Catalytic synthesis of methanethiol from hydrogen sulfide and carbon monoxide over vanadium-based catalysts

The direct synthesis of methanethiol, CH₃SH, from CO and H₂S was investigated using sulfided vanadium catalysts based on TiO₂ and Al₂O₃. These catalysts yield high activity and selectivity to methanethiol at an optimized temperature of 615 K. Carbonyl sulfide and hydrogen are predominant products below 615 K, whereas above this temperature methane becomes the preferred product. Methanethiol is formed by hydrogenation of COS, via surface thioformic acid and methylthiolate intermediates. Water produced in this reaction step is rapidly converted into CO₂ and H₂S by COS hydrolysis.

Titania was found to be a good catalyst for methanethiol formation. The effect of vanadium addition was to increase CO and H₂S conversion at the expense of methanethiol selectivity. High activities and selectivities to methanethiol were obtained using a sulfided vanadium catalyst supported on Al₂O₃. The TiO₂, V₂O₅/TiO₂ and V₂O₅/Al₂O₃ catalysts have been characterized by temperature programmed sulfidation (TPS). TPS profiles suggest a role of V₂O₅ in the sulfur exchange reactions taking place in the reaction network of H₂S and CO.     

On the reaction mechanism of MoS42− with nitric oxide

MoS₄^2? (TTM) reacts in deaerated neutral phosphate buffered aqueous solutions with NO. Three consecutive reactions are observed. The two first ones obey pseudo first order rate laws with observed rates proportional to [NO]. The third reaction obeys a first order rate law. Nitrite is one of the final products.The mechanism proposed involves thiol nitrosation via a radical mechanism followed by hydrolysis of the S–N bond. The findings indicate that if MoS₄^2? will be used as a drug, it will not affect considerably the NO concentration in the system.     

Resistance to oxidation of graphite silicided by reactive infiltration

In order to improve the oxidation resistance of graphite at high temperatures, graphite surfaces are modified by two silicidation processes involving reactive infiltration of molten silicon or, alternatively, of gaseous silicon monoxide.The resistance to oxidation of silicided graphite is studied by cyclic oxidation tests performed under a dry air flux at temperatures in the range 1550-1600 °C.During oxidation three successive regimes are evidenced: (i) initial growth of a passive silica layer on the continuous SiC superficial layer responsible for the remarkable oxidation resistance at 1550 °C; (ii) then, slow mass consumption in the intermediate composite SiC-C region; (iii) and finally, rapid local consumption of the underlying non-infiltrated porous graphite.The temporal stability of the silicided layer in air is analyzed and the reactions affecting this stability are identified. The analysis interprets the experimental findings in a satisfactory manner, especially the dramatical variation with temperature of silicided graphite lifetime observed in this temperature range.     

Large PAHs Derived from Benzanthrone and Naphthanthrone. A Semiempirical Study

In a systematic semiempirical study, AM1 and PM3 methods are employed to calculate minimum, transition state, and higher order saddle point conformations of planar and overcrowded C₃₄H₁₈ 4–15 and C₃₈H₁₈ 16–21 LPAHs, potential products of peri - peri reductive coupling of benzanthrone (2) and naphthanthrone (3), respectively. The most stable LPAHs in these series are planar C_2v-4 (C₃₄H₁₈) and twisted - folded C₂-21 (C₃₈H₁₈). Among overcrowded regions, two coves are consistently more destabilizing than one fjord, according to both semiempirical methods. The non-planar members of the C₃₄H₁₈ and C₃₈H₁₈ series adopt twisted - folded conformations as global minima and folded - twisted conformations as transition states. The barriers for enantiomerization of the LPAHs with a fjord, cove, and two coves are 18.5–24.0 (AM1) and 16.0–19.3 (PM3), 11.1–11.8 (AM1) and 7.2–7.5 (PM3), 12.0–12.4 (AM1) and 8.1–8.4kcal/mol (PM3), respectively. A comparison of calculated and experimentally determined geometries of LPAHs shows a good correlation between the two types of methods.     

Synthesis,characterization, and magnetic properties of α-MnS nanocrystals

MnS nanocrystals have been prepared by a colloidal synthesis route through the reaction of MnCl₂ and S[Si(CH₃)₃]₂ in trioctylphosphineoxide. The nanocrystals were characterized using X-ray diffraction and transmission electron microscopy. The magnetic properties were studied with SQUID magnetometry. X-ray diffraction shows that the nanocrystals are of the thermodynamically stable α-MnS (alabandite) structure. Size control was achieved by changing the concentration of the precursors. Nanocrystal sizes were measured by transmission electron microscopy, and three samples of average diameters 20, 40, and 80 nm were obtained, with narrow size distribution (σ˜9%). The zero field cooled magnetization curves for the 80-, 40-, and 20-nm samples showed a cusp at 116 K, 97 K, and 50 K respectively, all smaller than the antiferromagnetic transition temperature, T_N = 130 K, of bulk α-MnS. Below T_N the magnetization exhibits a paramagnetic behavior unlike typical antiferromagnetic materials. These results indicate that there is a mixture of paramagnetic and antiferromagnetic phases in the nanocrystals. The size dependence shows a general trend of decrease of T_N with reduced particle size, indicating size dependent magnetic ordering.     

Automatic progressive damage detection of rotor bar in induction motor using vibration analysis and multiple classifiers

There is an increased interest in developing reliable condition monitoring and fault diagnosis systems of machines like induction motors; such interest is not only in the final phase of the failure but also at early stages. In this paper, several levels of damage of rotor bars under different load conditions are identified by means of vibration signals. The importance of this work relies on a simple but effective automatic detection algorithm of the damage before a break occurs. The feature extraction is based on discrete wavelet analysis and autocorrelation process. Then, the automatic classification of the fault degree is carried out by a binary classification tree. In each node, comparing the learned levels of the breaking off correctly identifies the fault degree. The best results of classification are obtained employing computational intelligence techniques like support vector machines, multilayer perceptron, and the k-NN algorithm, with a proper selection of their optimal parameters.     

Role of chromium in Cr–Fe oxide catalysts for high temperature water-gas shift reaction – A DFT study

Carcinogenic hexavalent Cr in the current iron oxide-based catalysts of high temperature water-gas shift (HT-WGS) reaction is great environmental concern. Interpreting the role of the Cr in this important industrial catalyst system is required. In the present study, we investigated substitution of Cr atoms into the most stable termination of Fe₃O₄ (111) slab surface by spin-polarized periodic DFT approach to comprehend the role of Cr. We applied the projector augmented-wave (PAW) method within the Perdew-Wang 1991 (PW91) form of the generalized gradient approximation (GGA) on the Vienna Ab-initio Simulation Package (VASP). The calculations point out that Cr atoms choose being below the surface FeO₆ sites. Cr slightly affects the dissociative H₂O adsorption. There is no effect on the CO adsorption. Oxygen vacancy is favored to form on the topmost layer with less vacancy formation energy. Substitution of Cr into the structure increases the oxygen vacancy formation energy. This indicates that Cr does not act as a chemical promoter and does not affect the catalytic activity positively, which is experimentally confirmed by the previous studies.