Electron beam induced changes in transition metal oxides

Electron beam induced changes in maximal valence transition metal oxides V(2)O(5), M(o)O(3) and TiO(2) (anatase) were studied by means of electron energy-loss spectroscopy and electron diffraction in transmission electron microscopy. For V(2)O(5), the observed chemical shifts of the L-edge reveal the reduction of V(5+) to V(2+). The structure changes from orthorhombic V(2)O(5) to cubic VO. MoO(3) can be reduced to a phase with an oxidation state less than that in MoO(2). No notable structural or electronic change in TiO(2) (anatase) is observed. The different behaviours of the studied oxides under an electron beam are discussed with respect to bonding energy and lattice structure.     

The gel route to transition metal oxides

The so-called “sol-gel” process offers new approaches to the synthesis of transition metal oxides. Based on inorganic polymerization from molecular precursors, it leads to highly condensed species or colloids. These colloids are actually two-phase systems in which small oxide particles are dispersed in a liquid medium. A very large interface separates both phases and interfacial phenomena, at the oxide-water interface, lead to new features in the physics and chemistry of transition metal oxides. Ordered aggregation of oxide particles may occur, giving rise to colloidal crystals or anisotropic tactoids in which the mean distance between particles can be of about 0, 1 μm. This distance can be decreased leading to ordered solid aggregates. Transition metal oxide gels exhibit the physical properties of both phases, i.e., electronic properties arising from electron hopping through the mixed valence oxide network and ionic properties arising from proton diffusion through the liquid phase. Electronic and ionic properties appear to be strongly related through the very large interface. Large coatings can be easily deposited from colloidal solutions and transition metal oxide gels should be very useful for making microionic devices.     

Ultrasound-assisted synthesis of nanostructured transition metal oxides

Ultrasound pretreatment of aqueous solutions of cetyltrimethylammonium bromide as a structure- directing agent has been applied to prepare nanostructured mesoporous Mn, Fe, and Ni oxides. After removal of the template by triple extraction with a water–ethanol solution of sodium acetate or ammonium chloride, air-calcined samples of oxide materials prepared in such a way possess surface areas of about 300–450 m²/g and are thermally stable up to 300°C.     

Oxidation of dodecane on transition metal oxides

The catalytic oxidation of dodecane on individual and mixed vanadium and molybdenum oxides is studied. Products of the oxidation of alkane are studied qualitatively and quantitatively. The activities of the samples of the catalysts with various ratios of vanadium and molybdenum oxides are compared. One possible scheme for the activation of reagents on a catalyst is given.     

Polymer flame inhibition in presence of transition metal oxides

Summary Transition metal oxides like Fe₂O₃, Ni₂O₃, Co₂O₃ and MnO₂ suppress the combustion of polystyrene. The effect has been explained on the basis of condensed-phase and gas-phase reactions.With 2 figures and 1 table     

The oxidation of toluene on transition metal oxides

The oxidation of toluene on pure vanadium and molybdenum oxides was found to follow independent paths; it was benzene ring oxidation on V₂O₅ and side chain oxidation on MoO₃. On mixed xV₂O₅ · yMoO₃ oxides, the main reaction was the addition at the double bond preferably positioned meta rather than one-electron oxidation.     

Catalysts based on cordierite modified with transition metal oxides

Catalysts active in ammonia oxidation have been obtained by the substitution of transition metal (Mn, Fe, Co, Ni, and Cu) ions for Mg ions in the cordierite structure 2MgO · 2Al₂O₃ · 5SiO₂ at 1100°C. Their phase composition, texture, and activity depend on the type and amount of introduced transition metal oxide. The Mn- and Cu-containing catalysts, which consist of substituted cordierites 2(Mg_{1 ? x} M _x )O · 2Al₂O₃ · 5SiO₂ and Mn₂O₃ or CuO crystallites located on their surface, are most active in ammonia oxidation. The catalysts are characterized by a small specific surface area and have large pores, whose total volume is small. The Fe-containing catalysts consist of the Fe-substituted cordierite phase and particles of an iron oxide phase. These particles are mostly located in internal pores of the catalysts and are, therefore, hardly accessible to ammonia molecules. The introduction of Co or Ni oxide leads to the formation of a low-active spinel phase rather than the cordierite phase.     

Oxidative desulfurization of benzene fraction on transition metal oxides

It is established that molecular oxygen is able to oxidize thiophene selectively in a mixture with benzene on V₂O₅ · MoO₃. The introduction of thiophene inhibits the oxidation of benzene. It is shown that the conversion of thiophene during operation of the catalyst is reduced at first and then increases until it reaches its initial value.     

Thermogravimetric determination of oxygen in transition metal oxides with a derivatograph

The thermal decompositions of a series of complex copper oxides and some other transition metal oxides were studied by simultaneous TG-DTA. The oxygen stoichiometry was determined by thermogravimetry. For the compound YBa₂Cu₃O_7–x, the electrical properties and some structural peculiarities depend on the oxygen content.

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Zusammenfassung Die thermische Zersetzung einer Reihe von komplexen oxidischen Kupferverbindungen sowie weiterer Übergangsmetallverbindungen wurde durch simultane TG-DTA untersucht. Die Sauerstoff-Stöchiometrie dieser Verbindungen wurde durch Thermogravimetrie ermittelt. Bei der Verbindung YBa₂Cu₃O_7–x wurde eine Abhängigkeit der elektrischen Eigenschaften und einiger struktureller Besonderheiten vom Sauerstoffgehalt gefunden.

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R . . , YBa₂Cu₃O_7–x .

     

Challenges in the understanding oxygen reduction electrocatalysis on transition metal oxides

Transition metal oxides are attractive noble metal-free catalysts of the oxygen reduction for application at the cathode of alkaline membrane fuel cells or metal-air batteries. However, despite of a rapidly increasing number of publications devoted to the oxygen electrocatalysis on transition metal oxides, a clear picture regarding the relations between their structure and composition on the one hand and electrocatalytic activity on the other hand is lacking. This short review discusses challenges facing researchers seeking to understand electrocatalysis of the oxygen reduction reaction on transition metal oxides.     

Method to affect catalytic properties of low-temperature transition metal oxides

A method to affect the catalytic properties of low-temperature oxide catalysts by their synthesis through hydroxide compounds of the required composition and structure is suggested.

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过渡金属氧化物催化剂上甲烷催化燃烧的研究

与传统燃烧方式相比 ,催化燃烧因具有能量利用率高 ,且基本无污染等突出优点而受到人们的重视。以Ｐｄ为代表的贵金属燃烧催化剂[1] 的研究发展得较为完善 ,但价格昂贵使这类催化剂难以被推向实用。而过渡金属氧化物则价格便宜 ,并且也被认为是很有潜力的燃烧催化剂[2 ] 。但这类催化剂的高温稳定性差 ,活性也远不及贵金属催化剂。为此 ,人们试图使用Ｌａ ,Ｓｒ等作为助剂 ,使其与活性组分生成具有钙钛矿结构的化合物 ;或使用六铝酸盐催化剂[3] 来克服过渡金属氧化物催化剂的缺点。这些方法对过渡金属氧化物催化剂的性能有所改善 ,但催化剂…     

Advances in catalytic elimination of atmospheric pollutants by two-dimensional transition metal oxides

Atmospheric pollutants can deteriorate air quality and put human health at risk. There is a growing need for green, economical, and efficient technologies, among which catalytic elimination technology is the most promising, to remove atmospheric pollutants. Two-dimensional transition metal oxides (2D TMOs) have recently become attractive catalysts due to their highly exposed active sites, excellent reactant transport properties, and extraordinary catalytic performance. This review systematically summarizes the top-down and bottom-up preparation methods of 2D TMOs and focuses on the specific applications of 2D TMOs in the catalytic elimination of atmospheric inorganic pollutants and volatile organic pollutants. The development of 2D TMOs in the catalytic elimination of atmospheric pollutants is prospected. This review is expected to provide design insights into efficient 2D TMOs to remove atmospheric pollutants.     

Investigation of exchange reactions between alkaline earth metal oxides and some transition metal halides

DTA and the simultaneous recording of electrical conductivity were applied for the investigation of exchange reactions between alkaline earth metal oxides and some lead, copper and nickel halides. Although the possibility of the gaseous phase is not excluded for the majority of the reactions investigated, it is either the appearance of a liquid phase or the polymorphous transformation of CaO that has the decisive effect on the interaction mechanism.     

Dehydrogenation activities of highly dispersed transition metal oxides on NaY zeolite

Highly dispersed α-Fe_2O_3/NaY,NiO/NaY,and CuO/NaY catalyst systems were pre-pared by impregnation method.Dispersion thresholds of the transition metal oxides on NaY" zeolitewere determined by XRD phase analysis.The dispersion capacities of the transition metal oxides on NaYzeolite are much lower than that estimated on the basis of a closed packed monolayer in the micropores.The catalytic activity and selectivity of the highly dispersed oxide catalyst systems for ethylben-zene and cyclohexane dehydrogenation reactions were reported.     

Water adsorption and hydrolysis on molecular transition metal oxides and oxyhydroxides

Addition of water to molecular transition metal oxides (TiO2(g) and CrO3(g)) and oxyhydroxides (ScO(OH)(g), VO2(OH)(g), and MnO3(OH)(g)) was studied by means of quantum chemistry. In the investigated reactions, each reaction step comprised the breaking of one M=O bond and the formation of two OH groups. Exothermicity was observed when the product had tetrahedral or lower oxygen coordination. The reactions were found to involve stable water complexes as intermediates. The stabilities of such complexes were accentuated in the addition reaction Sc(OH)3(g) + H2O(g), in which the formation of a tetrahedral complex was found exothermic. For VO(OH)3(g), CrO2(OH)2(g), and MnO3(OH)(g), water addition to the remaining M=O bonds was found endothermic, whereas the formation of water complexes, using hydrogen bonds and preserving the oxyhydroxide kernel, was preferred. Thus, the sequence of such kernels for water clustering in the investigated reactions was found to be Sc(OH)3.H2O(g), Ti(OH)4(g), VO(OH)3(g), CrO2(OH)2(g), and MnO3(OH)(g). These stability considerations are important, as CrO2(OH)2(g) is believed to be the product of water-induced degradation of the protective chromium oxide scale on stainless steel at elevated temperatures.     

Magnetic susceptibility studies of lead oxyhalide glasses containing transition metal oxides

Magnetic susceptibility studies of lead oxyhalide glasses containing high concentrations of transition metal oxides such as MnO and Fe₂O₃ have been performed. While they exhibit predominantly antiferromagnetic interactions, the low temperature (<100K) region is dominated by paramagnetic contributions. The behaviour in these glasses is found to be similar to that of covalent oxide glasses and is different from that of purely ionic sulphate glasses. Communication No. 324 from the Solid State and Structural Chemistry Unit.