Determinação dos elementos-traço (Zn, Co e Ni) em sedimentos da Plataforma Continental Amazônica sob influência da descarga do rio Amazonas

Determination of trace elements (Zn, Co and Ni) in sediments at the Amazon Continental Shelf on influence of the Amazon River discharge This research was developed at Amazon Continental Shelf (ACS) between the Orange Cape (Amapá State) and the Maguari Cape (Pará State), to determine the levels of zinc, cobalt and nickel in the superficial sediments identifying its inputs and outputs. The rate of these metallic elements in the sediments decreases following Zn> Ni> Co. The concentrations of total metals in the surface sediments range from Zn = 52.8-159.5; Ni = 21.7-47.4 and Co = 14.3-48.8 (mg.kg-1). The levels of trace elements observed in the sediments at ACS are of natural occurrence. Pollution sources of metallic elements were not observed

Application of in situ dispersive XAFS studies in catalysis

Useful application of in situ dispersive XAFS spectroscopy to time-resolved processes, e.g. activation, solid-state ion exchange of and catalytic reaction over noble metal containing zeolites are described

DEXAFS studies on the reactivity of exfoliated graphite intercalation compounds

The exfoliation (rapid thermal decomposition) of a H2PtCl6 GIC was investigated using time resolved X-ray absorption spectroscopy in an energy dispersive mode. The experiments were carried out at the DEXAFS beamline at HASYLAB (Hamburg). The process results under nitrogen atmosphere as well as in air in a reduction of the platinum to graphite fixed metal clusters. During the exfoliation under nitrogen atmosphere the occurrence of PtCl4−x GIC, which cannot be prepared by application of the standard synthesis conditions, can be observed

In Situ XAS Investigations of Chemical Oscillations in the Oxidation of Co on Supported Pd Catalysts

Self-sustained temperature as well as rate oscillations during the oxidation of CO at atmospheric pressure on supported Pd catalysts were studied in-situ at the Pd K edge by employing energy-dispersive XAS. From the absorption spectra a phase transition from palladium oxide to palladium clusters can be excluded as driving force of the observed oscillations. Nevertheless, a constant phase correlation between edge position and the observed temperature oscillations could be revealed. This points toward a periodical surface oxidation / reduction process in phase with deactivation / activation cycles of the catalyst. Furthemore, an oscillatory behaviour in height and position of the first Pd peak could be seen in the radial distribution function. From theoretical cluster calculations using FEFF6 it can be deduced, that the evaluated oscillations in Pd coordination number and Pd-Pd distance indicate an oscillatory change of surface coverage from CO to oxygen and vice-versa. Taking these surface-coverage oscillations and the periodical oxidation / reduction into account, a surface activation / deactivation process is proposed as kinetic mechanism for the observed chemical oscillations

Time-resolved XAS and XRD studies on the solid-state ion exchange of platinum into zeolite Y

The course of solid-state ion exchange of NH4Y zeolite with platinum oxide, chloride and bromide was monitored by TPR, XRD and - for the first time - by DEXAFS. In the absence of air PtO2 and PtCl2 are reduced at about 300°C to oxidation state zero by ammonia from the zeolite under formation of highly dispersed platinum clusters. In the presence of air this reduction is inhibited and the chlorine neighbour of Pt is replaced by oxygen. Hydrogen treatment resulted in a reduction to a formal oxidation number lower than zero which might be tentatively explained either by formation of a hydride phase or by quantum-size effects. In situ XRD experiments prove the migration of the salt components into the channels of the zeolite, although partial reaction at the outer surface cannot be excluded. In the case of platinum bromide, after reaction bromine cannot be completely removed from the sample, even at 500°C

Investigations of exfoliating graphite intercalation compounds using time-resolved DEXAFS

The exfoliation processes (rapid thermal decomposition) of CuCl2 graphite intercalation compounds (GICs) cointercalated with FeCl3 or AlCl3 were investigated using time resolved X-ray absorption spectroscopy in an energy dispersive mode. The experiments were carried out at the DEXAFS beam line at HASYLAB (Hamburg). The evolution of the absorption spectra of the Cu K-edge was compared with the reaction of pure CuCl2 under the same conditions. From the exfoliation of the CuCl2/AlCl3-GIC a CuO graphite compound can be obtained. The formation resembles the reaction of pure CuCl2, whereas the exfoliation of the CuCl2/FeCl3-GIC results in a mixture of Cu2O, CuO and Cu clusters. The reduction of the Cu(II) at a reaction temperature of 500°C is correlated with an oxidation of Fe(II) to Fe(III)

Selected applications of time-resolved in situ investigations using dispersive X-ray absorption spectroscopy

In this report the capability of dispersive x-ray absorption spectroscopy is demonstrated for four examples selected from a variety of chemical applications