 1205) used atomic-layer deposition to apply a uniform overlayer of alumina onto supported palladium nanoparticles. This coating greatly increased the resistance of the nanoparticles to coking and sintering during the oxidative dehydration of ethane to ethylene. Uniform oxide coating on palladium nanoparticles prevents carbon accumulation and particle growth during chemical reactions. We showed that alumina (Al2O3) overcoating of supported metal nanoparticles (NPs) effectively reduced deactivation by coking and sintering in high-temperature applications of heterogeneous catalysts. We overcoated palladium NPs with 45 layers of alumina through an atomic layer deposition (ALD) process that alternated exposures of the catalysts to trimethylaluminum and water at 200°C. When these catalysts were used for 1 hour in oxidative dehydrogenation of ethane to ethylene at 650°C, they were found by thermogravimetric analysis to contain less than 6% of the coke formed on the uncoated catalysts. Scanning transmission electron microscopy showed no visible morphology changes after reaction at 675°C for 28 hours. The yield of ethylene was improved on all ALD Al2O3 overcoated Pd catalysts.

Supported Au catalysts for low temperature CO oxidation

Supported Au catalysts are very active for low temperature CO oxidation. However, the reported activity from different laboratories for apparently similar catalysts can differ quite substantially. Recent progress in resolving this difficulty is summarized. Residual chloride in the sample is a very effective poison of the active site. The effect of water vapor on the catalytic activity differs depending on the support and the residual chloride content. A model of the active site, which consists of an ensemble of metallic Au atoms and Au cations with hydroxyl ligands, the reaction mechanism for CO oxidation, and the mechanism for deactivation during reaction as well as regeneration are discussed with respect to the available data.

Crumpled Graphene-Encapsulated Si Nanoparticles for Lithium Ion Battery Anodes

Submicrometer-sized capsules made of Si nanoparticles wrapped by crumpled graphene shells were made by a rapid, one-step capillary-driven assembly route in aerosol droplets. Aqueous dispersion of micrometer-sized graphene oxide (GO) sheets and Si nanoparticles were nebulized to form aerosol droplets, which were passed through a preheated tube furnace. Evaporation-induced capillary force wrapped graphene (a.k.a., reduced GO) sheets around the Si particles, and heavily crumpled the shell. The folds and wrinkles in the crumpled graphene coating can accommodate the volume expansion of Si upon lithiation without fracture, and thus help to protect Si nanoparticles from excessive deposition of the insulating solid electrolyte interphase. Compared to the native Si particles, the composite capsules have greatly improved performance as Li ion battery anodes in terms of capacity, cycling stability, and Coulombic efficiency.

Flexible Holey Graphene Paper Electrodes with Enhanced Rate Capability for Energy Storage Applications

The unique combination of high surface area, high electrical conductivity and robust mechanical integrity has attracted great interest in the use of graphene sheets for future electronics applications. Their potential applications for high-power energy storage devices, however, are restricted by the accessible volume, which may be only a fraction of the physical volume, a consequence of the compact geometry of the stack and the ion mobility. Here we demonstrated that remarkably enhanced power delivery can be realized in graphene papers for the use in Li-ion batteries by controlled generation of in-plane porosity via a mechanical cavitation-chemical oxidation approach. These flexible, holey graphene papers, created via facile microscopic engineering, possess abundant ion binding sites, enhanced ion diffusion kinetics, and excellent high-rate lithium-ion storage capabilities, and are suitable for high-performance energy storage devices.

Selective oxidative dehydrogenation of butane over VMgO catalysts

Abstract Vanadium-magnesium oxides were found to be selective oxidative dehydrogenation catalysts for butane. The selectivity for dehydrogenation increased with increasing vanadium content until an optimum was reached for samples containing 24 to 54 wt% V 2 O 5 . Infrared spectroscopy, X-ray diffraction, Auger electron spectroscopy, and scanning electron microscopy studies of the catalysts suggested that the active component was the compound magnesium orthovanadate. For a given catalyst at about 540 °C, the selectivity for oxidative dehydrogenation increased with decreasing oxygen-to-butane ratio, decreasing conversion, and decreasing temperature. A selectivity of up to 60% was obtained. The high selectivity for oxidative dehydrogenation instead of oxygenate production is attributed to two factors: the basic surface facilitates desorption of basic butenes and butadiene, and the absence of VO lowers the oxidation activity of the surface.

Oxidative Dehydrogenation of Light (C2 to C4) Alkanes

Publisher Summary This chapter summarizes the data and current understanding regarding the oxidative dehydrogenation reaction of alkanes. The reaction mechanism, the nature of the catalysts, and factors that determine selectivity for dehydrogenation versus formation of oxygen-containing products are discussed in the chapter. From the pattern of product distribution in the oxidation of C 2 to C 6 alkanes obtained with supported vanadium oxide, orthovanadates of cations of different reduction potentials, and vanadates of different bonding units of VO x in the active sites, it is shown that the selectivities can be explained by the probability of the surface alkyl species (or the surface alkene formed from the alkyl) to react with a reactive surface lattice oxygen. Catalysts for which this occurs with a high probability would show low selectivities. This probability increases for vanadates that have low heats of removal of lattice oxygen, which are those that contain easily reducible cations in the active sites and for vanadates whose active sites can bind the surface alkyl species (or alkene) in a way that bring the surface intermediate close to the reactive lattice oxygen. The dependence of selectivity for dehydrogenation on the conversion of alkane shows that for the more selective catalysts known, the reaction proceeds with a sequential mechanism.

Semiconducting oxide anodes in photoassisted electrolysis of water

The electrochemical properties of semiconducting anodes of TiO2, SrTiO3, BaTiO3, Fe2O3, CdO, CdFe2O4, WO3, PbFe12O19, Pb2Ti1.5W0.5O6.5, Hg2Ta2O7, and Hg2Nb2O7 in photoassisted electrolysis of water were determined. All of these oxides formed a rectifying junction with the electrolyte and anodic photocurrents were generated only with larger‐than‐band‐gap illumination. For Fe2O3, the optical absorption spectrum was different from the photoelectrochemical spectrum due to crystal field transitions. These oxides were found to be stable over certain range of pH. In a given electrolyte, the flatband potential Vfb varied linearly with the band gap. A good correlation was obtained between Vfb and the heat of formation of the oxide per metal atom per metal‐oxygen bond, but not between Vfb and the calculated Fermi energy of the oxide. This suggests that a semiconductor‐electrolyte interface may be approximated by a semiconductor‐metal junction where the barrier height is determined by the heat of formation of the me...

On the potential role of hydroxyl groups in CO oxidation over Au/Al2O3

The deuterium isotope effect in the steady state CO oxidation rate over Au/-Al2O3 in the presence of H2 or H2O and the effect of pretreatment on an uncalcined catalyst were studied. In a reaction feed containing 1% CO, 0.5% O2, and 40.5% H2 at room temperature, CO oxidation exhibited a deuterium isotope effect (kH/kD )o f 1.4 ± 0.2. The rate of D2 oxidation was also slower than the oxidation of H2, such that the selectivity for CO oxidation was 86% in the presence of D2 versus 77% in the presence of H2. In contrast, there was no deuterium isotope effect in a feed containing 1% CO, 0.5% O2, and 1.5% H2O. H2 was also more effective in regenerating a CO oxidation reaction deactivated catalyst than D 2, whereas H2O and D2O were equally effective. The difference was attributed to the different mechanisms with which H 2 or H2O prevented deactivation of the catalyst during CO oxidation. An uncalcined Au/-Al2O3 was rather inactive. It could be activated by treatment with a mixture of H2 and H2 Oa t 100 ◦ C, although treatment by either H2 or H2O alone was ineffective. The observations are consistent with the model of the active site consisting of an ensemble of metallic Au atoms and a cationic Au with a hydroxyl group.

IR Studies of NH3, Pyridine, CO, and NO Adsorbed on Transition Metal Oxides

Abstract Chemisorption of small molecules is often used as a probe for By probing the surface properties of transition metal oxides. the interaction of molecules with the surface, information is often obtained on the oxidation state, the coordination symmetry, the degree of coordination unsaturation of the surface cations, the acid-base properties of the surface hydroxyl groups, and the presence and the nature of surface Lewis acid and B rosnsted acid sites. This information is deduced from experimental measurements of the adsorption isotherms, the heats of adsorption, the thermal desorption spectra, and the vibrational spectra of the adsorbate. Until recently, when high resolution electron energy loss spectroscopy became available, vibrational spectra were obtained with infrared spectroscopy. Laser Raman spectroscopy has seldom been used because of the low Raman scattering cross section of most molecules.