Molecular mechanism of propane oxidative dehydrogenation on surface oxygen radical sites of VO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/TiO<Subscript>2</Subscript> catalysts

Minimum energy pathways of propane oxidative dehydrogenation to propene and propanol on supported vanadium oxide catalyst VO _x /TiO₂ were studied by periodic discrete Fourier transform (DFT) using a surface oxygen radical as the active site. The propene formation pathway was shown to consist of two consecutive hydrogen abstraction steps. The first step includes C_β–H bond activation of propane followed by the formation of a surface hydroxyl group V–O _t H and a propyl radical n-C₃H₇. This step with the activation energy E* = 0.56 eV (54.1 kJ/mol) appears to be rate-determining. The second step involves the reaction of the bridging O _b oxygen atom with the methylene C–H bond of propyl radical n-C₃H₇ followed by the formation of a hydroxylated surface site HO _t –V⁴⁺–O _b H and propene. The initial steps of the C–H bond activation during propane conversion to propanol and propene by ODH on V⁵⁺–(O _t O _b )^? active sites are identical. The obtained results demonstrate that participation of surface oxygen radicals as the active sites of propane ODH makes it possible to explain relatively low activation energies observed for this reaction on the most active catalysts. The presence of very active radical species in low concentration seems to be the key factor for obtaining high selectivity.     

Neutron diffraction study of dehydrogenation of titanium carbohydrides TiC<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>H<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>

The possibility of the synthesis of hydrogen-nonstoichiometric cubic titanium carbide ТiС _х of high purity from powdery nonstoichiometric cubic titanium carbohydride ТiС _х H _y or nonstoichiometric titanium carbide with admixture hydrogen by annealing in a continuously maintained vacuum of no worse than 1.33 × 10^–3 Pa at temperatures of 600–750°C for several hours has been shown. Similar annealing at higher temperatures (T ≥ 800°C) does not lead to the complete removal of hydrogen from a sample due to intensive sintering. In this case, it seems that pores between sintered particles are hydrogen traps, and the release of hydrogen through the surface of sintered particles is hindered.     

Monolithic FeO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalysts for ammonia oxidation and nitrous oxide decomposition

(1.2–8.3)%FeO_х/Al₂O₃ monolith catalysts have been prepared by impregnating alumina with aqueous solutions of iron(III) nitrate and oxalate and have been tested in NH₃ oxidation and in the selective decomposition of N₂O in mixtures resulting from ammonia oxidation over a Pt–Rh gauze pack under conditions of nitric acid synthesis (800–900°C). In the case of the support calcined at 1200°C, the catalyst is dominated by bulk Fe₂O₃ particles localized on the Al₂O₃ surface. The activity of these samples in both reactions decreases with a decreasing active component content, thus limiting the potential of Fe₂(C₂O₄)₃ · 5H₂O, an environmentally friendlier but poorly soluble compound, as a substitute for Fe(NO₃)₃ · 9H₂O. Decreasing the support calcination temperature to 1000°C or below leads to the formation of a highly defective Fe–Al–O solid solution in the (1.2–2.7)%FeO_х/Al₂O₃ catalysts. The surface layers of the solid solution are enriched with iron ions or stabilize ultrafine FeO_х particles. The catalytic activity of these samples in both reactions is close to the activities measured for ~8%FeO_х/Al₂O₃ samples prepared using iron nitrate.     

Review of Ag/Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-Reductant System in the Selective Catalytic Reduction of NO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>

Ag/Al₂O₃ is a promising catalyst for the selective catalytic reduction (SCR) by hydrocarbons (HC) of NO _x in both laboratory and diesel engine bench tests. New developments of the HC-SCR of NO _x over a Ag/Al₂O₃ catalyst are reviewed, including the efficiencies and sulfur tolerances of different Ag/Al₂O₃-reductant systems for the SCR of NO _x ; the low-temperature activity improvement of H₂-assisted HC-SCR of NO _x over Ag/Al₂O₃; and the application of a Ag/Al₂O₃-ethanol SCR system with a heavy-duty diesel engine. The discussions are focused on the reaction mechanisms of different Ag/Al₂O₃-reductant systems and H₂-assisted HC-SCR of NO _x over Ag/Al₂O₃. A SO₂-resistant surface structure in situ synthesized on Ag/Al₂O₃ by using ethanol as a reductant is proposed based on the study of the sulfate formation. These results provide new insight into the design of a high-efficiency NO _x reduction system. The diesel engine bench test results showed that a Ag/Al₂O₃-ethanol system is promising for catalytic cleaning of NO _x in diesel exhaust.     

Oxidative dehydrogenation of <Emphasis Type="Italic">n</Emphasis>-butenes to 1,3-butadiene over BiMoFe<Subscript>0.65</Subscript>P<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> catalysts: effect of phosphorous contents

A series of BiMoFe_0.65P _x oxide catalysts with varying phosphorous contents from 0.0 to 0.6 mol ratio were prepared by a co-precipitation method, and oxidative dehydrogenation (ODH) was carried out to produce 1,3-butadiene (BD) from n-butenes. The physico-chemical properties of the oxide catalysts were characterized by X-ray diffraction (XRD), Raman spectroscopy, N₂ sorption, and NH₃ and 1-butene temperature-programmed desorption (TPD). Among the catalysts studied here, BiMoFe_0.65P_0.1 oxide catalyst showed the highest conversion and selectivity to BD. From the result of 1-butene TPD, the higher catalytic activity is related to the amount of weakly bounded intermediate and the desorbing temperature of strongly bounded intermediates. Also, the higher catalytic activity likely originates from the acidity of the BiMoFe_0.65P_0.1 oxide catalyst; its acidity was higher than that of phosphorous-free oxide catalyst and further contained other oxide catalysts. BiMoFe_0.65P_0.1 oxide catalyst is stable and no significant deactivation for 100 h ODH reaction was shown.     

Kinetics of Na|CF<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> and Li|CF<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> systems

Properties of CF _x /Li and CF _x /Na cells were examined while using galvanostatic charging/discharging, electrochemical impedance spectroscopy and scanning electron microscopy (SEM). The capacity during the first cycle was as high as ca. 1000 mAh g^?1. Such an electrode is suitable for primary CF _x /Li and CF _x /Na batteries. SEM images of CF _x cathode showed that during discharging it was transformed into amorphous carbon and LiF or NaF crystals (of diameter of ca. 5–20 μm). These systems (C?+?LiF or C?+?NaF) cannot be reversibly converted back into CF _x /Li or CF _x /Na, respectively. Exchange current densities are between 10^?7 Acm^?2 and 10^?9 Acm^?2 when working with LiPF₆ and NaPF₆ electrolytes (1.12?×?10^?7 Acm^?2 and 6.82?×?10^?9 Acm^?2, respectively). Those values are low and indicate that the charge transfer process may be the rate-determining step. Activation energies for the charge transfer process were 57 and 72 kJ mol^?1 for CF _x /LiPF₆ and CF _x /NaPF₆ systems, respectively. Higher activation energy barrier for the CF/Na⁺?+?e^??→?C?+?NaF reaction results in lower observed exchange current density in comparison to the system with lithium ions.     

Thermal activation effect on catalytic systems MnO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> for deep oxidation of hydrocarbons

The effect of thermal activation, sharp increase in the catalytic activity of the system MnO _x -Al₂O₃ in reactions of deep oxidation of CO and hydrocarbons after calcination of the catalyst at 900–1000°C was discovered and investigated. With the use of X-ray phase analysis, X-ray electron spectroscopy, EXAFS, IR spectroscopy, electronic spectroscopy of diffuse reflections, electron microscopy etc. it was established that the effect of thermal activation is related to reversible phase transitions in the system at heating and cooling. On cooling from 1100°C to 650°C disperse particles of cubic spinel of composition Mn_{2.1 ? x} · Al_{0.9 + x} O₄ are conserved on the corundum surface. On further cooling the spinel decomposes and finally the nanocristalline species of β-Mn₃O*₄ containing up to 15 at% of Al³⁺ form and govern the activity.The thermal activation effect was implemented in an industrial catalyst IK-12-40. Joint Stocks Co “KATALIZATOR” produced and supplied to customers hundreds of tons of this catalyst. The catalyst was awarded with a silver medal of the International exhibition EUREKA in Brussels (1995).     

Chemical composition-structure-properties relations in a series of Sr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ba<Subscript>1-<Emphasis Type="Italic">x</Emphasis></Subscript>Nb<Subscript>2</Subscript>O<Subscript>6</Subscript> compounds

Based on structural studies of Sr _x Ba_1-x Nb₂O₆ crystals with different concentrations of strontium and barium, the structural causality of optical nonlinearity of these crystalline materials is established. YAG:Nd laser radiation of the crystals results in a monotonic decrease in the second harmonic intensity with increasing strontium concentration in a sample. Fine details of the structure responsible for this effect are determined.     

Effect of the nature of the active-component precursor on the properties of Pt/MgAlO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> catalysts in propane and <Emphasis Type="Italic">n</Emphasis>-decane dehydrogenation

The Pt/MgAlO _x catalysts, in which a mixed aluminum–magnesium oxide obtained by the heat treatment of corresponding layered double hydroxides was used as a support, were studied. The effects of the nature of active-component precursors ([PtCl₆]^2–, [PtCl₄]^2–, and [Pt₃(CO)₆] ₆ ^2- ), and the Mg/Al ratio in the support on the anchoring of complexes, the disperse state of platinum, and the properties of the obtained catalysts in the reactions of propane and n-decane dehydrogenation were found.     

Nano-composites SnO(VO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>) as anodes for lithium ion batteries

Nano-composites of SnO(V₂O₃) _x (x = 0, 0.25, and 0.5) and SnO(VO)_0.5 are prepared from SnO and V₂O₃/VO by high-energy ball milling (HEB) and are characterized by X-ray diffraction (XRD), scanning electron microscopy, and high-resolution transmission electron microscopy techniques. Interestingly, SnO and SnO(VO)_0.5 are unstable to HEB and disproportionate to Sn and SnO₂, whereas HEB of SnO(V₂O₃) _x gives rise to SnO₂.VO _x . Galvanostatic cycling of the phases is carried out at 60 mA g⁻¹ (0.12 C) in the voltage range 0.005–0.8 V vs. Li. The nano-SnO(V₂O₃)_0.5 showed a first-charge capacity of 435 (±5) mAh g⁻¹ which stabilized to 380 (±5) mAh g⁻¹ with no noticeable fading in the range of 10–60 cycles. Under similar cycling conditions, nano-SnO (x = 0), nano-SnO(V₂O₃)_0.25, and nano-SnO(VO)_0.5 showed initial reversible capacities between 630 and 390 (±5) mAh g⁻¹. Between 10 and 50 cycles, nano-SnO showed a capacity fade as high as 59%, whereas the above two VO _x -containing composites showed capacity fade ranging from 10% to 28%. In all the nano-composites, the average discharge potential is 0.2–0.3 V and average charge potential is 0.5–0.6 V vs. Li, and the coulombic efficiency is 96–98% after 10 cycles. The observed galvanostatic cycling, cyclic voltammetry, and ex situ XRD data are interpreted in terms of the alloying–de-alloying reaction of Sn in the nano-composite “Sn-VO _x -Li₂O” with VO _x acting as an electronically conducting matrix.     

Structural features of bismuth borates in the system <Emphasis Type="Italic">n</Emphasis>Bi<Subscript>2</Subscript>O<Subscript>3</Subscript>-<Emphasis Type="Italic">m</Emphasis>B<Subscript>2</Subscript>O<Subscript>3</Subscript>

The crystal structures of general composition nBi₂O₃-mB₂O₃ were analyzed and systematized with the use of the structures of borate groups. Based on the CNs calculated by the bond valence method, the shapes of bismuth coordination polyhedra derived from an octahedron were suggested. A correlation was found between the number of BO₃ triangles and BO₄ tetrahedra in borate groups, the average CN of Bi atoms, and the degree of distortion of Bi polyhedra as a function of the m: n ratio, as well as between the polarity of BO₄ tetrahedra and noncentrosymmetry of the structures. The role of Bi³⁺ with the active E pair in the manifestation of specific features of the forms of bismuth polyhedra and the types of connection of boron polyhedra was elucidated.     

Electrodeposition of Fe<Subscript> <Emphasis Type="Italic">x</Emphasis> </Subscript>Se<Subscript> <Emphasis Type="Italic">y</Emphasis> </Subscript> films from acidic solutions

Iron selenide (Fe_xSe_y) thin films were electrodeposited on a glassy carbon electrode (GCE) surface under constant potential and pulse potential modes. The deposition mechanism was investigated using cyclic voltammetry. Electrochemical processes at room temperature are accompanied by adsorption of selenium on the electrode surface and complicated by chemical reactions in the solution bulk. Several approaches to control the film stoichiometry were applied: varying of electrodeposition potential; the use of elevated temperatures (60–80°C) to decrease the electrode passivation and electrodissolution of interfering elements under pulse mode. The composition of Fe_xSe_y thin films was analyzed using an energy dispersive X-rays (EDX) analysis.     

Si<Subscript>1-<Emphasis Type="Italic">x</Emphasis></Subscript>Ge<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>/Si Interface Profiles Measured to Sub-Nanometer Precision Using uleSIMS Energy Sequencing

The utility of energy sequencing for extracting an accurate matrix level interface profile using ultra-low energy SIMS (uleSIMS) is reported. Normally incident O₂ ⁺ over an energy range of 0.25–2.5 keV were used to probe the interface between Si_0.73Ge_0.27/Si, which was also studied using high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). All the SIMS profiles were linearized by taking the well understood matrix effects on ion yield and erosion rate into account. A method based on simultaneous fitting of the SIMS profiles measured at different energies is presented, which allows the intrinsic sample profile to be determined to sub-nanometer precision. Excellent agreement was found between the directly imaged HAADF-STEM interface and that derived from SIMS.

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Graphical Abstract ?

     

Degradation of the Pt-MoO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> composite catalyst in methanol solutions

Pt-MoO _x deposits synthesized by an electrochemical method are shown to be instable in methanol solutions at potentials ≥0.3 V. It is assumed that molybdenum oxides in the deposit composition react with methanol. The reduction of molybdenum compounds with methanol to form soluble Mo(+3) complexes in the presence of platinum is confirmed by spectrophotometric data. The MoO _x reaction with methanol leads to the removal of molybdenum compounds from the electrode surface, which is accompanied by the loss of its catalytic activity in the methanol oxidation reaction.     

Biomimetic synthesis of the arachidic acid/Ag<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Cd<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>S nanocomposite films

Ag _x Cd _y S nanoparticles were obtained in arachidic acid (AA) monolayer containing Ag⁺ and Cd²⁺ under H₂S flow. The AA/Ag _x Cd _y S monolayers were deposited onto solid substrate to prepare LB films. The UV-vis spectrum showed that the LB film exhibited notable quantum-size effect. The small-angle X-ray diffraction revealed periodic structure of the LB films. The molar ratio of Ag to Cd in AA/Ag _x Cd _y S film was ca. 1 : 5 as measured by the XPS. TEM and FTIR spectroscopy showed that the head-groups of arachidic acid molecules controlled formation of Ag _x Cd _y S nanoparticles in the monolayer.     

A study of structural non-stoichiometry with respect to oxygen in <Emphasis Type="Italic">R</Emphasis>BaCo<Subscript>4</Subscript>O<Subscript>7+<Emphasis Type="Italic">x</Emphasis></Subscript> single crystals

Impurity-free RBaCo₄O_7+x (R = Y, Lu) crystals are grown and the evolution of their structure with varying oxygen concentration in the range 0 ≤ x ≤ 1.3 is studied. The structural features of the studied samples required to develop an extended strategy of the X-ray crystallographic experiment, which made it possible to obtain more accurate data on unit cell parameters and additional information on the features of superstructure modulation and diffuse scattering, along with the phase composition of crystalline samples. A crystal chemical analysis of the known structural models of RBaCo₄O_7+x is performed and suggestions are made about possible structural changes occurring when the oxygen concentration increases up to the limit.     

Conformations and properties of <Emphasis Type="Italic">O</Emphasis>-Alkyl-<Emphasis Type="Italic">S</Emphasis>-(2-<Emphasis Type="Italic">N</Emphasis>,<Emphasis Type="Italic">N</Emphasis>-dialkylamino)-ethylmethylthiophosphonates

Conformers of the biologically active compounds CH₃P(O)(OR)(SCH₂CH₂NR ₂ ^′ ), where (I) R = i-C₄H₉, R′ = C₂H₅ and (II) R = C₂H₅, R′ = i-C₃H₇, are calculated within the AM1 level of theory. The elongated and twisted forms with maximum and minimum distances between a nitrogen atom and those of a phosphorus tetrahedron, respectively, and bearing a syn and anti oriented alkoxy group relative to a phosphoryl oxygen, are studied. It is found that the differences between the energy, electronic, and geometric parameters of these forms are apparent in differences between their properties, e.g., the ability to participate in complexation and protonation, reactions that to some extent simulate the interaction between a substance and a biological object.     

<Emphasis Type="Italic">N</Emphasis>-doped carbon supported Pd catalysts for <Emphasis Type="Italic">N</Emphasis>-formylation of amines with CO<Subscript>2</Subscript>/H<Subscript>2</Subscript>

Using mesoporous N-doped carbons (NCs) derived from glucose and melamine as the supports, a series of Pd/NC catalysts were prepared, in which Pd nanoparticles with average size<2.0 nm were uniformly distributed on the supports. It was indicated that the resultant Pd/NC catalysts were effective for N-formylation of amines with CO₂ and H₂ in ethanol without any additives. Especially, the catalyst Pd/NC-800-6.9% containing quaternary N showed the best performance, affording a series of formylamides in good or even excellent yields. Further investigation reveals that the interaction between the Pd nanoparticles and quaternary nitrogen in the NC support was responsible for the good performance of the catalyst.     

Phase transitions of the ZrNiH<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> hydride

The hydrogen pressure-ZrNiH _x hydride composition isotherms were determined under isoplethic experiment conditions during the desorption and sorption of hydrogen. According to the experimental data, the ZrNiH _x -H₂ system (0 < x < 2.229) experienced two phase transitions, hydrogenolysis with the absorption of large amounts of hydrogen and the reverse transition to the initial state. It was found that the β phase existed in the ZrNiH _x -H₂ system at least up to 410°C, and the region of its homogeneity was shifted toward the α solution. The homogeneity region of the γ phase at 250°C on the side depleted of hydrogen extended to the composition x = 1.50, and the temperature of its peritectoid decomposition was above 250°C. The hydrogenolysis reaction proceeded with the formation of the ?-ZrH_3.153?x -NiZr_0.7H_0.3x system, which had a much higher (higher by 200–300 torr) partial hydrogen pressure over the temperature range 0–240°C, that is, was unstable with respect to the initial system.     

Synthesis of <Emphasis Type="Italic">O</Emphasis>,<Emphasis Type="Italic">O</Emphasis>-Dialkyl <Emphasis Type="Italic">S</Emphasis>-(1,1-dimethyl-2-oxoethyl) dithiophosphates and their reactions with N-nucleophiles

O,O-Dialkyl S-(1,1-dimethyl-2-oxoethyl) dithiophosphates, 2-(dialkoxythiophosphorylthio)-substituted aldehydes with branched carbon groups, were obtained by hydrolysis of phosphoryl-containing iminium salts. The reaction with primary amines results in imines containing acetal group in different positions. Tri- and tetra-substituted perhydro-1,3-diazoles and oxazoles containing a phosphorus atom in the side chain were prepared by reacting the aldehydes with O,Nand N,N-dinucleophiles.