Solid State Phenomena Vol. 128

Abstract: An external electrostatic field of the order of a few tens of a volt per nanometer causes significant changes in the electron density distribution near a metal surface. Because of differing electronic distributions and varying responses of electrons to the applied field for various metals, the resulting local field distribution in the close vicinity of the surface should depend on the electronic properties of the particular metal, even for flat surfaces. Field-free and field-modified electron density distributions for different metal surfaces were calculated using the functional integration method. This approach enables the exchange-correlation effects to be correctly considered and makes it possible to account for the proper field-effect for broad field ranges without using the perturbation theory. The results of calculations are compared with the field-ion microscopic observations.

Abstract: p-type semiconducting perovskites Sm1-xAxFe1-yByO3 (A=La, Sr, x=0, 10, 20 at%, B=Cr, Co, Mn, y=0, 10, 20 at%) were synthesised via the polyol method. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to confirm the structure and morphology of the materials. Thick films of the materials prepared together with surface doped SmFeO3 (with Au, Ce, Ir, Pd, Pt, Rh, Ru) samples were electrically characterised using high throughput impedance spectroscopy (HT-IS). The gas sensing behaviour was tested at temperatures ranging from 200 to 500°C. For La and Cr doping a model describing a correlation between the (M-O) binding energy and the sensing properties was verified.

Abstract: Nanocrystalline CeO2-ZrO2 (Ce:Zr 1:1) samples doped with La, Pr or Gd cations (containing up to 30 at.%) were prepared via the Pechini route. Pt (1.4 wt.%) was supported via impregnation with H2PtCl6 solution followed by drying and calcination. The samples’ surface features were studied by SIMS and FTIRS of adsorbed CO. The oxygen mobility was characterized by the dynamic oxygen isotope exchange and H2 TPR. Catalytic activity was studied in the flow installation using diluted feeds (0.7% CH4 +0.5% O2 or 1% C3H6O + 0.5% O2 +0.5% H2O in He). In the selective oxidation of methane (POM), the catalytic activity correlates with Pt dispersion controlled by the oxidized sample’s ability to stabilize Pt2+ cations as precursors of small reactive Pt clusters formed under reaction conditions. This is favoured by a larger doping cation (La) and a developed network of nanodomain boundaries. At comparable Pt dispersion, the highest performance was demonstrated by a La-doped system, which correlates with the highest surface/near-surface oxygen mobility controlled by the strength of Ce-O bonds in the surface layer. In the autothermal reforming of acetone, the activity trends differ from those in POM because of the more prominent role of the oxygen mobility required to prevent surface coking.

Abstract: Nanocrystalline cobalt was carburised with ethylene in the range 340– 500°C to obtain Co(C) nanocapsules. The carbon deposit was reduced by a flow of hydrogen in the range 500– 560°C. The reduction kinetics were studied using thermogravimetry, described by the equation: α = Α[1-exp(-kt)n]. The apparent activation energy of the reduction process of the carbon deposit was determined. After carburisation and reduction the samples were examined by XRD and HRTEM.

Abstract: High surface area nano-sized catalysts with the general formula La0.9M0.1CoO3 (M=Ce, Pr, Tb) were synthesised by a novel flame-spray-pyrolysis method. All the prepared samples possessed the LaCoO3 perovskite-like structure and consisted of 30-60 nm particles, but existing as large (80-200 nm) agglomerates. Their surface area ranged from 45 to 60 m2/g. All the catalysts showed a very high activity for methane flameless combustion, attaining 100% conversion at temperatures Tf, between 495 and 515 0C, depending on the nature of the M doping ion. The results of life-tests did not show any decreasing of catalytic activity after 50 h under the reaction conditions at Tf. The thermal stability under high temperature application was investigated by overheating the catalysts twice at 800 0C for 1 hr in a flowing reacting gas mixture, fast deactivation cycles, and then measuring methane conversion at Tf. All the catalysts showed a significant decrease of activity, depending on the chemical composition. Taking account of the results of SEM analysis, this was attributed to sintering, connected with the Gibbs instability of nano-sized materials.

Abstract: Multi-wall carbon nanotubes (MWCNTs) were used as a support for the deposition of highly dispersed platinum. After characterization by several physical techniques, the catalyst was studied in reactions for: hydrodechlorination of carbon tetrachloride and the hydrodechlorination of dichlorodifluoromethane. For the first reaction Pt/MWCNTs were very effective catalysts in terms of both the overall activity and the selectivity to CHCl3; both quantities appeared high and stable. For CCl2F2 hydrodechlorination the catalyst was rather moderate, although very stable, activity and product selectivities were established at a constant level in a relatively short time-on-stream. The MWCNTs-supported Pt particles do not undergo great changes during the reactions, i.e. neither substantial metal sintering occurred nor extensive surface carbonization/chloriding took place.