Properties Of Mixed Metal Oxides Research Articles

Thermodynamic properties for metal oxides from first-principles

In this study, an efficient first-principles approach for calculating the thermodynamic properties of mixed metal oxides at high temperatures is demonstrated. More precisely, this procedure combines density functional theory and harmonic phonon calculations with tabulated thermochemical data to predict the heat capacity, formation energy, and entropy of important metal oxides. Alloy cluster expansions are, moreover, employed to represent phases that display chemical ordering as well as to calculate the configurational contribution to the specific heat capacity. The methodology can, therefore, be applied to compounds with vacancies and variable site occupancies. Results are, moreover, presented for a number of systems of high practical relevance: FeKTiO, KMnO, and CaMnO. For the reference materials, the agreement with experimental measurements is exceptional in the case of ilmenite (FeTiO3) and good for CaMnO3. When the generated data is used in multi-phase thermodynamic calculations to represent materials for which experimental data is not available, the predicted phase-diagrams for the KMnO and KTiO systems change dramatically. The demonstrated methodology is highly useful for obtaining approximate values on key thermodynamic properties in cases where experimental data is hard to obtain, inaccurate or missing.

Structure-property-performance relationship of transition metal doped WO3 mixed oxides for catalytic degradation of organic pollutants

Transition metal doped WO3 mixed oxides (named as W-M-O, M = Nb, Fe, Cr, Cu, Ti or Sn, respectively) with high structure stability were synthesized by modified sol-gel method using citric acid as organic crosslinking agent, and were evaluated for catalytic elimination of low-concentration toluene, monochlorobenzene and 1,2-dichloroethance with high toxicity and relatively stable molecule structure, as the typical examples for the pollutants of various volatile organic compounds (VOCs). Results of the structure-property-performance relationship research showed that mesoporous structure and nanocrystalline/amorphous state were formed, and binary metal components were dispersed into each other, which contributed to promoting the metal/metal electron interaction and adjusting the physicochemical properties of mixed metal oxides. The sequence of apparent catalytic activity for toluene degradation was: W–Nb–O＞W–Fe–O＞W–Cr–O, W–Cu–O＞W–Ti–O＞W–Sn–O＞WO3, and the sequence for monochlorobenzene degradation was: W–Nb–O＞W–Fe–O＞W–Cr–O, W–Ti–O＞W–Cu–O＞W–Sn–O＞WO3. There existed cooperative catalytic effect: mesopore and surface acid sites of catalysts facilitated adsorption, activation and breakage of the C-X bond, and then redox sites of catalysts promoted deep oxidation of a series of reaction intermediates to transform into CO2 and H2O. Especially, the optimized W–Nb–O catalyst deserved more attention, since it represented remarkable catalytic activity, selectivity and durability for three typical VOCs degradation along with good resistance to water vapor and corrosion of HCl.

Optical, electrical and dielectric properties of mixed metal oxides derived from Mg-Al Layered Double Hydroxides based solid solution series

In this work, we investigated the effect of aluminum content on optical, electrical and dielectric properties of mixed metal oxides (MMOs), obtained from calcination at 726 K of solid solution series based on nitrate intercalated Mg1-xAlx Layered Double Hydroxides (x = 0.167, 0.20, 0.25, 0.33). Subsequently, the MMOs were characterized by X-Ray Diffraction, and analyzed by Inductively Coupled Plasma, Scanning Electron Microscopy, Diffuse Reflectance Spectroscopy and Impedance Spectroscopy. It was found a shift of light absorption towards shorter wavelengths, and an increase of conductivity and dielectric constant when the aluminum content increased. Higher aluminum content exhibited strong ultraviolet light absorption with average band gap of 6.29 eV, and also manifested high reflectance in visible and near-infrared wavelengths, whereas lower aluminum content showed lower values of conductivity, dielectric constant and dielectric loss tangent. Moreover, the electrical, dielectric and optical parameters of MMOs were reported and discussed.

China coal-to-chems margins pressured as crude plunges

The structure, porosity, and adsorption properties of mixed metal oxides (MMO) were investigated according to the physical property of the starting materials. Layered double hydroxides (LDH), the pristine MMO, were prepared to have the same composition of Mg2Al(OH)6(CO3)0.5, but with different particle size. The prepared LDH were calcined at 400 °C to obtain MMO named CMO, HMO, and UMO corresponding to the pristine MMO synthesized by coprecipitation, hydrothermal treatment, and urea hydrolysis method, respectively. The particle sizes of CMO, HMO, and UMO were comparable to those of the pristine MMO in the order UMO > HMO > CMO; on the other hand, the crystallite sizes were almost the same as each other, regardless of the pristine MMO. According to the high-resolution transmission electron microscopy and X-ray diffraction, we could suggest that a single particle of MMO was composed of smaller domains of metal oxide crystallite. Consequently, small MMO particle tended to have interparticulate macropore, while large MMO particle had more possibility of containing intraparticulate mesopores, which was cross-confirmed by the N2 adsorption–desorption isotherms. Toluene and water vapor adsorption tests on three different MMO showed that the intraparticulate mesopores were advantageous to adsorb a variety of adsorbate. It was therefore concluded that MMO obtained by calcination of LDH would have high adsorption property when the pristine LDH has large particle dimension, thanks to the large portions of intraparticulate mesopores.

Particle size effect of layered double hydroxide on the porosity of calcined metal oxide

The structure, porosity, and adsorption properties of mixed metal oxides (MMO) were investigated according to the physical property of the starting materials. Layered double hydroxides (LDH), the pristine MMO, were prepared to have the same composition of Mg2Al(OH)6(CO3)0.5, but with different particle size. The prepared LDH were calcined at 400 °C to obtain MMO named CMO, HMO, and UMO corresponding to the pristine MMO synthesized by coprecipitation, hydrothermal treatment, and urea hydrolysis method, respectively. The particle sizes of CMO, HMO, and UMO were comparable to those of the pristine MMO in the order UMO > HMO > CMO; on the other hand, the crystallite sizes were almost the same as each other, regardless of the pristine MMO. According to the high-resolution transmission electron microscopy and X-ray diffraction, we could suggest that a single particle of MMO was composed of smaller domains of metal oxide crystallite. Consequently, small MMO particle tended to have interparticulate macropore, while large MMO particle had more possibility of containing intraparticulate mesopores, which was cross-confirmed by the N2 adsorption–desorption isotherms. Toluene and water vapor adsorption tests on three different MMO showed that the intraparticulate mesopores were advantageous to adsorb a variety of adsorbate. It was therefore concluded that MMO obtained by calcination of LDH would have high adsorption property when the pristine LDH has large particle dimension, thanks to the large portions of intraparticulate mesopores.

Selective Synthesis of Dimethyl Carbonate via Transesterification of Propylene Carbonate with Methanol Catalyzed by Bifunctional Li‐Al Nano‐composite

AbstractA series of binary mixed metal oxides (MMOs) were synthesized by combination of Mn+/Al3+ (Mn+= Li, Mg, Co, Ni, Zn) via co‐precipitation method and screened for transesterification of propylene carbonate (PC) with methanol. The aim of this study was to understand the role of acid‐base properties of MMOs on the activity and selectivity towards dimethyl carbonate (DMC) synthesis. The MMOs were characterized in detail by XRD, BET, TEM, NH3 and CO2‐TPD techniques. Catalysts synthesized by various combination of Mn+/Al3+ showed different textural and surface properties and the strength of acid‐base sites were also found to be significantly altered. These changes in physicochemical properties significantly affected the PC conversion and selectivity to DMC. Li+ incorporated in Al3+ showed the formation of octahedral α‐LiAlO2 phase having more amounts of active sites (O2−) present on the edges and corner of the structure. This resulted in easy accessibility of active sites and higher activity compared to other catalysts screened. Furthermore the catalyst retained high activity and selectivity for six consecutive recycle experiments; which is highly desirable for large‐scale application.

Structural Diversity in Heterobimetallic Complexes of Alkaline Earth Metals and Molybdenum, and Selective Dye Adsorption Properties of Mixed Metal Oxides Synthesized from the Heterobimetallic Complexes

AbstractFour heterobimetallic complexes of group 2 metal (Mg2+, Ca2+, Sr2+, and Ba2+) and molybdenum metalloligand ([Mo2O5L2]2− {H2L=2‐(3‐tert‐butyl‐2‐hydroxy‐5‐methoxybenzylamino) acetic acid}) have been synthesized and structurally characterized by single crystal X‐ray diffraction. The size‐dependent structural diversity has been observed. The coordination number of alkaline earth metal has gradually been increased from hexacoordinate to octa‐coordinate and as a result, we are able to get monomer to dimer to one‐dimensional (1D) coordination polymer. The lattice water molecule further connects two 1D chains and generated a two‐dimensional (2D) network. The heterobimetallic complexes afford mixed‐metal oxides of the type MMoO4⋅MoO3 (M = Mg2+, Ca2+, Sr2+, and Ba2+). The dye and gas adsorption properties of the mixed metal oxides have been studied. They exhibit selective cationic dyes adsorption from the mixture of anionic and cationic dye.

Influence of the preparation method on the structural properties of mixed metal oxides

In this paper, we report a study of the influence of the synthesis method on the properties of mixed metal oxides. For this, the syntheses of the mixed oxide by chemical coprecipitation and modified polymeric precursor (MPP) methods containing Ni2+, Zn2+, Al2+, and Zr4+ were conducted. For this purpose, the samples were characterized by X-ray powder diffraction, which showed defined peaks for the mixed metal oxides (NiO, ZnO, and ZrO2). The interaction of H2 with mixed oxides has been investigated using temperature-programmed reduction (H2-TPR); this result showed that the modified polymer precursor method favored the presence of the peak at elevated temperatures. The N2 physisorption (BET method) showed that the polymer precursor method provided higher specific surface areas and basic site numbers in relation to the coprecipitation method. Thermogravimetric analysis of the systems decomposition revealed their disintegration at ~550 °C and resulted in mixed metal oxides in both methods.

Influence of heating rate on the physical and electrochemical properties of mixed metal oxides anodes synthesized by thermal decomposition method applying an ionic liquid

Electrochemical oxidation has been focus of recent study in order to supply the growing demand for environmentally friendly wastewater treatment systems. In that sense, the development of efficient and durable mixed metal oxides (MMO) is a growing research field. Herein we propose a novel thermal decomposition methodology applying ionic liquid (IL) for the synthesis of Ti/Sn50SbxCe50−x anodes (where x = 10, 20, 30 or 40). The different MMOs were synthesized through thermal decomposition applying various heating rates (3, 6 and 9 °C min−1) reaching a calcination temperature of 600 °C. Furthermore, X-ray diffraction, scanning electron microscopy, cyclic voltammetry, electrochemical impedance spectroscopy, and electrolysis measurements were carried out. For the synthesized electrodes it was possible to notice a higher superficial area, porosity and electrochemical conductivity for the Sn50Sb40Ce10 electrode. Additionally, the redox couple Ce+4/Ce+3 was more intensely seen for this electrode which presented the best outcomes for Atrazine electrolysis, promoting a 58% concentration removal after 1 h electrolysis at current density as low as 1 mA cm−2.

High-temperature thermochemical energy storage based on redox reactions using Co-Fe and Mn-Fe mixed metal oxides

Metal oxides are potential materials for thermochemical heat storage via reversible endothermal/exothermal redox reactions, and among them, cobalt oxide and manganese oxide are attracting attention. The synthesis of mixed oxides is considered as a way to answer the drawbacks of pure metal oxides, such as slow reaction kinetics, loss-in-capacity over cycles or sintering issues, and the materials potential for thermochemical heat storage application needs to be assessed. This work proposes a study combining thermodynamic calculations and experimental measurements by simultaneous thermogravimetric analysis and calorimetry, in order to identify the impact of iron oxide addition to Co and Mn-based oxides. Fe addition decreased the redox activity and energy storage capacity of Co3O4/CoO, whereas the reaction rate, reversibility and cycling stability of Mn2O3/Mn3O4 was significantly enhanced with added Fe amounts above ~15mol%, and the energy storage capacity was slightly improved. The formation of a reactive cubic spinel explained the improved re-oxidation yield of Mn-based oxides that could be cycled between bixbyite and cubic spinel phases, whereas a low reactive tetragonal spinel phase showing poor re-oxidation was formed below 15mol% Fe. Thermodynamic equilibrium calculations predict accurately the behavior of both systems. The possibility to identify other suitable mixed oxides becomes conceivable, by enabling the selection of transition metal additives for tuning the redox properties of mixed metal oxides destined for thermochemical energy storage applications.

Synthesis–structure correlations of manganese–cobalt mixed metal oxide nanoparticles

Mixed metal oxides in the nanoscale are of great interest for many aspects of energy related research topics as water splitting, fuel cells and battery technology. The development of scalable, cost-efficient and robust synthetic routes toward well-defined solid state structures is a major objective in this field. While monometallic oxides have been studied in much detail, reliable synthetic recipes targeting specific crystal structures of mixed metal oxide nanoparticles are largely missing. Yet, in order to meet the requirements for a broad range of technical implementation it is necessary to tailor the properties of mixed metal oxides to the particular purpose. Here, we present a study on the impact of the nature of the gas environment on the resulting crystal structure during a post-synthesis thermal heat treatment of manganese–cobalt oxide nanoparticles. We monitor the evolution of the crystal phase structure as the gas atmosphere is altered from pure nitrogen to synthetic air and pure oxygen. The particle size and homogeneity of the resulting nanoparticles increase with oxygen content, while the crystal structure gradually changes from rocksalt-like to pure spinel. We find the composition of the particles to be independent of the gas atmosphere. The manganese–cobalt oxide nanoparticles exhibited promising electrocatalytic activity regarding oxygen evolution in alkaline electrolyte. These findings offer new synthesis pathways for the direct preparation of versatile utilizable mixed metal oxides.

Preparation and properties of mixed metal oxides based layered double hydroxide as anode materials for dye-sensitized solar cell

Abstract In this paper, a new simple approach has been developed for the preparation of anode materials for dye-sensitized solar cell by a facile calcination method using the layered double hydroxide (LDH) as a precursor. The ZnAl-LDH with molar ratio Zn:Al = 3:1 is prepared by urea method. The mixed metal oxides (MMO) are prepared by calcining the LDH at different temperatures and a series of dye-sensitized solar cells (DSSC) are assembled by the corresponding oxides and the dye Ruthenizer 535-bisTBA (N719). The basic parameters are investigated by X-ray diffraction (XRD), scanning electron microscope (SEM), thermogravimetric and differential thermal analysis (TG–DTA), nitrogen sorption analysis and UV–Vis absorption spectrum. The band gap of the MMO (about 3.13 eV) obtained from ZnAl-LDH is similar to that of the pure ZnO (about 3.2 eV). The photovoltaic behaviors of solar cells are characterized and the best efficiency is 0.0129% when the calcining temperature is 500 °C.

Reduction Properties of Physically Mixed Metallic Oxide Oxygen Carriers in Chemical Looping Combustion

A combined experimental and theoretical study was carried out to examine the reactivity of physically mixed metallic oxide oxygen carriers under conditions pertinent to chemical looping combustion. This was motivated by the notion that mixed metal oxides can resolve many of the shortcomings associated with the conventional oxygen carriers. Experiments were conducted on binary mixtures of Cu, Fe, and Ni oxides under reducing environments of H2, CO, and CH4. Experiments were performed in a TGA (thermogravimetric analyzer) at atmospheric pressure over a range of temperatures between 500 and 950 °C. The concentration of the reducing gas (volume fraction of H2, CO, and CH4) and the weight percentage of metal oxides were systematically varied between 20 and 70% volume fraction (VF) and 0−100% wt, respectively. For each mixture, the activation energy, reaction order, and pre-exponential factor were extracted from experimental measurements of the weight loss using the shrinking core model. Experimental observations suggested that for any given mixture, the conversion time (i.e., time corresponding to full conversion) was the geometric mean of the conversion times of the parent materials. On the basis of this observation, an in-depth theoretical analysis of the kinetic properties of mixed carriers was carried out which, in turn, led to the development of a simple methodology for determining the kinetic properties of mixed metal oxides from those of its parent materials. The methodology significantly reduces the need for extensive reactivity measurements when dealing with metal oxide mixtures. The validity of the methodology was verified through a series of comparisons with experimental data.

Tuning of textural properties of mixed metal oxides by changes in anionic composition of hydrotalcite-like precursors

Benzoate anions were introduced into the interlayer space of Mg–Al, Mg–Ni–Al and Mg–Cu–Al hydrotalcite-like compounds. The structure of the obtained samples was studied by XRD and UV–vis-DR spectroscopy. Differences in an orientation of the interlayer anions influenced by the cationic composition of the brucite-like sheets were observed. The mechanism of thermal decomposition of layered double hydroxides intercalated with benzoates was examined by thermal analysis methods (TG-SDTA-EGA). It was found that the thermal treatment of the hydrotalcite-like materials containing organic anions led to a formation of mixed metal oxides characterized by a very uniform pore size distribution. Moreover, these materials exhibited the BET surface area significantly higher compared to the calcined Mg–Al–CO 3 hydrotalcite. The generation of the uniform porosity was strongly promoted by a modification of the starting layered structure with transition metal cations (Ni 2+ and Cu 2+). This effect was attributed to a catalytic role of transition metal cations in the burning of interlayer organic species. CO 2 and H 2O formed during the removal of the interlayer anions were found to be responsible for an improvement of textural properties of the hydrotalcite-derived samples.

Mössbauer spectroscopic investigations of spin glass and other magnetic properties of mixed metal oxides

The application of Mossbauer spectroscopy to the study of the megnetic properties of mixed metal oxides is illustrated here by descriptions of its use in the investigation of three different systems. Firstly, its use to examine relaxation processes and supertransferred magnetic hyperfine interactions in the compound iron antimonate which exhibits unusual spin glass behaviour is described. Secondly, the role of Mossbauer spectroscopy in detecting the nature of iron species in solid solution with zirconium dioxide and the segregation of phases during phase transitions is outlined. Finally, the important contribution which Mossbauer spectroscopy can make to the characterisation of new materials in which iron is located within the three dimensional channels of niobium titanium phosphate is described.