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Abstract: Unsupported, bulk phase palladium sulfide has been studied for the selective hydrogenation of acetylene. The sample underwent significant change during thermal pretreatments, the extent of which depends on temperature. Exposure to hydrogen at temperatures of 150 °C or above results in the loss of sulfur from the sample, primarily as hydrogen sulfide. As sulfur is lost, the sample is progressively transformed from a sulfur rich phase (PdS) to a sulfur lean phase (Pd4S) via an intermediate phase (Pd16S7). Reduction at 250 °C produces a material, which contains Pd4S as the surface phase, whereas reduction at 350 °C results in a largely pure Pd4S phase. Thermal treatments which produce a Pd4S surface display excellent catalytic properties. At complete acetylene conversion at 250 °C, ethylene selectivities of 82.8% and 90.0% were obtained under non-competitive and competitive conditions, respectively. Beneficial catalytic properties arise from the uniformity of Pd sites due to the crystal structure of Pd4S along with electronic influences on the adsorption/desorption processes arising from sulfur neighbours. No indications of deactivation or significant deposition of carbon were observed over Pd4S sample after 50 h on stream.

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Abstract: We investigated the oxidation behaviour of a synthetic potassic-ferro-richterite up to 750 °C by using simultaneous X-ray absorption spectroscopy and X-ray diffraction experiments with synchrotron radiation. From the X-ray diffraction results, we observed an abrupt decrease of cell dimensions at ∼335 °C accompanied by an anomalous increase in the monoclinic cell angle β. From the analysis of the XANES spectra at the iron K-edge, we observed that the structural shrinkage is due to the iron oxidation process, coupled to hydrogen loss, occurring at ∼315 °C, slightly before the cell contraction. Combining these results with previous studies performed on similar samples by single-crystal structure refinement, Mössbauer, high temperature-Fourier transform IR and Raman spectroscopies, we show that the temperature evolution in Fe-amphiboles is a multi-step process. Following the iron oxidation driven by temperature, the structural dynamics in this double-chain silicate is ruled by local strains in the ribbon of iron-filled octahedra, mainly due to the contraction of the M(1) site.

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Abstract: We describe the atomic structure and magnetism in Fe nanoparticles (∼2 nm) ebedded in a Cu1-xAlx alloy matrix. Nanocomposite films for these studies were prepared directly from gas phase using a flexible co-deposition technique under Ultra-High Vacuum (UHV) conditions. Fe nanoparticles and the alloy matrix were prepared using a gas aggregation source and MBE sources respectively. Extended x-ray absorption fine structure (EXAFS) experiments indicate that the embedded Fe nanoparticles retain a bcc structure for Al-contents greater than x = 0.13 but, for Al-contents lower than this value, Fe nanoparticles have both fcc and bcc structures. The magnetic moment per Fe atom initially increases with increasing Al-content due to the bcc structure becoming more dominant in the embedded nanoparticles as the Al-content is increased, but then decreases with further increase in Al-content. This decrease is consistent with a modest degree of alloying between Fe and Al atoms at the particle/matrix interfaces.

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Abstract: The rhodium(III) hydrogarnets Ca3Rh2(OH)12 and Sr3Rh2(OH)12 crystallize as polycrystalline powders under hydrothermal conditions at 200 °C from RhCl3·3H2O and either Ca(OH)2 or Sr(OH)2 in either 12 M NaOH or KOH. Rietveld refinements against synchrotron powder X-ray diffraction (XRD) data allow the first crystal structures of the two materials to be determined. If BaO2 is used as a reagent and the concentration of hydroxide increased to hydroflux conditions (excess NaOH), then single crystals of a new complex rhodium hydroxide, BaNaRh(OH)6, are formed in a phase-pure sample, with sodium included from the flux. Structure solution from single-crystal XRD data reveals isolated octahedral Rh centers that share hydroxides with 10-coordinate Ba and two independent 8-coordinate Na sites. 23Na magic-angle spinning NMR confirms the presence of the two crystallographically distinct Na sites and also verifies the diamagnetic nature of the sample, expected for Rh(III). The thermal behavior of the hydroxides on heating in air was investigated using X-ray thermodiffractometry, showing different decomposition pathways for each material. Ca3Rh2(OH)12 yields CaRh2O4 and CaO above 650 °C, from which phase-pure CaRh2O4 is isolated by washing with dilute nitric acid, a material previously only reported by high-pressure or high-temperature synthesis. Sr3Rh2(OH)12 decomposes to give a less crystalline material with a powder XRD pattern that is matched to the 2H-layered hexagonal perovskite Sr6Rh5O15, which contains mixed-valent Rh3+/4+, confirmed by Rh K-edge XANES spectroscopy. On heating BaNaRh(OH)6, a complex set of decomposition events takes place via transient phases.

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Abstract: Changes in elemental and mineralogical composition, and lead speciation, of air pollution control residue (APCR) from municipal solid waste incineration, due to treatment by water washing, were investigated in this work and are reported in the context of a review of the literature. Water washing was shown to substantially modify the nature of APCR by: 1) removing 23% dry mass soluble salts to disagglomerate particles and significantly reduce concentrations of the associated major elements, and increase concentrations of insoluble matrix elements and potential pollutants; and 2) respeciating elements to form new phases. X-ray absorption near edge spectroscopy (XANES) showed that the 500 mg/kg of Pb in raw and washed APCR were comprised mainly of Pb-glass, with some PbSO4, and small amounts of PbO and PbCl2. Semi-quantitative linear combination fitting suggests that the glass in the APCR may be unstable and release Pb under the alkaline pH of water washing, to reprecipitate as PbO. Chemical analysis suggests that some Pb may be removed by washing. Scientific understanding of the composition of raw and washed APCR, and particularly the speciation of potentially toxic metals, such as Zn and Pb, can help in developing effective element recovery and residue treatment, utilization or disposal strategies.