Quantum chemical study of NO reduction mechanism on Ag /Al2O3 catalysts

It was shown that N2O content among NO reduction products increases with an increase of the silver concentration in the catalyst because the nature of the catalytic centers changes and leads to a subsequent change in the mechanism of the reaction. Two reaction mechanisms were proposed and studied by means of quantum chemistry: a two-stage mechanism that proceeds via NO dimer formation on catalysts with high (above 2 wt. %) silver concentration and a parallel mechanism with isocyanates involved on catalysts with low (below 2 wt. %) silver concentration. It was demonstrated that on catalysts with high silver concentration mechanism that involves stepwise NO reduction via N2O to N2 is realised. Moreover, the final stage is complicated by the fact that formed intermediates and N2O are likely to desorb from the catalyst surface. In the case of catalysts with low silver concentration, the formation of both products (N2O and N2) proceeds in parallel and the lower activation barriers of the reaction leading to N2, as well as the thermodynamic profitability of its formation, lead to the predominance of the target product. The competition between the proposed mechanisms was studied in the case of catalytic centers represented by silver dimers. It was shown that activation barriers of reaction proceeding via NO dimer formation are lower than the corresponding barriers of the reaction with isocyanates involved, which confirms the prevalent realisation of the first process and the predominance of N2O among the final products. The obtained results explain the experimental data and are significant for further modelling of the mechanism of nitrogen oxides catalytic reduction considering the Al2O3 support.

Long-Term Embrittlement of Ancient Copper and Silver Alloys

The manifestations of ancient metals’ embrittlement, cracking and fracture, are challenging problems for restorers and conservators, yet the scientific understanding of these problems is limited. In particular, the study and interpretation of fracture surfaces, fractography, is a minor or non-existent consideration for most archaeometallurgical investigations. This paper presents a survey of fractographic analyses, in combination with the more widely used disciplines of microstructural studies, metallography, and chemical analyses for some Old-World copper alloy (bronzes) and high-silver alloy artifacts that have undergone long-term corrosion and embrittlement damage. We show that fractography, as an adjunct to metallography, can improve the interpretation of these types of damage and assist in selecting the best methods for restoration and conservation of the objects made from these alloys.

Ligand Size and Chain Length Study of Silver Carboxylates in Focused Electron Beam Induced Deposition

Focused electron beam induced deposition is a versatile tool for the direct fabrication of complex-shaped nanostructures with unprecedented shape fidelity and resolution. While the technique is well-established for various materials, the direct electron beam writing of silver is still in its infancy. Here, five different silver carboxylates, three fluorinated: [Ag2(µ-O2CCF3)2], [Ag2(µ-O2CC2F5)2], [Ag2(µ-O2CC3F7)2] and two containing hydrogen: [Ag2(µ-O2CCMe2Et)2] and [Ag2(µ-O2CtBu)2] were examined and compared as potential precursors for focused electron beam induced deposition. All of the compounds show high sensitivity to electron dissociation and efficient dissociation of Ag-O bonds. The as-deposited materials showed high silver content from 42 at.% to above 70at.% and silver nano-crystal formation with impurities of carbon and fluorine incorporated between metal grains. A correlation of the number of carbon atoms in the precursor ligands and silver content of the deposited structures was found. The highest silver contents were achieved for compounds with the shortest CF chains. The deposited silver content depends on the balance of electron induced ligand co-deposition and thermal ligand desorption. Low electron flux is advantageous for high silver content. Our findings demonstrate that silver carboxylates constitute a promising group of precursors for focused electron beam nanoprinting of high silver content materials.

Simple Preparation of Ceramic-Like Materials Based on 1D-Agx(x=0, 5, 10, 20, 40 mM)/TiO2 Nanostructures and Their Photocatalysis Performance

Vertical Agx/TiO2 nanorods were successfully grown by a simple oxidation method of a Ti-Ag coating. The samples were grown in the phase of ceramic-like materials, which can be reusable for many cycles for photocatalysis applications. These ceramic-like Agx/TiO2 nanostructures were prepared by the spin-coating of silver nitrate onto Ti sheets. The presence of silver on the surface of the Ti sheet during the oxidation process helped in the growth of one-dimensional nanostructures. The physical properties of the fabricated ceramic-like nanostructures were studied by varying the concentration of silver on the Ti-sheet before the oxidation. One-dimensional nanostructures with an average size varying within the range of 200–500 nm were grown. The presence of silver made the nanostructure vertically directed. The nanorods were dense at the low and medium concentrations of 5, 10, and 20 mM of silver in contrary to high silver concentrations, where the nanorods were very sparse at 40 mM. Structural analysis showed the anatase and rutile structure of pure TiO2 with distinguishing diffraction lines A(101) and R(110); however, Agx/TiO2 showed a dominant orientation of A(101), confirming the 1D growth. Raman spectra confirmed the presence of TiO2 via the observation of its corresponding phonon modes. The photocatalysis properties of the fabricated ceramic-like nanostructures were performed on methylene blue (MB) as a known target dye. The low- and medium-silver-concentration samples showed a high photocatalytic activity compared to the pure and high-silver-concentration samples.

Materials for the Study of Items from Non-Ferrous and Precious Metals of Selitrennoe Settlement: Chemical Composition

Introduction. This paper presents the results of the study of the chemical composition of nonferrous and precious metal items from the excavations of the Selitrennoe settlement by the Volga Region Archaeological Expedition in 1986–1989 stored in the State Historical Museum. Methods and materials. XRF was used as a method of determining the elemental composition; the main technological operations were determined using the trasology method. A total of 63 subjects were analyzed, including domestic items, jewelry and items related to production. Analysis and Results. The alloys have been distinguished depending on the presence and concentration of alloying elements: brass (two-component, tin, lead, multicomponent), bronzes (tin, lead, leadarsenic, tin-lead, tin-zinc, and multicomponent) and silver alloys (multicomponent high silver bronzes and goldsilver alloy). Inclusions of soldering with silver in the solder and gilding according to the amalgamation have been also recorded. The results of the analysis show the prevalence of two-component brass and multicomponent alloys, a small amount of tin bronzes and the complete absence of items from “pure” metals – copper, tin and lead. The common practice of using recycled metal can be judged by the relatively low content of zinc in brass and a large proportion of multicomponent alloys. Despite the presence of distinguishable stable groups, a wide range of concentrations suggests the absence of clear recipes, including for certain categories of objects. The dependence of the applied technology on the composition of metal is also revealed. The sample of the Selitrennoe settlement is significantly different from other large series of analyses of the Golden Horde time (Bolgar Region, Uvek and its neighborhood).

SILVER-GOLD FORMATION TYPE ORE MINERALIZATION IN TAJIKISTAN

Серебро-золоторудный формационный тип минерализации в Таджикистане известен в пределах Табошар-Канджольского рудного узла (Карамазар) и на Памире. В Карамазаре к этому типу относятся месторождения Школьное, Четсу и Караулхона, а на Памире рудопроявления Сассык, Лянгар, Бугучиджилга, Курустык и др. Серебро-золоторудный формационный тип представлен убогосульфидными кварц-золоторудными жилами с высоким содержанием серебра. Рудные минералы представлены пиритом, тетраэдритом, халькопиритом, пираргиритом, фрейбергитом, миаргиритом, самородным золотом, электрумом и кюстелитом. Формационными особенностями этого типа являются предрудная пропилитизация, синрудная березитизация, многостадийный характер минерализации, простой минеральный состав, крайне неравномерное распределение серебра и золота, а также близповерхностное образование золота и его низкопробность. Продуктивное оруденение в них образовалось при сравнительно низких температурах (300–150ο) и давлениях (500 бар и ниже). Silver-gold ore-formation type mineralization in Tajikistan known within Taboshar-Kanjol – ore unit (Karamazar) and the Pamirs. The most known deposits KaramazarScholnoe, Chetsy and Karaulhona and the Pamirs to this type of ore can be attributed Sassyk, Langar, Buguchidzhilga, Kurustyk. Silver-gold ore-formation type is represented by poorly-high silver sulfide-quartz veins of gold mining. The ore minerals are pyrite, tetrahedrite, chalcopyrite, pyrargyrite, freibergite, miargyrite, native gold, electrum and kyustelitе. Formational peculiarities of this type are pre-ore propylitization, sin-ore beresitization, multi-stage nature of the mineralization, simple mineral composition, extremely uneven distribution of silver and gold, as well as subsurface formation of gold and its sleaze. Productive mineralization formed there in at relatively low temperatures (300–150ο) and pressures (500 bar or less).

GEOCHEMICAL CHARACTERISTIC OF AG-BEARING MINERALS OCCURRING IN COPPER ORE DEPOSIT AT THE FORE-SUDETIC MONOCLINE

Silver is the most important associated element in the copper ore deposit of the Fore-Sudetic Monocline. Besides its own minerals, such as native silver, amalgams, stromeyerite, mackinstryite, jalpaite and Cu-Fe-Ag system sulphides, the silver occurs as isomorphic substitutions in copper minerals. The aim of this work included geochemical characterization of Ag-bearing minerals and correlation of the occurrence of Ag-enriched Cu sulphides with the areas of native silver and amalgam mineralization. Distribution of silver in the ore deposit distinctly correlates with Cu concentrations, and the areas of the strongest Cu mineralization are accompanied by high silver concentrations. Results of the scanning electron microscopy and electron microprobe analysis indicate that the Cu sulphides contain up to several wt.% Ag as isomorphic substitutions. Distribution of Ag in Cu sulfides does not show regularity, as demonstrated by wide diversity of Ag contents in various minerals, ranging from 0.0X to dozen wt.% Ag.