Browsing by Subject "GOLD"

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  • Sharygin, Victor V.; Kamenetsky, Vadim S.; Zhitova, Liudmila M.; Belousov, Alexander B.; Abersteiner, Adam (2018)
    Cu-rich magnesioferrite was found in vesicular basaltic trachyandesite in one of lava tubes (Duplex) that formed during the 2012-2013 eruption of the Tolbachik volcano, Kamchatka. This mineral is commonly associated with hematite, tenorite, halite, sylvite, and Ca-rich silicates (mainly, esseneite and Na-rich melilite) in high-temperature (800-1000 degrees C) reactionary zones (up to 100 mu m) covering vesicular rocks and lava stalactites in the Duplex tube. The mineral relationships of this assemblage indicate the following crystallization sequence: Ca-rich silicates + hematite -> Cu-rich magnesioferrite -> tenorite -> chlorides. This formed due to the reaction of hot gases containing Cu, alkalis, and Cl with solidified lava rock. The composition of magnesioferrite varies strongly in CuO (5.8-17.3 wt %; cuprospinel end-member-15-47 mol %), whereas the contents of other oxides are minor, indicating the main isomorphic substitution is Mg2+ Cu2+. Compositions with maximal CuO content nominally belong to Mg-rich cuprospinel: (Cu0.48Mg0.41Mn0.09Zn0.02Ca0.02) (Fe1.943+Al0.03Ti0.02)O-4. Increasing CuO content of the Duplex Cu-rich magnesioferrite is reflected in Raman spectra by moderate right shifting bands at approximate to 700-710 and 200-210 cm(-1) and the appearance of an additional band at 596 cm(-1). This supports the main isomorphic scheme and may indicate a degree of inversion in the spinel structure.
  • Teixeira, Ivo F.; Homsi, Mauricio S.; Geonmonond, Rafael S.; Rocha, Guilherme F. S. R.; Peng, Yung-Kang; Silva, Ingrid F.; Quiroz, Jhon; Camargo, Pedro H. C. (2020)
    Solar-to-chemical conversion via photocatalysis is of paramount importance for a sustainable future. Thus, investigating the synergistic effects promoted by light in photocatalytic reactions is crucial. The tandem oxidative coupling of alcohols and amines is an attractive route to synthesize imines. Here, we unravel the performance and underlying reaction pathway in the visible-light-driven tandem oxidative coupling of benzyl alcohol and aniline employing Au/CeO(2)nanorods as catalysts. We propose an alternative reaction pathway for this transformation that leads to improved efficiencies relative to individual CeO(2)nanorods, in which the localized surface plasmon resonance (LSPR) excitation in Au nanoparticles (NPs) plays an important role. Our data suggests a synergism between the hot electrons and holes generated from the LSPR excitation in Au NPs. While the oxygen vacancies in CeO(2)nanorods trap the hot electrons and facilitate their transfer to adsorbed O(2)at surface vacancy sites, the hot holes in the Au NPs facilitate the alpha-H abstraction from the adsorbed benzyl alcohol, evolving into benzaldehyde, which then couples with aniline in the next step to yield the corresponding imine. Finally, cerium-coordinated superoxide species abstract hydrogen from the Au surface, regenerating the catalyst surface.
  • Trochowski, Mateusz; Kobielusz, Marcin; Mroz, Krystian; Surowka, Marcin Karol; Hämäläinen, Jani; Iivonen, Tomi; Leskelä, Markku; Macyk, Wojciech (2019)
    Synthetic procedures, including doping, sintering and surface coating, can noticeably affect the physicochemical properties of semiconductors. Introduced changes very often translate into photocatalytic and photoelectrochemical activity alterations. However, in this work we have focused on more subtle treatments, which result in lack of changes observed using XRD, UV-vis, porosimetry, TEM or SEM. We have subjected titanium dioxide (P25, UV100) to a treatment with reducing agents used in procedures of noble metal deposition (citrate, borohydride, and photoreduction), or surface decoration with small amounts of TiO2 by atomic layer deposition (ALD; 10 to 200 deposition cycles), which presumably should be neutral to its activity. Although the "classical" characterization methods did not show any differences between the original and treated samples, spectroelectrochemical (SE-DRS) determination of the density of states (DOS) and catechol adsorption tests revealed a significant influence of such treatments on the photocatalytic activity (photogeneration of HO radicals, water reduction, and herbicide degradation) and photoelectrochemical behaviour of the studied samples. We have shown that the applied slight surface modifications of titanium dioxide ("insignificant" at the first glance) may strongly affect the activity of this material. Such often overlooked effects must be taken into account during a comparative photoactivity analysis of various semiconductors, since an insignificant surface treatment may noticeably influence surface chemistry. We have also demonstrated that SE-DRS can be considered as a useful tool to study these effects, although it can be difficult to correlate a particular treatment with recorded changes in the density of states.
  • Andersson, Stefan S.; Wagner, Thomas; Jonsson, Erik; Fusswinkel, Tobias; Leijd, Magnus; Berg, Johan T. (2018)
    The Swedish part of the Fennoscandian Shield hosts a variety of rare earth element (REE) deposits, including magmatic to magmatic-hydrothermal types. This paper focuses on the origin of the Olserum-Djupedal REE-phosphate mineralisation located in the sparsely studied Västervik region, SE Sweden. Here, mineralisation occurs in three main areas, Olserum, Djupedal and Bersummen. Primary hydrothermal REE mineralisation formed at high temperatures (about 600°C), leading to precipitation of monazite-(Ce), xenotime-(Y), fluorapatite and minor (Y,REE,U,Fe)-(Nb,Ta)-oxides in veins and vein zones dominated by biotite, amphibole, magnetite and quartz. The veins are hosted primarily by metasedimentary rocks present close to, or within, the contact aureole of a local 1.8 Ga ferroan alkali feldspar granite pluton, but also occur within in the chemically most primitive granite in the outermost part of that pluton. In the Djupedal area, REE-mineralised metasedimentary bodies are extensively migmatised, with migmatisation post-dating the main stage of mineralisation. In the Olserum and Bersummen areas, the REE-bearing veins are cross-cut by abundant pegmatitic to granitic dykes. The field relationships demonstrate a protracted magmatic evolution of the granitic pluton and a clear spatial and temporal relationship of the REE mineralisation to the granite. The major and trace element chemistry of ore-associated biotite and magnetite support genetic links between all mineralised areas. Biotite mineral chemistry data further demonstrate a distinct chemical trend from metasediment-hosted ore-associated biotite distal to the major contact of the granite to the biotite in the granite-hosted veins. This trend is characterised by a systematic decrease in Mg and Na and a coupled increase in Fe and Ti with proximity to the granite-hosted veins. The halogen compositions of ore-associated biotite indicate elevated contents of HCl and HF in the primary REE mineralising fluid. Calculated log(fHF/fHCl) values in the Olserum area suggest a constant ratio of about -1 at temperatures of 650-550°C during the evolution of the primary hydrothermal system. In the Djupedal and Bersummen areas, the fluid locally equilibrated at lower log(fHF/fHCl) values down to -2. High Na contents in ore-associated biotite and amphibole, and the abundance of primary ore-associated biotite indicate a K- and Na-rich character of the primary REE mineralising fluid and suggest initial high-temperature K-Na metasomatism. With subsequent cooling of the system, the fluid evolved locally to more Ca-rich compositions as indicated by the presence of the Ca-rich minerals allanite-(Ce) and uvitic tourmaline and by the significant calcic alteration of monazite-(Ce). The later Ca-rich stages were probably coeval with low-temperature (200-500°C) Na-Ca metasomatism variably affecting the granite and the wall rocks, which in the latter case produced distinct white quartz-plagioclase rocks. All observations and data lead us to discard the prevailing model that the REE mineralisation in the Olserum-Djupedal district represents assimilated and remobilised former heavy mineral-rich beds. Instead, we propose that the primary REE mineralisation formed by granite-derived fluids enriched in REE and P that were expelled early during the evolution of a local granitic pluton. The REE mineralisation developed primarily in the contact aureole of this granite and represents the product of a high temperature contact metamorphic-hydrothermal mineralising system. The REE mineralisation probably formed synchronously with K-Na and subsequent Na-Ca metasomatism affecting the granite and the wall rocks. The later Na-Ca metasomatic stage is probably related to a regional Na ± Ca metasomatic and associated U ± REE mineralising system operating concurrently with granitic magmatism at c. 1.8 Ga in the Västervik region. This highlights the potential for discovering hitherto unknown REE deposits and for the reappraisal of already known deposits in this part of the Fennoscandian Shield.