Methane conversion by various metal, metal oxide and metal carbide catalysts

The progress in the field of methane conversion into higher hydrocarbons including aromatics and oxygenated compounds in the recent five years will be reviewed shortly, together with a new type of the methane conversion reaction with carbon monoxide at lower temperatures (600–700 K) by supported group VIII metal catalysts. Benzene was formed selectively among hydrocarbons in the CH₄–CO reaction over silica-supported Rh, Ru, Pd and Os catalysts under atmospheric pressure. Both CH₄ and CO were required for benzene formation, and only ethane and ethylene were formed besides benzene. The amount of C₃–C₅ hydrocarbons was negligible, which suggests that a completely different mechanism from the CO–H₂ reaction may be operating over these catalysts despite of the similarity in the reaction conditions with the CO–H₂ reaction. The mechanism of benzene formation was studied deeply by means of kinetical investigation as well as infrared spectroscopy and isotopic tracer method in connection with that of CO hydrogenation.     

Electrochemistry of metal adlayers on metal chalcogenides

Electrodeposition of metal adlayers on semiconductor metal chalcogenides (CdSe, CdS, PbTe, PbSe, PbS, Bi₂Te₃) is reviewed. Cathodic underpotential deposition of metal adlayer on metal chalcogenide is the electrochemically irreversible surface limited reaction. The irreversibility of the upd increases in the row from tellurides to selenides and further to sulfides. The underpotential shift on chalcogenide nanoparticles increases with particle size. Metal upd on chalcogenides is applied as a means of measurement of electroactive surface area of chalcogenide electrodes. The method is especially advantageous for multicomponent systems with other component not supporting upd, such as CdSe-TiO₂, CdSe-ZnO. Differences of voltammetric profiles of Pb upd on Bi₂Te₃ and Te are applied for detection of Bi₂Te₃ surface contamination by elemental tellurium. The further tasks in the electrochemistry of metal adlayers are their incorporation as interlayers in layered chalcogenides and electrodeposition of superlattices.

Graphical abstract

     

Three-coordinate metal centers in extended transition metal oxides

The n = 3 Ruddlesden-Popper phase Sr3LaFe1.5Co1.5O10+/-delta is capable of sustaining O contents as low as O7.5 with a mean metal oxidation state of +2 and three coordination at the central site in the trilayer of originally octahedral transition metal sites. The shortening of the axial bonds to the flanking octahedral layers stabilizes the low oxidation state and consequent unusual low coordination number of the Fe2+ and Co2+ cations within the extended structure.     

Kinetics of dissolution of metal powders and metal oxides

Current approaches to study of the kinetics of the dissolution of metallic powders and metal oxides are discussed. The applicability of the various kinetic equations to the rate of a topochemical reaction is discussed. It is determined by features of the appearance and growth of the nuclei of the solid product, by the change in the size of the interface of the solid phases, and by the effect of the reaction product. For the case of the kinetics of reaction of copper, nickel, cobalt, zinc, cadmium, and lead powders and their oxides with aqueous and nonaqueous solutions of ammonium salts it was shown that the conditions of formation, the properties, and the behavior of the product layer formed on the surface of the reacting particle can have a determining effect on the kinetics and mechanism of the reaction. The results from the kinetic experiment are examined in terms of a model described by the generalized topochemical equation =1 – exp(–ktⁿ) and also a model based on an analysis of the reaction rate constants at various sections of the kinetic curve. Correlations were established between the coefficient n in the generalized topochemical equation and also the rate constant and density or solubility of the reaction product formed on the surface of the reacting particle.Translated from Teoreticheskaya i Éksperimental'naya Khimiya, Vol. 31, No. 5, pp. 284–297, September–October, 1995.     

On-line dialysis of some metal ions and metal complexes

The effect of ionic strength, pH and complexing ligands on the dialysis of metal ions, particularly zinc(II), through cellulose acetate membranes, was studied under flow conditions. The dialysis factor, which depends on both the mass transfer and the membrane permeability, was found to be independence of ionic strength in the interaval 0.05–0.3 M and to increase only slightly with pH 4.6 and 7.0. Some common buffer constituents had no effect, but chloride and calcium ions affected the transfer rates. The rate of transfer of the ligands histidine, NTA and EDTA was of the same order of magnitude as that of the metal ions. The transfer rate of the Ni(II)-EDTA complex was the same as that of a mixture of Ni(II) and EDTA. Generally, addition of the chelating agents decreased the metal-ion transfer rates. Partition coefficients between the membrane polymer and the buffers were evaluated and compared with the dialysis factors and there were significant differences with regard to the range of variation, effect of concentration and pH-dependence. The time scale of uptake or release of metal ions by the polymer is much longer than that of a steady-state transfer in dialysis. Partition and dialysis may not be directly related to each other.     

Polyoxometalate tri-supported transition metal complexes containing mixed-valent transition metal ions

Three compounds, [AsMo₈V₆O₄₂][Cu(2,2?-bpy)₂]₂[Cu(2,2?-bpy)]·4H₂O (1), [PMo₈V₆O₄₂][Cu(2,2?-bpy)₂]₂[Cu(2,2?-bpy)]·3H₂O (2) and [PMo₈V₆O₄₂][Cu(2,2?-bpy)₂]₂[Cu(2,2?-bpy)]·3.5H₂O (3), have been synthesized under hydrothermal conditions and characterized by IR, UV–vis, XRD, TG, elemental analysis, and X-ray diffraction analysis. Single-crystal X-ray structure analysis reveals that 1 and 2 are isostructural and isomorphous, whereas 2 and 3 are polymorphs. Polymorphs of 1 have not been synthesized yet. The mixed-valent transition metal ion in 1–3 has been further confirmed by TG analyses. Catalytic properties of 1 and 2 have also been studied.     

Thermal investiongation of metal fluoberyllate hydrates and metal fluoride hydrates

The thermal decompositions of MBeF₄ · H₂O and MF₂ · x H₂O, where M = Ni(II), Co(II) or Cu(II) and x = 0–6, have been carried out using a MOM derivatograph. Dehydration takes place in multiple steps. Anhydrous fluoberyllates dissociate first to MF₂ and BeF₂ and then decompose to MO and BeO in two steps. Metal fluorides give ultimately oxyfluoride in one step. None of the intermediate hydrates is thermally stable, except CuF₂ · 2 H₂O. The probable mechanisms of thermal decomposition have been reported. Pyrolysed products are characterised by elemental analysis and X-ray powder patterns. The value of enthalpy change is reported for each decomposition step.     

Mass spectrometry of metal chelates-III Further studies on metal oxinates

The mass spectra of the oxmates of gallium, dysprosium, beryllium, samarium, cadmium, neodymium, indium, bismuth, lanthanum, yttrium and gadolinium have been recorded. The results have been correlated with the structures proposed m the literature. The integrated ion current method has been applied to determine submicrogram quantities of some of these compounds. Oxinates insoluble in organic solvents or undergoing reaction on solution were precipitated directly in the evaporation probe. The factors which determine the suitability of metal chelates for the estimation of metals by this method are discussed together with the instrumental factors which determine the ultimate sensitivity.     

Growth modes of ultrathin metal films on dissimilar metal substrates

This paper outlines some of the outstanding problems associated with the experimental determination of the growth modes of thin metal films on dissimilar metal substrates, and suggests approaches for overcoming them. Specifically, it discusses ways to differentiate between Stranski-Krastanov and alloy growth, the quantitative fitting of “Auger signaltime” plots, and the determination of the stoichiometry of surface alloys. A comparison of the observed growth modes with those predicted by the Macroscopic Atom Method shows that the results predicted by this method must be treated with extreme caution.     

Nucleophilic cyclocarbenes as ligands in metal halides and metal oxides

The nucleophilic cyclocarbene 1,3-dimethylimidazolin-2-ylidene (1) reveals universal ligand properties in metal coordination chemistry: in addition to the well-known stabilization of low oxidation-state transition metal fragments, this particular class of carbenes also coordinates with metal halides and metal oxides thus resembling the properties of conventional ether (O), amine (N), and phosphane (P) ligands. Complexes of titanium(IV), zirconium(IV), hafnium(IV), vanadium(II), niobium(IV), tantalum(IV) of general formula MX_nL_m (3a–f) and the rhenium(VII) of formula CH₃ReO₃L₂ (3g) are reported (L = 1,3-dimethylimidazolin-2-ylidene).     

Amine-templated metal squarates

Two layered amine-templated cobalt squarates, [C₆N₂H₁₄]₂[Co₂(C₄O₄)₃(H₂O)₄], I, and [C₃N₂H₅]₂[Co₂(C₄O₄)₃(H₂O)₄], II, have been prepared under hydrothermal conditions. Both I and II contain chains formed by dimers comprising two cobalt atoms bound to the squarate units, the chains being connected through hydrogen bond interactions. An amine-templated cobalt squarate of the formula [C₄N₂H₁₂][Co(C₄O₄)₂(H₂O)₄][H₂O]₂, III, as well as its Ni, Zn and Cd analogues have been prepared by room temperature reactions. III has a layered architecture wherein the cobalt-squarate monomers are linked by the amine molecules. Co and Zn analogues of [Ni(C₄O₄)(H₂O)₂(C₃N₂H₄)] with ligating imidazole units have also been prepared and characterized.     

Template-grown metal nanowires

The growth of metal nanowires using membranes as hard templates is reviewed. The method provides access to arrays of single-crystal metal nanowires and to quasi-one-dimensional metal nanostructures with controlled compositional variation along their length. Recent applications of these kinds of nanowires to problems in superconductivity, optical spectroscopy and sensing, and catalytic conversion of chemical to mechanical energy are reviewed.     

Simple metal clusters

The response of alkali cluster ions to an optical excitation is investigated for two different photon energy domains. Below the ionization potential giant resonances in the photoabsorption cross-section are observed for closed shell species. Above the ionization potential, the ionization process competes with the photofragmentation process. The number of valence electrons determines both the behavior of the photoabsorption spectrum and the evolution of the ionization cross-section with the cluster size. The stability of the clusters against an excess of charge is examined through the observation of an asymmetric fission of Na _n ⁺⁺ . Experimental results are discussed in term of an electrostatic model giving an estimate of the critical size of stability and of the height of the coulombic barrier.     

Precious metal polymers

The synthesis and properties of polymers containing precious metal centres in the backbone are described. Polymers with labile transition metal centres can be prepared by ring-opening polymerization of macrocycles, and examples are given with silver, gold and palladium as the metal centres. In some cases, the polymers can be further organized by using ligands with hydrogen bonding substituents, and self-assembly into sheet or network structures can then occur. Secondary bonding between inorganic centres can also lead to ordered self-assembly. Bicyclic precursors can ring open to form either chains or sheets.