Theoretical study of the energies and activation barriers of elementary reactions of hydrogenation of the Ti-doped closo-aluminide cluster Al@TiAl11 and its anion Al@TiAl 11?

The potential energy surfaces, energies E, and activation barriers h of elementary reactions of addition of an H₂ molecule to the Ti-doped closo-aluminide cluster Al@TiAl₁₁ and its anion Al@Ti₁₁⁻ with an icosahedral and marquee structure in the states with different multiplicity were calculated within the B3LYP approximation of the density functional theory using the 6–31G* and 6–311+G* basis sets. The results were compared with the data calculated at the same level of theory for the related reactions of hydrogenation of bare closo-aluminides Al₁₃ and Al₁₃⁻ and their B-, C-, Si-, and Ge-doped derivatives. The computations demonstrated that, depending on the structure, charge, and multiplicity of the Al@TiAl₁₁ cluster, the hydrogenation energy varies in the range 15–23 kcal/mol. At the first stage of addition (chemisorption) of H₂, a μ-H₂ complex at the Ti atom (intermediate) forms with the distance R(Ti-H₂) ∼ 1.9–2.0 ?, which is accompanied by an energy decrease of ∼4–10 kcal/mol. The H-H bond in the μ-H₂ complex is ∼0.1 ? longer and the stretching vibration frequency V_val(HH) is ∼700–1500 cm⁻¹ (or more) lower than the corresponding characteristics of the isolated H₂ molecule. In the transition state with an imaginary frequency of ∼600i–1100i, the H₂ molecule is coordinated to the attacked edge Ti-Al_r, and its length increases to ∼0.9–1.1 ?. The activation barrier height h varies from a few kcal/mol to ∼8–10 kcal/mol when measured from the μ-H₂ complex and is within 18–22 kcal/mol when measured from the product (dihydride Al@TiAl₁₁H₂). The latter barrier (to the reverse reaction of dehydrogenation) is considerably higher than the barriers to migration of hydrogen atoms around the metal cage in the Al@TiAl₁₁H₂ dehydrides. There is a correlation between the energies E and barriers h of hydrogenation reactions and the structure, external charge, and multiplicity of the Al@TiAl₁₁ cluster. In all cases, the hydrogenation should occur significantly more readily than dehydrogenation. It was shown that these reactions can be both irreversible (for example, for an icosahedron in the singlet state) and reversible (for a marquee in the triplet state and others). The conclusion was drawn that the elementary reactions of hydrogenation and dehydrogenation for Ti-doped aluminides should occur considerably faster and under milder conditions than for bare aluminides or their analogues doped with main-group atoms. Original Russian Text ? V.K. Charkin, O.P. Kochnev, N.M. Klimenko, 2009, published in Zhurnal Neorganicheskoi Khimii, 2009, Vol. 54, No. 8, pp. 1345–1354.     

Electrocatalytic construction of the C–N bond from the derivates of CO2 and N2

Electrochemical synthesis of C-N bond-containing compounds(e.g.,urea,amino acid,amide,amine,and their derivates)from CO₂/N₂and their derivates is emerging as a promising sustainable strategy[1-7].CO₂and its derived products,CO,HCOOH,(COOH)₂,etc.,could serve as carbon sources(Figure 1)[8].N₂,making up 80%of air,is an appealing nitrogen source.However,the low solubility of N2 and the high dissociation energy for the N≡N bond limit its application.     

Role of support nature （γ-Al2O3 and SiO2-Al2O3） on the performances of rhenium oxide catalysts in the metathesis of ethylene and 2-pentene

The metathesis of ethylene and 2-pentene was studied as an alternative route for propylene production over Re2O7/γ-Al2O3 and Re2O7/SiO2-Al2O3 catalysts. Both NH3 temperature-programmed desorption (NH3-TPD) and H2 temperature-programmed reduction (H2-TPR) results showed that Re2O7/SiO2-Al2O3 exhibited stronger acidity and weaker metal-support interaction than Re2O7/γ-Al2O3. At 35 60℃, isomerization free metathesis was observed only over Re2O7/γ-Al2O3, suggesting that the formation of metal-carbene metathesis active sites required only weak acidity. Our results suggest that on the Re2O7/SiO2-Al2O3, hydrido-rhenium species ([Re]-H) were formed in addition to the metathesis active sites, resulting in the isomerization of the initial 1-butene product into 2-butenes. A subsequent secondary metathesis reaction between these 2-butenes and the excess ethylene could explain the enhanced yields of propylene observed. The results demonstrate the potential for high yield of propylene from alternative feedstocks.     

α-Iodination of enaminones using the modified Johnson's procedure: the use of I2 and Et3N

Facile synthesis of α-iodo enaminones 2 was carried out by the use of iodine and triethylamine in methylene chloride in short time in excellent yields.     

H/D exchange between CD4 and the OH-groups of H-zeolites and γ-Al2O3

The H/D exchange between CD₄ and OH-groups was studied over zeolite H-ZSM-5, H-mordenite, H-beta and over -Al₂O₃ using the pulsed microcatalytic method. The apparent activation energy of the exchange was found to be independent of the Brönsted acidity of the solid. Results suggested that methane was activated for the reaction by dissociative adsorption over Lewis acid-Lewis base pair sites.     

Acid-base properties of the surface of the α-Al2O3 suspension

The distribution of the acid-base centers on the surface of α-Al₂O₃ suspension particles was studied by potentiometric titration, and the corresponding pK spectra were constructed. It was inferred that the double electric layer created by the supporting electrolyte substantially affected the screening of the acid-base centers on the particle surface of the suspension.     

A Raman spectroscopic study of the XeOF4/XeF2 system and the X-ray crystal structure of α-XeOF4·XeF2

The mixed oxidation state complexes, α-XeOF₄·XeF₂ and β-XeOF₄·XeF₂, result from the interaction of XeF₂ with excess XeOF₄. The X-ray crystal structure of the more stable α-phase shows that the XeF₂ molecules are symmetrically coordinated through their fluorine ligands to the Xe(VI) atoms of the XeOF₄ molecules which are, in turn, coordinated to four XeF₂ molecules. The high-temperature phase, β-XeOF₄·XeF₂, was identified by low-temperature Raman spectroscopy in admixture with α-XeOF₄·XeF₂; however, the instability of the β-phase precluded its isolation and characterization by single-crystal X-ray diffraction. The Raman spectrum of β-XeOF₄·XeF₂ indicates that the oxygen atom of XeOF₄ interacts less strongly with the XeF₂ molecules in its crystal lattice than in α-XeOF₄·XeF₂. The ¹⁹F and ¹²⁹Xe NMR spectra of XeF₂ in liquid XeOF₄ at −35 °C indicate that any intermolecular interactions that exist between XeF₂ and XeOF₄ are weak and labile on the NMR time scale. Quantum-chemical calculations at the B3LYP and PBE1PBE levels of theory were used to obtain the gas-phase geometries and vibrational frequencies as well as the NBO bond orders, valencies, and NPA charges for the model compounds, 2XeOF₄·XeF₂, and XeOF₄·4XeF₂, which provide approximations of the local XeF₂ and XeOF₄ environments in the crystal structure of α-XeOF₄·XeF₂. The assignments of the Raman spectra (−150 °C) of α- and β-XeOF₄·XeF₂ have been aided by the calculated vibrational frequencies for the model compounds. The fluorine bridge interactions in α- and β-XeOF₄·XeF₂ are among the weakest for known compounds in which XeF₂ functions as a ligand, whereas such fluorine bridge interactions are considerably weaker in β-XeOF₄·XeF₂.     

Effect of oxygen non-stoichiometry on the structural and magnetotransport properties of LaMn0.85Cr0.15O3+δ

Thermogravimetry, X-ray diffraction (XRD), d.c. magnetization, high-temperature susceptibility and electrical resistivity measurements were performed for LaMn_0.85Cr_0.15O_3+δ perovskites with accurate control of the oxygen content (0?δ?0.11). For 0?δ<0.09, three orthorhombic structures (Pnma) are found: for 0?δ<0.045, the O′ phase (b/√2<c<a), for 0.045?δ<0.06, the O″ (b/√2<a<c) and for 0.06?δ<0.09, the O^? (a<b/√2<c). For 0.09?δ?0.11, a rhombohedral symmetry coexists with O^? in a biphasic field. Magnetic measurements revealed the ferromagnetic interactions (FM) character of the Mn³⁺-O-Cr³⁺ interaction, but also the intricate magnetic phase diagram due to the presence of multiple interactions (Mn^3+,4+-O-Mn^3+,4+, Cr³⁺-O-Mn³⁺, etc.). The comparison of the results for LaMn_0.85Cr_0.15O_3+δ with those of LaMn_0.9Cr_0.1O_3+δ allows discuss the role of Cr³⁺ on the structural, magnetic and magnetotransport properties of the LaMn_1−xCr_xO_3+δ perovskites.     

Influence of preparation on the structure and co conversion of γ-Bi2MoO6 catalysts

The effect of treatment conditions during preparation and activation of γ-Bi₂MoO₆ catalysts was investigated. Catalytic evaluation shows that CO conversion is much higher for catalysts obtained under vapor controlled evaporation conditions (CE) than for standard evaporation catalysts (SE). Characterization results show that preparation under SE conditions induces segregation of Bi₂O₃ on the surface of Bi₂MoO₆ catalysts decreasing their catalytic activity. Catalysts treatment under hydrogen reactivates the catalytic properties of SE samples.     

Effects of the support crystallite size and the reduction temperature on the properties of Pd/α-Al2O3 catalysts in selective acetylene hydrogenation

Pd catalysts supported on the solvothermal-derived nanocrystalline α-Al₂O₃ (45 nm) exhibited superior performances in the selective acetylene hydrogenation than those supported on micron-sized ones (44–149 μm). Reduction at 500°C led to an improvement of the ethylene yield for the Pd/nanocrystalline α-Al₂O₃, but not for the Pd/micron-sized α-Al₂O₃.     

Gas-phase measurements of the surface basicity of AlPO4?TiO2 and AlPO4?ZrO2 catalysts

The amount and strength of basic sites of AlPO₄–TiO₂ and AlPO₄–ZrO₂ catalysts over a different range of AlPO₄/metal oxide weight ratios were measured by studying the adsorption of acid molecules (acrylic acid and phenol) in the gas phase (473–673 K) by using the gas-chromatographic pulse method. The results obtained show that the basicity of AlPO₄–TiO₂ and AlPO₄–ZrO₂ catalysts is far lower than that of pure AlPO₄, and with an increase in the metal oxide (TiO₂ or ZrO₂) weight ratio, the basicity decreases. Besides, the basicity of AlPO₄–ZrO₂ is fairly low compared with that AlPO₄–TiO₂. In both cases, the total basicity (measured at 473 K vs. acrylic acid) gradually decreases with the calcination temperature while the stronger basic sites (measured at 573 K vs. phenol) remained unchanged up to calcination temperatures of 1073 K. Some weak surface basic sites remained in catalysts pretreated at 1273 K.

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- AlPO₄–TiO₂ AlPO₄–ZrO₂ AlPO₄/ , ( ) (473–673 K). , AlPO₄–TiO₂ AlPO₄–ZrO₂ AlPO₄ TiO₂ ZrO₂. , AlPO₄–ZrO₂ AlPO₄–TiO₂. — 473 K — , , 573 K , 1073 K. , 1273 K, .

     

Benzidine and diphenylamine radical formation on the surface of NaY-zeolite and γ-alumina with and without V2O5

Solutions of benzidine and diphenylamine in benzene produce cation radicals on the surface of zeolite, -alumina and V₂O₅ doped alumina and zeolite. The sequence of the electron acceptor site strength on the surface is as follows:     

Formation and properties of the new Al8V10W16O85 and Fe8?xAlxV10W16O85 phases with the M–Nb2O5 structure

A new, previously unknown phase Al₈V₁₀W₁₆O₈₅ has been obtained from reaction taking place in the solid state. It forms continuous solid solution with Fe₈V₁₀W₁₆O₈₅ of the Fe_8−x Al _x V₁₀W₁₆O₈₅ general formula. All these phases are isostructural with M–Nb₂O₅ and (W_0.35V_0.65)₂O₅ and belong to a block structure phases with ReO₃ type blocks of 4 × 4×∞ dimensions. Al₈V₁₀W₁₆O₈₅ is tetragonal and has the lattice constants a = b = 1.9487(1) nm and c = 0.36706(4) nm. It melts incongruently at 1,183 K depositing Al₂(WO₄)₃ and WO₃. The increase of the Al³⁺ ions content in the crystal lattice of Fe₈V₁₀W₁₆O₈₅ causes the melting point increasing, and decreasing of a = b unit cell parameters with c being almost constant. IR spectra of Al₈V₁₀W₁₆O₈₅ and Fe_8−x Al _x V₁₀W₁₆O₈₅ phases have been recorded.     

Synthesis of High-purity Fe2AlB2 and the Effect as Sintering Additive for ZrB2

Fe₂AlB₂ powder material was prepared by the direct reaction of iron,aluminum and boron powders in a tubular furnace.The effects of different Al contents,temperature and raw material pretreatment on the purity of product were studied.The mixed powder with the stoichiometric ratio of 1.5Al/2Fe/2B was processed by CIP (Cold Isostatic Pressing),and then calcined at 1150℃ for 120 min.The product containing a small amount of impurities is treated with alkaline solution to obtain high-purity Fe₂Al B₂ powder.Zr B₂-Fe₂Al B₂ composite ceramic was successfully prepared at 1250℃ by hot pressing sintering.The density,hardness and fracture toughness were 96.2%,22±0.3 GPa and 5.78±0.5 MPa·m^1/2,respectively.     

Influence of MgO loading on the structure and catalytic performance of Pd/γ-Al2O3, I. Effect of interaction between MgO and γ-Al2O3

The influence of MgO dispersed on -Al₂O₃ in different amounts on the structure and performances of Pd/-Al₂O₃ catalysts has been studied by means of XRD, H₂–O₂ titration, BET and catalytic activity test for CO oxidation. It was found that introduction of MgO enhanced greatly the CO oxidation activity of catalyst. It seems that the enhanced activity stems from the stronger interaction between MgO and -Al₂O₃ at a given temperature (e.g. 450 °C).     

Structure and physical properties of the new telluride BaAg2Te2 and its quaternary variants BaCuδAg2-δTe2

The new materials BaCu_δAg_2-δTe₂ (0?δ?2) were prepared from the elements at 800 °C in evacuated silica tubes. BaAg₂Te₂ crystallizes in the α-BaCu₂S₂ type, space group Pnma, with lattice parameters a=10.8897(3) Å, b=4.6084(1) Å, c=11.8134(3) Å (Z=4). The structure consists of a three-dimensional network of vertex- and edge-condensed AgTe₄ tetrahedra, which includes the Ba²⁺ cations in linear channels running along the short b-axis. Half of the Ag atoms participate in an Ag atom zigzag chain extended parallel to the channels. BaAg₂Te₂ is a p-type semiconductor with large Seebeck coefficient. Within the series BaCu_δAg_2−δTe₂, the electrical conductivity increases and the Seebeck coefficient decreases strongly with increasing Cu content.     

1H NMR refinement of the structure of the guest sublattice and molecular dynamics in the ultrathin channels of [Zn2(C8H4O4)2(C6H12N2)]·n(H3C)2NCHO

Localization and molecular mobility of the ligands ([C₈H₄O₄]²⁺ and [C₆H₁₂N₂]⁰) of the host lattice and (CH₃)₂NCHO dimethyl formamide guest molecules in the inclusion compound [Zn₂(C₈H₄O₄)₂(C₆H₁₂N₂)]·n(H₃C)₂NCHO were studied on the basis of ¹H NMR data. At room temperature, the longest axes of the dimethyl formamide guest molecules are ordered in parallel to the C ₄ symmetry axes, and the symmetry planes of these molecules are disordered, while preserving the tetragonal crystal system of the inclusion compound. At lower temperatures, a phase transition takes place in view of the ordering in the guest sublattice. Original Russian Text Copyright ? 2009 by A. V. Sabylinskii, S. P. Gabuda, S. G. Kozlova, D. N. Dybtsev, and V. P. Fedin __________ Translated from Zhurnal Strukturnoi Khimii, Vol. 50, No. 3, pp. 443–450, May–June, 2009.     

Selective solution-phase synthesis of BiOCl, BiVO4 and δ-Bi2O3 nanocrystals in the reaction system of BiCl3-NH4VO3-NaOH

In this study, we demonstrate a straightforward solution-phase method for the selective synthesis of BiOCl, BiVO₄ and δ-Bi₂O₃ nanocrystals by simply manipulating the reaction temperature and the BiCl₃-to-NaOH mole ratio in the reaction system of BiCl₃-NH₄VO₃-NaOH. The experimental results revealed that BiOCl, as the sole product, was prepared when designating the reaction temperature ranging from room temperature to 100 °C, regardless of the BiCl₃-to-NaOH mole ratio; on the other hand, BiOCl, BiVO₄, and δ-Bi₂O₃ nanocrystals could be selectively prepared at 140-180 °C, depending on the BiCl₃-to-NaOH mole ratio in solution. Significantly, we first report on fabricating δ-Bi₂O₃ sample, the high-temperature cubic phase commonly stabilized at 730-824 °C, at the low reaction temperature of 140-180 °C under solution-phase synthetic conditions. In addition, the δ-Bi₂O₃ sample exhibits strong emission at room temperature.     

The effect of preparation method on the defect structure and luminescence properties of γ-Al2O3

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Intermolecular cycloalumination of cyclic and acyclic alkynes with Et n AlCl3? n in the presence of Cp2ZrCl2

Intramolecular cycloalumination of cyclic and acyclic alkynes with Et _n AlCl_3−n (n = 0, 1) in the presence of Cp₂ZrCl₂ gave previously unknown unsaturated bi-and tricyclic organoaluminum compounds in up to 80% yield. Original Russian Text ? V.A. D’yakonov, L.F. Galimova, A.G. Ibragimov, U.M. Dzhemilev, 2008, published in Zhurnal Organicheskoi Khimii, 2008, Vol. 44, No. 9, pp. 1308–1312.