Придністровський конфлікт: чинники існування напруги

Стаття присвячена аналізу та систематизації чинників, що обумовлюють збереження статус-кво у процесі придністровського врегулювання на глобальному, регіональному та локальному рівні.The article is devoted to the analysis and systematization of factors leading to the preservation of the status-quo in the Transnistrian settlement process on global, regional and local level

Site-Specific Iron Substitution in STA-28, a Large Pore Aluminophosphate Zeotype Prepared by Using 1, 10-Phenanthrolines as Framework-Bound Templates

An AlPO4 zeotype has been prepared using the aromatic diamine 1, 10-phenanthroline and some of its methylated analogues as templates. In each case the two template N atoms bind to a specific framework Al site to expand its coordination to the unusual octahedral AlO4N2 environment. Furthermore, using this framework-bound template, Fe atoms can be included selectively at this site in the framework by direct synthesis, as confirmed by annular dark field scanning transmission electron microscopy and Rietveld refinement. Calcination removes the organic molecules to give large pore framework solids, with BET surface areas up to 540 m2 g-1 and two perpendicular sets of channels that intersect to give pore space connected by 12-ring openings along all crystallographic directions

High-pressure sulfidation of hydrotreating catalysts: Genesis and properties of the active phase

The global petroleum demand is projected to increase in the next decennia as oil is expected to remain the primary source of energy around the globe contributing to approximately 40% of the total consumption. The production of high-quality fuels from crude oil involves physical separation steps but also catalytic processes to remove contaminants (hydrotreating), shift the boiling point (alkylation and hydrocracking) and increase fuel quality (isomerisation). In hydrotreating sulfur, nitrogen, oxygen and metal atoms are removed from the different petroleum streams and unsaturated hydrocarbons are hydrogenated. The main reasons for refineries to perform hydrotreating are of environmental and economic nature. Besides protection of downstream catalysts from poisoning by sulfur, stringent environmental legislation has been aimed at the reduction of sulfur oxide emissions from fuel combustion. The environmental regulations on transportation fuel quality and the diminishing supplies of lighter types of crude oil necessitate further improvement of hydrotreating processes. An important approach is then to further optimize the catalytic activity of hydrotreating catalysts. In spite of the substantial progress made in the fundamental understanding of the active phase morphology, the metal-support interaction and reaction mechanisms, many details about the nature and stability of the active sites have not yet been elucidated. One specific issue not widely addressed is the influence of the sulfidation pressure on the active phase in hydrotreating catalysts. In industrial practice, catalysts are brought in their active, sulfided form at elevated pressure. The main objective of the present study was to understand the effect of the sulfidation pressure on the active phase structure in CoMo and NiW catalystsApplied Science

Effect of pressure on the sulfidation behavior of NiW catalysts: A 182W Mössbauer spectroscopy study

The sulfidation of Al2O3- and ASA-supported NiW catalysts under conditions relevant to industrial practice was studied for the first time by 182W Mössbauer spectroscopy. Only limited number of 182W Mössbauer experiments have been performed previously, mainly due to the need of having specialized equipment for such measurements. 182W MAS can clearly distinguish between WO3- and WS2-type phases encountered in the calcined and sulfided forms of W-based hydrotreating catalysts. NiW/Al2O3 catalysts are more difficult to sulfide than their Mo-based counterparts and, hence, intermediate stages of sulfidation can be studied as separate phases. At low sulfidation rates, an intermediate WS3-type phase is identified at temperatures as high as 673 K. The presence of Ni atoms facilitates sulfidation and in this case the formation of an oxysulfidic intermediate is observed. Sulfidation at 673 K and 0.1 MPa leads to a poorly crystalline WS2 phase, whereas subsequent sulfidation at 4.0 MPa results in the development of better-defined WS2 structures. The merits of 182W Mössbauer spectroscopy to study W-based hydrotreating catalysts are discussed

High-pressure sulfidation of a calcined CoMo/Al2O3 hydrodesulfurization catalyst

The influence of the pressure during the sulfidation on the structure and activity of a calcined CoMo/Al2O3 catalyst was studied by Mössbauer emission spectroscopy (MES), extended X-ray absorption fine structure (EXAFS), transmission electron microscopy (TEM) and dibenzothiophene hydrodesulfurization (HDS) activity measurements. Sulfidation at elevated pressure (4 MPa) leads to a much higher HDS activity than upon 0.1 MPa sulfidation. Similarly, the HDS activity increases when after 0.1 MPa sulfidation (673 K) the sulfidation pressure is increased to 4 MPa. The average slab size (not, vert, similar2.8 nm) and stacking degree (not, vert, similar1.4) do not depend on the sulfidation pressure. EXAFS data point to a higher rate of Co and Mo sulfidation at elevated pressure. Although this leads to a somewhat more aggregated form of Co-sulfide particles at intermediate temperatures compared to the case of 0.1 MPa sulfidation, redispersion takes place to small Co-sulfide species on the MoS2 edges. The spectroscopic data of such stepwise sulfided series support the supposition that sulfidation at 4 MPa leads to a Type II Co–Mo–S phase whereas 0.1 MPa sulfidation results in a less active Type I phase. In addition, upon direct high-pressure sulfidation all Co atoms end up in the Co–Mo–S phase, whereas atmospheric pressure sulfidation leaves a small fraction of Co in close interaction with the support

Role of surface carboxylate deposition on the deactivation of cobalt on titania Fischer-Tropsch catalysts

Operando spectroscopic techniques (Diffusive Reflective Infrared Fourier-Transform and Mössbauer emission spectroscopy) were combined to investigate the role of oxygenates deposition on deactivation of cobalt on titania Fischer-Tropsch catalysts at high pressure. Clear formation of carboxylates was seen for catalysts prepared via both impregnation and precipitation, but more and heavier carboxylates were seen on the impregnated catalyst. This effect is related to a higher olefin content in the products obtained with the impregnated sample, resulting to increased formation of oxygenates through the hydroformylation side reaction. The combined gas chromatography/infrared spectroscopy data demonstrated that the surface carboxylate species are not involved in the catalyst deactivation, being most likely spectator species on the titania support.</p

Magnetoelastic transition and negative thermal expansion of Fe₂Hf_0.83Ta_0.17 ribbons

In this work, the magnetocaloric effect and negative thermal expansion in melt-spun Fe2Hf0.83Ta0.17 Laves phase alloys were studied. Compared to arc-melted alloys, which undergo a first-order magnetoelastic transition from the ferromagnetic to the antiferromagnetic phase, melt-spun alloys exhibit a second-order transition. For Fe2Hf0.83Ta0.17 ribbons, we observed a large volumetric coefficient of negative thermal expansion of −19 × 10−6 K−1 over a wide temperature range of 197 – 297 K and a moderate adiabatic temperature change of 0.7 K at 290 K for a magnetic field change of 1.5 T. The magnetic field dependence of the transition temperature (dTt/dµ0H = 4.4 K/T) for the melt-spun alloy is about half that of the arc-melted alloy (8.6 K/T). The origin of second-order phase transition of the melt-spun alloy is attributed to the partially suppressed frustration effect, which is due to the atomic disorder introduced by the rapid solidification.</p

Elucidating deactivation of titania-supported cobalt Fischer-Tropsch catalysts under simulated high conversion conditions

The study of titania-supported cobalt nanoparticles is relevant for industrial Fischer-Tropsch synthesis (FTS). Herein, we report about various deactivation pathways of cobalt supported on P25 titania (cobalt loading 2–8 wt%) under simulated high conversion conditions using in situ Mössbauer spectroscopy. A fraction of metallic cobalt was oxidized under humid FTS conditions. The absolute amount of oxidized cobalt was ∼ 1.2 wt% independent of the cobalt loading, indicating that specific cobalt-titanol interactions are involved in the oxidation process. The formation of cobalt-titanate-like compounds was only observed under very high water-to-hydrogen ratios in the absence of carbon monoxide. Steam considerably enhances cobalt sintering under FTS conditions. As such, deactivation under humid FTS conditions is not only caused by cobalt oxidation but also by enhancing sintering of the active phase.</p

Chemical looping capabilities of olivine, used as a catalyst in indirect biomass gasification

Indirect biomass gasification systems consist of two reactors: an oxidation reactor and a gasification reactor. A bed-material is used to transfer heat from the oxidation to the gasification reactor. Olivine has been widely studied as a reactive bed-material for this process. The iron in olivine can act as a catalyst for the decomposition of tars, produced during the gasification process. Moreover, iron is capable of transferring oxygen to the gasification reactor. In this study, we elucidate the role of iron in this chemical looping process. Mössbauer spectra show that during oxidation in O2/Ar at 750 °C, iron segregates out of the olivine matrix forming free iron oxide phases. These free iron phases form metallic iron upon subsequent reduction in hydrogen. Thermo gravimetric analysis (TGA) is used to quantify oxygen transport under alternating oxidizing/reducing conditions. TGA results indicate that at least 18% of all the iron, present in olivine, is capable of transferring oxygen on the time scale of minutes. X-ray photoelectron spectroscopy (XPS) combined with depth profiling provides insight in the dynamic behavior of olivine under relevant conditions. Iron enrichment at the surface is observed; oxidized olivine has an iron rich surface layer of 400 nm. The increased iron concentration is particularly pronounced at the outermost surface. Upon subsequent reduction, the iron quickly redistributes in the olivine toward the original, homogeneous distribution. These results show that oxygen transport should be taken into account when olivine is evaluated as a catalyst for indirect biomass gasification. Furthermore, both oxygen transport and catalytic properties are heavily dependent on the iron phases present in the material, which in turn depend on the gas environment