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Percepire significa costruire un percetto

Fe-based catalysts for the reduction of oxygen in polymer electrolyte fuel cells (PEFCs) have been prepared from a precursor containing 10 wt% Fe as-Fe(OH)(2) adsorbed on carbon black (Fe(OH)(2)/C). Activation of the precursor was performed in two steps: (i) H-2 reduction at 600 degrees C; (ii) pyrolysis in acetonitrile (AN) vapor at various temperatures (400, 600, 800, 1000 degrees C). The electrocatalytic properties of all catalysts were tested in rotating disk electrode (RDE) experiments and in single H-2/O-2 gas diffusion electrode (GDE) assemblies. Catalysts for O-2 reduction were obtained for Fe(OH)(2)/C pyrolyzed at 600 degrees C and higher. Stable currents in the fuel cell assembly were observed for the catalysts prepared at 800 degrees C and higher. Leaching the excess iron from the catalysts by exposure to an H2SO4 solution increased their catalytic activities. Exposure of the acid leached catalyst prepared at 1000 degrees C to Cl(2 )at 650 degrees C removed additional quantities of excess iron and increased the catalyst activity even further. The catalyst obtained after these treatments retained an iron content of 3.3 wt%. The survival of the catalytic activity in that material even after the Cl-2 treatment suggests that either Fe is present in the active site in a high oxidation state unleachable under the form of FeCl3, or that Fe is not a constituent of the active site. In this case, its role would be limited to catalyzing the formation of the carbon and nitrogen based active site. The XRD, XPS, ToF-SIMS and TEM analyses performed on the catalysts prepared during this study were inconclusive in resolving this issue as they all were dominated by the presence of inactive iron particles and AN pyrolysis products. (C) 1998 Elsevier Science Ltd. All rights reserved.

A brain-damaged patient is described whose pattern of performance provides insight into both the functional mechanisms and the neural structures involved in visual mental imagery. The patient became severely agnosic, alexic, achromatopsic and prosopagnosic following bilateral brain lesions in the temporo-occipital cortex. However, her mental imagery for the same visual entities that she could not perceive was perfectly preserved. This clear-cut dissociation held across all the major domains of high-level vision: object recognition, reading, colour and face processing. Our findings, together with other reports on domain-specific dissociations and functional brain imaging studies, provide evidence to support the view that visual perception and visual mental imagery are subserved by independent functional mechanisms, which do not share the same cortical implementation. In particular, our results suggest that mental imagery abilities need not be mediated by early visual cortices.

We report the case of a patient who, after sequential bilateral strokes in the occipital regions sparing the primary visual cortex, developed a severe deficit of colour perception. At variance with other reports of acquired achromatopsic patients, she showed a perfectly vivid visual imagery for colours. These findings, together with similar data in domains other than colour processing, challenge the theories which posit that the same cognitive processes are involved in both the perception and the retrieval from memory of a given stimulus.

Iron and cobalt tetraphenylporphyrins (FeTPP and CoTPP, respectively) have been adsorbed on carbon black (C). The resulting FeTPP/C and CoTPP/C were heat-treated in Ar at various temperatures ranging from 100-1100 degrees C in order to produce catalysts for the electroreduction of oxygen in polymer electrolyte fuel cells. The catalysts have been characterized by XRD, XPS, ToF-SIMS, and bulk analyses. Their electrocatalytic properties have been evaluated by rotating disk electrode (rde) and gas diffusion electrode (gde) measurements. The highest rde activities at 0.70 V vs nhe were recorded for CoTPP/C and FeTPP/C heat-treated in the 500-700 degrees C range. In that temperature range, the increased catalytic activity originates from the well dispersed N-4-metal moiety or from fragments of the original molecule still containing the metal bound to nitrogen. Short term stability tests on the initially most active catalysts revealed the instability of these catalysts in comparison to those obtained at higher pyrolysis temperatures (900-1000 degrees C). At these pyrolysis temperatures the active site of the catalysts is inorganic in nature. The presence of iron and cobalt in their metallic states in these catalysts has been confirmed using XRD. TEM of the catalysts pyrolyzed at these higher temperatures revealed that most of the iron and cobalt are encapsulated in a graphite-like coating. The better stability of these catalysts in the acidic environment of a working fuel cell may be ascribed to the presence of this protective coating. The activity of FeTPP/C pyrolyzed at 1000 degrees C was observed to increase with time in a manner similar to that of platinum-based catalysts. The activity of the former catalyst was determined to be approximately one half that of a commercial platinum catalyst containing the same amount of metal (2 wt%). Copyright (C) 1996 Elsevier Science Ltd.

Different preparation methods of Pt(IV) chloride-graphite intercalation compounds from micrometer sized graphite powder and their subsequent chemical reduction are described. The reaction products were investigated by XRD, SEM, EDX and neutron activation analysis. Pt(IV) chloride-GIC was obtained by the isothermal vapour transport method of PtCl4 between 350 and 500 degrees C under a pressure of chlorine gas. Reduction of Pt(IV) chloride-GIC in a tetrahydrofuran (THF)-naphthalene-K solution led to a compound containing both included potassium and potassium chloride particles and a large proportion of well distributed platinum clusters supported on the graphite flakes. The latter originate from the unremovable excess of platinum chloride systematically observed in all samples prepared by this method. The reaction of H2PtCl6 with graphite in SOCl2 under an inert atmosphere also yields a Pt(IV) chloride-GIC of formula C74PtCl3.84. The hexachloroplatinic acid lost two HCl molecules during the intercalation process in thionyl chloride. This Pt(IV) chIoride-GIC prepared in SOCl2 was reduced either by potassium in a naphthalene-THF solution, by potassium in liquid ammonia, or by NaBHEt(3) in toluene. Irrespective of the reduction method, only Pt and KCl included in the graphite matrix were detected. The average diameter of the platinum particles was evaluated from the X-ray diffraction patterns and ranged from 27 to 38 Angstrom. When heated at 400 degrees C for 1 hour under a flow of argon, cubic KCl crystals and minute platinum particles emerged on the edges of the graphite flakes. It is concluded that the reduction occurs in the graphite galleries, and the reaction mechanism is discussed for each method.