Mechanism of the reduction of molecular nitrogen to hydrazine by niobium (iii) hydroxide

The effect of the concentration of water on the rate of reduction of molecular nitrogen to hydrazine by niobium(iii) hydroxide in alkaline H₂O−MeOH and D₂O−MeOD mixtures was studied. In both cases, the reaction rate is maximum when [H₂O]=4 mol L⁻¹, and the inverse isotopic effect (K ^D/k ^H>1) is observed when [H₂O]<20 mol L⁻¹. Similar regularity was observed for the reaction of hydrogen elimination. It was found that HD is formed in the H₂O−MeOH system in the presence of D₂. The conclusion was made that the ratedetermining stage in hydrazine formation is the transfer of a hydride ion to the dinitrogen molecule coordinated to the binuclear Nb^III center. A kinetic scheme satisfactorily explaining the effect of the concentration of water ([H₂O]=1.5−49.0 mol L⁻¹) on the reaction rate constant was proposed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 9, pp. 1600–1604, September, 1997.     

Catalytic reduction of molecular nitrogen in solutions

Reports on nitrogen fixation in solution are reviewed. The optimum catalyst is the polynuclear complex. The reaction proceeds as a multielectron process, and the limiting step involves the electron transfer from a reducing agent.     

Acetylene reduction by hydroxides of transition metals

Ethylene and ethane are formed during reduction of acetylene by aqueous hydroxides of Ti(III), V(II), Cr(II) and Mo(III). Under these conditions ethylene is not reduced as a rule, which indicates direct formation of ethane from acetylene with the participation of four electrons of a reducing agent. The results are explained by the collective effect of metal ions in the hydroxides, leading to four-electron reduction of a substrate within the coordination sphere.

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Ti(III). V(II), Cr(II) Mo(III) . , , , . , .

     

Reduction of nitrogen oxides with ammonia over complex vanadium and chromium oxides

Catalytic properties of -Al₂O₃ -supported complex vanadium and chromium oxides V_2–x Cr _x O_5– (0 < × s 1.3), amorphous to X-rays, in the reduction of nitrogen oxides with ammonia were studied. Vanadium exists in these catalysts mostly in a pentavalent state and chromium exists as Cr³⁺ and Cr⁶⁺. As the content of chromium in the catalysts increases, the optimal temperature of the process decreases, and the degree of conversion of nitrogen oxides increases.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 2, pp. 339–342, February, 1996.     

Activation and functionalization of molecular nitrogen by metal complexes

Molecular nitrogen is intrinsically unreactive, so much so that it has confounded chemists for decades in attempts to functionalize this abundant diatomic molecule. While biological systems and industrial processes can fix nitrogen to form ammonia, the challenge is to discover a process that involves a homogeneous catalyst that can utilize N(2) as a feedstock to generate higher value organonitrogen materials. In this review, the activation of molecular nitrogen by transition metal complexes is reviewed with the view to present new kinds of transformations for coordinated dinitrogen. Moreover, some reaction types that are as yet unknown are outlined to try and stimulate further research in this area.     

Coprecipitation with metal hydroxides for the determination of beryllium in seawater by graphite furnace atomic absorption spectrometry

Coprecipitation first with magnesium hydroxide, next with tin(IV) hydroxide is developed for the determination of traces of beryllium in sea-water. To a 200-ml sample is added a sodium hydroxide solution to form magnesium hydroxide at pH 11.5, on which beryllium is quantitatively coprecipitated. The precipitate is separated by centrifugation and dissolved in 2 ml of 12 mol/l hydrochloric acid. The resulting solution (ca. 10 ml) is mixed with 2 mg of tin (IV) carrier and the pH is adjusted to 5.0 to collect the beryllium on tin (IV) hydroxide, leaving magnesium ions in the solution. The tin (IV) hydroxide is centrifuged, dissolved in 0.1 ml of 5 mol/l hydrobromic acid, and then diluted to 1 ml with water. Magnesium is so added as to be 500 g/ml for increasing the sensitivity about four times, and the beryllium in the solution is determined by graphite furnace atomic absorption spectrometry. The experiments with synthetic seawater samples showed that pg — g amounts of beryllium can be coprecipitated on the metal hydroxides and beryllium at the low ng/1 level can be determined with reasonable precision (RSD < 10%). The detection limit of the proposed method is 0.5 ng/l of beryllium in seawater.     

Investigations on the selectivity of catalytic no reduction by ammonia

The activity and selectivity of V₂O₅/γ-Al₂O₃ catalyst were studied in the catalytic reduction of nitrogen oxide by ammonia. The activity of the catalyst monotonically increases as a function of temperature, however, its selectivity decreases. The DeNOx reaction of nitrogen oxides with ammonia can be described well by a mathematical model, which considers selectivity-decreasing side reactions as well in a wide temperature range (220–420°C).     

Sorption properties of hydrogels of transition and p-metal oxide hydroxides

The experimental data on the sorption properties of hydrogels of Fe^III, Ti^IV, Zr^IV, Sn^IV, In^III, Cr^III, Sm^III, Co^II, and Zn^II oxide hydroxides are summarized. The data were obtained by the point of zero-charge method and by measurin the rates of the indicator reaction,viz., heterogeneous hydrolysis of the chloro complexes of platinum group metals. The compositions of the oxide hydroxides, the pH of the point of zero charge in various ionic media, and the rate constants of the indicator reactions were determined. The values obtained were analyzed in terms of their correlation with the charge, the radius, and the electronic struture of the central ion and with the composition and the preparation conditions of the oxide hydroxide. A data base that allows one to choose an appropriate oxide hydroxide sorbent for extraction of particular complex-forming metals from complexing media is presented. The data obtained confirm the model of the mechanism of heterogeneous hydrolysis and extend its predicting ability. The model was tested experimentally in relation to tin(IV) oxide hydroxide. Translated fromIzvestiya Akademii Nauk, Seriya Khimicheskaya, No. 2, pp. 228–238, February, 1999.     

A new fluorescent chemosensor for transition metal cations and on/off molecular switch controlled by pH

A new fluorescent chemosensor with imidazole as ionophore was synthesized by the selective derivation of calixarene, which can effectively recognize Cu²⁺ and Zn²⁺ leading to different fluoroscopic behaviors in CH₃OH-H₂O. This system could be considered as a molecular switch. By modulating the pH of the solution, on-off-on fluorescent switching is carried out upon combinatory addition of acid, base and Cu²⁺.     

Keggin结构过渡金属取代硼钨酸盐的电化学及电催化还原

采用循环伏安法,研究了Keggin结构过渡金属取代硼钨酸盐K7[MBW11(H2O)O39].H2O(简写为MBW11,M=Cu,Co,Ni)的电化学性质及电催化活性.结果表明,CuBW11在水溶液中表现出较为特殊的电化学性质,MBW11(M=Cu,Co,Ni)对亚硝酸根及碘酸根离子的还原反应具有较好的催化作用,取代金属对该体系的电催化活性有很大影响.     

用于抑制海洋微生物附着的过渡金属配合物研究进展

许多金属配合物具有杀菌、抗病毒和抗癌的活性,在医疗卫生等领域占有重要地位,其生物活性机理也得到了较深入系统的研究.虽然一些过渡金属配合物也在防止海洋微生物附着方面得到应用,但是专门从配位化学和生物无机化学的角度对其进行总结分析的研究还未见报道.以配体的不同类型对这类过渡金属配合物进行分类,简介了其用于防海洋微生物附着的实验室研究和实际应用情况,并对未来研究前景进行了展望.     

Investigations of the thermal transformations of precipitated mixed transition metal hydroxides

The thermal behaviour and phase composition of mixed oxides obtained by oxidation of iron(II) hydroxide in the presence of Mg, Zn, Co, Cu and Ni, is investigated by thermogravimetry, and conventional and high-temperature X-ray diffractometry.     

Selective catalytic reduction of nitrogen monoxide with propene over CuCl/MCM-41 catalysts

Selective catalytic reduction of nitrogen monoxide with propene over two kinds of CuCl/MCM-41 catalysts prepared by a dispersion method has been studied. It was found that CuCl/AlMCM-41 exhibits substantially higher activity over CuCl/SiMCM-41. Characterization of these samples by H₂-TPR, IR and XRD showed that the active copper species were mainly related to Cu²⁺ and Cu⁺ ions in CuCl/AlMCM-41 catalyst.     

Theoretical screening of the transition metal heteronuclear dimer anchored graphdiyne for electrocatalytic nitrogen reduction

Developing efficient electrocatalysts for nitrogen reduction reaction(NRR)is crucial to replace the both energy-intensive and environment-malignant Haber-Bosch process.Here using density functional theory calculations,we systematically studied the potential of the heteronuclear 3 d transition metal dimers anchored graphdiyne monolayers(FeM@and NiM@GDY,M=Ti,V,Cr,Mn,Fe,Co,Ni,and Cu)as efficient NRR catalysts.Among all the studied double-atom catalysts(DACs),FeCo@and NiCo@GDY are the most promising with excellent NRR catalytic activity,high ability to suppress the competing hydrogen evolution reaction(HER),and good stability.For both FeCo@and NiCo@GDY,NRR prefers to the distal pathway with the calculated onset potentials of -0.44 and -0.36 V,respectively,which are comparable and even better than their homonuclear counterparts.Moreover,FeCo@and NiCo@GDY have higher ability to suppress HER than Fe₂@ and Co₂@GDY,which may result from the modulated d state electronic structure due to the synergy effect of the heteronuclear atoms in the DACs.Our work not only suggests that FeCo@and NiCo@GDY hold great promises as efficient,low-cost,and stable DACs for NRR,but also further provides a strategy,i.e.alloying the atomic metal catalysts,to improve the NRR catalytic activity and/or selectivity.     

Computational investigation of MnIr and FeIr electrocatalyst for nitrogen reduction reaction

NH₃ is not only an important component of agricultural and industrial production, but also an extremely promising energy carrier and storage intermediate. Currently, the Haber-Bosch process used in industry for NH₃ production has shortcomings such as high energy consumption and low output. The electrocatalytic nitrogen reduction reaction (NRR) can improve the route and conditions of NH₃ synthesis through high-efficient electrocatalyst, and realize the production mode of high efficiency and low energy consumption. Therefore, the design and synthesis of the NRR electrocatalysts with high catalytic performance are very important. Here, the first principles calculation based on density functional theory was used to form alloy catalysts by using Mn and Fe atoms instead of nine Ir atoms on the surface of Ir(100), and the electrocatalytic performance of the NRR was systematically studied. The results showed that N₂ could be stably adsorbed on Mn₉@Ir(100) and Fe₉@Ir(100) in the side-on configuration. The possible reaction pathways were analyzed and discussed, and the enzymatic pathway was determined to be the best. Through the simulation of the entire NRR process, it was found that the limit potential was only −0.659 and −0.647 V for Mn₉@Ir(100) and Fe₉@Ir(100). In addition, the electronic properties of Mn₉@Ir(100) and Fe₉@Ir(100) were analyzed utilizing charge density difference and density of states, and the reasons for their high activity were obtained. We hope this work can not only reduce the number of noble metals and develop highly active catalysts, but also provide theoretical support and guidance for the catalytic mechanism of alloy electrocatalysts.     

Conformational searching of transition metal compounds

To date, no conformational search of inorganic complexes has been reported that searches for all the conformations and configurations available to the complex. This is due to the various coordination geometries that transition metal ions can adopt and the difficulties in conducting conformational searches with systems that have connected ring systems, such as the ones formed when a metal ion binds a multidentate ligand. Using three test complexes {[Co(dien)₂]³⁺, [Co(dien)(dpt)]³⁺, and [Co(hexamethylcyclam)(Cl)} the ability of the random kick (Cartesian stochastic Monte Carlo search) method and the Monte Carlo dihedral and positional method to find all conformations and geometric isomers was tested (dien, diethylenetriamine; dpt, di(3-aminopropyl)amine; hexamethylcyclam: tet-a, meso-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane; tet-b, racemic-5,5,7,12,12,14-hexamethyl-1,4,8,11-tetraazacyclotetradecane). Both methods are significant improvements on the current method by which all possible isomers are entered graphically and minimized individually. The major difficulty that was encountered was how to differentiate between the large number of similar conformations found. ©1999 John Wiley & Sons, Inc. J Comput Chem 20: 1549–1558, 1999     

过渡金属替代的CeO_2(111)表面上NO+CO反应机理的理论研究(英文)

采用DFT+U方法研究了过渡金属替代的CeO2(111)表面上的NO+CO反应机理,以探求不同过渡金属对N2选择性的影响.结果表明,在反应过程中,反应活性中心由过渡金属单原子与其最近邻的氧空位组成.NO在过渡金属-氧空位上发生N–O断键,不同过渡金属上该还原步骤的难易程度不同.计算发现,右过渡金属Rh,Pd和Pt替代的CeO2(111)表面可以与吸附物之间形成较强的吸附作用,进而可以达到较高的N2选择性.其主要原因是右过渡金属具有较多的d电子,可以与吸附小分子之间形成有效的反馈键.而左过渡金属拥有较少的d电子,难以有效抓住吸附物,最终导致较低的N2选择性.