Dy2O3 as a catalyst for a Meerwein-Ponndorf-Verley reaction

Dy₂O₃ activated at high temperature is reported as a catalyst for the liquid phase reduction of cyclohexanone. The catalytic activity of Dy₂O₃ activated at 300, 500 and 800°C and its mixed oxides with alumina for the reduction of cyclohexanone with 2-propanol has been reported. The data have been correlated with the electron donating properties of the catalysts which were reported from the adsorption of electron acceptors [EA] of various electron affinity on the surface of these oxides.     

Electron donor properties and catalytic activity of manganese ferrospinels

The electron donating properties of manganese ferrospinels of various compositions (MnFe₂O₄, Mn_1.2Fe_1.8O₄, Mn₂FeO₄ and Mn_2.5Fe_0.5O₄) were studied from the adsorption of electron acceptors of various electron affinity values from acetonitrile as solvent. The limit of electron transfer from the oxide surface is from 1.77 to 2.40 eV in terms of the electron affinity of the electron acceptor. The data have been correlated with the catalytic activity of these oxides towards autoxidation of sulfites. Both weak and strong electron donor sites catalyze the reaction.     

Polytetrafluoroethylene surface modification by impregnation with iron and manganese oxides

Polytetrafluoroethylene (PTFE) surfaces can be modified by impregnation with Fe and Mn oxides by procedures based on Fe(CO)₅ sorption in the polymer and followed by oxidation with H₂O₂ or KMnO₄. The effect of some variables (reagent concentrations and temperature) on the amount of oxides deposited was examined. The oxides incorporated in PTFE can be only partly removed by acid cleaning. Water–PTFE contact angles and PTFE surface resistivity are altered. Electron and x-ray diffractograms of the oxides were obtained but unambiguous identification was not achieved. Transmission electron microscopy examination of surface replicas showed that impregnation with oxides which covered most of the PTFE surface was fairly uniform.     

Surface electron donor properties of Dy2O3, Y2O3 and their mixed oxides with alumina catalyst

The limit of electron transfer in electron affinity from the oxide surface to the electron acceptor (EA) are reported from the adsorption of EA on Dy₂O₃, mixed oxides of Dy₂O₃ with alumina and mixed oxides of Y₂O₃ with -alumina. The extent of electron transfer is understood from magnetic measurements.     

Reduction reactivity of CuO-Cr2O3 mixed oxides with hydrogen and the effect of gamma pre-irradiation

The physico-chemical properties and parameters of CuO-Cr₂O₃ mixed oxides as well as their reactivity toward reduction with hydrogen have been investigated by means of various methods (X-ray analysis, electron spectroscopy for chemical analysis (ESCA), DTA, IR spectroscopy and scanning electron microscopy). A series of mixed oxides of various compositions in the range 0–100% of each component were prepared by calcinating the mass of coprecipitated basic carbonates in air. The reduction kinetics were studied by isothermal thermogravimetry in the temperature range 180–470 ° C. The effect of the origin of the oxides on the reactivity of the oxide system during its hydrogen reduction was proved by comparing its properties with those of a previously studied series of analogous mixed oxides, which were prepared from different precursors.The pre-irradiation of the system by ⁶⁰Co γ radiation results in a positive kinetic effect (an increase in the reduction rate) if the lowest dose applied is 2.57 × 10⁶ Gy. The magnitude of the radiation effect is greatly dependent on the composition of the mixed oxides.     

Characterization of chemical information and morphology for in‐depth oxide layers in decarburized electrical steel with glow discharge sputtering

A combination of scanning electron microscopy, X‐ray photoelectron spectroscopy, Fourier transform infrared spectroscopy, and transmission electron microscopy with a sampling method by glow discharge sputtering was successfully employed to characterize the chemical information and microscopic features of oxide layers formed during decarburization annealing of electrical steel in the depth direction at high resolution. The discontinuous surface oxides consisted of SiO₂, (Fe,Mn)SiO₃/(Fe,Mn)₂SiO₄, and FeO. SiO₂ embedded in the (Fe,Mn)₂SiO₄ at the surface may be developed by the preferential nucleation and growth kinetics. The discrete or often relatively spherical oxides of internal oxidation by the energetically favorable surface effect were identified as a mixture of SiO₂ and (Fe,Mn)₂SiO₄ at a depth of ~0.5 µm from the surface. The oxides of networks and small particles at a depth greater than ~1 µm were solely silica, of which the morphologies were possibly caused by the enhanced diffusion of oxygen atoms and Si atoms at grain boundaries or sub‐grain boundaries. The equilibrium and kinetic considerations served by theoretical calculations were introduced to understand the formation and behavior of the observed in‐depth oxidation. Copyright © 2013 John Wiley & Sons, Ltd.     

Preparation, characterization and gas-sensitive properties of nano-crystalline Cr2O3Fe2O3 mixed oxides

A series of Fe₂-_xCr_xO₃(x = 0, 0.4, 0.8, 1.2, 1.6, 2.0) mixed oxides have been prepared with the chemical coprecipitation method and characterized by specific surface area, transmission electron microscopy (TEM), powder X-ray diffraction (XRD), IR and Mössbauer spectroscopy. Single-phase Fe-Cr mixed oxide nano-crystalline powders with corundum structure are obtained, and the results of the five characterization methods are well accordant with each other. Furthermore, gas-sensitive properties of the sensors made of the oxide powders have been studied.     

Combustion synthesis and characterization of porous perovskite catalysts

Porous perovskite-type complex oxides LaCoO₃ and La_0·95Sr_0·05Ni_0·05Co_0·95O₃ were produced by combustion method. The properties of these porous materials such as crystal structures, particle sizes, surface patterns, pore size, surface area and pore volume were characterized by X-ray diffraction( XRD), scanning electron microscopy(SEM) and BET measurements. The results indicated that all porous materials are of the perovskite-type complex oxides. Doping Sr²⁺ ions on site A and doping Ni²⁺ ions on site B entered the crystal lattices of LaCoO₃ in the place of La³⁺ and Co³⁺, respectively, and the maximum peak of XRD patterns of doping sample was weaken and broaden. Morphological microscopy demonstrated agglomerates involved mostly thin smooth flakes and layers perforated by a large number of pores and its lamella decreased with the introduction of Sr²⁺ and Ni²⁺. Hysteresis loop in the N₂ adsorption-desorption isotherm of samples indicated its porous structures and the doping effect on its pore size, surface area and pore volume were improved. The porous catalysts have been tested for methane catalytic combustion and the results showed that these catalysts possessed high catalytic activity.     

Effect of the chemical nature of metal oxide on adhesion to the polyacrylate matrix in filled nanocomposites

The morphology, particle size, and thermochemical properties of the surface of oxides Al₂O₃, NiO, TiO₂, ZnO, and ZrO₂ obtained by the wire electroexplosion method were studied. The nanoparticles are spherical, with a mean diameter of 54–86 nm depending on the nature of the oxide. The hydrophilicity of the surface of metal oxide nanopowders was found to change in the series NiO-ZrO₂-TiO₂-ZnO-Al₂O₃. Nanocomposites with widely varied compositions were obtained from butyl methacrylate copolymer with 5 wt % methacrylic acid and the oxides under study. The enthalpies of dissolution of the composites in chloroform were determined by Calvet calorimetry. The enthalpies of copolymer mixing with oxides were calculated using the thermochemical cycle. The limiting enthalpies of copolymer adhesion to the oxide surface were calculated from the thermochemical data. The limiting adhesion enthalpy was shown to be negative for all oxides under study; these values decreased in magnitude as the surface hydrophilicity increased. The results were analyzed from the viewpoint of balance between the specific and dispersion interactions at the interface.     

Specifics of adsorption and chemical processes on the surface of gamma-irradiated vanadium dioxide

The effect of γ-irradiation on the electrophysical properties and processes of thermal desorption of water from the surface of vanadium oxides V₂O₃-VO_2?δ-VO_2+δ-V₂O₅ were studied by the thermal analysis and electric conductivity techniques. It was shown that the amount of adsorbed water on the surface and the phase composition of the surface changed under exposure to low doses. The observed effects for vanadium oxides as semiconductors were correlated with analogous processes for alumina as a dielectric. It was found that the surface conductivity in irradiated VO_2?δ samples during the chemical reaction of ethanol with benzoic acid adsorbed after irradiation was substantially higher than that in their unirradiated counterparts. It was assumed that irradiation enhances the metal-semiconductor phase transition on the VO_2?δ surface during the chemical reaction.     

Active sites on the surface of nanocrystalline semiconductor oxides ZnO and SnO<Subscript>2</Subscript> and gas sensitivity

The data on active sites on the surface of nanocrystalline semiconductor oxides ZnO and SnO₂ are reviewed. Their interrelation to the gas sensitivity of the materials toward the main air pollutants, viz., CO, NO₂, NH₃, and H₂S, is analyzed. The influence of the synthesis conditions, microstructure parameters, content of dopant impurities, and the presence of catalytic modifiers on the concentration of various active sites on the oxide surface is considered. Relationships between the concentration of the surface sites and sensitivity of the oxides to gases with various chemical properties are revealed. The active sites responsible for the formation of a sensory signal upon the selective interaction with molecules of the detected gases are determined.     

Smoothing and passivation of special Si(111) substrates: studied by SPV,PL, AFM and SEM measurements

Surface sensitive techniques, the field-modulated surface photovoltage, photoluminescence measurements, atomic force microscopy and scanning electron microscopy, were employed to yield detailed information on the influence of wet-chemical treatments on the preparation induced microroughness and electronic properties of wet-chemically passivated Si(111) substrates with special surface morphology. Stepped substrates with evenly distributed atomically flat terraces were prepared and passivated by thin oxide layers, which were used as a starting point for the subsequent H-termination after long storage in air. It was shown that their surface morphology and electronic properties do not degrade. Applying this preparation method to solar cell substrates with randomly distributed Si(111) pyramids, we achieved significantly lower densities of surface states and reduced recombination loss at a-Si:H/c-Si interfaces, compared with conventional pretreatments. The surface microroughness, the density of rechargeable states and the resulting recombination loss on a-Si:H/c-Si heterojunctions were found to be mainly influenced by two steps of surface pretreatment: firstly, the wet-chemical smoothing procedure of structured substrates and, secondly, the removal of native and wet-chemical oxides during the final etching in HF- or NH₄F- containing solutions. Figure After wet-chemical oxidation in H₂SO₄/H₂O₂ and storage in air     

DFT Study of Methylene Blue Adsorption on ZnTiO3 and TiO2 Surfaces (101)

The search for alternative materials with high dye adsorption capacity, such as methylene blue (MB), remains the focus of current studies. This computational study focuses on oxides ZnTiO₃ and TiO₂ (anatase phase) and on their adsorptive properties. Computational calculations based on DFT methods were performed using the Viena Ab initio Simulation Package (VASP) code to study the electronic properties of these oxides. The bandgap energy values calculated by the Hubbard U (GGA + U) method for ZnTiO₃ and TiO₂ were 3.17 and 3.21 eV, respectively, which are consistent with the experimental data. The most favorable orientation of the MB adsorbed on the surface (101) of both oxides is semi-perpendicular. Stronger adsorption was observed on the ZnTiO₃ surface (−282.05 kJ/mol) than on TiO₂ (–10.95 kJ/mol). Anchoring of the MB molecule on both surfaces was carried out by means of two protons in a bidentate chelating (BC) adsorption model. The high adsorption energy of the MB dye on the ZnTiO₃ surface shows the potential value of using this mixed oxide as a dye adsorbent for several technological and environmental applications.     

“嵌入模型”在γ-Al2O3负载单组分和双组分金属氧化物催化剂中的应用

探讨负载型金属氧化物催化剂的表面组分与载体之间的相互作用, 有助于理解相关催化剂的催化作用本质. 近年来, 我们对单组分CuO以及双组分CuO-Mn₂O₃, CuO-CoO等金属氧化物在γ-Al₂O₃载体表面的分散行为和存在状态, 及其物理化学性质和催化性能(CO+O₂和NO+CO模型反应)进行了研究. 结果表明, 这些金属氧化物在γ-Al₂O₃载体表面的分散行为和所得负载型催化剂样品的一些物理化学性质及其催化性能均可参照“嵌入模型”来解释. 在此基础上, 我们讨论了这些样品的“组成-结构-性质”间的关系, 并针对表面负载双组分金属氧化物样品提出了表面协同氧空位参与的NO+CO反应机理.     

Heterogeneity of surface colour centres on alkaline earth metal oxides as revealed through EPR/ENDOR spectroscopy

A variety of surface anion vacancies, or point defects, are created by high‐temperature activation of a series of polycrystalline alkaline earth metal oxides (MgO, CaO and SrO). Subsequent UV irradiation of the activated oxide under a hydrogen atmosphere results in the generation of surface colour centres [F_S⁺(H)], by electron trapping at these anion vacancies. The paramagnetic properties of these colour centres were studied by EPR and ENDOR spectroscopy. ¹H ENDOR spectroscopy revealed that a well defined heterogeneity of trapped electron species exists on each oxide surface, as characterized by the different superhyperfine couplings between the trapped electron and the nearby proton of the F_S⁺ (H) centre. On MgO and CaO two dominant F_S⁺ (H) centres were identified (labelled sites I and II) whereas on SrO three F_S⁺ (H) species were found (sites I, II and III). The possible surface sites responsible for electron stabilization are discussed, and include a 3C corner mono‐vacancy, a 4C mono‐vacancy and an anion–cation di‐vacancy. The results indicate that regardless of the oxide used, a common degree of morphological similarities exists on each oxide. Copyright © 2002 John Wiley & Sons, Ltd.     

Challenges in the use of density functional theory to examine catalysis by M‐doped ceria surfaces

For CeO₂ or M‐doped CeO₂ catalysts, reliable energetics associated with surface reactivity requires accurate representation of oxidized and reduced metal states. Density functional theory (DFT) is used extensively for metals and metal oxides; however, for strongly correlated electron materials, conventional DFT fails to predict both qualitative and quantitative properties. This is the result of a localized electron self‐interaction error that is inherit to DFT. DFT+U has shown promise in correcting energetic errors due to the self‐interaction error, however, its transferability across processes relevant to surface catalysis remains unclear. Hybrid functionals, such as HSE06, can also be used to correct this self‐interaction error. These hybrid functionals are computationally intensive, and especially demanding for periodic surface slab models. This perspective details the challenges in representing the energetics of M‐doped ceria catalyzed processes and examines using DFT extensions to model the localized electronic properties. © 2013 Wiley Periodicals, Inc.     

Effect of the oxidation state of manganese in the manganese oxides used in the synthesis of Mn-substituted cordierite on the properties of the product

Catalysts based on Mn-substituted cordierite 2MnO · 2Al₂O₃ · 5SiO₂ have been synthesized using different manganese oxides (MnO, Mn₂O₃, and MnO₂) at a calcination temperature of 1100°C. The catalysts differ in their physicochemical properties, namely, phase composition (cordierite content and crystallinity), manganese oxide distribution and dispersion, texture, and activity in high-temperature ammonia oxidation. The synthesis involving MnO yields Mn-substituted cordierite with a defective structure, because greater part of the manganese cations is not incorporated in this structure and is encapsulated and the surface contains a small amount of manganese oxides. This catalyst shows the lowest ammonia oxidation activity. The catalysts prepared using Mn₂O₃ or MnO₂ are well-crystallized Mn-substituted cordierite whose surface contains different amounts of manganese oxides differing in their particle size. They ensure a high nitrogen oxides yield in a wide temperature range. The product yield increases with an increasing surface concentration of Mn³⁺ cations. The highest NO_x yield (about 76% at 800–850°C) is observed for the MnO₂-based catalyst, whose surface contains the largest amount of manganese oxides.     

镁基复合氧化物催化烯烃环氧化反应

 制备了一系列镁基复合氧化物 MgM-n 样品 (M = Sn, Al, Ti, La, Ce, Zr; n 为 Mg/M 原子比), 用 X 射线衍射、N2 吸附-脱附、CO2 程序升温脱附、紫外可见漫反射光谱和电子自旋共振等手段表征了它们的结构和表面性质, 并考察了其以过氧化氢为氧化剂催化烯烃环氧化反应性能. 结果表明, MgM-n 样品表面碱量和催化性能与其中 M 的种类及含量密切相关. MgSn-4 样品的表面碱量比 MgAl-4 低, 虽两者在催化苯乙烯环氧化反应中, 苯乙烯转化率和环氧化物选择性均为 95% 左右, 过氧化氢利用率大于 80%, 但在循环使用过程中 MgSn-4 的催化性能更为稳定, 并在不同结构烯烃的环氧化反应中表现出优良的催化性能. 这除与 MgSn-4 表面碱强度适当有关外, 还与其中存在高分散的 Sn4+物种及其结构特性有关.     

Paramagnetic complexes of 9, 10-anthraquinone and 9-fluorenone on the metal oxide surface

Paramagnetic complexes of 9, 10-anthraquinone and 9-fluorenone adsorbed on the surface of calcium, magnesium, zinc, zirconium, and aluminum oxides and modified Al₂O₃ as well as on mixed oxides were studied by ESR and electron-nuclear double resonance. Radical anions that do not interact with Lewis acid sites are generated on the surfaces of oxides with electrondonating properties (CaO, MgO). Paramagnetic complexes of the anthraquinone or fluorenone radical anion with Lewis acid sites (coordinatively unsaturated metal cations) are formed in other cases. Several types of similar complexes can be formed. Mechanisms of interaction of the probe molecules with the metal oxide surface were proposed.     

Crystal chemistry and magnetic properties of layered metal oxides possessing the K2NiF4 or related structures

There is increasing interest in recent years in the structural chemistry and properties of layered metal oxides possessing the K₂NiF₄ or related structures. Many new oxides of this structure exhibiting novel properties are being reported from time to time in the literature. The crystal chemistry of the oxides of the general formula A₂BO₄ with particular reference to the stability of the K₂NiF₄ structure and the relations between the different structures exhibited by this family of oxides is discussed. Non-stoichiometry in these oxides is another aspect of interest discussed in the article. While K₂NiF₄ itself is a well-known two-dimensional antiferromagnet, oxides of this structure with a variety of magnetic properties are examined in some detail. Besides the ternary A₂BO₄ oxides, the structure and magnetic properties of complex oxides, where the A or/and the B ions are partly substituted by other cations, is discussed. Some of the problems related to this family of oxides that are worth investigating are indicated. Much of the discussion in this article would have relevance in understanding the structure and properties of layered materials.