Mayfair C. Kung

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.

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.

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.

Oxidative dehydrogenation of alkanes over vanadium-magnesium-oxides

Abstract Vanadium-magnesium-oxides are among the most selective and active catalysts for the oxidative dehydrogenation of alkane. The selectivity for dehydrogenation depends strongly on the alkane, the Mg vanadate phase, the presence of modifiers, and to a lesser extent, the method of preparation. The results of studies on these variables are reviewed and discussed with respect to the current understanding of the nature of the active site and requirements for selective dehydrogenation.

Metal oxide catalysts for lean NOx reduction

Abstract A summary of the current understanding of the lean NOx reduction reaction on various metal oxides, particularly Cu-ZrO2, is presented. Whenever appropriate, a comparison with the behavior of the corresponding ZSM-5-based catalysts is made. It is concluded that there are many mechanistic similarities between metal oxide-based and zeolite-based catalysts. Thus, the catalytic properties are determined more by the nature of the active centers than by the nature of the support. Potential advantages offered by metal oxides should make them attractive candidates for practical applications.

The role of NO2 in the reduction of NO by hydrocarbon over Cu-ZrO2 and Cu-ZSM-5 catalysts

The reduction of NOx with propene or propane in the presence of 1 or 4% O2 was studied at low conversions over a 7.4 wt% Cu-ZrO2 and a 3.2 wt% Cu-ZSM-5 catalyst. The rates of N2 production were compared in experiments using only NO or a mixture of NO and NO2 in the feed. They were also compared with the rates of NO2 reduction to NO under the same conditions, and of NO oxidation to NO2 in the absence of hydrocarbon. It was found that the reduction of NO2 to NO was very fast, consistent with literature data. The data were best explained by a reaction scheme in which the hydrocarbon was activated primarily by reaction with adsorbed NO2 to form an adsorbed oxidized N-containing hydrocarbon intermediate, the reaction of which with NO was the principal route to produce N2 under lean NOx conditions.

Free-standing nitrogen-doped graphene paper as electrodes for high-performance lithium/dissolved polysulfide batteries.

Free-standing N-doped graphene papers (NGP), generated by pyrolysis of polydiallyldimethylammonium chloride, were successfully used as binder-free electrodes for the state-of-the-art Li/polysulfide-catholyte batteries. They exhibited high specific capacities of approximately 1000 mA h g(-1) (based on S) after 100 cycles and coulombic efficiencies great than 98%, significantly better than undoped graphene paper (GP). These NGP were characterized with XRD, X-ray photoelectron spectroscopy, thermogravimetric analysis, AFM, electron microscopy, and Raman and impedance spectroscopy before and after cycling. Spectroscopic evidence suggested stronger binding of sulfide to NGP relative to GP, and modelling results from DFT calculation, substantiated with experimental data, indicated that pyrrolic and pyridinic N atoms interacted more strongly with Li polysulfides than quaternary N atoms. Thus, more favorable partition of polysulfides between the electrode and the electrolyte and the corresponding effect on the morphology of the passivation layer were the causes of the beneficial effect of N doping.