Ambesh G. Shastri

Gold-titania interactions: Temperature dependence of surface area and crystallinity of TiO2 and gold dispersion

Abstract The influence of temperature on the BET surface area, crystallinity, and anatase/rutile phase transformation of blank TiO 2 and Au/TiO 2 catalysts is studied. Presence of gold delays the recrystallization of anatase and the phase transformation into rutile. In turn, high gold dispersions are stabilized by TiO 2 up to a temperature of 700 °C. Agglomeration of gold into large particles coincides with the phase transformation into rutile at 800 °C. The stability of the gold dispersion does not seem to be due to an SMSI effect. The low metal loading used to impregnate a high-surface-area TiO 2 may be responsible for either an incorporation of gold atoms in interstitial positions of the TiO 2 lattice, or the trapping of small gold particles in micropores.

Influence of chlorine on the surface area and morphology of TiO2

Abstract Changes in BET surface area and morphology of TiO2 (anatase) were studied as a function of temperature and level of chlorine contamination. The obj

The microstructure of bimetallic Ru---Cu/SiO2 catalysts: A chemisorption and analytical electron microscopy study

Abstract Supported bimetallic RuCu SiO 2 catalysts are characterized by transmission electron microscopy, energy dispersive X-ray spectroscopy, electron microdiffraction, and chemisorption. Metal particles up to 4 nm in diameter are bimetallic, while particles larger than 4 nm contain only Cu. Considerable compositional nonuniformity is observed from one individual metal particle to the next. Microdiffraction patterns obtained from individual particles can be attributed to either Ru or Cu suggesting no significant modification in crystallographic structure of either metal component. Addition of Cu to Ru results in a drastic suppression of H2 chemisorption while the extent of O2 chemisorption is not as strongly affected. The suppressed H2 chemisorption capability of Ru in the bimetallic catalysts is an indication of atomic interdispersion of Ru and Cu on the surface of the bimetallic clusters, leading to the break-up of the Ru ensembles which would be necessary for dissociation of molecular hydrogen. The influence of catalyst preparation techniques on the relative interdispersion of Ru and Cu and consequent discrepancies in the RuCu literature are discussed.

The Influence of Hydrazine Reduction on Metal Dispersion and Support Morphology in Bimetallic Ru-Au/MgO Catalysts

The influence of hydrazine reduction on metal dispersion, particle size distribution, and support morphology was investigated for a series of bimetallic Ru-AuiMgO catalysts. The catalysts were prepared by impregnation of MgO with chlorine-containing precursor salt solutions. Hydrazine reduction significantly lowered the residual chlorine content of the catalysts and allowed the formation of high-surface-area MgO. This, in turn, led to much higher Ru metal dispersions as compared to that obtained after HZ reduction alone. X-Ray energy dispersive spectroscopy (EDS) proved that bimetallic Ru-Au aggregates were confined to a particle size range of less than 5 nm. Particles larger than 10 nm contained without exception monometallic Au. The hydrazine-treated catalysts did not show the suppression in catalytic activity for CO hydrogenation that was observed in HZ-reduced Ru/MgO catalysts. The increased metal dispersion and restored catalytic activity of Ru in the hydrazine-treated catalysts may be linked to the absence of chlorine contamination of the support.

Metal dispersion of bimetallic catalysts via stepwise chemisorption and surface titration : I. Ru---Au/SiO2

Abstract A stepwise chemisorption and titration procedure involving H 2 and O 2 as adsorbates is used to measure the dispersion of the individual metal components in bimetallic RuAu SiO 2 catalysts. The experimental approach is based upon differences in O 2 adsorption on Ru and Au, respectively, as a function of adsorption temperature. At room temperature, O 2 is adsorbed on Ru but not on Au. However, at 473 K both metals chemisorb O 2 , albeit with a change in adsorption stoichiometry in the case of Ru. Finally, oxygen that is adsorbed on Ru can be titrated with H 2 at 373 K, while oxygen that is adsorbed on Au sites does not react with H 2 under these conditions. The reliability of the technique is first tested on physical mixtures of Ru SiO 2 and Au SiO 2 of known dispersion and then applied to a series of bimetallic RuAu SiO 2 catalysts. The results from this stepwise chemisorption procedure are critically evaluated and compared with previously obtained extensive characterization data on these catalysts. In conjunction with analytical electron microscopy and WAXS results, an estimate of the average surface composition of bimetallic particles is derived.

Adsorption-induced conductance changes of thin Pt films and PtPd/TiO2 gas sensors

Chemisorption of H, and Oa and resulting changes in electrical conductance of a typical gas sensing material, PtPd/TiO,, and thin Pt films on glass are studied and compared. The activation energy of conduction increases as Pt film thickness decreases. Chemisorption of Ha on thin Pt films causes an increase in conductance and activation energy of conduction. 0, chemisorption results in a decrease in conductance and increase in activation energy of conduction. Alteration in the number of charge carriers and reduction in charge carrier mobility are the mechanisms proposed for the observed changes. Compared to thin Pt films, relatively large changes in electrical conductance are observed upon chemisorption of gases on TiO, supported PtPd. The role of the oxide substrate in the observed chemical interaction and electronic response is discussed. The electronic changes upon adsorption/desorption of gases are reversible for thin Pt films but only partially reversible for TiO, supported PtPd.

Electron microdiffraction study of bimetallic Ru---Au/MgO catalysts

A microdiffraction study of bimetallic RuAuMgO catalysts is carried out in order to discriminate between the random adsorption versus atomic ordering model proposed for the structure of bimetallic clusters. The microdiffraction patterns from small metal clusters could be ascribed to either Au or Ru indicating an absence of structural modification of individual metal components in bimetallic clusters. Local variations in the MgO planes exposed are seen within areas of 1 μm in diameter which are free of metal particles. Large Au particles (>10 nm) are randomly aligned on MgO whereas small Au particles are aligned such that the [110] zone axis of Au is parallel to the [111] zone axis of MgO. Small Ru particles are aligned so that in most cases the [0001] zone axis of Ru is parallel to the [111] zone axis of MgO. The random adsorption model, where one metal component is chemisorbed on top of the other, is consistent with the experimental observations by EDS and microdiffraction.

Electron microscopy and krypton adsorption characterization of high-purity MgO powder

A promising method for the preparation of sizeable quantities of structurally well-defined, high-purity MgO powder is reported. The morphology and surface uniformity of the powder is comparable to that of MgO smokes but with narrow size distribution of particles. Sample characterization of these oxide powders is accomplished by combining structural TEM/STEM examination with krypton gas adsorption isotherms. The latter technique is sensitive to the presence of surface hydroxyl groups and of surface roughness on an atomic scale. High-resolution TEM indicates a perfect cubic morphology, intra-crystallite orientation, and dendritic sintering of cubes. In the STEM mode sharp convergent beam electron diffraction patterns are obtained, and in thick specimen regions Kikuchi lines appear, indicating the absence of crystal defects. After prolonged outgassing to remove surface hydroxyl groups, a krypton adsorption isotherm contains a near vertical submonolayer riser and second layer step along with partial wetting features near saturation. These near-ideal dendritic ceramic powders, therefore, provide a research bridge between single crystal surface studies and large-scale powder technology.